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Extrachromosomal DNA
Extrachromosomal DNA
Extrachromosomal DNA
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Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosomes, either inside or outside the nucleus of a cell. Most DNA in an individual genome is found in chromosomes contained in the nucleus. Multiple forms of extrachromosomal DNA exist, and, while some of these serve important biological functions,[1] they can also play a role in diseases such as cancer.[2][3][4]

In prokaryotes, nonviral extrachromosomal DNA is primarily found in plasmids, whereas, in eukaryotes extrachromosomal DNA is primarily found in organelles.[1] Mitochondrial DNA is a main source of this extrachromosomal DNA in eukaryotes.[5] The fact that this organelle contains its own DNA supports the hypothesis that mitochondria originated as bacterial cells engulfed by ancestral eukaryotic cells.[6] Extrachromosomal DNA is often used in research into replication because it is easy to identify and isolate.[1]

Although extrachromosomal circular DNA (eccDNA) is found in normal eukaryotic cells, extrachromosomal DNA (ecDNA) is a distinct entity that has been identified in the nuclei of cancer cells and has been shown to carry many copies of driver oncogenes.[7][8][3] ecDNA is considered to be a primary mechanism of gene amplification, resulting in many copies of driver oncogenes and very aggressive cancers.

Extrachromosomal DNA in the cytoplasm has been found to be structurally different from nuclear DNA. Cytoplasmic DNA is less methylated than DNA found within the nucleus. It was also confirmed that the sequences of cytoplasmic DNA were different from nuclear DNA in the same organism, showing that cytoplasmic DNAs are not simply fragments of nuclear DNA.[9] In cancer cells, ecDNA have been shown to be primarily isolated to the nucleus (reviewed in [2]).

In addition to DNA found outside the nucleus in cells, infection by viral genomes also provides an example of extrachromosomal DNA.

Prokaryotic

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pBR32 plasmid of E. coli

Although prokaryotic organisms do not possess a membrane-bound nucleus like eukaryotes, they do contain a nucleoid region in which the main chromosome is found. Extrachromosomal DNA exists in prokaryotes outside the nucleoid region as circular or linear plasmids. Bacterial plasmids are typically short sequences, consisting of 1 to a few hundred kilobase (kb) segments, and contain an origin of replication which allows the plasmid to replicate independently of the bacterial chromosome.[10] The total number of a particular plasmid within a cell is referred to as the copy number and can range from as few as two copies per cell to as many as several hundred copies per cell.[11] Circular bacterial plasmids are classified according to the special functions that the genes encoded on the plasmid provide. Fertility plasmids, or f plasmids, allow for conjugation to occur whereas resistance plasmids, or r plasmids, contain genes that convey resistance to a variety of different antibiotics such as ampicillin and tetracycline. Virulence plasmids contain the genetic elements necessary for bacteria to become pathogenic. Degradative plasmids that contain genes that allow bacteria to degrade a variety of substances such as aromatic compounds and xenobiotics.[12] Bacterial plasmids can also function in pigment production, nitrogen fixation and the resistance to heavy metals.[13]

Naturally occurring circular plasmids can be modified to contain multiple resistance genes and several unique restriction sites, making them valuable tools as cloning vectors in biotechnology.[10] Circular bacterial plasmids are also the basis for the production of DNA vaccines. Plasmid DNA vaccines are genetically engineered to contain a gene which encodes for an antigen or a protein produced by a pathogenic virus, bacterium or other parasites.[14] Once delivered into the host, the products of the plasmid genes will then stimulate both the innate immune response and the adaptive immune response of the host. The plasmids are often coated with some type of adjuvant prior to delivery to enhance the immune response from the host.[15]

Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia (to which the pathogen responsible for Lyme disease belongs), several species of the gram positive soil bacteria of the genus Streptomyces, and in the gram negative species Thiobacillus versutus, a bacterium that oxidizes sulfur. Linear plasmids of prokaryotes are found either containing a hairpin loop or a covalently bonded protein attached to the telomeric ends of the DNA molecule. The adenine-thymine rich hairpin loops of the Borrelia bacteria range in size from 5 kilobase pairs (kb) to over 200 kb[16] and contain the genes responsible for producing a group of major surface proteins, or antigens, on the bacteria that allow it to evade the immune response of its infected host.[17] The linear plasmids which contain a protein that has been covalently attached to the 5' end of the DNA strands are known as invertrons and can range in size from 9 kb to over 600 kb consisting of inverted terminal repeats.[16] The linear plasmids with a covalently attached protein may assist with bacterial conjugation and integration of the plasmids into the genome. These types of linear plasmids represent the largest class of extrachromosomal DNA as they are not only present in certain bacterial cells, but all linear extrachromosomal DNA molecules found in eukaryotic cells also take on this invertron structure with a protein attached to the 5' end.[16][17]

The long, linear "borgs" that co-occur with a species of archaeon – which may host them and shares many of their genes – could be an unknown form of extrachromosomal DNA structures.[18][19][20]

Eukaryotic

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Mitochondrial

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Human mitochondrial DNA showing the 37 genes

Mitochondria present in eukaryotic cells contain multiple copies of mitochondrial DNA (mtDNA) in the mitochondrial matrix.[21] In multicellular animals, including humans, the circular mtDNA chromosome contains 13 genes that encode proteins that are part of the electron transport chain and 24 genes for mitochondrial RNAs; these genes are broken down into 2 rRNA genes and 22 tRNA genes.[22] The size of an animal mtDNA plasmid is roughly 16.6 kb and, although it contains genes for tRNA and mRNA synthesis, proteins coded for by nuclear genes are still required for the mtDNA to replicate or for mitochondrial proteins to be translated.[23] There is only one region of the mitochondrial chromosome that does not contain a coding sequence, the 1 kb region known as the D-loop to which nuclear regulatory proteins bind.[22] The number of mtDNA molecules per mitochondrion varies from species to species, as well as between cells with different energy demands. For example, muscle and liver cells contain more copies of mtDNA per mitochondrion than blood and skin cells do.[23] Due to the proximity of the electron transport chain within the mitochondrial inner membrane and the production of reactive oxygen species (ROS), and due to the fact that the mtDNA molecule is not bound by or protected by histones, the mtDNA is more susceptible to DNA damage than nuclear DNA.[24] In cases where mtDNA damage does occur, the DNA can either be repaired via base excision repair pathways, or the damaged mtDNA molecule is destroyed (without causing damage to the mitochondrion since there are multiple copies of mtDNA per mitochondrion).[25]

The standard genetic code by which nuclear genes are translated is universal, meaning that each 3-base sequence of DNA codes for the same amino acid regardless of what species from which the DNA comes. However, this code is quite universal and is slightly different in mitochondrial DNA of fungi, animals, protists and plants.[21] While most of the 3-base sequences (codons) in the mtDNA of these organisms do code for the same amino acids as those of the nuclear genetic code, a few are different.

Coding differences found in the mtDNA sequences of various organisms
Genetic code Translation table DNA codon involved RNA codon involved Translation with this code Comparison with the universal code
Vertebrate mitochondrial 2 AGA AGA Ter (*) Arg (R)
AGG AGG Ter (*) Arg (R)
ATA AUA Met (M) Ile (I)
TGA UGA Trp (W) Ter (*)
Yeast mitochondrial 3 ATA AUA Met (M) Ile (I)
CTT CUU Thr (T) Leu (L)
CTC CUC Thr (T) Leu (L)
CTA CUA Thr (T) Leu (L)
CTG CUG Thr (T) Leu (L)
TGA UGA Trp (W) Ter (*)
CGA CGA absent Arg (R)
CGC CGC absent Arg (R)
Mold, protozoan, and coelenterate mitochondrial 4 and 7 TGA UGA Trp (W) Ter (*)
Invertebrate mitochondrial 5 AGA AGA Ser (S) Arg (R)
AGG AGG Ser (S) Arg (R)
ATA AUA Met (M) Ile (I)
TGA UGA Trp (W) Ter (*)
Echinoderm and flatworm mitochondrial 9 AAA AAA Asn (N) Lys (K)
AGA AGA Ser (S) Arg (R)
AGG AGG Ser (S) Arg (R)
TGA UGA Trp (W) Ter (*)
Ascidian mitochondrial 13 AGA AGA Gly (G) Arg (R)
AGG AGG Gly (G) Arg (R)
ATA AUA Met (M) Ile (I)
TGA UGA Trp (W) Ter (*)
Alternative flatworm mitochondrial 14 AAA AAA Asn (N) Lys (K)
AGA AGA Ser (S) Arg (R)
AGG AGG Ser (S) Arg (R)
TAA UAA Tyr (Y) Ter (*)
TGA UGA Trp (W) Ter (*)
Chlorophycean mitochondrial 16 TAG UAG Leu (L) Ter (*)
Trematode mitochondrial 21 TGA UGA Trp (W) Ter (*)
ATA AUA Met (M) Ile (I)
AGA AGA Ser (S) Arg (R)
AGG AGG Ser (S) Arg (R)
AAA AAA Asn (N) Lys (K)
Scenedesmus obliquus mitochondrial 22 TCA UCA Ter (*) Ser (S)
TAG UAG Leu (L) Ter (*)
Thraustochytrium mitochondrial 23 TTA UUA Ter (*) Leu (L)
Pterobranchia mitochondrial 24 AGA AGA Ser (S) Arg (R)
AGG AGG Lys (K) Arg (R)
TGA UGA Trp (W) Ter (*)
Amino acids biochemical properties nonpolar polar basic acidic Termination: stop codon

The coding differences are thought to be a result of chemical modifications in the transfer RNAs that interact with the messenger RNAs produced as a result of transcribing the mtDNA sequences.[26]

Chloroplast

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Eukaryotic chloroplasts, as well as the other plant plastids, also contain extrachromosomal DNA molecules. Most chloroplasts house all of their genetic material in a single ringed chromosome, however in some species there is evidence of multiple smaller ringed plasmids.[27][28][29] A recent theory that questions the current standard model of ring shaped chloroplast DNA (cpDNA), suggests that cpDNA may more commonly take a linear shape.[30] A single molecule of cpDNA can contain anywhere from 100 to 200 genes[31] and varies in size from species to species. The size of cpDNA in higher plants is around 120–160 kb.[21] The genes found on the cpDNA code for mRNAs that are responsible for producing necessary components of the photosynthetic pathway as well as coding for tRNAs, rRNAs, RNA polymerase subunits, and ribosomal protein subunits.[32] Like mtDNA, cpDNA is not fully autonomous and relies upon nuclear gene products for replication and production of chloroplast proteins. Chloroplasts contain multiple copies of cpDNA and the number can vary not only from species to species or cell type to cell type, but also within a single cell depending upon the age and stage of development of the cell. For example, cpDNA content in the chloroplasts of young cells, during the early stages of development where the chloroplasts are in the form of indistinct proplastids, are much higher than those present when that cell matures and expands, containing fully mature plastids.[33]

Circular

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Main article: Extrachromosomal circular DNA

Extrachromosomal circular DNA (eccDNA) are present in all eukaryotic cells, are usually derived from genomic DNA, and consist of repetitive sequences of DNA found in both coding and non-coding regions of chromosomes. EccDNA can vary in size from less than 2000 base pairs to more than 20,000 base pairs.[34] In plants, eccDNA contain repeated sequences similar to those that are found in the centromeric regions of the chromosomes and in repetitive satellite DNA.[35] In animals, eccDNA molecules have been shown to contain repetitive sequences that are seen in satellite DNA, 5S ribosomal DNA and telomere DNA.[34] Certain organisms, such as yeast, rely on chromosomal DNA replication to produce eccDNA[35] whereas eccDNA formation can occur in other organisms, such as mammals, independently of the replication process.[36] The function of eccDNA have not been widely studied, but it has been proposed that the production of eccDNA elements from genomic DNA sequences add to the plasticity of the eukaryotic genome and can influence genome stability, cell aging and the evolution of chromosomes.[37]

A distinct type of extrachromosomal DNA, denoted as ecDNA, is commonly observed in human cancer cells.[2][3][4] ecDNA found in cancer cells contain one or more genes that confer a selective advantage. ecDNA are much larger than eccDNA, and are visible by light microscopy. ecDNA in cancers generally range in size from 1-3 MB and beyond.[2] Large ecDNA molecules have been found in the nuclei of human cancer cells and are shown to carry many copies of driver oncogenes, which are transcribed in tumor cells. Based on this evidence it is thought that ecDNA contributes to cancer growth.

Specialized tools exist that allow ecDNA to be identified, such as

  • software developed by Paul Mischel and Vineet Bafna that allows ecDNA to be identified in microscopic images
  • "Circle-Seq, a method for physically isolating ecDNA from cells, removing any remaining linear DNA with enzymes, and sequencing the circular DNA that remains", developed by Birgitte Regenberg and her team at the University of Copenhagen.[38]

Viral

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Viral DNA are an example of extrachromosomal DNA. Understanding viral genomes is very important for understanding the evolution and mutation of the virus.[39] Some viruses, such as HIV and oncogenic viruses, incorporate their own DNA into the genome of the host cell.[40] Viral genomes can be made up of single stranded DNA (ssDNA), double stranded DNA (dsDNA) and can be found in both linear and circular form.[41]

One example of infection of a virus constituting as extrachromosomal DNA is the human papillomavirus (HPV). The HPV DNA genome undergoes three distinct stages of replication: establishment, maintenance and amplification. HPV infects epithelial cells in the anogenital tract and oral cavity. Normally, HPV is detected and cleared by the immune system. The recognition of viral DNA is an important part of immune responses. For this virus to persist, the circular genome must be replicated and inherited during cell division.[42]

Recognition by host cell

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Cells can recognize foreign cytoplasmic DNA. Understanding the recognition pathways has implications towards prevention and treatment of diseases.[43] Cells have sensors that can specifically recognize viral DNA such as the Toll-like receptor (TLR) pathway.[44]

The Toll Pathway was recognized, first in insects, as a pathway that allows certain cell types to act as sensors capable of detecting a variety of bacterial or viral genomes and PAMPS (pathogen-associated molecular patterns). PAMPs are known to be potent activators of innate immune signaling. There are approximately 10 human Toll-Like Receptors (TLRs). Different TLRs in human detect different PAMPS: lipopolysaccharides by TLR4, viral dsRNA by TLR3, viral ssRNA by TLR7/TLR8, viral or bacterial unmethylated DNA by TLR9. TLR9 has evolved to detect CpG DNA commonly found in bacteria and viruses and to initiate the production of IFN (type I interferons ) and other cytokines.[44]

Inheritance

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Mitochondrial inheritance in humans: the mtDNA and its mutations are maternally transmitted.

Inheritance of extrachromosomal DNA differs from the inheritance of nuclear DNA found in chromosomes. Unlike chromosomes, ecDNA does not contain centromeres and therefore exhibits a non-Mendelian inheritance pattern that gives rise to heterogeneous cell populations. In humans, virtually all of the cytoplasm is inherited from the egg of the mother.[45] For this reason, organelle DNA, including mtDNA, is inherited from the mother. Mutations in mtDNA or other cytoplasmic DNA will also be inherited from the mother. This uniparental inheritance is an example of non-Mendelian inheritance. Plants also show uniparental mtDNA inheritance. Most plants inherit mtDNA maternally with one noted exception being the redwood Sequoia sempervirens that inherit mtDNA paternally.[46]

There are two theories why the paternal mtDNA is rarely transmitted to the offspring. One is simply the fact that paternal mtDNA is at such a lower concentration than the maternal mtDNA and thus it is not detectable in the offspring. A second, more complex theory, involves the digestion of the paternal mtDNA to prevent its inheritance. It is theorized that the uniparental inheritance of mtDNA, which has a high mutation rate, might be a mechanism to maintain the homoplasmy of cytoplasmic DNA.[46]

Clinical significance

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Sometimes called EEs, extrachromosomal elements, have been associated with genomic instability in eukaryotes. Small polydispersed DNAs (spcDNAs), a type of eccDNA, are commonly found in conjunction with genome instability. SpcDNAs are derived from repetitive sequences such as satellite DNA, retrovirus-like DNA elements, and transposable elements in the genome. They are thought to be the products of gene rearrangements.

Extrachromosomal DNA (ecDNA) found in cancer have historically been referred to as Double minute chromosomes (DMs), which present as paired chromatin bodies under light microscopy. Double minute chromosomes represent ~30% of the cancer-containing spectrum of ecDNA, including single bodies and have been found to contain identical gene content as single bodies.[3] The ecDNA notation encompasses all forms of the large, oncogene-containing, extrachromosomal DNA found in cancer cells.  This type of ecDNA is commonly seen in cancer cells of various histologies, but virtually never in normal cells.[3] ecDNA are thought to be produced through double-strand breaks in chromosomes or over-replication of DNA in an organism. Studies show that in cases of cancer and other genomic instability, higher levels of EEs can be observed.[5]

Mitochondrial DNA can play a role in the onset of disease in a variety of ways. Point mutations in or alternative gene arrangements of mtDNA have been linked to several diseases that affect the heart, central nervous system, endocrine system, gastrointestinal tract, eye, and kidney.[22] Loss of the amount of mtDNA present in the mitochondria can lead to a whole subset of diseases known as mitochondrial depletion syndromes (MDDs) which affect the liver, central and peripheral nervous systems, smooth muscle and hearing in humans.[23] There have been mixed, and sometimes conflicting, results in studies that attempt to link mtDNA copy number to the risk of developing certain cancers. Studies have been conducted that show an association between both increased and decreased mtDNA levels and the increased risk of developing breast cancer. A positive association between increased mtDNA levels and an increased risk for developing kidney tumors has been observed but there does not appear to be a link between mtDNA levels and the development of stomach cancer.[47]

Extrachromosomal DNA is found in Apicomplexa, which is a group of protozoa. The malaria parasite (genus Plasmodium), the AIDS-related pathogen (Toxoplasma and Cryptosporidium) are both members of the Apicomplexa group. Mitochondrial DNA (mtDNA) was found in the malaria parasite.[48] There are two forms of extrachromosomal DNA found in the malaria parasites. One of these is 6-kb linear DNA and the second is 35-kb circular DNA. These DNA molecules have been researched as potential nucleotide target sites for antibiotics.[49]

Role of ecDNA in cancer

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Gene amplification is among the most common mechanisms of oncogene activation. Gene amplifications in cancer are often on extrachromosomal, circular elements.[50][4] One of the primary functions of ecDNA in cancer is to enable the tumor to rapidly reach high copy numbers, while also promoting rapid, massive cell-to-cell genetic heterogeneity.[3][8] The most commonly amplified oncogenes in cancer are found on ecDNA and have been shown to be highly dynamic, re-integrating into non-native chromosomes as homogeneous staining regions (HSRs)[51][3] and altering copy numbers and composition in response to various drug treatments.[52][7][53]

The circular shape of ecDNA differs from the linear structure of chromosomal DNA in meaningful ways that influence cancer pathogenesis.[54] Oncogenes encoded on ecDNA have massive transcriptional output, ranking in the top 1% of genes in the entire transcriptome. In contrast to bacterial plasmids or mitochondrial DNA, ecDNA are chromatinized, containing high levels of active histone marks, but a paucity of repressive histone marks. The ecDNA chromatin architecture lacks the higher-order compaction that is present on chromosomal DNA and is among the most accessible DNA in the entire cancer genome. This less-compact organization permits greater access for transcription factors and transcription machinery, directly contributing to the increased expression of oncogenes.[54]

Furthermore, research on the three-dimensional genomic landscape of cancer has shown that ecDNAs frequently cluster together within the nucleus, forming "ecDNA hubs."[55] Spatially, these hubs facilitate intermolecular enhancer–promoter interactions to promote oncogene overexpression. Research has also demonstrated that cancers containing ecDNA generate the a higher frequency of new enhancer-promoter interactions compared to other types of structural variations, making ecDNA a potent driver of epigenetic rewiring.[56] This "enhancer rewiring" mechanism allows oncogenes to interact with multiple distant regulatory elements, increasing transcriptional activity, cancer growth, and heterogeneity.

Consequently, the presence of ecDNA in cancerous tumors is associated with a poor prognosis and significantly shorter survival.[50] This aggressiveness is attributed to massive oncogene copy number increases, regulatory rewiring, and intratumoral heterogeneity, which together accelerate tumor evolution and resistance to various cancer treatments.[57][3]

References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Extrachromosomal DNA

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from Grokipedia
Extrachromosomal DNA (ecDNA), also referred to as extrachromosomal circular DNA (eccDNA), consists of circular, double-stranded DNA molecules that exist independently of the chromosomal genome in cells, including both prokaryotes (such as plasmids) and eukaryotes, typically ranging in size from dozens of base pairs to several megabases. First identified in bacteria as plasmids in the 1950s and later in eukaryotic cancer cells as double-minute chromosomes in the 1960s, these structures are found in both normal and diseased tissues but are particularly abundant in cancer cells, where they often contain amplified oncogenes and regulatory elements, driving rapid tumor evolution and heterogeneity. Unlike linear chromosomal DNA, ecDNA lacks centromeres and telomeres, enabling non-Mendelian inheritance patterns that facilitate high copy number variation and genetic instability. The biogenesis of ecDNA involves multiple mechanisms, including chromosomal breakage events such as , breakage-fusion-bridge cycles, and replication fork stalling, which generate acentric circular fragments that are subsequently amplified. Structurally, ecDNA exhibits highly accessible , allowing for enhanced transcriptional activity of contained , and can form dynamic hubs that interact with chromosomal DNA to co-regulate expression. In normal cells, small eccDNAs (e.g., microDNAs) may arise from gene excision and contribute to plasticity or maintenance, but in cancer, larger forms predominate and are often derived from a single arm. In , ecDNA plays a pivotal role in tumorigenesis by amplifying key drivers like , EGFR, and , promoting aggressive phenotypes, , and immune evasion through altered . Detected in approximately 17% of tumor samples across various cancers, including high prevalence in glioblastomas (up to 49%) and liposarcomas (up to 55%), ecDNA is strongly associated with poor , with patients showing reduced survival (hazard ratio 1.44) and increased treatment resistance to therapies like and targeted inhibitors. Emerging as a , ecDNA can be identified in from plasma, offering potential for non-invasive diagnostics, while therapeutic strategies targeting its loss—such as hydroxyurea or —hold promise for overcoming resistance.

Overview

Definition and Characteristics

Extrachromosomal DNA (ecDNA), often specifically referring to extrachromosomal circular DNA (eccDNA), consists of DNA molecules that exist independently of the primary chromosomal complement within a cell, typically in circular form and not integrated into the main . In contrast to chromosomal DNA, which is organized into linear chromosomes within the nucleus of eukaryotes or the of prokaryotes, ecDNA can reside either within the nucleus as separate entities or extranuclearly (e.g., in organelles), often adopting a circular to support autonomous replication without reliance on chromosomal machinery. This independence allows ecDNA to propagate separately, though it typically lacks centromeric sequences in nuclear contexts, resulting in unstable inheritance during due to random segregation rather than equitable distribution. Key characteristics of ecDNA include its relatively small size, ranging from hundreds of base pairs to several megabases (kb to Mb), which facilitates high copy numbers and rapid amplification compared to the larger chromosomal genomes. It possesses the potential for autonomous replication through incorporated origins of replication, enabling persistence across generations without integration, and is prevalent in diverse biological contexts across prokaryotes and eukaryotes. While predominantly circular—enhancing stability and replication efficiency—linear extrachromosomal elements, such as double-minute chromosomes in cancer cells, also occur and contribute to . Representative examples of ecDNA include plasmids in prokaryotes, which serve as vehicles for resistance and metabolic functions; (mtDNA) in eukaryotic organelles, encoding essential respiratory proteins; viral episomes, such as those from herpesviruses that maintain latency; and cancer-associated circular ecDNA, often amplifying oncogenes like or EGFR to drive tumor progression. EcDNA is ubiquitous in , from bacterial populations to pathologies, with detection in up to 17% of tumor samples across various cancer types, underscoring its role in normal and .

Historical Discovery

The concept of extrachromosomal DNA emerged from early 20th-century experiments demonstrating genetic transformation in bacteria. In 1928, Frederick Griffith observed that heat-killed smooth pneumococci could transfer virulence traits to non-virulent rough strains in mice, providing the first evidence of a heritable factor independent of the bacterial chromosome. This laid the groundwork for understanding non-chromosomal genetic elements, later identified as plasmids. The 1940s and 1950s brought key breakthroughs in bacterial genetics, solidifying the existence of extrachromosomal DNA. Joshua Lederberg and Edward Tatum's 1946 experiments revealed genetic recombination in Escherichia coli through conjugation, indicating the transfer of extrachromosomal factors between cells. In 1952, Norton Zinder and Lederberg discovered transduction, where bacteriophages mediate the transfer of bacterial DNA fragments, further highlighting mobile genetic elements. That same year, Lederberg coined the term "plasmid" to describe these self-replicating, extrachromosomal DNA molecules, linking them to traits like antibiotic resistance observed in clinical isolates. In eukaryotes, the discovery of extrachromosomal DNA in organelles marked significant milestones in the 1960s. Margit M. K. Nass and Sylvan Nass identified DNA within mitochondria using electron microscopy in 1963, visualizing DNase-sensitive fibers distinct from nuclear DNA. For chloroplasts, biochemical evidence of DNA appeared in 1959 through incorporation of into Spirogyra chloroplasts by Ralph Stocking and Ernest Gifford, with electron microscopic confirmation in 1962 by Hans Ris and Walther Plaut, establishing plastid autonomy. By the , these findings supported the endosymbiotic theory, recognizing organellar genomes as separate from nuclear chromosomes. In the cancer context, extrachromosomal DNA was first observed as double-minute chromosomes in tumor cells during the . A. I. Spriggs and colleagues reported these small, paired bodies in a human pleural effusion sample in 1962, initially without recognizing their DNA . By the 1980s, double minutes were linked to in tumors, serving as precursors to circular extrachromosomal DNA (ecDNA). The 2010s revived interest through next-generation sequencing (NGS), with a 2020 study confirming circular ecDNA in , driving amplification and genome remodeling. Post-2020 research, including a 2024 , has underscored ecDNA's role in treatment resistance across cancers, amplifying its genomic impact.

Biogenesis

Mechanisms of Formation

Extrachromosomal DNA (ecDNA) arises through several molecular processes that disrupt chromosomal , primarily involving the excision or misrepair of genomic segments. Key mechanisms include chromosomal excision, replication errors, and failures in double-strand break repair, each contributing to the release of non-integrated DNA elements that can persist extrachromosomally. These pathways often intersect with broader genomic instability, enabling the formation of circular ecDNA structures. Chromosomal excision typically occurs via microhomology-mediated deletion (MMEJ), where short homologous sequences (1–20 bp) at DNA break sites facilitate the deletion of intervening segments, releasing acentric DNA fragments that can circularize. This process is error-prone and generates small ecDNA molecules, often termed microDNA, without leaving prominent chromosomal scars. Replication errors, such as fork stalling and template switching (FoSTeS), further promote ecDNA formation during S-phase; stalled replication forks switch to alternative templates, leading to the excision of looped-out DNA segments that form extrachromosomal circles via or ligation. These mechanisms are particularly active in contexts of replication stress, allowing rapid generation of ecDNA from amplified regions. Failures in double-strand break repair play a central role in ecDNA biogenesis, with (NHEJ) or alternative end joining (alt-EJ) pathways ligating broken DNA ends to form circular structures. In NHEJ, Ku proteins and DNA-PK recruit ligases to join incompatible ends, often resulting in small deletions, while alt-EJ relies on microhomologies for more precise but still mutagenic circularization. Genomic instability amplifies these processes; for instance, within micronuclei—small, extranuclear compartments formed from chromosome fragments—shatters DNA into pieces that reassemble into complex ecDNA via erroneous repair. Circular forms predominate due to head-to-tail ligation of multiple fragments, stabilizing the molecule against degradation. Recent studies have identified additional mechanisms, such as YY1-mediated DNA looping that facilitates religation by the Lig3-YY1 complex, further contributing to ecDNA biogenesis. Several factors enhance ecDNA formation, including defects in DNA damage response pathways such as /2 mutations, which impair and favor error-prone NHEJ, increasing ecDNA prevalence. Similarly, replication stress induced by activation (e.g., overexpression) causes fork collapse and DSBs, promoting excision events. Quantitatively, ecDNA amplifies rapidly post-formation, achieving high copy numbers—often tens to hundreds per cell—through activation of multiple extrareplicative origins, far exceeding chromosomal duplication rates and enabling swift increases. Depletion of classical NHEJ-promoting components, like 53BP1, increases ecDNA levels by favoring alt-EJ and MMEJ pathways, underscoring the role of error-prone repair in ecDNA biogenesis.

Structural Properties

Extrachromosomal DNA (ecDNA) predominantly adopts a circular, double-stranded configuration, which provides and allows for independent replication outside of conventional chromosomes. These circular forms typically incorporate origins of replication and promoter sequences, enabling autonomous propagation within the cell. The size of ecDNA molecules spans a broad spectrum, ranging from approximately 100 base pairs for small circular DNAs to more than 1 megabase for larger entities often termed ecDNA in cancer contexts. These structures frequently harbor functional gene clusters, including oncogenes like , EGFR, or FGFR2, along with associated regulatory elements such as enhancers. Circular ecDNA generally does not require telomeric caps, while centromeric sequences appear variably, though functional centromeres are rare, contributing to patterns. ecDNA associates with proteins to form -based structures, albeit with distinct organizational features compared to linear chromosomes. These associations include binding by canonical s, facilitating a chromatin-like architecture that supports regulatory functions. Epigenetically, ecDNA displays heightened accessibility and pronounced enrichment in active marks, particularly H3K27ac, which correlates with enhanced promoter and enhancer activity. This open configuration often results in the absence of repressive marks and incomplete occupancy, promoting rapid transcriptional activation and elevated relative to chromosomally integrated sequences.

Occurrence in Prokaryotes

Plasmids

Plasmids are small, circular, double-stranded DNA molecules that exist independently of the bacterial , typically ranging in size from a few kilobases to several hundred kilobases. These extrachromosomal elements are self-replicating and maintain multiple copies within a single bacterial cell, enabling rapid dissemination of genetic information. In prokaryotes, plasmids are ubiquitous across bacterial and archaeal species, serving as key vehicles for that drives microbial and adaptation. Plasmid replication occurs autonomously through specific origins of replication, distinct from those of the host chromosome. For instance, the ColE1 origin, commonly found in plasmids of Escherichia coli, initiates replication via RNA priming and proceeds in a theta mode, producing bidirectional replication forks. Copy number control is achieved through regulatory mechanisms, including antisense RNA inhibition and partitioning systems involving proteins such as ParA and ParB, which actively segregate plasmids to daughter cells during division to prevent loss. Alternative replication modes include rolling-circle replication, seen in smaller plasmids like pC194 in Gram-positive bacteria, where a single-strand displacement mechanism generates linear intermediates that are later circularized. Plasmids confer diverse functional advantages to their bacterial hosts, often encoding traits that enhance survival in challenging environments. Resistance (R) plasmids carry genes for antibiotic resistance, such as those encoding beta-lactamases or efflux pumps, allowing to evade agents. The F () plasmid in E. coli enables conjugation, facilitating the transfer of genetic material between cells and often including factors that promote pathogenicity. Similarly, the Ti (tumor-inducing) plasmid in encodes enzymes for metabolism and genes that integrate into plant genomes, enabling symbiotic or pathogenic interactions with host plants. Plasmid diversity is reflected in their mobility and transfer capabilities, broadly classifying them as conjugative or non-conjugative. Conjugative plasmids, like the , possess a complete set of transfer (tra) genes that form a type IV secretion system for direct cell-to-cell DNA transfer via conjugation. Non-conjugative plasmids lack these tra genes but can be mobilized if co-resident with a conjugative plasmid, relying on shared origins of transfer (oriT) for hitchhiking during conjugation events. This mobility underpins plasmids' essential role in , allowing the spread of adaptive traits across bacterial populations without vertical inheritance.

Bacteriophage DNA

In the lysogenic cycle of temperate bacteriophages, the viral genome can exist transiently as an extrachromosomal circular DNA molecule within the prokaryotic host before potential integration into the bacterial chromosome. For bacteriophage lambda, which infects Escherichia coli, the linear double-stranded DNA genome of approximately 48.5 kb enters the host cell and rapidly circularizes via its cohesive ends, forming a covalently closed circular plasmid-like structure. This extrachromosomal form serves as the substrate for the lysogeny decision, where the phage either establishes dormancy or proceeds to the lytic cycle. The cI repressor protein, encoded by the cI gene, plays a central role in maintaining repression of lytic genes during this phase, binding to operator sites on the circular DNA to prevent expression of early lytic promoters. Unlike , which typically integrates its genome to complete lysogeny, P1 maintains its extrachromosomal state as a stable, low-copy-number throughout the lysogenic phase, with a of about 94 kb. P1 replication is tightly coupled to the host , occurring once per division to ensure equitable partitioning to daughter cells, facilitated by the ParA and ParB proteins that act analogously to chromosomal segregation systems. Repression in P1 lysogens is similarly enforced by a cI-like that inhibits lytic , contributing to the phage's decision between lysogeny and upon infection, often influenced by host multiplicity of infection. Lysogenic bacteriophages confer significant biological advantages to their prokaryotic hosts through extrachromosomal maintenance. In lysogens, the cI-mediated repression provides immunity against by homologous phages, as incoming viral DNA cannot initiate lytic replication due to the established . Similarly, P1 plasmids impart exclusion, enhancing host survival in phage-rich environments. Beyond defense, some lysogenic phages carry accessory genes that alter host physiology; for instance, the beta-phage in encodes the tox gene for , expressed only under specific iron-limiting conditions in the lysogen, thereby converting non-pathogenic strains to toxigenic ones. The extrachromosomal phage DNA can transition to the via induction, often triggered by host DNA damage that activates the response, leading to inactivation, genome excision (if previously integrated), and subsequent virion production. In P1, this process mobilizes the for rolling-circle replication and packaging into new particles, highlighting the dynamic equilibrium between and in prokaryotic systems.

Occurrence in Eukaryotes

Organellar DNA

Organellar DNA refers to the genetic material found in eukaryotic organelles, primarily mitochondria and chloroplasts, which exist as extrachromosomal elements semi-autonomous from the nuclear genome. (mtDNA) in humans is a circular, double-stranded approximately 16,569 base pairs in length. It encodes 13 proteins essential for , along with 2 ribosomal RNAs (rRNAs) and 22 transfer RNAs (tRNAs). This compact genome is maternally inherited, with paternal mtDNA typically eliminated during fertilization. Chloroplast DNA (cpDNA), present in photosynthetic eukaryotes, consists of larger circular molecules ranging from 120 to 160 kilobase pairs. These genomes encode around 100-130 genes, many involved in , including components of the , ribosomal proteins, rRNAs, and tRNAs. Unlike the strictly maternal of mtDNA, cpDNA exhibits biparental transmission in certain plant species, such as some angiosperms, though maternal inheritance predominates in most cases. Both mtDNA and cpDNA maintain autonomy through dedicated replication and transcription machineries. For mtDNA, replication is mediated by , the sole replicase in mitochondria, which operates independently of nuclear replication processes. Transcription of mtDNA occurs within the using nuclear-encoded factors imported into mitochondria, but the process is decoupled from nuclear transcription regulation. Similarly, cpDNA replication employs -specific polymerases and proteins, often nuclear-encoded but functioning locally in the . Transcription in chloroplasts involves both plastid-encoded (PEP) and nuclear-encoded (NEP), enabling independent . The evolutionary origins of organellar DNA trace back to endosymbiotic events, with mtDNA deriving from an alpha-proteobacterium and cpDNA from a cyanobacterium. These genomes retain prokaryotic-like features, including their circular and, in the case of cpDNA, a general absence of introns in many protein-coding genes. Variants such as nuclear mitochondrial DNA segments (NUMTs) represent transferred mtDNA fragments integrated into the nuclear genome, but true extrachromosomal mtDNA persists as the functional organellar form.

Nuclear Extrachromosomal DNA

Nuclear extrachromosomal DNA (ecDNA) in eukaryotes primarily consists of non-viral circular forms known as extrachromosomal circular DNAs (eccDNAs), which range in size from approximately 100 base pairs to 1 megabase and arise through mechanisms such as gene excision from chromosomal DNA. These eccDNAs are double-stranded, closed-circle molecules that exist independently of the linear chromosomes within the nucleus. Double minutes (DMs) are small, acentric circular chromatin bodies that occur as extrachromosomal elements, particularly as precursors in cancer development, where they amplify oncogenes. EccDNAs are prevalent in specific eukaryotic cell types, including post-mitotic neurons and immune cells, where they contribute to cellular heterogeneity. In neurons, eccDNAs are abundant in both healthy and aged tissues, with tens of thousands detected in brain tissue samples from models. Their levels increase during aging, accumulating in senescent cells and promoting age-related phenotypes in and mammals. Similarly, eccDNA upregulation occurs under cellular stress conditions, such as or , serving as signaling molecules in immune responses. In immune cells, eccDNAs enriched with repetitive sequences activate innate immunity pathways during stress. Functionally, nuclear eccDNAs act as "gene parking" structures, enabling rapid amplification of gene copy numbers to facilitate quick expression of adaptive without altering the chromosomal . This allows for accelerated cellular responses, such as in evolutionary or stress survival, by increasing or essential gene dosage transiently. In B-cells, eccDNAs play a specific role in , where they are generated during heavy-chain recombination to support antibody diversification. Regulation of nuclear eccDNAs involves dynamic interactions with nuclear structures, including association with nuclear pore complexes (NPCs) via nuclear actin filaments, which facilitate their export or positioning for degradation. Epigenetic marks on eccDNAs differ from those on chromosomal DNA, often exhibiting reduced histone association and variable modifications like altered DNA methylation or histone acetylation patterns, which influence their stability and transcriptional activity. These distinct epigenetic profiles allow eccDNAs to evade typical chromosomal silencing mechanisms. A notable non-cancer example of nuclear eccDNA occurs during Drosophila development, where eccDNAs form multimers of tandemly repeated genes, including those involved in chorion production for eggshell formation, supporting developmentally timed gene amplification.

Viral Extrachromosomal DNA

Episomal Forms

Episomal forms of viral extrachromosomal DNA refer to covalently closed circular genomes of certain DNA viruses that persist in the nucleus of eukaryotic host cells without integrating into the host chromosomes. These episomes, derived from viruses such as Epstein-Barr virus (EBV), human papillomavirus (HPV), and simian virus 40 (SV40), maintain latency by replicating in synchrony with the host cell cycle using hijacked cellular machinery. Replication of these episomes relies on specific viral origins of replication (ori) and associated proteins that recruit host replication factors. In EBV, the oriP element consists of the family of repeats (FR) and dyad symmetry (DS) regions; EBNA-1 binds to DS to facilitate replication once per , mimicking chromosomal origins. Similarly, HPV episomes use the upstream regulatory region as an ori, where the E2 protein binds multiple sites to recruit the , initiating bidirectional replication dependent on host polymerases. For SV40, the viral large T antigen binds the core ori to unwind DNA and assemble the with host proteins like RPA and . During latency, copy numbers remain low, typically 1-50 episomes per cell for EBV and HPV in infected tissues, ensuring stable persistence without triggering lytic cycles; in contrast, lytic replication can amplify copies to hundreds or thousands to support virion production. Representative examples illustrate episomal maintenance in distinct contexts. HPV episomes persist at low copy numbers in basal of , driving epithelial proliferation through E6 and E7 oncoproteins while replicating with host DNA. EBV episomes are maintained in B lymphocytes during latent infection, with EBNA-1 ensuring segregation; this form underlies conditions like and certain lymphomas. SV40 episomes, often studied in transformed rodent or human cells, achieve higher copy numbers (50-1000 per cell) via large T and small t antigens, contributing to cellular immortalization in experimental models. Stability of these episomes during host cell division is achieved through partitioning mechanisms where viral proteins tether the circular DNA to host chromosomes, emulating centromeric functions. EBNA-1 in EBV links to mitotic chromosomes via interactions with host factors like hEBP2, ensuring equitable distribution to daughter cells. In HPV, E2 mediates attachment to host chromatin through BRD4 binding, promoting faithful segregation in dividing . SV40 employs large T antigen for similar host interactions, maintaining episomal integrity in non-permissive human cells during persistent infection. These strategies allow long-term viral persistence without genomic integration.

Host Cell Interactions

Host cells recognize viral DNA, including that from extrachromosomal , through receptors. For nuclear-replicating viruses like EBV and HPV, nuclear sensors such as IFI16 detect viral genomes in the nucleus, while the cGAS-STING pathway senses cytosolic double-stranded DNA that may arise from or episome leakage during . Upon binding to foreign DNA, cyclic GMP-AMP synthase (cGAS) produces the second messenger 2'3'-cGAMP, activating (STING) to trigger type I production and release, thereby initiating innate immune defense against viruses like herpesviruses. This recognition mechanism is crucial for distinguishing self from non-self DNA, with cGAS preferentially sensing longer DNA fragments (>500–1,000 base pairs) typical of viral DNA in the . Viruses counteract this detection through immune evasion strategies, often employing viral proteins to suppress interferon responses and NF-κB signaling. For instance, Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) activates both canonical and non-canonical NF-κB pathways via its C-terminal activating regions (CTAR1 and CTAR2), recruiting TRAF adapters to induce anti-apoptotic and immunomodulatory cytokines like IL-10 and IL-6, which dampen type I interferon signaling and promote B-cell survival. LMP1 further inhibits retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 9 (TLR9) expression, reducing innate sensing of viral DNA and facilitating persistent infection. Viral ecDNA interacts with host cellular machinery to ensure stability, such as tethering episomes to mitotic chromosomes for faithful segregation during . In EBV, the Epstein-Barr nuclear antigen 1 (EBNA1) binds to specific AT-rich docking sites on host chromosomes marked by , using its arginine-glycine repeats and AT-hook domains to mediate homotypic interactions and , often repressing nearby host genes like neuronal factors (e.g., NRXN1). Similarly, human (HCMV) genomes associate with chromosome peripheries in infected cells, supported by immediate-early protein 1 (IE1), enabling genome retention without integration. If undetected, these interactions promote viral persistence; however, failure to evade host can induce arrest or via activation and DNA damage responses. A key example is human papillomavirus (HPV), where E6 and E7 oncoproteins degrade the tumor suppressor through E6AP-mediated ubiquitination, disrupting and to allow episomal maintenance in epithelial cells. This evasion enables latency establishment by inhibiting host clearance mechanisms like ATM-CHK2 signaling, contrasting with effective immune clearance in immunocompetent hosts that limits viral spread. Unchecked persistence contributes to oncogenesis, as seen in EBV-driven lymphomas and HPV-associated cervical cancers, where dysregulated latency programs induce genomic instability and proliferation without triggering full immune elimination.

Inheritance and Dynamics

Segregation and Stability

Extrachromosomal DNA (ecDNA) exhibits distinct segregation mechanisms depending on its context within prokaryotic or eukaryotic cells. In nuclear ecDNA, commonly observed in cancer cells, segregation occurs randomly during due to the absence of centromeres, resulting in unequal partitioning to daughter cells. This distribution contrasts with chromosomal inheritance and promotes rapid heterogeneity. In contrast, bacterial plasmids employ active segregation systems, such as the ParMRC apparatus, where ParM actin-like filaments push plasmid copies toward opposite cell poles, ensuring equitable division and high fidelity inheritance. Stability of ecDNA is maintained through compensatory mechanisms that offset segregation losses. High copy number amplification allows nuclear ecDNA to persist despite random partitioning, as elevated copies increase the probability of retention in both daughter cells; for instance, oncogene-bearing ecDNA in tumors can amplify under selective pressure to sustain function. Additionally, some ecDNA elements tether to chromosomes, enhancing co-segregation and reducing missegregation into the . For (), a genetic bottleneck during reduces effective copy number to a few dozen, enabling near-Mendelian segregation and rapid resolution across generations. Segregation rates vary significantly, with nuclear eccDNA experiencing 10-50% loss per in the absence of selection, leading to progressive dilution unless counteracted by replication. These rates are influenced by cell type—plasmids remain stable in through par-mediated partitioning, while nuclear ecDNA shows high variability in tumor cells—and environmental factors, such as stress or exposure, which accelerate loss by suppressing replication. modeling, using algorithms like Gillespie simulations, demonstrates how random segregation generates broad copy number distributions and intratumoral heterogeneity, facilitating adaptive evolution in dynamic environments.

Evolutionary Roles

Extrachromosomal DNA (ecDNA), including plasmids and DNA, plays a pivotal role in (HGT), enabling rapid dissemination of adaptive traits across bacterial populations. Plasmids, as self-replicating extrachromosomal elements, facilitate conjugation-mediated transfer of genes conferring antibiotic resistance, accelerating evolutionary adaptation in response to selective pressures like antimicrobial exposure. further enhance this process through transduction, packaging and delivering bacterial DNA, including resistance genes, between hosts, which has been instrumental in the global spread of multidrug-resistant strains. Phage-plasmids, hybrid elements combining features of both, promote recombination and the emergence of novel resistance cassettes, underscoring ecDNA's contribution to microbial evolution beyond vertical inheritance. In unicellular organisms, ecDNA drives plasticity by serving as a substrate for duplications, deletions, and rearrangements, allowing swift adjustments to environmental challenges. These circular molecules can amplify copy numbers without disrupting chromosomal integrity, providing a mechanism for rapid in fluctuating conditions, such as or exposure. For instance, eccDNA formation enables unicellular eukaryotes and prokaryotes to generate genetic variants at higher rates than chromosomal mutations, fostering population-level diversity and survival advantages. Among eukaryotes, mitochondrial DNA (mtDNA), a vestigial extrachromosomal element derived from an ancient bacterial endosymbiont, harbors mutations that propel metabolic evolution by altering energy production pathways. These mutations, often heteroplasmic, influence cellular respiration efficiency and have shaped eukaryotic diversification across lineages. Nuclear ecDNA, meanwhile, contributes to immune system evolution; byproducts of V(D)J recombination in lymphocytes form circular DNAs that diversify antigen receptor genes, enhancing adaptive immunity and host defense variability. In cancer contexts, recent 2025 analyses reveal that ecDNA enables non-Mendelian inheritance in tumors, allowing oncogene amplification to propagate asymmetrically during cell division and accelerate tumor evolution under therapeutic selection, including through RNA-mediated tethering and selective degradation. On a broader scale, ecDNA has influenced through the retention of endosymbiont-derived genomes, as seen in the endosymbiotic origin of mitochondria, where partial retention in mtDNA facilitated eukaryotic cellular complexity and divergent evolutionary trajectories. Recent 2024 reviews highlight ecDNA's role in microbial , where plasmids and phage-derived elements mediate community dynamics, , and niche adaptation in diverse environments like and aquatic systems.

Detection Methods

Traditional Approaches

Early methods for detecting and characterizing extrachromosomal DNA (ecDNA) relied on and basic biochemical separation techniques, which laid the foundation for understanding its structure and presence in eukaryotic cells. In the 1960s, electron emerged as a pivotal tool for visualizing circular DNA configurations, particularly in organelles. Margit M. K. Nass and Sylvan Nass first observed intramitochondrial fibers with DNA-like staining properties in mouse liver cells using electron and specific fixation techniques, providing initial evidence of extrachromosomal DNA outside the nucleus. Subsequent studies confirmed the circular nature of through electron micrographs of extracted molecules, revealing theta structures indicative of replication and closed-loop forms distinct from linear chromosomal DNA. These observations extended to chloroplast DNA in plants, where similar circular forms were identified via electron in the late 1960s. In cancer during the 1970s, light microscopy identified double minutes—small, paired extrachromosomal chromatin bodies lacking centromeres—in tumor cell metaphase spreads, marking early recognition of nuclear ecDNA in mammalian cells. (FISH), an advancement building on these cytogenetic approaches, began to be applied in the late 1970s and early 1980s to map specific sequences on double minutes, confirming their extrachromosomal origin and gene content in cancer lines like COLO 320. However, FISH at the time was limited by probe resolution and required fixed cell preparations, often complementing traditional staining methods. Density centrifugation provided a key biochemical approach for isolating ecDNA based on buoyant density differences from nuclear DNA. In the , cesium chloride (CsCl) equilibrium density centrifugation separated , enabling purification from total cellular extracts. This method, often combined with to distinguish supercoiled circular forms, was routinely used to isolate chloroplast DNA in plants, isolating closed circular molecules of 100-150 kb. Such techniques confirmed the organellar localization and purity of ecDNA fractions for downstream analyses. Southern blotting, developed in the mid-1970s, allowed detection of non-integrated viral ecDNA by hybridizing restriction-digested DNA with radiolabeled probes. Edwin Southern's 1975 method transferred electrophoresed DNA fragments to membranes, enabling specific identification of episomal viral genomes like Epstein-Barr virus in latently infected cells, where supercoiled forms produced characteristic band patterns distinct from integrated sequences. This approach was widely adopted for quantifying extrachromosomal viral DNA copies in host cells by the late 1970s. Pulsed-field gel electrophoresis (PFGE), introduced in the 1980s, facilitated separation of large ecDNA molecules from chromosomal DNA. By alternating directions, PFGE resolved DNA fragments up to several megabases, allowing isolation of intact extrachromosomal circles or double minutes from yeast artificial chromosomes or cancer cell lines without shearing. This technique proved essential for characterizing large nuclear ecDNA in mammalian cells, distinguishing them from linear chromosomes based on migration patterns. Despite these advances, traditional approaches had notable limitations, including low sensitivity for detecting low-copy-number ecDNA amid high nuclear DNA background and challenges in sequencing small circular molecules due to the lack of high-throughput tools. often required labor-intensive and provided only structural insights without sequence information, while separation methods like density gradients and PFGE were prone to and inefficient for trace amounts. Southern blotting, though specific, depended on prior knowledge of sequences and could not resolve structural variants in ecDNA.

Contemporary Techniques

Next-generation sequencing (NGS) has revolutionized ecDNA detection through read-depth analysis, which quantifies copy number variations by comparing sequencing coverage across genomic regions to identify focal amplifications indicative of extrachromosomal elements. This approach is particularly effective in cancer samples, where ecDNA often drives overexpression via high copy numbers, as demonstrated in cohorts where read-depth thresholds above 10-fold amplification signal ecDNA presence. To specifically enrich circular DNA prior to NGS, Circle-seq utilizes digestion to degrade linear DNA while preserving circles, followed by library preparation and sequencing; this method achieves over 100-fold enrichment for eccDNA ranging from 100 bp to megabases, uncovering genome-wide circular structures in tumors. An enhanced 2025 protocol for Circle-seq addresses biases in enrichment efficiency, improving detection sensitivity for low-abundance ecDNA in heterogeneous samples. Long-read sequencing platforms, including PacBio HiFi and , enable resolution of complete ecDNA structures by producing reads exceeding 10 kb, which span junction breakpoints and repetitive regions inaccessible to short-read NGS. These technologies identify chimeric junctions formed during ecDNA biogenesis, such as head-to-tail concatemers in oncogene-amplified circles. In 2024 protocols applied to advanced cancer cohorts, long-read sequencing resolved ecDNA harboring viral integrations and complex rearrangements in over 80% of predicted cases, with tools like assembling full ecDNA contigs from raw reads using graph-based algorithms that model discordant alignments. This has facilitated de novo annotation of ecDNA in patient-derived models, revealing structural heterogeneity not captured by short reads. Microscopy advancements, particularly super-resolution techniques like STED and SIM, provide nanometer-scale visualization of ecDNA localization and dynamics within the nucleus, distinguishing extrachromosomal hubs from chromosomal integrations. HaloTag labeling enables live-cell tracking of ecDNA partitioning during , showing asymmetric inheritance in cancer cells. Complementing this, assays accessibility on ecDNA, revealing elevated open regions in amplified oncogenes like , with peak signals 2-5 times higher than in linear DNA; this method detected pre-amplification ecDNA in therapy-resistant tumors, predicting resistance emergence. Bioinformatics pipelines exploit discordant read mapping in NGS data to pinpoint ecDNA, where reads with unexpected orientations or spanning non-adjacent genomic loci indicate circular junctions. Tools such as ECCsplorer process BAM files to filter split and paired-end discordant reads, achieving >95% specificity in identifying ecDNA under 1 Mb across diverse tissues. Similarly, ecc_finder models read-pair distances to cluster potential circles, validated on simulated datasets with 90% recall for low-coverage samples. models further enhance prediction from whole-genome sequencing by training on features like read-depth variance and density; for instance, a 2024 classifier detects ecDNA in whole-exome data with 85% accuracy, prioritizing high-impact amplifications in pan-cancer analyses. Innovations in 2025 include single-cell ecDNA profiling via scATAC-seq, which captures accessibility patterns at cellular resolution to infer ecDNA in heterogeneous populations, identifying amplified foci in 20-30% of tumor cells missed by bulk methods. The ATACAmp algorithm analyzes scATAC-seq peaks for copy number anomalies, enabling ecDNA/HSR discrimination in single nuclei. Integration with CRISPR-Cas9 supports functional validation by targeted cleavage or enrichment of specific ecDNA, as in CRISPR-CATCH protocols that isolate oncogene-bearing circles for downstream phenotyping, confirming their role in proliferation with >50% fitness reduction upon disruption.

Biological Significance

Functions in Normal Cells

In normal cells, extrachromosomal DNA (ecDNA) serves as a transient amplifier for during developmental processes, particularly in oocytes where it facilitates rapid production of essential proteins. For instance, in amphibian oocytes such as those of Xenopus laevis, (rDNA) is amplified extrachromosomally to generate multiple copies of the genes encoding , supporting the high demand for synthesis during . This amplification occurs through rolling-circle replication, producing free extrachromosomal circles that are not integrated into the nuclear genome, thereby enabling efficient, stage-specific without altering chromosomal structure. In the immune system, ecDNA arises as a byproduct of V(D)J recombination, a physiological process that assembles diverse antigen receptor genes in developing lymphocytes. During this recombination, the RAG1 and RAG2 proteins cleave DNA at recombination signal sequences, excising intervening segments that form stable extrachromosomal signal joint circles. These circles, such as T-cell receptor excision circles (TRECs) in T cells and kappa-deleting recombination excision circles (KRECs) in B cells, persist transiently without integrating into the genome, contributing to immune repertoire diversity by allowing precise joining of variable (V), diversity (D), and joining (J) segments while maintaining genomic stability. ecDNA also supports in prokaryotes through plasmids, which are autonomously replicating extrachromosomal elements that confer environmental responsiveness. In like species, plasmids carry genes for metabolic adjustments and stress resistance, enabling rapid adaptation to host intracellular niches via and coevolution with chromosomal elements. Similarly, (mtDNA), a circular extrachromosomal , maintains in eukaryotic cells by encoding components of the system, with its copy number regulated to match cellular metabolic demands through fission and fusion dynamics. Tissue-specific variations in ecDNA abundance reflect physiological roles, with higher levels observed in dynamic tissues like the compared to stable somatic cells. In murine brain tissues, such as cortex and hippocampus, eccDNA correlates with open regions marked by H3K27ac and H3K4me1, potentially supporting neuronal plasticity by influencing near immediate-early genes involved in synaptic remodeling. In contrast, tissues like exhibit lower eccDNA levels, consistent with their post-mitotic stability. In plants, chloroplast DNA (cpDNA), an extrachromosomal genome, sustains photosynthetic efficiency by encoding proteins for the and , optimizing light energy conversion in leaf mesophyll cells. Certain viral episomes maintain persistent infections in normal host cells without disrupting physiology, acting as stable extrachromosomal replicons. For example, (HSV-1) persists as low-copy episomes in sensory neurons, regulated by latency-associated transcripts to ensure lifelong carriage with minimal host impact until reactivation triggers. This episomal form allows the virus to evade immune clearance while integrating into the host's cellular .

Implications in Disease

Extrachromosomal DNA (ecDNA), including mitochondrial DNA (mtDNA) deletions and nuclear eccDNA, plays a significant role in neurodegenerative diseases such as Parkinson's disease. In Parkinson's, the 4977-bp common mtDNA deletion, spanning from nucleotide 8470 to 13447, accumulates in substantia nigra neurons and is associated with mitochondrial dysfunction and dopaminergic cell loss. This deletion disrupts oxidative phosphorylation by affecting genes like MT-ATP8 and MT-ND5, contributing to energy deficits observed in affected brain regions. Additionally, nuclear eccDNA accumulates in aging brains, potentially exacerbating genomic instability and neuronal vulnerability, as evidenced by increased eccDNA profiles in aged mouse brain structures compared to young ones. In infectious diseases, viral episomes serve as extrachromosomal elements that promote persistence. Epstein-Barr virus (EBV) maintains latency in B cells during by circularizing into episomal form, tethered to host via EBNA1, allowing lifelong infection without integration. Similarly, unintegrated HIV-1 DNA contributes to viral latency by persisting as extrachromosomal forms in infected cells, enabling low-level and evasion of immune detection, which sustains chronic infection. These episomal structures facilitate reactivation and complicate eradication efforts in latency reservoirs. Genetic disorders involving ecDNA often arise from nuclear-mitochondrial transfers, such as numts (nuclear mitochondrial DNA segments), which integrate mtDNA fragments into the nuclear genome as nonfunctional pseudogenes. Numts can introduce sequencing artifacts or mimic mtDNA variants, complicating diagnostics in mitochondrial disorders, and their accumulation is linked to diseases through erroneous or genomic instability. In mitochondrial diseases like , caused by mtDNA point mutations such as m.3243A>G in MT-TL1, extrachromosomal mtDNA dynamics—resembling plasmid-like circular elements—contribute to shifts and impaired tRNA function, leading to encephalomyopathy and . EcDNA also influences autoimmunity by acting as autoantigens that trigger aberrant immune responses. Circulating eccDNA, derived from apoptotic cells, acts as a novel autoantigen in systemic lupus erythematosus (SLE), where plasma eccDNA profiles differ from healthy controls and correlate with immunological markers such as complement C3 and . In chronic infections, bacterial plasmids persist as extrachromosomal elements, conferring antibiotic resistance and virulence factors that sustain reservoirs.

Role in Cancer

Oncogene Amplification

Extrachromosomal DNA (ecDNA) serves as a primary mechanism for oncogene amplification in cancer, enabling high copy numbers of key driver genes that drive tumorigenesis through overexpression. Unlike chromosomal amplifications, ecDNA can exist in tens to hundreds of copies per cell, such as for oncogenes like MYC and EGFR, resulting in substantially elevated transcript levels compared to equivalent linear DNA amplifications. This amplification occurs via focal excision and circularization of genomic regions containing oncogenes, often facilitated by chromothripsis—a catastrophic shattering of chromosomes within micronuclei during mitosis—that generates the initial DNA fragments for ecDNA formation. Under selective pressure in the tumor microenvironment, clones harboring these amplified ecDNAs proliferate preferentially, accelerating tumor progression and heterogeneity. In specific cancers, ecDNA-mediated oncogene amplification is particularly prevalent and impactful. For instance, in , approximately 90% of MYCN amplifications occur on ecDNA, contributing to aggressive disease phenotypes and poor by hijacking distal enhancers that boost MYCN expression. Similarly, in , the EGFRvIII variant—a constitutively active —is frequently amplified on ecDNA, where it associates with active enhancers to drive rapid tumor evolution and therapy resistance. These examples highlight how ecDNA enables dynamic, high-level oncogene dosage that outpaces chromosomal mechanisms, with ecDNA detected across 10–60% of cases in diverse tumor types, including up to 49% in glioblastomas and 55% in liposarcomas. The circular structure of ecDNA confers advantages over linear amplifications, including evasion of TP53-dependent degradation pathways that target unstable linear DNA, thereby promoting persistence and accumulation in cancer cells. Additionally, ecDNA's lack of centromeres leads to random, heterogeneous segregation during , generating subclonal variation in oncogene copy number that fuels rapid tumor and . Recent findings further reveal that ecDNA integrates into nuclear condensates via association with the MED1 transcription coactivator, forming hubs that reorganize and enhance oncogenic transcription in a cancer-type-specific manner. This condensate-mediated boosting of underscores ecDNA's role in sustaining high output.

Therapeutic Resistance

Extrachromosomal DNA (ecDNA) facilitates rapid evolutionary adaptation in cancer cells, enabling quick shifts in expression under therapeutic pressure. This non-chromosomal allows for asymmetric segregation during , promoting stochastic changes in that accelerate the selection of resistant clones. For instance, in response to targeted therapies, ecDNA can drive the emergence of alternative oncogenic pathways, enhancing tumor survival and progression. The mosaic distribution of ecDNA within tumors generates significant intratumoral heterogeneity, fostering subpopulations with varying sensitivities to treatment. This uneven partitioning leads to phenotypic diversity, where cells with higher ecDNA copy numbers exhibit enhanced proliferation and survival advantages during . A 2024 pan-cancer analysis revealed that ecDNA presence correlates with poor , including shorter overall survival (hazard ratio 1.44) and increased likelihood of advanced disease stages, underscoring its role in driving resistant subpopulations. In MYCN-amplified neuroblastoma, ecDNA-driven oncogene dosage heterogeneity promotes rapid adaptation to , with senescent cells harboring lower ecDNA copies contributing to . Key mechanisms of ecDNA-mediated resistance include the amplification of drug efflux genes, such as MDR1, which encodes to expel chemotherapeutic agents like and . In human epidermal carcinoma cells, MDR1 amplification on ecDNA increases gene copy number and expression, directly enhancing multidrug resistance through ATP-dependent efflux pumps. Additionally, ecDNA carrying enables bypass signaling pathways by upregulating activity, allowing cells to evade therapy-induced dependencies; for example, in pancreatic ductal , ecDNA confers independence from stromal WNT signals, promoting plasticity and survival under stress. Clinically, ecDNA is strongly associated with relapse in various cancers, including ovarian and breast tumors, where its retention in recurrent lesions outpaces chromosomal amplifications. In high-grade serous ovarian cancer, ecDNA contributes to clonal evolution and therapeutic failure, while in breast cancer, it drives oncogene amplification linked to distant metastasis in up to 30% of early-stage cases. A 2025 preprint highlights ecDNA's role in fast-tracking evolutionary adaptability at the population level, making tumors resilient to interventions and prone to recurrence.01620-X/fulltext) A major challenge in targeting ecDNA arises from its dynamic nature, where copy numbers may decline post-treatment due to selective pressures but re-emerge from residual heterogeneous cells during . This transient loss, observed in models of on MDR1-amplified ecDNA, complicates eradication efforts, as surviving subpopulations can regenerate ecDNA under renewed stress, perpetuating resistance.

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