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Transversion
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Transversion, in molecular biology, refers to a point mutation in DNA in which a single (two ring) purine (A or G) is changed for a (one ring) pyrimidine (T or C), or vice versa.[1] A transversion can be spontaneous, or it can be caused by ionizing radiation or alkylating agents. It can only be reversed by a spontaneous reversion.
Ratio of transitions to transversions
[edit]Although there are two possible transversions but only one possible transition per base, transition mutations are more likely than transversions because substituting a single ring structure for another single ring structure is more likely than substituting a double ring for a single ring. Also, transitions are less likely to result in amino acid substitutions (due to wobble base pair), and are therefore more likely to persist as "silent substitutions" in populations as single nucleotide polymorphisms (SNPs).[2] A transversion usually has a more pronounced effect than a transition because the second and third nucleotide codon position of the DNA, which to a large extent is responsible for the degeneracy of the code, is more tolerant of transition than a transversion: transitions are more likely to be synonymous substitutions than transversions, as one observes in the codon table.
Spontaneous germline transversion
[edit]8-oxo-2'-deoxyguanosine (8-oxodG) is an oxidized derivative of deoxyguanosine, and is one of the major products of DNA oxidation. During DNA replication in the germ line of mice, the oxidized base 8-oxoguanine (8-oxoG) causes spontaneous and heritable G to T transversion mutations.[3] These mutations occur in different stages of the germ cell lineage and are distributed throughout the chromosomes.
Consequences of transversion mutations
[edit]The location of a transversion mutation on a gene coding for a protein correlates with the extent of the mutation. If the mutation occurs at a site that is not involved with the shape of a protein or the structure of an enzyme or its active site, the mutation will not have a significant effect on the cell or the enzymatic activity of its proteins. If the mutation occurs at a site that changes the structure or function of a protein, therefore changing its enzymatic activity, the mutation can have significant effects on the survival of the cell.[4]
Transversions due to products of oxidative guanine damage
[edit]Of the natural nitrogenous bases of DNA, guanine is most prone to oxidation. Oxidation of guanine, also known as oxidative guanine damage, results in the formation of many products. These products trigger mutations, leading to DNA damage, and can pair with adenine and guanine through hydrogen bonding causing G-T transversions and G-C transversions, respectively.[5]
Transversion and p53 mutations in smoking-associated cancers
[edit]The mutation of the P53 gene is the most common gene mutation found in cancer cells. A study has shown that p53 mutations are common in tobacco-related cancers, with a variation in the amount of G-T transversions in lung cancer from smokers and non-smokers. In smokers’ lung cancer, the prevalence of G-T transversions is 30% compared to that of 12% in non-smokers. At many p53 mutational hotspots, a large number of the mutations are G-T events in lung cancers but almost exclusively G-A transitions in non-tobacco-related cancers.[6]
See also
[edit]- Transition
- Aristolochic acid, a natural plant chemical causing A → T and T → A transversions in humans
- P53
- Guanine
- DNA oxidation
- Oxidative stress
References
[edit]- ^ Futuyma, D.J. (2013). Evolution (3rd ed.). Sinauer. ISBN 978-1605351155.
- ^ Diagram at mun.ca
- ^ Ohno M, Sakumi K, Fukumura R, Furuichi M, Iwasaki Y, Hokama M, Ikemura T, Tsuzuki T, Gondo Y, Nakabeppu Y (2014). "8-oxoguanine causes spontaneous de novo germline mutations in mice". Sci Rep. 4: 4689. Bibcode:2014NatSR...4E4689O. doi:10.1038/srep04689. PMC 3986730. PMID 24732879.
- ^ Landolph, J.R. (2014-01-01). "Genetic Toxicology". Encyclopedia of Toxicology. pp. 715–725. doi:10.1016/B978-0-12-386454-3.00392-4. ISBN 9780123864550.
- ^ Kino, Katsuhito; Kawada, Taishu; Hirao-Suzuki, Masayo; Morikawa, Masayuki; Miyazawa, Hiroshi (2020-10-15). "Products of Oxidative Guanine Damage Form Base Pairs with Guanine". International Journal of Molecular Sciences. 21 (20): 7645. doi:10.3390/ijms21207645. ISSN 1422-0067. PMC 7589758. PMID 33076559.
- ^ Pfeifer, Gerd P.; Denissenko, Mikhail F.; Olivier, Magali; Tretyakova, Natalia; Hecht, Stephen S.; Hainaut, Pierre (2002-10-15). "Tobacco smoke carcinogens, DNA damage and p53 mutations in smoking-associated cancers". Oncogene. 21 (48): 7435–7451. doi:10.1038/sj.onc.1205803. ISSN 1476-5594. PMID 12379884.
External links
[edit]
Wikimedia Commons has media related to Transversion.
- Carr, Steven M. "Transition versus Transversion mutations". Memorial University of Newfoundland.
- Nikolay's Genetics Lessons (2014). Transition versus Transversion mutations (how to memorize). Youtube. Archived from the original on 2021-12-22.
Transversion
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Fundamentals
Definition
A transversion is a type of point mutation in which a purine nucleotide (adenine, A, or guanine, G) is replaced by a pyrimidine nucleotide (cytosine, C, or thymine, T), or vice versa, resulting in a change at a single position in the DNA sequence.[1] This substitution alters the base pairing properties, as purines pair with pyrimidines, potentially disrupting the genetic code during replication or transcription.[7] Specific examples of transversions include A to C, G to T, C to A, or T to G substitutions.[8] In molecular notation, such as sequence alignments or genetic analyses, transversions are denoted by the original and substituted bases; for instance, a change from AGG to ACG represents a G to C transversion.[9] The terms "transition" and "transversion" to classify point mutations were first introduced by Ernst Freese in 1959, based on studies of spontaneous and base-analogue-induced mutations in bacteriophage T4. This distinction became foundational for understanding nucleotide substitution patterns in molecular evolution.[10]Comparison to Transitions
A transition mutation is defined as a point substitution in which a purine base (adenine or guanine) is replaced by another purine, or a pyrimidine base (cytosine or thymine) by another pyrimidine, such as A to G or C to T.[11] In contrast, a transversion involves the replacement of a purine by a pyrimidine or vice versa, such as A to C or G to T.[11] Structurally, transitions occur between bases of similar chemical shape—both purines being two-ring structures and both pyrimidines one-ring—facilitating easier mispairing during replication with fewer distortions to the DNA helix.[2] Transversions, however, require pairing between dissimilar shapes, often involving the breaking of more hydrogen bonds in the original base pair (typically two for A-T pairs) and leading to greater steric hindrance, which makes them biochemically less favorable.[11] Tautomeric shifts, where a base temporarily adopts a rare enol or imino form, predominantly enable transitions by allowing compatible hydrogen bonding patterns, further reducing the likelihood of transversions.[11] In most organisms, transversions occur at approximately one-third to one-half the frequency of transitions due to these biochemical constraints on polymerase fidelity and base-pairing stability.[12] For example, in the human genome, the observed transition-to-transversion ratio is around 2.1, indicating transitions are roughly twice as common.[13] Base-pairing diagrams illustrate this distinction: transitions depict substitutions within the same base class (e.g., A-T pair shifting to G-T via wobble, maintaining roughly two hydrogen bonds), while transversions show cross-class mismatches (e.g., A-T to C-T, forcing a pyrimidine-pyrimidine pair that distorts the double helix and weakens bonding).[11] Evolutionarily, transversions tend to produce more nonsynonymous mutations in protein-coding regions compared to transitions, as the degeneracy of the genetic code allows many transitions to be silent (synonymous), whereas transversions more frequently alter amino acids and face stronger purifying selection.[14]Molecular Mechanisms
Types of Base Substitutions
Transversions represent a category of point mutations in which a purine base (adenine [A] or guanine [G]) is substituted by a pyrimidine base (cytosine [C] or thymine [T]), or vice versa.[5] These mutations can be categorized into purine-to-pyrimidine and pyrimidine-to-purine subtypes, yielding a total of eight possible changes.[3] The four purine-to-pyrimidine transversions are A→C, A→T, G→C, and G→T.[3] The four pyrimidine-to-purine transversions are C→A, C→G, T→A, and T→G.[3] These subtypes alter the chemical structure of the DNA by exchanging the two-ring purine for the one-ring pyrimidine or the reverse.[5] Due to the complementary nature of double-stranded DNA, transversions occur as reciprocal pairs across strands, resulting in equivalent base pair substitutions. For example, an A→C transversion on one strand (changing an A·T pair to a C·G pair) is the inverse of a T→G transversion on the complementary strand (also yielding a C·G pair from A·T).[15] Similarly, G→T pairs with C→A (both converting G·C to T·A), A→T with T→A (A·T to T·A), and G→C with C→G (G·C to C·G).[15] The following table summarizes all eight transversions, including the corresponding base pair substitutions:| Subtype | Mutation | Original Base Pair | New Base Pair |
|---|---|---|---|
| Purine-to-pyrimidine | A → C | A·T | C·G |
| A → T | A·T | T·A | |
| G → C | G·C | C·G | |
| G → T | G·C | T·A | |
| Pyrimidine-to-purine | C → A | C·G | A·T |
| C → G | C·G | G·C | |
| T → A | T·A | A·T | |
| T → G | T·A | C·G |
Biochemical Processes Leading to Transversions
Transversion mutations arise primarily through biochemical pathways that disrupt normal base pairing during DNA replication or subsequent repair processes. One key mechanism involves tautomeric shifts, where nucleobases transiently adopt rare isomeric forms, such as keto-enol or amino-imino tautomers, altering their hydrogen-bonding patterns and promoting non-standard mispairing. Although tautomeric shifts more commonly lead to transitions, rare configurations can facilitate purine-purine or pyrimidine-pyrimidine mispairs that result in transversions.[18][19] DNA polymerases contribute to transversions via inherent limits in their fidelity mechanisms during replication. High-fidelity polymerases, such as those in the A and B families, rely on an induced-fit conformational change to select correct nucleotides, where the active site closes tightly around Watson-Crick base pairs but accommodates mismatches less efficiently. Transversions often result from wobble base pairing involving purine-purine (e.g., A·G) or pyrimidine-pyrimidine (e.g., C·T) mispairs, which cause significant geometric distortion but can occasionally be extended if the polymerase fails to reject the incorrect nucleotide, occurring at rates approximately 10^4 to 10^6 times lower than correct incorporation. Induced-fit failures exacerbate this, particularly under replication stress, allowing transversion-prone misinsertions to proceed.[20][21] Post-replication, the mismatch repair (MMR) system attempts to correct these errors, but it is less effective against transversion mismatches. The MutS protein recognizes distortions in the DNA helix caused by mismatched bases, recruiting MutL to initiate excision; however, transversion mismatches, like purine-purine or pyrimidine-pyrimidine pairs, induce greater helical distortions and are repaired with lower efficiency compared to transition mismatches, which more closely mimic standard geometry. Studies indicate that transition mismatches are repaired up to several-fold more effectively, allowing transversions to evade correction more frequently.[22][23] The frequency of transversions driven by such mispairing processes can be modeled simply as a function of the probability of rare forms or errors. Let denote the transversion mutation rate, and the frequency of error-prone configurations (typically to ). A basic derivation assumes that mispairing occurs proportionally to , with polymerase incorporation and repair inefficiency factors and (where for misinsertion and for incomplete repair). Thus,
This yields on the order of to per base pair per replication, aligning with observed spontaneous rates, though stochastic fluctuations in introduce variability.[24][25]
Experimental validation of these processes comes from site-directed mutagenesis studies using base analogs like 2-aminopurine (2-AP), which mimics adenine but promotes mispairing. In vitro assays with bacteriophage T4 and E. coli systems demonstrate that 2-AP primarily induces transitions but can also enhance transversions at certain sites (up to 10^{-6} per site) by forming wobble pairs during replication, bypassing full MMR scrutiny and confirming roles of mispairing in transversion generation. Gap misrepair protocols further enable targeted induction of transversions, highlighting enzymatic pathways without external mutagens.[26][27]
Causes
Spontaneous Origins
Spontaneous transversions arise from intrinsic cellular processes that introduce base substitution errors without external influences, contributing to baseline genomic instability across organisms. These mutations primarily stem from hydrolytic reactions, endogenous chemical modifications, and replication inaccuracies, which occur at low but measurable frequencies during normal cellular metabolism. In the absence of repair, such events can fix transversions into the genome, particularly during DNA replication or repair synthesis. One key mechanism is depurination and depyrimidination, where purine (adenine or guanine) or pyrimidine (cytosine or thymine) bases are spontaneously lost from the DNA backbone due to hydrolysis of the glycosidic bond, generating apurinic (AP) or apyrimidinic sites. These abasic sites are non-instructive during replication, prompting DNA polymerases to follow the "A-rule," preferentially inserting adenine opposite the lesion. For instance, depurination of guanine (creating an AP site opposite cytosine) followed by adenine insertion results in an A-C mismatch; subsequent replication can yield a G-to-T transversion on the original strand. Similarly, depyrimidination of cytosine (AP site opposite guanine) with adenine insertion leads to a potential C-to-A transversion. These processes occur at rates of approximately 5,000 to 10,000 depurination events per mammalian genome per day, though most are repaired; unrepaired lesions contribute significantly to spontaneous mutagenesis.[28][29][30] Endogenous alkylating agents, such as the cellular metabolite S-adenosylmethionine (SAM), also drive transversions by non-enzymatically methylating DNA bases, forming adducts like O6-methylguanine or 7-methylguanine. These modified bases mispair during replication; for example, O6-methylguanine pairs with thymine instead of cytosine, leading to a G-to-A transition, but further depurination of alkylated purines can result in transversion mutations upon repair or bypass, such as G-to-T. In Escherichia coli, defects in alkyltransferases like Ada and Ogt reveal that endogenous alkylation accounts for a substantial portion of spontaneous base substitutions, including transversions, underscoring SAM's role as an intracellular mutagen. Such events highlight how routine metabolic byproducts compromise DNA fidelity.[31][32][33] Replication slippage, though more commonly associated with insertions or deletions in repetitive sequences, can rarely occur in non-repetitive regions, where transient misalignment of the polymerase active site during synthesis leads to base skipping or misincorporation, potentially resolving as transversion substitutions after mismatch repair or proofreading failure. This mechanism contributes to a small fraction of spontaneous point mutations, as polymerase pausing and dissociation allow erroneous base pairing, such as purine-to-pyrimidine swaps.[34][35] Spontaneous transversion rates vary by organism and context. In E. coli, the overall base substitution rate is approximately 2 × 10^{-10} per base pair per generation, with transversions comprising about 30-40% of these, or roughly 10^{-10} per site, reflecting efficient repair but persistent intrinsic errors. In humans, germline de novo mutation rates are around 1.2 × 10^{-8} per base pair per generation, with transversions making up approximately 30% (transition/transversion ratio ~2:1), yielding about 4 × 10^{-9} transversions per site; these are predominantly spontaneous, as external mutagens are minimized in protected gametes. Transversions are enriched in germline cells during gametogenesis, particularly spermatogenesis, where increased replication rounds (up to 600 per male lifetime) and transiently reduced mismatch repair efficiency elevate spontaneous mutation burdens compared to somatic tissues.[36][37][38][39]Oxidative and Environmental Damage
Oxidative damage to DNA primarily arises from reactive oxygen species (ROS), which are generated endogenously through metabolic processes such as mitochondrial respiration and enzymatic reactions. These species, including superoxide anion, hydrogen peroxide, and hydroxyl radical, can oxidize DNA bases, leading to modifications that promote transversion mutations during replication or repair. A prominent example is the oxidation of guanine to 8-oxoguanine (8-oxoG), one of the most abundant oxidative lesions, which adopts a syn conformation and preferentially pairs with adenine instead of cytosine. This mispairing results in G:C to T:A transversions, with the mutation manifesting as G→T on the original strand or C→A on the complementary strand. In human cells, an estimated 100–1,000 8-oxoG lesions form per cell per day under normal physiological conditions, though steady-state levels are maintained lower through base excision repair. Other ROS-induced modifications, such as thymine glycol formed from thymine oxidation, can obstruct replication forks and, when bypassed by translesion synthesis polymerases, contribute to C→A transversions by favoring adenine insertion opposite the lesion. Environmental exposures exacerbate oxidative DNA damage, introducing additional transversion-inducing agents. Ultraviolet (UV) radiation from sunlight generates cyclobutane pyrimidine dimers (CPDs), primarily between adjacent thymines or cytosines, which distort the DNA helix and are repaired through nucleotide excision repair or bypassed via error-prone translesion synthesis. While CPDs most commonly lead to C→T transitions, certain repair pathways or tandem lesions can yield transversions, particularly G→T under UVA exposure, due to secondary oxidative effects. Tobacco smoke represents a major environmental contributor, containing polycyclic aromatic hydrocarbons (PAHs) like benzo[a]pyrene that are metabolically activated to benzo[a]pyrene diol epoxide (BPDE). BPDE forms bulky adducts preferentially at the N2 position of guanine, impeding replication and triggering translesion synthesis by polymerases such as Pol η or Pol ζ, which often insert adenine opposite the adduct, resulting in G→T transversions. These mutations are characteristic of smoking-associated lung cancers, with BPDE adduct levels correlating directly with exposure intensity. Overall, such oxidative and environmental insults collectively elevate transversion rates, underscoring the interplay between endogenous ROS and exogenous stressors in genomic instability.Biological Consequences
General Impacts on DNA and Proteins
Transversions, by substituting a purine for a pyrimidine or vice versa, induce greater structural distortions in the DNA double helix compared to transitions, which involve exchanges within the same base class. These distortions primarily affect parameters such as minor groove width and helical roll, leading to increased DNA fragility and potential instability during replication or transcription.[23][14] For instance, purine transversions in mismatched base pairs cause more pronounced local alterations in DNA conformation, exacerbating helix bending and potentially hindering efficient enzymatic processing.[23] In protein-coding regions, transversions exhibit a higher propensity for nonsynonymous substitutions than transitions due to the organization of the genetic code, often resulting in missense or nonsense mutations that alter protein function.[40][14] Analysis of the codon table reveals that transversions in the first or second positions are particularly disruptive; for example, a transversion in the start codon ATG (methionine) to ATC (isoleucine) changes the amino acid identity, potentially affecting translation initiation.[40] In contrast, transversions in the third position may occasionally be silent, but overall, they are less likely to preserve the original amino acid compared to transitions, amplifying the risk of dysfunctional proteins.[14] Beyond coding sequences, transversions in regulatory elements like promoters can disrupt transcription factor (TF) binding sites, leading to altered gene expression levels.[14] The structural changes induced by transversions more severely impact TF-DNA interactions than those from transitions, often resulting in reduced binding affinity and downstream transcriptional dysregulation.[14] Additionally, transversion-induced base mismatches impose a greater burden on DNA repair systems, such as mismatch repair (MMR), which recognizes these lesions but may signal apoptosis if the damage persists unresolved.[41][42] This heightened repair demand underscores the cellular stress associated with transversions, promoting programmed cell death to prevent propagation of severely altered genomes.[43]Role in Disease and Cancer
Transversions play a significant role in the pathogenesis of various diseases, particularly cancers linked to environmental exposures. In smoking-induced lung cancers, transversions are predominant, especially G:C→T:A mutations at CpG dinucleotides within the TP53 gene (encoding p53), reflecting direct DNA damage from tobacco carcinogens such as benzo[a]pyrene. These mutations disrupt p53's tumor suppressor function, promoting oncogenesis.[44][45] Detailed analysis of p53 mutations reveals a strong association with smoking: approximately 30% of p53 mutations in lung cancers from smokers are transversions, compared to less than 12% in nonsmokers, highlighting the mutagenic impact of cigarette smoke. Hotspots for these G→T transversions occur at codons 157, 248, and 273, where guanine residues are particularly susceptible to adduction by polycyclic aromatic hydrocarbons in tobacco. This bias is evident in epidemiological studies from the 1990s to the 2020s, which consistently show transversion enrichment in cancers exposed to environmental carcinogens like those in tobacco.[44][46][47] Beyond lung cancer, transversions contribute to mitochondrial disorders, such as MELAS syndrome, where novel transversion mutations in tRNA genes (e.g., A>C in tRNA^Leu) impair mitochondrial protein synthesis and energy production, leading to encephalomyopathy, lactic acidosis, and stroke-like episodes. In aging, somatic transversions accumulate in various tissues, exacerbating cellular dysfunction and contributing to age-related decline, with studies indicating higher transversion rates in post-mitotic cells like neurons.[48][49][50] Therapeutically, understanding transversion-prone pathways informs chemotherapy strategies; alkylating agents, such as those used in cancer treatment, induce transversions (e.g., A→T) by forming DNA adducts that, if unrepaired, lead to cytotoxic mutations, allowing targeted exploitation of repair deficiencies in tumor cells. More recently, gene editing tools like prime editing have emerged to directly correct transversion mutations in genetic diseases, offering potential for precise therapeutic interventions as of 2025.[51][52][53] Oxidative damage from smoking, a key inducer of transversions, underscores the need for preventive interventions.[51][52]Population and Evolutionary Aspects
Transition-Transversion Ratios
The transition-transversion (Ti/Tv) ratio is defined as the number of transition mutations divided by the number of transversion mutations observed in sequence data, providing a key metric for assessing mutational spectra and evolutionary processes.[54] In most biological contexts, this ratio favors transitions over transversions, typically ranging from 2:1 to 3:1, reflecting inherent biases in DNA replication and repair mechanisms.[55] In the human genome, whole-genome sequencing of single nucleotide polymorphisms (SNPs) yields a Ti/Tv ratio of approximately 2.1 for both known and novel variants, while coding regions exhibit a higher ratio around 3.0 due to purifying selection against more disruptive transversions.[55] In contrast, RNA virus genomes often display a lower Ti/Tv ratio, indicating relatively higher transversion frequencies; for example, SARS-CoV-2 variants show a Ti/Tv of about 1.2 across global sequences, attributed to the error-prone nature of RNA-dependent RNA polymerases that reduce transition bias.[56] Several factors influence the Ti/Tv ratio, primarily stemming from biochemical and structural properties of DNA. Transitions are favored due to the smaller geometric distortion required in the DNA double helix, as purine-to-purine or pyrimidine-to-pyrimidine changes involve less steric hindrance during replication.[40] Spontaneous deamination, particularly of 5-methylcytosine in CpG dinucleotides, predominantly generates C→T transitions, elevating the ratio in methylated genomes like mammals.[57] Conversely, transversion biases arise in contexts of oxidative damage, where lesions like 8-oxoguanine pair erroneously with adenine, leading to G→T transversions during replication.[58] Environmental factors such as UV radiation primarily induce C→T transitions via cyclobutane pyrimidine dimers but can indirectly promote transversions through secondary oxidative stress.[59] Evolutionary models incorporate the Ti/Tv ratio to estimate genetic distances while accounting for multiple substitutions. The Jukes-Cantor model assumes equal rates for all nucleotide changes, predicting a neutral Ti/Tv ratio of 0.5, with the evolutionary distance calculated as:
where is the observed proportion of differing sites; this underestimates distances when transition bias is present. The Kimura two-parameter model addresses this by distinguishing transition rate from transversion rate , with the Ti/Tv rate ratio typically estimated at 2–5 across taxa. The distance is given by:
where is the proportion of transitional differences and the proportion of transversional differences; this correction better fits observed data by separately modeling the two substitution types.