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Complex germline and somatic mutation processes at a haploid human minisatellite shown by single-molecule analysis.

Shanks ME, May CA, Dubrova YE, Balaresque P, Rosser ZH, Adams SM, Jobling MA - Mutat. Res. (2008)

Bottom Line: Sperm DNA showed significantly more length mutants than blood DNA, with mutants in both tissues involving small-scale (1-3 repeat units in a 77 repeat progenitor allele) increases or decreases in repeat block lengths, with no gain or loss bias.Isometric mutations altering structure but not length were found in both tissues, and involved either the apparent shift of a boundary between repeat unit blocks (a 'boundary switch') or the conversion of a repeat within a block to a different repeat type ('modular structure' mutant).There was a significant excess of boundary switch mutants and deficit of modular structure mutants in sperm.

View Article: PubMed Central - PubMed

Affiliation: Department of Genetics, University of Leicester, Leicester, UK.

ABSTRACT
Mutation at most human minisatellites is driven by complex interallelic processes that give rise to a high degree of length polymorphism and internal structural variation. MSY1, the only highly variable minisatellite on the non-recombining region of the Y chromosome, is constitutively haploid and therefore precluded from interallelic interactions, yet maintains high diversity in both length and structure. To investigate the basis of its mutation processes, an unbiased structural analysis of >500 single-molecule MSY1 PCR products from matched sperm and blood samples from a single donor was undertaken. The overall mutation frequencies in sperm and blood DNAs were not significantly different, at 2.68% and 1.88%, respectively. Sperm DNA showed significantly more length mutants than blood DNA, with mutants in both tissues involving small-scale (1-3 repeat units in a 77 repeat progenitor allele) increases or decreases in repeat block lengths, with no gain or loss bias. Isometric mutations altering structure but not length were found in both tissues, and involved either the apparent shift of a boundary between repeat unit blocks (a 'boundary switch') or the conversion of a repeat within a block to a different repeat type ('modular structure' mutant). There was a significant excess of boundary switch mutants and deficit of modular structure mutants in sperm. A comparison of mutant structures with phylogenetically matched alleles in population samples showed that alleles with structures resembling the blood mutants were unlikely to arise in populations. Mutation seems likely to involve gene conversion via synthesis-dependent strand annealing, and the blood-sperm differences may reflect more relaxed constraint on sister chromatid alignment in blood.

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Distribution of the length of the interstitial type 4 repeat block in population samples and mutants.
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fig4: Distribution of the length of the interstitial type 4 repeat block in population samples and mutants.

Mentions: While each of the mutants represents a unique and independent event, the population samples are the result of successive mutation processes, and subsets of them are likely to be relatively closely related, carrying structural features that are identical by descent. This makes a fair comparison between the alleles in the population and the blood and sperm mutants difficult. However, with this caveat in mind, there are 14 alleles in the population sample that have non-1, 3 ,4 structures (Supplementary Table), and can be compared with the modular structural mutants. In the population sample, and in the one example of a sperm mutant, the interstitial block (or blocks) of type 4 repeats is between one and three repeat units in length. However, among the 10 examples of such blocks in the blood mutants, six are ≥4 repeats in length (Fig. 4), suggesting that the somatic processes giving rise to these mutants are qualitatively different from those underlying germline mutation.


Complex germline and somatic mutation processes at a haploid human minisatellite shown by single-molecule analysis.

Shanks ME, May CA, Dubrova YE, Balaresque P, Rosser ZH, Adams SM, Jobling MA - Mutat. Res. (2008)

Distribution of the length of the interstitial type 4 repeat block in population samples and mutants.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2599865&req=5

fig4: Distribution of the length of the interstitial type 4 repeat block in population samples and mutants.
Mentions: While each of the mutants represents a unique and independent event, the population samples are the result of successive mutation processes, and subsets of them are likely to be relatively closely related, carrying structural features that are identical by descent. This makes a fair comparison between the alleles in the population and the blood and sperm mutants difficult. However, with this caveat in mind, there are 14 alleles in the population sample that have non-1, 3 ,4 structures (Supplementary Table), and can be compared with the modular structural mutants. In the population sample, and in the one example of a sperm mutant, the interstitial block (or blocks) of type 4 repeats is between one and three repeat units in length. However, among the 10 examples of such blocks in the blood mutants, six are ≥4 repeats in length (Fig. 4), suggesting that the somatic processes giving rise to these mutants are qualitatively different from those underlying germline mutation.

Bottom Line: Sperm DNA showed significantly more length mutants than blood DNA, with mutants in both tissues involving small-scale (1-3 repeat units in a 77 repeat progenitor allele) increases or decreases in repeat block lengths, with no gain or loss bias.Isometric mutations altering structure but not length were found in both tissues, and involved either the apparent shift of a boundary between repeat unit blocks (a 'boundary switch') or the conversion of a repeat within a block to a different repeat type ('modular structure' mutant).There was a significant excess of boundary switch mutants and deficit of modular structure mutants in sperm.

View Article: PubMed Central - PubMed

Affiliation: Department of Genetics, University of Leicester, Leicester, UK.

ABSTRACT
Mutation at most human minisatellites is driven by complex interallelic processes that give rise to a high degree of length polymorphism and internal structural variation. MSY1, the only highly variable minisatellite on the non-recombining region of the Y chromosome, is constitutively haploid and therefore precluded from interallelic interactions, yet maintains high diversity in both length and structure. To investigate the basis of its mutation processes, an unbiased structural analysis of >500 single-molecule MSY1 PCR products from matched sperm and blood samples from a single donor was undertaken. The overall mutation frequencies in sperm and blood DNAs were not significantly different, at 2.68% and 1.88%, respectively. Sperm DNA showed significantly more length mutants than blood DNA, with mutants in both tissues involving small-scale (1-3 repeat units in a 77 repeat progenitor allele) increases or decreases in repeat block lengths, with no gain or loss bias. Isometric mutations altering structure but not length were found in both tissues, and involved either the apparent shift of a boundary between repeat unit blocks (a 'boundary switch') or the conversion of a repeat within a block to a different repeat type ('modular structure' mutant). There was a significant excess of boundary switch mutants and deficit of modular structure mutants in sperm. A comparison of mutant structures with phylogenetically matched alleles in population samples showed that alleles with structures resembling the blood mutants were unlikely to arise in populations. Mutation seems likely to involve gene conversion via synthesis-dependent strand annealing, and the blood-sperm differences may reflect more relaxed constraint on sister chromatid alignment in blood.

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