References to STR marker mutability

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This is one of a series of articles on Genealogical Methods, prepared in association with The Tapestry. See Index for a list of related articles.
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Abstracts

AuthorDateTitleSourceLinkNotes
Ballantyne, et al. 2010 Mutability of Y-Chromosomal Microsatellites The American Journal of Human Genetics, Volume 87, Issue 3, 341-353, 02 September 2010 Cell.com, abstract, link to PDF "Abstract

Nonrecombining Y-chromosomal microsatellites (Y-STRs) are widely used to infer population histories, discover genealogical relationships, and identify males for criminal justice purposes. Although a key requirement for their application is reliable mutability knowledge, empirical data are only available for a small number of Y-STRs thus far. To rectify this, we analyzed a large number of 186 Y-STR markers in nearly 2000 DNA-confirmed father-son pairs, covering an overall number of 352,999 meiotic transfers. Following confirmation by DNA sequence analysis, the retrieved mutation data were modeled via a Bayesian approach, resulting in mutation rates from 3.78 × 10−4 (95% credible interval [CI], 1.38 × 10−5 − 2.02 × 10−3) to 7.44 × 10−2 (95% CI, 6.51 × 10−2 − 9.09 × 10−2) per marker per generation. With the 924 mutations at 120 Y-STR markers, a nonsignificant excess of repeat losses versus gains (1.16:1), as well as a strong and significant excess of single-repeat versus multirepeat changes (25.23:1), was observed. Although the total repeat number influenced Y-STR locus mutability most strongly, repeat complexity, the length in base pairs of the repeated motif, and the father's age also contributed to Y-STR mutability. To exemplify how to practically utilize this knowledge, we analyzed the 13 most mutable Y-STRs in an independent sample set and empirically proved their suitability for distinguishing close and distantly related males. This finding is expected to revolutionize Y-chromosomal applications in forensic biology, from previous male lineage differentiation toward future male individual identification."

Dupuy et al. 2004 Y-chromosomal microsatellite mutation rates: Differences in mutation rate between and within loci. Hum. Mutat. 23, 117–124. Online Library "Abstract

Precise estimates of mutation rates at Y-chromosomal microsatellite STR (short tandem repeat) loci make an important basis for paternity diagnostics and dating of Y chromosome lineage origins. There are indications of considerable locus mutation rate variability between (inter-) and within (intra-) loci. We have studied nine Y-STR loci—DYS19, DYS389I/II, DYS390, DYS391, DYS392, DYS393, DYS385, and DYS388—in 1,766 father–son pairs of confirmed paternity (a total of 15,894 meioses). Five biallelic markers were also analyzed in the fathers—Tat, YAP, 12f2, SRY1532, and 92R7—defining haplogroups 1, 2, 3, 4, 9, and 16, respectively. A total of 36 fragment length mutations were observed: 24 gains (22 single-step, two double-step) and 12 single-step losses. Thus, there was a significant surplus of gains (p=0.045). Overall, the mutation rate was positively correlated to STR repeat length and there was a significant relative excess of losses in long alleles and gains in short alleles (p=0.043). In contrast to the situation in autosomal STR loci and in MSY-1, no noteworthy correlation between mutation rate and the father's age at the child's birth was observed. We observed significant interlocus differences in Y-STR mutation rates (p<0.01). The number of observed mutations ranged from zero in DYS392 to eight in DYS391 and DYS390. We have also demonstrated obvious differences in mutation rates between the haplogroups studied (p=0.024), a phenomenon that is a reflection of the dependence of mutation rate on allele size. Our study has thus demonstrated the necessity of not only locus-specific, but even allele-specific, mutation rate estimates for forensic and population genetic purposes, and provides a considerable basis for such estimates. Hum Mutat 23:117–124, 2004. © 2003 Wiley-Liss, Inc.

Makova et al. 2008 Factors That Contribute To DNA Mutations  ? science daily "There have been reports indicating that individual factors might be affecting microsatellite mutability, but nobody has looked at how these factors interact with each other and which factors are more important," said Makova. "This is the first study to bring together multiple factors affecting microsatellite evolution."

"An example is the microsatellite sequence ACACACACAC. The repeat number of this example sequence is five; in other words, the repeated nucleotides A and C (adenine and cytosine) occur together five times, each repeat length is two, and the length of the full microsatellite is 10. The team found that these three factors -- repeat number, repeat length, and microsatellite length -- were the most significant factors affecting microsatellite mutability."
Igor L. Rozhanskii, Anatole A. Klyosov 2011 Mutation Rate Constants in DNA Genealogy (Y Chromosome) Advances in Anthropology 2011. Vol.1, No.2, 26-34 The basic principles of DNA genealogy and the mutation rate constants for haplotypes of Y chromosome are considered. They are exemplified with 3160 haplotypes, 2489 of those in the 67 marker format, with 55 DNA lineages, 11 of them having documented confirmed common ancestors. In total, they cover 8 haplogroups and the time range from 225 to ca. 8000 years before present. A series (including 67 marker, 37 marker, 25 marker, 16 marker mostly of the Y filer haplotype panel, 12 marker, as well as the “slowest” 22 marker and its subset of 6 marker haplotypes) were calibrated using documented genealogies (with a number of lineages which allegedly descended from some legendary and/or mythical historical figures that were examined and verified employing the calibration plots). The study principally confirms a number of previously made or assumed theoretical foundations of DNA genealogy, such as a postulated stochastic character of mutations in non-recombining parts of DNA, the first-order kinetics of mutations in the DNA, the same values of the mutation rate constants for different haplogroups and lineages, and the principles of calculating timespans to the most recent common ancestors taking into account corrections for back (reverse) mutations.
Concetta Burgarella and Miguel Navascués 2010 Mutation rate estimates for 110 Y-chromosome STRs combining population and father–son pair data European Journal of Human Genetics (2011) 19, 70–75; Nature Abstract

Y-chromosome microsatellites (Y-STRs) are typically used for kinship analysis and forensic identification, as well as for inferences on population history and evolution. All applications would greatly benefit from reliable locus-specific mutation rates, to improve forensic probability calculations and interpretations of diversity data. However, estimates of mutation rate from father–son transmissions are available for few loci and have large confidence intervals, because of the small number of meiosis usually observed. By contrast, population data exist for many more Y-STRs, holding unused information about their mutation rates. To incorporate single locus diversity information into Y-STR mutation rate estimation, we performed a meta-analysis using pedigree data for 80 loci and individual haplotypes for 110 loci, from 29 and 93 published studies, respectively. By means of logistic regression we found that relative genetic diversity, motif size and repeat structure explain the variance of observed rates of mutations from meiosis. This model allowed us to predict locus-specific mutation rates (mean predicted mutation rate 2.12 × 10−3, SD=1.58 × 10−3), including estimates for 30 loci lacking meiosis observations and 41 with a previous estimate of zero. These estimates are more accurate than meiosis-based estimates when a small number of meiosis is available. We argue that our methodological approach, by taking into account locus diversity, could be also adapted to estimate population or lineage-specific mutation rates. Such adjusted estimates would represent valuable information for selecting the most reliable markers for a wide range of applications.

Brinkmann, B., Klintschar, M., Neuhuber, F., Hühne, J., and Rolf, B. 1998 Mutation rate in human microsatellites: Influence of the structure and length of the tandem repeat. Am. J. Hum. Genet. 62, 1408–1415. AJHG In 10,844 parent/child allelic transfers at nine short-tandem-repeat (STR) loci, 23 isolated STR mismatches were observed. The parenthood in each of these cases was highly validated (probability >99.97%). The event was always repeat related, owing to either a single-step mutation (n=22) or a double-step mutation (n=1). The mutation rate was between 0 and 7×10−3 per locus per gamete per generation. No mutations were observed in three of the nine loci. Mutation events in the male germ line were five to six times more frequent than in the female germ line. A positive exponential correlation between the geometric mean of the number of uninterrupted repeats and the mutation rate was observed. Our data demonstrate that mutation rates of different loci can differ by several orders of magnitude and that different alleles at one locus exhibit different mutation rates.
Rubinsztein DC, Amos B, Cooper G. 1999 Microsatellite and trinucleotide-repeat evolution: evidence for mutational bias and different rates of evolution in different lineages. Philos Trans R Soc Lond B Biol Sci. 1999 Jun 29;354(1386):1095-9. PubMed Abstract

Microsatellites are stretches of repetitive DNA, where individual repeat units comprise one to six bases. These sequences are often highly polymorphic with respect to repeat number and include trinucleotide repeats, which are abnormally expanded in a number of diseases. It has been widely assumed that microsatellite loci are as likely to gain and lose repeats when they mutate. In this review, we present population genetic and empirical data arguing that microsatellites, including normal alleles at trinucleotide-repeat disease loci, are more likely to expand in length when they mutate. In addition, our experiments suggest that the rates of expansion of such sequences differ in related species.


Schlötterer C. 2000 Evolutionary dynamics of microsatellite DNA. Chromosoma. 2000 Sep;109(6):365-71. Erratum in Chromosoma 2001 Feb;109(8):571. Chomosoma Abstract

Within the past decade microsatellites have developed into one of the most popular genetic markers. Despite the widespread use of microsatellite analysis, an integral picture of the mutational dynamics of microsatellite DNA is just beginning to emerge. Here, I review both generally agreed and controversial results about the mutational dynamics of microsatellite DNA. Microsatellites are short DNA sequence stretches in which a motif of one to six bases is tandemly repeated. It has been known for some time that these sequences can differ in repeat number among individuals. With the advent of polymerase chain reaction (PCR) technology this property of microsatellite DNA was converted into a highly versatile genetic marker (Litt and Luty 1989; Tautz 1989; Weber and May 1989). Polymerase chain reaction products of different length can be amplified with primers flanking the variable microsatellite region. Due to the availability of high-throughput capillary sequencers or mass spectrography the sizing of alleles is no longer a bottleneck in microsatellite analysis. The almost random distribution of microsatellites and their high level of polymorphism greatly facilitated the construction of genetic maps (Dietrich et al. 1994; Dib et al. 1996) and enabled subsequent positional cloning of several genes. Almost at the same time, microsatellites were established as the marker of choice for the identification of individuals and paternity testing. The high sensitivity of PCR-based microsatellite analysis was not only of great benefit for forensics, but opened completely new research areas, such as the analysis of samples with limited DNA amounts (e.g., many social insects) or degraded DNA (e.g., feces, museum material) (Schlötterer and Pemberton 1998). More recently, microsatellite analysis has also been employed in population genetics (Goldstein and Schlötterer 1999). Compared with allozymes, microsatellites offer the advantage that, in principle, several thousand potentially polymorphic markers are available. Nevertheless, the application of microsatellites to population genetic questions requires a more detailed understanding of the mutation processes of microsatellite DNA as the evolutionary time frames covered in population genetics are often too long to allow novel microsatellite mutations to be ignored. Additional interest in the evolution of microsatellite DNA comes from the discovery that trinucleotide repeats, a special class of microsatellites, are involved in human neurodegenerative diseases (e.g., fragile X and Huntington's disease). A detailed understanding of the processes underlying microsatellite instability is therefore an important contribution toward a better understanding of these human neurodegenerative diseases.

Goedbloed, Miriam et al. 2009 Comprehensive mutation analysis of 17 Y-chromosomal short tandem repeat polymorphisms included in the AmpFlSTR® Yfiler® PCR amplification kit International Journal of Legal MedicineNovember 2009, Volume 123, Issue 6, pp 471-482, Springer Abstract

The Y-chromosomal short tandem repeat (Y-STR) polymorphisms included in the AmpFlSTR® Yfiler® polymerase chain reaction amplification kit have become widely used for forensic and evolutionary applications where a reliable knowledge on mutation properties is necessary for correct data interpretation. Therefore, we investigated the 17 Yfiler Y-STRs in 1,730–1,764 DNA-confirmed father–son pairs per locus and found 84 sequence-confirmed mutations among the 29,792 meiotic transfers covered. Of the 84 mutations, 83 (98.8%) were single-repeat changes and one (1.2%) was a double-repeat change (ratio, 1:0.01), as well as 43 (51.2%) were repeat gains and 41 (48.8%) repeat losses (ratio, 1:0.95). Medians from Bayesian estimation of locus-specific mutation rates ranged from 0.0003 for DYS448 to 0.0074 for DYS458, with a median rate across all 17 Y-STRs of 0.0025. The mean age (at the time of son’s birth) of fathers with mutations was with 34.40 (±11.63) years higher than that of fathers without ones at 30.32 (±10.22) years, a difference that is highly statistically significant (p < 0.001). A Poisson-based modeling revealed that the Y-STR mutation rate increased with increasing father’s age on a statistically significant level (α = 0.0294, 2.5% quantile = 0.0001). From combining our data with those previously published, considering all together 135,212 meiotic events and 331 mutations, we conclude for the Yfiler Y-STRs that (1) none had a mutation rate of >1%, 12 had mutation rates of >0.1% and four of <0.1%, (2) single-repeat changes were strongly favored over multiple-repeat ones for all loci but 1 and (3) considerable variation existed among loci in the ratio of repeat gains versus losses. Our finding of three Y-STR mutations in one father–son pair (and two pairs with two mutations each) has consequences for determining the threshold of allelic differences to conclude exclusion constellations in future applications of Y-STRs in paternity testing and pedigree analyses.


Ge J, Budowle B, Aranda XG, Planz JV, Eisenberg AJ, Chakraborty R. 2009 Mutation rates at Y chromosome short tandem repeats in Texas populations. Forensic Sci Int Genet. 2009 Jun;3(3):179-84. PubMed

Abstract Father-son pairs from three populations (African American, Caucasian, and Hispanic) of Texas were typed for the 17 Y STR markers DYS19, DYS385, DYS389I, DYS389II, DYS390, DYS391, DYS392, DYS393, DYS437, DYS438, DYS439, DYS456, DYS458, DYS635, DYS448, and Y GATA H4 using the AmpFlSTR YfilerTM kit. With 49,578 allele transfers, 102 mutations were detected. One three-step and four two-step mutations were found, and all others (95.1%) were one-step mutations. The number of gains (48) and losses (54) of repeats were nearly similar. The average mutation rate in the total population is 2.1 x 10(-3) per locus (95% CI (1.7-2.5)x10(-3)). African Americans showed a higher mutation rate (3.0 x 10(-3); 95% CI (2.4-4.0)x10(-3)) than the Caucasians (1.7 x 10(-3); 95% CI (1.1-2.5)x10(-3)) and Hispanics (1.5 x 10(-3); 95% CI (1.0-2.2)x10(-3)), but grouped by repeat-lengths, such differences were not significant. Mutation is correlated with relative length of alleles, i.e., longer alleles are more likely to mutate compared with the shorter ones at the same locus. Mutation rates are also correlated with the absolute number of repeats, namely, alleles with higher number of repeats are more likely to mutate than the shorter ones (p-value=0.030). Finally, occurrences of none, one, and two mutations over the father-son transmission of alleles were consistent with the assumption of independence of mutation rates across loci.

Eckert KA, Hile SE. 2009 Every microsatellite is different: Intrinsic DNA features dictate mutagenesis of common microsatellites present in the human genome. Mol Carcinog. 2009 Apr;48(4):379-88. PubMed

Microsatellite sequences are ubiquitous in the human genome and are important regulators of genome function. Here, we examine the mutational mechanisms governing the stability of highly abundant mono-, di-, and tetranucleotide microsatellites. Microsatellite mutation rate estimates from pedigree analyses and experimental models range from a low of approximately 10(-6) to a high of approximately 10(-2) mutations per locus per generation. The vast majority of observed mutational variation can be attributed to features intrinsic to the allele itself, including motif size, length, and sequence composition. A greater than linear relationship between motif length and mutagenesis has been observed in several model systems. Motif sequence differences contribute up to 10-fold to the variation observed in human cell mutation rates. The major mechanism of microsatellite mutagenesis is strand slippage during DNA synthesis. DNA polymerases produce errors within microsatellites at a frequency that is 10- to 100-fold higher than the frequency of frameshifts in coding sequences. Motif sequence significantly affects both polymerase error rate and specificity, resulting in strand biases within complementary microsatellites. Importantly, polymerase errors within microsatellites include base substitutions, deletions, and complex mutations, all of which produced interrupted alleles from pure microsatellites. Postreplication mismatch repair efficiency is affected by microsatellite motif size and sequence, also contributing to the observed variation in microsatellite mutagenesis.' Inhibition of DNA synthesis within common microsatellites is highly sequence-dependent, and is positively correlated with the production of errors. DNA secondary structure within common microsatellites can account for some DNA polymerase pause sites, and may be an important factor influencing mutational specificity.


Pumpernik D, Oblak B, Borstnik B. 2007 Replication slippage versus point mutation rates in short tandem repeats of the human genome. Mol Genet Genomics. 2008 Jan;279(1):53-61. PubMed Epub 2007 Oct 10.

Abstract Short tandem repeats (STRs) are subjected to two kinds of mutational modifications: point mutations and replication slippages. The latter is found to be the more frequent cause of STR modifications, but a satisfactory quantitative measure of the ratio of the two processes has yet to be determined. The comparison of entire genome sequences of closely enough related species enables one to obtain sufficient statistics by counting the differences in the STR regions. We analyzed human-chimpanzee DNA sequence alignments to obtain the counts of point mutations and replication slippage modifications. The results were compared with the results of a computer simulation, and the parameters quantifying the replication slippage probability as well as the probabilities of point mutations within the repeats were determined. It was found that within the STRs with repeated units consisting of one, two or three nucleotides, point mutations occur approximately twice as frequently as one would expect on the basis of the 1.2% difference between the human and chimpanzee genomes. As expected, the replication slippage probability is negligible below a 10-bp threshold and grows above this level. The replication slippage events outnumber the point mutations by one or two orders of magnitude, but are still lower by one order of magnitude relative to the mutability of the markers that are used for genotyping purposes.

Gusmão L, et al. 2005 Mutation rates at Y chromosome specific microsatellites. Hum Mutat. 2005 Dec;26(6):520-8. PubMed Abstract

A collaborative work was carried out by the Spanish and Portuguese ISFG Working Group (GEP-ISFG) to estimate Y-STR mutation rates. Seventeen Y chromosome STR loci (DYS19, DYS385, DYS389I and II, DYS390, DYS391, DYS392, DYS393, DYS437, DYS438, DYS439, DYS460, DYS461, DYS635 [GATA C4], GATA H4, and GATA A10) were analyzed in a sample of 3,026 father/son pairs. Among 27,029 allele transfers, 54 mutations were observed, with an overall mutation rate across the 17 loci of 1.998 x 10(-3) (95% CI, 1.501 x 10(-3) to 2.606 x 10(-3)). With just one exception, all of the mutations were single-step, and they were observed only once per gametogenesis. Repeat gains were more frequent than losses, longer alleles were found to be more mutable, and the mutation rate seemed to increase with the father's age.

Dupuy BM, Stenersen M, Egeland T, Olaisen B. 2004 Y-chromosomal microsatellite mutation rates: differences in mutation rate between and within loci. >Hum Mutat. 2004 Feb;23(2):117-24 PubMed Abstract

Precise estimates of mutation rates at Y-chromosomal microsatellite STR (short tandem repeat) loci make an important basis for paternity diagnostics and dating of Y chromosome lineage origins. There are indications of considerable locus mutation rate variability between (inter-) and within (intra-) loci. We have studied nine Y-STR loci-DYS19, DYS389I/II, DYS390, DYS391, DYS392, DYS393, DYS385, and DYS388-in 1,766 father-son pairs of confirmed paternity (a total of 15,894 meioses). Five biallelic markers were also analyzed in the fathers-Tat, YAP, 12f2, SRY1532, and 92R7-defining haplogroups 1, 2, 3, 4, 9, and 16, respectively. A total of 36 fragment length mutations were observed: 24 gains (22 single-step, two double-step) and 12 single-step losses. Thus, there was a significant surplus of gains (p=0.045). Overall, the mutation rate was positively correlated to STR repeat length and there was a significant relative excess of losses in long alleles and gains in short alleles (p=0.043). In contrast to the situation in autosomal STR loci and in MSY-1, no noteworthy correlation between mutation rate and the father's age at the child's birth was observed. We observed significant interlocus differences in Y-STR mutation rates (p<0.01). The number of observed mutations ranged from zero in DYS392 to eight in DYS391 and DYS390. We have also demonstrated obvious differences in mutation rates between the haplogroups studied (p=0.024), a phenomenon that is a reflection of the dependence of mutation rate on allele size. Our study has thus demonstrated the necessity of not only locus-specific, but even allele-specific, mutation rate estimates for forensic and population genetic purposes, and provides a considerable basis for such estimates.


Lai Y, Sun F. 2003 The relationship between microsatellite slippage mutation rate and the number of repeat units. Mol Biol Evol. 2003 Dec;20(12):2123-31 PubMed

Abstract Microsatellite markers are widely used for genetic studies, but the relationship between microsatellite slippage mutation rate and the number of repeat units remains unclear. In this study, microsatellite distributions in the human genome are collected from public sequence databases. We observe that there is a threshold size for slippage mutations. We consider a model of microsatellite mutation consisting of point mutations and single stepwise slippage mutations. From two sets of equations based on two stochastic processes and equilibrium assumptions, we estimate microsatellite slippage mutation rates without assuming any relationship between microsatellite slippage mutation rate and the number of repeat units. We use the least squares method with constraints to estimate expansion and contraction mutation rates. The estimated slippage mutation rate increases exponentially as the number of repeat units increases. When slippage mutations happen, expansion occurs more frequently for short microsatellites and contraction occurs more frequently for long microsatellites. Our results agree with the length-dependent mutation pattern observed from experimental data, and they explain the scarcity of long microsatellites.

Xu X, Peng M, Fang Z. 2000 The direction of microsatellite mutations is dependent upon allele length. Nat Genet. 2000 Apr;24(4):396-9. PubMed

Abstract Microsatellites, comprising tandemly repeated short nucleotide sequences, are ubiquitous in eukaryotic genomes. Mutations within microsatellites are frequent, altering their overall length by insertion or deletion of a small number of repeat units, with a rate as high as 10(-3) in humans. Despite their high mutability, stable allele frequency distributions are typically observed for microsatellites in humans as well as other primates, although the mechanism maintaining these stable distributions remains unclear. Previous studies have suggested that microsatellite mutations occur more frequently in longer alleles and favour expansion. Generalizing these results has been hindered because the sample sizes were small, only a small subset of alleles for any marker was studied and the direction of mutation (expansion or contraction) was not rigorously determined. Here we examine 236 mutations at 122 tetranucleotide repeat markers and find that the rate of contraction mutations increases exponentially with allele size, whereas the rate of expansion mutations is constant across the entire allele distribution. The overall rate of expansion mutations does not differ from that of contractions. Our findings offer an explanation for the stationary allele distribution of microsatellites.

Whittaker JC, Harbord RM, Boxall N, Mackay I, Dawson G, Sibly RM. 2003 Likelihood-based estimation of microsatellite mutation rates. Genetics. 2003 Jun;164(2):781-7. PubMed Abstract

Microsatellites are widely used in genetic analyses, many of which require reliable estimates of microsatellite mutation rates, yet the factors determining mutation rates are uncertain. The most straightforward and conclusive method by which to study mutation is direct observation of allele transmissions in parent-child pairs, and studies of this type suggest a positive, possibly exponential, relationship between mutation rate and allele size, together with a bias toward length increase. Except for microsatellites on the Y chromosome, however, previous analyses have not made full use of available data and may have introduced bias: mutations have been identified only where child genotypes could not be generated by transmission from parents' genotypes, so that the probability that a mutation is detected depends on the distribution of allele lengths and varies with allele length. We introduce a likelihood-based approach that has two key advantages over existing methods. First, we can make formal comparisons between competing models of microsatellite evolution; second, we obtain asymptotically unbiased and efficient parameter estimates. Application to data composed of 118,866 parent-offspring transmissions of AC microsatellites supports the hypothesis that mutation rate increases exponentially with microsatellite length, with a suggestion that contractions become more likely than expansions as length increases. This would lead to a stationary distribution for allele length maintained by mutational balance. There is no evidence that contractions and expansions differ in their step size distributions.


Huang QY, Xu FH, Shen H, Deng HY, Liu YJ, Liu YZ, Li JL, Recker RR, Deng HW. 2002 Mutation patterns at dinucleotide microsatellite loci in humans. Am J Hum Genet. 2002 Mar;70(3):625-34 PubMed Abstract

Microsatellites are a major type of molecular markers in genetics studies. Their mutational dynamics are not clear. We investigated the patterns and characteristics of 97 mutation events unambiguously identified, from 53 multigenerational pedigrees with 630 subjects, at 362 autosomal dinucleotide microsatellite loci. A size-dependent mutation bias (in which long alleles are biased toward contraction, whereas short alleles are biased toward expansion) is observed. There is a statistically significant negative relationship between the magnitude (repeat numbers changed during mutation) and direction (contraction or expansion) of mutations and standardized allele size Contrasting with earlier findings in humans, most mutation events (63%) in our study are multistep events that involve changes of more than one repeat unit. There was no correlation between mutation rate and recombination rate. Our data indicate that mutational dynamics at microsatellite loci are more complicated than the generalized stepwise mutation models.

Chakraborty R, Kimmel M, Stivers DN, Davison LJ, Deka R. 1997 Relative mutation rates at di-, tri-, and tetranucleotide microsatellite loci. Proc Natl Acad Sci U S A. 1997 Feb 4;94(3):1041-6. pubmed

Abstract Using the generalized stepwise mutation model, we propose a method of estimating the relative mutation rates of microsatellite loci, grouped by the repeat motif. Applying ANOVA to the distributions of the allele sizes at microsatellite loci from a set of populations, grouped by repeat motif types, we estimated the effect of population size differences and mutation rate differences among loci. This provides an estimate of motif-type-specific mutation rates up to a multiplicative constant. Applications to four different sets of di-, tri-, and tetranucleotide loci from a number of human populations reveal that, on average, the non-disease-causing microsatellite loci have mutation rates inversely related to their motif sizes. The dinucleotides appear to have mutation rates 1.5-2 times higher than the tetranucleotides, and the non-disease-causing trinucleotides have mutation rates intermediate between the di- and tetranucleotides. In contrast, the disease-causing trinucleotides have mutation rates 3.9-6.9 times larger than the tetranucleotides. Comparison of these estimates with the direct observations of mutation rates at microsatellites indicates that the earlier suggestion of higher mutation rates of tetranucleotides in comparison with the dinucleotides may stem from a nonrandom sampling of tetranucleotide loci in direct mutation assays.


Kruglyak S, Durrett RT, Schug MD, Aquadro CF. 1998 Equilibrium distributions of microsatellite repeat length resulting from a balance between slippage events and point mutations. Proc Natl Acad Sci U S A. 1998 Sep 1;95(18):10774-8. PubMed Abstract

We describe and test a Markov chain model of microsatellite evolution that can explain the different distributions of microsatellite lengths across different organisms and repeat motifs. Two key features of this model are the dependence of mutation rates on microsatellite length and a mutation process that includes both strand slippage and point mutation events. We compute the stationary distribution of allele lengths under this model and use it to fit DNA data for di-, tri-, and tetranucleotide repeats in humans, mice, fruit flies, and yeast. The best fit results lead to slippage rate estimates that are highest in mice, followed by humans, then yeast, and then fruit flies. Within each organism, the estimates are highest in di-, then tri-, and then tetranucleotide repeats. Our estimates are consistent with experimentally determined mutation rates from other studies. The results suggest that the different length distributions among organisms and repeat motifs can be explained by a simple difference in slippage rates and that selective constraints on length need not be imposed.


Bell GI, Jurka J. The length distribution of perfect dimer repetitive DNA is consistent with its evolution by an unbiased single-step mutation process. >J Mol Evol. 1997 Apr;44(4):414-21. PubMed Abstract

We have examined the length distribution of perfect dimer repeats, where perfect means uninterrupted by any other base, using data from GenBank on primates and rodents. Virtually no lengths greater than 30 repeats are found, except for rodent AG repeats, which extend to 35. Comparable numbers of long AC and AG repeats suggest that they have not been selected for special functions or DNA structures. We have compared the data with predictions of two models: (1) a Bernoulli Model in which bases are assumed equally likely and distributed at random and (2) an Unbiased Random Walk Model (URWM) in which repeats are permitted to change length by plus or minus one unit, with equal probabilities, and in which base substitutions are allowed to destroy long perfect repeats, producing two shorter perfect repeats. The source of repeats is assumed to be from single base substutions from neighboring sequences, i.e., those differing from the perfect repeat by a single base. Mutation rates either independent of repeat length or proportional to length were considered. An upper limit to the lengths L approximately 30 is assumed and isolated dimers are assumed unable to expand, so that there are absorbing barriers to the random walk at lengths 1 and L + 1, and a steady state of lengths is reached. With these assumptions and estimated values for the rates of length mutation and base substitution, reasonable agreement is found with the data for lengths > 5 repeats. Shorter repeats, of lengths </= 3 are in general agreement with the Bernoulli Model. By reducing the rate of length mutations for n </= 5, it is possible to obtain reasonable agreement with the full range of data. For these reduced rates, the times between length mutations become comparable to those suggested for a bottleneck in the evolution of Homo sapiens, which may be the reason for low heterozygosity of short repeats.

Heyer E, Puymirat J, Dieltjes P, Bakker E, de Knijff P. 1997 Estimating Y chromosome specific microsatellite mutation frequencies using deep rooting pedigrees. Hum Mol Genet. 1997 May;6(5):799-803. PubMed Abstract

Recently, a set of highly polymorphic chromosome Y specific microsatellites became available for forensic, population genetic and evolutionary studies. However, the lack of a mutation frequency estimate for these loci prevents a reliable application. We therefore used seven chromosome Y tetranucleotide repeat loci to screen 42 males who are descendants from 12 'founding fathers' by a total number of 213 generations. As a result, we were able to estimate an average chromosome Y tetranucleotide mutation frequency of 0.20% (95% CIL 0.05-0.55). This closely matches the often cited Weber and Wong estimate of 0.21% for a set of autosomal tetranucleotide repeats. Expanding the set of microsatellites with two more loci (a tri- and a penta-nucleotide repeat locus) an average chromosome Y microsatellite mutation frequency of 0.21% (95% CIL 0.06-0.49) was found. These estimates suggest that microsatellites on the Y chromosome have mutation frequencies comparable to those on the autosomes. This supports the hypothesis that slippage-generated growth is the driving force behind the microsatellite variability.


Ellegren H. 2000 Heterogeneous mutation processes in human microsatellite DNA sequences. Nat Genet. 2000 Apr;24(4):400-2. PubMed Abstract

Although microsatellite polymorphisms are one of the most commonly used tools in genetic analyses, it remains to be understood how microsatellite DNA has evolved as a ubiquitous and highly abundant class of repetitive sequences in eukaryotic genomes. On the basis of analyses of spontaneous human microsatellite mutations of germline origin, I show here that different mutation biases underlie the evolution of microsatellite repeats. The within-locus mutation rate increases with allele length, but is not affected by the size difference between an individual's two alleles (allele span). Within loci, long alleles tend to mutate to shorter lengths, thereby acting to prevent infinite growth. Expansions are more common than contractions among dinucleotide repeats, whereas no such trend is evident among tetranucleotide repeats. This observation is consistent with the longer repeat lengths and higher frequency of di- compared with tetranucleotide repeats. An excess of paternally transmitted mutations (male-to-female ratio of 4.9) supports a male-biased mutation rate in the human genome.



Definitiions

Microsatellites...., also known as Simple Sequence Repeats (SSRs) or short tandem repeats (STRs), are repeating sequences of 2-6 base pairs of DNA.[1] It is a type of variable number tandem repeat (VNTR). Microsatellites are typically co-dominant. They are used as molecular markers in genetics, for kinship, population and other studies. They can also be used for studies of gene duplication or deletion, marker assisted selection, and fingerprinting. In a recent discovery by Centre for Cellular and Molecular Biology the functionality of simple sequence repeats among non-coding DNA, has been identified. Wikipedia:Microsatellite

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