A multistep mutation mechanism drives the evolution of the CAG repeat at MJD/SCA3 locus

Abstract

Despite the intense debate around the repeat instability reported on the large group of neurological disorders caused by trinucleotide repeat expansions, little is known about the mutation process underlying alleles in the normal range that, ultimately, expand to pathological size. In this study, we assessed the mutation mechanisms by which wild-type Machado -Joseph disease (MJD) alleles have been generated throughout human evolution. Haplotypes including the CAG repeat, six intragenic SNPs and four flanking microsatellites were analysed in 431 normal chromosomes of European, Asian and African origin. A bimodal CAG repeat length frequency distribution was found in the four most frequent wild-type lineages (H1-GCGGCA; H2-GTGGCA; H3-TTAGAC and H4-TTACAC). Based on flanking microsatellite haplotypes, the variance calculated by analysis of molecular variance between modal (CAG) n alleles was little or null in lineages H1, H2 and H4, as were the pairwise differences. Moreover, genetic distances among all the alleles from each lineage did not reflect the allele sizes differences, as expected if a stepwise mutation model was the main process of evolution. On the contrary, when exposed in maximum parsimonious phylogenetic trees, a large number of mutation steps separated same-size alleles, whereas several microsatellite haplotypes were shared by modal CAGs. In conclusion, our results suggest that the main mutation mechanism occurring in the evolution of the polymorphic CAG region at MJD/SCA3 locus is a multistep one, either by gene conversion or DNA slippage; repeats with 14, 21, 23 and 27 CAGs are the main alleles involved in this process.

Asian Origin for the Worldwide-Spread Mutational Event in Machado-Joseph Disease

“…Amplification and genotyping conditions were performed as previously described. 20 PHASE software version 2.0 (http: //www.stat.washington.edu/stephens/software.html) was used to reconstruct haplotypes from genotypic data when the phase could not be directly inferred by family structure or by allelespecific amplification. Allele frequencies and phase-known haplotypes were taken into account, but only haplotype pairs with a probability greater than 0.6 were used for further analyses.…”

Cis‐acting factors promoting the CAG intergenerational instability in Machado–Joseph disease

“…Another mechanism proposed to explain CAG/CTG instability was the meiotic recombination and/or gene conversion, evidenced from clinical cases in DM1 O'Hoy et al, 1993], HD [Warner et al, 1996], and MJD [Mittal et al, 2005]. More recently, a multistep mutational mechanism, either by gene conversion or DNA slippage, has also been proposed by us for the evolution of normal alleles at the ATXN3 locus [Martins et al, 2006].…”

Non-Mendelian Genetic Aspects in Spinocerebellar Ataxias (SCAS): The Case of Machado-Joseph Disease (MJD)

“…On the contrary, the report from Lima and colleagues (2005), shows that the allelic distribution was skewed towards smaller size alleles. In a subsequent study, Martins et al (2006) have integrated not only the analysis of the CAG repeat, but also information on haplotypes built using intragenic and flanking markers; the conclusions indicate that a multistep mechanism is on the basis of the evolution of the CAG repeats in MJD, originated either by gene conversion or DNA slippage.…”