Intermediate repeat alleles at polyglutamine repeat loci in the Indian population

An expansion of trinucleotide repeats beyond a threshold, at particular loci, is linked to several diseases. The prevalence of these expansion-linked disorders varies across populations, and may be influenced by the prevalence of sub-threshold intermediate alleles, which are prone to expand. The effect of somatic instability of these repeats, adds further complexity to the role of intermediate repeats. We studied the prevalence of intermediate CAG repeat alleles at SCA1, SCA2, SCA3 and HTT in several hundred individuals. These included patients with clinical suspicion of movement disorder (N[~]725), those with severe mental illness (N[~] 420), and healthy individuals from the general population (N[~]300). Intermediate alleles were defined as those in the range of 36 -38 CAG repeats for ATXN1, 45 -59 CAG repeats for ATXN3, 27 - 35 CAG repeats for HTT, and 27-32 CAG repeats for ATXN2. In those with neuromotor symptoms, we found intermediate alleles in 26 out of 561 (4.6 %) individuals genotyped at ATXN1, 10 out of 437 (2.2%) individuals at ATXN3, 16 out of 686 (2.3%) individuals at HTT and 1 out of 484(0.2%) individuals at ATXN2. In individuals without a diagnosis of movement disorder (patients with SMI and healthy controls), intermediate alleles were detected in 19 out of 725 (2.6%) individuals at ATXN1 and 14 out of 687 (2.0%) individuals at HTT loci. No intermediate alleles were detected at the ATXN2 and ATXN3 loci. The impact of these intermediate alleles on risk of disease in future generations, and prevalence of disease, needs to be evaluated.