Molecular diagnosis of myotonic dystrophy and Huntington's disease from Calcutta, India

It has been identified that the expansion of normally polymorphic CTG repeats in myotonin protein kinase (DM-PK) gene in myotonic dystrophy (DM) and CAG repeats in huntingtin gene in Huntington's disease (HD) acts as the causative mutation. This hasimproved the ability of molecular diagnosis of these diseases. On molecular genetic screening of 40 normal chromosomes from people living in and around Calcutta, we have obtained 10 distinct alleles at the DM locus, with an estimated heterozygosity of 0.83. We have also observed 10 alleles at the HD locus on 32 chromosomes, with an estimated heterozygosity of 0.79. We further report here that at the DM locus, in two clinically diagnosed heterozygous individuals, the expanded alleles were of sizes 330 repeats and 1400 repeats respectively; the allele on the homologous chromosome was within the normal repeat range for both these individuals. Of two HD patients, one carried an expanded allele of size of 57 repeats, the other allele being in the normal range; while the second patient had both the alleles in the normal range.