Distribution of CGG/GCC Repeats at the FMR1 and FMR2 Genes in an Indian Population with Mental Retardation of Unknown Etiology

Abstract

Fragile X syndrome is one of the X-linked disorders associated with moderate to severe mental retardation. Fragile X A syndrome (FRAXA) and fragile X E syndrome (FRAXE) are caused by trinucleotide repeat expansion of CGG and GCC repeats at the 5' untranslated region of the FMR1 and FMR2 genes, respectively. The present study was undertaken to identify the repeat polymorphism and to estimate the risk of transmission in Andhra Pradesh and surrounding states of South India. The FRAXA and FRAXE allelic polymorphisms were studied by radioactive polymerase chain reaction that revealed 25 FRAXA among 344 X-chromosomes and 20 FRAXE allelic variants among 212 X-chromosomes in our population. The most frequent FRAXA allele size was of 29 CGG repeats (27.5%) followed by allele sizes of 28 (20.8%) and 31 (7.2%), and that of FRAXE was 15 GCC repeats (24.0%) followed by allele containing 18 repeats (18.4%) and 16 repeats (11.3%). CGG/GCC repeat polymorphism at the FMR1 and FMR2 loci observed in this study demonstrated a racial and ethnic variation among the populations.