ABSTRACT

Mental retardation affects about 3% of the population. Final diagnosis could be reached in approximately one third of all cases, and no diagnosis is available in the rest of the patients, which is followed by repeated laboratory tests and a long time exhausting both parents and the patients. Recent studies have established that chromosomal abnormalities comprise 30-40% of moderate to severe mental retardation; and nearly 30% of mild mental retardation. Standard cytogenetic analysis is capable of detecting DNA rearrangements of larger than 3 Mb. Subtelomeric region is the part of the chromosome just before the telomere which is the end structures of the chromosomes. Fluorescent in situ hybridisation (FISH) is a molecular cytogenetic technique in which deletions or rearrangements smaller than 2-3 Mb could be detectable. Subtelomeric region accounts for a significant proportion (2.2%-23%) of cases with undiagnosed mental retardation because of high concentration of genes in this region. In this article we aimed to discuss the reasons of mental retardation and the studies performed in mentally retarded cases related to the subtelomeric regions of the chromosomes.