Cytogenetics in Mental Retardation: An Overview

It is important to ascertain the cause of mental retardation, so that recurrence risk can be reduced and where MR is transmissible, prenatal diagnosis and proper genetic counseling could be provided. The paper highlights the importance of cytogenetic investigations in detecting abnormalities like rings, and mosaics that may be missed even with the CMA assays. Mental retardation is defined as an inadequate development of mental capacities and related behavioural problems. It is the most common neuropsychiatric illness in all civilised societies, affecting 2.5-3.0% of the population. Chromosomal abnormalities are the important cause of mental retardation. In the first report from North India cytogenetic investigations were carried out on 143 mentally subnormal individuals that were referred to the Centre for Genetic Disorders, Guru Nanak Dev University, Amritsar, India, during 1996 to 2002. These cases were referred mainly as suspected Down syndrome, delayed milestones, mental retardation, etc. The age group of the patients ranged from 1 month to 18 years. Interestingly, maximum number of patients i.e., 58/143 (40.5%) were the firstborns and the average maternal age was 27.6 years. Free trisomy 21 was found to be the most frequent autosomal aberration, both amongst males and females (45.4% males, 18.8% females). Free trisomy was seen in 92/143 (64.3%) cases while translocations were seen in 2.7% cases. The latter included 45,XY,+t(13;14), 46,XY,+t(14;21), 45,XX,+t(14;21) and 46,XX,+t(14;21) karyotypes. The cytogenetic analysis in patients with intellectual disability (ID)/MR should nowadays be applied when aneuploidies are suspected, and Array Comparative Genomic Hybridization should be used as the first-line genetic test for patients with ID/MR, especially non-syndromic cases.