© Kamla-Raj 2010 Int J Hum Genet, 10(1-3): 21-32 (2010) Chromosomal Abnormalities in Mental Retardation: Indian Experience Usha Dave and Dhanlaxmi Shetty R & D, Super Religare Labs (SRL), Goregaon (W), Mumbai 400 062, Maharashtra, India Telephone: 091 022 267801311, E-mail: usha.dave@srl.in KEYWORDS Mental Retardation. Chromosomal Abnormalities. Genetic Syndromes ABSTRACT At a Tertiary Genetic Centre, children with mental retardation (MR) (also referred as intellectual disability) and associated developmental disabilities were investigated for genetic diagnosis which is important in prevention and genetic counseling while offering the risk of recurrence to the family. A prospective and retrospective cytogenetic study was conducted on 1760 MR cases for chromosomal abnormalities using routine GTG and high resolution banding methods of karyotyping. Out of 1760 MR cases, 555 cases showed abnormal chromosomal constitution (31.5%), and males were more than females (2.1: 1). Numerical chromosomal abnormalities were detected in 40.4% (224 of 555), out of which autosomal abnormalities were 36% (199 of 555) and sex chromosomal abnormalities were 4.5% (25 of 555). Structural chromosomal abnormalities were detected in 52% (289 of 555), out of which autosomal abnormalities were 28.6% (159 of 555) and sex chromosomal abnormalities were 29.5% (164 of 555), with some having both numerical-structural (7.6%) and autosomal-sex abnormalities (1.4%). The chromosomal study revealed Down syndrome as the most common chromosomal abnormality i.e. 45% (250 of 555). The children varied from mild to severe mental retardation with and without multiple congenital anomalies and dysmorphism. A few genetic syndromes with characteristic clinical features were also confirmed due to chromosomal aberrations. Genetic counseling was provided to the family members explaining the importance of recurrence risk, the need for prenatal diagnosis in subsequent pregnancies, along with the management of MR children in Indian set-up. INTRODUCTION Genetic disorders have been a great burden on the society since the beginning of the civilization and with the human genome project completed, the genetic services are being integrated with the other health care services. Mental retardation (MR), also referred as ‘Intellectual Disability’, ‘Mental Deficit’, ‘Mental Subnormality’ or ‘Mental Handicap’ means delay in mental development; it means an impairment of the intellectual processes of the mind, making it difficult for the person to cope with environment in which they find themselves. In 1992, the American Association on Mental Retardation (AAMR) revised the definition as significantly sub average intellectual functioning (defined as an I.Q. score below 70) existing concurrently with limitations in two or more of the following adaptive areas like communication, self care, home living, social skills, self direction, health and safety, leisure, work and functional academics (Grossman 1977; Epstein 1996). Despite extensive studies in area of mental retardation the overall prevalence of MR is still not known with certainty. It is approximately 1- 3% (Munro 1986; DeVries et al. 2005) and in India too it is estimated to be 2 – 3% of the population (Kaur et al. 2003). Chromosome abnormalities are visible alteration of chromosomes. Alternatively these are produced by specific chromosomal mecha- nism. Errors in mitosis and meiosis may result in chromosomally abnormal daughter nuclei containing either the wrong number of chromo- somes or structurally altered chromosomal complement (Hamerton 1971). Most aberrations are produced by mis-repair of broken chromo- somes, improper recombination or improper segregation of chromosomes. Chromosome complements are subject to two kinds of changes-(1) numerical and (2) structural and they may affect either sex chromosomes or autosomes. In rare cases both kinds of chromosomes are affected. There are over 100 chromosomal syndromes, which have been reported. While, on the individual basis many of these are rare, together they make a major contribution to human Corresponding author: Dr. Usha P. Dave Principal Scientist- R & D Super Religare Labs (SRL) Prime Square, Plot No. 1 S. V. Road, Goregaon (W), Maharashtra, India Telephone: 091 022 28750576 E-mail: ushadave26@gmail.com