Genetics of mental retardation
AS Ahuja, Anita Thapar, MJ Owen
Department of Psychological Medicine, Cardiff University, Heath Park, Cardiff, United Kingdom
A S Ahuja
Department of Psychological Medicine, Cardiff University, Heath Park, Cardiff CF14 4XN
Indian J Med Sci 2005;59:407-17. [Open Access Article]
Mental retardation can follow any of the biological, environmental and psychological events that are capable of producing deficits in cognitive functions. Recent advances in molecular genetic techniques have enabled us to understand more about the molecular basis of several genetic syndromes associated with mental retardation. In contrast, where there is no discrete cause, the interplay of genetic and environmental influences remains poorly understood. This article presents a critical review of literature on genetics of mental retardation.
Keywords: chromosomal abnormalities; genetic; mental retardation; X-linked
Mental retardation is characterized by significantly below average intellectual functioning existing concurrently with related impaired limitations in two or more of the following applicable adaptive skills areas: communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure and work, manifest before the age of 18. The term mental retardation is not in itself a diagnosis, as it does not inform about aetiology, prognosis, or specific treatments. Rather, it refers to a clinical state that is developmental in origin and which affects intellectual and social functioning.
Early literature made a distinction between 'pathological' severe mental retardation (SMR) and 'familial' (usually mild) mental retardation. More recently it has been customary to divide it into two groups according to IQ. Those who show IQ scores between 50 and 70 are categorized as having mild mental retardation (MMR) and those with IQ scores of below 50 are considered as having moderate-severe mental retardation (SMR).'
The impact of molecular genetics on our understanding of disease processes has been enormous and is likely to increase even further. The resurgence of interest in phenotypes combined with the new genetic techniques has reduced the proportion of those with moderate/severe mental retardation in whom the cause is unknown to 20%. Genetic causes may be hereditary or nonhereditary and may not produce specific syndromes. Over 500 recognized syndromes involving a genetic disorder have now been isolated and many have behavioural epiphenomena. A specific cause for mental retardation can be identified in approximately 80% of people with SMR (IQ<50) and 50% of people with MMR (IQ 50-70).
In this article, we will begin by considering what is known about the genetics of idiopathic mental retardation and then move onto discuss specific genetic causes of mental retardation.
Electronic searching was done and the relevant studies were identified by searching Medline, ClinPSYC, CINAHL, EMBASE, PubMed and The Cochrane library using the keywords: mental retardation, genetics, learning disability, chromosomal abnormalities, single-gene defect and X-linked mental retardation. The references cited in the above studies were also searched in order to identify more studies. Also high yield journals related to the topic were identified and were hand searched for relevant articles.