The genetics of mental retardation

Scope for more genetic testing in learning disability. Case report of an inherited duplication on the X-chromosome

There have been major advances in the past few years in our understanding of the X-linked learning disabilities. The most common of these is the fragile-X syndrome, but the number of other gene defects that are now recognised to be linked with learning disability is increasing year on year. We describe one family displaying a family displaying a rare X-linked abnormality. Repeat genetic testing was requested for a family member with mild learning disability when, following chromosomal analysis for her brother, it became known that he had a genetic defect. The genetic defect 46, Xdup(X) (p22.13 p22.31) was identified. To our knowledge this is the first time this precise configuration has been demonstrated. We conclude that genetic testing for individuals with learning disability is worthwhile, even when there may be only a low index of suspicion.

Neurodevelopmental Disabilities

The topic of this chapter is neurodevelopmental disabilities, with discussion focused on three prototypic neurodevelopmental disorders: mental retardation, autism, and cerebral palsy. These disorders are highlighted because the epidemiologic knowledge is more advanced; however, many of the important methodologic issues apply to other neurodevelopmental disorders. The chapter discusses the particular research challenges in achieving complete case ascertainment due to the problems of case finding and case confirmation. It presents information about the prevalence of each of these disorders and temporal changes in frequency over time, as well as the possible factors contributing to increased recognition of these disorders. The chapter summarizes investigations of risk factors for each disorder, including genetic factors and possible environmental etiologies.

Chromosome 22qII deletions. An under-recognised cause of idiopathic learning disability

BACKGROUND

Velo-cardio-facial syndrome (VCFS), a syndrome of multiple congenital abnormalities including characteristic dysmorphology, congenital heart defects and learning disability, is associated with small interstitial deletions of chromosome 22qII. We tested the hypothesis that VCFS may be significantly under-diagnosed by screening a learning disabled population for chromosome 22qII deletions.

METHOD

Two hundred and sixty-five people with learning disability residing in two learning disability hospitals in South Wales were reviewed. They were selected for inclusion in the study if they fulfilled any of the following criteria: psychotic disorder (schizophrenia or affective disorder), family history of psychotic disorder, cleft palate and/or lip, congenital heart disease, broadly defined facial dysmorphism or a history of hypocalcaemia. Fluorescence in situ hybridisation studies were performed on 74 selected individuals.

RESULTS

Cytogenetic analysis revealed that two people demonstrated a previously undetected chromosome 22qII deletion. A third person demonstrated a previously undetected cytogenetically visible deletion on chromosome 15.

CONCLUSIONS

VCFS appears to be aetiologically significant in a proportion of individuals with idiopathic learning disability, especially in those where psychosis is associated with mild learning disability. We suggest that clinicians should consider a chromosome 22qII deletion in people who meet selection criteria.

Substantial genetic influence on cognitive abilities in twins 80 or more years old

General and specific cognitive abilities were studied in intact Swedish same-sex twin pairs 80 or more years old for whom neither twin had major cognitive, sensory, or motor impairment. Resemblance for 110 identical twin pairs significantly exceeded resemblance for 130 fraternal same-sex twin pairs for all abilities. Maximum-likelihood model-fitting estimates of heritability were 62 percent for general cognitive ability, 55 percent for verbal ability, 32 percent for spatial ability, 62 percent for speed of processing, and 52 percent for memory. There was also evidence for the significant influence of idiosyncratic experience as the environmental component that most determines individual differences in cognitive abilities late in life.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.