761 MULTIPLE SIBLING MENTAL RETARDATION AND THE IMPACT OF THE FRAGILE X CHROMOSOME

Abstract

Mental retardation in two or more siblings with two normal parents represents an area about which little is known and is a difficult problem with regard to accurate diagnosis, elucidation of genetic mechanisms, and genetic counseling.

Sixty-one families with two or more mentally retarded siblings with two normal parents were ascertained from a large institution for the mentally retarded and a clinical genetics service. Investigation included pedigree analyses, physical examination, biochemical/metabolic analyses and chromosome analyses. A specific diagnosis and/or genetic mechanism was found in only 25% (15/61) of the families.

Of the sixty-one families, 22 had affected males only, of which two had definite X-linked recessive mental retardation. When chromosome analysis utilizing Medium 199 was done, an Xq27 fragile site was found in 36% (8/22) of the families with affected males only, including one of the known X-linked recessive families. Thus, the ability to detect the Xq27 fragile site resulted in an increase to 36% (22/61) of definite diagnoses for all families.

Multiple sibling mental retardation represents a major clinical problem for which the detection of the Xq27 fragile site has an important impact.

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