Current Medical Diagnosis & Treatment in Psychiatry

Autistic Disorder

Population Genetics

The pooled frequency of autistic disorder in siblings of autistic probands is about 3%, which is 50–100 times the frequency in the general population. A twin study reported an MZ concordance of 36% and a DZ concordance of 0%. When the phenotype was extended to include language and cognitive abnormalities, concordance rates rose to 82% and 10%. This sample of twins, though carefully selected, was small (N = 21), but the essential conclusions regarding heritability were borne out in later studies.

Genetically Related Disorders

What is perhaps most striking about the known genetics of autistic disorder is the association of multiple single-gene disorders with the syndrome. The most clearly documented of these disorders is the fragile X syndrome; perhaps 8% of autistic subjects have the cytogenetic fragile X, and 16% of males with fragile X syndrome are autistic. However, studies in a series of families did not provide evidence for a major role of fragile X mutations in autistic disorder in general. There are also probable associations with tuberous sclerosis, neurofibromatosis, and phenylketonuria. Thus a variety of single-gene abnormalities may serve as the first step in the pathophysiology of autistic syndrome. Although these abnormalities account for only a fraction of the total number of cases of autistic disorder, they may provide important biochemical and anatomic clues.

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