Family-based and population study of a functional promoter-region monoamine oxidase A polymorphism in autism: possible association with IQ.

Although the etiology of autism remains to be elucidated, genetic elements significantly contribute to this disorder, and genes on the X chromosome are of special interest because there is a 4:1 predominance of male probands in autism. In the current study, we therefore examined, using the robust transmission disequilibrium test (TDT), possible preferential transmission of variants of a functional monoamine oxidase A (MAO A) promoter region polymorphism for linkage to autism. In the 49 families examined (33 families with one proband and 15 families with two affected siblings), we did not find preferential transmission of MAO A from 33 heterozygous mothers to affected child (TDT chi-square = 0.29, NS). Nor was any significant difference in MAO A allele frequency observed between 43 male autism subjects versus a group of 108 non-autism control subjects (chi-square = 1.23, P = 0.27, NS). However, a trend was observed for an association between IQ in the probands and the MAO A genotype that just attained significance (F = 3.5, P = 0.046, N = 28) in the small group of autism subjects recruited from families with two affected siblings.