title: Imprinting, the X-chromosome, and the male brain: explaining sex differences in the liability to autism.
abstract: Males are at least four times more likely to develop autism than females. Among relatives with a broader autistic phenotype, males predominate too. Autism is a highly heritable disorder, yet genome scans have not revealed any predisposing loci on the sex chromosomes. A nongenetic explanation for male vulnerability, such as exposure to prenatal androgens, is unlikely for a variety of reasons. A novel genetic mechanism that resolves many of the outstanding difficulties is outlined here. The imprinted-X liability threshold model hypothesizes that the threshold for phenotypic expression of many autistic characteristics is influenced by an imprinted X-linked gene(s) that is protective in nature. Imprinted genes are known to play an important role in normal fetal and behavioral development. The gene is expressed only on the X-chromosome that is inherited from the father and raises the threshold for phenotypic expression. It is normally silenced when transmitted maternally. Because only females have a paternal X-chromosome, the threshold for phenotypic expression is higher in them than in males. Evidence for the existence of the genetic locus was found in a study of females with X-monosomy (Turner's syndrome) in which females had either a single paternal or maternal X-chromosome. Identifying the sites of action of this X-linked gene could lead to the discovery of autosomal loci that confer more directly a predisposition to autism.
authors.affiliation: Institute of Child Health, London, United Kingdom.
authors.forename: D H
authors.initials: DH
authors.lastname: Skuse
mesh_terms: D001321:Autistic Disorder / Q000235:genetics*; D001921:Brain / Q000503:physiopathology*; D005260:Female; D018392:Genomic Imprinting; D006801:Humans; D008297:Male; D010641:Phenotype; D012737:Sex Factors; D014960:X Chromosome