X-linked hydrocephalus: clinical heterogeneity at a single gene locus.

X-linked hydrocephalus-stenosis of the aqueduct of Sylvius sequence (H-SAS, MIM number 30007) is a rare genetic disorder characterized by hydrocephalus, macrocephaly, adducted thumbs, spasticity, agenesis of corpus callosum and mental retardation. We confirm here the localisation of the mutant gene on Xq (Xq 2.8) by linkage analysis in a 5-generation pedigree (maximum lod score of Z = 4.57 at theta = 0.04 with probe St14 at locus DXS52) and emphasise the phenotypic variability of the disease. Ventricular dilatation in affected males was either severe and diagnosed antenatally or moderate and consistent with a long survival with little or no macrocephaly. Since other X-linked syndromes of mental retardation with spasticity and flexion deformities of the thumbs have previously been shown to map to the Xq 2.8 region as well (e.g. MASA syndrome and spastic paraplegia), the present results raise the question of whether H-SAS syndrome, MASA syndrome and spastic paraplegia with mental retardation might represent different phenotypic expression of various mutations at the same locus.