Linkage mapping of a severe X-linked mental retardation syndrome.

A four-generation Swedish family with a new type of X-linked mental retardation syndrome was recently reported by Gustavson et al. The complex syndrome includes microcephaly, severe mental retardation, optical atrophy with decreased vision or blindness, severe hearing defect, characteristic facial features, spasticity, seizures, and restricted joint motility. The patients die during infancy or early in childhood. Twenty-one family members, including two affected males, were available for study. Linkage analysis was conducted in the family by using 11 RFLP markers and 10 VNTR markers spread along the X chromosome. A hypervariable short tandem repeat of DXS294 at Xq26 showed a peak two-point lod score of 3.35 at zero recombination fraction. Calculations using the same markers revealed a multipoint peak lod score of 3.65 at DXS294. Crossover events with the centromeric marker DXS424 and the telomeric marker DXS297 delimit a probable region for the gene localization. It is noteworthy that hte disease loci of two other syndromes with overlapping clinical manifestations recently were shown by Turner et al. and Pettigrew et al. to be linked to markers at Xq26.