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Neurogenetics

X-linked mental retardation (MRX) is affecting approximately 1 in 300 males. While syndromic forms are associated with clinically heterogeneous dysmorphic features, affected males in families with non-syndromic MRX have no consistent phenotypic manifestations other than mental retardation. More than 20 MRX loci have been mapped along the X chromosome with clustering in the pericentromeric region and in bands Xp22.3, Xp2l, and Xq28.
We have performed linkage analysis in two large pedigrees with non-syndromic MRX using 35 DNA markers situated along the entire length of the X-chromosome. In the first family (19 members, 8 affected males), no recombinants were found with the VNTR markers DXS52 and STR9120/9121, thus locating the responsible gene within a 3.5 Mbp interval in Xq28 (z=2.8). Expressed sequences of two GABA-receptor subunits adjacent to DXS52 have been used for mutation screening in the affected males. No sequence alterations have been detected so far. In a second family (22 members, 6 affected males), no recombinants were found with the VNTR loci DXS1068, CYBB, DXS1236 and DXS1237, indicating a map location within a 20cM interval in Xp11.4-Xp22.12. This region includes an MRX locus defined by deletions in patients with adrenal hypoplasia and mental retardation. Comparative mapping studies are currently performed using probes isolated from a YAC contig covering this deletion interval.


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X-linked mental retardation (MRX)

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