Mapping to distal Xq28 of nonspecific X-linked mental retardation MRX72: linkage analysis and clinical findings in a three-generation Sardinian family

Families with mentally retarded males found to be negative for FRAXA and FRAXE mutations are useful in understanding the genetic basis of X-linked mental retardation. According to the most recent data (updated to 1999), 69 MRX loci have been mapped and 6 genes cloned. Here we report on a linkage study performed on 20 subjects from a 4-generation Sardinian family segregating a non-specific X-linked recessive mental retardation (XLMR)(MRX72) associated with global delay of all psychomotor development. Five of 8 affected males have been tested for mental age, verbal and performance skills and behavioral anomalies; mental impairment ranged from mild to severe. Only minor anomalies were present in the affected subjects. Two-point linkage analysis based on 28 informative microsatellites spanning the whole X chromosome demonstrated linkage between the disorder and markers DXS1073 and F8c in Xq28 (maximum Lod score of 2. 71 at straight theta = 0.00). Multipoint linkage analysis confirmed the linkage with a Z(max) of 3.0 at straight theta = 0.00 at DXS1073 and F8c. Recombination in an affected male at DXS1073 and F8c allowed us to delimit centromerically and telomerically the region containing the putative candidate gene. The region, where MRX72 maps, overlaps that of another MRX families previously mapped to Xq28, two of which harbored mutations in GDI. Involvement of this gene was excluded in our family, suggesting another MRX might reside in Xq28.