A Search for Genetic Markers in Disease Linkage Loci of distal Xq28 region

The cytogenetic region q28 of the human X chromosome has great health relevance in Medical Genetics because it contains several disease loci whose causative genes are still unknown. In particular, two hereditary diseases of the central nervous system (CNS) map in the distal Xq28, X-linked polymicrogyria (PMGX) and a form of non specific X mental retardation (MRX72). The difficulty to isolate new Xq28 disease genes is remarkable because of the paucity of polymorphic markers available for linkage study. Thus, we have initiated a systematic search of new short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs) in 1.6 Mb of distal Xq28, between G6PD gene and the telomeric repeat TTAGGG. Starting from the nucleotide sequence, performed in our laboratory, we have selected new microsatellites. Sequence analysis of the genomic clones RP11-211L10, RP11-196H18, RP11-273L24, RP11-405N23 and RP11-53A12 was performed by CENSOR software looking for DNA repeated elements. Specific primer pairs were designed for each potential STRs and used in QF-PCR reactions. DNA pools from males of 40 CEPH families were genotyped to establish the number of alleles and their frequencies in the Caucasian population. Based on this strategy, we are able to establish the index of heterozigosity and consequently the power of each new Xq28 marker to be used in linkage studies. We found significant evidences of the presence of several new STRs that we are currently using to narrow down the MRX72 locus. Furter study are in progress to construct haplotypes of distal Xq28 to evaluate linkage disiquilibrium.