Mapping of the X-linked recessive retinitis pigmentosa gene. A review

X-linked retinitis pigmentosa is caused by (a) mutation(s) on the X chromosome. One of the problems encountered in the genetic counselling of this disease is the identification of carrier females who appear ophthalmologically and electrodiagnostically normal. Despite normal testing these women are at risk of transmitting the XLRP gene to their children. Since the biochemical basis of XLRP is unknown, prenatal diagnosis and definitive carrier detection remain elusive. Existing methods of diagnosis and carrier detection are subject to limitations and are dependent on X-inactivation. The application of recombinant DNA probes to families with XLRP has provided a large number of genetic marker loci at the level of DNA. These markers are called restriction fragment length polymorphisms (RFLPs). By analysis of linkage relationships in affected kindreds, the XLRP gene(s) has (have) been localized to two subregions of the short arm of the X chromosome, Xp11 and Xp21. These findings suggest that there may be more than one retinitis pigmentosa gene on the X chromosome. Until further families are studied to clarify the localization(s) of XLRP, neither locus can be excluded if prenatal diagnosis and accurate carrier detection is to be established.