Evidence for a new locus for X-linked retinitis pigmentosa (RP23)

X-linked retinitis pigmentosa (XLRP) is a degenerative disease of the retina characterized in the early stages of disease by night blindness as a result of rod photoreceptor loss, progressing to severe disease with loss of central vision by the third decade in affected males. XLRP displays exceptional genetic heterogeneity, with five reported loci on the human X-chromosome. To investigate the level of heterogeneity for XLRP in the patient pool in the current study, extensive haplotype analysis, linkage analysis, and mutation screening were performed.

Haplotype analysis of a family with diagnosed XLRP was scored with more than 34 polymorphic markers spanning the entire X-chromosome, including regions already identified as harboring XLRP genes and retina-specific genes. Two-point and multipoint lod scores were calculated. Affected male DNA was amplified with primers specific for the retinoschisis gene (XLRS1), and the products were screened for nucleic acid alterations by direct automated sequencing.

In this article haplotype and linkage data are presented identifying a new locus for XLRP on the short arm of the X-chromosome, distinct from previously reported gene localizations for XLRP. The phenotype is atypical, in that the onset of vision loss in the male members of this family is unusually early, and female obligate carriers have normal fundi and waveforms. Informative recombination events in this family define a locus for XLRP (RP23) on Xp22 between the markers DXS1223 and DXS7161, spanning approximately 15 cM. A maximum lod score of 2.1 was calculated for the locus order DXS7103-8 cM-(RP23/DXS1224)-4 cM-DXS999. This new locus (RP23) encompasses the retinoschisis disease gene; therefore, XLRS1 was screened for a mutation. No sequence alteration was identified indicating that mutations in the coding region of the gene responsible for retinoschisis do not cause RP23.

The results describe evidence for a new locus for XLRP (RP23), adding to the established genetic heterogeneity for this disease and the number of genes expressed in ocular tissue residing on the X-chromosome.