Autosomal dominant retinitis pigmentosa (adRP): exclusion of a gene from three mapped loci provides evidence for the existence of a fourth locus.

Retinitis Pigmentosa (RP) is a group of inherited retinopathies which affect approximately 1 in 4,000 individuals. The disorder can be classified on the basis of inheritance; dominant, recessive and X-linked forms have been well documented. The existence of genetic heterogeneity within autosomal dominant RP (adRP) had been previously demonstrated. As a result of extensive linkage studies in 2 large Irish families and 1 American pedigree three adRP genes have been mapped. adRP genes have been localised to chromosome 3q close to the rod photoreceptor gene, rhodopsin; to chromosome 6p close to another transmembrane photoreceptor gene, peripherin/RDS and to the pericentric region of chromosome 8, although the causative gene in this region has not yet been identified. Here we report the results of a linkage study in a Spanish family, who exhibit an early-onset form of adRP. The adRP gene segregating in this family has been excluded from the three known adRP loci on chromosomes 3q, 6p and 8 using a series of both intragenic microsatellite markers from the rhodopsin and peripherin/RDS genes and markers flanking the three known loci. These results provide definitive evidence for the existence of a fourth adRP locus, further emphasising the genetic heterogeneity that exists within adRP.