Retinitis pigmentosa (RP) affects approximately 1.5 million people worldwide.
1 Patients with RP experience night blindness, a gradual loss of peripheral visual field, and eventually loss of central vision, in most cases due to degeneration of the photoreceptor cells of the retina. The defective genes in RP are strikingly heterogeneous. There are autosomal dominant (adRP), autosomal recessive (arRP), X-linked (xlRP), and rare mitochondrial and digenic forms.
2 To date, 13 genes are known to cause adRP, 21 cause arRP, and 5 cause xlRP according to RetNet (http://www.sph.uth.tmc.edu/RetNet/, provided in the public domain by the University of Texas Houston Health Science Center, Houston, TX) and other publications.
3 4 Of the genes causing adRP, all but one (
RP17) have been cloned: rhodopsin (
RHO, also
RP4 as earlier locus symbol),
5 peripherin/RDS (
RDS,
RP7),
6 7 neural retina leucine-zipper (
NRL,
RP27),
8 RP1,
9 cone-rod homeobox-containing (
CRX),
3 retinal outer segment membrane protein 1 (
ROM1),
10 retinal fascin (
FSCN2),
11 inosine monophosphate dehydrogenase 1 (
IMPDH1,
RP10),
12 13 and Pim-1 kinase (
PIM1K,
RP9),
14 and genes encoding pre-mRNA splicing factors
PRPC8 (
RP13),
PRP31 (
RP11), and
HPRP3 (
RP18), also designated as
PRPF8,
PRPF31 and
HPRP3P, respectively, in GenBank (http://www.ncbi.nlm.nih.gov/Genbank/; provided in the public domain by the National Center for Biotechnology Information, Bethesda, MD).
15 16 17 Identifying the causative gene of RP is an essential step in prenatal diagnosis, accurate prognosis, and proper genetic counseling, as well as for possible gene-specific and/or mutation-specific treatments of RP.
18 19 Although the significance of the molecular diagnosis of the gene defect causing RP has been emphasized, and various tests are available for all the cloned genes, gene heterogeneity has made the molecular diagnosis complicated and often nondefinitive.