Retinitis pigmentosa (RP) is a genetically and clinically heterogeneous disease leading to the degeneration of photoreceptors and can cause complete blindness. The prevalence of the disease is about 1 in 3000 to 4000 people with more than 1.5 million affected worldwide. RP can be a part of several syndromic disorders, as for example the Bardet-Biedl syndrome (BBS). We have identified mutations in the genes RPGR and BBS1 which are important for the functioning of the photoreceptor‟s connecting cilium. The mutations lead to defects in pre-mRNA splicing. Mutations in BBS1 are generally associated with BBS, whereas most of the mutations in RPGR cause non-syndromic RP. However, our
patients show different phenotypes. In the family of one of the two male RP patients who show RPGR splice defects, heterozygous female carriers are affected. The other patient suffers from mild hearing impairments in addition to RP. Furthermore, the two patients with a splice site mutation in BBS1 show RP without any further signs of BBS. Our findings implicate that defects in mRNA splicing might modify progression and expression of the disease. The two mutations found in RPGR change the amount of novel, alternative transcript isoforms. We have found that the novel RPGR isoforms were expressed in a tissue-dependent manner in healthy individuals which suggests that misregulation of these variants could affect other tissues or increase disease severity. In contrast, the splice site mutation in BBS1 causes RP, most probably by reducing the amount of functional protein, whereas other tissues remain
unaffected. To correct spliced defects a gene therapeutic approach using a modified form of the U1 small nuclear ribonucleoprotein has been developed in our group. U1 is a splicing factor directly involved in the general splicing process. The mutation in BBS1 causes the splice defect most probably by interfering with U1 binding to the transcript. To increase its binding affinity to the transcript and thereby improve recognition of the affected exon, U1 was adapted to the mutation. Treatment of patient-derived fibroblasts with the adapted form of U1 increased the amount of correctly spliced BBS1 transcripts and completely abolished skipping of exon 5. These results could help to establish a gene therapeutic treatment to correct splice defects in patients.