Compound Heterozygosity of Two Novel Truncation Mutations inRP1Causing Autosomal Recessive Retinitis Pigmentosa

Abstract

Purpose. To evaluate the phenotypic effects of two novel frameshift mutations in the RP1 gene in a Chinese pedigree of autosomal recessive retinitis pigmentosa (ARRP). Methods. Family members of a proband with ARRP were screened for RP1, RHO, NR2E3, and NRL mutations by direct sequencing. Detected RP1 mutations were genotyped in 225 control subjects. Since one family member with the RP1 deletion mutation in exon 2 was found to have age-related macular degeneration (AMD) but not RP, exons 2 and 3 of RP1 were screened in 120 patients with exudative AMD. Major AMD-associated SNPs in the HTRA1 and CFH genes were also investigated. Results. Two novel frameshift mutations in RP1, c.5_6delGT and c.4941_4942insT, were identified in the pedigree. They were absent in 225 control subjects. Family members who were compound heterozygous for the nonsense mutations had early-onset and severe RP, whereas those with only one mutation did not have RP. No mutations in RHO, NR2E3, and NRL were identified in the pedigree. Subject I:2 with AMD carried both at-risk genotypes at HTRA1 rs11200638 and CFH rs800292. No mutation in RP1 exons 2 and 3 was identified in 120 AMD patients. Conclusions. This report is the first to associate ARRP with compound heterozygous nonsense mutations in RP1. Identification of the nonsense-mediated mRNA decay (NMD)-sensitive mutation c.5_6delGT provided further genetic evidence that haploinsufficiency of RP1 is not responsible for RP. The authors propose four classes of truncation mutations in the RP1 gene with different effects on the etiology of RP.

RP1 and retinitis pigmentosa: report of novel mutations and insight into mutational mechanism

“…It has been suggested that RP due to mutations in RP1 is a result of haploinsufficiency, but our previous observation that carriers of the recessive p.A221G fs X20 mutation in exon 3 (predicted to be subjected to NMD) were asymptomatic suggests an alternative mechanism. This is further supported by the report of compound heterozygosity in arRP, where carriers of one allele predicted to be subjected to NMD were also asymptomatic 26. The four novel arRP protein truncating mutations we describe are downstream of the DCX domain and carrier individuals show no evidence of a retina phenotype (figure 3) suggesting each allele in a heterozygous state is not acting in a dominant negative manner, and one normal allele is sufficient for retina function.…”

Expression of Wild-Type Rp1 Protein in Rp1 Knock-in Mice Rescues the Retinal Degeneration Phenotype

“…First, we evaluated the family of an adult patient with recessive RP due to a homozygous mutation in exon 3 of RP1 , which is predicted to be null. The sibling and parents of this patient, who all carry a single mutant RP1 allele, do not exhibit evidence of RP, confirming that functional hemizygosity of RP1 does not cause disease [41]. Next, we generated and characterized Rp1 knock-in mice with a Q662X nonsense point mutation in exon 4 of human RP1 .…”

“…In that family, the carrier of the p.S2RfsX16 allele, which creates a premature termination codon in exon 2, did not show evidence of RP either [41]. Since the nonsense mutations, c.5_6delGT (p.S2RfsX16, exon 2) and c.686delC (p.P229QfsX35, exon 3), occur in the 2 nd or 3 rd rather than the final exon in these two families, they are expected to lead to NMD of the mutant mRNAs, and thus create null alleles [43].…”

“…These are all nonsense or frameshift mutations clustered at the beginning of the 4 th and final exon of RP1 (Figure 1A) [21]–[33]. Compared to the autosomal dominant form, only a small number of families with arRP due to mutations in RP1 have been reported, including 4 homozygous mutations (c.1606insTGAA, c.2847delT, c.4703delA, and c.5400delA) and one pair of compound heterozygous mutations (c.5_6delGT and c.4941_4942insT) (Figure 1A)[34], [39]–[41]. These are also frameshift mutations, with 5 out of 6 located in the final exon as well.…”

Mutations in Splicing Factor Genes Are a Major Cause of Autosomal Dominant Retinitis Pigmentosa in Belgian Families

“…The numerous deletions and insertions can be explained by the multiple A nucleotides flanking the mutation sites, possibly causing slipped strand mispairing during replication [43]. Chen et al proposed four classes of RP1 mutations [44]. Truncations located between amino acid 500 and 1053 within the last exon are NMD-insensitive and belong to ‘Class II’ mutations, making up the majority of PTC mutations.…”