The Severity of Retinal Degeneration in Rp1h Gene-Targeted Mice Is Dependent on Genetic Background

Inherited retinal degenerations are important causes of blindness.1–3 For the most part, these disorders are thought to be caused by mutations in single genes. Great progress has been made identifying the genes that harbor mutations that cause inherited retinal degenerations; more than 150 disease genes have been identified to date.4 Although the identified mutations are clearly pathogenic, a growing body of evidence suggests that mutations or sequence variants in genes other than the primary disease gene make important contributions to disease phenotypes.5,6 For example, the severity of retinal disease has been reported to vary significantly in patients who share primary mutations in a number of other retinal degeneration disease genes, including CRX, PRPH2, RHO, and RPGR.7–13 Variation in disease expression is especially evident in patients with retinitis pigmentosa 1 (RP1) disease. Mutations in RP1 are a common cause of dominant RP.14–18 There are also reports of mutations in RP1 causing recessive and simplex RP.19–21 Clinical characterization of patients with RP1 disease showed notable differences in visual field diameters and electroretinographic (ERG) amplitudes in patients of the same age who shared the same primary mutations.18,22 In general, variation in the phenotypes of genetic disorders can be attributed to allelic heterogeneity, environmental factors, and genetic modifiers.5,6 For RP1 disease, genetic modifiers are thought to be especially important because variation in the severity of disease is observed in patients with the same primary mutation and because environmental exposures are likely to be relatively similar within families.18,22 Identification of sequence variations in genes that modify disease severity is an important goal because the modifier genes may provide insight into the pathogenesis of disease. Modifier genes may also provide additional targets for therapeutic interventions.5,6 Although they are important factors in determining disease severity, few human modifier genes have been identified. One of the best examples comes from Bardet Biedl syndrome (BBS), a pleiotropic cilia disorder that includes retinal degeneration.23,24 BBS is an autosomal recessive disorder, but much of the observed phenotypic variability in patients with BBS cannot be explained by mutations at a single locus. It has been observed that some BBS patients have mutations at two BBS loci, indicating that inheritance of this disorder can be oligogenic.25–27 Further, a hypomorphic sequence variant in the CCDC28B gene has been observed to modify the severity of BBS caused by primary mutations in other BBS genes.28 Given that mutations in CCDC28B itself were not found to cause BBS, this is a BBS modifier gene.28 Similarly, cases of digenic RP caused by mutations in the PRPH2 and ROM1 genes have been reported.29 Sequence variants of the ROM1 gene serve as the modifiers. Several modifier loci for inherited retinal degenerations in mice have been identified. This includes quantitative trait loci for the Rd3, Nr2e3, and Rs1 genes and for age-related retinal degeneration.30–33 To investigate the basis of the variation in disease expression for RP1 disease, we generated congenic mice with the retinitis pigmentosa 1 homolog (Rp1h)-myc allele on several different genetic backgrounds.34 The official designation for this allele is Rp1htm1Eap (http://www.informatics.jax.org/). Rp1htm1Eap is a gene-targeted allele in which the mouse Rp1h gene is truncated to mimic most human RP1 mutations, which are either nonsense or frameshift mutations predicted to result in truncation of the RP1 protein after the N-terminal one-third.18,34 Homozygous Rp1htm1Eap/tm1Eap mice on a mixed 129/B6 background experience a relatively rapid photoreceptor degeneration, characterized by loss of organization of outer segment discs. In contrast, heterozygous Rp1h+/tm1Eap mice on the mixed 129/B6 background do not show signs of retinal degeneration up to 1 year of age.34 We generated several strains of Rp1h+/tm1Eap congenic mice and analyzed their retinal phenotypes. Results show that A.129S(B6)-Rp1h+/tm1Eap congenic mice, with the heterozygous mutant allele transferred to the A/J background, experienced photoreceptor degeneration whereas pigmented B6.129S-Rp1h+/tm1Eap congenic mice did not.