This study represents the largest comprehensive clinical analysis of patients with causative RP2
mutations (the cohort from our institution alone). Previous studies have either been case reports with phenotype descriptions22–27
or comparative analyses of many XLRP gene subtypes.12, 21
We have gathered supplemental information from previously published cases yielding meta-analysis-type data on the RP2 clinical phenotype. The present report describes a recognizable phenotype consisting of early onset of macular atrophy and poor visual acuity combined with high myopia. This phenotype runs contrary to the typical forms of RP, where the macula is often spared until late in the disease course. We propose that screening of RP2
should be prioritized in male patients presenting with an X-linked pedigree, high myopia, poor visual acuities, and early-onset macular atrophy.
In addition, screening for RP2
mutations is appropriate in the rare male patients who fail CHM
mutation screening. Consistent with the fundus findings in patient 933–2420 with an Arg118Cys mutation, Vorster et al26
noted a similar-like phenotype in a male patient with an Arg120stop mutation. Patient 971–2490 also has a similar choroideremia-like phenotype, and the mutation (Ser172fs) shares the same exon (2) and functional ARL3-binding domain). These data suggest that mutations in this domain (Arg118Cys, Arg120stop, and Ser172fs) can lead to a choroideremia-like phenotype.
Female patients from X-linked RP pedigrees who have high myopia, asymmetrical retinal involvement, macular atrophy, or reduced central visual acuity may also have RP2 mutations. Mutational screening of RP2 is warranted in these cases, which are exemplified by our female carrier patients 1015–2553, 1029–2585, and 948–2443.
Although previously published reports have shown macular atrophy atypical of classic RP23
with poor visual acuity in patients with RP2
a clear clinical phenotype for RP2
mutations has not been described. An overwhelming majority of patients (10/11, 91%) in the cohort of male patients from our institution demonstrated macular atrophy starting at an early age (before age 12). This atrophy progressed into central scotomata in 50% of the patients, and runs counter to the typical RP presentation where the macula is spared until late in the natural history of disease progression. Our results indicate that early macular involvement is a distinguishing clinical feature of disease due to RP2
The severe degree of cone photoreceptor dysfunction in RP2
mutations is further supported by the ERG data demonstrating large delays in the photopic b-wave implicit times in all 12/12 (100%) patients in our combined male and female cohorts for whom data was available. This data corroborates the implicit times found by Sharon et al. in patients with RP2
However, only one patient had a clear cone-rod dysfunction pattern on ERG testing, suggesting that rod photoreceptor degeneration is still a prominent feature in this disease.
Predilection for superior visual field loss (inferior retinal disease) attributed to retinal phototoxicity has been described in autosomal dominant RP associated with RHO
We encountered a similar superior field loss in four patients: two patients evaluated at our institution, and two in the cohort of published cases, but the role of sunlight in the disease mechanism for RP2
mutations is currently unknown.
The association of high myopia with RP2
mutations has been demonstrated in other studies,17
and we have confirmed this finding in our group of patients. Interestingly, the female carrier (1015–2553) manifesting asymmetrical disease had anisometropia of approximately 8.00 D with the severely affected eye having myopia (), further supporting the concomitance of myopia and RP2
Correlating the wide spectrum of clinical phenotypes in RP2 patients to their genotypes has been an intriguing puzzle. In general, missense or in-frame deletion mutations are considered hypomorphic as they may result in a mutant protein with reduced function, whereas truncation mutations in RP2 (frameshift or splice site defects) cause severe phenotypes likely due to loss of protein function. However, our examination revealed that missense RP2 mutations are also associated with a severe phenotype. As most of the truncation mutations are found in the amino-terminal domain of RP2, the carboxyl-terminal region may be involved in either providing stability to the protein or is important for maintaining a functional conformation of RP2.
The Arg118His and Arg118Cys mutations are associated with a severe phenotype although previous in vitro
biochemical studies predict that mutations at Arg118 result in residual, but not abolished, activity of RP2 and its affinity to Arl3. On the other hand, RP2 Cys3Ser or Ser6del mutations have previously been shown to affect the localization of RP2 to plasma membrane in cultured cells.16, 19
In fact, RP2 Ser6del mutant protein is present at relatively low levels likely due to decreased stability. These results demonstrate that the localization of RP2 to plasma membrane may not be critical for its function. Clinically, we successfully correlated the genotypes from our patient with a Cys3Ser mutation (1090–2262) and a patient from the published literature with a Ser6del mutation (Literature Patient 115
) with a less severe phenotype. It is also possible that alternative localization of RP2 within the cells may be affected by some of the mutations. Since Arl3 localizes to photoreceptor sensory cilium and the mouse mutant of Arl3
develops a ciliary phenotype,29
RP2 may be involved in the targeting of Arl3 or modulating its activity at the cilium. Further studies are necessary to resolve these issues.
Splice mutations present another level of complexity associated with the prediction of the phenotype. Such mutations can result in a severe phenotype if they occur early in the gene, resulting in premature truncation.
Taken together, our data provide a platform for clinical identification of XLRP patients with RP2 mutations that can assist in better disease management and genetic counseling. We propose that RP2 be the first gene screened in male patients presenting with an X-linked pedigree, high myopia, poor visual acuities, and macular atrophy in childhood. Future therapeutic modalities for RP2-XLRP should carefully consider the quality and character of the mutant protein expressed in the diseased photoreceptors. Resolving the crystal structure of RP2 has increased our understanding of the role of different aminoacid residues in the protein’s function and the probable effect of disease-associated mutations on its three-dimensional structure and putative function. Our genotype-phenotype analysis has shown that a mutant RP2 protein with reduced activity can result in the same severe phenotype caused by mutations that result in protein degradation. As the biochemical activity of RP2 has not been demonstrated in vivo, further investigations are necessary to carefully analyze the correlation between RP2 mutations and their associated phenotypes, which will aid the design of appropriate clinical treatments.