In this study, clinical measurements of Japanese patients with RP were shown according to each identified RP gene using column scatter plots. For genes mutated in a relatively large number of patients, such as EYS, we could estimate the associated age of decreasing quality of vision and thereby decreasing quality of life. Similar information for each gene would allow more objective and accurate consultations for individual patients than currently is available. However, the most common causal genes identified in Japanese patients with RP tended to be positioned toward the center of the x-axis. Thus, statistical analysis did not result in the identification of significant differences of clinical measurements between representative autosomal and recessive or recessive and dominant genes. Therefore, in terms of comparing the potential clinical outcome of each gene, our results suggested that many genes, especially those commonly mutated in RP, might overlap in terms of their associated clinical course, as many clinicians have previously considered to be the case.
The visual acuity of patients with RP usually remains within the normal range, even in the advanced stage of the disease. Half of our patients (61/121) had good visual acuity, sufficient to obtain a driver's license, and approximately 75% of patients (94/121) had the minimum acuity necessary to read letters. Nonearly onset RP genes, such as
PDE6B,
PRPH2,
USH2A, and
EYS, showed 0.1 or better visual acuity at the age of 45 years or younger. These observations indicated that most Japanese patients with RP can continue their employment by selecting deskwork incorporating monitors or low vision aid devices by the age of 45. Notably,
RHO and
RPGR, which are considered to cause earlier onset RP than the formerly listed four genes, were associated with relatively good visual acuity, even in older patients. There is little information about
RP1L1-associated RP, which was reported in the UK only relatively recently.
17 However, according to our previous work,
RP1L1 accounts for 8.6% of AR-RP in Japan.
7 The decline in visual acuity we had observed was similar to that observed with
USH2A or
EYS mutations. In comparison, the good visual acuity suggested by our results to be associated with the
BEST1 gene or the poor visual acuity suggested to be associated with the
RPE65 gene might be over- or underestimated because of the small sample size used in this study.
The HFA 10-2 program measures represent the central visual functioning of patients with RP, even at the advanced stage.
18 In X-linked RP (such as that caused by
RPGR mutation), visual disturbances are thought to be more severe than in other forms of RP.
19 Conversely, good visual acuity of
RPGR-associated RP was found in our study, supported by an MD of more than −20 dB. However, this good central visual function might be caused by the sampling bias of our patients with the
RPGR mutation. In this study, the five patients with
RPGR mutations were male and the study did not include female carriers. However, only one patient had a mutation in ORF15, while 60% of disease-causing mutations have been shown to be located in this region.
20 The cohort in this study included patients diagnosed genetically by previous targeted exome sequencing, and the median coverage of the ORF15 was zero, which might explain the small number of patients shown to have ORF15 mutations. Indeed, the shortcomings of next-generation sequencing with respect to the sequencing of ORF15 in
RPGR have been reported.
20
Cataracts are the common complication of RP. The scatter plots indicated that in most patients with RP cataracts develop before 60 years of age. All RP genes had similar tendencies. Because fewer than 16% of patients require cataract surgery before 65 years of age in the normal Japanese population,
21 cataracts impair visual function earlier in patients with RP than in the general population.
The residual EZ is known to be associated strongly with visual acuity and to decline with disease progression.
22,23 Phenotypic variety is common in dystrophies associated with
PRPH2; however, all patients diagnosed with RP with
PRPH2 mutation in this study had EZ measures of longer than 1000 μm. This is concordant with a recent report of patients with RP and
PRPH2 mutations from France.
24 On the other hand,
EYS and
USH2A were common AR-RP genes in Japan. The subfoveal EZ associated with mutation of these genes is likely to be retained through the fourth decade. This is very informative when considering public health or the support of Japanese patients with RP.
RP is complicated by CME and ERM. The higher prevalence of ERM in older patients with RP was consistent with that in the normal population. Interestingly, no patients with
PDE6B and
PRPH2 had ERM. Moreover, to our knowledge, no previous reports have shown the absence of ERM in the context of
PDE6B and
PRPH2. Our previous study using spectral domain OCT estimated the prevalence of CME as 26.9% in a genotype-undetermined RP cohort.
25 The current results showed that 18 of 116 patients (15.5%) had CME, which was a lower percentage than that in a previous report, potentially because the previous report included both eyes and investigated volume scans instead of cross scans. CME occurred in younger patients than ERM; thus, CME may occur during an earlier stage of RP. Two older patients with CME with
RHO mutation (66 and 62 years old) retained the cone response in ERG, suggesting a mild phenotype. This is consistent with our previous report showing that the CME was located predominantly in an area in which the external limiting membrane remained.
25 The absence of the CME in patients with
RPGR and
RP1L1 mutations was interesting. The most plausible explanation for this observation was selection bias due to the small sample size. Birch et al.
26 did not refer to CME in their OCT study in a cohort with RPGR-RP.
In our cohort, patients with RP and
RHO mutations had relatively large visual fields. This also is consistent with a previous report in French patients with RP.
27 Sandberg et al.
28 demonstrated that the severity of the disease was correlated with the location of the amino acid residue altered by the mutation, and showed that the decline in the visual field was mild for patients carrying gene mutations at the transmembrane domain. However, the four patients with a preserved visual field in our study carried mutations in other domains. Therefore, more cases are needed to allow a detailed discussion of the effects of each mutation.
We often see patients with RP with moderately preserved ERG in the clinical setting. These patients include young patients and those with sectorial RP. In this study, however, only one of 10 and one of four patients showed measurable peaks in rod and cone responses, respectively. The patients with preserved responses were not limited to youth but rather showed mutations in specific genes, such as EYS or RHO. These findings suggested that electrophysiological findings might be more effective for identification of causative genes than other ophthalmologic measures.
Statistical analysis is the gold standard of scientific reports for comparing two groups consisting of large samples. However, not more than 35 patients had even the most commonly mutated RP gene within our cohort. As a result, we presented the results of paired
t-tests using 14 and six sample sets. The small sample groups did not show normal distributions in some measurements, requiring us to use the Wilcoxon signed rank test. Accordingly, we did not identify any significant differences between the parameters associated with mutations in
EYS and
USH2A or between the parameters associated with mutations in
RHO and
USH2A. Although the phenotypes associated with these genes might, indeed, overlap, we recommend that these results be treated with caution because of the lack of statistical power owing to the small sample sizes. The formation of a consortium would be the key to overcoming this problem. It is relatively straightforward to obtain standardized information regarding genomics or visual acuity between institutions, and it is possible to compare these findings. However, ERG and the Goldmann perimeter still show variable results between hospitals, even if obtained using standardized protocols. These difficulties might result in the performance of few genotype–phenotype correlation studies of RP using statistical methods.
29–32
One limitation of this study is that our scatter plots were integrated based not on specific mutations but on mutated genes. The type of mutation (nonsense, missense, insertion, deletion, or copy number variation) and the location of the mutation (splice site, intron, exon, or protein domain) within a gene are thought to influence the RP phenotype. The mixture of these mutations in this study possibly confused the determination of genotype–phenotype correlations. However, the scatter plots could be applied to mutation-based analysis if additional numbers of patients with RP are diagnosed molecularly in a large consortium.
In conclusion, the use of column scatter plots enabled the comparison of the severity associated with RP genes and the visualization of the ophthalmological measurements of RP for every gene. This could not be achieved currently by statistical approaches because of the large number of causative genes. The results suggest that the phenotypes of the common causative genes in Japan might overlap in the standard clinical course; however, the prognosis of visual function based on the known outcomes associated with each causative gene would be helpful for daily consultations. Another advantage of the scatter plot is that we can add points or even plots for each new patient, which would result in more accurate collective information. Thus, the column scatter plots might be an effective tool for managing the clinical course of RP based on causative genes.