Using whole-exome and Sanger sequencing, we identified a novel hemizygous CHM mutation, c.1475_1476insCA, in a family with retinal degenerative disease initially diagnosed as RP. This novel CHM insertion mutation, rather than being a rare polymorphism in the general population, resulted in a truncated protein, commonly observed in CHM families. By combining the clinical data and initial genetic findings, the diagnosis for disease in this family was suggested to be an atypical form of CHM.
CHM is a rare X-linked retinal degenerative disease caused by mutations in the CHM gene that encodes REP-1 [12]. CHM mutations cause loss of functioning REP-1, an essential component of an enzyme complex formed with Rab geranylgeranyltransferase. Without functioning REP-1, RABs cannot participate in pathways of intracellular vesicular transport [12]. REP-1 is normally expressed in humans, and loss of REP-1 protein can be compensated by REP-2 in all tissues, except in the eye [15]. Functioning REP-1 is crucial for normal biological function of the retinal pigment epithelium and photoreceptors. Ultimately, lack of REP-1 results in the degeneration of these cells, as well as associated choroidal tissue [16].
The REP-1 protein-coding gene CHM spans 186,383 bp on Xq21.2 (based on NC_000023.11). A wide variety of CHM-causing mutations include small deletions, nonsense mutations, missense mutations, frame shifts, splice site defects, retrotransposon insertions and deletion of the entire CHM gene [17]. At least 147 CHM mutations have been reported in patients with choroideremia [5]. Thus, sequencing of the CHM gene has emerged as a diagnostic tool to identify mutations causing CHM [18]. There are two transcript variants for CHM gene. The 5442-bp CHM transcript variant 1 mRNA consists of 15 exons (NM_000390.2) with an open reading frame of 1962 bp and encodes a 653-aa REP-1 protein (95 kDa), while the 2856-bp CHM transcript variant 2 mRNA consists of 5 exons (NM_001145414.2) with an open reading frame of 333 bp and encodes a 110-aa REP-1 isoform protein. The two transcript variants share the same four 5′ exons and the exon 5 of the shorter variant is actually located in the intron 4 of the long transcript. As summarized in the CHM database (http://www.lovd.nl/CHM), no mutation in the exon 5 of the shorter transcript has been reported to cause choroideremia. And about 75 % (209/279) of the diseasing causing CHM gene mutations summarized are located in the latter 5 exons of the longer variant (exon 5 to 15). And totally 9 known mutations in exon 12, in which the novel c.1475_1476insCA mutation is located, have been identified to cause choroideremia in literature according to above database.
The c.1475_1476insCA insertion mutation we identified in exon 12 induced a frame shift which caused a new premature stop codon. Subsequently, the 156 C-terminal residues of REP-1 protein were truncated in the encoded mutant protein, leaving only 497 residues of the 653-aa protein. Most of the CHM-causing mutations result in lack of REP-1 due to a premature stop codon and degradation of the inappropriately folded protein or truncated mRNA [19, 20]. Our data demonstrating the truncation in the CHM gene in CHM patients suggest that a truncated REP-1 protein of 497 aa is unable to function as a normal escort protein of Rab proteins in vivo. The truncated REP-1 protein is likely degraded enzymatically in vivo in the affected members of this CHM family.
CHM is a rare eye disease with clinical features similar to those of RP. So far, no effective treatment exists for either disease. Transplantation of autologous transduced iris pigment epithelial cells into the subretinal space might help CHM patients [21]. Clinically, CHM and RP share several features common to retinal degenerative disorders, including night blindness, visual field constriction, visual acuity reduction and retinal degeneration, which may lead to difficulties in the differential diagnosis and even cause diagnosis confusion, especially with lack of typical fundus appearance [14]. On fundus examination, CHM is clinically characterized by chorioretinal scalloped atrophy initiated from the mid-peripheral fundus without affecting the macula [5, 7, 8]. However, these typical fundus changes in CHM may not be apparent when the patient visits the physician. Considering the diverse appearance of fundus in RP patients, CHM patients without typical fundus changes may be easily given a diagnosis of RP [5]. Actually, about 6 % of patients with a diagnosis of RP-related disorders have choroideremia [14].
Consistent with the above reports, the typical fundus changes for CHM including chorioretinal scalloped atrophy with preservation of the macula was not found in the proband with CHM mutations in our family. Instead, fundus examination revealed the typical bone-spicule pigment deposits of RP in both the proband and another brother. Thus as mentioned earlier, this family was initially given a diagnosis of RP based on night blindness, decreasing visual acuity, loss of peripheral vision, and typical bone-spicule pigment deposits. Recently, Li et al. reported mutations in the CHM gene in 6 of 157 families with RP by whole-exome sequencing [5]. However, the fundus changes in the 6 probands with CHM mutations were also atypical as compared with those seen in classical RP, and no potential pathogenic mutations in RP-associated genes were found in the 6 families [5]. Similarly, clinical and experimental data for our family suggest an atypical phenotype of CHM. Together with previous reports, our findings indicate that CHM may be misdiagnosed as RP with lack of a typical fundus appearance and the CHM gene should be included as a candidate in genetic studies for atypical RP.