The Spectrum of CHM Gene Mutations in Choroideremia and Their Relationship to Clinical Phenotype

Matthew P Simunovic, Jasleen K Jolly, Kanmin Xue, Thomas L Edwards, Markus Groppe, Susan M Downes, Robert E MacLaren, Matthew P Simunovic, Jasleen K Jolly, Kanmin Xue, Thomas L Edwards, Markus Groppe, Susan M Downes, Robert E MacLaren

Abstract

Purpose: We report the underlying genotype and explore possible genotypic-phenotypic correlations in a large cohort of choroideremia patients.

Methods: We studied prospectively a cohort of 79 patients diagnosed within a tertiary referral service for patients with retinal dystrophies. Phenotypic evaluation consisted of clinical examination, including visual acuity and residual retinal area by fundus autofluorescence (FAF). Genotype was established by sequencing. We also investigated whether particular genotypes were associated with more severe phenotypes by performing analysis of covariance (ANCOVA), with visual acuity and FAF as the dependent variables and age as the covariant.

Results: A total of 74 (94%) of patients in our cohort had causative mutations by sequencing, the majority of which were anticipated to be null. Of these, 35 (47%) had insertions and deletions, 13 (18%) had mutations predicted to affect splicing, and 26 (35%) had single point mutations. In the latter case, 13 of 21 (62%) pedigrees with single point mutations were C to T transitions at C-phosphate-G (CpG) dinucleotides. These mutations were spread across 5 of only 24 CpG dinucleotides in the entire CHM cDNA. Furthermore, these 5 locations are the only sites at which C to T transitions result in a stop codon. No clear evidence was found for genotype-phenotype correlation except in the instance of a patient with a large deletion involving neighbouring sequences.

Conclusions: In patients with a diagnosis of choroideremia made by a specialty service, there is a high likelihood of establishing a genetic diagnosis. The majority of causative mutations appear to be null and, therefore, may benefit from gene replacement therapy. A disproportionate number of single point mutations observed were C to T transitions, consistent with the evolutionary decay of CpG dinucleotides through methylation and subsequent deamination. Hence, the development of choroideremia in such patients may represent the unwanted consequence of human evolution; de novo mutations are predicted to arise at these sites in future generations. (ClinicalTrials.gov number, NCT01461213.).

Figures

Figure 1
Figure 1
Number of pedigrees by cDNA location of mutations. The two mutations between 1701 and 1800 occur within 55 bp of the final exonic junction and are anticipated to result in expression of a truncated REP1 protein.
Figure 2
Figure 2
Visual acuity (logMAR) versus age (years). PTC, premature termination codon; LD, large deletion (≥1 exon); SD, small deletion; MIS, missense mutation; SI, small insertion or insertion/deletion; SS, splice site mutation; NMF, no mutation found; LI, large insertion.
Figure 3
Figure 3
Fundus autofluorescence area (mm2) versus age (years).

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Source: PubMed

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