Choroideremia: The Endpoint Endgame

Choroideremia is an X-linked retinal degeneration resulting from the progressive, centripetal loss of photoreceptors and choriocapillaris, secondary to the degeneration of the retinal pigment epithelium. Affected individuals present in late childhood or early teenage years with nyctalopia and progressive peripheral visual loss. Typically, by the fourth decade, the macula and fovea also degenerate, resulting in advanced sight loss. Currently, there are no approved treatments for this condition. Gene therapy offers the most promising therapeutic modality for halting or regressing functional loss. The aims of the current review are to highlight the lessons learnt from clinical trials in choroideremia, review endpoints, and propose a future strategy for clinical trials.

Autofluorescence in female carriers with choroideremia: A familial case with a novel mutation in the CHM gene

BACKGROUND

Choroideremia is an X-linked chorioretinal dystrophy caused by mutations in the CHM gene. The main differential diagnosis is X-linked retinitis pigmentosa. Clinically, male patients that are affected by these two diseases have similar symptoms. This work aims to report a familial case of choroideremia initially diagnosed as X-linked retinitis pigmentosa with a novel mutation in the CHM gene, and the relevance of fundus autofluorescence (FAF) in female carriers.

MATERIALS AND METHODS

A complete ophthalmological evaluation was done in a 37-year-old woman and her 53-year-old maternal uncle; the uncle had been diagnosed previously with X-linked retinitis pigmentosa. A visual field test, FAF imaging, full-field electroretinography, and a genetic test were performed.

RESULTS

In the proband, the fundoscopy revealed diffuse changes in the retinal pigment epithelium in both eyes, and the FAF showed a speckled pattern of low- and high-density. The maternal uncle's ophthalmological evaluation showed choroidal and retinal atrophy consistent with choroideremia. The molecular analysis revealed a pathogenic variant in the CHM gene, c.190-1 G > T.

CONCLUSIONS

In female carriers of choroideremia and X-linked retinitis pigmentosa, differential diagnosis may be challenging. A speckled pattern of low- and high-density in autofluorescence is commonly found in female carriers of choroideremia. FAF is a powerful tool for making a correct clinical diagnosis because the pattern in FAF is much more apparent than the visible retinal changes obtained by fundoscopy. Although it is crucial to perform molecular analysis to confirm the diagnosis, FAF is useful when genetic testing may not be readily available.

Progress in the development of novel therapies for choroideremia

INTRODUCTION

There are no currently approved treatments for choroideremia, an X-linked progressive inherited retinal degeneration that leads to blindness by middle age. Several treatment options are being explored, but with major advances in adeno-associated vector (AAV) gene replacement therapy that has reached phase III clinical trials.

AREAS COVERED

In this review we discuss new insights into the clinical phenotyping and genetic testing of choroideremia patients, that aid disease characterisation, progression and patient inclusion into clinical trials. Recent advances in in-vitro studies have resulted in the development of functional assays that can be used to confirm the diagnosis in challenging cases and to quantify vector potency for use in clinical trials. We review the progress in current gene therapy trials and some considerations towards gene therapy approval for the treatment of choroideremia. Lastly, we discuss developments in alternative therapies including optogenetics.

EXPERT COMMENTARY

AAV gene replacement therapy is the most promising treatment strategy for choroideremia, that has developed exponentially over the last few years with a phase III clinical trial now underway. Optogenetics is a promising alternative strategy that might be applicable in late stages of degeneration.

Novel CHM mutations in Polish patients with choroideremia - an orphan disease with close perspective of treatment

Background

Choroideremia (CHM) is a rare X-linked recessive retinal dystrophy characterized by progressive chorioretinal degeneration in the males affected. The symptoms include night blindness in childhood, progressive peripheral vision loss and total blindness in the late stages. The disease is caused by mutations in the CHM gene encoding Rab Escort Protein 1 (REP-1). The aim of the study was to identify the molecular basis of choroideremia in five families of Polish origin.

Methods

Six male patients from five unrelated families of Polish ethnicity, who were clinically diagnosed with choroideremia, were examined in this study. An ophthalmologic examination performed in all the probands included: best-corrected visual acuity, slit-lamp examination, funduscopy, fluorescein angiography and perimetry. The entire coding region encompassing 15 exons and the flanking intronic sequences of the CHM gene were amplified with PCR and directly sequenced in all the patients.

Results

Five variants in the CHM gene were identified in the five families examined. Two of the variants were new: c.1175dupT and c.83C > G, while three had been previously reported.

Conclusions

This study provides the first molecular genetic characteristics of patients with choroideremia from the previously unexplored Polish population.

Evaluation of amplification refractory mutation system (ARMS) technique for quick and accurate prenatal gene diagnosis of CHM variant in choroideremia

Choroideremia is a rare X-linked recessive inherited disorder that causes chorioretinal dystrophy leading to visual impairment in its early stages which finally causes total blindness in the affected person. It is caused due to mutations in the CHM gene. In this study, we have recruited a pedigree with choroideremia and detected a nonsense variant (c.C799T:p.R267X) in CHM of the proband (I:1). Different primer sets for amplification refractory mutation system (ARMS) were designed and PCR conditions were optimized. Then, we evaluated the sequence variant in the patient, carrier, and a fetus by using ARMS technique to identify if they inherited the pathogenic gene from parental generation; we used amniotic fluid DNA for the diagnosis of the gene in the fetus. The primer pairs, WT2+C and MT+C, amplified high specific products in different DNAs which were verified by Sanger sequencing. Based on our results, ARMS technique is fast, accurate, and reliable prenatal gene diagnostic tool to assess CHM variants. Taken together, our study indicates that ARMS technique can be used as a potential molecular tool in the diagnosis of prenatal mutation for choroideremia as well as other genetic diseases in undeveloped and developing countries, where there might be shortage of medical resources and supplies.

Genetic study in a tunisian family revealed IVS1+1G>A mutation in the CHM gene

Choroideremia is a rare X-linked recessive, hereditary retinal pigment epithelial dystrophy, characterized by night blindness and progressive constriction of the visual fields leading to blindness in young adulthood. In this study, we reported three cases of choroideremia belonging to a Tunisian family. Patients complained of vision loss and night blindness. Fundus examination revealed diffused chorioretenal atrophy. In all cases, there was a visual field constriction and an undetectable electroretinography. Direct sequencing of the CHM gene detected a guanine to adenine transition (G>A) into the donor splice site of intron 1 leads to aberrantly spliced mRNA producing a premature stop codon and therefore functional loss of the CHM gene product, REP-1. The diagnosis should be considered in patients with a suitable family history and fundus findings.