Missense mutation in the choroideremia gene

The small GTPase RAB28 is required for phagocytosis of cone outer segments by the murine retinal pigmented epithelium

RAB28, a member of the RAS oncogene family, is a ubiquitous, farnesylated, small GTPase of unknown function present in photoreceptors and the retinal pigmented epithelium (RPE). Nonsense mutations of the human RAB28 gene cause recessive cone-rod dystrophy 18 (CRD18), characterized by macular hyperpigmentation, progressive loss of visual acuity, RPE atrophy, and severely attenuated cone and rod electroretinography (ERG) responses. In an attempt to elucidate the disease-causing mechanism, we generated Rab28-/- mice by deleting exon 3 and truncating RAB28 after exon 2. We found that Rab28-/- mice recapitulate features of the human dystrophy (i.e. they exhibited reduced cone and rod ERG responses and progressive retina degeneration). Cones of Rab28-/- mice extended their outer segments (OSs) to the RPE apical processes and formed enlarged, balloon-like distal tips before undergoing degeneration. The visual pigment content of WT and Rab28-/- cones was comparable before the onset of degeneration. Cone phagosomes were almost absent in Rab28-/- mice, whereas rod phagosomes displayed normal levels. A protein-protein interaction screen identified several RAB28-interacting proteins, including the prenyl-binding protein phosphodiesterase 6 δ-subunit (PDE6D) and voltage-gated potassium channel subfamily J member 13 (KCNJ13) present in the RPE apical processes. Of note, the loss of PDE6D prevented delivery of RAB28 to OSs. Taken together, these findings reveal that RAB28 is required for shedding and phagocytosis of cone OS discs.

A Comprehensive Analysis of Choroideremia: From Genetic Characterization to Clinical Practice

Choroideremia (CHM) is a rare X-linked disease leading to progressive retinal degeneration resulting in blindness. The disorder is caused by mutations in the CHM gene encoding REP-1 protein, an essential component of the Rab geranylgeranyltransferase (GGTase) complex. In the present study, we evaluated a multi-technique analysis algorithm to describe the mutational spectrum identified in a large cohort of cases and further correlate CHM variants with phenotypic characteristics and biochemical defects of choroideremia patients. Molecular genetic testing led to the characterization of 36 out of 45 unrelated CHM families (80%), allowing the clinical reclassification of four CHM families. Haplotype reconstruction showed independent origins for the recurrent p.Arg293* and p.Lys178Argfs*5 mutations, suggesting the presence of hotspots in CHM, as well as the identification of two different unrelated events involving exon 9 deletion. No certain genotype-phenotype correlation could be established. Furthermore, all the patients´ fibroblasts analyzed presented significantly increased levels of unprenylated Rabs proteins compared to control cells; however, this was not related to the genotype. This research demonstrates the major potential of the algorithm proposed for diagnosis. Our data enhance the importance of establish a differential diagnosis with other retinal dystrophies, supporting the idea of an underestimated prevalence of choroideremia. Moreover, they suggested that the severity of the disorder cannot be exclusively explained by the genotype.

Clinical characteristics of a large choroideremia pedigree carrying a novel CHM mutation

OBJECTIVE

To describe a large family with a novel mutation in CHM.

METHODS

Family members were characterized using clinical examination, wide-field fundus photography, wide-field autofluorescence, and spectral domain optical coherence tomography. The CHM mutation was identified with the National Institutes of Health-sponsored eyeGene program.

RESULTS

A novel nonsense CHM mutation (T1194G), resulting in a premature stop (Y398X) and loss of the final one-third C-terminal portion of the protein, was identified. A large pedigree was generated from information provided by the twice-married proband. Seven men (aged 27-39 years) and 7 women (aged 22-89 years) were evaluated. Affected men showed characteristic peripheral chorioretinal atrophy with islands of macular sparing. Female carriers exhibited a wide range of variability, from mild pigmentary alterations to significant chorioretinal atrophy with severe vision loss. Older women tended to have a more severe phenotype. Autofluorescence demonstrating subfoveal loss or absence of retinal pigment epithelium correlated with vision loss in both sexes. Spectral domain optical coherence tomography demonstrated dynamic changes and remodeling of the outer retina over time, including focal thickening, drusenlike deposits, and disruption to photoreceptor inner segment and outer segment junctions in young female carriers.

CONCLUSIONS

CHM (T1194G) is a novel mutation that manifests a wide range of phenotypic variability in a single family with a trend toward more severe phenotypes in older female carriers. Our findings emphasize the importance of considering X-linked diseases by carefully evaluating pedigrees in women with severe manifestations of disease.

CLINICAL RELEVANCE

These findings demonstrate a novel CHM mutation that emphasizes severe posterior pole carrier phenotypes, age-related changes, and early choroideremia disease.

Translational bypass of nonsense mutations in zebrafish rep1, pax2.1 and lamb1 highlights a viable therapeutic option for untreatable genetic eye disease

The extensive molecular genetic heterogeneity seen with inherited eye disease is a major barrier to the development of gene-based therapeutics. The underlying molecular pathology in a considerable proportion of these diseases however are nonsense mutations leading to premature termination codons. A therapeutic intervention targeted at this abnormality would therefore potentially be relevant to a wide range of inherited eye diseases. We have taken advantage of the ability of aminoglycoside drugs to suppress such nonsense mutations and partially restore full-length, functional protein in a zebrafish model of choroideraemia (chm(ru848); juvenile chorio-retinal degeneration) and in two models of ocular coloboma (noi(tu29a) and gup(m189); congenital optic fissure closure defects). In vitro cell-based assays showed significant readthrough with two drugs, gentamicin and paromomycin, which was confirmed by western blot and in vitro prenylation assays. The presence of either aminoglycoside during zebrafish development in vivo showed remarkable prevention of mutant ocular phenotypes in each model and a reduction in multisystemic defects leading to a 1.5-1.7-fold increase in survival. We also identified a significant reduction in abnormal cell death shown by TUNEL assay. To test the hypothesis that optic fissure closure was apoptosis-dependent, the anti-apoptotic agents, curcumin and zVAD-fmk, were tested in gup(m189) embryos. Both drugs were found to reduce the size of the coloboma, providing molecular evidence that cell death is required for optic fissure remodelling. These findings draw attention to the value of zebrafish models of eye disease as useful preclinical drug screening tools in studies to identify molecular mechanisms amenable to therapeutic intervention.

A novel mutation (967-970+2)delAAAGGT in the choroideremia gene found in a Japanese family and related clinical findings

PURPOSE

To investigate the choroideremia (CHM) gene of one affected male and one obligate carrier in a Japanese family with choroideremia, and to characterize the related clinical features.

METHODS

We examined one affected man and one carrier woman from a Japanese family. Genomic DNA was extracted from leukocytes of peripheral blood collected from the affected man and his daughter, who is an obligate carrier of choroideremia. Exons 1-15 of the CHM gene were amplified by polymerase chain reaction (PCR) and directly sequenced. We performed ophthalmic examinations including best-corrected visual acuity, slit-lamp examination, fundus examination, electroretinography, and Goldmann perimetry.

RESULTS

A novel (967-970+2)delAAAGGT mutation was detected in the CHM gene. The affected man was hemizygous and had night-blindness, chorioretinal atrophy spreading from the posterior pole to the mid-periphery, and bareness of the sclera. His daughter was a heterozygous carrier who had chorioretinal atrophy and mottled appearance of the retinal pigment epithelium.

CONCLUSION

A novel (967-970+2)delAAAGGT mutation existed in the CHM gene of a Japanese family with choroideremia.

Clinical findings in a carrier of a new mutation in the choroideremia gene

OBJECTIVE

To describe the clinical and molecular findings of a female carrier of a new mutation in the choroideremia (CHM) gene.

DESIGN

Single interventional case report.

METHODS

A 27-year-old woman was seen with mild difficulties with dark adaptation and a history of a retinal degeneration in her father and choroideremia in 3 male paternal first cousins. Visual acuity measurements, peripheral and color vision tests, electroretinography (ERG), Goldmann visual fields, fluorescein angiogram, computed tomography scan, and DNA analysis were performed.

MAIN OUTCOME MEASURES

(1) Visual fields, (2) fluorescein angiography, and (3) DNA analysis.

RESULTS

Visual acuity decreased from 20/30 to 10/200 in the right eye abruptly over 2 months, then remained stable over 2 years of follow-up and remained 20/25 in the left eye. Goldmann visual fields showed development of a central scotoma in the right eye concurrent with the rapid decline. A small amount of subretinal hemorrhage was visible on dilated fundus examination at that time, but definite leakage was not evident on fluorescein angiography; afterwards, a choroidal neovascular membrane (CNV) was suspected. The ERG was normal. DNA analysis revealed that the patient was heterozygous for a previously undescribed substitution mutation at the 3'-splice site of intron 6 of the CHM gene (850-1 G to C), confirmed by mRNA analysis with reverse transcriptase polymerase chain reaction.

CONCLUSIONS

Severe visual acuity loss rarely occurs in female carriers of choroideremia mutations. The diagnosis should be considered in patients with a suitable family history and fundus findings. Physicians should consider the possibility of CNV development in such patients, which may be a response to abnormal retinal pigment epithelium. Recognition of this new mutation may help identify patients who could benefit from current and future treatments to protect against vision loss.

Organization of the Rab-GDI/CHM superfamily: the functional basis for choroideremia disease

Choroideremia is an X-chromosome-linked disease that leads to the degeneration of the choriocapillaris, the retinal pigment epithelium and the photoreceptor layer in the eye. The gene product defective in choroideremia, CHM, is identical to Rab escort protein 1 (REP1). CHM/REP1 is an essential component of the catalytic geranylgeranyltransferase II complex (GGTrII) that delivers newly synthesized small GTPases belonging to the RAB gene family to the catalytic complex for post-translational modification. CHM/REP family members are evolutionarily related to members of the guanine nucleotide dissociation inhibitor (GDI) family, proteins involved in the recycling of Rab proteins required for vesicular membrane trafficking through the exocytic and endocytic pathways, forming the GDI/CHM superfamily. Biochemical and structural analyses have now revealed a striking parallel in the organization and function of these two families allowing us to generate a general model for GDI/CHM superfamily function in health and disease.

Molecular basis of choroideremia (CHM): mutations involving the Rab escort protein-1 (REP-1) gene

Choroideremia (CHM) is an X-linked recessive eye disease that results from mutations involving the Rab escort protein-1 (REP-1) gene. In 18 patients deletions of different sizes have been found. Two females suffering from CHM were reported to have translocations that disrupt the REP-1 gene. In 22 patients, small mutations have been identified. Interestingly, these are all nonsense, frameshift or splice-site mutations; with one possible exception, missense mutations have not been found. This comprises all the known mutations in the disease.

Visual impairment and REP-1 gene mutations in Japanese choroideremia patients

Choroideremia (CHM), an X-linked recessive hereditary disease, is an intractable chorioretinal dystrophy. The rate of disease progression of CHM reportedly shows considerable variability. A number of mutations involving the gene that codes for Rab escort protein-1 (REP-1) have been detected in CHM patients. We have analyzed REP-1 gene mutations of Japanese CHM patients. The present study was designed to investigate the clinical variability and the genotype to phenotype relationship in 15 Japanese CHM patients referred to the Department of Ophthalmology of Juntendo University Hospital. The clinical investigation of visual acuity, visual field, color vision and refraction revealed inter-individual variability. Mutation analyses of the REP-1 gene revealed 10 types of mutations in 13 patients from 11 families, including an insertion, small deletions, nonsense mutations and an A to CC mutation. In 13 CHM patients with detectable REP-1 gene mutations, no relationship of genotype to phenotype was detected. At present, we consider the REP-1 genotype to be an unreliable prognostic factor for counseling of CHM patients. In two patients from one family, no mutations were detected in coding regions of the REP-1 gene. These patients may have intron mutations of the REP-1 gene, not detectable by the techniques employed in this study, or other causative genes. Both were observed to have somewhat slower disease progression than the other 13 patients. More advanced analyses are necessary to answer questions regarding the genotype-phenotype relationship in CHM patients.

No missense mutation in choroideremia patients analyzed to date

PURPOSE

To elucidate the status of a previously described missense mutation (1442A>T) reported in the Rab Escort Protein 1 gene of a patient with choroideremia.

METHODS

The base substitution previously described by Donnelly et al. (Hum Mol Genet 1994;3:1017) was first confirmed by direct genomic DNA sequencing. The REP-1 cDNA region encompassing exons 10-14 was then specifically amplified from lymphocyte-derived mRNA. The effect on mRNA splicing of the mutation was analyzed by RT-PCR and cDNA sequencing.

RESULTS

The 1442A>T change located at the penultimate nucleotide of exon 11 causes complete skipping of this exon during the processing of REP-1 mRNA. Loss of exon 11 leads to the translation of a premature termination codon within exon 12.

CONCLUSION

RT-PCR analyses demonstrated that the 1442A>T transversion previously described as a possible causative missense mutation does act as a splice-site error and gives rise to a truncated REP-1 protein. The virtual absence of any missense mutation found to be responsible for choroideremia makes the RT-PCR-based protein truncation test the most relevant genotypic diagnostic procedure for identifying mutations in the CHM gene.

Molecular basis of choroideremia (CHM): mutations involving the Rab escort protein-1 (REP-1) gene

Choroideremia (CHM) is an X-linked recessive eye disease that results from mutations involving the Rab escort protein-1 (REP-1) gene. In 18 patients deletions of different sizes have been found. Two females suffering from CHM were reported to have translocations that disrupt the REP-1 gene. In 22 patients, small mutations have been identified. Interestingly, these are all nonsense, frameshift or splice-site mutations; with one possible exception, missense mutations have not been found. This comprises all the known mutations in the disease.

Amino- and carboxy-terminal domains of the yeast Rab escort protein are both required for binding of Ypt small G proteins

The Rab escort protein (REP) is an essential component of the heterotrimeric enzyme Rab geranylgeranyl transferase that modifies the carboxy-terminal cysteines of the Ras-like small G proteins belonging to the Rab/Ypt family. Deletions in the human CHM locus, encoding one of the two REPs known in humans, result in a retinal degenerative syndrome called choroideremia. The only known yeast homologue of the choroideremia gene product is encoded by an essential gene called MRS6. Besides three structurally conserved regions (SCRs) previously detected in the amino-terminal half of REPs and RabGDIs, three other regions in the carboxy-terminal domain (RCR 1-3) are here identified as being characteristic of REPs alone. We have performed the first mutational analysis of a REP protein to experimentally define the regions functionally important for Rab/Ypt protein binding, making use of the genetic system of the yeast Saccharomyces cerevisiae. This analysis has shown that the SCRs are necessary but not sufficient for Ypt1p binding by the yeast REP, the carboxy-terminal region also being required.

Mutation analysis in Canadian families with choroideremia

Choroideremia (CHM) is an X-linked heritable progressive dystrophy of the choroid and retina. The condition predominantly affects males beginning in early childhood and eventually results in blindness after a period of 30-40 years. The CHM gene was localized to Xq21 and cloned in the past few years. The gene encodes for Rab escort protein-I, a protein involved in the isoprenylation of intracellular proteins. With the isolation of the gene, a number of mutations have been identified in patients affected by CHM using molecular techniques. Our group reports the characterization of mutations in four Canadian families affected by CHM. In addition, an intragenic polymorphism was identified in exon 5. Finding the mutations in these families will result in accurate predictive testing for carriers, avoid unnecessary repeated examination of at-risk individuals, and add to our understanding of the cause of this disorder.

Phenotype variations within a choroideremia family lacking the entire CHM gene

A Swedish family with choroideremia and a deletion of the CHM gene has been studied with ophthalmological examination, full-field electroretinography, and DNA analysis in order to characterize the phenotype of the disease. Although all four patients studied had a complete deletion of the gene, they showed a considerable variability regarding the phenotype, including the electroretinogram tracings. Two of the affected males demonstrated a severe form of choroideremia with low or nondetectable ERG recordings, while the other two affected males showed a less severe phenotype with only a slight reduction of the ERG amplitudes. The variation of the clinical phenotype among family members carrying the same mutation indicates that the severity of choroideremia is not solely a function of the CHM gene.