Large Gene Deletion and Changes in Corneal Endothelial Cells in a Family With Choroideremia

Abstract

Purpose.: We provided the first report of an association between changes in corneal endothelial cells, retina, and choriocapillaris in a choroideremia family.

Methods.: Four members of an Asian choroideremia family, comprising two affected patients and two carriers, were evaluated. All participants underwent complete eye examinations, including visual acuity (VA), slit-lamp examination, ophthalmoscopy, perimetry, and electrophysiology tests. In addition, images of corneal endothelium were captured with a noncontact specular microscope. Genomic DNA amplification and whole-genome cytogenic array analysis were used to confirm the diagnosis of choroideremia and determine the molecular basis of the phenotype.

Results.: In the affected patients, funduscopy revealed characteristic features of RPE and chorioretinal atrophy. The slit-lamp biomicroscopy disclosed unexpected pigmented punctate lesions in the corneal endothelium in one of them. Surprisingly, specular microscopy detected decreased endothelial cell density (ECD) with features of pleomorphism and polymegethism. Genomic DNA analysis revealed large deletion (∼4.5 mega base pairs) of the entire CHM gene and encompassed region. In carriers, funduscopy revealed stippling pigmentary change despite normal electrophysiological results. Specular microscopy also disclosed reduced ECD with features of pleomorphism and polymegethism.

Conclusions.: To our knowledge, this is the first description of corneal changes in choroideremia patients. The loss of corneal ECD is conspicuous and is accompanied by pleomorphism and polymegethism in this family. The observed changes in corneal endothelium may be associated with larger encompassed regions of the CHM gene defect or dysfunction in the blood–aqueous barrier. It warrants further investigation and clarification of the pathophysiology and associations between retinal and corneal changes in choroideremia.

Choroideremia, a rare X-linked recessive chorioretinal dystrophy, produces diffuse and progressive degeneration of the RPE, retina, and choriocapillaris. It is characterized by night blindness, visual field constriction, and confluent scalloped areas of RPE atrophy and choriocapillaris loss. Only males are affected, while female carriers usually are asymptomatic with variable chorioretinal changes in the fundus.^1,2 Now it is known that choroideremia is caused by deletion or mutation of the CHM gene, encoding Ras-associated binding (Rab) escort protein-1 (REP-1).^1,3 Rab and Rab-associated proteins are key regulators of vesicle transport, which is essential for the delivery of proteins to specific intracellular locations. Rab and REP-1 proteins form a stable complex that is the substrate for the phenylation via Rab geranylgeranyl transferase; the complex interacts with its effectors to perform its physiological functions.⁴ Seabra et al.⁵ proposed that a lack of REP-1 leads to a lack of functional Rab27a specifically in the RPE. The degeneration of RPE and its adjacent layers may be due to deficient melanosome transport and, consequently, a lack of protection against harmful light exposure.⁴ 

The most remarkable clinical findings of choroideremia are hypopigmented and atrophic fundi with exposure of choroidal vessels, eventually leaving only scattered small areas of intact choroid in the macula and periphery.^1,6 It has been reported that retinal disorders may be associated with keratopathy in Bietti crystalline dystrophy and Fabry's disease.^7,8 However, there are no corneal abnormalities mentioned in choroideremia in the previous literature. Herein, we present a rare association between chorioretinal degeneration and corneal endotheliopathy in a choroideremia family. Changes in corneal endothelium with reduced endothelial cell counts and altered cellular morphology are clearly manifested in the patients with choroideremia and carriers we report. In the affected patients with typical clinical manifestations of choroideremia, the reduction of corneal endothelial cell density (ECD) was more conspicuous with marked pleomorphism and polymegethism. 

Methods

This study followed the tenets of the Declaration of Helsinki and was approved by the Institutional Review Board of Taipei Veterans General Hospital, Taipei, Taiwan. Informed consent was obtained from all subjects before entry into the study. 

Patients and Clinical Investigations

Four patients in a family, 2 males and 2 females, with suspected choroideremia were referred to a medical center between 2010 and 2011. They were generally healthy except for presumed retinal disease. All participants underwent complete eye examinations, including best-corrected visual acuity (BCVA), slit-lamp biomicroscopy, indirect ophthalmoscopy, fundus color photography (Canon CF-60DSi digital mydriatic fundus camera; Canon, Inc., Melville, NY, USA), and electrophysiology examinations. Fluorescein angiography (FA) was performed in three patients. In addition, a noncontact specular microscope (Noncon ROBO Pachy SP-9000; Konan Medical, Inc., Tokyo, Japan) was used to capture the images of the endothelium in the central cornea in all patients. Endothelial cell density was assessed using Frame method, by projecting photographs to a known magnification and counting cells in an area of known size (0.24 × 0.4 mm) by computer. Coefficient variation (CV) of cell area and percentage of hexagonal cells were also analyzed. The diagnosis of choroideremia was based on clinical findings and family history in accordance with an X-linked inheritance, and was confirmed by genetic analysis, as described in the following section. 

Molecular Analysis of Candidate Genes

Genomic DNA of the four patients was extracted from blood samples using a Genomic DNA Extraction Kit (Blood-Bacteria-Cultured Cells; RBC Bioscience, Taiwan, ROC) according to the manufacturer's protocol. DNA was quantified by spectrophotometry (NanoDrop Products, Wilmington, DE, USA) and integrity was evaluated by agarose gel electrophoresis. All coding exons and their flanking regions were amplified by PCR with the primers listed in the Table. Polymerase chain reaction conditions were as follows: activation of the DNA polymerase for 10 minutes at 95°C, followed by denaturing at 95°C, annealing at 60°C, and extending at 72°C in 35 cycles, each step lasting 1 minute. Final extension at 72°C was continued for 10 minutes. Amplified DNA fragments were visualized after gel electrophoresis through 1% agarose in the presence of ethidium bromide. For each primer combination, a nontemplate control sample was included. 

Table

 

Primer Used for PCR

 

To search for the breaking points, array comparative genomic hybridization (array CGH) was performed at GenePhile BioScience Laboratory (Taiwan, ROC), using combined genome-wide and targeted oligo microarrays, the 8 × 60 K ISCA v2 array (Agilent Technologies, Santa Clara, CA, USA), in which 60,000 oligo probes tile along the human genome at a mean backbone spacing of 60 kb. Genomic DNA extracted from patients and controls were labeled, hybridized, washed, and scanned following the manufacturer's protocol. The arrays were scanned using the Agilent G4900DA SureScan Microarray Scanner System (Agilent Technologies). The scanned arrays were analyzed using Feature Extraction (version 10.7.3.1) and CytoGenomics (version 1.5.1) analytic software (Agilent Technologies) to determine copy number losses and gains. 

To confirm our claim of the first description of unusual corneal findings in choroideremia patients, we searched the online database PubMed for all reports of choroideremia, keratopathy in choroideremia, and endotheliopathy. We have read all appropriate English literature on this topic and found no evidence of articles reporting any associated keratopathy in patients with choroideremia. 

Results

Figure 1 illustrates the pedigree of this family. The pedigree was in accordance with X-linked transmission. All the participants denied ocular history of trauma or surgery, and no concomitant ocular disease except choroideremia was found. 

Figure 1

 

Pedigree of the family with choroideremia. An X-linked inheritance pattern is clear. Filled symbols indicate individuals affected by choroideremia and unfilled symbols indicate unaffected individuals. Dotted circles indicate female carriers. Arrows indicate probands. Slash indicates a deceased person.

Ophthalmic Manifestations in Patients With Choroideremia

Patient III–3.

This 58-year-old man suffered from progressive poor vision in both eyes for decades. Night blindness was noted since high school, and then he was diagnosed with retinal dystrophy. His medical history was notable for diabetes mellitus. 

His BCVA was hand motion (HM) in both eyes. Intraocular pressure (IOP) was normal. Horizontal nystagmus also was noted. 

Slit-lamp biomicroscopy showed normal cornea and mild cataract in both eyes. Fundus examination showed typical, diffuse atrophy of the RPE and choriocapillaris in the posterior pole and midperiphery. The macula also was involved. The large retinal vessels and optic nerve were relatively normal (Fig. 2A). Fluorescein angiography disclosed diffuse loss of choriocapillaris with very poor chorioretinal dye perfusion. Electroretinogram (ERG) showed flat response with abnormal cone and rod waveforms. 

Figure 2

 

Clinical images of patient III–3. (A) Diffuse atrophy in the RPE and choriocapillaris over the posterior pole and midperiphery was noted in color fundus photographs. Macula also was involved. (B) Pleomorphism and polymegethism of endothelial cells were clearly evident in the images of noncontact specular microscopy. Endothelial cell density was 1650 and 1912 cells/mm² in the right and left eyes, respectively. CD, cell density; 6A, percentage of six-sided cells.

Under specular microscope, there was a noticeable decrease in ECD of 1650 cells/mm² and 1912 cells/mm² in the right and left eyes, respectively. In addition, pleomorphism and polymegethism of the corneal endothelial cells were present in both eyes (Fig. 2B). Central corneal thickness (550 and 543 μm in the right and left eyes, respectively) was within the normal population range (535 ± 62 μm).⁹ 

Patient IV–1.

This 32-year-old male patient suffered from night blindness since high school, and he was diagnosed as choroideremia at the age of 17. His medical history was otherwise unremarkable. His uncle suffered from choroideremia (patient III–3). 

His BCVA was 20/50 and 20/25 in the right and left eyes, respectively. Intraocular pressure was normal. Slit-lamp examination revealed multiple small punctate endothelial lesions disseminated in the whole corneal endothelium. They appeared yellow, and did not look like guttae (Fig. 3A). The anterior chamber was silent, and the lens was clear. Fundus examination disclosed diffuse chorioretinal atrophy that spared the macular area (Fig. 3B). Fluorescein angiography showed clear hypofluorescence in areas of missing choriocapillaris with patches of bright hyperfluorescence over the macula. 

Figure 3

 

Clinical images of patient IV–1. (A) In cornea slit-lamp photography, multiple small punctate endothelial lesions disseminated in the whole corneal endothelium. (B) Color fundus photography showed diffuse chorioretinal atrophy with large choroidal vessel exposure. Macular area was relatively preserved. (C) Great variation in endothelial cell shape and size was obvious in the specular microscopy images. Endothelial cell density was 2036 and 1996 cells/mm² in the right and left eyes, respectively.

Visual field examination (Humphrey Field Analyzed; Carl Zeiss Meditec, Dublin, CA, USA) showed loss of peripheral visual fields in both eyes. Electroretinogram showed decreased cone and rod response. 

Specular microscopy revealed features of mild pleomorphism and polymegethism of the corneal endothelium. Endothelial cell density was reduced in both eyes (2036 and 1996 cells/mm² in the right and left eyes, respectively, Fig. 3C) compared to normal adult values (2700 to 2900 cells/mm² in middle-aged adults).¹⁰ Central corneal thickness was 606 and 599 μm in the right and left eyes, respectively. 

Ophthalmic Observations in the Carriers

These carriers (patients III–2 and IV–2) did not have visual symptoms related to choroideremia. Visual acuity, IOP, anterior segment, visual field, and electrophysiological tests were normal. However, fundus examination showed diffuse tiny yellowish spots over the posterior pole. 

Specular microscopy revealed reduced endothelial cell counts in the carriers. Patient III–2 had an ECD of 1776 and 1838 cells/mm² in the right and left eyes, respectively, while patient IV–2 had an ECD of 2136 and 2336 cells/mm² in the right and left eyes, respectively. Features of polymegethism of corneal endothelial cells also were noted (CV was 0.43 in patient III–2 and 0.49 in patient IV–2, respectively). However, no corneal guttae were found. The corneal thickness was normal. 

Genetic Analysis of This Family

Genetic analysis of this family is shown in Figure 4. DNA amplifications of all REP-1 coding exons (1–15) from the affected males (patients III–3 and IV–1) and carriers (patients III–2 and IV–2) revealed that DNA fragments of expected sizes were lost in probands, indicative of a deletion of entire coding regions of REP-1. To search for the breaking points, we used whole-genome cytogenic array analysis with markers flanking the deletions. This revealed a large gene deletion in Xq 21.1 to 21.31, approximately 4.5 mega base pairs (Mbp) in length, including genes of UBE2DNL, APOOL, SATL1, ZNF711, POF1B, CHM, DACH2, KLHL4, and CPXCR1 missed in probands and carriers. 

Figure 4

 

Genetic analysis of candidate gene. (A) Results of microarray analysis in a proband (patient III–3). Copy number loss from q21.1 to 21.31 in chromosome X is clearly shown, which signifies a large deletion of the entire CHM gene and encompassed region. Gel electrophoresis results of PCR fragments in this family are shown in (B) and (C). Bands shown on agarose indicate the corresponding gene product is present. Polymerase chain reaction products from exons 1 to 15 were missing in 1A and 1D, implicating a complete absence of REP-1 protein (arrowhead). DNA template source: normal adult (N), patient III–3 (1A), patient III–2 (1B), patient IV–2 (1C), patient IV–1 (1D), an unaffected member in this family (1E), no-DNA control (B). A molecular weight standard marker (M) was run on each gel to estimate fragment size.

Review of Systems

Because large deletion of the CHM gene and encompassed regions were detected, and complex syndromic choroideremia phenotypes associated with deletions of the X chromosome have been reported,^11,12 a detailed review of systems was performed in affected males and female carriers. None of the family members showed any contributory systemic findings, such as mental retardation, sensorineural deafness, agenesis of the corpus callosum, or cleft lip. Hence, the deletion affecting the CHM gene in this family appeared not to be associated with any overt syndromic features. 

Discussion

Choroideremia has been linked to the REP-1 gene located on the Xq 21.2 region, which contains 15 exons that span a genomic sequence of approximately 150 kb. Different mutations, including deletion, insertion, translocation, or aberrant splicing at the mRNA level, lead to nonfunctional or complete absence of REP-1 protein and are responsible for the disease.^13-15 Patients with classic choroideremia have shown deletions that vary in size from 45 kbp to several Mbp in previous studies.^11,12 Large deletions of various parts of Xq21 usually are associated with complex or syndromic phenotypes.¹² Nevertheless, two families with approximately 6-Mbp large deletions surrounding the CHM gene have been described as having choroideremia with rather mild systemic manifestations.¹² The loss of the entire CHM gene and encompassed regions of comparable size and location in our patients do not show overt systemic comorbidities. 

In accordance with the X-linked inheritance, choroideremia affects males with progressive RPE and choroid atrophy, concentric visual field loss, and finally, visual impairment. In contrast, female carriers present various abnormal fundus findings, such as pigmented fundus periphery; however, their retinal function remains normal. The variability of phenotype in the carrier can be explained by lyonization. It is a random inactivation of one of the X chromosomes in females, thus explaining the cellular phenotypic variability.² Patients with choroideremia have been reported variously to have comorbid posterior polar cataract,¹⁶ cystoid macular edema,¹⁷ and recurrent uveitis.¹⁸ However, abnormal corneal findings have not been reported. The unexpected corneal abnormalities in probands and carriers in this family are difficult to explain solely by the X-linked inheritance pattern and lyonization. The penetrance pattern of keratopathy in our cases also is unclear. 

The normal corneal endothelium is a monolayer of uniformly sized cells with a predominantly hexagonal shape. Endothelial cell density is more than 3000 cell/mm² in newborns, and gradually declines with an average rate of approximately 0.6% per year throughout adult life.¹⁹ Reduced ECD is associated with age, presence of guttae, and an increase in cellular pleomorphism, while corneal thickness is a poor predictor of cell density.²⁰ It is known that trauma, surgery, uveitis, or Fuchs endothelial dystrophy may result in significant corneal endothelial changes in cell density and morphology.^21,22 However, the patients in this study do not have any of the aforementioned factors to which to attribute endothelial cell loss. In addition, there are no reports of corneal endothelial abnormalities in choroideremia in the previous literature. 

A potential etiology for the decreased ECD observed in our patients may have been inflammation. T-lymphocytic infiltration and gliosis within the choroid and surrounding vessels have been observed previously in patients with choroideremia, which implies inflammation could have a role in the pathogenesis of the disease.³ In addition, Sung et al.¹⁸ described a case report of choroideremia with recurrent anterior uveitis. Perhaps most tellingly, Chen et al.²³ described an increased aqueous flare intensity in a patient with choroideremia, and suggested this increase might be associated with altered blood–aqueous barrier function in choroideremia. The dysfunction of the blood–aqueous barrier may lead to long-term low grade inflammation in the anterior chamber, which puts the endothelial cells under physiological stress and results in endothelial cell loss accompanied by increased cell pleomorphism and polymegethism. 

The endotheliopathy in our cases also may relate to the large deletion of CHM. Disruption of the CHM gene in mice results in photoreceptor degeneration and severe defects in vasculogenesis in the yolk sac and placenta.²⁴ Changes in the vascular endothelium of choroidal and iris stromal vessels also have been observed in human choroideremia from histopathological examinations.²⁵ It is known that chronic hypoperfusion of the iris with microneovascularization is a major factor affecting corneal endothelial cell counts and morphology in several anterior segment diseases.²¹ Combining the results of these studies, we postulated the genetic defects in choroideremia patients may affect not only the intracellular trafficking in the RPE and photoreceptors, but also the vascular system of the choroid and ciliary body. The impaired vascular circulation in the ciliary body leads to relative hypoperfusion and nutrient deficiency in the anterior chamber, affecting the endothelial cells' ability to thrive, and decreasing cell density during prenatal development. 

Conclusions

To our knowledge, this study is the first description of corneal endothelial changes in choroideremia patients. We described an Asian choroideremia family with classic clinical manifestations and unexpected corneal findings. Multiple pigmented punctate lesions on the endothelium in both eyes were revealed for one of the affected probands. The reduction of corneal ECD is obvious and accompanied by pleomorphism and polymegethism. We speculated that the decrease in endothelial cells may be attributable to blood–aqueous barrier dysfunction and relative hypoperfusion of ocular circulation, perhaps resulting from a genetic defect of the CHM gene. Changes in corneal endothelium may be associated with choroideremia. Compared to retinal manifestations, however, these changes are subtle and usually overlooked. The current study reminds clinicians to examine carefully not only the retina, but also the cornea in choroideremia patients. Our results warrant further investigation and clarification of the pathophysiology and associations between retinal and corneal changes in choroideremia. 

Acknowledgments

The authors thank Jeremy Huang and GenePhile Bioscience Company for their support with gene analysis. 

Disclosure: S.-Y. Lee, None; W.-K. Yu, None; P.-K. Lin, None