Mouse choroideremia gene mutation causes photoreceptor cell degeneration and is not transmitted through the female germline

Female carriers of X-linked inherited retinal diseases - Genetics, diagnosis, and potential therapies

Inherited retinal diseases (IRDs) are a group of heterogeneous conditions that cause progressive vision loss, typically due to monogenic mutations. Female carriers of X-linked IRDs have a single copy of the disease-causing gene, and therefore, may exhibit variable clinical signs that vary from near normal retina to severe disease and vision loss. The relationships between individual genetic mutations and disease severity in X-linked carriers requires further study. This review summarises the current literature surrounding the spectrum of disease seen in female carriers of choroideremia and X-linked retinitis pigmentosa. Various classification systems are contrasted to accurately grade retinal disease. Furthermore, genetic mechanisms at the early embryonic stage are explored to potentially explain the variability of disease seen in female carriers. Future research in this area will provide insight into the association between genotype and retinal phenotypes of female carriers, which will guide in the management of these patients. This review acknowledges the importance of identifying which patients may be at high risk of developing severe symptoms, and therefore should be considered for emerging treatments, such as retinal gene therapy.

A patient advocating for transparent science in rare disease research

300 million people live with at least one of 6,000 rare diseases worldwide. However, rare disease research is not always reviewed with scrutiny, making it susceptible to what the author refers to as nontransparent science. Nontransparent science can obscure animal model flaws, misguide medicine regulators and drug developers, delay or frustrate orphan drug development, or waste limited resources for rare disease research. Flawed animal models not only lack pharmacologic relevance, but also give rise to issue of clinical translatability. Sadly, these consequences and risks are grossly overlooked. Nontransparency in science can take many forms, such as premature publication of animal models without clinically significant data, not providing corrections when flaws to the model are discovered, lack of warning of critical study limitations, missing critical control data, questionable data quality, surprising results without a sound explanation, failure to rule out potential factors which may affect study conclusions, lack of sufficient detail for others to replicate the study, dubious authorship and study accountability. Science has no boarders, neither does nontransparent science. Nontransparent science can happen irrespective of the researcher's senority, institutional affiliation or country. As a patient-turned researcher suffering from Bietti crystalline dystrophy (BCD), I use BCD as an example to analyze various forms of nontransparent science in rare disease research. This article analyzes three papers published by different research groups on Cyp4v3^-/-, high-fat diet (HFD)-Cyp4v3^-/-, and Exon1-Cyp4v3^-/- mouse models of BCD. As the discussion probes various forms of nontransparent science, the flaws of these knockout mouse models are uncovered. These mouse models do not mimic BCD in humans nor do they address the lack of Cyp4v3 (murine ortholog of human CYP4V2) expression in wild type (WT) mouse retina which is markedly different from CYP4V2 expression in human retina. Further, this article discusses the impact of nontransparent science on drug development which can lead to significant delays ultimately affecting the patients. Lessons from BCD research can be helpful to all those suffering from rare diseases. As a patient, I call for transparent science in rare disease research.

Widespread subclinical cellular changes revealed across a neural-epithelial-vascular complex in choroideremia using adaptive optics

Choroideremia is an X-linked, blinding retinal degeneration with progressive loss of photoreceptors, retinal pigment epithelial (RPE) cells, and choriocapillaris. To study the extent to which these layers are disrupted in affected males and female carriers, we performed multimodal adaptive optics imaging to better visualize the in vivo pathogenesis of choroideremia in the living human eye. We demonstrate the presence of subclinical, widespread enlarged RPE cells present in all subjects imaged. In the fovea, the last area to be affected in choroideremia, we found greater disruption to the RPE than to either the photoreceptor or choriocapillaris layers. The unexpected finding of patches of photoreceptors that were fluorescently-labeled, but structurally and functionally normal, suggests that the RPE blood barrier function may be altered in choroideremia. Finally, we introduce a strategy for detecting enlarged cells using conventional ophthalmic imaging instrumentation. These findings establish that there is subclinical polymegathism of RPE cells in choroideremia.

Molecular Therapy for Choroideremia: Pre-clinical and Clinical Progress to Date

Choroideremia is an inherited retinal disease characterised by a degeneration of the light-sensing photoreceptors, supporting retinal pigment epithelium and underlying choroid. Patients present with the same symptoms as those with classic rod-cone dystrophy: (1) night blindness early in life; (2) progressive peripheral visual field loss, and (3) central vision decline with a slow progression to legal blindness. Choroideremia is monogenic and caused by mutations in CHM. Eight clinical trials (three phase 1/2, four phase 2, and one phase 3) have started (four of which are already finished) to evaluate the therapeutic efficacy of gene supplementation mediated by subretinal delivery of an adeno-associated virus serotype 2 (AAV2/2) vector expressing CHM. Furthermore, one phase 1 clinical trial has been initiated to evaluate the efficiency of a novel AAV variant to deliver CHM to the outer retina following intravitreal delivery. Lastly, a non-viral-mediated CHM replacement strategy is currently under development, which could lead to a future clinical trial. Here, we summarise the rationale behind these various studies, as well as any results published to date. The diversity of these trials currently places choroideremia at the forefront of the retinal gene therapy field. As a consequence, the trial outcomes, regardless of the results, have the potential to change the landscape of gene supplementation for inherited retinal diseases.

Overcoming translational barriers in modeling macular degenerations

Advances in tissue mimetics are paving the way for interrogating both the pathobiology of human disease and innovative therapeutic paradigms. In this issue of Cell Stem Cell, Manian et al. (2021) develop a novel iPSC-derived retinal pigment epithelium (RPE)-choriocapillaris (CC) complex that recapitulates key features of macular degenerations.

CHM mutation spectrum and disease: An update at the time of human therapeutic trials

Choroideremia is an X-linked inherited retinal disorder (IRD) characterized by the degeneration of retinal pigment epithelium, photoreceptors, choriocapillaris and choroid affecting males with variable phenotypes in female carriers. Unlike other IRD, characterized by a large clinical and genetic heterogeneity, choroideremia shows a specific phenotype with causative mutations in only one gene, CHM. Ongoing gene replacement trials raise further interests in this disorder. We describe here the clinical and genetic data from a French cohort of 45 families, 25 of which carry novel variants, in the context of 822 previously reported choroideremia families. Most of the variants represent loss-of-function mutations with eleven families having large (i.e. ≥6 kb) genomic deletions, 18 small insertions, deletions or insertion deletions, six showing nonsense variants, eight splice site variants and two missense variants likely to affect splicing. Similarly, 822 previously published families carry mostly loss-of-function variants. Recurrent variants are observed worldwide, some of which linked to a common ancestor, others arisen independently in specific CHM regions prone to mutations. Since all exons of CHM may harbor variants, Sanger sequencing combined with quantitative polymerase chain reaction or multiplex ligation-dependent probe amplification experiments are efficient to achieve the molecular diagnosis in patients with typical choroideremia features.

A Novel Chromosomal Translocation Identified due to Complex Genetic Instability in iPSC Generated for Choroideremia

Induced pluripotent stem cells (iPSCs) have revolutionized the study of human diseases as they can renew indefinitely, undergo multi-lineage differentiation, and generate disease-specific models. However, the difficulty of working with iPSCs is that they are prone to genetic instability. Furthermore, genetically unstable iPSCs are often discarded, as they can have unforeseen consequences on pathophysiological or therapeutic read-outs. We generated iPSCs from two brothers of a previously unstudied family affected with the inherited retinal dystrophy choroideremia. We detected complex rearrangements involving chromosomes 12, 20 and/or 5 in the generated iPSCs. Suspecting an underlying chromosomal aberration, we performed karyotype analysis of the original fibroblasts, and of blood cells from additional family members. We identified a novel chromosomal translocation t(12;20)(q24.3;q11.2) segregating in this family. We determined that the translocation was balanced and did not impact subsequent retinal differentiation. We show for the first time that an undetected genetic instability in somatic cells can breed further instability upon reprogramming. Therefore, the detection of chromosomal aberrations in iPSCs should not be disregarded, as they may reveal rearrangements segregating in families. Furthermore, as such rearrangements are often associated with reproductive failure or birth defects, this in turn has important consequences for genetic counseling of family members.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

Oncogenic role of rab escort protein 1 through EGFR and STAT3 pathway

Rab escort protein-1 (REP1) is linked to choroideremia (CHM), an X-linked degenerative disorder caused by mutations of the gene encoding REP1 (CHM). REP1 mutant zebrafish showed excessive cell death throughout the body, including the eyes, indicating that REP1 is critical for cell survival, a hallmark of cancer. In the present study, we found that REP1 is overexpressed in human tumor tissues from cervical, lung, and colorectal cancer patients, whereas it is expressed at relatively low levels in the normal tissue counterparts. REP1 expression was also elevated in A549 lung cancer cells and HT-29 colon cancer cells compared with BEAS-2B normal lung and CCD-18Co normal colon epithelial cells, respectively. Interestingly, short interfering RNA (siRNA)-mediated REP1 knockdown-induced growth inhibition of cancer cell lines via downregulation of EGFR and inactivation of STAT3, but had a negligible effect on normal cell lines. Moreover, overexpression of REP1 in BEAS-2B cells enhanced cell growth and anchorage-independent colony formation with little increase in EGFR level and STAT3 activation. Furthermore, REP1 knockdown effectively reduced tumor growth in a mouse xenograft model via EGFR downregulation and STAT3 inactivation in vivo. These data suggest that REP1 plays an oncogenic role, driving tumorigenicity via EGFR and STAT3 signaling, and is a potential therapeutic target to control cancers.

REP1 inhibits FOXO3-mediated apoptosis to promote cancer cell survival

Rab escort protein 1 (REP1) is a component of Rab geranyl-geranyl transferase 2 complex. Mutations in REP1 cause a disease called choroideremia (CHM), which is an X-linked eye disease. Although it is postulated that REP1 has functions in cell survival or death of various tissues in addition to the eye, how REP1 functions in normal and cancer cells remains to be elucidated. Here, we demonstrated that REP1 is required for the survival of intestinal cells in addition to eyes or a variety of cells in zebrafish, and also has important roles in tumorigenesis. Notably, REP1 is highly expressed in colon cancer tissues and cell lines, and silencing of REP1 sensitizes colon cancer cells to serum starvation- and 5-FU-induced apoptosis. In an effort to elucidate the molecular mechanisms underlying REP1-mediated cell survival under those stress conditions, we identified FOXO3 as a binding partner of REP1 using a yeast two-hybrid (Y2H) assay system, and we demonstrated that REP1 blocked the nuclear trans-localization of FOXO3 through physically interacting with FOXO3, thereby suppressing FOXO3-mediated apoptosis. Importantly, the inhibition of REP1 combined with 5-FU treatment could lead to significant retarded tumor growth in a xenograft tumor model of human cancer cells. Thus, our results suggest that REP1 could be a new therapeutic target in combination treatment for colon cancer patients.

Hereditary Retinal Dystrophy

As our understanding of the genetic basis for inherited retinal disease has expanded, gene therapy has advanced into clinical development. When the gene mutations associated with inherited retinal dystrophies were identified, it became possible to create animal models in which individual gene were altered to match the human mutations. The retina of these animals were then characterized to assess whether the mutated genes produced retinal phenotypes characteristic of disease-affected patients. Following the identification of a subpopulation of patients with the affected gene and the development of techniques for the viral gene transduction of retinal cells, it has become possible to deliver a copy of the normal gene into the retinal sites of the mutated genes. When this was performed in animal models of monogenic diseases, at an early stage of retinal degeneration when the affected cells remained viable, successful gene augmentation corrected the structural and functional lesions characteristic of the specific diseases in the areas of the retina that were successfully transduced. These studies provided the essential proof-of-concept needed to advance monogenic gene therapies into clinic development; these therapies include treatments for: Leber's congenital amaurosis type 2, caused by mutations to RPE65, retinoid isomerohydrolase; choroideremia, caused by mutations to REP1, Rab escort protein 1; autosomal recessive Stargardt disease, caused by mutations to ABCA4, the photoreceptor-specific ATP-binding transporter; Usher 1B disease caused by mutations to MYO7A, myosin heavy chain 7; X-linked juvenile retinoschisis caused by mutations to RS1, retinoschisin; autosomal recessive retinitis pigmentosa caused by mutations to MERTK, the proto-oncogene tyrosine-protein kinase MER; Leber's hereditary optic neuropathy caused by mutations to ND4, mitochondrial nicotinamide adenine dinucleotide ubiquinone oxidoreductase (complex I) subunit 4 and achromatopsia, caused by mutations to CNGA3, cyclic nucleotide-gated channel alpha 3 and CNGB3, cyclic nucleotide-gated channel beta 3. This review includes a tabulated summary of treatments for these monogenic retinal dystrophies that have entered into clinical development, as well as a brief summary of the preclinical data that supported their advancement into clinical development.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Zebrafish Models of Retinal Disease

Visual defects affect a large proportion of humanity, have a significant negative impact on quality of life, and cause significant economic burden. The wide variety of visual disorders and the large number of gene mutations responsible require a flexible animal model system to carry out research for possible causes and cures for the blinding conditions. With eyes similar to humans in structure and function, zebrafish are an important vertebrate model organism that is being used to study genetic and environmental eye diseases, including myopia, glaucoma, retinitis pigmentosa, ciliopathies, albinism, and diabetes. This review details the use of zebrafish in modeling human ocular diseases.

Human iPSC derived disease model of MERTK-associated retinitis pigmentosa

Retinitis pigmentosa (RP) represents a genetically heterogeneous group of retinal dystrophies affecting mainly the rod photoreceptors and in some instances also the retinal pigment epithelium (RPE) cells of the retina. Clinical symptoms and disease progression leading to moderate to severe loss of vision are well established and despite significant progress in the identification of causative genes, the disease pathology remains unclear. Lack of this understanding has so far hindered development of effective therapies. Here we report successful generation of human induced pluripotent stem cells (iPSC) from skin fibroblasts of a patient harboring a novel Ser331Cysfs*5 mutation in the MERTK gene. The patient was diagnosed with an early onset and severe form of autosomal recessive RP (arRP). Upon differentiation of these iPSC towards RPE, patient-specific RPE cells exhibited defective phagocytosis, a characteristic phenotype of MERTK deficiency observed in human patients and animal models. Thus we have created a faithful cellular model of arRP incorporating the human genetic background which will allow us to investigate in detail the disease mechanism, explore screening of a variety of therapeutic compounds/reagents and design either combined cell and gene- based therapies or independent approaches.

Adeno-associated virus 8-mediated gene therapy for choroideremia: preclinical studies in in vitro and in vivo models

BACKGROUND

Choroideremia (CHM) is a slowly progressive X-linked retinal degeneration that results in a loss of photoreceptors, retinal pigment epithelium and choroid. CHM, the gene implicated in choroideremia, encodes Rab escort protein-1 (REP-1), which is involved in the post-translational activation via prenylation of Rab proteins.

METHODS

We evaluated AAV8.CBA.hCHM, a recombinant adeno-associated virus serotype 8 (rAAV8) vector, which targets retinal cells efficiently, for both therapeutic effect and safety in vitro and in vivo in a murine model. In vitro studies included western blot analyses and prenylation assays. In vivo studies included ophthalmoscopy, pupillometry, histology and immunofluorescence analysis.

RESULTS

Infection with AAV8.CBA.hCHM induced the expression of REP-1 protein in a dose-responsive fashion. Transduction with AAV8.CBA.hCHM reverses the biochemical and pathogenetic defects in CHM both in vitro and in vivo and showed no safety concerns in the in vivo investigations performed in the present study.

CONCLUSIONS

AAV8 is a promising vector for human clinical gene therapy trials for choroideremia.

Gene therapy for choroideremia using an adeno-associated viral (AAV) vector

Choroideremia is an outer retinal degeneration with a characteristic clinical appearance that was first described in the nineteenth century. The disorder begins with reduction of night vision and gradually progresses to blindness by middle age. The appearance of the fundus in sufferers is recognizable by the characteristic pale color caused by the loss of the outer retina, retinal-pigmented epithelium, and choroidal vessels, leading to exposure of the underlying sclera. Choroideremia shows X-linked recessive inheritance and the choroideremia gene (CHM) was one of the first to be identified by positional cloning in 1990. Subsequent identification and characterization of the CHM gene, which encodes Rab escort protein 1 (REP1), has led to better comprehension of the disease and enabled advances in genetic diagnosis. Despite several decades of work to understand the exact pathogenesis, no established treatments currently exist to stop or even slow the progression of retinal degeneration in choroideremia. Encouragingly, several specific molecular and clinical features make choroideremia an ideal candidate for treatment with gene therapy. This work describes the considerations and challenges in the development of a new clinical trial using adeno-associated virus (AAV) encoding the CHM gene.

Proof of concept for AAV2/5-mediated gene therapy in iPSC-derived retinal pigment epithelium of a choroideremia patient

Inherited retinal dystrophies (IRDs) comprise a large group of genetically and clinically heterogeneous diseases that lead to progressive vision loss, for which a paucity of disease-mimicking animal models renders preclinical studies difficult. We sought to develop pertinent human cellular IRD models, beginning with choroideremia, caused by mutations in the CHM gene encoding Rab escort protein 1 (REP1). We reprogrammed REP1-deficient fibroblasts from a CHM^-/y patient into induced pluripotent stem cells (iPSCs), which we differentiated into retinal pigment epithelium (RPE). This iPSC-derived RPE is a polarized monolayer with a classic morphology, expresses characteristic markers, is functional for fluid transport and phagocytosis, and mimics the biochemical phenotype of patients. We assayed a panel of adeno-associated virus (AAV) vector serotypes and showed that AAV2/5 is the most efficient at transducing the iPSC-derived RPE and that CHM gene transfer normalizes the biochemical phenotype. The high, and unmatched, in vitro transduction efficiency is likely aided by phagocytosis and mimics the scenario that an AAV vector encounters in vivo in the subretinal space. We demonstrate the superiority of AAV2/5 in the human RPE and address the potential of patient iPSC–derived RPE to provide a proof-of-concept model for gene replacement in the absence of an appropriate animal model.

Rab and Arf proteins in genetic diseases

Rab and ADP-ribosylation factor (Arf) family proteins are master regulators of membrane trafficking and are involved in all steps of vesicular transport. These families of small guanine-nucleotide-binding (G) proteins are well suited to regulate membrane trafficking processes since their nucleotide state determines their conformation and the capacity to bind to a multitude of effectors, which mediate their functions. In recent years, several inherited diseases have been associated with mutations in genes encoding proteins belonging to these two families or in proteins that regulate their GTP-binding cycle. The genetic diseases that are caused by defects in Rabs, Arfs or their regulatory proteins are heterogeneous and display diverse symptoms. However, these diseases mainly affect two types of subcellular compartments, namely lysosome-related organelles and cilia. Also, several of these diseases affect the nervous system. Thus, the study of these diseases represents an opportunity to understand their etiology and the molecular mechanisms involved, as well as to develop novel therapeutic strategies.

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.

Choroideremia: a review of general findings and pathogenesis

Choroideremia (CHM) is an X-linked retinal dystrophy belonging to the family of blinding disorders. It is characterized by progressive degeneration of the choriocapillaris, retinal pigment epithelium and photoreceptors. CHM is caused by mutations in the Rab Escort Protein 1 (REP-1) gene, which encodes a protein involved in vesicular trafficking. This paper gives an overview of the clinical features, visual function, biochemistry, histology, molecular genetics, pathogenesis, diagnosis and treatment of CHM.

The single active X in human cells: evolutionary tinkering personified

All mammals compensate for sex differences in numbers of X chromosomes by transcribing only a single X chromosome in cells of both sexes; however, they differ from one another in the details of the compensatory mechanisms. These species variations result from chance mutations, species differences in the staging of developmental events, and interactions between events that occur concurrently. Such variations, which have only recently been appreciated, do not interfere with the strategy of establishing a single active X, but they influence how it is carried out. In an overview of X dosage compensation in human cells, I point out the evolutionary variations. I also argue that it is the single active X that is chosen, rather than inactive ones. Further, I suggest that the initial events in the process-those that precede silencing of future inactive X chromosomes-include randomly choosing the future active X, most likely by repressing its XIST locus.

Retinal pigment epithelium defects accelerate photoreceptor degeneration in cell type-specific knockout mouse models of choroideremia

PURPOSE

Choroideremia (CHM) is a progressive X-linked degeneration of three ocular layers (photoreceptors, retinal pigment epithelium, and choroid), with a complex and still largely unclear pathogenesis. To investigate the pathophysiology of CHM, the authors engineered mice with a cell type-specific Chm/Rep1 knockout (KO).

METHODS

A mouse line carrying a conditional allele Chm(Flox) was crossed with the transgenic line IRBP-Cre to achieve Chm KO, specifically in the photoreceptor layer, and Tyr-Cre to produce Chm KO, specifically in the retinal pigment epithelial and other pigmented cells. Chm(Flox), Tyr-Cre+ and Chm(Flox), IRBP-Cre+ mice were mated to produce mice with Chm KO in both layers. All mouse lines were studied by histology, electron microscopy, electroretinography (ERG), scanning laser ophthalmoscopy (SLO), and biochemical

METHODS

RESULTS

In Chm(Flox), IRBP-Cre+ mice the authors observed the progressive degeneration of photoreceptors in the presence of normal retinal pigment epithelium (RPE). Chm(Flox), Tyr-Cre+ mice exhibited coat color dilution and pigment abnormalities of the RPE in the presence of an intact outer nuclear layer. In 6- to 8-month-old Chm(Flox), Tyr-Cre+, IRBP-Cre+ mice, the degeneration of photoreceptors was accelerated compared with Chm(Flox), IRBP-Cre+ mice but became leveled with age, such that it was comparable at 12 to 14 months. Detailed ERG and SLO analysis supported the histopathologic findings.

CONCLUSIONS

Defects in photoreceptors and RPE can arise because of intrinsic defects caused cell autonomously by the Chm KO. However, when both photoreceptors and RPE are diseased, the dynamics of the degenerative process are altered. Photoreceptor functional deficit and cell death manifest much earlier, suggesting that the diseased RPE accelerates photoreceptor degeneration.

Choroideremia: new findings from ocular pathology and review of recent literature

Histopathology of young individuals affected by choroideremia is rarely available to allow correlation with the clinical presentation. A 30-year-old man with choroideremia died in a motor vehicle accident and one eye was subjected to histopathological examination. Immunoblot analysis of protein derived from white blood cells of a living brother, also affected with choroideremia, confirmed the absence of Rab escort protein-1, the normal CHM gene product. Direct sequencing of the coding region and adjacent splice sites of the CHM gene was undertaken on genomic DNA from the living brother and revealed a transition mutation, C to T, in exon 6 (R253X) which resulted in a stop codon and was predicted to truncate the protein product. Histopathological examination of the eye of the deceased brother showed relative independent degeneration of choriocapillaris, retinal pigment epithelium, and retina, similar to observations in the mouse model of choroideremia. In addition, mild T-lymphocytic infiltration was found within the choroid. The ophthalmic features and the pathology of choroideremia are discussed in light of new findings in the current case.

Rab escort protein 1 (REP1) in intracellular traffic: a functional and pathophysiological overview

The intracellular distribution of proteins, compartments, substrates, and products is an active process called intracellular traffic. Control of intracellular traffic is established by small GTP-binding proteins (Rab proteins). Rab proteins are modified by geranyl-geranyl moieties necessary for membrane association and target-protein recognition. Geranyl-geranyl groups are transferred to Rab proteins by geranyl-geranyl transferase 2 (GGTase2). GGTase2 requires Rab escort protein 1 (REP1) to bind Rab proteins. REP1 null mutations underlie an X-linked retinal degeneration called choroideremia (CHM). This review summarizes the current biochemical and clinical knowledge on REP1 and CHM.

The functional effect of pathogenic mutations in Rab escort protein 1

Choroideremia (CHM) is a chorioretinal degeneration with an X-linked pattern of inheritance. Affected males experience progressive atrophy of the choroid, retinal pigment epithelium and retina leading to eventual blindness. The CHM gene encodes Rab escort protein 1 (REP-1). REP-1 is involved in trafficking of Rab proteins in the cell. To date, the majority of reported mutations in the CHM gene cause a complete loss of REP-1 function. Here we report pathogenic mutations: a novel missense mutation, L550P; a truncation c.1542T>A, STOP; and two deletions (c.525_526delAG and c.1646delC) in the CHM gene and their phenotypic effect. To analyze the effect of mutations, the 3D structure of human REP-1 and the proteins associated with REP-1 function were modeled using sequence homology with rat proteins. In silico analysis of the missense mutation L550P suggests that the proline residue at position 550 destabilizes the beta-structural elements, and the REP-1 tertiary structure. Truncation and deletion mutants are associated with a partial or total loss of the REP-1 essential activity and protein-protein interactions as predicted by the analysis of the structure and stability of these protein products. The presumptive loss of protein was confirmed by Western Blot analysis of protein from mononuclear cells and fibroblasts (FB) from CHM patients.

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.

Analysis of kinesin-2 function in photoreceptor cells using synchronous Cre-loxP knockout of Kif3a with RHO-Cre

PURPOSE

To determine the relationship between the presence of kinesin-2 and photoreceptor cell viability and opsin transport, by generating RHO-Cre transgenic mice and breeding them to mice with a floxed kinesin-2 motor gene.

METHODS

Different lines of RHO-Cre transgenic mice were generated and characterized by transgene expression, histology, and electrophysiology. Mice from one line, showing uniform transgene expression, were crossed with Kif3a(flox)/Kif3a(flox) mice. The time courses of photoreceptor Cre expression, KIF3A loss, ectopic opsin accumulation, and photoreceptor cell death were determined by Western blot analysis and microscopy.

RESULTS

One of the RHO-Cre lines effected synchronous expression of Cre and thus uniform excision of Kif3a(flox) in rod photoreceptors across the retina. After the neonatal production of CRE and the initiation of KIF3A loss, ectopic accumulation of opsin was detected by postnatal day (P)7, and ensuing photoreceptor cell death was evident after P10 and almost complete by P28. Of importance, the photoreceptor cilium formed normally, and the disc membranes of the nascent outer segment remained normal until P10.

CONCLUSIONS

The RHO-Cre-8 mice provide an improved tool for studying gene ablation in rod photoreceptor cells. Regarding kinesin-2 function in photoreceptor cells, the relatively precise timing of events after CRE excision of Kif3a(flox) allows us to conclude that ectopic opsin is a primary cellular lesion of KIF3A loss, consistent with the hypothesis that opsin is a cargo of kinesin-2. Moreover, it demonstrates that KIF3A loss results in very rapid photoreceptor cell degeneration.

Intragenomic politics

The mammalian genome contains multiple genetic factions with distinct interests in the outcomes of interactions among kin. In the context of an offspring's relations with its mother, these factions are proposed to align into two 'parties', one favoring increased demand by offspring and the other favoring reduced demand. A possible alignment has inhibitors of demand located on the X chromosome and enhancers of demand located on autosomes, because X-linked loci are maternally derived two-thirds of the time by contrast to autosomal loci which are maternally derived half of the time.

Independent degeneration of photoreceptors and retinal pigment epithelium in conditional knockout mouse models of choroideremia

Choroideremia (CHM) is an X-linked degeneration of the retinal pigment epithelium (RPE), photoreceptors, and choroid, caused by loss of function of the CHM/REP1 gene. REP1 is involved in lipid modification (prenylation) of Rab GTPases, key regulators of intracellular vesicular transport and organelle dynamics. To study the pathogenesis of CHM and to develop a model for assessing gene therapy, we have created a conditional mouse knockout of the Chm gene. Heterozygous-null females exhibit characteristic hallmarks of CHM: progressive degeneration of the photoreceptors, patchy depigmentation of the RPE, and Rab prenylation defects. Using tamoxifen-inducible and tissue-specific Cre expression in combination with floxed Chm alleles, we show that CHM pathogenesis involves independently triggered degeneration of photoreceptors and the RPE, associated with different subsets of defective Rabs.

Choroideremia gene product affects trophoblast development and vascularization in mouse extra-embryonic tissues

Choroideremia (CHM) is a hereditary eye disease caused by mutations in the X-linked CHM gene. Disruption of the Chm gene in mice resulted in prenatal death of Chm-/Y males and Chm-/Chm+ females that had inherited the mutation from their mothers. Male chimeras and Chm+/Chm- females with paternal transmission of the mutation were viable and had photoreceptor degeneration reminiscent of human choroideremia. Here, we show that Chm-/Y males and Chm-/Chm+ females were retarded at e7.5 and died before e11.5 due to multiple defects of the extra-embryonic tissues. Mutant embryos exhibited deficiency of diploid trophoblasts associated with overabundance of giant cells. In yolk sac and placenta, severe defects in vasculogenesis were obvious. Chm-/Y males exhibited more pronounced phenotypes than Chm-/Chm+ females. The lethal genotypes could be rescued by tetraploid aggregation. Chm-/Chm+ females, but not Chm-/Y males, could also be rescued when their Chm+/Chm- mothers were mated with Mus spretus males. Backcross analysis suggested that the viability of interspecies hybrid Chm-/Chm+ females may be due to expression from the Chm allele on the M. spretus X-chromosome rather than a modifier effect. Our results demonstrate that Chm is essential for diploid trophoblast development and plays a role in the vascularization in placenta and yolk sac.

Mutation of the zebrafish choroideremia gene encoding Rab escort protein 1 devastates hair cells

To identify genes important for hair-cell function, we conducted a mutagenic screen in zebrafish. Larvae from one mutant line, ru848, were unresponsive to acoustic stimuli and unable to balance. The mutation results in a 90% reduction in hair-cell number and partial retinal degeneration by 5 days postfertilization. We localized the recessive ru848 mutation by positional cloning to the zebrafish homolog of the human Choroideremia gene, which encodes Rab escort protein 1. This protein is essential for the normal prenylation of Rabs. Mutations in the human gene induce choroideremia, a disease marked by slow-onset degeneration of rod photoreceptors and retinal pigment epithelial cells. The degenerative phenotype resulting from a null mutation in the zebrafish gene indicates that hair cells and retinal cells require Rab escort protein 1 for survival.

Gene therapy for choroideremia: in vitro rescue mediated by recombinant adenovirus

Choroideremia (CHM) is an X-linked retinal degenerative disease resulting from a lack of functional Rab Escort Protein-1 (REP-1). As a first step in developing gene-based therapies for this disease, we evaluated the feasibility of delivering functional REP-1 to defective lymphocytes and fibroblasts isolated from individuals with CHM. A recombinant adenovirus delivering the full-length human cDNA encoding REP-1 under the control of a cytomegalovirus promoter was generated. Adenovirus-mediated delivery of REP-1 rescued the defective cells as assessed through protein and enzymatic assays. Ultimately, it may be possible to use virus-mediated delivery of REP-1 to evaluate disease intervention in vivo.

Genetic animal models for retinal degeneration

Inherited retinal degenerations are a common cause of blindness in Western countries. A mechanism for most retinal degenerations is still unknown; hence, a suitable treatment for most of these diseases has yet to be found. Before one can rationally design a treatment, it is necessary to understand the pathway from a gene mutation to the phenotype in patients. Animal models are crucial to understand this process and to develop a treatment. Some naturally occurring animal models are known. However, over the past few years, transgenic engineering has allowed the generation of a rapidly growing number of animal models. In this review, we give an overview of the broad variety of genetic animal models for retinal degeneration.

Identification of TSIX, encoding an RNA antisense to human XIST, reveals differences from its murine counterpart: implications for X inactivation

X inactivation is the mammalian method for X-chromosome dosage compensation, but some features of this developmental process vary among mammals. Such species variations provide insights into the essential components of the pathway. Tsix encodes a transcript antisense to the murine Xist transcript and is expressed in the mouse embryo only during the initial stages of X inactivation; it has been shown to play a role in imprinted X inactivation in the mouse placenta. We have identified its counterpart within the human X inactivation center (XIC). Human TSIX produces a >30-kb transcript that is expressed only in cells of fetal origin; it is expressed from human XIC transgenes in mouse embryonic stem cells and from human embryoid-body-derived cells, but not from human adult somatic cells. Differences in the structure of human and murine genes indicate that human TSIX was truncated during evolution. These differences could explain the fact that X inactivation is not imprinted in human placenta, and they raise questions about the role of TSIX in random X inactivation.

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.

Pathways to photoreceptor cell death in inherited retinal degenerations

The mutations that cause many forms of inherited retinal degenerations have been identified, yet the mechanisms by which these mutations lead to death of photoreceptor cells of the retina are not completely understood. Investigations of the pathways from mutation to retinal degeneration have focused on spontaneous and engineered animal models of disease. Based on the studies performed to date, four major categories of degeneration mechanism can be identified. These include disruption of photoreceptor outer segment morphogenesis, metabolic overload, dysfunction of retinal pigment epithelial cells, and chronic activation of phototransduction. Future investigations will likely identify additional mechanisms of photoreceptor damage. This review will summarize what has been learned from studying animal models of non-syndromic inherited retinal degenerations.

Ypt protein prenylation depends on the interplay among levels of Rab escort protein and geranylgeranyl diphosphate in yeast cells

Farnesyl diphosphate (FPP), an intermediate of the sterol biosynthetic pathway, is used by farnesyl transferase to farnesylate, among others, the Ras proteins, and by geranylgeranyl diphosphate synthase to produce geranylgeranyl diphosphate (GGPP). GGPP is then transferred by geranylgeranyl transferase II (GGTase II) to Rab/Ypt members of the Ras superfamily known to be required at all stages of vesicle transport in both mammals and yeast. Formation of a complex between a Rab/Ypt protein and an accessory protein named the Rab escort protein (REP) is a prerequisite for GGTase II substrate recognition. Little is known about the factors that regulate GGTase II activity in living cells but, based on available data, it seems possible that vesicle transport in higher eukaryotes is regulated by the levels of prenylated Rab/Ypt proteins in the cells. Here we show that the levels of REP play an important role in regulating GGTase II activity in yeast cells if sufficient substrates are present. Moreover, overexpression of REP causes, directly or indirectly, an increased level of Ypt substrates available for prenylation, which in turn leads to the depletion of the GGPP pool in the cell. Overall our data suggest that the levels of REP and the availability of GGPP play a role in regulating Ypt protein prenylation.

Evaluation of retinal photoreceptors and pigment epithelium in a female carrier of choroideremia

PURPOSE

To clarify the pathogenesis of choroideremia.

STUDY DESIGN

Human tissue study. TISSUES: Eyes of an 88-year-old symptomatic female carrier of choroideremia (CHM) and six normal, age-matched donors.

METHODS

The eyes were processed for histopathologic examination, including immunocytochemistry with an antibody against the CHM gene product, REP-1, and retinal cell-specific markers.

RESULTS

The CHM carrier retina showed patchy degeneration, but the photoreceptor and retinal pigment epithelium (RPE) loss appeared to be independent. The choriocapillaris was normal except where retinal areas were severely degenerate. The CHM gene product, REP-1, was localized to the cytoplasm of rods but not cones.

CONCLUSIONS

It has generally been considered that photoreceptor degeneration in CHM is secondary to loss of the choriocapillaris or RPE. This study suggests that the rod photoreceptors are a primary site of disease in CHM.

A mutation in Rab27a causes the vesicle transport defects observed in ashen mice

The dilute (d), leaden (ln), and ashen (ash) mutations provide a unique model system for studying vesicle transport in mammals. All three mutations produce a lightened coat color because of defects in pigment granule transport. In addition, all three mutations are suppressed by the semidominant dilute-suppressor (dsu), providing genetic evidence that these mutations function in the same or overlapping transport pathways. Previous studies showed that d encodes a major vesicle transport motor, myosin-VA, which is mutated in Griscelli syndrome patients. Here, using positional cloning and bacterial artificial chromosome rescue, we show that ash encodes Rab27a. Rab GTPases represent the largest branch of the p21 Ras superfamily and are recognized as key players in vesicular transport and organelle dynamics in eukaryotic cells. We also show that ash mice have platelet defects resulting in increased bleeding times and a reduction in the number of platelet dense granules. These defects have not been reported for d and ln mice. Collectively, our studies identify Rab27a as a critical gene for organelle-specific protein trafficking in melanocytes and platelets and suggest that Rab27a functions in both MyoVa dependent and independent pathways.

Rab geranylgeranyl transferase alpha mutation in the gunmetal mouse reduces Rab prenylation and platelet synthesis

Few molecular events important to platelet biogenesis have been identified. Mice homozygous for the spontaneous, recessive mutation gunmetal (gm) have prolonged bleeding, thrombocytopenia, and reduced platelet alpha- and delta-granule contents. Here we show by positional cloning that gm results from a G-->A substitution mutation in a splice acceptor site within the alpha-subunit of Rab geranylgeranyl transferase (Rabggta), an enzyme that attaches geranylgeranyl groups to Rab proteins. Most Rabggta mRNAs from gm tissues skipped exon 1 and lacked a start codon. Rabggta protein and Rab geranylgeranyl transferase (GGTase) activity were reduced 4-fold in gm platelets. Geranylgeranylation and membrane association of Rab27, a Rab GGTase substrate, were significantly decreased in gm platelets. These findings indicate that geranylgeranylation of Rab GTPases is critical for hemostasis. Rab GGTase inhibition may represent a new treatment for thrombocytosis and clotting disorders.

Low levels of Ypt protein prenylation cause vesicle polarization defects and thermosensitive growth that can be suppressed by genes involved in cell wall maintenance

The Rab/Ypt small G proteins are essential for intracellular vesicle trafficking in mammals and yeast. The vesicle-docking process requires that Ypt proteins are located in the vesicle membrane. C-terminal geranylgeranyl anchors mediate the membrane attachment of these proteins. The Rab escort protein (REP) is essential for the recognition of Rab/Ypt small G proteins by geranylgeranyltransferase II (GGTase II) and for their delivery to acceptor membranes. What effect an alteration in the levels of prenylated Rab/Ypt proteins has on vesicle transport or other cellular processes is so far unknown. Here, we report the characterization of a yeast REP mutant, mrs6-2, in which reduced prenylation of Ypt proteins occurs even at the permissive temperature. A shift to the restrictive temperature does not alter exponential growth during the first 3 h. The amount of Sec4p, but not Ypt1p, bound to vesicle membranes is reduced 2.5 h after the shift compared with wild-type or mrs6-2 cells incubated at 25 degrees C. In addition, vesicles fail to be polarized towards the bud and small budded binucleate cells accumulate at this time point. Growth in 1 M sorbitol or overexpression of MLC1, encoding a myosin light chain able to bind the unconventional type V myosin Myo2, or of genes involved in cell wall maintenance, such as SLG1, GFA1 and LRE1, suppresses mrs6-2 thermosensitivity. Our data suggest that, at least at high temperature, a critical minimal level of Ypt protein prenylation is required for maintaining vesicle polarization.

A phenotype map of the mouse X chromosome: models for human X-linked disease

The identification of many of the transcribed genes in man and mouse is being achieved by large scale sequencing of expressed sequence tags (ESTs). Attention is now being turned to elucidating gene function and many laboratories are looking to the mouse as a model system for this phase of the genome project. Mouse mutants have long been used as a means of investigating gene function and disease pathogenesis, and recently, several large mutagenesis programs have been initiated to fulfill the burgeoning demand of functional genomics research. Nevertheless, there is a substantial existing mouse mutant resource that can be used immediately. This review summarizes the available information about the loci encoding X-linked phenotypic mutants and variants, including 40 classical mutants and 40 that have arisen from gene targeting.

Parental origin-dependent, male offspring-specific transmission-ratio distortion at loci on the human X chromosome

We have analyzed the transmission of maternal alleles at loci spanning the length of the X chromosome in 47 normal, genetic disease-free families. We found a significant deviation from the expected Mendelian 1:1 ratio of grandpaternal:grandmaternal alleles at loci in Xp11.4-p21.1. The distortion in inheritance ratio was found only among male offspring and was manifested as a strong bias in favor of the inheritance of the alleles of the maternal grandfather. We found no evidence for significant heterogeneity among the families, which implies that the major determinant involved in the generation of the non-Mendelian ratio is epigenetic. Our analysis of recombinant chromosomes inherited by male offspring indicates that an 11.6-cM interval on the short arm of the X chromosome, bounded by DXS538 and DXS7, contains an imprinted gene that affects the survival of male embryos.