Our laboratory is interested in developing sight-saving treatments for inherited eye diseases. We are especially interested in understanding genetic diseases that affect retinal photoreceptors and the underlying retinal pigment epithelium. We are working to develop gene therapy for the early stages of these disorders and stem cell–based therapy for patients with more advanced disease.
A utosomal recessive Stargardt disease is a disorder caused by mutations in the ATP-binding cassette transporter of the retina (ABCA4). This transmembrane protein of the photoreceptor outer segments is a key component of the visual cycle, and significant reductions of its function result in the accumulation of bisretinoids that are toxic to both the photoreceptor cells and the underlying retinal pigment epithelium. Although the ABCA4 gene was identified as the cause of Stargardt disease more than 15 years ago, several important questions remain to be answered before safe and effective therapy can be delivered to the majority of affected individuals.
The first challenge is to accurately predict the clinical course based upon the patient's genotype. Treatments that would be considered a reasonable risk for patients with the most severe disease could be completely unsuitable for patients with the mildest disease. Also, for any trial that employs an untreated control group, it will be important to enroll treated and untreated individuals with disease of similar severity. The challenge of predicting clinical severity from the patient's genotype stems from the fact that the ABCA4 gene is extraordinarily polymorphic in the human population and hundreds of disease-causing alleles have been identified. We recently addressed this problem by studying 80 individuals with a completely defined ABCA4 genotype who had been followed at the University of Iowa for as long as 50 years. Using multiple linear regression analysis of visual acuity and visual field data, we were able to assign pathogenicity scores to 16 of the most common ABCA4 alleles in the population. This analysis also revealed evidence of two modifying factors: one controls the sensitivity of foveal photoreceptors to accumulating bisretinoids, and another controls the sensitivity of the underlying RPE.
We gained additional insight into the pathogenesis of ABCA4-associated retinal disease when one of my patients, who had arranged to donate his eyes for research, passed away. His eyes were obtained and processed for histopathology and RNA analysis within 7 hours of death. We had previously genotyped him and his family and found him to harbor two severe ABCA4 alleles. He had lost all perception of light at age 64, 14 years before he died. Histopathologic study of these eyes revealed complete loss of photoreceptors but some preservation of the retinal pigment epithelium, confirming that the most severe ABCA4 mutations cause a photoreceptor-selective disease. Microarray analysis of RNA extracted from the retina revealed transcripts typically found in bipolar cells, suggesting that inner retinal circuitry necessary for successful stem cell–based photoreceptor replacement therapy persists for more than a decade after all light perception is lost.
Retinitis pigmentosa (RP) is an inherited condition in which rod photoreceptor cells degenerate prior to (or more than) cone photoreceptor cells. More than 70 genes are already known to cause this phenotype, and it is likely that more than 100 others will eventually be identified. We used next-generation sequencing of a 69-year-old individual with RP to discover a homozygous 353–base-pair insertion in exon 9 of the gene encoding the male germ cell–associated kinase (MAK).
To investigate the mechanism of MAK-associated RP in humans, we made patient-derived pluripotent stem cells from the proband and differentiated them into retinal progenitor cells. We discovered that MAK transcripts are differentially spliced during development and that undifferentiated cells lack exon 9 (the site of the mutation). We then screened 1,798 unrelated individuals with RP for this mutation and found 20 additional individuals who were homozygous for it, the majority of whom had surnames that suggested Ashkenazi Jewish (AJ) ancestry. We screened 1,207 unrelated control individuals of AJ ancestry and identified a carrier frequency of 1/55 for this mutation, making MAK-associated RP the sixth most common Mendelian condition in this population. This carrier frequency is so high that this one MAK mutation is likely to be responsible for more than 30 percent of all cases of RP among AJ individuals. As a result, we created an inexpensive test for this and five other AJ retinal degeneration alleles and made it available to all physicians through the John and Marcia Carver Nonprofit Genetic Testing Laboratory.
Heritable Uveitis veitis is a term used to describe a group of conditions characterized by noninfectious inflammation of the iris and/or choroid of the eye. Although some genetic factors (such as HLA haplotypes) may predispose patients to uveitis, the disease is not commonly inherited in a high-penetrance fashion. In 1992, my laboratory identified the chromosomal location of a gene responsible for a Mendelian form of uveitis known as autosomal dominant neovascular inflammatory vitreoretinopathy (ADNIV). This condition begins in the second decade of life as a mild inflammation of the vitreous gel and progresses though a series of stages that mimic uveitis, retinitis pigmentosa, and diabetic retinopathy before progressing to complete blindness. We used next-generation sequencing of three affected families to identify two different missense mutations in the CAPN5 gene. CAPN5 encodes calpain-5, a calcium-activated cysteine protease that is expressed in retinal photoreceptor cells. Both mutations cause mislocalization from the cell membrane to the cytosol, and structural modeling reveals that both mutations lie within a calcium-sensitive domain near the active site. CAPN5 is only the second member of the large calpain gene family to cause a human Mendelian disorder, and this is the first report of a specific molecular cause for autoimmune eye disease. Further investigation of these mutations is likely to provide insight into the pathophysiologic mechanisms of common diseases, ranging from autoimmune disorders to diabetic retinopathy
This work was supported in part by the National Eye Institute, the Foundation Fighting Blindness, the Carver Endowment for Molecular Ophthalmology, and the Grousbeck Family Foundation.
As of December 04, 2012