Associate Professor, Genetics
Board of Scientific Counselors, National Eye Institute-NIH (2008 - 2013)
BS, University of Wisconsin-Madison, Biochemistry (1981)
PhD, Stanford University, Biochemistry (1988)
MD, Stanford University, Medicine (1989)
Current Research and Scholarly Interests
Work in the Vollrath laboratory is focused on understanding processes in the eye that are relevant to human health and disease. The eye is an organ particularly amenable to genetic analysis because its accessibility facilitates detection and characterization of a variety of disease states, yet such diseases rarely impair life span or fertility. We frequently start with genes known to be important in the eye because of their association with ocular disease phenotypes, and then investigate molecular mechanisms by uncovering pathways and processes relevant to normal eye function and pathogenesis.
RPE mitochondrial dysfunction is thought to play a causative role in retinal degenerative diseases such as mitochondrial retinopathy and age-related macular degeneration. As a test of this hypothesis, we generated mice with an RPE-selective postnatal loss of mitochondrial oxidative phosphorylation (OXPHOS). OXPHOS-deficient RPE cells are surprisingly long-lived, but lose critical epithelial characteristics through cellular dedifferentiation and, later, an epithelial to mesenchymal-like transition. OXPHOS-deficient RPE cells initiate a stress response that includes dependence upon the HGF/c-Met pathway, upregulation of aerobic glycolysis, activation of the mTOR signaling pathway, and cellular hypertrophy. Activation of mTOR and subsequent dedifferentiation can also be triggered by acute chemical oxidative damage to the RPE in vivo. For both chronic metabolic and acute oxidative RPE stress, the consequences for adjacent photoreceptors are profoundly negative, resulting in a gradual or rapid (respectively) retinal degeneration. Strikingly, treatment of animals with the mTOR inhibitor, rapamycin, blunts RPE dedifferentiation and hypertrophy and preserves photoreceptor numbers and function for both stressors. We would like to understand the mechanism of mTOR-mediated RPE dedifferentiation and determine whether this new in vivo RPE stress response is activated in human retinal disease.
Phagocytosis is an example of a basic process that we study. Every morning in mammalian eyes, the distal portion of the light sensing outer segments of photoreceptors are phagocytized by adjacent cells of the retinal pigment epithelium (RPE). Phagocytosis is balanced by new synthesis at the proximal end of the outer segment. Together, these two processes lead to constant turnover of outer segments and serve to repair light- and oxygen-induced damage. The daily big breakfast of outer segment material, summed over the life of an animal, distinguishes the post-mitotic RPE cell as the most phagocytic cell in the body. Photoreceptor degeneration in mutant rats and mice with defective RPE phagocytosis demonstrates that this process is essential for the normal functioning of the mammalian retina. By genetic analysis of these mutant rodents, we identified the receptor tyrosine kinase, MERTK, as a critical part of the phagocytic mechanism. We also identified mutations in the human MERTK gene in individuals with a retinal degenerative disease known as retinitis pigmentosa. We have elaborated our understanding of the mechanism of phagocytosis by demonstrating that MERTK acts locally, at the site of phagocytosis, to promote ingestion of bound outer segment tips. MERTK does so by triggering a striking redistribution of myosin II from the cell periphery to sites of ingestion. We are continuing to investigate the mechanism of RPE phagocytosis with an emphasis on identifying new protein components and understanding its circadian regulation.
- Genetics and Developmental Biology Training Camp
DBIO 200, GENE 200 (Aut)
Independent Studies (6)
- Directed Reading in Genetics
GENE 299 (Aut, Win, Spr, Sum)
- Graduate Research
GENE 399 (Aut, Win, Spr, Sum)
- Medical Scholars Research
GENE 370 (Aut, Win, Spr, Sum)
- Out-of-Department Advanced Research Laboratory in Experimental Biology
BIO 199X (Sum)
- Supervised Study
GENE 260 (Aut, Win, Spr, Sum)
- Undergraduate Research
GENE 199 (Aut, Win, Spr, Sum)
- Directed Reading in Genetics
- Prior Year Courses
Graduate and Fellowship Programs
An ENU-Induced Mutation in the Mertk Gene (Mertk(nmf12)) Leads to a Slow Form of Retinal Degeneration
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE
2011; 52 (7): 4703-4709
To determine the basis and to characterize the phenotype of a chemically induced mutation in a mouse model of retinal degeneration.Screening by indirect ophthalmoscopy identified a line of N-ethyl-N-nitrosourea (ENU) mutagenized mice demonstrating retinal patches. Longitudinal studies of retinal histologic sections showed photoreceptors in the peripheral retina undergoing slow, progressive degeneration. The mutation was named neuroscience mutagenesis facility 12 (nmf12), and mapping localized the critical region to Chromosome 2.Sequencing of nmf12 DNA revealed a point mutation in the c-mer tyrosine kinase gene, designated Mertk(nmf12). We detected elevated levels of tumor necrosis factor (Tnf, previously Tnfa) in retinas of Mertk(nmf12) homozygotes relative to wild-type controls and investigated whether the increase of TNF, an inflammatory cytokine produced by macrophages/monocytes that signals intracellularly to cause necrosis or apoptosis, could underlie the retinal degeneration observed in Mertk(nmf12) homozygotes. Mertk(nmf12) homozygous mice were mated to mice lacking the entire Tnf gene and partial coding sequences of the Lta (Tnfb) and Ltb (Tnfc) genes.(2) B6.129P2-Ltb/Tnf/Lta(tm1Dvk)/J homozygotes did not exhibit a retinal degeneration phenotype and will, hereafter, be referred to as Tnfabc(-/-) mice. Surprisingly, mice homozygous for both the Mertk(nmf12) and the Ltb/Tnf/Lta(tm1Dvk) allele (Tnfabc(-/-)) demonstrated an increase in the rate of retinal degeneration.These findings illustrate that a mutation in the Mertk gene leads to a significantly slower progressive retinal degeneration compared with other alleles of Mertk. These results demonstrate that TNF family members play a role in protecting photoreceptors of Mertk(nmf12) homozygotes from cell death.
View details for DOI 10.1167/iovs.10-7077
View details for Web of Science ID 000293332500103
View details for PubMedID 21436282
- mTOR pathway activation in age-related retinal disease AGING-US 2011; 3 (4): 346-347
Generation of Cre Transgenic Mice with Postnatal RPE-Specific Ocular Expression
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE
2011; 52 (3): 1378-1383
To generate and characterize a constitutively active, RPE-specific, cre-expressing transgenic mouse line. This line can be used to create RPE-specific knockouts by crossing with mice harboring loxP-flanked (floxed) genes.A transgene construct was assembled with the BEST1 promoter driving cre expression. Transgenic mice were generated on a C57BL/6 background. Cre expression was assessed by immunofluorescence and Western blot analysis. Cre enzymatic activity was tested by crossing to three lines with floxed DNA regions and detecting deletion of the intervening sequences or through histochemical detection of lacZ activity. Potential cre-mediated toxicity was assessed by retinal histology up to 24 months of age and by electroretinography.The BEST1-cre line with expression in the highest percentage of RPE cells displayed a patchy mosaic expression pattern, with 50% to 90% of RPE cells expressing cre. In mice outcrossed to a mixed B6/129 background, expression was consistently found in 90% of RPE cells. Within the eye, only the RPE cells were immunoreactive with an anti-cre antibody. Maximum cre expression quantified by Western blot analysis occurred at P28. Crosses with three lines containing floxed sequences revealed RPE-specific cre activity in the eye and extraocular expression limited to the testes. Histology and electroretinography showed no cre-mediated RPE toxicity.This BEST1-cre transgenic line enables generation of RPE-specific knockout mice. The mosaic expression pattern provides an internal control; the non-cre-expressing RPE cells continue to express the floxed genes. These mice should facilitate study of the multifunctional RPE and the generation of mouse models of human retinal disease.
View details for DOI 10.1167/iovs.10-6347
View details for Web of Science ID 000288965300023
View details for PubMedID 21212186
mTOR-mediated dedifferentiation of the retinal pigment epithelium initiates photoreceptor degeneration in mice
JOURNAL OF CLINICAL INVESTIGATION
2011; 121 (1): 369-383
Retinal pigment epithelial (RPE) cell dysfunction plays a central role in various retinal degenerative diseases, but knowledge is limited regarding the pathways responsible for adult RPE stress responses in vivo. RPE mitochondrial dysfunction has been implicated in the pathogenesis of several forms of retinal degeneration. Here we have shown that postnatal ablation of RPE mitochondrial oxidative phosphorylation in mice triggers gradual epithelium dedifferentiation, typified by reduction of RPE-characteristic proteins and cellular hypertrophy. The electrical response of the retina to light decreased and photoreceptors eventually degenerated. Abnormal RPE cell behavior was associated with increased glycolysis and activation of, and dependence upon, the hepatocyte growth factor/met proto-oncogene pathway. RPE dedifferentiation and hypertrophy arose through stimulation of the AKT/mammalian target of rapamycin (AKT/mTOR) pathway. Administration of an oxidant to wild-type mice also caused RPE dedifferentiation and mTOR activation. Importantly, treatment with the mTOR inhibitor rapamycin blunted key aspects of dedifferentiation and preserved photoreceptor function for both insults. These results reveal an in vivo response of the mature RPE to diverse stressors that prolongs RPE cell survival at the expense of epithelial attributes and photoreceptor function. Our findings provide a rationale for mTOR pathway inhibition as a therapeutic strategy for retinal degenerative diseases involving RPE stress.
View details for DOI 10.1172/JCI44303
View details for Web of Science ID 000285892300039
View details for PubMedID 21135502
- Focus on Molecules: MERTK EXPERIMENTAL EYE RESEARCH 2010; 91 (6): 786-787
Candidate genes for chromosomes 6 and 10 quantitative trait loci for age-related retinal degeneration in mice
2010; 16 (111-13): 1004-1018
In a previous study, several quantitative trait loci (QTL) that influence age-related degeneration (ageRD) were identified in a cross between the albino strains B6(Cg)-Tyr(c-2J)/J (B6a) and BALB/cByJ (C). The Chromosome (Chr) 6 and Chr 10 QTL were the strongest and most highly significant loci and both involved B6a protective alleles. The QTL were responsible for 21% and 9% of the variance in phenotypes, respectively. We focused on these two QTL to identify candidate genes.DNA microarrays were used for the two mouse strains at four and eight months of age to identify genes that are differentially regulated and map to either QTL. Gene Ontology (GO) analysis of the differentially expressed genes was performed to identify possible processes and pathways associated with ageRD. To identify additional candidates, database analyses (Positional Medline or PosMed) were used. Based on differential expression, PosMed, and the presence of reported polymorphisms, five genes per QTL were selected for further study by sequencing analysis and qRT-PCR. Tumor necrosis factor, alpha- induced protein 3 (Tnfaip3; on a C57BL/6J (B6) background) was phenotypically tested. Single nucleotide polymorphisms (SNPs) flanking this gene were correlated with outer nuclear layer thickness (ONL), and eight-month-old Tnfaip3(+/-) mice were tested for ageRD.Polymorphisms were found in the coding regions of eight genes. Changes in gene expression were identified by qRT-PCR for Hexokinase 2 (Hk2) and Docking protein 1 (Dok1) at four months and for Dok1 and Tnfaip3 at eight months. Tnfaip3 was selected for phenotypic testing due to differential expression and the presence of two nonsynonymous mutations. However, when ONL thickness was compared in eight-month-old congenic Tnfaip3(+/-) and Tnfaip3(+/+) mice, no differences were found, suggesting that Tnfaip3 is not the quantitative trait gene (QTG) for the Chr 10 QTL. The GO analysis revealed that GO terms associated with stress and cell remodeling are overrepresented in the ageRD-sensitive C strain compared with the B6a strain with age (eight months). In the ageRD-resistant B6a strain, compared with the C strain, GO terms associated with antioxidant response and the regulation of blood vessel size are overrepresented with age.The analyses of differentially expressed genes and the PosMed database yielded candidate genes for the Chr 6 and Chr 10 QTL. HtrA serine peptidase 2 (Htra2), Dok1, and Tnfaip3 were deemed most promising because of their known roles in apoptosis and our finding of nonsynonymous substitutions between B6a and C strains. While Tnfaip3 was excluded as the QTG for the Chr 10 QTL, Dok1 and Htra2 remain good candidates for the Chr 6 QTL. Finally, the GO term analysis further supports the general hypothesis that oxidative stress is involved in ageRD.
View details for Web of Science ID 000279676700001
View details for PubMedID 20577653
Rescue of Glaucoma-Causing Mutant Myocilin Thermal Stability by Chemical Chaperones
ACS CHEMICAL BIOLOGY
2010; 5 (5): 477-487
Mutations in myocilin cause an inherited form of open angle glaucoma, a prevalent neurodegenerative disorder associated with increased intraocular pressure. Myocilin forms part of the trabecular meshwork extracellular matrix presumed to regulate intraocular pressure. Missense mutations, clustered in the olfactomedin (OLF) domain of myocilin, render the protein prone to aggregation in the endoplasmic reticulum of trabecular meshwork cells, causing cell dysfunction and death. Cellular studies have demonstrated temperature-sensitive secretion of myocilin mutants, but difficulties in expression and purification have precluded biophysical characterization of wild-type (wt) myocilin and disease-causing mutants in vitro. We have overcome these limitations by purifying wt and select glaucoma-causing mutant (D380A, I477N, I477S, K423E) forms of the OLF domain (228-504) fused to a maltose binding protein (MBP) from E. coli . Monomeric fusion proteins can be isolated in solution. To determine the relative stability of wt and mutant OLF domains, we developed a fluorescence thermal stability assay without removal of MBP and provide the first direct evidence that mutated OLF is folded but less thermally stable than wt. We tested the ability of seven chemical chaperones to stabilize mutant myocilin. Only sarcosine and trimethylamine N-oxide were capable of shifting the melting temperature of all mutants tested to near that of wt OLF. Our work lays the foundation for the identification of tailored small molecules capable of stabilizing mutant myocilin and promoting secretion to the extracellular matrix, to better control intraocular pressure and to ultimately delay the onset of myocilin glaucoma.
View details for DOI 10.1021/cb900282e
View details for Web of Science ID 000277865500006
View details for PubMedID 20334347
Mertk Drives Myosin II Redistribution during Retinal Pigment Epithelial Phagocytosis
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE
2009; 50 (5): 2427-2435
Mertk is a key phagocytic receptor in the immune, male reproductive, and visual systems. In the retinal pigment epithelium, Mertk is required for the daily ingestion of photoreceptor outer segment (OS) tips. Loss of Mertk function causes retinal degeneration in rats, mice, and humans; however, little is known about the mechanism by which Mertk regulates the ingestion phase of retinal pigment epithelial (RPE) phagocytosis. To address this, the authors sought proteins that associated with Mertk during OS phagocytosis.Lysates of RPE-J cells challenged with OS for various times were immunoprecipitated with Mertk antibody. Potential interacting proteins were identified by mass spectrometry and characterized with confocal microscopy, pharmacologic inhibition, and siRNA knockdown coupled with an in vitro phagocytic assay in primary RPE cells.Myh9, the non-muscle myosin II-A heavy chain, was enriched in immunoprecipitates from OS-treated samples. Myosin II-A and II-B isoforms exhibited a striking redistribution in wild-type rat primary RPE cells challenged with OS, moving from the cell periphery to colocalize with ingested OS over time. In contrast, myosin II-A redistribution in response to OS was blunted in primary RPE cells from RCS rats, which lack functional Mertk. Wild-type rat primary RPE cells treated with the myosin II-specific inhibitor blebbistatin or myosin II siRNAs exhibited a significant phagocytic defect.Mertk mobilizes myosin II from the RPE cell periphery to sites of OS engulfment, where myosin II function is essential for the normal phagocytic ingestion of OS.
View details for DOI 10.1167/iovs.08-3058
View details for Web of Science ID 000265451000060
View details for PubMedID 19117932
Rapid and stable knockdown of an endogenous gene in retinal pigment epithelium
HUMAN GENE THERAPY
2007; 18 (10): 871-880
The selective silencing of target genes in specific cell types by RNA interference (RNAi) represents a powerful approach both to gene therapy of dominantly active mutant alleles, and to the investigation of normal gene function in animal models in vivo. We established a simple and versatile in vitro method for screening the efficacy of DNA-based short hairpin RNAs (shRNAs), and identified a highly effective shRNA targeting basic fibroblast growth factor (bFGF), a gene thought to play important roles in endogenous neuroprotective responses in the rat retina. We used two viral vectors, based on lentivirus and adeno-associated virus (AAV), to deliver shRNAs and silence bFGF in retinal pigment epithelial cells in vivo. The AAV experiments made use of a "stabilized double-stranded" version of these vectors with rapid onset of gene expression. In the rat retinal pigment epithelium, shRNAs delivered by either vector reduced bFGF immunoreactivity to undetectable levels in transduced cells, whereas a nonfunctional control construct incorporating a two-base pair mutation had no measurable effect on bFGF expression. Silencing commenced within a few days after injection of virus and remained stable throughout the period of observation, as long as 60 days. Viral delivery of RNAi constructs offers a powerful and versatile approach for both gene therapy and the analysis of fundamental questions in retinal biology.
View details for DOI 10.1089/hum.2007.065
View details for Web of Science ID 000250339500001
View details for PubMedID 17892416
A novel His158Arg mutation in TIMP3 causes a late-onset form of Sorsby fundus dystrophy
AMERICAN JOURNAL OF OPHTHALMOLOGY
2006; 142 (5): 839-848
To describe the phenotype and genotype of a family with suspected Sorsby fundus dystrophy (SFD).Case reports and results of deoxyribonucleic acid (DNA) analysis.Clinical features were determined by complete ophthalmologic examination or by review of medical records. Mutational analysis of the tissue inhibitor of metalloproteinase (TIMP)3 gene was performed by DNA resequencing. Biochemical properties of the mutant TIMP3 protein were studied, and phylogenetic and molecular modeling analyses of TIMP proteins were performed.Fundi of four affected family members demonstrated active or regressed bilateral choroidal neovascularization, whereas another affected individual displayed severe diffuse pigmentary degeneration associated with nyctalopia characteristic of SFD. Onset of disease occurred in the fifth to seventh decades of life. A heterozygous His158Arg mutation was found in seven affected family members and was absent from an unaffected member and 98 unrelated controls. Bioinformatic analyses indicate that histidine 158 is an evolutionarily conserved residue in most vertebrate TIMP homologs and predict that substitution by arginine disrupts TIMP3 function. The mutant protein appears to be expressed by fibroblasts from an affected family member. Molecular modeling suggests that TIMP3 residue 158 may be part of a protein-protein interaction interface.A novel mutation in TIMP3 causes a late-onset form of SFD in this family. His158Arg is the first reported TIMP3 SFD coding sequence mutation that does not create an unpaired cysteine. Further study of this unusual mutation may provide insight into the mechanism of SFD pathogenesis.
View details for DOI 10.1016/j.ajo.2006.06.003
View details for Web of Science ID 000242142900019
View details for PubMedID 16989765
Temperature sensitive secretion of mutant myocilins
EXPERIMENTAL EYE RESEARCH
2006; 82 (6): 1030-1036
Recent studies have demonstrated that glaucoma-causing mutant myocilin proteins are misfolded and retained in the endoplasmic reticulum of cells. We showed previously that P370L mutant myocilin is poorly secreted at 37 degrees C and prolonged expression of the protein in differentiated human trabecular meshwork cells results in abnormal morphology and cell killing. Culturing cells at a lower temperature, a condition known to facilitate protein folding, enhances secretion and reverses the cytotoxic effects. We wanted to determine if temperature sensitive secretion is a general property of myocilin missense mutants. Wild-type or mutant forms of myocilin were transiently expressed in HEK 293 cells cultured at either 37 or 30 degrees C and protein secretion was assessed by immunoblotting. Of 15 myocilin missense mutants tested, representing a range in severity of associated glaucoma phenotypes, 14 displayed increased secretion at 30 degrees C. The sole exception was K423E, which is associated with an unusual mode of glaucoma inheritance. Generally, there is an inverse relationship between the degree of mutant myocilin secretion at 30 degrees C and the severity of the associated glaucoma phenotype. Mutants that show abundant secretion at 30 degrees C such as T377M, G364V, I499F and D380A are associated with less virulent glaucoma phenotypes, while mutants such as P370L, I477N, and Y437H display little secretion at 30 degrees C and are associated with more virulent glaucoma phenotypes. We conclude that temperature sensitive secretion is a property of most olfactomedin-domain myocilin mutants. The correlation between temperature sensitive secretion and glaucoma phenotype likely reflects the intrinsic susceptibility to misfolding of individual mutant proteins. These results support the hypothesis that myocilin-induced glaucoma is a protein conformational disease. Facilitating mutant protein folding could be a new approach to development of therapies for this disease.
View details for DOI 10.1016/j.exer.2005.10.007
View details for Web of Science ID 000237935200015
View details for PubMedID 16297911
phi C31 integrase confers genomic integration and long-term transgene expression in rat retina
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE
2005; 46 (6): 2140-2146
Gene therapy has shown promise in animal models of retinal disease, with the most success achieved to date with viral vectors used for gene delivery. Viral vectors, however, have side effects and limitations and are difficult to manufacture. The present study was conducted in an attempt to develop a novel system for long-term gene transfer in rat retinal pigment epithelium (RPE), by using nonviral transfection methods for gene transfer and the integrase from the bacteriophage phiC31 to confer long-term gene expression by means of genomic integration.Efficient nonviral delivery of plasmid DNA to rat RPE in vivo was achieved by using subretinal injection of plasmid DNA, followed by in situ electroporation. Gene delivery was evaluated by analyzing enhanced green fluorescent protein (eGFP) expression in frozen sections. In subsequent experiments, a plasmid expressing luciferase, with or without a plasmid encoding the phiC31 integrase, was delivered to rat RPE. Luciferase expression was followed over time by using in vivo luciferase imaging.Subretinal injection followed by electroporation yielded abundant transgene expression in the rat RPE. Expression was strongest 48 hours after delivery. In the absence of phiC31 integrase, transgene expression declined to near-background levels within 3 to 4 weeks after treatment. By contrast, coinjection of the integrase plasmid led to long-term stable transgene expression throughout the 4.5-month test period. Eyes injected with phiC31 integrase showed approximately 85-fold higher long-term transgene expression in the retina than eyes without integrase.Subretinal injection of DNA followed by electroporation affords abundant transfer of plasmid DNA in rat RPE. phiC31 integrase confers robust long-term transgene expression by mediating genomic integration of the transgene. These findings suggest that phiC31 integrase may be a simple and effective tool for nonviral long-term gene transfer in the eye.
View details for DOI 10.1167/iovs.04-1252
View details for Web of Science ID 000229504600038
View details for PubMedID 15914635
Reversal of mutant myocilin non-secretion and cell killing: implications for glaucoma
HUMAN MOLECULAR GENETICS
2004; 13 (11): 1193-1204
Glaucoma is a progressive blinding disease characterized by gradual loss of vision due to optic neuropathy and retinal ganglion cell death. Increased intraocular pressure is a common feature of glaucoma that is thought to arise from an increased resistance to outflow of aqueous humor through the trabecular meshwork. Mutations of the myocilin gene are one cause of autosomal dominant juvenile- and adult-onset primary open angle glaucoma, but the mechanism by which mutant myocilins cause disease is poorly understood. We have found that disease-causing myocilin mutants are misfolded, are highly aggregation-prone and accumulate in large aggregates in the endoplasmic reticulum (ER) of human embryonic kidney cells and differentiated primary human trabecular meshwork (HTM) cells. In HTM cells, Pro370Leu mutant myocilin is not secreted under normal culture conditions and prolonged expression results in abnormal cell morphology and cell killing. Culturing HTM cells at 30 degrees C, a condition known to facilitate protein folding, promotes secretion of mutant myocilin, normalizes cell morphology and reverses cell lethality. Our results indicate that myocilin-associated glaucoma is an ER storage disease and suggest a progression of events in which chronic expression of misfolded, non-secreted myocilin leads to HTM cell death, trabecular meshwork dysfunction and, ultimately, a dominant glaucoma phenotype. The beneficial effects of facilitating folding and secretion of mutant myocilin suggest a new type of treatment for this form of glaucoma.
View details for DOI 10.1093/hmg/ddh128
View details for Web of Science ID 000221406000010
View details for PubMedID 15069026
An RCS-like retinal dystrophy phenotype in Mer knockout mice
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE
2003; 44 (2): 826-838
To determine whether mice that are homozygous for a targeted disruption of the Mer receptor tyrosine kinase gene (mer(kd)) manifest a retinal dystrophy phenotype similar to RCS rats, which carry a mutation in the orthologous gene MERTK:Eyes of mer(kd) and C57BL/6 wild-type (WT) mice were examined by light and electron microscopy, whole-eye rhodopsin measurement, and Ganzfeld electroretinography (ERG).The mer(kd) mice showed rapid, progressive degeneration of the photoreceptors (PRs). Features of the phenotype common to mer(kd) mice and RCS rats included the absence or near absence of phagosomes in the retinal pigment epithelium (RPE) at the peak of outer segment (OS) disc shedding, accumulation of debris and whorls of membranes at the RPE-OS interface, transient supernormal rhodopsin content and OS lengths, the presence of OS vacuoles beginning at early ages, and a relatively slow removal of pyknotic PR nuclei. Most PRs were missing, and OS debris was removed by approximately postnatal day (P)45. Scotopic ERG responses were lower than age-matched WT responses and declined with PR loss. Photopic responses were preserved better than scotopic responses, corresponding with preferential cone preservation as judged histologically. ERG amplitudes were usually unmeasurable beyond P40, although a small-amplitude scotopic threshold response (STR) could still be elicited at P253 in some mice when only scattered PR nuclei remained.Ablation of Mer function in mer(kd) mice results in a retinal phenotype almost identical with that of RCS rats. The similarity in phenotypes between the two rodent models suggests that an RPE phagocytic defect is a feature of all types of retinal degeneration caused by loss of function of Mer tyrosine kinase, perhaps including mutations in human MERTK.
View details for DOI 10.1167/iovs.02-0438
View details for Web of Science ID 000180966800052
View details for PubMedID 12556419
Mertk triggers uptake of photoreceptor outer segments during phagocytosis by cultured retinal pigment epithelial cells
JOURNAL OF BIOLOGICAL CHEMISTRY
2002; 277 (19): 17016-17022
The RCS rat is a widely studied model of recessively inherited retinal degeneration. The genetic defect, known as rdy (retinal dystrophy), results in failure of the retinal pigment epithelium (RPE) to phagocytize shed photoreceptor outer segment membranes. We previously used positional cloning and in vivo genetic complementation to demonstrate that Mertk is the gene for rdy. We have now used a rat primary RPE cell culture system to demonstrate that the RPE is the site of action of Mertk and to obtain functional evidence for a key role of Mertk in RPE phagocytosis. We found that Mertk protein is absent from RCS, but not wild-type, tissues and cultured RPE cells. Delivery of rat Mertk to cultured RCS RPE cells by means of a recombinant adenovirus restored the cells to complete phagocytic competency. Infected RCS RPE cells ingested exogenous outer segments to the same extent as wild-type RPE cells, but outer segment binding was unaffected. Mertk protein progressively co-localized with outer segment material during phagocytosis by primary RPE cells, and activated Mertk accumulated during the early stages of phagocytosis by RPE-J cells. We conclude that Mertk likely functions directly in the RPE phagocytic process as a signaling molecule triggering outer segment ingestion.
View details for DOI 10.1074/jbc.M107876200
View details for Web of Science ID 000175564500090
View details for PubMedID 11861639
Correction of the retinal dystrophy phenotype of the RCS rat by viral gene transfer of Mertk
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2001; 98 (22): 12584-12589
The Royal College of Surgeons (RCS) rat is a widely studied animal model of retinal degeneration in which the inability of the retinal pigment epithelium (RPE) to phagocytize shed photoreceptor outer segments leads to a progressive loss of rod and cone photoreceptors. We recently used positional cloning to demonstrate that the gene Mertk likely corresponds to the retinal dystrophy (rdy) locus of the RCS rat. In the present study, we sought to determine whether gene transfer of Mertk to a RCS rat retina would result in correction of the RPE phagocytosis defect and preservation of photoreceptors. We used subretinal injection of a recombinant replication-deficient adenovirus encoding rat Mertk to deliver the gene to the eyes of young RCS rats. Electrophysiological assessment of animals 30 days after injection revealed an increased sensitivity of treated eyes to low-intensity light. Histologic and ultrastructural assessment demonstrated substantial sparing of photoreceptors, preservation of outer segment structure, and correction of the RPE phagocytosis defect in areas surrounding the injection site. Our results provide definitive evidence that mutation of Mertk underlies the RCS retinal dystrophy phenotype, and that the phenotype can be corrected by treatment of juvenile animals. To our knowledge, this is the first demonstration of complementation of both a functional cellular defect (phagocytosis) and a photoreceptor degeneration by gene transfer to the RPE. These results, together with the recent discovery of MERTK mutations in individuals with retinitis pigmentosa, emphasize the importance of the RCS rat as a model for gene therapy of diseases that arise from RPE dysfunction.
View details for Web of Science ID 000171806100054
View details for PubMedID 11592982
Mutations in MERTK, the human orthologue of the RCS rat retinal dystrophy gene, cause retinitis pigmentosa
2000; 26 (3): 270-271
Mutation of a receptor tyrosine kinase gene, Mertk, in the Royal College of Surgeons (RCS) rat results in defective phagocytosis of photoreceptor outer segments by the retinal pigment epithelium (RPE) and retinal degeneration. We screened the human orthologue, MERTK, located at 2q14.1 (ref. 10), in 328 DNA samples from individuals with various retinal dystrophies and found three mutations in three individuals with retinitis pigmentosa (RP). Our findings are the first conclusive evidence implicating the RPE phagocytosis pathway in human retinal disease.
View details for Web of Science ID 000165176500010
View details for PubMedID 11062461
Age-dependent prevalence of mutations at the GLC1A locus in primary open-angle glaucoma
AMERICAN JOURNAL OF OPHTHALMOLOGY
2000; 130 (2): 165-177
To screen a population with primary open-angle glaucoma for mutations in the gene that encodes the trabecular meshwork inducible glucocorticoid response protein (TIGR), also known as myocilin (MYOC).Ophthalmologic information was collected for study subjects with primary open-angle glaucoma and their relatives. Mutation screening of 74 primary open-angle glaucoma probands was conducted by sequencing TIGR/MYOC coding sequence and splice sites.In 23 families we detected 13 nonsynonymous sequence changes, nine of which appear to be mutations likely to cause or contribute to primary open-angle glaucoma. Two mutations, Arg272Gly and Ile499Ser, and one nonsynonymous sequence variant, Asn57Asp, are novel. We found mutations in nine of 25 juvenile glaucoma probands (36%) and two of 49 adult-onset glaucoma probands (4%). Age classification of families rather than individual probands revealed mutations in three of nine families with strictly juvenile primary open-angle glaucoma (33%), and no mutations in 39 families with strictly adult-onset primary open-angle glaucoma (0%). In families with mixed-onset primary open-angle glaucoma containing both juvenile primary open-angle glaucoma and adult-onset primary open-angle glaucoma cases, we found mutations in eight of 26 families (31%).Our data suggest that Gly252Arg, Arg272Gly, Glu323Lys, Gln368STOP, Pro370Leu, Thr377Met, Val426Phe, Ile477Asn, and Ile499Ser are likely to play roles that cause or contribute to the etiology of autosomal dominant primary open-angle glaucoma. Our finding of more TIGR/MYOC mutations in families with mixed-onset primary open-angle glaucoma than in the families with strictly adult-onset primary open-angle glaucoma implies that the presence of relatives with juvenile primary open-angle glaucoma in a family could be used as a basis for identifying a subset of the population with adult-onset primary open-angle glaucoma with higher prevalence of TIGR/MYOC mutations. To address this issue, and to refine estimations of mutation prevalence in these age-defined subpopulations, prospective study of a larger population ascertained entirely through adult-onset primary open-angle glaucoma probands will be needed.
View details for Web of Science ID 000089601800004
View details for PubMedID 11004290
Mutation of the receptor tyrosine kinase gene Mertk in the retinal dystrophic RCS rat
HUMAN MOLECULAR GENETICS
2000; 9 (4): 645-651
Vertebrate photoreceptor cells are the basic sensory apparatus of the retina, capable of converting the energy of absorbed photons into neuronal signals. The proximal portions of mammalian photoreceptor outer segments are synthesized daily by cell bodies, and outer segment tips are shed with a circadian rhythm, resulting in a complete turnover of outer segments about every 9 days. The shed outer segments are phagocytosed by adjacent retinal pigment epithelial (RPE) cells, and metabolites are recycled to photoreceptors. The Royal College of Surgeons (RCS) rat is a widely studied, classic model of recessively inherited retinal degeneration in which the RPE fails to phagocytose shed outer segments, and photoreceptor cells subsequently die. We have used a positional cloning approach to study the rdy (retinal dystrophy) locus of the RCS rat. Within a 0.3 cM genetic inclusion interval, we have discovered a small deletion of RCS DNA that disrupts the gene encoding the receptor tyrosine kinase Mertk. The deletion includes the splice acceptor site upstream of the second coding exon of Mertk and results in a shortened transcript that lacks this exon. The aberrant transcript joins the first and third coding exons, leading to a frameshift and a translation termination signal 20 codons after the AUG. The concordance of these and other data indicate that Mertk is probably the gene for rdy. Our results provide genetic evidence for an essential role of a receptor tyrosine kinase in a specialized form of phagocytosis and suggest a molecular model for ingestion of outer segments by RPE cells.
View details for Web of Science ID 000085781000020
View details for PubMedID 10699188
A cellular assay distinguishes normal and mutant TIGR/myocilin protein
HUMAN MOLECULAR GENETICS
1999; 8 (12): 2221-2228
Glaucoma is a blinding eye disease that affects approximately 70 000 000 people world-wide. Mutations in the gene TIGR / MYOC have been shown to cause the most common form of the disease, primary open angle glaucoma, in selected families. Amino acid sequence variants of the gene have been found in 2-4% of sporadic primary open angle glaucoma cases. Most variants are rare and it is often difficult to definitively distinguish between a deleterious mutation and a benign variant solely on the basis of relative frequencies in patient and control groups. The function of the TIGR/myocilin protein is unknown and an assay to functionally classify variants is lacking. We sought to develop a biochemical assay to distinguish different forms of TIGR/myocilin. We investigated the Triton X-100 detergent solubility characteristics of mutant and normal forms of the protein, expressed by transfection in cultured cells. We observed a clear difference in the behavior of the two types of TIGR/myocilin; all confirmed mutant proteins tested were substantially Triton insoluble, while normal protein and controls were completely soluble. We also tested seven ambiguous variant proteins and classified them as mutant or normal on the basis of their Triton solubility. The results in some cases validated, and in other cases contradicted, earlier classifications of these variants. To our knowledge, Triton solubility is the first example of a general difference in the properties of mutant and normal forms of TIGR/myocilin. The assay we have developed will be useful for discerning protein functional information from the location of mutations, will aid genetic counseling of individuals with TIGR/myocilin variants and may provide a clue to understanding a mechanism by which mutations in TIGR / MYOC cause glaucoma.
View details for Web of Science ID 000083658800011
View details for PubMedID 10545602
- Characterization of the murine TIGR/myocilin gene MAMMALIAN GENOME 1998; 9 (8): 673-675
Loss-of-function mutations in the LIM-homeodomain gene, LMX1B, in nail-patella syndrome
HUMAN MOLECULAR GENETICS
1998; 7 (7): 1091-1098
Nail-patella syndrome (NPS) is an inherited developmental disorder most commonly involving maldevelopment of the fingernails, kneecaps and elbow joints. NPS exhibits wide variation in phenotypic expression within and among families with respect to these features. Other skeletal abnormalities such as hip dislocation and club foot have also been reported in some individuals with NPS. There is an association between NPS and renal disease, and between NPS and open-angle glaucoma (OAG), but it is not known whether mutations in a single gene cause the observed skeletal, renal and ophthalmic abnormalities. Recently, LMX1B , a transcription factor of the LIM-homeodomain type with homologs that are important for limb development in vertebrates, was mapped to the same general location as NPS at 9q34. We sequenced a large segment of LMX1B from the genomic DNA of probands from four families with NPS and OAG, and identified four mutations: two stop codons, a deletion causing a frameshift and a missense mutation in a functionally important residue. The presence of these putative loss-of-function mutations in the DNA of individuals with NPS indicates that haploinsufficiency of LMX1B underlies this disorder. These findings help to explain the high degree of variability in the NPS phenotype, and suggest that the skeletal defects in NPS are a result of the diminished dorsoventral patterning activity of LMX1B protein during limb development. The results further suggest that the NPS and OAG phenotypes in the families studied result from mutations in a single gene, LMX1B.
View details for Web of Science ID 000074571900004
View details for PubMedID 9618165
TANDEM ARRAY OF HUMAN VISUAL PIGMENT GENES AT XQ28
1988; 240 (4859): 1669-1672
Unequal crossing-over within a head-to-tail tandem array of the homologous red and green visual pigment genes has been proposed to explain the observed variation in green-pigment gene number among individuals and the prevalence of red-green fusion genes among color-blind subjects. This model was tested by probing the structure of the red and green pigment loci with long-range physical mapping techniques. The loci were found to constitute a gene array with an approximately 39-kilobase repeat length. The position of the red pigment gene at the 5' edge of the array explains its lack of variation in copy number. Restriction maps of the array in four individuals who differ in gene number are consistent with a head-to-tail configuration of the genes. These results provide physical evidence in support of the model and help to explain the high incidence of color blindness in the human population.
View details for Web of Science ID A1988N833700035
View details for PubMedID 2837827