Correlation of phenotype with genotype in inherited retinal degeneration

Mutations in four glycosyl hydrolases reveal a highly coordinated pathway for rhodopsin biosynthesis and N-glycan trimming in Drosophila melanogaster

As newly synthesized glycoproteins move through the secretory pathway, the asparagine-linked glycan (N-glycan) undergoes extensive modifications involving the sequential removal and addition of sugar residues. These modifications are critical for the proper assembly, quality control and transport of glycoproteins during biosynthesis. The importance of N-glycosylation is illustrated by a growing list of diseases that result from defects in the biosynthesis and processing of N-linked glycans. The major rhodopsin in Drosophila melanogaster photoreceptors, Rh1, is highly unique among glycoproteins, as the N-glycan appears to be completely removed during Rh1 biosynthesis and maturation. However, much of the deglycosylation pathway for Rh1 remains unknown. To elucidate the key steps in Rh1 deglycosylation in vivo, we characterized mutant alleles of four Drosophila glycosyl hydrolases, namely α-mannosidase-II (α-Man-II), α-mannosidase-IIb (α-Man-IIb), a β-N-acetylglucosaminidase called fused lobes (Fdl), and hexosaminidase 1 (Hexo1). We have demonstrated that these four enzymes play essential and unique roles in a highly coordinated pathway for oligosaccharide trimming during Rh1 biosynthesis. Our results reveal that α-Man-II and α-Man-IIb are not isozymes like their mammalian counterparts, but rather function at distinct stages in Rh1 maturation. Also of significance, our results indicate that Hexo1 has a biosynthetic role in N-glycan processing during Rh1 maturation. This is unexpected given that in humans, the hexosaminidases are typically lysosomal enzymes involved in N-glycan catabolism with no known roles in protein biosynthesis. Here, we present a genetic dissection of glycoprotein processing in Drosophila and unveil key steps in N-glycan trimming during Rh1 biosynthesis. Taken together, our results provide fundamental advances towards understanding the complex and highly regulated pathway of N-glycosylation in vivo and reveal novel insights into the functions of glycosyl hydrolases in the secretory pathway.

Retinitis pigmentosa-associated rhodopsin mutations in three membrane-located cysteine residues present three different biochemical phenotypes

A large number of mutations in rhodopsin are associated with autosomal dominant retinitis pigmentosa (ADRP). We analyzed the biochemical phenotypes of the ADRP-associated cysteine mutants C167R, C222R, and C264del. C222R behaved as wild type in every aspect testable and is classified as a class I mutant. C167R produced intact protein but did not regenerate with 11-cis retinal and was not transported to the plasma membrane. We confirm its classification as a class IIa mutant. C264del represents a novel phenotype, which we propose to call class III. It produced a truncated protein of 27kDa that failed to regenerate with 11-cis retinal and was not targeted to the plasma membrane.

Genetic variations and polymorphisms of G protein-coupled receptors: functional and therapeutic implications

G protein-coupled receptors (GPCRs) represent a major class of proteins in the genome of many species, including humans. In addition to the mapping of a number of human disorders to regions of the genome containing GPCRs, a growing body of literature has documented frequently occurring variations (i.e. polymorphisms) in GPCR loci. In this article, we use a domain-based approach to systematically examine examples of genetic variation in the coding and noncoding regions of GPCR loci. Data to date indicate that residues in GPCRs are involved in ligand binding and coupling to G proteins and that regulation can be altered by polymorphisms. Studies of GPCR polymorphisms have also uncovered the functional importance of residues not previously implicated from other approaches that are involved in the function of GPCRs. We predict that studies of GPCR polymorphisms will have a significant impact on medicine and pharmacology, in particular, by providing new means to subclassify patients in terms of both diagnosis and treatment.

Role of asparagine-linked oligosaccharides in rhodopsin maturation and association with its molecular chaperone, NinaA

Many proteins require N-linked glycosylation for conformational maturation and interaction with their molecular chaperones. In Drosophila, rhodopsin (Rh1), the most abundant rhodopsin, is glycosylated in the endoplasmic reticulum (ER) and requires its molecular chaperone, NinaA, for exit from the ER and transport through the secretory pathway. Studies of vertebrate rhodopsins have generated several conflicting proposals regarding the role of glycosylation in rhodopsin maturation. We investigated the role of Rh1 glycosylation and Rh1/NinaA interactions under in vivo conditions by analyzing transgenic flies expressing Rh1 with isoleucine substitutions at each of the two consensus sites for N-linked glycosylation (N20I and N196I). We show that Asn(20) is the sole site for glycosylation. The Rh1(N20I) protein is retained within the secretory pathway, causing an accumulation of ER cisternae and dilation of the Golgi complex. NinaA associates with nonglycosylated Rh1(N20I); therefore, retention of nonglycosylated rhodopsin within the ER is not due to the lack of Rh1(N20I)/NinaA interaction. We further show that Rh1(N20I) interferes with wild type Rh1 maturation and triggers a dominant form of retinal degeneration. We conclude that during maturation Rh1 is present in protein complexes containing NinaA and that Rh1 glycosylation is required for transport of the complexes through the secretory pathway. Failure of this transport process leads to retinal degeneration.

Retinal degeneration in retinitis pigmentosa and neuronal ceroid lipofuscinosis: An overview

Retinal degeneration is an early consequence of the group of lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). This review details specialized techniques that have evolved for retinal assessment in patients with hereditary retinal degeneration. A standard ERG protocol is described for assessing rod- and cone-mediated function. Standardization will be crucial for planning and implementing multicenter trials as rational therapeutic intervention becomes available. In recent years, there has been a dramatic increase in knowledge of the molecular biological bases of retinitis pigmentosa and allied retinal degenerations. Rather than attempting a comprehensive summary, this review stresses the concepts of genetic, allelic, and clinical heterogeneity, which have obvious parallels in the NCLs. Many of the mutations that cause retinal degeneration are in genes that encode photoreceptor cascade proteins; others are in genes that encode photoreceptor structural proteins. Recent advances in linking the retinal degeneration slow (RDS) and ATP-binding cassette transporter retina (ABCR) genes to a variety of disease phenotypes will be summarized. Clinical heterogeneity even among family members with the same mutation raises the possibility that modifying factors, either genetic or environmental, could influence the severity of the disease. Here, we focus on vitamin A and docosahexaenoic acid, two potential nutritional modifiers that have received considerable attention in recent years.

Detection of alterations in all three exons of the peripherin/RDS gene in Swedish patients with retinitis pigmentosa using an efficient DGGE system

AIMS

To develop a sensitive mutation screening procedure suitable for routine analysis of the peripherin/RDS gene, and to estimate the nature and prevalence of peripherin/RDS gene mutations in Swedish patients with autosomal dominant retinitis pigmentosa.

METHODS

To make the method as sensitive as possible, as many as eight segments, covering the three exons and the flanking intron sequences of the peripherin/RDS gene, were analysed by denaturing gradient gel electrophoresis. A group of 38 Swedish patients with a clinical diagnosis of autosomal dominant retinitis pigmentosa were screened for mutations in the peripherin/RDS gene.

RESULTS

Three point mutations were found in four of the patients and five polymorphisms were defined. One mutation in exon 1, R172W, has been described previously in other ethnic groups as causing a macular degeneration. Another mutation, in exon 2 and causing the substitution F211L, was found in two unrelated patients. A third mutation, resulting in the likely non-pathogenic substitution S289L, as well as a polymorphism not reported previously, was found in exon 3.

CONCLUSIONS

The screening procedure described allows detection of mutations in all of the exons, including the polymorphic 5' and 3' ends of the gene, and is therefore suitable for routine screening of peripherin/RDS gene defects in patients with autosomal dominant retinitis pigmentosa. The frequency of mutations found in the Swedish patient group indicates that defects in the peripherin/RDS gene might be a more common cause of autosomal dominant retinitis pigmentosa than was thought previously.

Effect of vitamin A supplementation on rhodopsin mutants threonine-17 --> methionine and proline-347 --> serine in transgenic mice and in cell cultures

A therapeutic effect of vitamin A supplementation on the course of photoreceptor degeneration, previously reported for patients with retinitis pigmentosa, was tested in two transgenic mouse models of this disease, each carrying a dominant rhodopsin mutation. The threonine-17 --> methionine (T17M) mutation is a class II rhodopsin mutation, characterized by a thermal instability/folding defect and minimal regeneration with the chromophore. The proline-347 --> serine (P347S) mutation belongs to class I, comprised of a smaller number of mutations that exhibit no recognized biochemical abnormality in vitro. In the present study, each of the two mouse models was fed a diet containing 2.5 mg of vitamin A palmitate (control) or 102.5 mg of vitamin A palmitate (high vitamin A) per kilogram of diet. Dark-adapted, full-field electroretinograms showed that the high vitamin A diet significantly reduced the rate of decline of a-wave and b-wave amplitudes in the T17M mice but had no significant effect on the decline of electroretinogram amplitude in the P347S mice. Correspondingly, histologic evaluation revealed that the treatment was associated with significantly longer photoreceptor inner and outer segments and a thicker outer nuclear layer in the T17M mice but had no effect on photoreceptor morphology in the P347S mice. In a separate series of experiments, the instability defect of the T17M mutant opsin expressed in vitro was partially alleviated by inclusion of 11-cis-retinal in the culture media. These results show that vitamin A supplementation slows the rate of photoreceptor degeneration caused by a class II rhodopsin mutation. Vitamin A supplementation may confer therapeutic benefit by stabilizing mutant opsins through increased availability of the chromophore.

Disease sequence from mutant rhodopsin allele to rod and cone photoreceptor degeneration in man

Mutations in the gene encoding rhodopsin, the visual pigment in rod photoreceptors, lead to retinal degeneration in species from Drosophila to man. The pathogenic sequence from rod cell-specific mutation to degeneration of rods and cones remains unclear. To understand the disease process in man, we studied heterozygotes with 18 different rhodopsin gene mutations by using noninvasive tests of rod and cone function and retinal histopathology. Two classes of disease expression were found, and there was allele-specificity. Class A mutants lead to severely abnormal rod function across the retina early in life; topography of residual cone function parallels cone cell density. Class B mutants are compatible with normal rods in adult life in some retinal regions or throughout the retina, and there is a slow stereotypical disease sequence. Disease manifests as a loss of rod photoreceptor outer segments, not singly but in microscopic patches that coalesce into larger irregular areas of degeneration. Cone outer segment function remains normal until >75% of rod outer segments are lost. The topography of cone loss coincides with that of rod loss. Most class B mutants show an inferior-nasal to superior-temporal retinal gradient of disease vulnerability associated with visual cycle abnormalities. Class A mutant alleles behave as if cytotoxic; class B mutants can be relatively innocuous and epigenetic factors may play a major role in the retinal degeneration.

Light, sight and fight for insight

The visual processes in the human retina begin with absorption of photons from light. This light energy is converted into electrical stimuli in a series of enzymatic steps which initiates neural responses to light. Therefore, any structural and functional abnormalities in these molecules will likely interfere with signal transduction which may ultimately lead to blindness. A new era began in 1990 with an ultimate goal of redefining and developing new treatments for the photoreceptor disorders by identifying mutations in the genes encoding phototransduction cascade enzymes. As a result of this intense investigation around the world, mutations have now been identified in eight genes, in several retinal dystrophies. Almost all of these genes encode signal transduction enzymes and all are highly expressed in photoreceptor cells. This effort has been further aided by gene disruption technology. Although there are many puzzles that need to be solved, these approaches have given some insight into the genetic eye disorders and will undoubtedly improve our understanding of inherited eye disorders in the future. This improved knowledge may eventually lead to prevention or a cure.

Gene-based approach to human gene-phenotype correlations

Elucidating the genetic basis of human phenotypes is a major goal of contemporary geneticists. Logically, two fundamental and contrasting approaches are available, one that begins with a phenotype and concludes with the identification of a responsible gene or genes; the other that begins with a gene and works toward identifying one or more phenotypes resulting from allelic variation of it. This paper provides a conceptual overview of phenotype-based vs. gene-based procedures with emphasis on gene-based methods. A key feature of a gene-based approach is that laboratory effort first is devoted to developing an assay for mutations in the gene under regard; the assay then is applied to the evaluation of large numbers of unrelated individuals with a variety of phenotypes that are deemed potentially resulting from alleles at the gene. No effort is directed toward chromosomally mapping the loci responsible for the phenotypes scanned. Example is made of my laboratory's successful use of a gene-based approach to identify genes causing hereditary diseases of the retina such as retinitis pigmentosa. Reductions in the cost and improvements in the speed of scanning individuals for DNA sequence anomalies may make a gene-based approach an efficient alternative to phenotype-based approaches to correlating genes with phenotypes.

Evaluation of recombinant adeno-associated virus as a gene transfer vector for the retina

PURPOSE

To evaluate recombinant adeno-associated virus (AAV) as an in vivo gene transfer vector for the retina.

METHODS

A recombinant AAV, vCMVp-lacZ, in which the bacterial beta-galactosidase reporter gene (lacZ) was placed under the control of a cytomegalovirus (CMV) early promoter, was injected into the vitreous body or the subretinal space of mouse eyes. The reporter gene expression was followed by histochemical analyses from 10 to 100 days post-injection. The effect of several variables on the extent of AAV-mediated gene transfer was examined, including routes of delivery, presence of an underlying mutation that caused retinal degeneration, and prior treatment with hydroxyurea.

RESULTS

As measured by reporter gene expression, the AAV vector mediated gene transfer to three major cell types in the retina: the retinal pigment epithelium (RPE), ganglion cells and photoreceptor cells. Following a single injection, more than half of the total retinal areas were typically positive for gene transfer. Reporter gene expression was stable for at least 3 months, the farthest time point examined. Gene transfer to photoreceptor cells was observed only following subretinal delivery, and was greatly enhanced in mice undergoing early retinal degeneration. Cells in the inner nuclear layer were rarely transduced. Systemic administration of a genotoxic drug, hydroxyurea, 2 days prior to AAV delivery did not affect the patterns and extent of reporter gene expression. There was minimal histopathology associated with AAV transduction in the retinas of recipient mice, as determined by light microscopy.

CONCLUSION

Recombinant AAV mediates efficient gene transfer to RPE and ganglion cells, and to photoreceptor cells under certain conditions. Persistence of transgene expression is of long duration and without apparent histopathology. The greater stability, lower cytopathicity, and the ability to transduce retinal ganglion cells are three distinct features of the AAV vector compared to current adenovirus-based vectors.

Rhodopsin replacement rescues photoreceptor structure during a critical developmental window

Rhodopsin is essential for normal photoreceptor development in Drosophila (O'Tousa et al., 1989; Leonard et al., 1992; Kumar and Ready, 1995) and in mice (Humphries et al., 1997). Here we report studies in which a rhodopsin transgene is expressed at restricted stages during the development of Drosophila photoreceptors otherwise lacking rhodopsin. Substantial rescue of normal photoreceptor structure and physiology is effected by rhodopsin expression during the time of the normal onset of rhodopsin synthesis. Expression shortly before or after this critical period does not rescue these deficits. There is a critical developmental period in which rhodopsin plays its key role in photoreceptor morphogenesis.

Efficient photoreceptor-targeted gene expression in vivo by recombinant adeno-associated virus

We describe a general approach for achieving efficient and cell type-specific expression of exogenous genes in photoreceptor cells of the mammalian retina. Recombinant adeno-associated virus (rAAV) vectors were used to transfer the bacterial lacZ gene or a synthetic green fluorescent protein gene (gfp) to mouse or rat retinas after injection into the subretinal space. Using a proximal murine rod opsin promoter (+86 to -385) to drive expression, reporter gene product was found exclusively in photoreceptors, not in any other retinal cell type or in the adjacent retinal pigment epithelium. GFP-expressing photoreceptors typically encompassed 10-20% of the total retinal area after a single 2-microl injection. Photoreceptors were transduced with nearly 100% efficiency in the region directly surrounding the injection site. We estimate approximately 2.5 million photoreceptors were transduced as a result of the single subretinal inoculation. This level of gene transfer and expression suggests the feasibility of genetic therapy for retinal disease. The gfp-containing rAAV stock was substantially free of both adenovirus and wild-type AAV, as judged by plaque assay and infectious center assay, respectively. Thus, highly purified, helper virus-free rAAV vectors can achieve high-frequency tissue-specific transduction of terminally differentiated, postmitotic photoreceptor cells.

Transgenic mice carrying the dominant rhodopsin mutation P347S: evidence for defective vectorial transport of rhodopsin to the outer segments

To explore the pathogenic mechanism of dominant mutations affecting the carboxyl terminus of rhodopsin that cause retinitis pigmentosa, we generated five lines of transgenic mice carrying the proline-347 to serine (P347S) mutation. The severity of photoreceptor degeneration correlated with the levels of transgene expression in these lines. Visual function as measured by the electroretinogram was approximately normal at an early age when there was little histologic evidence of photoreceptor degeneration, but it deteriorated as photoreceptors degenerated. Immunocytochemical staining showed the mutant rhodopsin predominantly in the outer segments prior to histologically evident degeneration, a finding supported by quantitation of signal intensities in different regions of the photoreceptor cells by confocal microscopy. A distinct histopathologic abnormality was the accumulation of submicrometer-sized vesicles extracellularly near the junction between inner and outer segments. The extracellular vesicles were bound by a single membrane that apparently contained rhodopsin as revealed by ultrastructural immunocytochemical staining with anti-rhodopsin antibodies. The outer segments, although shortened, contained well-packed discs. Proliferation of the endoplasmic reticulum as reported in Drosophila expressing dominant rhodopsin mutations was not observed. The accumulation of rhodopsinladen vesicles likely represents aberrant transport of rhodopsin from the inner segments to the nascent disc membranes of the outer segments. It is possible that photoreceptor degeneration occurs because of a failure to renew outer segments at a normal rate, thereby leading to a progressive shortening of outer segments, or because of the loss of cellular contents to the extracellular space, or because of both.

Defective subunit assembly underlies a digenic form of retinitis pigmentosa linked to mutations in peripherin/rds and rom-1

Retinitis pigmentosa (RP) is a group of progressive retinal dystrophies that include the most common hereditary degenerative disease affecting the retina. Although most disease phenotypes appear to result from defects at single genetic loci (monogenic), at least one instance of RP appears to require a coinheritance of defects in the unlinked peripherin/rds and rom-1 alleles (digenic), which encode the polypeptide subunits of an oligomeric transmembrane protein complex present at photoreceptor outer segment disc rims. Sedimentation velocity analysis was performed upon the affected gene products expressed heterologously in COS-1 cells to examine the assembly of the subunit polypeptides. The results indicate that the missense peripherin/rds mutant, L185P, which segregates with instance of digenically inherited RP, is conditionally defective with respect to its subunit assembly. Unlike wild-type peripherin/rds, the L185P mutant does not form native-like homotetramers on its own; however, the L185P mutant can assemble with wild-type rom-1 to form a structurally normal heterotetrameric complex. These findings provide a novel molecular-based rationale for the unusual digenic disease inheritance pattern and offer insight into regions of peripherin/rds and rom-1, which contribute to subunit-subunit interactions.

Intraocular light scatter in patients with retinitis pigmentosa

We evaluated the level of intraocular light scatter in a group of patients with retinitis pigmentosa (RP) who had minimal or no lens opacities, since such patients not infrequently complain of photoaversion. Intraocular light scatter was measured in 20 patients with RP who were < 60 years of age and who had no more than a trace of posterior subcapsular (PSC) lens opacity by slit-lamp evaluation. Measurements of intraocular straylight were made using a van den Berg Straylightmeter. Results from the patients with RP were compared with those of a control group of 30 subjects with normal vision whose ages were similar to those of the patients with RP. Seventeen of the 20 patients with RP had straylight levels that were above the range of age-similar normal control subjects. In some patients, the straylight parameter was increased by a factor of 2.5 above the normal mean for the patient's age and by as much as four to five times the normal mean for 20-yr-old subjects. There was a statistically significant correlation (r = -0.73, P < 0.01) between the patients' log relative elevation in the straylight parameter and their log visual field areas. Our findings indicate that patients with RP can have increased levels of intraocular light scatter despite minimal or no clinically observable PSC lens opacities. The increased intraocular straylight, which is likely due at least in part to subclinical abnormalities in lens morphology, can accentuate the visual disability of patients with RP in the presence of glare sources.

Inherited retinal degeneration: exceptional genetic and clinical heterogeneity

The function of the retina is to detect light and to send appropriate signals to the brain in response. Inherited diseases that cause the retina to degenerate, leading to either partial or total blindness, affect approximately 1 in 3000 people. Rapid progress is being made in identifying the genetic causes of common, inherited retinal diseases, such as retinitis pigmentosa and macular degeneration, as well as some of the rare forms of retinal disease. Linkage studies of large families and candidate-gene screening of known retinal genes have already identified 59 independent genetic loci that can cause retinal degeneration. The astounding genetic and clinical heterogeneity that is being revealed is a 'nightmare' for those interested in molecular diagnostics but, at the same time, provides great insight into functional aspects of the normal retina.

Metabolism of omega-3 fatty acids in patients with autosomal dominant retinitis pigmentosa

Abnormalities in lipid metabolism have been reported in numerous patients with retinitis pigmentosa. As an initial step in evaluating these anomalies, two trials of fatty acid intervention were conducted with autosomal dominant retinitis pigmentosa (adRP) patients and controls. The first trial addressed absorption and incorporation of omega 3 long-chain fatty acids from a fish-oil concentrate into red blood cell (RBC) lipids. The utilization of omega 3 long-chain fatty acids by adRP patients was found to be equivalent to that of controls. The second trial addressed the conversion of precursor, eicosapentaenoic acid (EPA, 20:5 omega 3), to end-product, docosahexaenoic acid (DHA, 22:6 omega 3), following oral supplementation of EPA ethyl ester. Although the levels of EPA and the intermediate, docosapentaenoic acid (22:5 omega 3), were both elevated by EPA supplementation in RBCs of adRP patients with rhodopsin gene mutations and controls, DHA production was elevated only in controls. Based on these results, we suggest the presence of a metabolic defect in the final stages of DHA biosynthesis.