A rare variant in CFH directly links age-related macular degeneration with rare glomerular nephropathies

A careful analysis of risk haplotypes in relation to age-related macular degeneration (AMD) susceptibility has led to the identification of a rare, high-penetrance variant in the complement factor H (CFH) gene that is also causally associated with atypical hemolytic uremic syndrome (aHUS) and related glomerulopathies. This finding provides a convincing causal mechanism linking the two diseases and develops a paradigm for the genetic architecture of a common and complex disease.     

A common CFH haplotype, with deletion of CFHR1 and CFHR3, is associated with lower risk of age-related macular degeneration

Age-related macular degeneration (AMD; OMIM #603075) is the most frequent cause of visual impairment in the elderly population, with severe disease affecting nearly 10% of individuals of European descent over the age of 75 years. It is a complex disease in which genetic and environmental factors contribute to susceptibility. Complement factor H (CFH) has recently been identified as a major AMD susceptibility gene, and the Y402H polymorphism has been proposed as the likely causative factor. We genotyped polymorphisms spanning the cluster of CFH and five CFH-related genes on chromosome 1q23 in 173 individuals with severe neovascular AMD and 170 elderly controls with no signs of AMD. Detailed analysis showed a common haplotype associated with decreased risk of AMD that was present on 20% of chromosomes of controls and 8% of chromosomes of individuals with AMD. We found that this haplotype carried a deletion of CFHR1 and CFHR3, and the proteins encoded by these genes were absent in serum of homozygotes. The protective effect of the deletion haplotype cannot be attributed to linkage disequilibrium with Y402H and was replicated in an independent sample.     

Biochemical defects in ABCR protein variants associated with human retinopathies

Mutations in the gene encoding ABCR (ABCA4), a photoreceptor-specific ATP-binding cassette (ABC) transporter, are responsible for autosomal recessive Stargardt disease (STGD), an early onset macular degeneration, and some forms of autosomal recessive cone-rod dystrophy and autosomal recessive retinitis pigmentosa. Heterozygosity for ABCA4 mutations may also represent a risk factor for age-related macular degeneration (AMD), although this idea is controversial. An ongoing challenge in the analysis of ABCA4-based retinopathies arises from the observation that most of the ABCA4 sequence variants identified so far are missense mutations that are rare in both patient and control populations. With the current sample size of most sequence variants, one cannot determine statistically whether a particular sequence variant is pathogenic or neutral. A related challenge is to determine the degree to which each pathogenic variant impairs ABCR function, as genotype-phenotype analyses indicate that age of onset and disease severity correlate with different ABCA4 alleles. To address these questions, we performed a functional analysis of human ABCR and its variants. These experiments reveal a wide spectrum of biochemical defects in these variants and provide insight into the transport mechanism of ABCR.     

Variation in factor B (BF) and complement component 2 (C2) genes is associated with age-related macular degeneration

Age-related macular degeneration (AMD) is the most common form of irreversible blindness in developed countries. Variants in the factor H gene (CFH, also known as HF1), which encodes a major inhibitor of the alternative complement pathway, are associated with the risk for developing AMD. Here we test the hypothesis that variation in genes encoding other regulatory proteins of the same pathway is associated with AMD. We screened factor B (BF) and complement component 2 (C2) genes, located in the major histocompatibility complex class III region, for genetic variation in two independent cohorts comprising approximately 900 individuals with AMD and approximately 400 matched controls. Haplotype analyses identify a statistically significant common risk haplotype (H1) and two protective haplotypes. The L9H variant of BF and the E318D variant of C2 (H10), as well as a variant in intron 10 of C2 and the R32Q variant of BF (H7), confer a significantly reduced risk of AMD (odds ratio = 0.45 and 0.36, respectively). Combined analysis of the C2 and BF haplotypes and CFH variants shows that variation in the two loci can predict the clinical outcome in 74% of the affected individuals and 56% of the controls. These data expand and refine our understanding of the genetic risk for AMD.     

A 5-bp deletion in ELOVL4 is associated with two related forms of autosomal dominant macular dystrophy

Stargardt-like macular dystrophy (STGD3, MIM 600110) and autosomal dominant macular dystrophy (adMD) are inherited forms of macular degeneration characterized by decreased visual acuity, macular atrophy and extensive fundus flecks. Genetic mapping data suggest that mutations in a single gene may be responsible for both conditions, already known to bear clinical resemblance. Here we limit the minimum genetic region for STGD3 and adMD to a 0.6-cM interval by recombination breakpoint mapping and identify a single 5-bp deletion within the protein-coding region of a new retinal photoreceptor-specific gene, ELOVL4, in all affected members of STGD3 and adMD families. Bioinformatic analysis of ELOVL4 revealed that it has homology to a group of yeast proteins that function in the biosynthesis of very long chain fatty acids. Our results are therefore the first to implicate the biosynthesis of fatty acids in the pathogenesis of inherited macular degeneration.     

Variation in complement factor 3 is associated with risk of age-related macular degeneration

The association of variants in complement factors H and B with age-related macular degeneration has led to more intense genetic and functional analysis of the complement pathway. We identify a nonsynonymous coding change in complement factor 3 that is strongly associated with risk of age-related macular degeneration in a large case-control sample.     

Bayesian inference of epistatic interactions in case-control studies

Epistatic interactions among multiple genetic variants in the human genome may be important in determining individual susceptibility to common diseases. Although some existing computational methods for identifying genetic interactions have been effective for small-scale studies, we here propose a method, denoted 'bayesian epistasis association mapping' (BEAM), for genome-wide case-control studies. BEAM treats the disease-associated markers and their interactions via a bayesian partitioning model and computes, via Markov chain Monte Carlo, the posterior probability that each marker set is associated with the disease. Testing this on an age-related macular degeneration genome-wide association data set, we demonstrate that the method is significantly more powerful than existing approaches and that genome-wide case-control epistasis mapping with many thousands of markers is both computationally and statistically feasible.     

Common variation in three genes, including a noncoding variant in CFH, strongly influences risk of age-related macular degeneration

Age-related macular degeneration (AMD) is a common, late-onset disease with seemingly typical complexity: recurrence ratios for siblings of an affected individual are three- to sixfold higher than in the general population, and family-based analysis has resulted in only modestly significant evidence for linkage. In a case-control study drawn from a US-based population of European descent, we have identified a previously unrecognized common, noncoding variant in CFH, the gene encoding complement factor H, that substantially increases the influence of this locus on AMD, and we have strongly replicated the associations of four other previously reported common alleles in three genes (P values ranging from 10(-6) to 10(-70)). Despite excellent power to detect epistasis, we observed purely additive accumulation of risk from alleles at these genes. We found no differences in association of these loci with major phenotypic categories of advanced AMD. Genotypes at these five common SNPs define a broad spectrum of interindividual disease risk and explain about half of the classical sibling risk of AMD in our study population.     

Mutations in a novel retina-specific gene cause autosomal dominant retinitis pigmentosa.

Inherited retinal diseases are a common cause of visual impairment in children and young adults, often resulting in severe loss of vision in later life. The most frequent form of inherited retinopathy is retinitis pigmentosa (RP), with an approximate incidence of 1 in 3,500 individuals worldwide. RP is characterized by night blindness and progressive degeneration of the midperipheral retina, accompanied by bone spicule-like pigmentary deposits and a reduced or absent electroretinogram (ERG). The disease process culminates in severe reduction of visual fields or blindness. RP is genetically heterogeneous, with autosomal dominant, autosomal recessive and X-linked forms. Here we have identified two mutations in a novel retina-specific gene from chromosome 8q that cause the RP1 form of autosomal dominant RP in three unrelated families. The protein encoded by this gene is 2,156 amino acids and its function is currently unknown, although the amino terminus has similarity to that of the doublecortin protein, whose gene (DCX) has been implicated in lissencephaly in humans. Two families have a nonsense mutation in codon 677 of this gene (Arg677stop), whereas the third family has a nonsense mutation in codon 679 (Gln679stop). In one family, two individuals homozygous for the mutant gene have more severe retinal disease compared with heterozygotes.     

Hypotrichosis with juvenile macular dystrophy is caused by a mutation in CDH3, encoding P-cadherin.

Congenital hypotrichosis associated with juvenile macular dystrophy (HJMD; MIM601553) is an autosomal recessive disorder of unknown etiology, characterized by hair loss heralding progressive macular degeneration and early blindness. We used homozygosity mapping in four consanguineous families to localize the gene defective in HJMD to 16q22.1. This region contains CDH3, encoding P-cadherin, which is expressed in the retinal pigment epithelium and hair follicles. Mutation analysis shows in all families a common homozygous deletion in exon 8 of CDH3. These results establish the molecular etiology of HJMD and implicate for the first time a cadherin molecule in the pathogenesis of a human hair and retinal disorder.     

Statistical false positive or true disease pathway?

Three very recent reports provide convincing statistical evidence (P < 10(-8)), at a genome-wide level, of the association of common polymorphisms with three different common diseases: systemic lupus erythematosus (IRF5), prostate cancer and type 1 diabetes (IFIH1 region). This adds to the trickle--soon to be a flood--of disease association results that are highly unlikely to be false positives. There are other convincing examples in the last 12 months: age-related macular degeneration (CFH), type 1 diabetes (IL2RA, also known as CD25) and type 2 diabetes (TCF7L2). Given 20 years of a literature full of irreproducible results, what has changed?     

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.     

Genetic linkage of vitelliform macular degeneration (Best's disease) to chromosome 11q13.

Macular degeneration is the most common cause of legal blindness in older patients in developed countries. Best's vitelliform dystrophy is an early-onset, autosomal dominant form of macular degeneration characterized by an egg-yolk-like collection of lipofuscin beneath the pigment epithelium of the retinal macula. Fifty-seven members of a five-generation family affected with this disease were studied. A combination of ophthalmoscopy and electro-oculography was used for diagnosis; 29 patients were found to be affected and 16 unaffected. Linkage analysis mapped the disease-causing gene to chromosome 11q13. Three markers in this region were found to be significantly linked (Zmax > 3.0) to the disease. Multipoint analysis yielded a maximum Lod score of 9.3 in the interval between markers INT2 and D11S871.     

Mutation of CDH23, encoding a new member of the cadherin gene family, causes Usher syndrome type 1D

Usher syndrome type I (USH1) is an autosomal recessive disorder characterized by congenital sensorineural hearing loss, vestibular dysfunction and visual impairment due to early onset retinitis pigmentosa (RP). So far, six loci (USH1A-USH1F) have been mapped, but only two USH1 genes have been identified: MYO7A for USH1B and the gene encoding harmonin for USH1C. We identified a Cuban pedigree linked to the locus for Usher syndrome type 1D (MIM 601067) within the q2 region of chromosome 10). Affected individuals present with congenital deafness and a highly variable degree of retinal degeneration. Using a positional candidate approach, we identified a new member of the cadherin gene superfamily, CDH23. It encodes a protein of 3,354 amino acids with a single transmembrane domain and 27 cadherin repeats. In the Cuban family, we detected two different mutations: a severe course of the retinal disease was observed in individuals homozygous for what is probably a truncating splice-site mutation (c.4488G-->C), whereas mild RP is present in individuals carrying the homozygous missense mutation R1746Q. A variable expression of the retinal phenotype was seen in patients with a combination of both mutations. In addition, we identified two mutations, Delta M1281 and IVS51+5G-->A, in a German USH1 patient. Our data show that different mutations in CDH23 result in USH1D with a variable retinal phenotype. In an accompanying paper, it is shown that mutations in the mouse ortholog cause disorganization of inner ear stereocilia and deafness in the waltzer mouse.     

Mutations in NYX, encoding the leucine-rich proteoglycan nyctalopin, cause X-linked complete congenital stationary night blindness

During development, visual photoreceptors, bipolar cells and other neurons establish connections within the retina enabling the eye to process visual images over approximately 7 log units of illumination. Within the retina, cells that respond to light increment and light decrement are separated into ON- and OFF-pathways. Hereditary diseases are known to disturb these retinal pathways, causing either progressive degeneration or stationary deficits. Congenital stationary night blindness (CSNB) is a group of stable retinal disorders that are characterized by abnormal night vision. Genetic subtypes of CSNB have been defined and different disease actions have been postulated. The molecular bases have been elucidated in several subtypes, providing a better understanding of the disease mechanisms and developmental retinal neurobiology. Here we have studied 22 families with 'complete' X-linked CSNB (CSNB1; MIM 310500; ref. 4) in which affected males have night blindness, some photopic vision loss and a defect of the ON-pathway. We have found 14 different mutations, including 1 founder mutation in 7 families from the United States, in a novel candidate gene, NYX. NYX, which encodes a glycosylphosphatidyl (GPI)-anchored protein called nyctalopin, is a new and unique member of the small leucine-rich proteoglycan (SLRP) family. The role of other SLRP proteins suggests that mutant nyctalopin disrupts developing retinal interconnections involving the ON-bipolar cells, leading to the visual losses seen in patients with complete CSNB.     

Rescue of embryonic lethality in Mdm4-null mice by loss of Trp53 suggests a nonoverlapping pathway with MDM2 to regulate p53

The relationship between the neurosensory photoreceptors and the adjacent retinal pigment epithelium (RPE) controls not only normal retinal function, but also the pathogenesis of hereditary retinal degenerations. The molecular bases for both primary photoreceptor and RPE diseases that cause blindness have been identified. Gene therapy has been used successfully to slow degeneration in rodent models of primary photoreceptor diseases, but efficacy of gene therapy directed at photoreceptors and RPE in a large-animal model of human disease has not been reported. Here we study one of the most clinically severe retinal degenerations, Leber congenital amaurosis (LCA). LCA causes near total blindness in infancy and can result from mutations in RPE65 (LCA, type II; MIM 180069 and 204100). A naturally occurring animal model, the RPE65-/- dog, suffers from early and severe visual impairment similar to that seen in human LCA. We used a recombinant adeno-associated virus (AAV) carrying wild-type RPE65 (AAV-RPE65) to test the efficacy of gene therapy in this model. Our results indicate that visual function was restored in this large animal model of childhood blindness.