Title: A Novel Locus for Leber Congenital Amaurosis Maps to Chromosome 6q
Abstract: To the Editor: Leber congenital amaurosis (LCA) (MIM 204000/204100) is a clinically and genetically heterogeneous retinal disorder that occurs in infancy and is accompanied by profound visual loss, nystagmus, poor pupillary reflexes, and either a normal retina or varying degrees of atrophy and pigmentary changes (Leber Leber, 1869Leber T Über retinitis pigmentosa und angeborene amaurose.Albrecht von Graefe's Arch Klin Exp Ophthalmol. 1869; 15: 1-25Crossref Scopus (172) Google Scholar, Leber, 1871Leber T Über anormale formen der retinitis pigmentosa.Arch für Ophthalmol. 1871; 17: 314-341Google Scholar; François François, 1968François J Leber's congenital tapeto-retinal degeneration.Int Ophthalmol Clin. 1968; 8: 929-947Crossref PubMed Google Scholar). The electroretinogram (ERG) is extinguished or severely reduced (Franceschetti Franceschetti and Dieterle, 1954Franceschetti A Dieterle P L'importance diagnostique de l'electrorétinogramme dans les dégénérescences tapéto-rétinennes avec rétrécissement du champ visuel et héméralopie.Conf Neurol. 1954; 14: 184-186Crossref PubMed Scopus (85) Google Scholar). LCA is largely a recessive disease, although autosomal dominant pedigrees have been identified (Sorsby et al. Sorsby and Williams, 1960Sorsby A Williams CE Retinal aplasia as a clinical entity.Br Med J. 1960; 1: 293-297Crossref PubMed Scopus (40) Google Scholar; Heckenlively Heckenlively, 1988Heckenlively JR Retinitis pigmentosa. Philadelphia, Lippincott1988: 125-149Google Scholar). To date, three genes for LCA have been identified and sequenced: retinal guanylate cyclase (GUCY2D) on chromosome 17p13; retinal pigment epithelium protein (RPE65) on chromosome 1p31; and cone-rod homeobox (CRX) on chromosome 19q13.3. One additional locus has been identified on chromosome 14q24 (Stockton et al. Stockton et al., 1998Stockton DW Lewis RA Abboud EB Rejhi-Manzer AA Anderson KH Lupski JR A novel locus for Leber congenital amaurosis on chromosome 14q24.Hum Genet. 1998; 103: 328-333Crossref PubMed Scopus (63) Google Scholar). We show evidence for linkage to chromosome 6q11-16 in a multigenerational kindred of Old Order River Brethren. The disease gene maps to a 23-cM interval flanked by DNA polymorphic markers D6S1551 and D6S1694, with a maximum two-point LOD score of 3.38 (recombination fraction [θ] zero) at D6S391. Two candidate genes on chromosome 6 were screened for mutations: gamma aminobutyric acid rho1 and rho2 (GABRR1 and GABRR2) at 6q14-21 (Cutting et al. Cutting et al., 1992Cutting GR Curristin S Zoghbi H O'Hara B Seldin MF Uhl GR Identification of a putative gamma-aminobutyric acid receptor subunit rho 2 cDNA and localization of the genes rho 2 (GABRR2) and rho 1 (GABRR1) to human chromosome 6q14-q21 and mouse chromosome 4.Genomics. 1992; 12: 801-806Crossref PubMed Scopus (171) Google Scholar), and interphotoreceptor matrix proteoglycan (IMPG1) at 6q13-15 (Gehrig et al. Gehrig et al., 1998Gehrig A Felbor U Kelsell R Hunt DM Maumenee IH Weber BHF Assessment of the interphotoreceptor matrix proteoglycan-1 (IMPG1) localized to 6q13-q15 in autosomal dominant Stargardt-like disease (ADSTGD), progressive bifocal chorioretinal atrophy (PBCRA), and North Carolina macular dystrophy (MCDR1).J Med Genet. 1998; 35: 641-645Crossref PubMed Scopus (17) Google Scholar). The incidence of LCA is 3 in 100,000 persons and accounts for ⩾5% of all inherited retinal dystrophies (Perrault et al. Perrault et al., 1996Perrault I Rozet JM Calvas P Gerber S Camuzat A Dollfus H Chatelin S et al.Retinal-specific guanylate cyclase gene mutations in Leber's congenital amaurosis.Nat Genet. 1996; 14: 461-466Crossref PubMed Scopus (379) Google Scholar). Clinical and genetic heterogeneity have been demonstrated (Wardenburg Waardenburg, 1961Waardenburg PJ Congenital and early infantile retinal dysfunction (high-graded) amblyopia and amaurosis Leber.in: Waardenburg PJ Franceschetti A Klein D Genetics and ophthalmology. 1st ed. Blackwell Scientific, Oxford1961: 1567-1581Google Scholar; Camuzat et al. Camuzat et al., 1996Camuzat A Rozet JM Dollfus H Gerber S Perrault I Weissenbach J Munnich A et al.Evidence of genetic heterogeneity of Leber's congenital amaurosis (LCA) and mapping of LCA1 to chromosome 17p13.Hum Genet. 1996; 97: 798-801Crossref PubMed Scopus (43) Google Scholar). The phenotype has been associated with familial juvenile nephronophthisis and cone-shaped epiphyses (Saldino-Mainzer syndrome) and with kidney disease (Senior-Loken syndrome), osteoporosis, metabolic diseases, and neurological abnormalities (Loken et al. Loken et al., 1961Loken AC Hanssen O Halvolsen S Jolster NB Hereditary renal dysplasia and blindness.Acta Paediatr. 1961; 50: 177-194Crossref PubMed Scopus (116) Google Scholar; Senior et al. Senior et al., 1961Senior B Friedman AI Brando JL Juvenile familial nephropathy with tapetoretinal degeneration.Am J Ophthalmol. 1961; 52: 625-633Abstract Full Text PDF PubMed Scopus (172) Google Scholar; Dekaban Dekaban, 1969Dekaban AS Hereditary syndrome of congenital retinal blindness (Leber), polycystic kidneys and maldevelopment of the brain.Am J Ophthalmol. 1969; 68: 1029-1037Abstract Full Text PDF PubMed Scopus (88) Google Scholar; Mainzer et al. Mainzer et al., 1970Mainzer F Saldino RM Ozonoff MB Minagi H Familial nephropathy associated with retinitis pigmentosa, cerebellar ataxia and skeletal abnormalities.Am J Med. 1970; 49: 556-562Abstract Full Text PDF PubMed Scopus (105) Google Scholar; Ellis et al. Ellis et al., 1984Ellis DS Heckenlively JR Martin CL Lachman RS Sakati NA Rimoin DL Leber's congenital amaurosis associated with familial juvenile nephronophthisis and cone-shaped epiphyses of the hands (the Saldino-Mainzer syndrome).Am J Ophthalmol. 1984; 97: 233-239Abstract Full Text PDF PubMed Scopus (34) Google Scholar). The first locus for LCA was mapped to 17p13 with the use of homozygosity mapping in consanguineous families of North African descent (Camuzat et al. Camuzat et al., 1996Camuzat A Rozet JM Dollfus H Gerber S Perrault I Weissenbach J Munnich A et al.Evidence of genetic heterogeneity of Leber's congenital amaurosis (LCA) and mapping of LCA1 to chromosome 17p13.Hum Genet. 1996; 97: 798-801Crossref PubMed Scopus (43) Google Scholar). Mutations in the retina-specific guanylate cyclase gene (RETGC 1), on chromosome 17p13, involved in phototransduction, were subsequently identified (Perrault et al. Perrault et al., 1996Perrault I Rozet JM Calvas P Gerber S Camuzat A Dollfus H Chatelin S et al.Retinal-specific guanylate cyclase gene mutations in Leber's congenital amaurosis.Nat Genet. 1996; 14: 461-466Crossref PubMed Scopus (379) Google Scholar). Mutations in RPE65 on chromosome 1p31, specific to the retinal pigment epithelium involved in retinoid metabolism, were reported in patients with LCA, thus establishing a second gene (LCA2) for this heterogeneous disease (Marlhens et al. Marlhens et al., 1997Marlhens F Bareil C Griffoin JM Zrenner E Amalric P Eliaou C Liu SY et al.Mutations in RPE65 cause Leber's congenital amaurosis.Nat Genet. 1997; 17: 139-141Crossref PubMed Scopus (486) Google Scholar). The photoreceptor-specific homeobox gene CRX, on chromosome 19q13.3, has been implicated as the third gene, since mutations were demonstrated (Freund et al. Freund et al., 1998Freund CL Wang QL Chen S Muskat B Wiles CD Sheffield VC Jacobson SG et al.De novo mutations in the CRX homeobox gene associated with Leber congenital amaurosis.Nat Genet. 1998; 18: 311-312Crossref PubMed Scopus (248) Google Scholar). A novel locus on chromosome 14q24 (LCA3) was identified in consanguineous Saudi Arabian families (Stockton et al. Stockton et al., 1998Stockton DW Lewis RA Abboud EB Rejhi-Manzer AA Anderson KH Lupski JR A novel locus for Leber congenital amaurosis on chromosome 14q24.Hum Genet. 1998; 103: 328-333Crossref PubMed Scopus (63) Google Scholar). We studied a consanguineous family belonging to the Old Order River Brethren, a religious isolate originating in eastern Pennsylvania. The Old Order River Brethren descended from the Swiss, who emigrated to America in the 1750s in pursuit of religious freedom (Breckvill Breckvill, 1972Breckvill LT History Old Order River Brethren. Breckvill and Stickler, Pennsylvania1972Google Scholar). The kindred includes three affected individuals in two related sibships (fig. 1) who were initially evaluated at the Johns Hopkins Center for Hereditary Eye Diseases (JHCHED) and who are being followed annually. The patients presented with visual acuities in the order of 20/100–20/400, nystagmus, high hypermetropia, poor pupillary reflexes, and normal fundi. Progressive hypermetropia and increasing peripheral retinal mottling, of varying degree, were noted. The ERG was abolished. Review of other systems was unremarkable. We report a novel locus for LCA (LCA5) in this pedigree, on chromosome 6q11-16, by linkage analysis and homozygosity mapping. Venous blood samples were obtained from 27 family members of the Old Order River Brethren community and a cheek brush sample was obtained from an infant (individual 29). Consents were obtained in accordance with regulations of the Johns Hopkins Medical Institutions' Joint Committee on Clinical Investigation. DNA was isolated from whole blood by means of the QIAamp Blood Kit (Qiagen), according to the manufacturer's instructions. The alkali method was used to obtain DNA from the single cheek sample. Initially, the affected members and their first-degree relatives were screened to exclude linkage to the regions of the previously described genes involved in LCA on chromosomes 1, 17, and 19. The screen was then extended, by use of the whole-genome 8A multiplex version of markers spaced at 20 cM (Research Genetics). A region of homozygosity was identified on chromosome 6q. Further analysis, with additional markers in all potentially significant family members, was undertaken. Marker information was obtained from the Genome Database. PCR-based genotyping, with fluorescent labeled markers, was performed by means of the Applied Biosystems 373 automated DNA sequencer. PCR reactions were performed in a 9600 Perkin Elmer thermocycler, and the PCR products were checked for amplification with a 3% agarose gel (Saiki et al. Saiki et al., 1988Saiki RK Gelfand DH Stoffel S Sharf SJ Higuchi R Horn GT Mullis KB et al.Primer-directed enzymatic amplification of DNA with a thermostable DNA polymerase.Science. 1988; 239: 487-491Crossref PubMed Scopus (13102) Google Scholar). The amplified PCR product was genotyped by means of the automated DNA sequencer. GENESCAN ANALYSIS 2.0.0 and GENOTYPER version 1.1 software were used, to size the PCR products and to analyze the data. Allele sizes were scored by two independent observers. Two-point linkage analysis was performed by use of the MLINK option of the FASTLINK program, version 5.1 (Lathrop et al. Lathrop et al., 1984Lathrop GM Lalouel JM Julier C Ott J Strategies for multilocus linkage analysis in humans.Proc Natl Acad Sci USA. 1984; 81: 3443-3446Crossref PubMed Scopus (2171) Google Scholar; Cottingham et al. Cottingham et al., 1993Cottingham Jr, RW Idury RM Schaffer AA Faster sequential genetic linkage computations.Am J Hum Genet. 1993; 53: 252-263PubMed Google Scholar.) In this pedigree, LCA was analyzed as an autosomal recessive trait with complete penetrance, with an assumed allele frequency of .0032. A total of 40 microsatellite markers (Research Genetics) on chromosome 6 were analyzed, to determine the minimum region containing the new gene (Lander and Botstein Lander and Botstein, 1987Lander ES Botstein D Homozygosity mapping: a way to map human recessive traits with the DNA of inbred children.Science. 1987; 236: 1567-1570Crossref PubMed Scopus (660) Google Scholar). Marker allele frequencies were estimated by means of the Genetic Analysis System, version 2.0 (Young), and GCONVERT (Duffy). The final LOD scores were computed by means of the allele frequencies generated by the GCONVERT program. Recombination frequencies for males and females were assumed to be equal. All inbreeding loops in the family were disconnected for computational reasons (Ott Ott, 1991Ott J Analysis of human genetic linkage. 2d ed. Johns Hopkins University Press, Baltimore1991Google Scholar) (fig. 1). The GENEHUNTER program was used to perform multipoint linkage analysis against a fixed map of 17 informative markers, with an assumed equilibrium between marker and test loci (Kruglyak et al. Kruglyak et al., 1996Kruglyak L Daly MJ Reeve-Daly MP Lander ES Parametric and nonparametric linkage analysis: a unified multipoint approach.Am J Hum Genet. 1996; 58: 1347-1363PubMed Google Scholar). These markers were selected from the original 40 microsatellite markers because they were highly polymorphic and their relative orders and map distances were well estimated in public databases. The comprehensive genetic map of the Center for Medical Genetics, Marshfield Medical Research Foundation, provided the sex-averaged genetic distances for the markers noted in figure 1. The same map provided the order for 37 markers. The remaining three markers were placed by means of the Genome Location Database. Subsequently, the genetic framework map from the Center for Medical Genetics, Marshfield Medical Research Foundation, reduced the 37 markers to 23, indicating that some markers occurred at identical positions. Thus, in the analyses in which multiple markers had identical positions, the markers with reduced information content were dropped in favor of those with better information content. Multipoint LOD scores were computed with the same model described above for two-point analysis. Multipoint nonparametric linkage (NPL) scores were also computed with the use of only the affected individuals. Because of the inherent limitation on pedigree size in the GENEHUNTER program, the large pedigree was trimmed and broken into two separate units, accounting for some potential loss of power in the analysis. The data were examined for regions of allelic homozygosity in the affected individuals (Dib et al. Dib et al., 1996Dib C Faune S Fizames C Samson D Dvout N Vignal A Millasseau P et al.A comprehensive genetic map of the human genome based on 5262 microsatellites.Nature. 1996; 380: 152-154Crossref PubMed Scopus (2665) Google Scholar). Haplotype analysis was used to further define the interval containing the disease locus (Lathrop et al. Lathrop et al., 1985Lathrop GM Lalouel JM Julier C Ott J Multipoint linkage analysis in humans: detection of linkage and estimation of recombination.Am J Hum Genet. 1985; 37: 482-498PubMed Google Scholar). The following retina-specific genes on chromosome 6 were evaluated for the presence of disease-causing mutations: the GABRR1 and GABRR2 genes on chromosome 6q14.1-21 (Cutting et al. Cutting et al., 1991Cutting GR Lu L O'Hara BF Kasch LM Montrose-Rafizadeh C Donovan DM Shimada S et al.Cloning of the gamma-aminobutyric acid GABA RHO 1 cDNA: a receptor subunit highly expressed in the retina.Proc Natl Acad Sci USA. 1991; 88: 2673-2677Crossref PubMed Scopus (401) Google Scholar, Cutting et al., 1992Cutting GR Curristin S Zoghbi H O'Hara B Seldin MF Uhl GR Identification of a putative gamma-aminobutyric acid receptor subunit rho 2 cDNA and localization of the genes rho 2 (GABRR2) and rho 1 (GABRR1) to human chromosome 6q14-q21 and mouse chromosome 4.Genomics. 1992; 12: 801-806Crossref PubMed Scopus (171) Google Scholar) and the IMPG1 gene on chromosome 6q13-15 (Gehrig et al. Gehrig et al., 1998Gehrig A Felbor U Kelsell R Hunt DM Maumenee IH Weber BHF Assessment of the interphotoreceptor matrix proteoglycan-1 (IMPG1) localized to 6q13-q15 in autosomal dominant Stargardt-like disease (ADSTGD), progressive bifocal chorioretinal atrophy (PBCRA), and North Carolina macular dystrophy (MCDR1).J Med Genet. 1998; 35: 641-645Crossref PubMed Scopus (17) Google Scholar) (fig. 2). GABRR1 and GABRR2 are assumed to have arisen by gene duplication and share a 50% homology with each other. GABA is a neuroinhibitory transmitter mediating fast synaptic inhibition by activating chloride channels. GABRR1 is expressed largely in the retina; GABRR2 is expressed primarily in the brain. GABRR1 expression in the developing retina suggests its possible role as a candidate gene. In exons 1 and 4, polymorphic changes were identified. This did not change in the amino acid. These polymorphic changes were also identified in the normal population. No sequence changes were noted in GABRR2. IMPG1, a novel gene encoding a major proteoglycan of the interphotoreceptor matrix, is expressed in the retina by both rods and cones and maps to 6q13-15; it was considered a further candidate gene for LCA (Gehrig et al. Gehrig et al., 1998Gehrig A Felbor U Kelsell R Hunt DM Maumenee IH Weber BHF Assessment of the interphotoreceptor matrix proteoglycan-1 (IMPG1) localized to 6q13-q15 in autosomal dominant Stargardt-like disease (ADSTGD), progressive bifocal chorioretinal atrophy (PBCRA), and North Carolina macular dystrophy (MCDR1).J Med Genet. 1998; 35: 641-645Crossref PubMed Scopus (17) Google Scholar). No significant changes were noted after the 17 coding exons of this gene were sequenced. Linkage of LCA in the Old Order River Brethren was found at chromosome 6q11-16, supported by statistically significant two-point LOD scores with maximum LOD score (Zmax) 3.38 (θ=0) at D6S391 (table 1). Haplotype analysis of recombination events localizes the disease locus to a region of 23 cM, flanked by D6S1551 and D6S1694, thus identifying a new locus for LCA. Critical recombinant events were observed at marker D6S1551 in individual 29, who is unaffected, and at marker D6S1694 in individual 24, who is affected, defining the centromeric and telomeric boundaries, respectively. A common haplotype covers the region in all the affected individuals (fig. 1). Homozygosity of other highly informative markers across the candidate region was noted.Table 1Two-Point LOD Scores for Linkage between LCA and Chromosome 6 MarkersLOD Score at θ =Order.00.01.50.10.20.30.40θmaxZmaxD6S257−1.33−.17.67.92.93.71.39.145.97D6S1628−1.241.001.491.521.29.93.49.0841.53D6S1658−1.23−.63−.15.04.19.18.11.239.20D6S430−1.171.411.851.821.461.00.51.0661.86D6S1551−1.32.27.82.94.88.67.37.117.95D6S16191.201.171.03.87.59.36.17.0011.20D6S15961.231.201.10.96.71.47.23.0011.23D6S3913.383.303.002.621.871.16.55.0013.38D6S17073.153.072.762.371.63.98.47.0013.15D6S2513.223.142.832.451.711.06.50.0013.22D6S4452.972.892.592.231.56.98.48.0012.97D6S16272.332.282.071.821.33.87.43.0012.33D6S16442.202.151.931.661.17.74.35.0012.20D6S16312.702.632.382.071.47.93.45.0012.70D6S4502.282.232.011.741.24.79.38.0012.28D6S10561.471.421.221.02.70.46.23.0011.47D6S3001.241.331.431.361.07.71.35.0501.43D6S1716.08.10.13.14.13.11.08.090.14D6S1694−.40−.31−.14−.07−.07−.07−.04.848.19D6S1717−.73−.74−.77−.77−.65−.43−.20.827.39D6S261−1.18−.60−.10.10.23.21.13.231.23 Open table in a new tab The maximum multipoint LOD score was 3.10 between D6S391 and D6S450, a 9.5-cM interval (fig. 2). The multipoint NPL score was significant, with P<.004 for a 23-cM region. Sequencing of candidate genes GABRR1 and GABRR2 and IMPG1 revealed polymorphic changes only. LCA in the Old Order River Brethren, a highly inbred community, maps to a 23-cM interval on chromosome 6q11-16, as defined by linkage analysis and homozygosity mapping. Since this population is genetically isolated, and since LCA is quite rare, we presume that a single common ancestor was a carrier for this recessive trait (Lander and Botstein Lander and Botstein, 1987Lander ES Botstein D Homozygosity mapping: a way to map human recessive traits with the DNA of inbred children.Science. 1987; 236: 1567-1570Crossref PubMed Scopus (660) Google Scholar). The large size of the region of homozygosity in the family and the history of migration indicate the recency of the mutation in the population (fig. 1). LCA in this pedigree was not associated with multisystem abnormalities. Renal function remains normal. Neurological and hepatic function were within normal limits. The patients are of normal stature and intelligence. Neither photophobia nor photoattraction was reported in infancy, although pressing on the globes (the digito-ocular phenomenon of Franceschetti-Bamatter) played a prominent part in childhood behavior (Franceschetti Franceschetti and Dieterle, 1954Franceschetti A Dieterle P L'importance diagnostique de l'electrorétinogramme dans les dégénérescences tapéto-rétinennes avec rétrécissement du champ visuel et héméralopie.Conf Neurol. 1954; 14: 184-186Crossref PubMed Scopus (85) Google Scholar). Visual dysfunction, nystagmus, and the digito-ocular phenomenon were noticed in early infancy. A high hyperopic refractive correction was noted in all the patients (Wagner et al. Wagner et al., 1985Wagner RS Caputo AR Nelson LB Zanoni D High hyperopia in Leber's congenital amaurosis.Arch Ophthalmol. 1985; 103: 1507-1509Crossref PubMed Scopus (39) Google Scholar). Ophthalmoscopic examination in infancy revealed normal fundi, but in childhood, attenuated retinal vasculature with a varying degree of pigmentary changes was noticed. Electroretinography showed a markedly reduced response in the affected individuals. Vision has been stable in all affected members of the family who have been followed clinically at JHCHED. Genetic studies have identified a large region on chromosome 6q responsible for several retinal dystrophies (Small et al. Small et al., 1992Small KW Weber JL Roses A Lennon F Vance JM Pericak-Vance MA North Carolina macular dystrophy is assigned to chromosome 6.Genomics. 1992; 13: 681-685Crossref PubMed Scopus (93) Google Scholar, Small et al., 1993Small KW Weber J Roses A Pericak-Vance P North Carolina macular dystrophy MCDR1: a review and refined mapping to 6q14-q16.2.Ophthalmic Paediatr Genet. 1993; 14: 143-150Crossref PubMed Scopus (36) Google Scholar, Small et al., 1997Small KW Puech B Mullen L Yelchits S North Carolina macular dystrophy phenotype in France maps to the MCDR1 locus.Mol Vis. 1997; 3: 1-6PubMed Google Scholar; Stone et al. Stone et al., 1994Stone EM Nichols BE Kimura AE Weingeist TA Drack A Sheffield VC Clinical features of a Stargardt-like dominant progressive macular dystrophy with genetic linkage to chromosome 6q.Arch Ophthalmol. 1994; 112: 765-772Crossref PubMed Scopus (138) Google Scholar; Kelsell et al. 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Ruiz et al., 1998Ruiz A Borrego S Marcos I Antinolo G A major locus for autosomal recessive retinitis pigmentosa on 6q, determined by homozygosity mapping of chromosomal regions that contain gamma-aminobutyric acid-receptor clusters.Am J Hum Genet. 1998; 62: 1452-1459Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar), progressive bifocal chorioretinal dystrophy (PBCRA) at 6q12-21 (Kelsell et al. Kelsell et al., 1995Kelsell RE Godley BF Evans K et al.Localization of the gene for progressive bifocal chorioretinal atrophy (PBCRA) to chromosome 6q.Hum Mol Genet. 1995; 4: 1653-1656Crossref PubMed Scopus (43) Google Scholar), North Carolina macular dystrophy at 6q14-16.2 (Small et al. Small et al., 1993Small KW Weber J Roses A Pericak-Vance P North Carolina macular dystrophy MCDR1: a review and refined mapping to 6q14-q16.2.Ophthalmic Paediatr Genet. 1993; 14: 143-150Crossref PubMed Scopus (36) Google Scholar), Stargardt-like dominant macular degeneration (STGD3) at 6q13 (Stone et al. Stone et al., 1994Stone EM Nichols BE Kimura AE Weingeist TA Drack A Sheffield VC Clinical features of a Stargardt-like dominant progressive macular dystrophy with genetic linkage to chromosome 6q.Arch Ophthalmol. 1994; 112: 765-772Crossref PubMed Scopus (138) Google Scholar; Griesinger et al. Griesinger et al., 1998Griesinger IB Sieving PA Chandrasekharappa SC Ayyagari R Macular degeneration with highly variable phenotype localized to chromosome 6q.Am J Hum Genet. 1998; 63: A30Google Scholar), and dominant cone-rod dystrophy (CORD7) at 6q13-15 (Kelsell et al. Kelsell et al., 1998Kelsell RE Gregory-Evans K Gregory-Evans CY Holder GE Jay MR Weber BH Moore AT et al.Localization of a gene (CORD7) for a dominant cone-rod dystrophy to chromosome 6q.Am J Hum Genet. 1998; 63: 274-279Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar) (fig. 3). The occurrence of multiple loci so closely spaced in the genome could indicate the presence of a number of retinal genes in continuum, since the phenotype of all these retinal dystrophies, their ophthalmologic appearance, age at onset, and the extent and pattern of visual loss are varied. On the other hand, like the ABCR gene mutations that cause autosomal recessive retinitis pigmentosa (Martinez-Mir et al. 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Lewis et al., 1999Lewis RA Shroyer NF Singh N Allikmets R Hutchinson A Li Y Lupski JR et al.Genotype/phenotype analysis of a photoreceptor-specific ATP-binding cassette transporter gene ABCR, in Stargardt disease.Am J Hum Genet. 1999; 64: 422-434Abstract Full Text Full Text PDF PubMed Scopus (239) Google Scholar), it is possible that a single large gene in the proximal centromeric portion of the long arm of chromosome 6 could cause a myriad of retinal dystrophies, LCA being the most severe. Perhaps LCA5 is allelic with STGD3, RP25, CORD7, MCDR, and PBCRA. It is conceivable that mutations in different sites cause different structural alterations in the predicted protein, predisposing to varying phenotypes (Rozet et al. Rozet et al., 1998Rozet JM Gerber S Souied E Perrault I Chatelin S Ghazi I Leowski C et al.Spectrum of ABCR gene mutations in autosomal recessive macular dystrophies.Eur J Hum Genet. 1998; 6: 291-295Crossref PubMed Scopus (131) Google Scholar). The three other genes causing LCA are known to cause other phenotypically varied retinal dystrophies as well, raising the possibility of a similar situation in the LCA5 gene. GUCY2D mutations (LCA1) have been identified in autosomal dominant cone-rod dystrophy (Kelsell et al. Kelsell et al., 1998Kelsell RE Gregory-Evans K Gregory-Evans CY Holder GE Jay MR Weber BH Moore AT et al.Localization of a gene (CORD7) for a dominant cone-rod dystrophy to chromosome 6q.Am J Hum Genet. 1998; 63: 274-279Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar), although RPE65 (LCA2) mutations cause autosomal recessive retinitis pigmentosa as well as LCA (Morimura et al. Morimura et al., 1998Morimura H Fishman GA Grover SA Fulton AB Berson EI Dryja TP Mutations in the RPE65 gene in patients with autosomal recessive retinitis pigmentosa or Leber congenital amaurosis.Proc Natl Acad Sci USA. 1998; 95: 3088-3093Crossref PubMed Scopus (367) Google Scholar). Mutations in the cone-rod homeobox gene are now known to cause autosomal dominant cone-rod dystrophy, LCA, and late-onset dominant retinitis pigmentosa (Sohocki et al. Sohocki et al., 1998Sohocki MM Sullivan LS Mintz-Hittner HA Birch D Heckenlively JR Freund CL McInnes RR et al.A range of clinical phenotypes associated with CRX, a photoreceptor transcription-factor gene.Am J Hum Genet. 1998; 63: 1307-1315Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar). It is currently possible to identify mutations of the known LCA genes in less than one-third of the patients with LCA (Dharmaraj et al. Dharmaraj et al., 1999Dharmaraj S Silva E Li YY Loyer M Koenekoop RK Maumenee IH Mutational analysis in one hundred consecutive patients with Leber congenital amaurosis.Invest Ophthalmol Vis Sci. 1999; 40: A2983PubMed Google Scholar). The isolation of another locus for this retinal disorder, LCA5, on chromosome 6q, will account for an additional proportion of patients with an identifiable gene mutation. Recruitment of additional families with LCA to further narrow the critical region is under way, and candidate gene analysis continues. We are grateful to the patients and their families for their cooperation and participation in the study. We would like to acknowledge the help of Garry Cutting, Marina Kniazeva, Joan Bailey-Wilson, Reza Vagefi, Naba Bora, Carrie Gruver, Kang Zhang, Suzanne M. Leal, Mary Anderson, Gregory Leppert, Karen A. Klima, and Pamela Maskell. This work was supported in part by grants from the Foundation for Retinal Research, The Grousbeck Foundation, and The Edel and Krieble Funds of The Johns Hopkins Center for Hereditary Eye Diseases.