Dominique Ducroq

Retinal Dehydrogenase 12 (RDH12) Mutations in Leber Congenital Amaurosis

Leber congenital amaurosis (LCA), the most early-onset and severe form of all inherited retinal dystrophies, is responsible for congenital blindness. Ten LCA genes have been mapped, and seven of these have been identified. Because some of these genes are involved in the visual cycle, we regarded the retinal pigment epithelium and photoreceptor-specific retinal dehydrogenase (RDH) genes as candidate genes in LCA. Studying a series of 110 unrelated patients with LCA, we found mutations in the photoreceptor-specific RDH12 gene in a significant subset of patients (4.1%). Interestingly, all patients harboring RDH12 mutations had a severe yet progressive rod-cone dystrophy with severe macular atrophy but no or mild hyperopia.

Complete exon-intron structure of the RPGR-interacting protein (RPGRIP1) gene allows the identification of mutations underlying Leber congenital amaurosis.

Leber congenital amaurosis (LCA) is a genetically heterogeneous autosomal recessive condition responsible for congenital blindness or greatly impaired vision since birth. So far, six LCA loci have been mapped but only 4 out of 6 genes have been identified. A genome-wide screen for homozygosity was conducted in seven consanguineous families unlinked to any of the six LCA loci. Evidence for homozygosity was found in two of these seven families at the 14q11 chromosomal region. Two retinal specific candidate genes were known to map to this region, namely the neural retina leucine zipper (NRL) and the retinitis pigmentosa GTPase regulator interacting protein (RPGRIP1). No mutation of the NRL gene was found in any of the two families. Thus, we determined the complete exon-intron structure of the RPGRIP1 gene. RPGRIP1 encompasses 24 coding exons, nine of which are first described here with their corresponding exon-intron boundaries. The screening of the gene in the two families consistent with linkage to chromosome 14q11 allowed the identification of a homozygous null mutation and a homozygous missense mutation, respectively. Further screening of LCA patients unlinked to any of the four already identified LCA genes (n=86) identified seven additional mutations in six of them. In total, eight distinct mutations (5 out of 8 truncating) in 8/93 patients were found. So far this gene accounts for eight out of 142 LCA cases in our series (5.6%).

Mutations of the retinal specific ATP binding transporter gene (ABCR) in a single family segregating both autosomal recessive retinitis pigmentosa RP19 and Stargardt disease: evidence of clinical heterogeneity at this locus

Stargardt disease (STGD) is an autosomal recessive macular dystrophy of childhood characterised by bilateral loss of central vision over a period of several months. STGD has been mapped to chromosome 1p22.1 and recently ascribed to mutations in the retinal specific ATP binding transporter gene (ABCR). The fundus flavimaculatus with macular dystrophy (FFM), an autosomal recessive condition responsible for gradual loss of visual acuity in adulthood (second to third decade) has also been mapped to the same locus. However, a gene for autosomal recessive retinitis pigmentosa with distinctive features of choriocapillaris atrophy at an advanced stage (RP19) has been mapped to the genetic interval encompassing the STGD gene on chromosome 1p (D1S435-D1S236), raising the question of whether, despite striking differences in clinical course and presentation, RP19 and STGD might be allelic disorders at the ABCR locus. In a family segregating RP and STGD in two first cousins, we found that heterozygosity for a splicing mutation in the ABCR gene (1938-1 G→A) resulted in STGD while hemizygosity for this splice mutation resulted in RP, and when studying the RP patient’s parents, we found a maternal non-contribution with apparent segregation of a null allele ascribed to a partial deletion of the ABCR gene. The present study shows that, despite striking clinical differences, RP19 and STGD are allelic disorders at the ABCR locus.

A Gene for X-Linked Idiopathic Congenital Nystagmus (NYS1) Maps to Chromosome Xp11.4-p11.3

Congenital nystagmus (CN) is a common oculomotor disorder (frequency of 1/1,500 live births) characterized by bilateral uncontrollable ocular oscillations, with onset typically at birth or within the first few months of life. This condition is regarded as idiopathic, after exclusion of nervous and ocular diseases. X-linked, autosomal dominant, and autosomal recessive modes of inheritance have been reported, but X-linked inheritance is probably the most common. In this article, we report the mapping of a gene for X-linked dominant CN (NYS1) to the short arm of chromosome X, by showing close linkage of NYS1 to polymorphic markers on chromosome Xp11.4-p11.3 (maximum LOD score of 3.20, over locus DXS993). Because no candidate gene, by virtue of its function, has been found in this region of chromosome Xp, further studies are required, to reduce the genetic interval encompassing the NYS1 gene. It is hoped that the complete gene characterization will address the complex pathophysiology of CN.

A first locus for isolated autosomal recessive optic atrophy (ROA1) maps to chromosome 8q

In contrast to the frequent dominant optic atrophies (DOAs) in which the neuropathy is usually an isolated event, isolated recessive optic atrophies (ROAs) are very uncommon and have been described as severe congenital or early infantile conditions. To date, two loci for isolated DOA have been mapped, of which one was ascribed to mutations in the OPA1 gene. Conversely, no isolated autosomal ROA locus had previously been localised. Here, we report a large multiplex consanguineous family of French origin affected with an early onset but slowly progressive form of isolated OA. A genome-wide search for homozygosity allowed the localisation of the disease-causing gene to chromosome 8q21–q22 (Zmax of 3.41 at θ=0 for D8S270), in a 12 Mb interval flanked by markers D8S1702 and D8S1794. This localisation excludes allelism of the disease with both isolated DOAs, on one hand, or all known syndromic forms of ROA, on the other hand, supporting the mapping of a first gene for isolated autosomal ROA (ROA1) on the long arm of chromosome 8.

Age-related Macular Degeneration in Grandparents of Patients With Stargardt Disease: Genetic Study

PURPOSE To report clinical features and molecular genetic study in three unrelated families in which age-related macular degeneration was observed in grandparents of patients with Stargardt disease. METHODS A complete ophthalmologic examination including best-corrected visual acuity measurement, fundus examination, and fluorescein angiography was performed on all members of the three families. The entire coding sequence of the ABCR gene was analyzed using a combination of single strand conformation polymorphism and direct sequence analysis of the 50 exons. RESULTS Compound heterozygous missense mutations were observed in patients with Stargardt disease (Arg212Cys, Argl107Cys, Gly1977Ser, Arg2107His, and le2113Met). Heterozygous missense mutations were observed in the grandparents with age-related macular degeneration (Arg212Cys and Arg1107Cys). CONCLUSIONS We report phenotype and genotype findings in three unrelated families segregating patients with Stargardt disease and age-related macular degeneration. The hypothesis that the Arg212Cys and Arg1107Cys ABCR gene mutations could be susceptibility factors for age-related macular degeneration is discussed. We speculate that the relatives of patients affected with Stargardt disease who are carriers of heterozygous ABCR gene mutations may have a higher risk of developing age-related macular degeneration.

The ABCA4 Gene in Autosomal Recessive Cone-Rod Dystrophies

To the Editor: Recently, Maugeri et al. (2000) reported on the screening of the ABCA4 gene in 5 patients with autosomal recessive cone-rod dystrophies (CRD) and 15 patients with sporadic CRD originating from Germany and the Netherlands. The identification of mutations in 13/20 patients (65%) led the authors to speculate that “Mutations in the ABCA4 (ABCR) gene are the major cause of autosomal recessive cone-rod dystrophy.” The present study was undertaken to evaluate the prevalence of ABCA4 mutations in a cohort of 55 patients affected with autosomal recessive or sporadic CRD. Within the huge family of inherited retinal dystrophies, the CRD phenotype indicates a specific form of retinal degeneration in which the cone degeneration appears early in life with a central involvement of the retina, followed by a degeneration of rods several years later (Klevering et al. 2002). This particular form of retinal dystrophy has long been regarded as “inverse retinitis pigmentosa” (RP) and can be misdiagnosed as macular dystrophy in the first stages of the disease. Indeed, the main symptoms at onset of the disease are decrease of visual acuity, loss of color discrimination, and photophobia. The b-wave of the photopic ERG (cone response) is severely reduced, although the b-wave of the scotopic ERG is still normal. As the disease progresses, nyctalopia, progressive peripheral visual field deficit, and decreasing scotopic electroretinogram (ERG) amplitudes are observed. Four genes (CRX [MIM 602225], GUCY2D [MIM 600179], GCAP1 [MIM 600364], and HRG4 [MIM 604011]) and two loci have been implicated in autosomal dominant CRD (CORD5 [MIM 600977] and CORD7 [MIM 603649]), whereas two other loci were reported for autosomal recessive CRD (CORD9 [Danciger et al. 2001] and CORD8 [MIM 605549]) and one for X-linked CRD (RPGR [MIM 312610]). Conversely, the ABCA4 gene, which was identified in 1997 as the Stargardt-causing gene, was later recognized as responsible for some forms of RP (RP19) and some CRD, depending on the nature of the ABCA4 mutations and on the remaining protein activity (Allikmets et al. 1997; Martinez-Mir et al. 1997; Cremers et al. 1998; Gerber et al. 1998; Rozet et al. 1998, 1999). Sixty-one individuals affected with CRD and 40 healthy relatives belonging to 55 families of various origin were recruited from genetic and ophthalmologic consultations. In 29/55 families, the disease was undoubtedly inherited as an autosomal recessive condition—23 multiplex families (11/23 consanguineous) and six simplex patients born to consanguineous parents. In the 26/55 remaining families, the patients were simplex cases. The time course of the disease was determined by interviewing at least one patient per family and, whenever possible, all affected siblings of the family. Minimal criteria for inclusion in the study were initial cone dysfunction and subsequent progressive peripheral disease. In one affected patient per family, we screened for mutations the 50 exons of the ABCA4 gene, as well as the flanking intronic sequences, using denaturing high-pressure liquid chromatography. On the basis of the secondary structure of each exon, the screening was performed at 1 or 2 temperatures (mutation detection rate estimated to be at least 0.98). Exons showing a shift were directly sequenced. Sixteen different mutant alleles were identified in 13/55 patients (i.e., 23.6% of all cases). Among these 13 patients, 2 were homozygotes (from two consanguineous families), 4 were compound heterozygotes, and 7 were single heterozygotes (see table 1). Among the 29 recognized autosomal recessive cases of CRD, only 6 were found to carry ABCA4 mutations (20.7%), whereas, of the 26 sporadic cases of CRD, 7 harbored mutations in the gene (26.9%). The frequencies of ABCA4 mutations in the two groups are not significantly different. Table 1 ABCA4 Mutations in Patients with CRD In a similar screen of 43 multiplex or consanguineous families with Stargardt disease showing genetic linkage to the ABCA4 locus on 1p22, we identified at least one mutated allele in 34 families (data not shown). This figure is broadly in line with the findings of other groups (Allikmets et al. 1997; Rozet et al. 1998; Lewis et al. 1999; Rivera et al. 2000; Yatsenko et al. 2001) and suggests that a proportion of ABCA4 mutations remain to be identified. These could lie in promotor or intron sequences or in undiscovered exons (e.g., RPGR [Vervoort et al. 2000]), or they could be deletions up to 1 mb away (e.g., PAX6 [Lauderdale et al. 2000]). We therefore conservatively estimate that this screen will have detected ∼80% of the mutations present in these families, giving a corrected implication of the ABCA4 gene in 29.5% of all cases (autosomal recessive CRD 25.9% and sporadic cases of CRD 33.6%). This study confirms that ABCA4 is a major gene responsible for CRD. Nevertheless, the frequency of mutations appears to be lower than reported (30% in our series vs. 65% in Maugeri’s series). Finally, this work might improve genetic counseling in this condition. Indeed, for a sporadic case of CRD with no ABCA4 mutation, the risk of the disease to be inherited nevertheless as an autosomal recessive condition can be estimated to be 15.6% using the Bayesian calculation (calculation details on request).

Phenotype-Genotype Correlations in Retinal Degenerations Caused by Abcr gene Mutations

Stargardt disease (STGD) and late-onset fundus flavimaculatus (FFM) are autosomal recessive conditions leading to macular degenerations in childhood and adulthood, respectively. Recently, mutations of the photoreceptor cell-specific ATP binding transporter gene (ABCR) have been reported in STGD on one hand, and in some age related macular dystrophies (AMD) on the other hand. In addition, autosomal recessive retinitis pigmentosa (RP19) has recently been ascribed to mutations in the same gene. To provide the first genotype-phenotype correlations in ABCR gene mutations, the screening of the 50 exons encoding ABCR have been performed in a large series of FFM and STGD families as well as in familial cases of AMD and in pedigrees segregating both STGD and RP19. Giving consideration to the results of this study, we suggest that homozygosity for truncating ABCR gene mutations result in severe RP while compound heterozygosity for frameshift and missense mutations at this locus result in a severe macular disorder, STGD. Conversely, all mutations identified in milder macular disorder, FFM, were missense mutations. Finally, some AMD patients were found to carry heterozygote ABCR gene mutations, suggesting that these mutations could represent in some cases a susceptibility factor for the disease.