Imad Ghazi

Spectrum of ABCR gene mutations in autosomal recessive macular dystrophies.

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 Stargardt disease. Here, we report on the screening of the whole coding sequence of the ABCR gene in 40 unrelated STGD and 15 FFM families and we show that mutations truncating the ABCR protein consistently led to STGD. Conversely, all mutations identified in FFM were missense mutations affecting uncharged amino acids. These results provide the first genotype-phenotype correlations in ABCR gene mutations.

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.

Dominant X linked retinitis pigmentosa is frequently accounted for by truncating mutations in exon ORF15 of the RPGR gene

Retinitis pigmentosa (RP) is a group of progressive hereditary disorders of the retina in which various modes of inheritance have been described. The X linked forms of retinitis pigmentosa (XLRP, MIM 268000) are among the most severe owing to their early onset, leading to significant vision loss before the fourth decade. Five XLRP loci have been localised by linkage: RP2 (MIM 312600), RP3 (MIM 312610), RP6 (MIM 312612), RP23,1 and RP24 (MIM 300155). The major loci, RP2 and RP3, map to Xp11.4 and Xp21.1, respectively. RP3 is accounted for by mutations in the retinitis pigmentosa GTPase regulator ( RPGR ) gene.2 RP3 accounts for 70% of XLRP,3, 4 but until recently only 20% of mutations were identified in RP3 families, suggesting genetic heterogeneity at this locus. This hypothesis has been excluded by the discovery of a mutational hot spot in a new RPGR exon, ORF15.5 In 1997, we reported on X linked RP in nine families with constant and severe expression in carrier females.6 In this series, onset was delayed and sometimes milder in females than in hemizygous males. …

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.

Severe manifestations in carrier females in X linked retinitis pigmentosa.

Retinitis pigmentosa (RP) is a group of progressive hereditary disorders of the retina in which various modes of inheritance have been described. Here, we report on X linked RP in nine families with constant and severe expression in carrier females. In our series, however, the phenotype was milder and delayed in carrier females compared to hemizygous males. This form of X linked RP could be regarded therefore as partially dominant. The disease gene maps to chromosome Xp2.1 in the genetic interval encompassing the RP3 locus (Zmax=13.71 at the DXS1100 locus). Single strand conformation polymorphism and direct sequence analysis of the retinitis pigmentosa GTPase regulator (RPGR) gene, which accounts for RP3, failed to detect any mutation in our families. Future advances in the identification of X linked RP genes will hopefully help to elucidate the molecular basis of this X linked dominant RP.

Exclusion of five subunits of cGMP phosphodiesterase in Leber’s congenital amaurosis

Leber’s congenital amaurosis (LCA) is the earliest and most severe of all inherited retinal dystrophies. Recently, we mapped an LCA gene to chromosome 17p13.1 (LCA1) and ascribed the disease to mutations of the retinal guanylate cyclase (ret GC) gene in a subset of families of North African ancestry. Owing to the genetic heterogeneity of LCA and considering that LCA1 results from an impaired production of cGMP in the retina (with permanent closure of cGMP-gated cation channels), we hypothesized that the activation of the cGMP phosphodiesterase (PDE) could trigger the disease by lowering the intracellular cGMP level in the retina. The rod and cone cGMP-PDE inhibitory subunits were regarded therefore as candidate genes in LCA. Here, we report the exclusion of five rod and cone cGMP-PDE subunits in LCA families unlinked to chromosome 17p13.

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.