A novel locus for Leber congenital amaurosis maps to chromosome 6q

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%).

Leber's congenital amaurosis with anterior keratoconus in Pakistani families is caused by the Trp278X mutation in the AIPL1 gene on 17p

BACKGROUND

Leber's congenital amaurosis (LCA) represents the earliest and severest form of retinal dystrophy leading to congenital blindness. A total of 20% of children attending blind schools have this disease. LCA has a multigenic basis and is proving central to our understanding of the development of the retina. We describe the clinical and molecular genetic features of four inbred pedigrees from neighbouring remote villages in northern Pakistan, in which some of the affected members have concurrent keratoconus.

METHODS

History-taking and physical and eye examinations were performed in the field. Venipuncture, DNA extraction, studies of linkage to known LCA genes, automated sequencing and polymorphism analyses for haplotype assessments were done.

RESULTS

We examined 12 affected and 15 unaffected family members. By history, there were an additional nine blind people in the four pedigrees. In each pedigree a consanguineous marriage was evident. We found a homozygous nonsense mutation in the AIPL1 gene, which replaces a tryptophan with a stop codon (Trp278X). The phenotype is severe and variable, despite the common molecular genetic etiology in each family. Affected patients had hand motion to no light perception vision and fundus findings ranging from maculopathy to diffuse pigmentary retinopathy. Three affected members had definite keratoconus, and two were suspects based on mild cone formation in the cornea of at least one eye.

INTERPRETATION

We have identified four Pakistani families with a severe form of LCA that is associated with severe keratoconus in some affected members. The molecular etiology in all four families is a homozygous nonsense mutation, Trp278X, in the photoreceptor-pineal gene AIPL1. To our knowledge, this is one of the first phenotype-genotype correlations of AIPL1-associated LCA.

Screening for CRX gene mutations in Chinese patients with Leber congenital amaurosis and mutational phenotype

PURPOSE

To screen for possible disease-causing mutations in the CRX gene in Chinese patients with Leber congenital amaurosis (LCA) and to enrich the understanding of its mutational phenotype.

METHODS

Genomic DNA was collected from 27 patients with LCA. The coding sequences of the CRX gene were analyzed using the PCR-heteroduplex-SSCP method. Mutations were confirmed by DNA sequencing.

RESULTS

We identified two heterozygous variations in the CRX gene in two patients with LCA. One was a deletion (GCC-->-CC, A181D1bp) leading to a frameshift and protein truncation. This mutation was present in a patient with LCA, but not in his healthy parents. The ocular manifestations of this A181Delta1bp mutation are described. An intronic variation (IVS1-13G-->C) was found in a patient with LCA as well as in his healthy father.

CONCLUSION

A heterozygous A181D1bp mutation in the CRX gene caused an LCA phenotype in a Chinese patient.

Autosomal dominant retinal degeneration and bone loss in patients with a 12-bp deletion in the CRX gene

PURPOSE

To define the phenotypic expression of a deletion in the gene encoding the transcription factor CRX in a large, seven-generation, white family.

METHODS

Fourteen affected individuals, all heterozygous for the Leu146del12 mutation in the cone-rod homeobox gene (CRX), and four nonaffected relatives from the same family were examined with visual function tests, and 10 underwent bone mineral density (BMD) measurement.

RESULTS

The ability of the mutated CRX protein to transactivate rhodopsin promoter was decreased by approximately 25%, and its ability to react synergistically with neural retinal leucine zipper (NRL) was reduced by more than 30%. The affected members of the family had an autosomal dominant ocular condition most closely resembling Leber congenital amaurosis (LCA) with severe visual impairment at an early age. Depending on age, affected members showed varying degrees of significant visual acuity loss, elevated dark-adaptation thresholds, significantly reduced cone and rod electroretinogram (ERG) amplitudes, and progressive constriction of the visual fields, in most cases leading to complete blindness. Six affected members had reduced levels of BMD in the spine and the hip (osteopenia). Four affected female members who were receiving long-term hormonal replacement therapy (HRT) demonstrated normal values of BMD.

CONCLUSIONS

This large deletion of the CRX gene is associated with a severe form of autosomal dominant retinal degeneration. Affected members not receiving HRT showed reduced BMD (osteopenia). This phenotype may reflect the abnormal influence of mutant CRX on both retinal and pineal development.

Autosomal dominant Stargardt-like macular dystrophy segregating in a large Canadian family

BACKGROUND

Inherited macular dystrophies account for a major fraction of the cases of retinal degenerative disease that lead to permanent blindness. We describe the clinical and genetic findings in a Canadian family with a form of macular dystrophy resembling autosomal dominant Stargardt-like macular dystrophy.

METHODS

Standard ophthalmologic examinations were performed in members of a single five-generation Alberta family. Tests of visual acuity and colour vision, fundus photography, fluorescein angiography and electroretinography were performed in 15 affected people. Blood was collected from 24 family members, and DNA was extracted for genotyping. Genetic linkage analysis was performed using polymorphic short tandem repeat microsatellite markers located on chromosome 6q, a region containing loci for several macular disorders.

RESULTS

Affected family members display clinical characteristics resembling autosomal dominant Stargardt-like macular dystrophy, previously assigned to chromosome 6q (STGD3). Linkage analysis generated a peak lod score of 5.50 at an estimated recombination fraction of 0.00 for marker locus D6S300.

INTERPRETATION

The family described has an autosomal dominant macular dystrophy that resembles Stargardt-like macular dystrophy. The disease locus for this family maps to an interval on chromosome 6q that overlaps that for STGD3 and other retinal dystrophy loci. These findings provide further evidence that human chromosome 6q represents a "hot spot" for retinal disorders.