Diverse prevalence of large deletions within the OA1 gene in ocular albinism type 1 patients from Europe and North America

Ocular albinism type 1 (OA1) is an X-linked disorder mainly characterized by congenital nystagmus and photodysphoria, moderate to severe reduction of visual acuity, hypopigmentation of the retina, and the presence of macromelanosomes in the skin and eyes. We have previously isolated the gene for OA1 and characterized its protein product as melanosomal membrane glycoprotein displaying structural and functional features of G protein-coupled receptors. We and others have identified mutations of various types within the OA1 gene in patients with this disorder, including deletions and splice site, frameshift, nonsense, and missense mutations. However, different prevalences of large intragenic deletions have been reported, ranging from 10% to 50% in independent studies. To determine whether these differences might be related to the geographic origin of the OA1 families tested, we performed a further extensive mutation analysis study leading to the identification of pathogenic mutations in 30 unrelated OA1 patients mainly from Europe and North America. These results, together with our earlier mutation reports on OA1, allow us to resolve the apparent discrepancies between previous studies and point to a substantial difference in the frequency of large intragenic deletions in European (<10%) compared with North American (>50%) OA1 families. These observations and our overall refinement of point mutation distribution within the OA1 gene have important implications for the molecular diagnosis of OA1 and for the establishment of any mutation detection program for this disorder.

Defective intracellular transport and processing of OA1 is a major cause of ocular albinism type 1

Ocular albinism type 1 (OA1) is an X-linked disorder mainly characterized by a severe reduction of visual acuity, hypopigmentation of the retina and the presence of macromelanosomes in the skin and eyes. Various types of mutation have been identified within the OA1 gene in patients with the disorder, including several missense mutations of unknown functional significance. In order to shed light into the molecular pathogenesis of ocular albinism and possibly define critical functional domains within the OA1 protein, we characterized 19 independent missense mutations with respect to processing and subcellular distribution on expression in COS-7 cells. Our analysis indicates the presence of at least two distinct biochemical defects associated with the different missense mutations. Eleven of the nineteen OA1 mutants (approximately 60%) were retained in the endoplasmic reticulum, showing defecNStive intracellular transport and glycosylation, consistent with protein misfolding. The remaining eight of the nineteen OA1 mutants (approximately 40%) displayed sorting and processing behaviours indistinguishable from those of the wild-type protein. Consistent with our recent findings that OA1 represents a novel type of intracellular G protein-coupled receptor (GPCR), we found that most of these latter mutations cluster within the second and third cytosolic loops, two regions that in canonical GPCRs are known to be critical for their downstream signaling, including G protein-coupling and effector activation. The biochemical analysis of OA1 mutations performed in this study provides important insights into the structure-function relationships of the OA1 protein and implies protein misfolding as a major pathogenic mechanism in OA1.

Oa1 knock-out: new insights on the pathogenesis of ocular albinism type 1

Ocular albinism type I (OA1) is an X-linked disorder characterized by severe reduction of visual acuity, strabismus, photophobia and nystagmus. Ophthalmologic examination reveals hypopigmentation of the retina, foveal hypoplasia and iris translucency. Microscopic examination of both retinal pigment epithelium (RPE) and skin melanocytes shows the presence of large pigment granules called giant melanosomes or macromelanosomes. In this study, we have generated and characterized Oa1-deficient mice by gene targeting (KO). The KO males are viable, fertile and phenotypically indistinguishable from the wild-type littermates. Ophthalmologic examination shows hypopigmentation of the ocular fundus in mutant animals compared with wild-type. Analysis of the retinofugal pathway reveals a reduction in the size of the uncrossed pathway, demonstrating a misrouting of the optic fibres at the chiasm, as observed in OA1 patients. Microscopic examination of the RPE shows the presence of giant melanosomes comparable with those described in OA1 patients. Ultrastructural analysis of the RPE cells, suggests that the giant melanosomes may form by abnormal growth of single melanosomes, rather than the fusion of several, shedding light on the pathogenesis of ocular albinism.

Ocular albinism: evidence for a defect in an intracellular signal transduction system

G protein-coupled receptors (GPCRs) participate in the most common signal transduction system at the plasma membrane. The wide distribution of heterotrimeric G proteins in the internal membranes suggests that a similar signalling mechanism might also be used at intracellular locations. We provide here structural evidence that the protein product of the ocular albinism type 1 gene (OA1), a pigment cell-specific integral membrane glycoprotein, represents a novel member of the GPCR superfamily and demonstrate that it binds heterotrimeric G proteins. Moreover, we show that OA1 is not found at the plasma membrane, being instead targeted to specialized intracellular organelles, the melanosomes. Our data suggest that OA1 represents the first example of an exclusively intracellular GPCR and support the hypothesis that GPCR-mediated signal transduction systems also operate at the internal membranes in mammalian cells.

Repeated transmission of X-linked ocular albinism type 1 by a carrier oocyte donor

OBJECTIVE

To report the transmission of unsuspected X-linked ocular albinism in an oocyte donor program.

DESIGN

Case report.

SETTING

University medical center.

PATIENT(S)

A 24-year-old white female oocyte donor and the outcomes of three recipient pregnancies.

INTERVENTION(S)

Clinical assessment and molecular diagnostic tests.

MAIN OUTCOME MEASURE(S)

Mutation detection.

RESULT(S)

Demonstration of carrier status and multiple transmissions of a mutant allele.

CONCLUSION(S)

We describe the transmission of a mutant allele for X-linked ocular albinism from an unsuspected carrier female oocyte donor to three independent pregnancies. We emphasize the need for diligent inquisition to clarify any unusual history of ocular or constitutional signs that might signify an X-linked disorder.

The ocular albinism type 1 gene product is a membrane glycoprotein localized to melanosomes

Ocular albinism type 1 (OA1) is an inherited disorder characterized by severe reduction of visual acuity, photophobia, and retinal hypopigmentation. Ultrastructural examination of skin melanocytes and of the retinal pigment epithelium reveals the presence of macromelanosomes, suggesting a defect in melanosome biogenesis. The gene responsible for OA1 is exclusively expressed in pigment cells and encodes a predicted protein of 404 aa displaying several putative transmembrane domains and sharing no similarities with previously identified molecules. Using polyclonal antibodies we have identified the endogenous OA1 protein in retinal pigment epithelial cells, in normal human melanocytes and in various melanoma cell lines. Two forms of the OA1 protein were identified by Western analysis, a 60-kDa glycoprotein and a doublet of 48 and 45 kDa probably corresponding to unglycosylated precursor polypeptides. Upon subcellular fractionation and phase separation with the nonionic detergent Triton X-114, the OA1 protein segregated into the melanosome-rich fraction and behaved as an authentic integral membrane protein. Immunofluorescence and immunogold analyses on normal human melanocytes confirmed the melanosomal membrane localization of the endogenous OA1 protein, consistent with its possible involvement in melanosome biogenesis. The identification of a novel melanosomal membrane protein involved in a human disease will provide insights into the mechanisms that control the cell-specific pathways of subcellular morphogenesis.

Analysis of the OA1 gene reveals mutations in only one-third of patients with X-linked ocular albinism

The locus for ocular albinism type 1 (OA1) has been assigned to the Xp22.3 region through both linkage and deletion mapping. The disorder was found to be genetically homogeneous, as all informative families showed convincing linkage data with markers on Xp22.3 and all identified deletions involved in the same region. The OA1 gene recently was cloned and several intragenic deletions were identified in affected individuals. We have characterized the genomic structure of the OA1 gene, which spans approximately 40 kb of genomic DNA and contains nine exons. A highly polymorphic dinucleotide repeat was identified in intron 1, that provides a useful tool for molecular diagnosis. Knowledge of the intron/exon boundaries allowed us to search for point mutations in patients' genomic DNA. All nine exons of the OA1 gene, as well as the 5' and 3' untranslated regions, were scanned for point mutations in PCR-amplified DNA from 60 OA1 patients. The mutations identified include: two frameshifts and a splice site mutation leading to truncated OA1 proteins; a deletion of a threonine codon at position 290; and four missense mutations,two of which involve amino acids located within putative transmembrane domains. Two of the mutations each occur in three apparently unrelated families, consistent with previous observations of a founder effect in OA1. Surprisingly, mutations were detected in only one-third of the patients (21 of 60) ascertained. We postulate that mutations not yet identified in either regulatory elements of the OA1 gene, or in other gene(s) located within the same chromosomal region, may be common cause of X-linked ocular albinism.

Cloning of the gene for ocular albinism type 1 from the distal short arm of the X chromosome

Ocular albinism type 1 (OA1) is an X-linked disorder characterized by severe impairment of visual acuity, retinal hypopigmentation and the presence of macromelanosomes. We isolated a novel transcript from the OA1 critical region in Xp22.3-22.2 which is expressed at high levels in RNA samples from retina, including the retinal pigment epithelium, and from melanoma. This gene encodes a protein of 424 amino acids displaying several putative transmembrane domains and sharing no similarities with previously identified molecules. Five intragenic deletions and a 2 bp insertion resulting in a premature stop codon were identified from DNA analysis of patients with OA1, indicating that we have identified the OA1 gene.

Cloning of a human homologue of the Xenopus laevis APX gene from the ocular albinism type 1 critical region

Ocular albinism type 1 (OA1) is an X-linked recessive disorder characterized by a major impairment of visual acuity, nystagmus, strabismus, photophobia and retinal hypopigmentation. From the analysis of patients carrying deletions and translocations involving the distal short arm of the X chromosome (Xp22.3) we have identified a region of approximately 110 kb in which the OA1 gene must lie. We have extensively searched for genes in this region using a variety of techniques which included exon amplification, cDNA selection and direct hybridization of cosmid inserts to cDNA libraries. Putative exons identified by exon amplification were used to screen a human retina cDNA library and several cDNA clones corresponding to an approximately 7.5 kb transcript were isolated and characterized. Transcripts of this newly identified gene were found to be abundant in retina and melanoma and could also be detected in brain, placenta, lung, kidney and pancreas. Interestingly, sequence analysis revealed that this new gene encodes a 1616 amino acid protein sharing significant similarities with the Apical Protein from Xenopus laevis (APX) which is implicated in amiloride-sensitive sodium channel activity. The gene, termed APXL (APX-Like), spans approximately 160 kb, contains 10 exons and covers over 70% of the 110 kb critical region for OA1. A truncated pseudogene sharing very high levels of homology with the rat eIF-5 gene, a eukaryotic translation initiation factor, was found to lie in the middle of intron 1. APXL was found deleted in two patients with contiguous gene syndromes including OA1 and in one patient with isolated OA1. Mapping, expression and patient analysis data led us to consider the APXL gene a strong candidate for the OA1 gene. DNA from 57 unrelated patients with OA1 was, therefore, scanned for mutations in the coding region, using both SSCP analysis and direct sequencing. No functionally significant mutation was identified, suggesting that APXL is not directly involved in OA1. Further studies are needed to clarify the physiologic role of this highly conserved gene.

A YAC-based binning strategy facilitating the rapid assembly of cosmid contigs: 1.6 Mb of overlapping cosmids in Xp22

We have applied a yeast artificial chromosome (YAC)-based cosmid isolation and binning strategy to convert a YAC contig in Xp22 into 1.6 Mb of overlapping cosmids. This strategy is based on the screening of a high-density arrayed X chromosome-specific cosmid library with large YAC-derived restriction fragments and entire YAC probes. Cosmids selected in this way were gridded on dot blots and further mapped into bins defined by the overlap intervals of the YACs and YAC fragments. This rapid binning of cosmids simplified the subsequent assembly of cosmid contigs by restriction fingerprint hybridization. In total, we identified 139 cosmids spanning the entire 1.6 Mb region with a minimal overlap set of 53 clones. These cosmids were assigned to 17 bins and 9 contigs. One of the contigs is 665 kb in length and is one of the largest uninterrupted cosmid contigs in humans reported to date. The gaps between the contigs are minor and, together, they represent less than 7% of the region covered. Two previously identified genes are contained in these cosmids, the gene for amelogenin (AMG) and the recently isolated putative chloride channel gene CICN4. In addition, two disease loci have been mapped to this region: X-linked ocular albinism type 1 (OA1) and the microphthalmia with linear skin defects (MLS) syndrome. The assembly of the cosmid maps allowed us to determine the size of the deletion intervals for these two loci, which were estimated to be 110 kb for OA1 and 570 kb for MLS.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene transfer in regenerating muscle

We have compared the efficiency of direct gene transfer in normal and regenerating rat skeletal muscle. Muscle necrosis and regeneration was induced by intramuscular injection of bupivacaine in the soleus muscle of adult rats. Plasmids containing beta-galactosidase (beta-gal) or chloramphenicol acetyltransferase (CAT) genes driven by viral promoters were injected 3 days after bupivacaine treatment into the regenerating and the contralateral uninjured muscles. Expression of CAT activity was > 80-fold higher in regenerating compared to control muscles at 7 days post-transfection, but decreased at 30 and 60 days. Southern blot analysis showed that the predominant form of CAT DNA was episomal in transfected muscles; however, CAT activity measurements performed on the same transfected muscles showed no precise correlation between enzymatic activity and amount of plasmid DNA. Expression of beta-gal was detected in numerous regenerating fibers of the injured soleus muscles at 7 days post-transfection; in contrast, only rare positive fibers were found in control muscles. Focal infiltrates of mononuclear cells, which surround and invade selectively beta-gal-positive fiber segments, were observed at 30 days post-transfection, suggesting that immune mechanisms are implicated in the progressive loss of transgenes with time. The finding that regenerating muscle fibers display a higher efficiency of transfection may be relevant to gene therapy of Duchenne muscular dystrophy, because regenerating fibers are numerous in the early stages of the disease.