Eight previously unidentified mutations found in the OA1 ocular albinism gene

Abstract

Background: Ocular albinism type 1 (OA1) is an X-linked ocular disorder characterized by a severe reduction in visual acuity, nystagmus, hypopigmentation of the retinal pigmented epithelium, foveal hypoplasia, macromelanosomes in pigmented skin and eye cells, and misrouting of the optical tracts. This disease is primarily caused by mutations in the OA1 gene.

Methods: The ophthalmologic phenotype of the patients and their family members was characterized. We screened for mutations in the OA1 gene by direct sequencing of the nine PCR-amplified exons, and for genomic deletions by PCR-amplification of large DNA fragments.

Results: We sequenced the nine exons of the OA1 gene in 72 individuals and found ten different mutations in seven unrelated families and three sporadic cases. The ten mutations include an amino acid substitution and a premature stop codon previously reported by our team, and eight previously unidentified mutations: three amino acid substitutions, a duplication, a deletion, an insertion and two splice-site mutations. The use of a novel Taq polymerase enabled us to amplify large genomic fragments covering the OA1 gene. and to detect very likely six distinct large deletions. Furthermore, we were able to confirm that there was no deletion in twenty one patients where no mutation had been found.

Conclusion: The identified mutations affect highly conserved amino acids, cause frameshifts or alternative splicing, thus affecting folding of the OA1 G protein coupled receptor, interactions of OA1 with its G protein and/or binding with its ligand.

Figure 1

point mutations. 1A: chromatogram of the 401T>C mutation (L134P), from a carrier, obtained with Chromas 2.3. The nucleotide change resulting from the mutation is shown on the chromatogram. 1B: pedigree of the whole family with its three branches: solid squares are affected people, dotted circles are molecularly proven carriers and hollow squares and circles are unaffected people. Here, I2 and II 1, 2 and 3 are carriers, III1 and 3 are affected. The mutation was previously described in II3 and III3 [12]. 1C: sequence alignment of OA1 between different species: MOUSE: Mus musculus, HUMAN: Homo sapiens, XENTR: Xenopus tropicalis, XENLA: Xenopus laevis, BRARE: Danio rerio, ANOGA: Anopheles gambiae. The arrows in the alignment show the amino acid affected by the mutation. 1D: chromatogram of the 853A->T mutation (R285X), from an affected individual. 1E: chromatogram of the 241G>T mutation (G81V), from an affected individual. 1F: pedigree of the family affected by G81V. 1G: sequence alignment around G81. 1H: chromatogram of the 348C>G mutation (C116W), from a carrier. 1I: pedigree of the family affected by C116W. 1J: sequence alignment around C116. 1K: chromatogram of the 497C>A mutation (T166N), from an affected individual. 1L: pedigree of the family affected by T166N. 1M: sequence alignment around T166.

Figure 2

duplication. 2A: chromatogram of the 163_170dup(GCGGGCCC) mutation, from an affected individual. 2B: pedigree.

Figure 3

deletion. 3A: chromatogram of the 504_505del(CT)mutation, from an affected individual. 3B: pedigree.

Figure 4

insertion. 4A: chromatogram of the 601_602ins(T) mutation, from an affected individual.

Figure 5

splice site mutations. 5A: chromatogram of the 547+2T->A mutation, from an affected individual. 5B: pedigree of the family affected by the 547+2T->A mutation. 5C: chromatogram of the 886-2A->T mutation, from an affected individual.