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A custom capture sequence approach for oculocutaneous albinism identifies structural variant alleles at the OCA2 locus.

ABSTRACT: Oculocutaneous albinism (OCA) is a heritable disorder of pigment production that manifests as hypopigmentation and altered eye development. Exon sequencing of known OCA genes is unsuccessful in producing a complete molecular diagnosis for a significant number of affected individuals. We sequenced the DNA of individuals with OCA using short-read custom capture sequencing that targeted coding, intronic, and noncoding regulatory regions of known OCA genes, and genome-wide association study-associated pigmentation loci. We identified an OCA2 complex structural variant (CxSV), defined by a 143 kb inverted segment reintroduced in intron 1, upstream of the native location. The corresponding CxSV junctions were observed in 11/390 probands screened. The 143 kb CxSV presents in one family as a copy number variant duplication for the 143 kb region. In the remaining 10/11 families, the 143 kb CxSV acquired an additional 184 kb deletion across the same region, restoring exons 3-19 of OCA2 to a copy-number neutral state. Allele-associated haplotype analysis found rare SNVs rs374519281 and rs139696407 are linked with the 143 kb CxSV in both OCA2 alleles. For individuals in which customary molecular evaluation does not reveal a biallelic OCA diagnosis, we recommend preliminary screening for these haplotype-associated rare variants, followed by junction-specific validation for the OCA2 143 kb CxSV.

SUBMITTER: Loftus SK

PROVIDER: S-EPMC8435020 | biostudies-literature | 2021 Oct

REPOSITORIES: biostudies-literature

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A custom capture sequence approach for oculocutaneous albinism identifies structural variant alleles at the OCA2 locus.

Loftus Stacie K SK Lundh Linnea L Watkins-Chow Dawn E DE Baxter Laura L LL Pairo-Castineira Erola E Nisc Comparative Sequencing Program Jackson Ian J IJ Oetting William S WS Pavan William J WJ Adams David R DR

Human mutation 20210801 10

Oculocutaneous albinism (OCA) is a heritable disorder of pigment production that manifests as hypopigmentation and altered eye development. Exon sequencing of known OCA genes is unsuccessful in producing a complete molecular diagnosis for a significant number of affected individuals. We sequenced the DNA of individuals with OCA using short-read custom capture sequencing that targeted coding, intronic, and noncoding regulatory regions of known OCA genes, and genome-wide association study-associat ...[more]

PMID: 34246199

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Project description:Oculocutaneous albinism (OCA) is a group of inherited disorders of melanin biosynthesis characterized by a generalized reduction in pigmentation of hair, skin and eyes. The prevalence of all forms of albinism varies considerably worldwide and has been estimated at approximately 1/17,000, suggesting that about 1 in 70 people carry a gene for OCA. The clinical spectrum of OCA ranges, with OCA1A being the most severe type with a complete lack of melanin production throughout life, while the milder forms OCA1B, OCA2, OCA3 and OCA4 show some pigment accumulation over time. Clinical manifestations include various degrees of congenital nystagmus, iris hypopigmentation and translucency, reduced pigmentation of the retinal pigment epithelium, foveal hypoplasia, reduced visual acuity usually (20/60 to 20/400) and refractive errors, color vision impairment and prominent photophobia. Misrouting of the optic nerves is a characteristic finding, resulting in strabismus and reduced stereoscopic vision. The degree of skin and hair hypopigmentation varies with the type of OCA. The incidence of skin cancer may be increased. All four types of OCA are inherited as autosomal recessive disorders. At least four genes are responsible for the different types of the disease (TYR, OCA2, TYRP1 and MATP). Diagnosis is based on clinical findings of hypopigmentation of the skin and hair, in addition to the characteristic ocular symptoms. Due to the clinical overlap between the OCA forms, molecular diagnosis is necessary to establish the gene defect and OCA subtype. Molecular genetic testing of TYR and OCA2 is available on a clinical basis, while, at present, analysis of TYRP1 and MATP is on research basis only. Differential diagnosis includes ocular albinism, Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, Griscelli syndrome, and Waardenburg syndrome type II. Carrier detection and prenatal diagnosis are possible when the disease causing mutations have been identified in the family. Glasses (possibly bifocals) and dark glasses or photocromic lenses may offer sufficient help for reduced visual activity and photophobia. Correction of strabismus and nystagmus is necessary and sunscreens are recommended. Regular skin checks for early detection of skin cancer should be offered. Persons with OCA have normal lifespan, development, intelligence and fertility.

Project description:BackgroundOculocutaneous albinism (OCA) is a group of rare autosomal recessive disorders characterized by clinical genetic heterogeneity. OCA type II (OMIM: 203200) is the most common subtype among African and African Americans, primarily caused by pathogenic variants in the OCA2 (HGNC ID: 8101) gene. In this study, we presented a Chinese family with OCA and reported two novel variants in the OCA2 gene.MethodsWhole-exome sequencing (WES) was performed to identify pathogenic variants in the proband. The candidate variants were subsequently validated using Sanger sequencing and QPCR assay. Additionally, bioinformatics analyses were employed to predict the deleteriousness and conservation of the identified mutations.ResultsIn the 16-year-old male proband, two novel compound heterozygous OCA2 variants, NM_000275.3: c.1640T>G (NP_000266.2: p.L547R) and an exons 10-19 deletion variant, were identified. Meanwhile, a reported heterozygous variant c.1441G>A/p.A481T (NM_000275.3, NP_000266.2) in the OCA2 gene was also found in the proband. Sanger sequencing confirmed that the two variants c.1441G>A/p.A481T and c.1640T>G/p.L547R were inherited from his father. Moreover, qPCR assay revealed that the exons 10-19 deletion was inherited from the mother, his sister also carried this variant. Fortunately, the variant was not detected in the amniotic fluid of the proband's sister. Multiple online bioinformatics tools predicted the variant c.1640T>G to be damaging, leading to the replacement of a highly conserved leucine with an arginine. The gross exon 10-19 deletion in the OCA2 gene resulted in a truncated, non-functional protein losing the 3-9 transmembrane α-helices domains. According to the American College of Medical Genetics and Genomics classification, these three variants in the OCA2 gene were evaluated as likely pathogenic.ConclusionThis study has identified two novel compound variants in the OCA2 gene and a previously reported variant in a Chinese family with OCA. By expanding the mutation spectrum of the OCA2 gene, our findings contribute to a better understanding of the genetic basis of OCA.

Project description:Autosomal recessive conditions are often associated with homozygous mutations showing common ancestral origins and are frequently linked to consanguinity. However, an increasing number of compound heterozygotes are found in diverse, admixed populations. Oculocutaneous albinism (OCA) is a recessive condition caused mainly by mutations in the TYR and OCA2 genes involved in skin pigmentation. We previously screened the TYR and OCA2 genes in Colombian OCA families, identifying both known and novel mutations. Affected family members were found to be either homozygous or compound heterozygous for these gene mutations. Compound heterozygosity, where two different recessive alleles inherited from each parent lead to the expression of an autosomal recessive trait, poses a challenge in genetic diagnosis. Estimating the ancestry of these disease-associated variants, in conjunction with understanding the colonization history of admixed populations, can enhance the precision of association mapping in genetic studies. The aim of this study was to determine the ancestral origins of TYR and OCA2 mutations for OCA patients from two populations located in the Andes region of Colombia-Altiplano Cundiboyacense and Marinilla-Santuario. Comparison of OCA patients, and their unaffected relatives, with global reference populations showed a pattern of European and Native American admixture, with little African ancestry, for these two populations. Mutation-bearing TYR and OCA2 haplotypes from Colombian OCA patients were compared against haplotypes from Spanish, Native American, and Sephardic Jewish reference populations to infer their ancestral origins. For 12 OCA1 patients from the Altiplano Cundiboyacense region, 21 out of 24 mutated TYR haplotypes show Spanish origins, two show Native American origins, and one shows a Sephardic Jewish origin. The two Native American TYR haplotypes, and the single Sephardic Jewish haplotype, are all found in compound heterozygote patients, paired with the predominant Spanish TYR haplotype G47D. OCA in these three patients is a result of genetic admixture that brought together disease-causing mutations with distinct ancestral origins. Both OCA2 patients from the Marinilla-Santuario region show homozygous OCA2 mutations with a Spanish origin. These findings underscore the complexity of the genetic architecture of Mendelian disease in admixed American populations, with both consanguinity and admixture contributing to the risk of autosomal recessive OCA in Colombia.

Dataset Information

A custom capture sequence approach for oculocutaneous albinism identifies structural variant alleles at the OCA2 locus.

Publications

A custom capture sequence approach for oculocutaneous albinism identifies structural variant alleles at the OCA2 locus.

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