Haplotype-based analysis resolves missing heritability in oculocutaneous albinism type 1B.

Area of Science:

• Genetics
• Ophthalmology
• Dermatology

Background:

• Oculocutaneous albinism (OCA) is a rare genetic disorder characterized by reduced melanin production, leading to hypopigmentation and visual impairment.
• Significant heritability remains unexplained in OCA, particularly in cases with residual pigmentation.
• Mutations in the Tyrosinase (TYR) gene, encoding the rate-limiting enzyme in melanin synthesis, are a frequent cause of OCA.

Purpose of the Study:

• To analyze high-depth TYR sequencing data from a cohort of 352 OCA probands to identify novel disease-causing variants.
• To investigate the contribution of a specific TYR haplotype (cis-YQ) to OCA pathogenesis.
• To assess the necessity of phased variant analysis for a complete genetic diagnosis of OCA.

Main Methods:

• High-depth short-read sequencing of the TYR gene in 352 OCA probands.
• Identification and characterization of single-nucleotide variants (SNVs), insertions/deletions (indels), and structural variants.
• Haplotype analysis to determine the origin and frequency of the cis-YQ allele and other complex alleles.

Main Results:

• Identified 66 TYR SNVs/indels, 3 structural variants, and a common cis-YQ haplotype in 149/352 probands.
• The cis-YQ haplotype was the most frequent pathogenic allele, accounting for 19.1% of TYR pathogenic alleles in type 1 OCA.
• Discovered additional complex alleles involving combinations of common and rare TYR variants in cis orientation.

Conclusions:

• The TYR gene harbors diverse pathogenic alleles, including common haplotypes like cis-YQ, contributing to OCA.
• Phased variant analysis of the entire TYR locus is crucial for accurate and comprehensive genetic diagnosis of OCA.
• Understanding complex allelic architectures improves diagnostic yield in OCA and addresses previously unexplained heritability.