Oculocutaneous Albinism, type III (OCA3) is a form of brown albinism that was first discovered in people from Southern Africa. It causes mild hypopigmentation in light-skinned individuals, while in dark-skinned individuals, the condition results in bright copper-red skin coloration with freckles as well as bright copper-red hair. Affected patients also exhibit red reflex on transillumination of the iris, color dilution of the iris, nystagmus and strabismus.
Unlike other forms of albinism, OCA3 is considered a mild manifestation of the disorder. Affected individuals have less severe vision abnormalities compared to OCA1 and OCA2. While the exact incidence rate of OCA3 is not known, it is a rare condition, mainly affecting dark-skinned people.
Diagnosis of the disorder depends on molecular testing of the TYRP1 gene. Affected individuals are advised to limit their exposure to the sun and to use broad-spectrum sunscreen to limit sun damage. They also require periodic ophthalmological evaluations and may benefit from low-vision aids.
The disorder follows an autosomal recessive pattern of inheritance and is caused by mutations in the TYRP1 gene. This gene encodes an oxidoreductase enzyme found specifically in melanocyte cells that is involved in the biosynthesis of melanin. Homozygous TYRP1 mutations associated with OCA3 include deletions, missense and nonsense variants.
Monies et al. (2017) described the genomic landscape of Saudi Arabia based on the findings of 1000 diagnostic panels and exomes. One patient, an 11-year-old male, suffered from hemimegalencephaly, developmental delay and ADHD. He also had abnormal pigmentation all over his body. Whole exome sequencing helped identify a dual molecular diagnosis in this patient. A heterozygous mutation (c.1557T>G, p.Y519X) was found in exon 8 of the patient’s TYRP1 gene, associated with oculocutaneous albinism type 3, and a heterozygous variant (c.90G>A, p.W30X) was uncovered in exon 1 of the TGIF1 gene, associated with holoprosencephaly 4. Such dual molecular diagnoses were rare and only occurred in 1.5% of the cohort.
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