P gene mutations in patients with oculocutaneous albinism and findings suggestive of Hermansky-Pudlak syndrome.

O culocutaneous albinism (OCA) is a genetically heterogeneous disorder characterised by abnormally low amounts of melanin in the eyes, skin, and hair. In addition to hypopigmentation of the skin and eyes, OCA patients have ocular manifestations including nystagmus, foveal hypoplasia with reduced visual acuity, and strabismus. Several subtypes of OCA exist. OCA2, the most common subtype, results from mutations in the P gene. 3 OCA2 patients have a broad range of phenotypes, with minimal to moderate pigmentation of the hair, skin, and iris that may darken with age. OCA1 is the second most common type and is caused by mutations in the tyrosinase gene, TYR. 5 The lack of functional tyrosinase results in the complete absence of pigmentation in hair and skin. Rarer forms of OCA include OCA3, also known as ‘‘rufous/red albinism’’ and associated with mutations in the TYRP1 gene, and OCA4, associated with mutations in the MATP gene. Finally, some genetic defects in intracellular vesicle formation and trafficking have OCA as a major clinical component. 9 For example, Chediak-Higashi syndrome (CHS) is characterised by giant intracellular granules, an often fatal diathesis to infection, and variable degrees of hypopigmentation. Hermansky-Pudlak syndrome (HPS) involves OCA as part of a constellation of findings that include platelet storage pool deficiency and, in some patients, accumulation of ceroid pigment, pulmonary fibrosis, and/or granulomatous colitis. The sine qua non of HPS is absence of platelet dense bodies on whole mount electron microscopy. CHS and HPS can have overlapping phenotypes. For example, HPS-2, associated with mutations in the b3A subunit of adaptor complex-3, manifests with neutropenia and childhood infections reminiscent of CHS. 15 Because of our interest in hypopigmentation and disorders of intracellular vesicles, we investigated all patients having OCA plus a history of bleeding, frequent infections, or some other HPS-related symptoms. We screened eight such individuals with clinical manifestations suggestive of HPS. Five of the eight had definitive molecular evidence for OCA2. We now describe those patients and their P gene mutations.