In the last decade, Hermansky-Pudlak syndrome (HPS) has arisen as an instructive disorder for cell biologists to study the biogenesis of lysosome-related organelles (LROs). Of the eight human HPS subtypes, only subtypes 1 through 5 are well described. Here, we carefully characterize the HPS-6 subtype, caused by defects in HPS6, a subunit of the biogenesis of lysosome-related organelles complex-2 (BLOC-2).
Mutation analysis for the HPS6 gene was performed on DNA from our group of unclassified HPS patients. The clinical phenotype of patients with HPS6 mutations was then carefully ascertained, and their cultured dermal melanocytes were employed for cellular immunofluorescence studies.
Molecular studies showed a variety of mutations in the single-exon HPS6 gene, including frame shift, missense, and nonsense mutations as well as a ~20-kb deletion spanning the entire HPS6 gene. Cellular studies revealed that the melanogenic proteins tyrosinase and tyrosinase-related protein 1 failed to be efficiently delivered to the melanosomes of HPS-6 patients, explaining their hypopigmentation. Clinical studies indicated that HPS-6 patients exhibit oculocutaneous albinism and a bleeding diathesis. Importantly, granulomatous colitis and pulmonary fibrosis, debilitating features present in HPS subtypes 1 and 4, were not detected in our HPS-6 patients.
In sum, the HPS-6 subtype resembles other BLOC-2 defective subtypes (i.e., HPS-3 and HPS-5) in its molecular, cellular and clinical findings. These findings are not only important for providing a prognosis to newly diagnosed HPS-6 patients, but also for further elucidation of HPS function in the biogenesis of LROs.
Keywords: albinism, BLOC, lysosome-related organelle, melanosome, platelet