Hermansky-Pudlak syndrome is caused by mutations in HPS4, the human homolog of the mouse light-ear gene.

Hermansky-Pudlak syndrome (HPS) is a disorder of organelle biogenesis in which oculocutaneous albinism, bleeding and pulmonary fibrosis result from defects of melanosomes, platelet dense granules and lysosomes. HPS is common in Puerto Rico, where it is caused by mutations in the genes HPS1 and, less often, HPS3 (ref. 8). In contrast, only half of non-Puerto Rican individuals with HPS have mutations in HPS1 (ref. 9), and very few in HPS3 (ref. 10). In the mouse, more than 15 loci manifest mutant phenotypes similar to human HPS, including pale ear (ep), the mouse homolog of HPS1 (refs 13,14). Mouse ep has a phenotype identical to another mutant, light ear (le), which suggests that the human homolog of le is a possible human HPS locus. We have identified and found mutations of the human le homolog, HPS4, in a number of non-Puerto Rican individuals with HPS, establishing HPS4 as an important HPS locus in humans. In addition to their identical phenotypes, le and ep mutant mice have identical abnormalities of melanosomes, and in transfected melanoma cells the HPS4 and HPS1 proteins partially co-localize in vesicles of the cell body. In addition, the HPS1 protein is absent in tissues of le mutant mice. These results suggest that the HPS4 and HPS1 proteins may function in the same pathway of organelle biogenesis.

p16(Ink4a) in melanocyte senescence and differentiation.

BACKGROUND: The Ink4a-Arf tumor suppressor locus encodes two growth inhibitors, p16 and Arf, both of which are also implicated as effectors in cellular senescence. Because human germline defects in the INK4A-ARF locus are associated with familial melanoma, melanocytes may have unusual INK4A-ARF functions or controls of cell senescence. Because senescence is believed to be an anticancer mechanism, we investigated the role of Ink4a-Arf and its individual components in melanocyte senescence. METHODS: Melanocytes were cultured from littermate mice with zero, one, or two functional copies of the Ink4a-Arf locus. Senescence was evaluated by cumulative population doubling curves and by the assessment of acidic beta-galactosidase (an indicator of senescence) expression. Pigmentation and cell size were evaluated by spectrophotometry and microscopy. p16 and Arf expression in primary and spontaneously immortalized melanocyte or melanocyte precursor cell lines were evaluated by immunoblotting. Retroviral vectors containing normal p16 and Arf complementary DNAs were used to restore expression of these genes in Ink4a-Arf(-/-) melanocytes. RESULTS: Wild-type melanocytes (i.e., Ink4a-Arf(+/+)) senesced within 4-5 weeks of culture. Ink4a-Arf(-/-) melanocytes did not senesce and readily became immortal. Ink4a-Arf(+/-) melanocytes showed defective senescence. Senescent Ink4a-Arf(+/+) melanocytes were heavily pigmented, but Ink4a-Arf(+/-) and Ink4a-Arf(-/-) melanocytes were less pigmented. All of six spontaneously immortalized melanocyte or melanocyte precursor lines from Ink4a-Arf(+/+) mice lacked p16 protein expression, although most retained Arf protein expression. After restoration of p16 but not Arf expression, Ink4a-Arf(-/-) melanocytes stopped growing, became highly melanized, and expressed acidic beta-galactosidase. By contrast, restoration of Arf but not p16 expression led to cell death without evidence of senescence. CONCLUSION: Normal mouse melanocyte senescence and associated pigmentation require both copies of Ink4a-Arf and appear to depend more on p16 than on Arf function. Mutations of the INK4A-ARF locus may favor tumorigenesis from melanocytes by impairing senescence, cell differentiation, and (where ARF is disrupted) cell death.

p16/cyclin-dependent kinase inhibitor 2A deficiency in human melanocyte senescence, apoptosis, and immortalization: possible implications for melanoma progression.

BACKGROUND: The melanoma susceptibility locus cyclin-dependent kinase inhibitor 2A encodes two unrelated cell growth inhibitors, p16 and alternative reading frame (ARF). In fibroblasts, both proteins are implicated in cellular senescence, a key barrier to tumor development. The p16 coding sequence is more often mutated in melanoma families than is the ARF sequence. To investigate the role of p16 in melanocytes, we assessed aspects of growth, apoptosis, and immortalization in melanocytes cultured from two melanoma patients, both of whom had two inactive p16 alleles but functional ARF. METHODS: Growth and senescence were evaluated by cumulative population-doubling curves, and apoptosis by terminal deoxytransferase labeling. Expression of p53 and p21, which are associated with fibroblast senescence, was assessed by immunoblotting. Amphotropic retroviruses were used to transfer exogenous gene sequences into the melanocytes. RESULTS: Both melanocyte cultures showed high rates of apoptosis, which were reduced when the cells were grown in the presence of keratinocyte feeder cells or human stem cell factor plus endothelin 1. With these growth factors, both cultures proliferated for 45-55 net population doublings, markedly longer than the maximum of 10 net population doublings of normal adult human melanocytes in similar media, indicating impaired senescence. One of the cultures developed chromosomal aberrations, with numerous dicentric chromosomes at senescence, consistent with telomere dysfunction. p53 and p21 levels were not elevated in senescent normal melanocytes but were elevated in senescent p16-deficient melanocytes. Interference with p53 function by transfer of human papillomavirus 16-E6 further extended the lifespan of p16-deficient melanocytes. Human telomerase reverse transcriptase was sufficient to immortalize both these cell strains but not normal melanocytes. CONCLUSION: Normal senescence in human melanocytes requires p16 activity. p53 contributes to a delayed form of senescence that requires telomere shortening, in p16-deficient melanocytes. These findings provide some basis for the role of p16 in melanoma susceptibility.