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.     

Human blood platelets showing no response to collagen fail to express surface glycoprotein Ia

The interaction of blood platelets with collagen is generally considered to be of primary importance in the arrest of bleeding and to have a role in the pathogenesis of thrombosis and atherosclerosis. Following damage to the vascular endothelium, circulating platelets come into contact with exposed collagen fibrils in the subendothelium and spread along it; this is followed by the secretion of several biologically active substances and by aggregation of platelets. The glycoproteins of the platelet plasma membrane have an important role in the mechanisms underlying these processes. So far, two specific defects of platelet function in patients with a bleeding disorder are known to be associated with a glycoprotein defect and the study of these patients has contributed significantly to present concepts of platelet function. The glycoprotein (GP) IIB-III complex, absent or deleted in the aggregation-defective Glanzmann's thrombasthenia, has been identified as the platelet fibrinogen receptor. GPIb, which is absent in the adhesion-defective Bernard-Soulier syndrome, has been identified as the von Willebrand factor receptor on platelets. We now report a defect of the platelet plasma membrane glycoprotein composition in a patient whose platelets are totally unresponsive to collagen.