Cappuccino, a mouse model of Hermansky-Pudlak syndrome, encodes a novel protein that is part of the pallidin-muted complex (BLOC-1)

Abstract

Hermansky-Pudlak syndrome (HPS) is a disorder of organelle biogenesis affecting 3 related organelles-melanosomes, platelet dense bodies, and lysosomes. Four genes causing HPS in humans (HPS1-HPS4) are known, and at least 15 nonallelic mutations cause HPS in the mouse. Where their functions are known, the HPS-associated proteins are involved in some aspect of intracellular vesicular trafficking, that is, protein sorting and vesicle docking and fusion. Biochemical and genetic evidence indicates that the HPS-associated genes encode components of at least 3 distinct protein complexes: the adaptor complex AP-3; the HPS1/HPS4 complex; and BLOC-1 (biogenesis of lysosome-related organelles complex-1), consisting of the proteins encoded at 2 mouse HPS loci, pallid (pa) and muted (mu), and at least 3 other unidentified proteins. Here, we report the cloning of the mouse HPS mutation cappuccino (cno). We show that the wild-type cno gene encodes a novel, ubiquitously expressed cytoplasmic protein that coassembles with pallidin and the muted protein in the BLOC-1 complex. Further, we identify a frameshift mutation in mutant cno/cno mice. The C-terminal 81 amino acids are replaced with 72 different amino acids in the mutant CNO protein, and its ability to interact in BLOC-1 is abolished. We performed mutation screening of patients with HPS and failed to identify any CNO defects. Notably, although defects in components of the HPS1/HPS4 and the AP-3 complexes are associated with HPS in humans, no defects in the known components of BLOC-1 have been identified in 142 patients with HPS screened to date, suggesting that BLOC-1 function may be critical in humans. IntroductionIn the autosomal recessive Hermansky-Pudlak syndrome (HPS) defects in lysosome-related organelles (melanosomes, platelet dense bodies, lysosomes) cause albinism, prolonged bleeding, and lysosomal storage disease. 1,2 The worldwide incidence of HPS is unknown, but it occurs in all ethnic groups and its frequency is very high in some populations due to founder effects (eg, Northwest Puerto Rico, 1/1800). 3 HPS is associated with considerable morbidity and mortality. Melanosome defects lead to oculocutaneous albinism resulting in nystagmus, decreased visual acuity, and skin damage on exposure to sunlight. 4 Platelet dense body defects result in prolonged bleeding, which can be severe. 4,5 Pulmonary fibrotic disease, possibly the result of ceroid lipofuscin accumulation in lysosomes, is progressive and is known to occur in patients with HPS-1 and HPS-4. In such individuals, death typically occurs in the fourth to fifth decades. [5][6][7] In mice, at least 15 nonallelic mutations cause HPS. 8 Eight are cloned, including the orthologues of the human genes HPS1-HPS4 (pale ear [ep], pearl [pe], cocoa [coa], light ear [le]), and mocha (mh), pallid (pa), ashen (ash), and muted (mu). [9][10][11][12][13][14][15][16][17] The known or suspected functions of the HPS proteins implicate them in some aspect of intracellular vesicular trafficking in the biogenesis of lysosome-rel...

Identification of Snapin and Three Novel Proteins (BLOS1, BLOS2, and BLOS3/Reduced Pigmentation) as Subunits of Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1)

“…The genes encoding the Pallidin, Muted, Cappuccino, and Dysbindin subunits are mutated in the pallid, muted, cappuccino, and sandy mouse strains, respectively, which display the most severe coat color dilution phenotypes among known mouse models of HPS (16,22,28,29). In addition, a nonsense mutation in human Dysbindin has been implicated in the pathogenesis of HPS type 7 (16).…”

BLOC-1 Is Required for Cargo-specific Sorting from Vacuolar Early Endosomes toward Lysosome-related Organelles

“…The genes disrupted in human HPS-7 (Li et al, 2003) and HPS-8 (Morgan et al, 2006) and in the mouse HPS models pallid, muted, reduced pigmentation (rp), cappuccino, and sandy encode five of the eight known subunits of a stable protein complex known as biogenesis of lysosome-related organelles complex (BLOC)-1 (Falcon-Perez et al, 2002;Moriyama and Bonifacino, 2002;Ciciotte et al, 2003;Li et al, 2003;Gwynn et al, 2004;Starcevic and Dell'Angelica, 2004). To date, no specific subcellular function has been assigned to BLOC-1.…”

“…Those LROs that are most severely affected in all forms of HPS-pigment cell melanosomes, platelet dense granules, and lung lamellar bodies-are unique in that they coexist with bona fide lysosomes in their respective cell types (Dell'Angelica et al, 2000;Marks and Seabra, 2001). The 15 known HPS-associated genes have been identified, and although the products of most are thought to participate in trafficking events that are uniquely required to form this class of LRO, the function of only a few is understood in detail.The genes disrupted in human HPS-7 (Li et al, 2003) and HPS-8 (Morgan et al, 2006) and in the mouse HPS models pallid, muted, reduced pigmentation (rp), cappuccino, and sandy encode five of the eight known subunits of a stable protein complex known as biogenesis of lysosome-related organelles complex (BLOC)-1 (Falcon-Perez et al, 2002;Moriyama and Bonifacino, 2002;Ciciotte et al, 2003;Li et al, 2003;Gwynn et al, 2004;Starcevic and Dell'Angelica, 2004). To date, no specific subcellular function has been assigned to BLOC-1.…”

An immunoblotting assay to facilitate the molecular diagnosis of Hermansky–Pudlak syndrome

“…At the very least, the eight genes already associated with the human disease (i.e., HPS1, AP3B1, HPS3, HPS4, HPS5, HPS6, DTNBP1 and BLOC1S3) should be included. It can also be argued that other genes encoding subunits of AP-3 or a BLOC should be taken into consideration as well, especially those for which mutations in the mouse counterpart have been shown to result in HPS-like phenotypes [34][35][36][37], thus potentially raising the number of candidate genes to eighteen (Table 1). On the other hand, one might consider excluding AP3S1 and AP3S2 from the list on the basis of an argument of genetic redundancy (i.e., both genes encode alternative AP-3 subunits with apparently the same molecular function [38]) or AP3D1 on the basis of the occurrence in Ap3d1-null mice of neurological phenotypes not yet observed in HPS patients [34].…”