Introduction to Hermansky–Pudlak syndrome
Clinical features and overview of Hermansky–Pudlak syndrome
Hermansky–Pudlak syndrome (HPS, OMIM 203300; Hermansky and Pudlak, 1959) is an autosomal recessive disorder characterized by oculocutaneous albinism (OCA), bleeding tendency, and ceroid deposition, which may cause lung fibrosis, colitis, and cardiomyopathy in some cases. Patients with HPS often die during the third to fifth decade (Huizing and Gahl, 2002; Huizing et al., 2000). The key pathological aspect of both human and mouse HPS is the disrupted biogenesis and/or function of lysosome-related organelles (LROs), including melanosomes and platelet-dense granules (DG) as well as secretory lysosomes (Dell'Angelica et al., 2000; Huizing et al., 2008; Li et al., 2004; Swank et al., 1998; Wei, 2006).
Hermansky–Pudlak syndrome is now known as a genetically heterogeneous, autosomal recessive inherited disorder. Nine genes (HPS1, AP3B1, and HPS3 to HPS9) have been identified as causative genes for HPS in humans (Anikster et al., 2001; Cullinane et al., 2011a; Dell'Angelica et al., 1999; Li et al., 2003; Morgan et al., 2006; Oh et al., 1996; Suzuki et al., 2002; Zhang et al., 2003). Additional six genes (Ap3d, Rabggta, Vps33a, Cno, Muted, Kxd1) cause mouse HPS (Li et al., 2004; Yang et al., 2012) and are listed in the HPS database (HPSD, http://liweilab.genetics.ac.cn/HPSD/; Li et al., 2006). As the HPS proteins have been categorized into several lysosomal-trafficking protein complexes such as AP-3, HOPS, BLOC-1, BLOC-2, and BLOC-3 (Di Pietro and Dell'Angelica, 2005; Li et al., 2004), genes encoding the subunits of these complexes that have not been defined as HPS proteins are putative HPS genes, as evidenced by the discovery of the HPS9 gene (Cullinane et al., 2011a).
Hermansky–Pudlak syndrome occurs in many countries, with more than 800 patients reported worldwide. It is almost certainly underestimated because of mis-diagnosis or un-diagnosis. The highest prevalence region of HPS is in Puerto Rico with founder effects. The incidence rate is estimated as 1:1800 with a carrier rate of 1:21 in Puerto Rico (Oh et al., 1996). HPS-1 and HPS-3 are the two common types of HPS in this region (Anikster et al., 2001). Non-Puerto Rican patients with HPS are scattered in many populations as listed in the HPSD database. HPS-1 is the relatively common subtype in Japanese (Ito et al., 2005) and Chinese OCA patients (Wei et al., 2010, 2011).
The pathogenesis underlying HPS results from defects in the biogenesis of LROs, which are described in more detail later.
Symptoms of HPS in humans have been reviewed extensively (DePinho and Kaplan, 1985; Huizing et al., 2008; Spritz, 2000). In 1959, two Czechoslovakian physicians Hermansky and Pudlak first described the pigmentary and non-pigmentary abnormalities in HPS (Hermansky and Pudlak, 1959). The most common symptoms of HPS are hypopigmentation, loss of visual acuity, prolonged bleeding, colitis, and, in some cases, fatal lung disease. Hemophagocytic lymphohistocytosis (Enders et al., 2006) and immune deficiency (Huizing et al., 2002) have been reported in patients with HPS-2. Neuronal symptoms are described in Ap3b1-deficient and Ap3b2-deficient mice (Seong et al., 2005). Participation of mouse AP-3 and BLOC-1 in synaptic vesicle biogenesis implicates potential neuronal dysfunction in patients with HPS (Chen et al., 2008; Larimore et al., 2011; Newell-Litwa et al., 2010). Although the dysbindin-null sdy mutant has been characterized as a mouse model of schizophrenia (Cox et al., 2009; Feng et al., 2008), the patient with HPS-7 did not show symptoms related to schizophrenia (Li et al., 2003). No pathological mutation of the HPS7/DTNBP1 gene has been identified yet in patients with schizophrenia.
Prolonged bleeding often requires multiple platelet transfusions, and the fibrotic lung disease may lead to death in midlife.
There is presently no cure for HPS. Only symptomatic (e.g., sunscreen to avoid sunburn, platelet transfusion and use of desmopressin in the correction of prolonged bleeding) treatments for the disease exist. Stem cell therapy may be promising in alleviating symptoms such as bleeding and visual loss.
The gold standard of HPS is the absence of platelet-DG upon electron microscopy (EM). Symptoms of hypopigmentation and bleeding support the diagnosis. Molecular diagnoses are now available with the identification of nine HPS genes in humans. Chediak–Higashi syndrome (CHS, OMIM #214500) exhibits similar defects on the biogenesis of LROs due to the mutation on the CHS1/LYST gene (Barbosa et al., 1996). Mortality of CHS in childhood often results from frequent bacterial infections due to immunodeficiency or from an ‘accelerated phase’ lymphoproliferation into the major organs of the body (Blume and Wolff, 1972). Patients exhibiting milder clinical phenotypes survive to adulthood but develop progressive and often fatal neurological dysfunction (Karim et al., 2002). Griscelli syndrome (GS, OMIM #214450) also presents with hypopigmentation, immunological impairment, lymphohistiocytosis, or defects in the central nervous system (Meeths et al., 2010), but lacks defects in platelet DG (Chintala et al., 2007). Griscelli syndrome is caused by mutation of GS1/MYO5A (Pastural et al., 1997), GS2/RAB27A (Menasche et al., 2000), or GS3/MLPH (Menasche et al., 2003). Molecular diagnosis and EM examination of platelet-DG in patients with CHS or GS allow accurate differentiation from HPS (Table 1).
|Melanosomes||Oculocutaneous albinism (hypopimentation)||HPS, CHS, GS|
|Platelet granules||Bleeding diathesis||HPS, CHS|
|Synaptic vesicles|| |
|HPS-2, CHS, GS|
|Lytic granules||Immunodeficiency||HPS-2, CHS, GS|
|Azurophil granules||Neutropenia||HPS-2, CHS|
|Lamellar bodies||Lung fibrosis||HPS-1, HPS-4|
Mutations in HPS genes
Through positional candidate cloning, the first human HPS gene, HPS1, was identified in 1996 (Oh et al., 1996). This prompted the identification of the first murine HPS gene, Hps1/ep (Gardner et al., 1997; Spritz, 2000), and the cloning of 14 other HPS genes in mouse and eight human HPS genes thereafter (Table 2). Currently, proteins encoded by these human or murine HPS genes fall into several protein complexes in regulating vesicle trafficking in the endo-lysosomal system as summarized in several reviews (Di Pietro and Dell'Angelica, 2005; Huizing et al., 2008; Li et al., 2004; Sitaram and Marks, 2012; Wei, 2006). That is HPS1 and HPS4 in BLOC-3; AP3B1/HPS2 and AP3D in AP-3; HPS3, HPS5, and HPS6 in BLOC-2; HPS7, HPS8, HPS9, MUTED and CNO in BLOC-1. VPS33A is a subunit of HOPS. RABGGTA is the α-subunit of Rab geranylgeranyl transferase which is involved in the prenylation of Rab proteins. KXD1 is a BLOC-1 interactor that likely causes mild-form HPS when mutated (Yang et al., 2012). The cloning history of these HPS genes is listed in Table 2.
|HPS subtype||Human locus||Mouse mutant||Protein function||References of cloning|
|HPS-1||HPS1||pale-ear (ep)||BLOC-3 subunit||Gardner et al. (1997), Oh et al. (1996)|
|HPS-2||HPS2/AP3B1||pearl (pe)||AP-3 subunit||Dell'Angelica et al. (1999), Feng et al. (1997)|
|HPS-3||HPS3||cocoa (coa)||BLOC-2 subunit||Anikster et al. (2001), Suzuki et al. (2001)|
|HPS-4||HPS4||light-ear (le)||BLOC-3 subunit||Suzuki et al. (2002)|
|HPS-5||HPS5||ruby-eye 2 (ru2)||BLOC-2 subunit||Zhang et al. (2003)|
|HPS-6||HPS6||ruby-eye (ru)||BLOC-2 subunit||Zhang et al. (2003)|
|HPS-7||HPS7/DTNBP1||sandy (sdy)||BLOC-1 subunit||Li et al. (2003)|
|HPS-8||HPS8/BLOC1S3||reduced pigmentation (rp)||BLOC-1 subunit||Morgan et al. (2006), Starcevic and Dell'Angelica (2004)|
|HPS-9||HPS9/PLDN||pallid (pa)||BLOC-1 subunit||Cullinane et al. (2011a), Huang et al. (1999)|
|–||MUTED||muted (mu)||BLOC-1 subunit||Zhang et al. (2002a)|
|–||CNO||cappuccino (cno)||BLOC-1 subunit||Ciciotte et al. (2003)|
|–||KXD1||Kxd1-KO||BLOC-1 interactor||Yang et al. (2012)|
|–||AP3D||mocha (mh)||AP-3 subunit||Kantheti et al. (1998)|
|–||VPS33A||buff (bf)||HOPS subunit||Suzuki et al. (2003b)|
|–||RABGGTA||gunmetal (gm)||Rab geranylgeranyl transferase alpha subunit||Detter et al. (2000)|
The HPS1 gene is located on chromosome 10q24.2. It contains 20 exons (NCBI RefSeq: NM_000195), encoding a 700-aa HPS1 protein. There are three other transcription variants. Thirty-one alleles that cause HPS-1 have been identified and listed in the HPSD database. Most of the HPS1 gene mutations are frameshift mutations or nonsense mutations to produce truncated HPS1 proteins that disrupt the function of the HPS1 protein (Hermos et al., 2002; Oh et al., 1996, 1998). HPS1, together with HPS4, is an obligate subunit of BLOC-3. Loss of either subunit results in destabilization of the remaining subunits (Suzuki et al., 2002). Two missense mutations (p.L239P and p.L668P) have been reported. Overexpression of L668P-mutant HPS1 protein in HPS1-null melanocytes did not restore the stability of endogenous HPS4, suggesting this missense substitution is pathologic (Ito et al., 2005). Interestingly, the c.1932delC mutation leads to a longer HPS1 protein in which a novel 79-residue peptide replaces the wild-type 56-residue peptide after the mutation site at D644 (Wei et al., 2009). A similar elongated HPS1 protein is predicted for the c.1885delC mutation (Wei et al., 2011). In our Western blots, the elongated HPS1 protein is absent (Figure 1), indicating that the extension destabilizes the protein and is pathologic. The patient who carries the homozygous c.1932delC mutation shows typical OCA symptoms and the absence of platelet-DG (Figure 1), a gold standard for the diagnosis of HPS.
The HPS2 gene is located on chromosome 5q14.1. It contains 27 exons (NCBI RefSeq: NM_003664), encoding a 1094-aa AP3B1 protein, the β-subunit of the ubiquitous AP-3 complex (β3A). Eleven alleles that cause HPS-2 have been identified and listed in the HPSD database. Most of the HPS2 gene mutations are frameshift mutations or nonsense mutations to produce truncated AP3B1 proteins that disrupt the function of the AP3B1 protein, leading to the complete absence of β3A subunit and the destabilization of other AP-3 subunits (Clark et al., 2003; Dell'Angelica et al., 1999; Fontana et al., 2006; Huizing et al., 2002).
The HPS3 gene is located on chromosome 3q24. It contains 17 exons (NCBI RefSeq: NM_032383), encoding a 1004-aa HPS3 protein. Eight alleles that cause HPS-3 have been identified and listed in the HPSD database. Most of the HPS3 gene mutations are frameshift mutations or splicing mutations to produce truncated HPS3 proteins that disrupt the function of the HPS3 protein and BLOC-2 (Anikster et al., 2001; Boissy et al., 2005; Huizing et al., 2001a).
The HPS4 gene is located on chromosome 22q12.1. It contains 14 exons (NCBI RefSeq: NM_022081), encoding a 708-aa HPS4 protein. Four alternative transcription variants exist that differ in the 5′-UTR and coding regions compared to NM_022081. Thirteen alleles that cause HPS-4 have been identified and listed in the HPSD database. Most of the HPS4 gene mutations are frameshift mutations or nonsense mutations to produce truncated HPS4 proteins that disrupt the function of the HPS4 protein and BLOC-3 (Anderson et al., 2003; Bachli et al., 2004; Carmona-Rivera et al., 2011; Suzuki et al., 2002).
The HPS5 gene is located on chromosome 11p15.1. It contains 23 exons (NCBI RefSeq: NM_181507), encoding a 1129-aa HPS5 protein. Two alternative transcription variants exist that encode a 1015-aa HPS5 isoform. Eleven alleles that cause HPS-5 have been identified and listed in the HPSD database. Most of the HPS5 gene mutations are frameshift mutations that show severely decreased HPS5 mRNA, attributable to nonsense-mediated decay (Huizing et al., 2004; Zhang et al., 2003).
The HPS6 gene is located on chromosome 10q24.32. It contains only one exon (NCBI RefSeq: NM_024747), encoding a 775-aa HPS6 protein. Nine alleles that cause HPS-6 have been identified and listed in the HPSD database. Most of the HPS6 gene mutations are frameshift or nonsense mutations that disrupt the function of HPS6 protein and BLOC-2 (Huizing et al., 2009; Zhang et al., 2003).
The HPS7 or DTNBP1 gene is located on chromosome 6p22.3. It contains 10 exons (NCBI RefSeq: NM_032122), encoding a 351-aa dysbindin-1a protein. NM_183040 contains an additional segment in the coding region compared to NM_032122. The resulting 303-aa dysbindin-1b contains a shorter and distinct C-terminus compared to dysbindin-1a. NM_183041 contains an alternate splice site in the 5′ coding region and uses a downstream start codon, compared to NM_032122. The encoded 270-aa isoform dybindin-1c has a shorter N-terminus compared to dysbindin-1a. In addition, three dysbindin-2 isoforms and two dysbindin-3 isoforms were documented (Tang et al., 2009a; Talbot et al., 2009a). To date, only one homozygous nonsense mutation, p.Q103X, has been reported in a patient with HPS-7 (Li et al., 2003).
The HPS8 gene is located on chromosome 19q13.32. It contains only one exon (NCBI RefSeq: NM_212550), encoding a 202-aa BLOS3 protein. Two alleles that cause HPS-8 have been identified and listed in the HPSD database. No nonsense-mediated decay and destabilization of the BLOC-1 complex was observed to the p.S44X and p.Q150delC mutation (Cullinane et al., 2012; Morgan et al., 2006).
The HPS9 gene is located on chromosome 15q21.1. It contains five exons (NCBI RefSeq: NM_012388), encoding a 172-aa HPS9 protein. To date, only one homozygous nonsense mutation, p.Q78X, has been reported in a patient with HPS-9 (Cullinane et al., 2011a). The p.Q78X mutation does not cause nonsense- mediated decay directly but results in the skipping of exon3 (Cullinane et al., 2011a).
Mouse models of HPS
A major contribution of Dr. Richard T. Swank's laboratory was the collection, since the 1970s, of mouse mutants that mimic the human HPS phenotypes. Additional phenotypes such as susceptibility to anesthetics, protection from atherosclerosis, and otolith deficiency have been documented only in mouse HPS mutants as summarized in (Li et al., 2004; Swank et al., 1998). The characterization of more than a dozen mouse HPS mutants (Swank et al., 1998) led to a series of successful identifications of murine and human HPS genes (Li et al., 2004; Table 2). Currently, the 15 cloned mouse HPS genes are known to reside in several HPS Protein Associated Complexes (HPAC, shown in Table 2 and Figure 2) to mediate the biogenesis of LROs.
With the discovery of murine and human HPS genes, three biogenesis of lysosome-related organelles complexes (BLOC-1, BLOC-2, and BLOC-3) have been defined. That is, pallidin, muted, dysbindin, cappuccino, snapin, BLOS1, BLOS2, and BLOS3 in BLOC-1 (Ciciotte et al., 2003; Falcon-Perez et al., 2002; Gwynn et al., 2004; Li et al., 2003; Starcevic and Dell'Angelica, 2004); HPS3, HPS5, and HPS6 in BLOC-2 (Di Pietro et al., 2004; Gautam et al., 2004; Zhang et al., 2003); and HPS1 and HPS4 in BLOC-3 (Chiang et al., 2003; Martina et al., 2003; Nazarian et al., 2003; Suzuki et al., 2002). The biochemical features and assembling machineries of these BLOC complexes remain to be defined, although some pioneering studies have revealed binding domains in building these BLOCs (Dell'Angelica, 2004; Li et al., 2007) and the linear assembly of the BLOC-1 complex in vitro (Lee et al., 2012). Together with the well-known AP-3 (adaptor protein complex-3) and HOPS (homotypic fusion and protein sorting complex) complexes, the BLOC complexes function in endo-lysosomal trafficking. Emerging evidence has shown that these complexes direct cargoes from either de novo synthesis or endocytosis into lysosomes and LROs. In addition, whether there is a master regulator to coordinate the action of these complexes remains unknown. A key to understanding LRO biogenesis is to define the behaviors of these interacting complexes. An intriguing question is whether these complexes act synergistically or sequentially on cargo transport. Double or multiple mouse mutants played a pivotal role in dissecting these interactions as evidenced by epistatic or synergistic effects on coat color or LRO phenotypes (Gautam et al., 2006; Hoyle et al., 2011). At the cellular level, the mouse HPS mutants provide powerful tools for the dissection of cargo-specific endo-lysosomal-trafficking pathways in different tissues.
As revealed by spontaneous murine HPS mutants, one would expect the development of HPS when generating knockout mutants for the remaining subunits of the HPACs. While the mutants of known subunits of BLOC-2 and BLOC-3 are murine HPS models (Li et al., 2004), it is unknown whether mutation of the genes encoding snapin, BLOS1, and BLOS2 in the BLOC-1 complex would cause typical HPS. In contrast, the snapin knockout (KO) mice died at the perinatal stage, which is unlike other BLOC-1 mutants and showed developmental defects of the central nervous system (Tian et al., 2005; Zhou et al., 2011). This suggests that snapin may play extra roles independent of BLOC-1. Whether the BLOS1 or BLOS2 knockout mice exhibit typical HPS phenotypes with comparable survival remains to be investigated. Interestingly, multiple isoforms of dysbindin have been shown both in human and mouse tissues (Talbot et al., 2009). Studies have shown the different distribution pattern of these isoforms in developmental stages and in sub-brain regions and sub-synaptic regions (Ito et al., 2010; Talbot et al., 2011; Tang et al., 2009a). The involvement of dysbindin in the dystrobrevin complex (Benson et al., 2001) and WAVE2-Abi-1 complex (Ito et al., 2010) has raised the question whether all these isoforms function in a BLOC-1-dependent manner.
The AP-3 complex is a heterotetramer composed of two large adaptins (AP3D1/δ and AP3B1/β3A or AP3B2/β3B), a medium adaptin (AP3M1/μ3A or AP3M2/μ3B), and a small adaptin (AP3S1/σ3A or AP3S2/σ3B). There exist two types of mammalian AP-3 complexes: a ubiquitous AP-3 comprising AP3D1-AP3B1-AP3M1-AP3S1 (or AP3S2) subunits and a brain-specific AP-3 complex containing AP3D1-AP3B2-AP3M2-AP3S1 (or AP3S2) subunits as summarized in a recent review (Dell'Angelica, 2009). In mice, only mutation of the Ap3b1 (in pearl mice) or Ap3d1 (in mocha mice) have been shown to present HPS phenotypes (see Table 2). Knockout of the neuron-specific Ap3b2 or Ap3m2 results in neurological impairments but not HPS (Nakatsu et al., 2004; Newell-Litwa et al., 2009; Seong et al., 2005). This leads to the notion that tissue-specific complexes are formed to execute special physiological functions.
The class C Vps complexes (HOPS and CORVET) are involved in homotypic fusion and tethering during endosomal trafficking (Nickerson et al., 2009). Both HOPS and CORVET complexes share the four Vps proteins (class C core), VPS11-VPS16-VPS18-VPS33. The HOPS complex contains in addition two Rab-binding proteins, VPS41 and VPS39, whereas the CORVET complex has the VPS41 homolog VPS8 and the VPS39 homolog VPS3. The CORVET and HOPS complexes interconvert through two intermediate complexes consisting of the class C core bound to VPS39-VPS8 or VPS3-VPS41 (Peplowska et al., 2007). In the HOPS complex, the large head contains VPS41, VPS33, and VPS16, whereas Vps39 is found in the tip of its tail, with VPS11 and VPS18 connecting the head and tail (Brocker et al., 2012). VPS33 is a Sec1-/Munc18-like protein that interacts with SNAREs. In metazoans, two homologs, VPS33A and VPS33B, are present. It is uncertain whether VPS33B is a component of the COVERT complex (Zlatic et al., 2011), while VPS33A is a part of the HOPS complex (Sriram et al., 2003). Mutation of Vps33a in the bf mice leads to HPS with additional neurological lesions, although the underlying mechanism leading to Purkinje cell loss and neurological atrophy is unknown (Chintala et al., 2009; Suzuki et al., 2003b). A mutation in human VPS33B causes arthrogryposis-renal dysfunction-cholestasis syndrome but not HPS (Gissen et al., 2004). Again, this suggests that distinct mammalian Vps-C complexes function differently.
The HPS mouse model gunmetal (gm) is deficient in the α-subunit of Rab GGTase II (RABGGTA; Detter et al., 2000). Together with the β-subunit and an escort protein, REP-1, RABGGTA adds two geranyl 10-carbon isoprenoid groups to the C-termini of Rab proteins (Anant et al., 1998). In gm mutant, the prenylation of Rab proteins (such as Rab27a, 11a, and 4) is deficient in platelets and melanocytes (Zhang et al., 2002b), which may explain the defects in the biogenesis of platelet granules and melanosomes, thus mimicking the symptoms of HPS. Currently, no RABGGTA mutation has been reported in patients with HPS or storage pool deficiency (Li et al., 2000). However, mutations in the CHM gene that encodes REP-1 are a cause for choroideremia (also known as tapetochoroidal dystrophy). This X-linked disease is characterized by progressive dystrophy of the choroid, retinal pigment epithelium, and retina (Sankila et al., 1992). Defective Rab prenylation and depigmentation occur in Chm knockout mice (Tolmachova et al., 2006).
RAB38 forms a complex with VARP in mediating the melanosomal transport of TYRP1 (Tamura et al., 2009; Wang et al., 2008). Mutation of the melanosomal protein RAB38 in chocolate (cht) mice causes dilution in coat color (Loftus et al., 2002) and ocular hypopigmentation (Brooks et al., 2007), suggesting that cht is a mouse model of OCA. In addition, defects in melanosomes (Brooks et al., 2007) and lamellar bodies (LBs; Osanai et al., 2008) were observed in cht mice, suggesting that RAB38 is a candidate HPS gene affecting multiple LROs (Brooks et al., 2007; Osanai and Voelker, 2008). Consistent with this possibility, the rat RAB38 is null in Fawn-Hooded and Tester-Moriyama rats which mimic the human HPS phenotype (Oiso et al., 2004). Other evidence to show the involvement of RAB38 in the development of HPS is the interactions between RAB38 and BLOC-1, BLOC-2, and AP-3, which likely transport the cargoes of BLOC-2, AP-3, and AP-1 into melanosomes (Bultema et al., 2012). In addition, BLOC-3 has been shown to function as a guanine nucleotide exchange factor for RAB38 and RAB32 (Gerondopoulos et al., 2012). Taken together, these data suggest that RAB38 and RAB32 function with other HPS protein complexes, implicating the loss-of-function of RAB38 or RAB32 may develop similar phenotypes as HPS. However, no prolonged bleeding times or blood defects occur in cht mice (Brooks et al., 2007; Loftus et al., 2002). One possible explanation is a hypermorphic effect in cht mice. The existing mutant hydrophilic RAB38 (Osanai et al., 2008) may function in platelets without affecting the biogenesis of DG. Another explanation is redundant function of a RAB38 homolog, RAB32, in platelets (Wasmeier et al., 2006). In a recent study, RAB38 and RAB32 are involved in the cargo vesicle fusion with mature organelles during the biogenesis of platelet-DG (Ambrosio et al., 2012). Currently, no mutation of the RAB38 gene has been reported in patients with HPS or OCA (Brooks et al., 2007; Suzuki et al., 2003a).
Misty (m) mice show generalized hypopigmentation and prolonged bleeding times similar to HPS mouse mutants (Sviderskaya et al., 1998; Swank et al., 1998), but no evidence of a generalized LRO anomaly (Blasius et al., 2009). Thus, the mutation of Dock7 (a Rho family guanine nucleotide exchange factor) in misty mice (Blasius et al., 2009) is unlikely to cause murine HPS. The subtle gray (sut) mouse was regarded as a model for a mild form of HPS (Swank et al., 1996, 1998). However, the hypopigmentation is mainly caused by the reduction in pheomelanin production due to the mutation of the Slc7a11 gene which encodes the functional subunit of the cystine/glutamate antiporter, xCT (Chintala et al., 2005). Although the function of xCT in the biogenesis of platelet-dense granule has not been intensively studied, no generalized LRO anomaly in sut mice has been described, excluding it as a typical mouse HPS model. The ashen (ash) line maintained at Roswell Park Cancer Institute (referred as ash-Roswell) that was regarded as a mouse model of HPS (Li et al., 2004; Wilson et al., 2000) has been identified to be a double mutant with mutations in both the Rab27a gene and the Slc35d3 gene (Chintala et al., 2007; Wilson et al., 2000). The deficiency in Rab27a causes a defect in melanosomal transport which leads to hypopigmentation (Wilson et al., 2000), while the deficiency of Slc35d3 leads to defects in the biogenesis of platelet-DG (Chintala et al., 2007; Meng et al., 2012). The Rab27a mutation does not cause defects in platelet-DG or prolonged bleeding (Barral et al., 2002). In contrast, hypopigmentation is not seen in the Roswell (ros)-mutant mice, which carry only the Slc35d3 mutation. SLC35D3 may not be expressed in melanocytes (our unpublished data). Thus, the identified HPS mouse models have been updated to 15 lines as listed in Table 2.