Related Organelles Research Articles

The lysosome or lysosome-related organelle may serve as a Ca 2+ store in the boutons of hippocampal pyramidal cells

Boutons are specialised presynaptic compartments that lie along the axons of central neurons. Release of neurotransmitter from boutons is tightly regulated by the level of intracellular calcium [Ca 2+] i. A rise in Ca 2+ level may be generated in several ways; entry of extracellular Ca 2+ via voltage gated calcium channels (VGCCs), entry via ligand-operated channels (LOCs) or the release of Ca 2+ from intracellular Ca 2+ stores. The role of Ca 2+ stores in boutons remains poorly understood, despite recent work indicating that the release of Ca 2+ from the endoplasmic reticulum (ER) may contribute to transmitter release. In this study we assess whether the lysosome or a closely related organelle functions as a Ca 2+ store in the boutons of hippocampal pyramidal neurones. Lysosomes are small acidic organelles more commonly known for their role in degrading redundant cellular constituents. Using a fluorescent lysosomal marker, we show that lysosomes are located in the axons of hippocampal CA3 neurones. Selective pharmacological lysis of the lysosomes with glycyl-phenylalanine 2-napthylamide (GPN) generates rapid, highly focal Ca 2+ transients within the axon and increases the frequency of spontaneous miniature excitatory post-synaptic currents (mEPSCs), revealing that the organelle contains Ca 2+ at a concentration sufficient to evoke transmitter release. Confocal laser scanning microscopy, combined with electrophysiology is used to monitor the action potential evoked increases in [Ca 2+] i in boutons. We show that disruption of lysosomes compromises action potential evoked [Ca 2+] i but this effect is occluded if the ER is discharged. Conversely, disruption of the lysosome does not appear to impact on the capacity of the ER to release Ca 2+. These results suggest that the lysosome may serve as a Ca 2+ store within hippocampal boutons, with a Ca 2+ signalling role that is unique from that of the ER.

Proteomics of the human brain: sub-proteomes might hold the key to handle brain complexity

Proteomics is a promising approach, which provides information about the expression of proteins and increasingly finds application in life science and disease research. Meanwhile, proteomics has proven to be applicable even on post mortem human brain tissue and has opened a new area in neuroproteomics. Thereby, neuroproteomics is usually employed to generate large protein profiles of brain tissue, which mostly reflect the expression of highly abundant proteins. As a complementary approach, the focus on sub-proteomes would enhance more specific insight into brain function. Sub-proteomes are accessible via several strategies, including affinity pull-down approaches, immunoprecipitation or subcellular fractionation. The extraordinary potential of subcellular proteomics to reveal even minute differences in the protein constitution of related cellular organelles is exemplified by a recent global description of neuromelanin granules from the human brain, which could be identified as pigmented lysosome-related organelles.

Innate immunity defects in Hermansky-Pudlak type 2 syndrome

AbstractAdaptor protein-3 (AP-3) is an ubiquitous cytoplasmic complex that shuttles cargo proteins from the trans-Golgi and a tubular-endosomal compartment to endosome-lysosome–related organelles. Lack of the β3A subunit of this complex causes Hermansky-Pudlak syndrome type 2, an autosomal recessive disease characterized by partial albinism, prolonged bleeding tendency, and immunodeficiency. To investigate the pathogenesis of immunodeficiency, we studied natural killer (NK) cells and neutrophil functions in 2 previously unreported siblings affected by Hermansky-Pudlak type 2 syndrome. In both patients we observed a dramatic reduction of cytolytic activity of freshly isolated and of IL-2–activated NK cells. Levels of perforin were reduced in unstimulated NK cells, thereby accounting for the impairment of NK cytolitic activity. In addition, analysis of neutrophils in these patients demonstrated that intracellular elastase content was largely reduced while CD63 expression on plasma membrane was substantially increased. Taken together, these observations suggest that type 2 Hermansky-Pudlak syndrome is characterized by defects of innate immunity.

BLOC-1 Interacts with BLOC-2 and the AP-3 Complex to Facilitate Protein Trafficking on Endosomes

The adaptor protein (AP)-3 complex is a component of the cellular machinery that controls protein sorting from endosomes to lysosomes and specialized related organelles such as melanosomes. Mutations in an AP-3 subunit underlie a form of Hermansky-Pudlak syndrome (HPS), a disorder characterized by abnormalities in lysosome-related organelles. HPS in humans can also be caused by mutations in genes encoding subunits of three complexes of unclear function, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2, and -3. Here, we report that BLOC-1 interacts physically and functionally with AP-3 to facilitate the trafficking of a known AP-3 cargo, CD63, and of tyrosinase-related protein 1 (Tyrp1), a melanosomal membrane protein previously thought to traffic only independently of AP-3. BLOC-1 also interacts with BLOC-2 to facilitate Tyrp1 trafficking by a mechanism apparently independent of AP-3 function. Both BLOC-1 and -2 localize mainly to early endosome-associated tubules as determined by immunoelectron microscopy. These findings support the idea that BLOC-1 and -2 represent hitherto unknown components of the endosomal protein trafficking machinery.

Role of impaired lysosomal trafficking in pulmonary fibrosis

Rationale: Hermansky-Pudlack Syndrome (HPS) is an autosomal recessive disorder in which mutations on one of the six human genes HPS1, ADB3A, HPS3, HPS4, HPS5 and HPS6 results in impairment of lysosomal transport. Along with melanosomes, platelet dense bodies and lysosomes, HPS also results in impaired processing of lamellar bodies of type II pneumocytes – the lysosome related organelles, responsible for surfactant trafficking in the lungs. The most serious complication of HPS is severe pulmonary fibrosis, which is almost indistinguishable from clinical Idiopathic Pulmonary Fibrosis (IPF). In the present study, we investigated biochemical surfactant alterations in lung tissue and bronchoalveolar lavage fluid (BALF) from HPS1/6 (ep/ru) double mutant mice. Results: Using immunoblotting, increased levels of pro SP-B and decreased levels of mature SP-B were found in the lung tissue and in the BALF respectively, of ep/ru double mutant mice as compared to wild type control mice of the same background. No major alterations of pro and mature forms of SP-C were detected. High Performance Thin Layer Chromatographic analysis showed that the levels of phosphatidylcholine (PC) and phosphatidylglycerol (PG) were strongly increased in the lung tissue of ep/ru double mutant mice as compared to control mice. In parallel, the fatty acid profiles by means of gas-liquid chromatography showed that the relative content of palmitic acid in lung tissue PC of ep/ru mice was significantly increased when compared to control mice. Conclusion: Altered post-translational processing of surfactant components leads to an impairment of lamellar body genesis, which might be involved in the pathogenesis of IPF and pulmonary fibrosis associated with HPS.

Role of impaired lysosomal trafficking in pulmonary fibrosis

Rationale: Hermansky-Pudlack Syndrome (HPS) is an autosomal recessive disorder in which mutations on one of the six human genes HPS1, ADB3A, HPS3, HPS4, HPS5 and HPS6 results in impairment of lysosomal transport. Along with melanosomes, platelet dense bodies and lysosomes, HPS also results in impaired processing of lamellar bodies of type II pneumocytes – the lysosome related organelles, responsible for surfactant trafficking in the lungs. The most serious complication of HPS is severe pulmonary fibrosis, which is almost indistinguishable from clinical Idiopathic Pulmonary Fibrosis (IPF). In the present study, we investigated biochemical surfactant alterations in lung tissue and bronchoalveolar lavage fluid (BALF) from HPS 1/6 (ep/ru) double mutant mice.

Differentiation of cell organelles during pollen formation?maturation and pollen tube elongation-focusing on the vacuolar system-

Summary: The ultra-structural changes and behavior of vacuoles and related organelles(rER and Golgi bodies)during pollen development and maturation of Arabidopsis thaliana, and pollen tube elongation of Tradescantia reflexa will be described. In microspores just before generative cell formation, a large vacuole appeared which was made by fusion of pre-existing vacuoles and probably absorption of solutions. In the young pollen grain after the first mitosis, a large vacuole was divided into small vacuoles. The manner of division was not by binary fission and centripetally, but by the invagination of tonoplasts from one side to the opposite side of a vacuole. After the second mitosis, somatic typed vacuoles disappeared. In mature pollen grains just before germination, membrane-bound structures containing fine fibrillar substances(MBFs)appeared.Because of the expression of the pB vacuolar processing enzyme gene in these mature pollen grains, the MBFs were considered as storage vacuoles. In pollen grains from flowers in bloom, MBFschanged to lysosomal structures with acid phosphatases(lytic vacuole). They gradually increased in number andvolume, and decomposed the cytoplasm. A large vacuole at the base of elongating pollen tube may develope by membrane supply controlled with the ER and lipid granules, in addition to the absorption of water.

Mutational data integration in gene-oriented files of the Hermansky-Pudlak Syndrome database

Hermansky-Pudlak Syndrome (HPS) is a genetically heterogeneous disorder characterized by oculocutaneous albinism and prolonged bleeding due to abnormal vesicle trafficking to lysosomes and related organelles such as melanosomes and platelet dense granules. This HPS database (HPSD; http://liweilab.genetics.ac.cn/HPSD/) provides integrated, annotatory, and curative data that is distributed in a variety of public databases or predicted by bioinformatics servers for the recently cloned human and mouse HPS genes, as well as for the genes responsible for HPSrelated syndromes, such as ChediakHigashi Syndrome (CHS), Griscelli syndrome (GS), oculocutaneous albinism (OCA), Usher syndrome type 1B (USH1B), and ocular albinism (OA). The HPSD is designed by using a unique GeneOriented File (GOF) format. Seven blocks (genomic, transcript, protein, function, mutation, phenotype, and reference) are carefully annotated in each userfriendly GOF entry. The HPSD emphasizes paired human and mouse GOF entries. The genes included in this database (currently 58 in total) are arbitrarily divided into four categories: 1) Human and Mouse HPS, 2) Mouse HPS Only, 3) Putative Mouse or Human HPS, and 4) HPS Related Syndromes. All the mutations in these genes are integrated in the GOFs. We expect that these very informative and peerreviewed GOFs will be shortcuts to utilize the webbased information for the emerging interdisciplinary studies of HPS.

Interrelations between the Parasitophorous Vacuole ofToxoplasma gondiiand Host Cell Organelles

Toxoplasma gondii, the causative agent of toxoplasmosis, is capable of actively penetrating and multiplying in any nucleated cell of warm-blooded animals. Its survival strategies include escape from fusion of the parasitophorous vacuole with host cell lysosomes and rearrangement of host cell organelles in relation to the parasitophorous vacuole. In this article we report the rearrangement of host cell organelles and elements of the cytoskeleton of LLCMK2 cells, a lineage derived from green monkey kidney epithelial cells, in response to infection by T. gondii tachyzoites. Transmission electron microscopy made on flat embedded monolayers cut horizontally to the apical side of the cells or field emission scanning electron microscopy of monolayers scraped with scotch tape before sputtering showed that association of mitochondria to the vacuole is much less frequent than previously described. On the other hand, all parasitophorous vacuoles were surrounded by elements of the endoplasmic reticulum. These data were complemented by observations by laser scanning microscopy using fluorescent probes from mitochondria and endoplasmic reticulum and reinforced by three-dimensional reconstruction from serial sections observed by transmission electron microscopy and labeling of mitochondria and endoplasmic reticulum by fluorescent probes.

Peroxisomes, glyoxysomes and glycosomes (Review)

Peroxisomes, glyoxysomes and glycosomes are related organelles found in different organisms. The morphology and enzymic content of the different members of this organelle family differ considerably, and may also be highly dependent on the cell's environmental conditions or life cycle. However, all peroxisome-like organelles have in common a number of characteristic enzymes or enzyme systems, notably enzymes dealing with reactive oxygen species. All organelles of the family follow essentially the same route of biogenesis, but with species-specific differences. Sets of proteins called peroxins are involved in different aspects of the formation and proliferation of peroxisomes such as import of proteins in the organellar matrix, insertion of proteins in the membrane, etc. In different eukaryotic lineages these functions are carried out by often--but not always--homologous yet poorly conserved peroxins. The process of biogenesis and the nature of the proteins involved suggest that all members of the peroxisome family evolved from a single organelle in an ancestral eukaryotic cell. This original peroxisome was possibly derived from a cellular membrane system such as the endoplasmic reticulum. Most of the organism-specific functions of the extant organelles have been acquired later in evolution.

Functional links between mucolipin-1 and Ca 2+-dependent membrane trafficking in mucolipidosis IV

Most of the membrane trafficking phenomena including those involving the interactions between endosomes and lysosomes are regulated by changes in intracellular Ca 2+ (Ca i). These processes are disturbed in some types of mucolipidoses and other lysosomal storage disorders, such as mucolipidosis IV (MLIV), a neurological disorder that usually presents during the first year of life with blindness, cognitive impairment, and psychomotor delays. It is caused by mutations in MCOLN1, the gene encoding mucolipin-1 (MLN1), which we have recently established to represent a Ca 2+-permeable cation channel that is transiently modulated by changes in Ca i. The cells of MLIV patients contain enlarged lysosomes that are likely associated with abnormal sorting and trafficking of these and related organelles. We studied fibroblasts from MLIV patients and found disturbed Ca 2+ signaling and large acidic organelles such as late endosomes and lysosomes (LEL) with altered cellular localization in these cells. The fusion between LEL vesicles in these cells was defective. This is a Ca 2+-dependent process related to signaling pathways involved in regulation of Ca 2+ homeostasis and trafficking. The MLN1 channels could play a key role in Ca 2+ release from LEL vesicles, which triggers the fusion and trafficking of these organelles. The characterization of this MLN1-mediated Ca 2+-dependent process should provide new insights into the pathophysiological mechanisms that lead to the development of MLIV and other mucolipidoses associated with similar disturbances in membrane trafficking.

Focus Issue: Tuning in to the Ion Channel

In this second Focus Issue in an ongoing series of articles on the regulation and function of ion channels, Science 's STKE presents four articles that highlight various aspects of ion channel function. A Review by Faundez and Hartzell concerns the functions of intracellular chloride channels in endosomes and related organelles, concluding with an intriguing scenario whereby chloride channels in the membranes of synaptic vesicles could help determine their neurotransmitter content. A Perspective by DeCoursey on the functions of ion channels in phagocytes critically evaluates the possible contributions of proton channels and large-conductance calcium- and voltage-activated potassium channels to phagosomal destruction of microbes. Trauner and Kramer discuss various examples of metabolic regulation of potassium channels, speculating that other classes of potassium channels may be similarly subject to regulation by various small metabolites. Wolf discusses recent research implicating the TRPM7 "chanzyme" in magnesium homeostasis, research that has turned our previous concepts of the regulation of magneisum homeostasis upside down.

Targeting of proteins involved in sterol biosynthesis to lipid particles of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae , three enzymes of the sterol biosynthetic pathway, namely Erg1p, Erg6p and Erg7p, are located in lipid particles. Whereas Erg1p (squalene epoxidase) is also present in the endoplasmic reticulum (ER) to a significant amount, only traces of Erg6p (sterol C-24 methyltransferase) and Erg7p (lanosterol synthase) are found in the ER. We have chosen these three Erg-proteins as typical representatives of lipid particle proteins to study targeting to their destination. Lipid particle proteins do not contain obvious targeting motifs, but the only common structural feature is the presence of one or two hydrophobic domains near the C-termini. We constructed truncated versions of Erg1p, Erg6p and Erg7p to test the role of these hydrophobic domains in subcellular distribution. Our results demonstrate that lack of the hydrophobic domains prevents at least in part the association of the proteins with lipid particles and causes their retention to the ER. This result strongly supports the view that ER and lipid particles are related organelles.

Targeting of proteins involved in sterol biosynthesis to lipid particles of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, three enzymes of the sterol biosynthetic pathway, namely Erg1p, Erg6p and Erg7p, are located in lipid particles. Whereas Erg1p (squalene epoxidase) is also present in the endoplasmic reticulum (ER) to a significant amount, only traces of Erg6p (sterol C-24 methyltransferase) and Erg7p (lanosterol synthase) are found in the ER. We have chosen these three Erg-proteins as typical representatives of lipid particle proteins to study targeting to their destination. Lipid particle proteins do not contain obvious targeting motifs, but the only common structural feature is the presence of one or two hydrophobic domains near the C-termini. We constructed truncated versions of Erg1p, Erg6p and Erg7p to test the role of these hydrophobic domains in subcellular distribution. Our results demonstrate that lack of the hydrophobic domains prevents at least in part the association of the proteins with lipid particles and causes their retention to the ER. This result strongly supports the view that ER and lipid particles are related organelles.

Behavior of vacuoles during microspore and pollen development in Arabidopsis thaliana.

Using the cryo-fixation/freeze-substitution method, we studied the ultrastructural changes and behavior of vacuoles and related organelles (rER and Golgi bodies) during microspore and pollen development, and pollen maturation of Arabidopsis thaliana. In young microspores forming exine (pollen outer cell wall), vacuoles looked like those of somatic cells. In microspores during the formation of intine (inner cell wall), a large vacuole appeared which was made by fusion of pre-existing vacuoles and probably absorption of solutions. In the young pollen grain after the first mitosis, a large vacuole was divided into small vacuoles. The manner of division was not by binary fission and centripetally, but by the invagination of tonoplasts from one side to the opposite side of a vacuole. After the second mitosis, somatic type vacuoles disappeared. In mature pollen grains just before germination, membrane-bound structures containing fine fibrillar substances (MBFs) appeared. The MBFs were considered to be storage vacuoles. In pollen grains from flowers in bloom, MBFs changed to lysosomal structures with acid phosphatases (lytic vacuole). They gradually increased in number and volume, and decomposed the cytoplasm. The autolysis of pollen grains is the first finding in this study, which may contribute to the loss of ability of pollen germination after anthesis.

Hermansky-Pudlak syndrome type 7 (HPS-7) results from mutant dysbindin, a member of the biogenesis of lysosome-related organelles complex 1 (BLOC-1).

Hermansky-Pudlak syndrome (HPS; MIM 203300) is a genetically heterogeneous disorder characterized by oculocutaneous albinism, prolonged bleeding and pulmonary fibrosis due to abnormal vesicle trafficking to lysosomes and related organelles, such as melanosomes and platelet dense granules. In mice, at least 16 loci are associated with HPS, including sandy (sdy; ref. 7). Here we show that the sdy mutant mouse expresses no dysbindin protein owing to a deletion in the gene Dtnbp1 (encoding dysbindin) and that mutation of the human ortholog DTNBP1 causes a novel form of HPS called HPS-7. Dysbindin is a ubiquitously expressed protein that binds to alpha- and beta-dystrobrevins, components of the dystrophin-associated protein complex (DPC) in both muscle and nonmuscle cells. We also show that dysbindin is a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1; refs. 9-11), which regulates trafficking to lysosome-related organelles and includes the proteins pallidin, muted and cappuccino, which are associated with HPS in mice. These findings show that BLOC-1 is important in producing the HPS phenotype in humans, indicate that dysbindin has a role in the biogenesis of lysosome-related organelles and identify unexpected interactions between components of DPC and BLOC-1.

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

AbstractHermansky-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.

Enhancer of garnet/deltaAP-3 is a cryptic allele of the white gene and identifies the intracellular transport system for the white protein.

The white gene encodes an ABC-type transmembrane transporter that has a role in normal eye pigment deposition. In addition, overexpression in Drosophila leads to homosexual male courtship. Its human homologue has been implicated in cholesterol transport in macrophages and in mood disorders in human males. The garnet gene is a member of a group of other Drosophila eye colour genes that have been shown, or proposed, to function in intracellular protein transport. Recent molecular analysis indicates that it encodes the delta subunit of the AP-3 adaptin complex involved in vesicle transport from the trans-Golgi network to lysosomes and related organelles, such as pigment granules. This identification revealed a novel role for intracellular vesicular transport in Drosophila pigmentation. To further analyze this intracellular transport system, we examined the genetic interactions between garnet and a second site enhancer mutation, enhancer of garnet (e(g)). We show here that e(g) is a cryptic allele of the white gene. The white-garnet interaction is highly sensitive to the levels of both gene products but also shows some allele specificity for the white gene. The additive effect on pigmentation and the predicted protein products of these genes suggest that the garnet/AP-3 transport system ensures the correct intracellular localization of the white gene product. This model is further supported by the observation of homosexual male courtship behavior in garnet mutants, similar to that seen in flies overexpressing, and presumably mis-sorting, the white gene product. The w(e(g)) allele also enhances mutations in the subset of other eye-color genes with phenotypes similar to garnet. This observation supports a role for these genes in intracellular transport and leads to a model whereby incorrect sorting of the white gene product can explain the pigmentation phenotypes of an entire group of eye-color genes.

Quantal release of free radicals during exocytosis of phagosomes.

Secretion of lysosomes and related organelles is important for immune system function. High-resolution membrane capacitance techniques were used to track changes in membrane area in single phagocytes during opsonized polystyrene bead uptake and release. Secretagogue stimulation of cells preloaded with beads resulted in immediate vesicle discharge, visualized as step increases in capacitance. The size of the increases were consistent with phagosome size. This hypothesis was confirmed by direct observation of dye release from bead-containing phagosomes after secretagogue stimulation. Capacitance recordings of exocytosis were correlated with quantal free radical release, as determined by amperometry. Thus, phagosomes undergo regulated secretion in macrophages, one function of which may be to deliver sequestered free radicals to the extracellular space.

The Hermansky-Pudlak syndrome 1 (HPS1) and HPS2 genes independently contribute to the production and function of platelet dense granules, melanosomes, and lysosomes

Hermansky-Pudlak syndrome (HPS) is an inherited hemorrhagic disease affecting the related subcellular organelles platelet dense granules, lysosomes, and melanosomes. The mouse genes for HPS, pale ear and pearl, orthologous to the human HPS1 and HPS2 (ADTB3A) genes, encode a novel protein of unknown function and the beta(3)A subunit of the AP-3 adaptor complex, respectively. To test for in vivo interactions between these genes in the production and function of intracellular organelles, mice doubly homozygous for the 2 mutant genes were produced by appropriate breeding. Cooperation between the 2 genes in melanosome production was evident in increased hypopigmentation of the coat together with dramatic quantitative and qualitative alterations of melanosomes of the retinal pigment epithelium and choroid of double mutant mice. Lysosomal and platelet dense granule abnormalities, including hyposecretion of lysosomal enzymes from kidneys and depression of serotonin concentrations of platelet dense granules were likewise more severe in double than single mutants. Also, lysosomal enzyme concentrations were significantly increased in lungs of double mutant mice. Interaction between the 2 genes was specific in that effects on organelles were confined to melanosomes, lysosomes, and platelet dense granules. Together, the evidence indicates these 2 HPS genes function largely independently at the whole organism level to affect the production and function of all 3 organelles. Further, the increased lysosomal enzyme levels in lung of double mutant mice suggest a cause of a major clinical problem of HPS, lung fibrosis. Finally, doubly mutant HPS mice are a useful laboratory model for analysis of severe HPS phenotypes.