Heterozygous HPS1 mutations in a case of Hermansky-Pudlak syndrome with giant melanosomes

Complement modulation reverses pathology in Y402H-retinal pigment epithelium cell model of age-related macular degeneration by restoring lysosomal function

Age-related macular degeneration (AMD) is a multifactorial disease, which is characterized by loss of central vision, affecting one in three people by the age of 75. The Y402H polymorphism in the complement factor H (CFH) gene significantly increases the risk of AMD. We show that Y402H-AMD-patient-specific retinal pigment epithelium (RPE) cells are characterized by a significant reduction in the number of melanosomes, an increased number of swollen lysosome-like-vesicles with fragile membranes, Cathepsin D leakage into drusen-like deposits and reduced lysosomal function. The turnover of C3 is increased significantly in high-risk RPE cells, resulting in higher internalization and deposition of the terminal complement complex C5b-9 at the lysosomes. Inhibition of C3 processing via the compstatin analogue Cp40 reverses the disease phenotypes by relieving the lysosomes of their overburden and restoring their function. These findings suggest that modulation of the complement system represents a useful therapeutic approach for AMD patients associated with complement dysregulation.

Melanosome transport and regulation in development and disease

Melanosomes are specialized membrane-bound organelles that synthesize and organize melanin, ultimately providing color to the skin, hair, and eyes. Disorders in melanogenesis and melanosome transport are linked to pigmentary diseases, such as Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, and Griscelli syndrome. Clinical cases of these pigmentary diseases shed light on the molecular mechanisms that control melanosome-related pathways. However, only an improved understanding of melanogenesis and melanosome transport will further the development of diagnostic and therapeutic approaches. Herein, we review the current literature surrounding melanosomes with particular emphasis on melanosome membrane transport and cytoskeleton-mediated melanosome transport. We also provide perspectives on melanosome regulatory mechanisms which include hormonal action, inflammation, autophagy, and organelle interactions.

Hermansky-Pudlak syndrome: Mutation update

Hermansky-Pudlak syndrome (HPS) is a group of 10 autosomal recessive multisystem disorders, each defined by the deficiency of a specific gene. HPS-associated genes encode components of four ubiquitously expressed protein complexes: Adaptor protein-3 (AP-3) and biogenesis of lysosome-related organelles complex-1 (BLOC-1) through -3. All individuals with HPS exhibit albinism and a bleeding diathesis; additional features occur depending on the defective protein complex. Pulmonary fibrosis is associated with AP-3 and BLOC-3 deficiency, immunodeficiency with AP-3 defects, and gastrointestinal symptoms are more prevalent and severe in BLOC-3 deficiency. Therefore, identification of the HPS subtype is valuable for prognosis, clinical management, and treatment options. The prevalence of HPS is estimated at 1-9 per 1,000,000. Here we summarize 264 reported and novel variants in 10 HPS genes and estimate that ~333 Puerto Rican HPS subjects and ~385 with other ethnicities are reported to date. We provide pathogenicity predictions for missense and splice site variants and list variants with high minor allele frequencies. Current cellular and clinical aspects of HPS are also summarized. This review can serve as a manifest for molecular diagnostics and genetic counseling aspects of HPS.

snow white, a zebrafish model of Hermansky-Pudlak Syndrome type 5

Hermansky-Pudlak Syndrome (HPS) is a set of genetically heterogeneous diseases caused by mutations in one of nine known HPS genes. HPS patients display oculocutaneous hypopigmentation and bleeding diathesis and, depending on the disease subtype, pulmonary fibrosis, congenital nystagmus, reduced visual acuity, and platelet aggregation deficiency. Mouse models for all known HPS subtypes have contributed greatly to our understanding of the disease, but many of the molecular and cellular mechanisms underlying HPS remain unknown. Here, we characterize ocular defects in the zebrafish (Danio rerio) mutant snow white (snw), which possesses a recessive, missense mutation in hps5 (hps5I76N). Melanosome biogenesis is disrupted in snw/hps5 mutants, resulting in hypopigmentation, a significant decrease in the number, size, and maturity of melanosomes, and the presence of ectopic multi-melanosome clusters throughout the mutant retina and choroid. snw/hps5I76N is the first Hps5 mutation identified within the N-terminal WD40 repeat protein-protein binding domain. Through in vitro coexpression assays, we demonstrate that Hps5I76N retains the ability to bind its protein complex partners, Hps3 and Hps6. Furthermore, while Hps5 and Hps6 stabilize each other's expression, this stabilization is disrupted by Hps5I76N. The snw/hps5I76N mutant provides a valuable resource for structure-function analyses of Hps5 and enables further elucidation of the molecular and cellular mechanisms underlying HPS.

Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin

Hair colour is one of the most conspicuous phenotypes in humans, ranging from black, brown, blond to red. This diversity arises mostly from the quantity and ratio of the black-dark brown eumelanin and the reddish-brown pheomelanin. To study the chemical basis underlying the diversity of hair colour, we have developed several chemical methods to quantify those two pigments. Alkaline H(2) O(2) oxidation affords pyrrole-2,3,5-tricarboxylic acid (PTCA) as a eumelanin marker and thiazole-2,4,5-tricarboxylic acid (TTCA) as a pheomelanin marker. Pheomelanin can also be analysed as 4-amino-3-hydroxyphenylalanine (4-AHP) after hydroiodic acid hydrolysis. Using those methods, we evaluated the contents of eumelanin and pheomelanin (the 'chemical' phenotype) in human hairs of black, dark brown, brown, light brown, blond and red colour (the 'visual' phenotype). Eumelanin contents decrease in that order, with a trace but constant level of pheomelanin, except for red hair which contains about equal levels of pheomelanin and eumelanin. Thus, the chemical phenotype correlates well with the visual phenotype. The genotype of melanocortin-1 receptor (MC1R), a gene regulating the red hair phenotype, is predictive of hair melanin expressed as the log value of eumelanin to pheomelanin ratio, with a dosage effect evident. Hair melanin contents were also analysed in patients with various hypopigmentary disorders including Hermansky-Pudlak syndrome, Menkes disease, proopiomelanocortin deficiency, cystinosis, malnutrition and trace metal deficiency. The chemical phenotype helped evaluate the precise effects of each disease on pigmentation. In studies of human hair, the chemical phenotype will find more and more application as an objective measure of pigmentation.

Alveolar macrophage dysregulation in Hermansky-Pudlak syndrome type 1

RATIONALE

Individuals with Hermansky-Pudlak syndrome type 1 (HPS-1), an autosomal recessive disorder characterized by defective biogenesis of lysosome-related organelles, develop an accelerated form of progressive fibrotic lung disease. The etiology of pulmonary fibrosis associated with HPS-1 is unknown.

OBJECTIVES

To investigate the potential pathogenesis of pulmonary fibrosis in HPS-1, lung cells and proteins from individuals with HPS-1 were studied.

METHODS

Forty-one subjects with HPS-1 with and without pulmonary fibrosis were evaluated with pulmonary function tests, high-resolution computed tomography scan, and bronchoscopy. Bronchoalveolar lavage cells and analytes were analyzed.

MEASUREMENTS AND MAIN RESULTS

Concentrations of total bronchoalveolar lavage cells and alveolar macrophages were significantly higher in epithelial lining fluid from subjects with HPS-1 with and without pulmonary fibrosis compared with healthy research volunteers. Concentrations of cytokines and chemokines (i.e., monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, and granulocyte-macrophage colony-stimulating factor) in alveolar epithelial lining fluid were significantly higher in subjects with HPS-1 with and without pulmonary fibrosis compared with healthy research volunteers (P < 0.001). In vitro, HPS-1 pulmonary fibrosis alveolar macrophages, which did not express HPS1 mRNA, secreted significantly higher concentrations of monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, and regulated upon activation, normal T cell expressed and secreted (RANTES) protein compared with normal cells (P = 0.001, P = 0.014, and P = 0.011, respectively). Pirfenidone suppressed HPS-1 alveolar macrophage cytokine and chemokine secretion in vitro in a dose-dependent manner.

CONCLUSIONS

In HPS-1, alveolar inflammation predominantly involves macrophages and is associated with high lung concentrations of cytokines and chemokines. HPS-1 alveolar macrophages provide a model system in which to study the pathogenesis and treatment of HPS pulmonary fibrosis.

Hermansky-Pudlak syndrome: a disease of protein trafficking and organelle function

The Hermansky-Pudlak syndrome (HPS) is a collection of related autosomal recessive disorders which are genetically heterogeneous. There are eight human HPS subtypes, characterized by oculocutaneous albinism and platelet storage disease; prolonged bleeding, congenital neutropenia, pulmonary fibrosis, and granulomatous colitis can also occur. HPS is caused primarily by defects in intracellular protein trafficking that result in the dysfunction of intracellular organelles known as lysosome-related organelles. HPS gene products are all ubiquitously expressed and all associate in various multi-protein complexes, yet HPS has cell type-specific disease expression. Impairment of specialized secretory cells such as melanocytes, platelets, lung alveolar type II epithelial cells and cytotoxic T cells are observed in HPS. This review summarizes recent molecular, biochemical and cell biological analyses together with clinical studies that have led to the correlation of molecular pathology with clinical manifestations and led to insights into such diverse disease processes such as albinism, fibrosis, hemorrhage, and congenital neutropenia.

A germline mutation in BLOC1S3/reduced pigmentation causes a novel variant of Hermansky-Pudlak syndrome (HPS8)

Hermansky-Pudlak syndrome (HPS) is genetically heterogeneous, and mutations in seven genes have been reported to cause HPS. Autozygosity mapping studies were undertaken in a large consanguineous family with HPS. Affected individuals displayed features of incomplete oculocutaneous albinism and platelet dysfunction. Skin biopsy demonstrated abnormal aggregates of melanosomes within basal epidermal keratinocytes. A homozygous germline frameshift mutation in BLOC1S3 (p.Gln150ArgfsX75) was identified in all affected individuals. BLOC1S3 mutations have not been previously described in patients with HPS, but BLOC1S3 encodes a subunit of the biogenesis of lysosome-related organelles complex 1 (BLOC-1). Mutations in other BLOC-1 subunits have been associated with an HPS phenotype in humans and/or mouse, and a nonsense mutation in the murine orthologue of BLOC1S3 causes the reduced pigmentation (rp) model of HPS. Interestingly, eye pigment formation is reported to be normal in rp, but we found visual defects (nystagmus, iris transilluminancy, foveal hypoplasia, reduced visual acuity, and evidence of optic pathway misrouting) in affected individuals. These findings define a novel form of human HPS (HPS8) and extend genotype-phenotype correlations in HPS.

High Frequency of Hermansky–Pudlak Syndrome Type 1 (HPS1) Among Japanese Albinism Patients and Functional Analysis of HPS1 Mutant Protein

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder characterized by oculocutaneous albinism (OCA), bleeding tendency, and lysosomal accumulation of ceroid-like material. Seven genetically distinct subtypes of HPS are known in humans; most are rare outside of Puerto Rico. Here, we describe the analysis of the HPS1 gene in 24 Japanese OCA patients who lacked mutations in the four genes known to cause OCA (TYR/OCA1, P/OCA2, TYRP1/OCA3, and MATP/OCA4), and the identification of eight different HPS1 mutations in ten of these patients, four of which were novel (W583X, L668P, 532insC, 1691delA). An IVS5+5G --> A splice consensus mutation was particularly frequent, the result of a founder effect for this allele in Japanese patients. Functional analysis by transfection of the L668P variant into Hps1-mutant melan-ep mouse melanocytes showed that this missense substitution is pathologic, resulting in an Hps-1 protein that is unable to assemble into the biogenesis of lysosome-related organelles complex-3.

Hermansky-Pudlak syndrome with a novel mutation

We report a case of Hermansky-Pudlak syndrome (HPS) with a novel mutation in the HPS1 gene. This case showed oculocutaneous albinism and lysosomal ceroid accumulation, however platelet dysfunction was not observed. Histopathological findings of the biopsied lung tissue were compatible with HPS. Sequencing analysis showed the insertion of C in the codon 178 (739 bp) of the HPS1 gene forming a stop codon at codon 181. To the best of our knowledge, this is a novel HPS1 gene mutation.

Ultrastructural Features of Trafficking Defects Are Pronounced in Melanocytic Nevus in Hermansky–Pudlak Syndrome Type 1

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder characterized by oculocutaneous albinism, a bleeding disorder, and ceroid lipofuscinosis in the lungs and gut. HPS is genetically heterogeneous and the most common variant, HPS type 1, is caused by mutations in HPS1 gene. The protein encoded by HPS1 is considered to facilitate the trafficking of melanocyte-specific gene products into the premelanosome. We report the ultrastructural findings in a melanocytic nevus seen in a 17-y-old Japanese female patient with HPS1 who is a compound heterozygote of HPS1 mutations, including a novel mutation. Electron microscopy of a pinkish papule corresponding to the melanocytic nevus revealed markedly aberrant, immature melanosomes, large membranous structures, and giant melanosomes in the vicinity of trans-Golgi network, the characteristic abnormalities because of protein trafficking defects in HPS1. These ultrastructural features were far more clearly demonstrated in the nevus cells than in the epidermal melanocytes. Thus, ultrastructural analysis of nevus cells may be an additional diagnostic tool for HPS1 and could give us important clues to further understanding of the pathomechanisms of HPS.

The CHiPS Domain - Ancient Traces for the Hermansky-Pudlak Syndrome

Hermansky-Pudlak syndrome (HPS) is a rare disorder caused by malfunctions of lysosomes and specialized lysosome-related organelles, resulting primarily in oculocutaneous albinism and bleeding diathesis. The majority of the HPS genes have been described as novel, but herein we report the identification of a conserved protein family which includes human HPS4, as well as distant homologs for other HPS genes. Our results suggest that the cellular machinery involved in the HPS syndrome is ancient.

Quantitative analysis of eumelanin and pheomelanin in humans, mice, and other animals: a comparative review

The color of hair, skin, and eyes in animals mainly depends on the quantity, quality, and distribution of the pigment melanin, which occurs in two types: black to brown eumelanin and yellow to reddish pheomelanin. Microanalytical methods to quantify the amounts of eumelanin and pheomelanin in biological materials were developed in 1985. The methods are based on the chemical degradation of eumelanin to pyrrole-2,3,5-tricarboxylic acid and of pheomelanin to aminohydroxyphenylalanine isomers, which can be analyzed and quantitated by high performance liquid chromatography. This review summarizes and compares eumelanin and pheomelanin contents in various pigmented tissues obtained from humans, mice, and other animals. These methods have become valuable tools to study the functions of melanin, the control of melanogenesis, and the actions and interactions of pigmentation genes. The methods have also found applications in many clinical studies. High levels of pheomelanin are found only in yellow to red hairs of mammals and in red feathers of birds. It remains an intriguing question why lower vertebrates such as fishes do not synthesize pheomelanin. Detectable levels of pheomelanin are detected in human skin regardless of race, color, and skin type. However, eumelanin is always the major constituent of epidermal melanin, and the skin color appears to be determined by the quantity of melanin produced but not by the quality.

Molecular, ultrastructural and functional characterization of a Spanish family with Hermansky-Pudlak syndrome: role of insC974 in platelet function and clinical relevance

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder, which is genetically heterogeneous. In humans, mutations associated with this syndrome have been identified that affect four genes, most of them located in the HPS-1 gene. We evaluated the clinical, molecular, platelet ultrastructure and platelet function data obtained from one Spanish HPS patient and his relatives. The proband was compound heterozygous for a de novo nonsense mutation (Arg-131Stop), which has not been described previously, and for a common frameshift mutation (insC974). These two mutations were also identified by reverse transcription polymerase chain reaction (RT-PCR) in half the RNA, supporting the premise that they have minor effects on either transcription or RNA stability. The patient had an almost complete absence of platelet-dense granules. Accordingly, his platelets showed a small aggregatory response, reduced CD63 surface expression after platelet activation and minor serotonin uptake. Interestingly, despite the absence of clinical symptoms, two relatives carrying only one HPS-1 mutation (insC974) presented a decreased content of platelet-dense granules and showed significant reductions in platelet aggregation, expression of CD63 after platelet activation and serotonin uptake. Data show that the presence of a single mutation affecting one allele of the HPS-1 gene might have relevance in the organogenesis of platelet-dense granules, affecting platelet function. However, these functional defects were not of a great enough magnitude to have clinical significance and, thus, these subjects were clinically asymptomatic.

The dark side of lysosome-related organelles: specialization of the endocytic pathway for melanosome biogenesis

Melanosomes are lysosome-related organelles within which melanin pigments are synthesized and stored in melanocytes and retinal pigment epithelial cells. Early ultrastructural studies of pigment cells revealed that melanosomes consist of a complex series of organelles; more recently, these structures have been correlated with cargo constituents. By studying the fate of melanosomal and endosomal cargo in melanocytic cells, the effects of disease-related mutations on melanosomal morphology, and the genes affected by these mutations, we are beginning to gain novel insights into the biogenesis of these complex organelles and their relationship to the endocytic pathway. These insights demonstrate how specialized cells integrate unique and ubiquitous molecular mechanisms in subverting the endosomal system to generate cell-type specific structures and their associated functions. Further dissection of the melanosomal system will likely shed light not only on the biogenesis of lysosome-related organelles but also on general aspects of vesicular transport in the endosomal system.

The melanosome: membrane dynamics in black and white

Melanosomes are morphologically and functionally unique organelles within which melanin pigments are synthesized and stored. Melanosomes share some characteristics with lysosomes, but can be distinguished from them in many ways. The biogenesis and intracellular movement of melanosomes and related organelles are disrupted in several genetic disorders in mice and humans. The recent characterization of genes defective in these diseases has reinvigorated interest in the melanosome as a model system for understanding the molecular mechanisms that underlie intracellular membrane dynamics.

Abnormal translocation of tyrosinase and tyrosinase-related protein 1 in cutaneous melanocytes of Hermansky-Pudlak Syndrome and in melanoma cells transfected with anti-sense HPS1 cDNA

Hermansky-Pudlak syndrome is an autosomal recessive disorder characterized by oculocutaneous albinism, a bleeding disorder, and, in some patients, ceroid storage and progressive lung disease. Although Hermansky-Pudlak syndrome exhibits locus heterogeneity, most patients have mutations in the HPS1 gene. Melanocytes in the basal epithelial layer of skin from patients with different mutations in the HPS1 gene exhibited occasional large complexes containing dihydroxyphenylalanine-positive cisterna and 50 nm vesicles. To characterize the role of the HPS1 protein in cells, human HPS1 cDNA was transfected into pigmented SK-MEL-188 melanoma cells (M-188) in either the sense (S-188) or the antisense (A-188) orientation. Expression of the 79 kDa HPS1 protein (in M-188 and S-188 cells) or lack of expression (in A-188 cells) was confirmed by Western blotting using two HPS1-protein-specific polyclonal antibodies. Significant reduction in expression of HPS1 protein in A-188 cells resulted in a significant decrease in tyrosinase activity and melanin content compared with M-188 and S-188 cells using an intact cell assay for tyrosinase. In contrast, tyrosinase activities in cell lysates of M-188, S-188, and A-188 cells were not significantly different. Knockout of HPS1 protein expression in A-188 cells caused both tyrosinase and tyrosinase-related protein 1 to be localized to large granular complexes in the cell cytosol and dendrites. Electron microscope analysis of the A-188 cells revealed that absence of HPS1 protein resulted in the deposition of dihydroxyphenylalanine reaction products (i.e., tyrosinase) confined to large membrane-bound structures with limiting membranes. We conclude that lack of HPS1 protein expression results in mistranslocation of tyrosinase and tyrosinase-related protein 1 to large granular complexes rather than melanosomes, compromising melanin synthesis.