BEACH (named after ‘Beige and Chediak‐Higashi’) is a conserved ∼280 residue domain, present in nine human BEACH domain containing proteins (BDCPs). Most BDCPs are large, containing a PH‐like domain ...for membrane association preceding their BEACH domain, and containing WD40 and other domains for ligand binding. Recent studies found that mutations in individual BDCPs cause several human diseases. BDCP alterations affect lysosome size (LYST and NSMAF), apoptosis (NSMAF), autophagy (LYST, WDFY3, LRBA), granule size (LYST, NBEAL2, NBEA) or synapse formation (NBEA). However, the roles of each BDCP in these membrane events remain controversial. After reviewing studies on individual BDCPs, we propose a unifying hypothesis that BDCPs act as scaffolding proteins that facilitate membrane events, including both fission and fusion, determined by their binding partners. BDCPs may also bind each other, enabling fusion or fission of vesicles that are not necessarily of the same type. Such mechanisms explain why different BDCPs may have roles in autophagy; each BDCP is specific for the cell type or the cargo, but not necessarily specific for attaching to the autophagosome. Further elucidation of these mechanisms, preferably carrying out the same experiment on multiple BDCPs, and possibly using patients' cells, may identify potential targets for therapy.
UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme of sialic acid biosynthesis in vertebrates. It catalyzes the first two steps of the cytosolic formation of ...CMP-N-acetylneuraminic acid from UDP-N-acetylglucosamine. In this review we give an overview of structure, biochemistry, and genetics of the bifunctional enzyme and its complex regulation. Furthermore, we will focus on diseases related to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase.
Lysosome-related organelles (LROs) are a heterogeneous group of vesicles that share various features with lysosomes, but are distinct in function, morphology, and composition. The biogenesis of LROs ...employs a common machinery, and genetic defects in this machinery can affect all LROs or only an individual LRO, resulting in a variety of clinical features. In this review, we discuss the main components of LRO biogenesis. We also summarize the function, composition, and resident cell types of the major LROs. Finally, we describe the clinical characteristics of the major human LRO disorders.
To evaluate the safety and efficacy of N-acetylmannosamine (ManNAc) in GNE myopathy, a genetic muscle disease caused by deficiency of the rate-limiting enzyme in N-acetylneuraminic acid (Neu5Ac) ...biosynthesis.
We conducted an open-label, phase 2, single-center (NIH, USA) study to evaluate oral ManNAc in 12 patients with GNE myopathy (ClinicalTrials.gov NCT02346461). Primary endpoints were safety and biochemical efficacy as determined by change in plasma Neu5Ac and sarcolemmal sialylation. Clinical efficacy was evaluated using secondary outcome measures as part of study extensions, and a disease progression model (GNE-DPM) was tested as an efficacy analysis method.
Most drug-related adverse events were gastrointestinal, and there were no serious adverse events. Increased plasma Neu5Ac (+2,159 nmol/L, p < 0.0001) and sarcolemmal sialylation (p = 0.0090) were observed at day 90 compared to baseline. A slower rate of decline was observed for upper extremity strength (p = 0.0139), lower extremity strength (p = 0.0006), and the Adult Myopathy Assessment Tool (p = 0.0453), compared to natural history. Decreased disease progression was estimated at 12 (γ = 0.61 95% CI: 0.09, 1.27) and 18 months (γ = 0.55 95% CI: 0.12, 1.02) using the GNE-DPM.
ManNAc showed long-term safety, biochemical efficacy consistent with the intended mechanism of action, and preliminary evidence clinical efficacy in patients with GNE myopathy.
Elevated serum vitamin D with hypercalciuria can result in nephrocalcinosis and nephrolithiasis. This study evaluated the cause of excess 1,25-dihydroxycholecalciferol (1α,25(OH)2D3) in the ...development of those disorders in two individuals.
Two patients with elevated vitamin D levels and nephrocalcinosis or nephrolithiasis were investigated at the National Institutes of Health (NIH) Clinical Center and the NIH Undiagnosed Diseases Program, by measuring calcium, phosphate, and vitamin D metabolites, and by performing CYP24A1 mutation analysis.
Both patients exhibited hypercalciuria, hypercalcemia, low parathyroid hormone, elevated vitamin D (1α,25(OH)2D3), normal 25-OHD3, decreased 24,25(OH)2D, and undetectable activity of 1,25(OH)2D-24-hydroxylase (CYP24A1), the enzyme that inactivates 1α,25(OH)2D3. Both patients had bi-allelic mutations in CYP24A1 leading to loss of function of this enzyme. On the basis of dbSNP data, the frequency of predicted deleterious bi-allelic CYP24A1 variants in the general population is estimated to be as high as 4%-20%.
The results of this study show that 1,25(OH)2D-24-hydroxylase deficiency due to bi-allelic mutations in CYP24A1 causes elevated serum vitamin D, hypercalciuria, nephrocalcinosis, and renal stones.
Neutrophils are the predominant phagocytes that provide protection against bacterial and fungal infections. Genetically determined neutrophil disorders confer a predisposition to severe infections ...and reveal novel mechanisms that control vesicular trafficking, hematopoiesis, and innate immunity.
We clinically evaluated seven children from five families who had neutropenia, neutrophil dysfunction, bone marrow fibrosis, and nephromegaly. To identify the causative gene, we performed homozygosity mapping using single-nucleotide polymorphism arrays, whole-exome sequencing, immunoblotting, immunofluorescence, electron microscopy, a real-time quantitative polymerase-chain-reaction assay, immunohistochemistry, flow cytometry, fibroblast motility assays, measurements of apoptosis, and zebrafish models. Correction experiments were performed by transfecting mutant fibroblasts with the nonmutated gene.
All seven affected children had homozygous mutations (Thr224Asn or Glu238Lys, depending on the child's ethnic origin) in VPS45, which encodes a protein that regulates membrane trafficking through the endosomal system. The level of VPS45 protein was reduced, as were the VPS45 binding partners rabenosyn-5 and syntaxin-16. The level of β1 integrin was reduced on the surface of VPS45-deficient neutrophils and fibroblasts. VPS45-deficient fibroblasts were characterized by impaired motility and increased apoptosis. A zebrafish model of vps45 deficiency showed a marked paucity of myeloperoxidase-positive cells (i.e., neutrophils). Transfection of patient cells with nonmutated VPS45 corrected the migration defect and decreased apoptosis.
Defective endosomal intracellular protein trafficking due to biallelic mutations in VPS45 underlies a new immunodeficiency syndrome involving impaired neutrophil function. (Funded by the National Human Genome Research Institute and others.).
ABSTRACT
Oculocutaneous albinism (OCA) is a rare genetic disorder of melanin synthesis that results in hypopigmented hair, skin, and eyes. There are four types of OCA caused by mutations in TYR ...(OCA‐1), OCA2 (OCA‐2), TYRP1 (OCA‐3), or SLC45A2 (OCA‐4). Here we report 22 novel mutations in the OCA genes; 14 from a cohort of 61 patients seen as part of the NIH OCA Natural History Study and eight from a prior study at the University of Minnesota. We also include a comprehensive list of almost 600 previously reported OCA mutations along with ethnicity information, carrier frequencies, and in silico pathogenicity predictions as a supplement. In addition to discussing the clinical and molecular features of OCA, we address the cases of apparent missing heritability. In our cohort, 26% of patients did not have two mutations in a single OCA gene. We demonstrate the utility of multiple detection methods to reveal mutations missed by Sanger sequencing. Finally, we review the TYR p.R402Q temperature‐sensitive variant and confirm its association with cases of albinism with only one identifiable TYR mutation.
Isolated oculocutaneous albinism (OCA) is an autosomal recessive condition causing low vision and hypopigmentation. The article provides a mutation update for the OCA genes and discusses several challenging aspects of OCA molecular diagnostics, including the temperature sensitive TYR p.R402Q mutation, OCA‐1 subtyping and the presence of a substantial minority of affected individuals with apparent missing heritability. A comprehensive list of previously reported mutations is provided as supplementary material, and includes collated ethnicity information, carrier frequencies, and in silico pathogenicity predictions.
Gray platelet syndrome (GPS) is an inherited bleeding disorder characterized by macrothrombocytopenia and absence of platelet α-granules resulting in typical gray platelets on peripheral smears. GPS ...is associated with a bleeding tendency, myelofibrosis, and splenomegaly. Reports on GPS are limited to case presentations. The causative gene and underlying pathophysiology are largely unknown. We present the results of molecular genetic analysis of 116 individuals including 25 GPS patients from 14 independent families as well as novel clinical data on the natural history of the disease. The mode of inheritance was autosomal recessive (AR) in 11 and indeterminate in 3 families. Using genome-wide linkage analysis, we mapped the AR-GPS gene to a 9.4-Mb interval on 3p21.1-3p22.1, containing 197 protein-coding genes. Sequencing of 1423 (69%) of the 2075 exons in the interval did not identify the GPS gene. Long-term follow-up data demonstrated the progressive nature of the thrombocytopenia and myelofibrosis of GPS resulting in fatal hemorrhages in some patients. We identified high serum vitamin B12 as a consistent, novel finding in GPS. Chromosome 3p21.1-3p22.1 has not been previously linked to a platelet disorder; identification of the GPS gene will likely lead to the discovery of novel components of platelet organelle biogenesis. This study is registered at www.clinicaltrials.gov as NCT00069680 and NCT00369421.
Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized by defective biogenesis of lysosome-related organelles. Clinical manifestations include a bleeding diathesis due ...to a platelet delta storage pool deficiency, oculocutaneous albinism, inflammatory bowel disease, neutropenia, and pulmonary fibrosis. Ten genes associated with HPS are identified to date, and each gene encodes a protein subunit of either Biogenesis of Lysosome-related Organelles Complex (BLOC)-1, BLOC-2, BLOC-3, or the Adaptor Protein-3 complex. Several genetic variants and phenotypic heterogeneities are reported in individuals with HPS, who generally exhibit easy bruisability and increased bleeding. Desmopressin, pro-coagulants, or platelet transfusion may be used as prophylaxis or treatment for excessive bleeding in patients with HPS. However, response to desmopressin can be variable. Platelets are effective in preventing or treating bleeding in individuals with HPS, but platelets should be transfused judiciously to limit alloimmunization in patients with HPS who are at risk of developing pulmonary fibrosis and may be potential candidates for lung transplantation. The discovery of new genes associated with HPS in people with excessive bleeding and hypopigmentation of unknown etiology may be facilitated by the use of next-generation sequencing or panel-based genetic testing.
Mutations in the key enzyme of sialic acid biosynthesis, uridine diphospho-N-acetylglucosamine 2-epimerase/N-acetylmannosamine (ManNAc) kinase (GNE/MNK), result in hereditary inclusion body myopathy ...(HIBM), an adult-onset, progressive neuromuscular disorder. We created knockin mice harboring the M712T Gne/Mnk mutation. Homozygous mutant (Gne(M712T/M712T)) mice did not survive beyond P3. At P2, significantly decreased Gne-epimerase activity was observed in Gne(M712T/M712T) muscle, but no myopathic features were apparent. Rather, homozygous mutant mice had glomerular hematuria, proteinuria, and podocytopathy. Renal findings included segmental splitting of the glomerular basement membrane, effacement of podocyte foot processes, and reduced sialylation of the major podocyte sialoprotein, podocalyxin. ManNAc administration yielded survival beyond P3 in 43% of the Gne(M712T/M712T) pups. Survivors exhibited improved renal histology, increased sialylation of podocalyxin, and increased Gne/Mnk protein expression and Gne-epimerase activities. These findings establish this Gne(M712T/M712T) knockin mouse as what we believe to be the first genetic model of podocyte injury and segmental glomerular basement membrane splitting due to hyposialylation. The results also support evaluation of ManNAc as a treatment not only for HIBM but also for renal disorders involving proteinuria and hematuria due to podocytopathy and/or segmental splitting of the glomerular basement membrane.
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