Molecular genetic findings and clinical correlations in 100 patients with Joubert syndrome and related disorders prospectively evaluated at a single center.

PURPOSE: Joubert syndrome (JS) is a genetically and clinically heterogeneous ciliopathy characterized by distinct cerebellar and brainstem malformations resulting in the diagnostic "molar tooth sign" on brain imaging. To date, more than 30 JS genes have been identified, but these do not account for all patients. METHODS: In our cohort of 100 patients with JS from 86 families, we prospectively performed extensive clinical evaluation and provided molecular diagnosis using a targeted 27-gene Molecular Inversion Probes panel followed by whole-exome sequencing (WES). RESULTS: We identified the causative gene in 94% of the families; 126 (27 novel) unique potentially pathogenic variants were found in 20 genes, including KIAA0753 and CELSR2, which had not previously been associated with JS. Genotype-phenotype correlation revealed the absence of retinal degeneration in patients with TMEM67, C5orf52, or KIAA0586 variants. Chorioretinal coloboma was associated with a decreased risk for retinal degeneration and increased risk for liver disease. TMEM67 was frequently associated with kidney disease. CONCLUSION: In JS, WES significantly increases the yield for molecular diagnosis, which is essential for reproductive counseling and the option of preimplantation and prenatal diagnosis as well as medical management and prognostic counseling for the age-dependent and progressive organ-specific manifestations, including retinal, liver, and kidney disease.Genet Med advance online publication 26 January 2017.

Joubert syndrome: neuroimaging findings in 110 patients in correlation with cognitive function and genetic cause.

BACKGROUND: Joubert syndrome is a clinically and genetically heterogeneous ciliopathy. Neuroimaging findings have not been systematically evaluated in a large cohort of patients with Joubert syndrome in correlation with molecular genetic cause and cognitive function. METHODS: Brain MRI of 110 patients with Joubert syndrome was included in this study. A comprehensive evaluation of brain MRI studies for infratentorial and supratentorial morphological abnormalities was performed. Genetic cause was identified by whole-exome sequencing, and cognitive functions were assessed with age-appropriate neurocognitive tests in a subset of patients. RESULTS: The cerebellar hemispheres were enlarged in 18% of the patients, mimicking macrocerebellum. The posterior fossa was enlarged in 42% of the patients, resembling Dandy-Walker malformation. Abnormalities of the brainstem, such as protuberance at the ventral contour of the midbrain, were present in 66% of the patients. Abnormalities of the supratentorial brain were present in approximately one-third of the patients, most commonly malrotation of the hippocampi. Mild ventriculomegaly, which typically did not require shunting, was present in 23% of the patients. No correlation between neuroimaging findings and molecular genetic cause was apparent. A novel predictor of outcome was identified; the more severe the degree of vermis hypoplasia, the worse the neurodevelopmental outcome was. CONCLUSIONS: The spectrum of neuroimaging findings in Joubert syndrome is wide. Neuroimaging does not predict the genetic cause, but may predict the neurodevelopmental outcome. A high degree of vermis hypoplasia correlates with worse neurodevelopmental outcome. This finding is important for prognostic counselling in Joubert syndrome.

CELSR2, encoding a planar cell polarity protein, is a putative gene in Joubert syndrome with cortical heterotopia, microophthalmia, and growth hormone deficiency.

Joubert syndrome is a ciliopathy characterized by a specific constellation of central nervous system malformations that result in the pathognomonic "molar tooth sign" on imaging. More than 27 genes are associated with Joubert syndrome, but some patients do not have mutations in any of these genes. Celsr1, Celsr2, and Celsr3 are the mammalian orthologues of the drosophila planar cell polarity protein, flamingo; they play important roles in neural development, including axon guidance, neuronal migration, and cilium polarity. Here, we report bi-allelic mutations in CELSR2 in a Joubert patient with cortical heterotopia, microophthalmia, and growth hormone deficiency. © 2017 Wiley Periodicals, Inc.

Natural killer cell activity and dysfunction in Hermansky-Pudlak syndrome.

Hermansky-Pudlak syndrome (HPS) encompasses disorders with abnormal function of lysosomes and lysosome-related organelles, and some patients who develop immunodeficiency. The basic mechanisms contributing to immune dysfunction in HPS are ill-defined. We analysed natural killer (NK) cells from patients diagnosed with HPS-1, HPS-2, HPS-4, and an unreported HPS subtype. NK cells from an HPS-2 and an unreported HPS subtype share a similar cellular phenotype with defective granule release and cytotoxicity, but differ in cytokine exocytosis. Defining NK cell activity in several types of HPS provides insights into cellular defects of the disorder and understanding of mechanisms contributing to HPS pathogenesis.

Identification of a novel mutation in HPS6 in a patient with hemophilia B and oculocutaneous albinism.

PURPOSE: Hemophilia B, an X-linked disease, manifests with recurrent soft tissue bleeding episodes. Hermansky-Pudlak syndrome, a rare autosomal recessive disorder, is characterized by oculocutaneous albinism and an increased tendency to bleed due to a platelet storage pool defect. We report a novel mutation in HPS6 in a Caucasian man with hemophilia B and oculocutaneous albinism. RESULTS: The patient was diagnosed with hemophilia B at age 4months due to recurrent soft tissue bleeding episodes, and he was also diagnosed with Hermansky-Pudlak syndrome at 32years of age due to unexplained oculocutaneous albinism. His factor IX level was markedly reduced at 13%; whole exome and Sanger sequencing showed the Durham mutation in F9 (NM_000133.3). The diagnosis of Hermansky-Pudlak syndrome subtype 6 was established by demonstrating absence of platelet delta granules on whole mount electron microscopy, an abnormal secondary wave in platelet aggregation studies, and a novel homozygous c.1114 C>T (p.Arg372*) mutation in HPS6 (NM_024747.5) on exome analysis and Sanger sequencing. Clinical phenotyping revealed no evidence of recurrent or unusual infections, interstitial lung disease or pulmonary fibrosis, or neurological disorders. The patient was treated with fresh frozen plasma, recombinant factor IX, and aminocaproic acid. Treatment with desmopressin was added to his regimen after he was diagnosed with Hermansky-Pudlak syndrome. Treatment of bleeding episodes results in effective hemostasis, and the patient has not required platelet or blood product transfusions. CONCLUSIONS: This report highlights the need to consider Hermansky-Pudlak syndrome as an etiology of oculocutaneous albinism even in patients with known hematologic disorders associated with bleeding. Identification of a novel mutation in HPS6 in an individual with hemophilia B shows that, although quite rare, patients may be diagnosed with two independent inherited bleeding disorders. No evidence of lung disease was found in this adult patient with Hermansky-Pudlak syndrome subtype 6.

Cystic cerebellar dysplasia and biallelic LAMA1 mutations: a lamininopathy associated with tics, obsessive compulsive traits and myopia due to cell adhesion and migration defects.

BACKGROUND: Laminins are heterotrimeric complexes, consisting of α, ß and γ subunits that form a major component of basement membranes and extracellular matrix. Laminin complexes have different, but often overlapping, distributions and functions. METHODS: Under our clinical protocol, NCT00068224, we have performed extensive clinical and neuropsychiatric phenotyping, neuroimaging and molecular analysis in patients with laminin α1 (LAMA1)-associated lamininopathy. We investigated the consequence of mutations in LAMA1 using patient-derived fibroblasts and neuronal cells derived from neuronal stem cells. RESULTS: In this paper we describe individuals with biallelic mutations in LAMA1, all of whom had the cerebellar dysplasia, myopia and retinal dystrophy, in addition to obsessive compulsive traits, tics and anxiety. Patient-derived fibroblasts have impaired adhesion, reduced migration, abnormal morphology and increased apoptosis due to impaired activation of Cdc42, a member of the Rho family of GTPases that is involved in cytoskeletal dynamics. LAMA1 knockdown in human neuronal cells also showed abnormal morphology and filopodia formation, supporting the importance of LAMA1 in neuronal migration, and marking these cells potentially useful tools for disease modelling and therapeutic target discovery. CONCLUSION: This paper broadens the phenotypes associated with LAMA1 mutations. We demonstrate that LAMA1 deficiency can lead to alteration in cytoskeletal dynamics, which may invariably lead to alteration in dendrite growth and axonal formation. Estimation of disease prevalence based on population studies in LAMA1 reveals a prevalence of 1-20 in 1 000 000. TRIAL REGISTRATION NUMBER: NCT00068224.

Neurologic involvement in patients with atypical Chediak-Higashi disease.

OBJECTIVE: To delineate the developmental and progressive neurodegenerative features in 9 young adults with the atypical form of Chediak-Higashi disease (CHD) enrolled in a natural history study. METHODS: Patients with atypical clinical features, but diagnostically confirmed CHD by standard evaluation of blood smears and molecular genotyping, underwent complete neurologic evaluation, MRI of the brain, electrophysiologic examination, and neuropsychological testing. Fibroblasts were collected to investigate the cellular phenotype and correlation with the clinical presentation. RESULTS: In 9 mildly affected patients with CHD, we documented learning and behavioral difficulties along with developmental structural abnormalities of the cerebellum and posterior fossa, which are apparent early in childhood. A range of progressive neurologic problems emerge in early adulthood, including cerebellar deficits, polyneuropathies, spasticity, cognitive decline, and parkinsonism. CONCLUSIONS: Patients with undiagnosed atypical CHD manifesting some of these wide-ranging yet nonspecific neurologic complaints may reside in general and specialty neurology clinics. The absence of the typical bleeding or infectious diathesis in mildly affected patients with CHD renders them difficult to diagnose. Identification of these individuals is important not only for close surveillance of potential CHD-related systemic complications but also for a full understanding of the natural history of CHD and the potential role of the disease-causing protein, LYST, to the pathophysiology of other neurodevelopmental and neurodegenerative disorders.

Chediak-Higashi syndrome: Lysosomal trafficking regulator domains regulate exocytosis of lytic granules but not cytokine secretion by natural killer cells.

BACKGROUND: Mutations in lysosomal trafficking regulator (LYST) cause Chediak-Higashi syndrome (CHS), a rare immunodeficiency with impaired cytotoxic lymphocyte function, mainly that of natural killer (NK) cells. Our understanding of NK cell function deficiency in patients with CHS and how LYST regulates lytic granule exocytosis is very limited. OBJECTIVE: We sought to delineate cellular defects associated with LYST mutations responsible for the impaired NK cell function seen in patients with CHS. METHODS: We analyzed NK cells from patients with CHS with missense mutations in the LYST ARM/HEAT (armadillo/huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) or BEACH (beige and Chediak-Higashi) domains. RESULTS: NK cells from patients with CHS displayed severely reduced cytotoxicity. Mutations in the ARM/HEAT domain led to a reduced number of perforin-containing granules, which were significantly increased in size but able to polarize to the immunologic synapse; however, they were unable to properly fuse with the plasma membrane. Mutations in the BEACH domain resulted in formation of normal or slightly enlarged granules that had markedly impaired polarization to the IS but could be exocytosed on reaching the immunologic synapse. Perforin-containing granules in NK cells from patients with CHS did not acquire certain lysosomal markers (lysosome-associated membrane protein 1/2) but were positive for markers of transport vesicles (cation-independent mannose 6-phosphate receptor), late endosomes (Ras-associated binding protein 27a), and, to some extent, early endosomes (early endosome antigen 1), indicating a lack of integrity in the endolysosomal compartments. NK cells from patients with CHS had normal cytokine compartments and cytokine secretion. CONCLUSION: LYST is involved in regulation of multiple aspects of NK cell lytic activity, ranging from governance of lytic granule size to control of their polarization and exocytosis, as well as regulation of endolysosomal compartment identity. LYST functions in the regulated exocytosis but not in the constitutive secretion pathway.

A clinical report of Chediak-Higashi syndrome in infancy with a novel genotype from the Indian subcontinent.

Chediak-Higashi syndrome (CHS; OMIM no. 214500) is an inherited multisystem disorder presenting with hypopigmentation and a propensity to infections due to immunological dysfunction. CHS generally presents in infancy with a fatal outcome, but less severe cases can present in adulthood. Treatment with bone marrow transplantation can be life-saving, so establishing a correct diagnosis is critical. The presence of large granules on examination of peripheral blood smears is suggestive of the diagnosis of CHS in most centers. However, sequencing of the lysosomal trafficking, LYST, gene confirms the diagnosis and can provide a prognosis regarding disease severity. In the case presented here, we performed molecular testing to identify the causative mutation and tabulated published mutation data from 2009 to 2014. We found a novel frameshift mutation in our case and concluded that frameshift and nonsense are the most common types of mutation in CHS, but this may be biased due to underdiagnosis of the milder and atypical forms of the disease.

York platelet syndrome is a CRAC channelopathy due to gain-of-function mutations in STIM1.

Store-operated Ca(2+) entry is the major route of replenishment of intracellular Ca(2+) in animal cells in response to the depletion of Ca(2+) stores in the endoplasmic reticulum. It is primarily mediated by the Ca(2+)-selective release-activated Ca(2+) (CRAC) channel, which consists of the pore-forming subunits ORAI1-3 and the Ca(2+) sensors, STIM1 and STIM2. Recessive loss-of-function mutations in STIM1 or ORAI1 result in immune deficiency and nonprogressive myopathy. Heterozygous gain-of-function mutations in STIM1 cause non-syndromic myopathies as well as syndromic forms of miosis and myopathy with tubular aggregates and Stormorken syndrome; some of these syndromic forms are associated with thrombocytopenia. Increased concentration of Ca(2+) as a result of store-operated Ca(2+) entry is essential for platelet activation. The York Platelet syndrome (YPS) is characterized by thrombocytopenia, striking ultrastructural platelet abnormalities including giant electron-opaque organelles and massive, multilayered target bodies and deficiency of platelet Ca(2+) storage in delta granules. We present clinical and molecular findings in 7 YPS patients from 4 families, demonstrating that YPS patients have a chronic myopathy associated with rimmed vacuoles and heterozygous gain-of-function STIM1 mutations. These findings expand the phenotypic spectrum of STIM1-related human disorders and define the molecular basis of YPS.

In vitro functional correction of Hermansky-Pudlak Syndrome type-1 by lentiviral-mediated gene transfer.

Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by oculocutaneous albinism, bleeding tendency and susceptibility to pulmonary fibrosis. No curative therapy is available. Genetic correction directed to the lungs, bone marrow and/or gastro-intestinal tract might provide alternative forms of treatment for the diseases multi-systemic complications. We demonstrate that lentiviral-mediated gene transfer corrects the expression and function of the HPS1 gene in patient dermal melanocytes, which opens the way to development of gene therapy for HPS.

Skin fibroblasts from individuals with Chediak-Higashi Syndrome (CHS) exhibit hyposensitive immunogenic response.

BACKGROUND: Chediak-Higashi Syndrome (CHS) is a rare autosomal recessive disease characterized by immunodeficiency, oculocutaneous albinism, neurological dysfunction, and early death. Individuals with CHS present with increased susceptibility to infections of the skin, upper-respiratory tract, gastrointestinal tract, and oral tissues. Classical CHS is caused by mutations in the gene encoding lysosomal trafficking regulator (LYST). Although defects in cytotoxic T cell lytic secretory granule secretion and neutrophil phagocytosis are suggested to contribute to the immunodeficiency in CHS, the underlying molecular mechanisms are unknown. We hypothesized that skin fibroblasts from CHS subjects exhibit impaired immune response due to defective trafficking of inflammatory factors. METHODS AND RESULTS: Primary skin fibroblasts from CHS subjects or healthy controls were assessed for genes encoding inflammatory response factors using PCR array. At baseline, we found CD14, IL1R1 and TLR-1 were down-regulated significantly (≥2 fold change) and the genes encoding TLR-3, IL-1ß and IL-6 were up-regulated in CHS cells compared to control cells. When challenged with E. coli lipopolysaccharide (LPS), CHS cells were less responsive than control cells, with only 8 genes significantly up-regulated (3-68 fold change) compared to baseline values, whereas 28 genes in control cells were significantly up-regulated at a much higher magnitude (3-4,629 fold change). In addition, 50% of the genes significantly up-regulated in LPS-treated control cells were significantly lower in LPS-treated CHS cells. IL-6, a fibroblast-derived proinflammatory cytokine essential for fighting infections was significantly lower in culture media of CHS cells with or without LPS. Furthermore, Western blot and immunofluorescent staining revealed that TLR-2 and TLR-4 were diminished on cell membranes of CHS cells and dissociated from Rab11a. CONCLUSIONS: For the first time, results from our study indicate defective trafficking of TLR-2 and TLR-4 contributes to the hyposensitive response of CHS skin fibroblasts to immunogenic challenge, providing a potential therapeutic target for clinical intervention in CHS.

Dysregulation of galectin-3. Implications for Hermansky-Pudlak syndrome pulmonary fibrosis.

The etiology of Hermansky-Pudlak syndrome (HPS) pulmonary fibrosis (HPSPF), a progressive interstitial lung disease with high mortality, is unknown. Galectin-3 is a ß-galactoside-binding lectin with profibrotic effects. The objective of this study was to investigate the involvement of galectin-3 in HPSPF. Galectin-3 was measured by ELISA, immunohistochemistry, and immunoblotting in human specimens from subjects with HPS and control subjects. Mechanisms of galectin-3 accumulation were studied by quantitative RT-PCR, Northern blot analysis, membrane biotinylation assays, and rescue of HPS1-deficient cells by transfection. Bronchoalveolar lavage galectin-3 concentrations were significantly higher in HPSPF compared with idiopathic pulmonary fibrosis or that from normal volunteers, and correlated with disease severity. Galectin-3 immunostaining was increased in HPSPF compared with idiopathic pulmonary fibrosis or normal lung tissue. Fibroblasts from subjects with HPS subtypes associated with pulmonary fibrosis had increased galectin-3 protein expression compared with cells from nonfibrotic HPS subtypes. Galectin-3 protein accumulation was associated with reduced Galectin-3 mRNA, normal Mucin 1 levels, and up-regulated microRNA-322 in HPSPF cells. Membrane biotinylation assays showed reduced galectin-3 and normal Mucin 1 expression at the plasma membrane in HPSPF cells compared with control cells, which suggests that galectin-3 is mistrafficked in these cells. Reconstitution of HPS1 cDNA into HPS1-deficient cells normalized galectin-3 protein and mRNA levels, as well as corrected galectin-3 trafficking to the membrane. Intracellular galectin-3 levels are regulated by HPS1 protein. Abnormal accumulation of galectin-3 may contribute to the pathogenesis of HPSPF.

Disorders with similar clinical phenotypes reveal underlying genetic interaction: SATB2 acts as an activator of the UPF3B gene.

Two syndromic cognitive impairment disorders have very similar craniofacial dysmorphisms. One is caused by mutations of SATB2, a transcription regulator and the other by heterozygous mutations leading to premature stop codons in UPF3B, encoding a member of the nonsense-mediated mRNA decay complex. Here we demonstrate that the products of these two causative genes function in the same pathway. We show that the SATB2 nonsense mutation in our patient leads to a truncated protein that localizes to the nucleus, forms a dimer with wild-type SATB2 and interferes with its normal activity. This suggests that the SATB2 nonsense mutation has a dominant negative effect. The patient's leukocytes had significantly decreased UPF3B mRNA compared to controls. This effect was replicated both in vitro, where siRNA knockdown of SATB2 in HEK293 cells resulted in decreased UPF3B expression, and in vivo, where embryonic tissue of Satb2 knockout mice showed significantly decreased Upf3b expression. Furthermore, chromatin immunoprecipitation demonstrates that SATB2 binds to the UPF3B promoter, and a luciferase reporter assay confirmed that SATB2 expression significantly activates gene transcription using the UPF3B promoter. These findings indicate that SATB2 activates UPF3B expression through binding to its promoter. This study emphasizes the value of recognizing disorders with similar clinical phenotypes to explore underlying mechanisms of genetic interaction.

Evidence for defective Rab GTPase-dependent cargo traffic in immune disorders.

A fully functional immune system is essential to protect the body against pathogens and other diseases, including cancer. Vesicular trafficking provides the correct localization of proteins within all cell types, but this process is most exquisitely controlled and coordinated in immune cells because of their specialized organelles and their requirement to respond to selected stimuli. More than 60 Rab GTPases play important roles in protein trafficking, but only five Rab-encoding genes have been associated with inherited human disorders, and only one of these (Rab27a) causes an immune defect. Mutations in RAB27A cause Griscelli Syndrome type 2 (GS2), an autosomal recessive disorder of pigmentation and severe immune deficiency. In lymphocytes, Munc13-4 is an effector of Rab27a, and mutations in the gene encoding this protein (UNC13D) cause Familial Hemophagocytic Lymphohistiocytosis Type 3 (FHL3). The immunological features of GS2 and FHL3 include neutropenia, thrombocytopenia, and immunodeficiency due to impaired function of cytotoxic lymphocytes. The small number of disorders caused by mutations in genes encoding Rabs could be due to their essential functions, where defects in these genes could be lethal. However, with the increasing use of next generation sequencing technologies, more mutations in genes encoding Rabs may be identified in the near future.

A congenital neutrophil defect syndrome associated with mutations in VPS45.

BACKGROUND: 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. METHODS: 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. RESULTS: 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 ß1 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. CONCLUSIONS: 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.).

The BEACH is hot: a LYST of emerging roles for BEACH-domain containing proteins in human disease.

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.

Associations among genotype, clinical phenotype, and intracellular localization of trafficking proteins in ARC syndrome.

Arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome is a rare autosomal recessive multisystem disorder caused by mutations in vacuolar protein sorting 33 homologue B (VPS33B) and VPS33B interacting protein, apical-basolateral polarity regulator (VIPAR). Cardinal features of ARC include congenital joint contractures, renal tubular dysfunction, cholestasis, severe failure to thrive, ichthyosis, and a defect in platelet alpha-granule biogenesis. Most patients with ARC do not survive past the first year of life. We report two patients presenting with a mild ARC phenotype, now 5.5 and 3.5 years old. Both patients were compound heterozygotes with the novel VPS33B donor splice-site mutation c.1225+5G>C in common. Immunoblotting and complementary DNA analysis suggest expression of a shorter VPS33B transcript, and cell-based assays show that c.1225+5G>C VPS33B mutant retains some ability to interact with VIPAR (and thus partial wild-type function). This study provides the first evidence of genotype-phenotype correlation in ARC and suggests that VPS33B c.1225+5G>C mutation predicts a mild ARC phenotype. We have established an interactive online database for ARC (https://grenada.lumc.nl/LOVD2/ARC) comprising all known variants in VPS33B and VIPAR. Also included in the database are 15 novel pathogenic variants in VPS33B and five in VIPAR.