GORAB Missense Mutations Disrupt RAB6 and ARF5 Binding and Golgi Targeting

Comprehensive transcriptomic analysis and machine learning reveal unique gene expression profiles in patients with immune-mediated necrotizing myopathy

BACKGROUND

Immune-mediated necrotizing myopathy (IMNM) is an autoimmune myopathy characterized by severe proximal weakness and muscle fiber necrosis, yet its pathogenesis remains unclear. So far, there are few bioinformatics studies on underlying pathogenic genes and infiltrating immune cell profiles of IMNM. Therefore, we aimed to characterize differentially expressed genes (DEGs) and infiltrating cells in IMNM muscle biopsy specimens, which may be useful for elucidating the pathogenesis of IMNM.

METHODS

Three datasets (GSE39454, GSE48280 and GSE128470) of gene expression profiling related to IMNM were obtained from the Gene Expression Omnibus database. Data were normalized, and DEG analysis was performed using the limma package. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of DEGs were performed using clusterProfiler. The CIBERSORT algorithm was performed to identify infiltrating cells. Machine learning algorithm and gene set enrichment analysis (GSEA) were performed to find distinctive gene signatures and the underlying signaling pathways of IMNM.

RESULTS

DEG analysis identified upregulated and downregulated in IMNM muscle compared to the gene expression levels of other groups. GO and KEGG analysis showed that the pathogenesis of IMNM was notable for the under-representation of pathways that were important in dermatomyositis and inclusion body myositis. Three immune cells (M2 macrophages, resting dendritic cells and resting natural killer cells) with differential infiltration and five key genes (NDUFAF7, POLR2J, CD99, ARF5 and SKAP2) in patients with IMNM were identified through the CIBERSORT and machine learning algorithm. The GSEA results revealed that the key genes were remarkably enriched in diverse immunological and muscle metabolism-related pathways.

CONCLUSIONS

We comprehensively explored immunological landscape of IMNM, which is indicative for the research of IMNM pathogenesis.

Vimentin intermediate filaments provide structural stability to the mammalian Golgi complex

The Golgi complex comprises a connected ribbon of stacked cisternal membranes localized to the perinuclear region in most vertebrate cells. The position and morphology of this organelle depends upon interactions with microtubules and the actin cytoskeleton. In contrast, we know relatively little about the relationship of the Golgi complex with intermediate filaments (IFs). In this study, we show that the Golgi is in close physical proximity to vimentin IFs in cultured mouse and human cells. We also show that the trans-Golgi network coiled-coil protein GORAB can physically associate with vimentin IFs. Loss of vimentin and/or GORAB had a modest effect upon Golgi structure at the steady state. The Golgi underwent more rapid disassembly upon chemical disruption with brefeldin A or nocodazole, and slower reassembly upon drug washout, in vimentin knockout cells. Moreover, loss of vimentin caused reduced Golgi ribbon integrity when cells were cultured on high-stiffness hydrogels, which was exacerbated by loss of GORAB. These results indicate that vimentin IFs contribute to the structural stability of the Golgi complex and suggest a role for GORAB in this process.

Arf5-mediated regulation of mTORC1 at the plasma membrane

The mechanistic target of rapamycin (mTOR) kinase regulates a major signaling pathway in eukaryotic cells. In addition to regulation of mTORC1 at lysosomes, mTORC1 is also localized at other locations. However, little is known about the recruitment and activation of mTORC1 at nonlysosomal sites. To identify regulators of mTORC1 recruitment to nonlysosomal compartments, novel interacting partners with the mTORC1 subunit, Raptor, were identified using immunoprecipitation and mass spectrometry. We show that one of the interacting partners, Arf5, is a novel regulator of mTORC1 signaling at plasma membrane ruffles. Arf5-GFP localizes with endogenous mTOR at PI3,4P2-enriched membrane ruffles together with the GTPase required for mTORC1 activation, Rheb. Knockdown of Arf5 reduced the recruitment of mTOR to membrane ruffles. The activation of mTORC1 at membrane ruffles was directly demonstrated using a plasma membrane-targeted mTORC1 biosensor, and Arf5 was shown to enhance the phosphorylation of the mTORC1 biosensor substrate. In addition, endogenous Arf5 was shown to be required for rapid activation of mTORC1-mediated S6 phosphorylation following nutrient starvation and refeeding. Our findings reveal a novel Arf5-dependent pathway for recruitment and activation of mTORC1 at plasma membrane ruffles, a process relevant for spatial and temporal regulation of mTORC1 by receptor and nutrient stimuli.

The factory, the antenna and the scaffold: the three-way interplay between the Golgi, cilium and extracellular matrix underlying tissue function

The growth and development of healthy tissues is dependent on the construction of a highly specialised extracellular matrix (ECM) to provide support for cell growth and migration and to determine the biomechanical properties of the tissue. These scaffolds are composed of extensively glycosylated proteins which are secreted and assembled into well-ordered structures that can hydrate, mineralise, and store growth factors as required. The proteolytic processing and glycosylation of ECM components is vital to their function. These modifications are under the control of the Golgi apparatus, an intracellular factory hosting spatially organised, protein-modifying enzymes. Regulation also requires a cellular antenna, the cilium, which integrates extracellular growth signals and mechanical cues to inform ECM production. Consequently, mutations in either Golgi or ciliary genes frequently lead to connective tissue disorders. The individual importance of each of these organelles to ECM function is well-studied. However, emerging evidence points towards a more tightly linked system of interdependence between the Golgi, cilium and ECM. This review examines how the interplay between all three compartments underpins healthy tissue. As an example, it will look at several members of the golgin family of Golgi-resident proteins whose loss is detrimental to connective tissue function. This perspective will be important for many future studies looking to dissect the cause and effect of mutations impacting tissue integrity.

A Case of Geroderma Osteodysplasticum Syndrome: Unique Clinical Findings

Geroderma osteodysplasticum (GO; MIM 231070) is characterized by a typical progeroid facial appearance, wrinkled, lax skin, joint laxity, skeletal abnormalities with variable degree of osteopenia, frequent fractures, scoliosis, bowed long bones, vertebral collapse, and hyperextensible fingers. The disorder results from mutations in the GORAB—golgin, RAB6 interacting. This gene encodes a member of the golgin family, a group of coiled-coil proteins on golgin that maps to chromosome 1q24. The encoded protein has a function in the secretory pathway, was identified by terminal kinase-like protein, and thus, it may function in mitosis. Mutations in this gene have been associated with GO. Herein, we describe the clinical presentation of one young male patient from related Saudi parents. Mutations, a homozygous frameshift mutation (c.306dup p.(pro 103 Thrfs*20)). Interestingly, phenotypic variability was observed in this patient with GO features that were more atypical than the cases reported in the literature as he looks tall stature where most of the cases reported were short and arachnodactyly fingers which mimic other syndromes.

Potential stress tolerance roles of barley germins and GLPs

Germins and germin-like proteins (GLPs) known as germination markers are encoded by multigene families in several plant species, including barley. To date, functional analysis has revealed germins and GLPs are involved in diverse processes such as embryonic development and stress responses. The aim of this study was the analysis of barley germins and GLPs. In this study, 80 putative germins and GLPs have been identified in barley by using known 17 germins and GLP sequences. Analysis of germins and GLPs showed all germins and GLPs are distributed on all seven chromosomes that most of them spread through chromosome 3, 4 and 7 with 16, 18 and 12 proteins on each, respectively. The protein sizes varied between 185 and 335 amino acids, with an average length of 225 aa. Twelve conserved motifs were found. While germin motifs 1 and 3 were detected in all germins and GLPs, some motifs were found to be related to signalization. Interestingly, protein-protein interaction analysis demonstrated some GLPs are associated with RAB6-interacting golgin, oligopeptide transmembrane transporter activity, beta-glucuronidase activity, protein N-linked glycosylation, multi-pass membrane protein, and proteins containing zinc finger (Znf) domain and RING (really interesting new gene)-type zinc finger domains. Our findings suggest that barley germins and GLPs may have diverse functions that make them important candidates for crop improvement.

An RNA tagging approach for system-wide RNA-binding proteome profiling and dynamics investigation upon transcription inhibition

RNA-protein interactions play key roles in epigenetic, transcriptional and posttranscriptional regulation. To reveal the regulatory mechanisms of these interactions, global investigation of RNA-binding proteins (RBPs) and monitor their changes under various physiological conditions are needed. Herein, we developed a psoralen probe (PP)-based method for RNA tagging and ribonucleic-protein complex (RNP) enrichment. Isolation of both coding and noncoding RNAs and mapping of 2986 RBPs including 782 unknown candidate RBPs from HeLa cells was achieved by PP enrichment, RNA-sequencing and mass spectrometry analysis. The dynamics study of RNPs by PP enrichment after the inhibition of RNA synthesis provides the first large-scale distribution profile of RBPs bound to RNAs with different decay rates. Furthermore, the remarkably greater decreases in the abundance of the RBPs obtained by PP-enrichment than by global proteome profiling suggest that PP enrichment after transcription inhibition offers a valuable way for large-scale evaluation of the candidate RBPs.

The dimeric Golgi protein Gorab binds to Sas6 as a monomer to mediate centriole duplication

The duplication and ninefold symmetry of the Drosophila centriole requires that the cartwheel molecule, Sas6, physically associates with Gorab, a trans-Golgi component. How Gorab achieves these disparate associations is unclear. Here, we use hydrogen-deuterium exchange mass spectrometry to define Gorab's interacting surfaces that mediate its subcellular localization. We identify a core stabilization sequence within Gorab's C-terminal coiled-coil domain that enables homodimerization, binding to Rab6, and thereby trans-Golgi localization. By contrast, part of the Gorab monomer's coiled-coil domain undergoes an antiparallel interaction with a segment of the parallel coiled-coil dimer of Sas6. This stable heterotrimeric complex can be visualized by electron microscopy. Mutation of a single leucine residue in Sas6's Gorab-binding domain generates a Sas6 variant with a sixteenfold reduced binding affinity for Gorab that cannot support centriole duplication. Thus, Gorab dimers at the Golgi exist in equilibrium with Sas6-associated monomers at the centriole to balance Gorab's dual role.

The Close Relationship between the Golgi Trafficking Machinery and Protein Glycosylation

Glycosylation is the most common post-translational modification of proteins; it mediates their correct folding and stability, as well as their transport through the secretory transport. Changes in N- and O-linked glycans have been associated with multiple pathological conditions including congenital disorders of glycosylation, inflammatory diseases and cancer. Glycoprotein glycosylation at the Golgi involves the coordinated action of hundreds of glycosyltransferases and glycosidases, which are maintained at the correct location through retrograde vesicle trafficking between Golgi cisternae. In this review, we describe the molecular machinery involved in vesicle trafficking and tethering at the Golgi apparatus and the effects of mutations in the context of glycan biosynthesis and human diseases.

Premature aging disorders: A clinical and genetic compendium

Progeroid disorders make up a heterogeneous group of very rare hereditary diseases characterized by clinical signs that often mimic physiological aging in a premature manner. Apart from Hutchinson-Gilford progeria syndrome, one of the best-investigated progeroid disorders, a wide spectrum of other premature aging phenotypes exist, which differ significantly in their clinical presentation and molecular pathogenesis. Next-generation sequencing (NGS)-based approaches have made it feasible to determine the molecular diagnosis in the early stages of a disease. Nevertheless, a broad clinical knowledge on these disorders and their associated symptoms is still fundamental for a comprehensive patient management and for the interpretation of variants of unknown significance from NGS data sets. This review provides a detailed overview on characteristic clinical features and underlying molecular genetics of well-known as well as only recently identified premature aging disorders and also highlights novel findings towards future therapeutic options.

GORAB promotes embryonic lung maturation through antagonizing AKT phosphorylation, versican expression, and mesenchymal cell migration

Embryonic development of the alveolar sac of the lung is dependent upon multiple signaling pathways to coordinate cell growth, migration, and the formation of the extracellular matrix. Here, we identify GORAB as a regulator of embryonic alveolar sac formation as genetically disrupting the Gorab gene in mice resulted in fatal saccular maturation defects characterized by a thickened lung mesenchyme. This abnormality is not associated with impairments in cellular proliferation and death, but aberrantly increased protein kinase B (AKT) phosphorylation, elevated Vcan transcription, and enhanced migration of mesenchymal fibroblasts. Genetically augmenting PDGFRα, a potent activator of AKT in lung mesenchymal cells, recapitulated the alveolar phenotypes, whereas disrupting PDGFRα partially rescued alveolar phenotypes in Gorab-deficient mice. Overexpressing or suppressing Vcan in primary embryonic lung fibroblasts could, respectively, mimic or attenuate alveolar sac-like phenotypes in a co-culture model. These findings suggest a role of GORAB in negatively regulating AKT phosphorylation, the expression of Vcan, and the migration of lung mesenchyme fibroblasts, and suggest that alveolar sac formation resembles a patterning event that is orchestrated by molecular signaling and the extracellular matrix in the mesenchyme.

A conserved retromer-independent function for RAB-6.2 in C. elegans epidermis integrity

Rab proteins are conserved small GTPases that coordinate intracellular trafficking essential to cellular function and homeostasis. RAB-6.2 is a highly conserved C. elegans ortholog of human RAB6 proteins. RAB-6.2 is expressed in most tissues in C. elegans and is known to function in neurons and in the intestine to mediate retrograde trafficking. Here, we show that RAB-6.2 is necessary for cuticle integrity and impermeability in C. elegans RAB-6.2 functions in the epidermis to instruct skin integrity. Significantly, we show that expression of a mouse RAB6A cDNA can rescue defects in C. elegans epidermis caused by lack of RAB-6.2, suggesting functional conservation across phyla. We also show that the novel function of RAB-6.2 in C. elegans cuticle development is distinct from its previously described function in neurons. Exocyst mutants partially phenocopy rab-6.2-null animals, and rab-6.2-null animals phenocopy mutants that have defective surface glycosylation. These results suggest that RAB-6.2 may mediate the trafficking of one or many secreted glycosylated cuticle proteins directly, or might act indirectly by trafficking glycosylation enzymes to their correct intracellular localization.

GORAB scaffolds COPI at the trans-Golgi for efficient enzyme recycling and correct protein glycosylation

COPI is a key mediator of protein trafficking within the secretory pathway. COPI is recruited to the membrane primarily through binding to Arf GTPases, upon which it undergoes assembly to form coated transport intermediates responsible for trafficking numerous proteins, including Golgi-resident enzymes. Here, we identify GORAB, the protein mutated in the skin and bone disorder gerodermia osteodysplastica, as a component of the COPI machinery. GORAB forms stable domains at the trans-Golgi that, via interactions with the COPI-binding protein Scyl1, promote COPI recruitment to these domains. Pathogenic GORAB mutations perturb Scyl1 binding or GORAB assembly into domains, indicating the importance of these interactions. Loss of GORAB causes impairment of COPI-mediated retrieval of trans-Golgi enzymes, resulting in a deficit in glycosylation of secretory cargo proteins. Our results therefore identify GORAB as a COPI scaffolding factor, and support the view that defective protein glycosylation is a major disease mechanism in gerodermia osteodysplastica.

Selected Golgi-Localized Proteins and Carcinogenesis: What Do We Know?

The role of the Golgi apparatus in carcinogenesis still remains unclear. A number of structural and functional cis-, medial-, and trans-Golgi proteins as well as a complexity of metabolic pathways which they mediate may indicate a central role of the Golgi apparatus in the development and progression of cancer. Pleiotropy of cellular function of the Golgi apparatus makes it a "metabolic heart" or a relay station of a cell, which combines multiple signaling pathways involved in carcinogenesis. Therefore, any damage to or structural abnormality of the Golgi apparatus, causing its fragmentation and/or biochemical dysregulation, results in an up- or downregulation of signaling pathways and may in turn promote tumor progression, as well as local nodal and distant metastases. Three alternative or parallel models of spatial and functional Golgi organization within tumor cells were proposed: (1) compacted Golgi structure, (2) normal Golgi structure with its increased activity, and (3) the Golgi fragmentation with ministacks formation. Regardless of the assumed model, the increased activity of oncogenesis initiators and promoters with inhibition of suppressor proteins results in an increased cell motility and migration, increased angiogenesis, significantly activated trafficking kinetics, proliferation, EMT induction, decreased susceptibility to apoptosis-inducing factors, and modulating immune response to tumor cell antigens. Eventually, this will lead to the increased metastatic potential of cancer cells and an increased risk of lymph node and distant metastases. This chapter provided an overview of the current state of knowledge of selected Golgi proteins, their role in cytophysiology as well as potential involvement in tumorigenesis.

Gorab is a Golgi protein required for structure and duplication of Drosophila centrioles

We demonstrate that a Drosophila Golgi protein, Gorab, is present not only in the trans-Golgi but also in the centriole cartwheel where, complexed to Sas6, it is required for centriole duplication. In addition to centriole defects, flies lacking Gorab are uncoordinated due to defects in sensory cilia, which lose their nine-fold symmetry. We demonstrate the separation of centriole and Golgi functions of Drosophila Gorab in two ways: first, we have created Gorab variants that are unable to localize to trans-Golgi but can still rescue the centriole and cilia defects of gorab null flies; second, we show that expression of C-terminally tagged Gorab disrupts Golgi functions in cytokinesis of male meiosis, a dominant phenotype overcome by mutations preventing Golgi targeting. Our findings suggest that during animal evolution, a Golgi protein has arisen with a second, apparently independent, role in centriole duplication.

Impaired proteoglycan glycosylation, elevated TGF-β signaling, and abnormal osteoblast differentiation as the basis for bone fragility in a mouse model for gerodermia osteodysplastica

Gerodermia osteodysplastica (GO) is characterized by skin laxity and early-onset osteoporosis. GORAB, the responsible disease gene, encodes a small Golgi protein of poorly characterized function. To circumvent neonatal lethality of the Gorab^Null full knockout, Gorab was conditionally inactivated in mesenchymal progenitor cells (Prx1-cre), pre-osteoblasts (Runx2-cre), and late osteoblasts/osteocytes (Dmp1-cre), respectively. While in all three lines a reduction in trabecular bone density was evident, only Gorab^Prx1 and Gorab^Runx2 mutants showed dramatically thinned, porous cortical bone and spontaneous fractures. Collagen fibrils in the skin of Gorab^Null mutants and in bone of Gorab^Prx1 mutants were disorganized, which was also seen in a bone biopsy from a GO patient. Measurement of glycosaminoglycan contents revealed a reduction of dermatan sulfate levels in skin and cartilage from Gorab^Null mutants. In bone from Gorab^Prx1 mutants total glycosaminoglycan levels and the relative percentage of dermatan sulfate were both strongly diminished. Accordingly, the proteoglycans biglycan and decorin showed reduced glycanation. Also in cultured GORAB-deficient fibroblasts reduced decorin glycanation was evident. The Golgi compartment of these cells showed an accumulation of decorin, but reduced signals for dermatan sulfate. Moreover, we found elevated activation of TGF-β in Gorab^Prx1 bone tissue leading to enhanced downstream signalling, which was reproduced in GORAB-deficient fibroblasts. Our data suggest that the loss of Gorab primarily perturbs pre-osteoblasts. GO may be regarded as a congenital disorder of glycosylation affecting proteoglycan synthesis due to delayed transport and impaired posttranslational modification in the Golgi compartment.

Gerodermia osteodysplastica (GO) is segmental progeroid disorder affecting connective tissues and bone, leading to extreme bone fragility. The cause are loss-of-function mutations in the Golgi protein GORAB, whose function has been only partially unravelled. Using several mouse models and patient-derived primary cells we elucidate that loss of Gorab elicits a defect in proteoglycan glycanation, which is associated with collagen disorganization in dermis and bone. We also found evidence for TGF-β upregulation and enhanced downstream signalling. If these changes occur in mesenchymal stem cells or early osteoblasts they impair osteoblast differentiation resulting in cortical thinning and spontaneous fractures. We thus match GO mechanistically with also phenotypically overlapping progeroid connective tissue disorders with glycanation defects.

Geroderma osteodysplasticum: Histological features and the role of panel-based exome sequencing in diagnosis

Geroderma osteodysplasticum (GO) has clinical and histological features that overlap with other causes of wrinkly skin. Here we present the case of a child diagnosed with GO following exome sequencing of a panel of genes covering the wide differential diagnosis. The histological features of the overlapping conditions are presented, highlighting the utility of panel testing for conditions of this type. This is relevant to many genetic conditions and can influence ongoing management as exemplified by this case.

Heritable Skeletal Disorders Arising from Defects in Processing and Transport of Type I Procollagen from the ER: Perspectives on Possible Therapeutic Approaches

Rare bone disorders are a heterogeneous group of diseases, initially associated with mutations in type I procollagen (PC) genes. Recent developments from dissection at the molecular and cellular level have expanded the list of disease-causing proteins, revealing that disruption of the machinery that handles protein secretion can lead to failure in PC secretion and in several cases result in skeletal dysplasia. In parallel, cell-based in vitro studies of PC trafficking pathways offer clues to the identification of new disease candidate genes. Together, this raises the prospect of heritable bone disorders as a paradigm for biosynthetic protein traffic-related diseases, and an avenue through which therapeutic strategies can be explored.Here, we focus on human syndromes linked to defects in type I PC secretion with respect to the landscape of biosynthetic and protein transport steps within the early secretory pathway. We provide a perspective on possible therapeutic interventions for associated heritable craniofacial and skeletal disorders, considering different orders of complexity, from the cellular level by manipulation of proteostasis pathways to higher levels involving cell-based therapies for bone repair and regeneration.

Examining tissue composition, whole-bone morphology and mechanical behavior of GorabPrx1 mice tibiae: A mouse model of premature aging

Gerodermia osteodysplastica (GO) is a segmental progeroid disorder caused by loss-of-function mutations in the GORAB gene, associated with early onset osteoporosis and bone fragility. A conditional mouse model of GO (Gorab^Prx1) was generated in which the Gorab gene was deleted in long bones. We examined the biomechanical/functional relevance of the Gorab^Prx1 mutants as a premature aging model by characterizing bone composition, tissue-level strains, and whole-bone morphology and mechanical properties of the tibia. MicroCT imaging showed that Gorab^Prx1 tibiae had an increased anterior convex curvature and decreased cortical cross-sectional area, cortical thickness and moments of inertia, compared to littermate control (LC) tibiae. Fourier transform infrared (FTIR) imaging indicated a 34% decrease in mineral/matrix ratio and a 27% increase in acid phosphate content in the posterior metaphyseal cortex of the Gorab^Prx1 tibiae (p < .05), suggesting delayed mineralization. In vivo strain gauge measurement and finite element analysis showed ∼two times higher tissue-level strains within the Gorab^Prx1 tibiae relative to LC tibiae when subjected to axial compressive loads of the same magnitude. Three-point bending tests suggested that Gorab^Prx1 tibiae were weaker and more brittle, as indicated by decreasing whole-bone strength (46%), stiffness (55%), work-to-fracture (61%) and post-yield displacement (47%). Many of these morphological and biomechanical characteristics of the Gorab^Prx1 tibia recapitulated changes in other animal models of skeletal aging. Future studies are necessary to confirm how our observations might guide the way to a better understanding and treatment of GO.

A de novo 1q23.3-q24.2 deletion combined with a GORAB missense mutation causes a distinctive phenotype with cutis laxa

Gerodermia osteodysplastica is a recessive segmental progeroid disorder mainly characterized by wrinkled skin, generalized connective tissue weakness, infantile onset osteoporosis and normal intelligence. Coding mutations in GORAB, localized on chromosome 1q24.2, are the cause of this disease. 1q24 deletions underlie a spectrum of disorders with intellectual disability and ear abnormalities as phenotypic hallmarks. Here we report on an individual from Azerbaijan originating from a non-consanguineous couple showing short stature, cutis laxa, frequent fractures, facial dysmorphism, cup-shaped ears and intellectual disability. Sanger sequencing of GORAB revealed the seemingly homozygous missense mutation p.Ser175Phe. This mutation was detected in a heterozygous state in the clinically unaffected mother, but was absent in the healthy father. We performed copy-number investigations by high-resolution array-CGH and PCR approaches and found an ~6 Mb de novo deletion spanning 1q23.3-q24.2 in the affected boy. This novel combination of genetic defects very well explains the phenotype that goes beyond the usual presentation of gerodermia osteodysplastica. Our data provide new insight into the phenotypic spectrum of 1q23-q25 deletions and shows that the combination with another pathogenic allele can lead to more severe clinical manifestations.

Mapping Variation in Cellular and Transcriptional Response to 1,25-Dihydroxyvitamin D3 in Peripheral Blood Mononuclear Cells

The active hormonal form of vitamin D, 1,25-dihydroxyvitamin D (1,25D) is an important modulator of the immune system, inhibiting cellular proliferation and regulating transcription of immune response genes. In order to characterize the genetic basis of variation in the immunomodulatory effects of 1,25D, we mapped quantitative traits of 1,25D response at both the cellular and the transcriptional level. We carried out a genome-wide association scan of percent inhibition of cell proliferation (Imax) induced by 1,25D treatment of peripheral blood mononuclear cells from 88 healthy African-American individuals. Two genome-wide significant variants were identified: rs1893662 in a gene desert on chromosome 18 (p = 2.32 x 10-8) and rs6451692 on chromosome 5 (p = 2.55 x 10-8), which may influence the anti-proliferative activity of 1,25D by regulating the expression of nearby genes such as the chemokine gene, CCL28, and the translation initiation gene, PAIP1. We also identified 8 expression quantitative trait loci at a FDR<0.10 for transcriptional response to 1,25D treatment, which include the transcriptional regulator ets variant 3-like (ETV3L) and EH-domain containing 4 (EHD4). In addition, we identified response eQTLs in vitamin D receptor binding sites near genes differentially expressed in response to 1,25D, such as FERM Domain Containing 6 (FRMD6), which plays a critical role in regulating both cell proliferation and apoptosis. Combining information from the GWAS of Imax and the response eQTL mapping enabled identification of putative Imax-associated candidate genes such as PAIP1 and the transcriptional repressor gene ZNF649. Overall, the variants identified in this study are strong candidates for immune traits and diseases linked to vitamin D, such as multiple sclerosis.

Gorab Is Required for Dermal Condensate Cells to Respond to Hedgehog Signals during Hair Follicle Morphogenesis

GORAB is a golgin that localizes predominantly at the Golgi apparatus and physically interacts with small guanosine triphosphatases. GORAB is ubiquitously expressed in mammalian tissues, including the skin. However, the biological function of this golgin in skin is unknown. Here, we report that disrupting the expression of the Gorab gene in mice results in hair follicle morphogenesis defects that were characterized by impaired follicular keratinocyte differentiation. This hair follicle phenotype was associated with markedly suppressed hedgehog (Hh) signaling pathway in dermal condensates in vivo. Gorab-deficient dermal mesenchymal cells also displayed a significantly reduced capability to respond to Hh pathway activation in vitro. Furthermore, we found that the formation of the primary cilium, a cellular organelle that is essential for the Hh pathway, was impaired in mutant dermal condensate cells, suggesting that Gorab may be required for the Hh pathway through facilitating the formation of primary cilia. Thus, data obtained from this study provided insight into the biological functions of Gorab during embryonic morphogenesis of the skin in which Hh signaling and primary cilia exert important functions.

Recurrent De Novo Mutations Affecting Residue Arg138 of Pyrroline-5-Carboxylate Synthase Cause a Progeroid Form of Autosomal-Dominant Cutis Laxa

Progeroid disorders overlapping with De Barsy syndrome (DBS) are collectively denoted as autosomal-recessive cutis laxa type 3 (ARCL3). They are caused by biallelic mutations in PYCR1 or ALDH18A1, encoding pyrroline-5-carboxylate reductase 1 and pyrroline-5-carboxylate synthase (P5CS), respectively, which both operate in the mitochondrial proline cycle. We report here on eight unrelated individuals born to non-consanguineous families clinically diagnosed with DBS or wrinkly skin syndrome. We found three heterozygous mutations in ALDH18A1 leading to amino acid substitutions of the same highly conserved residue, Arg138 in P5CS. A de novo origin was confirmed in all six probands for whom parental DNA was available. Using fibroblasts from affected individuals and heterologous overexpression, we found that the P5CS-p.Arg138Trp protein was stable and able to interact with wild-type P5CS but showed an altered sub-mitochondrial distribution. A reduced size upon native gel electrophoresis indicated an alteration of the structure or composition of P5CS mutant complex. Furthermore, we found that the mutant cells had a reduced P5CS enzymatic activity leading to a delayed proline accumulation. In summary, recurrent de novo mutations, affecting the highly conserved residue Arg138 of P5CS, cause an autosomal-dominant form of cutis laxa with progeroid features. Our data provide insights into the etiology of cutis laxa diseases and will have immediate impact on diagnostics and genetic counseling.