Sotorasib for Vascular Malformations Associated with KRAS G12C Mutation.

KRAS gain-of-function mutations are frequently observed in sporadic arteriovenous malformations. The mechanisms underlying the progression of such KRAS-driven malformations are still incompletely understood, and no treatments for the condition are approved. Here, we show the effectiveness of sotorasib, a specific KRAS G12C inhibitor, in reducing the volume of vascular malformations and improving survival in two mouse models carrying a mosaic Kras G12C mutation. We then administered sotorasib to two adult patients with severe KRAS G12C-related arteriovenous malformations. Both patients had rapid reductions in symptoms and arteriovenous malformation size. Targeting KRAS G12C appears to be a promising therapeutic approach for patients with KRAS G12C-related vascular malformations. (Funded by the European Research Council and others.).

Deficiency of the minor spliceosome component U4atac snRNA secondarily results in ciliary defects in human and zebrafish.

In the human genome, about 750 genes contain one intron excised by the minor spliceosome. This spliceosome comprises its own set of snRNAs, among which U4atac. Its noncoding gene, RNU4ATAC, has been found mutated in Taybi-Linder (TALS/microcephalic osteodysplastic primordial dwarfism type 1), Roifman (RFMN), and Lowry-Wood (LWS) syndromes. These rare developmental disorders, whose physiopathological mechanisms remain unsolved, associate ante- and post-natal growth retardation, microcephaly, skeletal dysplasia, intellectual disability, retinal dystrophy, and immunodeficiency. Here, we report bi-allelic RNU4ATAC mutations in five patients presenting with traits suggestive of the Joubert syndrome (JBTS), a well-characterized ciliopathy. These patients also present with traits typical of TALS/RFMN/LWS, thus widening the clinical spectrum of RNU4ATAC-associated disorders and indicating ciliary dysfunction as a mechanism downstream of minor splicing defects. Intriguingly, all five patients carry the n.16G>A mutation, in the Stem II domain, either at the homozygous or compound heterozygous state. A gene ontology term enrichment analysis on minor intron-containing genes reveals that the cilium assembly process is over-represented, with no less than 86 cilium-related genes containing at least one minor intron, among which there are 23 ciliopathy-related genes. The link between RNU4ATAC mutations and ciliopathy traits is supported by alterations of primary cilium function in TALS and JBTS-like patient fibroblasts, as well as by u4atac zebrafish model, which exhibits ciliopathy-related phenotypes and ciliary defects. These phenotypes could be rescued by WT but not by pathogenic variants-carrying human U4atac. Altogether, our data indicate that alteration of cilium biogenesis is part of the physiopathological mechanisms of TALS/RFMN/LWS, secondarily to defects of minor intron splicing.

Mutations in the non-coding RNU4ATAC gene affect the homeostasis and function of the Integrator complex.

Various genetic diseases associated with microcephaly and developmental defects are due to pathogenic variants in the U4atac small nuclear RNA (snRNA), a component of the minor spliceosome essential for the removal of U12-type introns from eukaryotic mRNAs. While it has been shown that a few RNU4ATAC mutations result in impaired binding of essential protein components, the molecular defects of the vast majority of variants are still unknown. Here, we used lymphoblastoid cells derived from RNU4ATAC compound heterozygous (g.108_126del;g.111G>A) twin patients with MOPD1 phenotypes to analyze the molecular consequences of the mutations on small nuclear ribonucleoproteins (snRNPs) formation and on splicing. We found that the U4atac108_126del mutant is unstable and that the U4atac111G>A mutant as well as the minor di- and tri-snRNPs are present at reduced levels. Our results also reveal the existence of 3'-extended snRNA transcripts in patients' cells. Moreover, we show that the mutant cells have alterations in splicing of INTS7 and INTS10 minor introns, contain lower levels of the INTS7 and INTS10 proteins and display changes in the assembly of Integrator subunits. Altogether, our results show that compound heterozygous g.108_126del;g.111G>A mutations induce splicing defects and affect the homeostasis and function of the Integrator complex.

Agonists of prostaglandin E2 receptors as potential first in class treatment for nephronophthisis and related ciliopathies.

Nephronophthisis (NPH) is an autosomal recessive tubulointerstitial nephropathy belonging to the ciliopathy disorders and known as the most common cause of hereditary end-stage renal disease in children. Yet, no curative treatment is available. The major gene, NPHP1, encodes a protein playing key functions at the primary cilium and cellular junctions. Using a medium-throughput drug-screen in NPHP1 knockdown cells, we identified 51 Food and Drug Administration-approved compounds by their ability to alleviate the cellular phenotypes associated with the loss of NPHP1; 11 compounds were further selected for their physicochemical properties. Among those compounds, prostaglandin E1 (PGE1) rescued ciliogenesis defects in immortalized patient NPHP1 urine-derived renal tubular cells, and improved ciliary and kidney phenotypes in our NPH zebrafish and Nphp1 knockout mouse models. Furthermore, Taprenepag, a nonprostanoid prostaglandin E2 receptor agonist, alleviated the severe retinopathy observed in Nphp1−/− mice. Finally, comparative transcriptomics allowed identification of key signaling pathways downstream PGE1, including cell cycle progression, extracellular matrix, adhesion, or actin cytoskeleton organization. In conclusion, using in vitro and in vivo models, we showed that prostaglandin E2 receptor agonists can ameliorate several of the pleotropic phenotypes caused by the absence of NPHP1; this opens their potential as a first therapeutic option for juvenile NPH-associated ciliopathies.

B-cell immune deficiency in twin sisters expands the phenotype of MOPDI.

Microcephalic osteodysplastic primordial dwarfism type I (MOPDI) is a very rare and severe autosomal recessive disorder characterized by marked intrauterine growth retardation, skeletal dysplasia, microcephaly and brain malformations. MOPDI is caused by biallelic mutations in RNU4ATAC, a non-coding gene involved in U12-type splicing of 1% of the introns in the genome, which are recognized by their specific splicing consensus sequences. Here, we describe a unique observation of immunodeficiency in twin sisters with mild MOPDI, who harbor a novel n.108_126del mutation, encompassing part of the U4atac snRNA 3' stem-loop and Sm protein binding site, and the previously reported n.111G>A mutation. Interestingly, both twin sisters show mild B-cell anomalies, including low naive B-cell counts and increased memory B-cell and plasmablasts counts, suggesting partial and transitory blockage of B-cell maturation and/or excessive activation of naive B-cells. Hence, the localization of a mutation in stem II of U4atac snRNA, as observed in another RNU4ATAC-opathy with immunodeficiency, that is, Roifman syndrome (RFMN), is not required for the occurrence of an immune deficiency. Finally, we emphasize the importance of considering immunodeficiency in MOPDI management to reduce the risk of serious infectious episodes.

Bi-allelic variants in WNT7B disrupt the development of multiple organs in humans.

BACKGROUND: Pulmonary hypoplasia, Diaphragmatic anomalies, Anophthalmia/microphthalmia and Cardiac defects delineate the PDAC syndrome. We aim to identify the cause of PDAC syndrome in patients who do not carry pathogenic variants in RARB and STRA6, which have been previously associated with this disorder. METHODS: We sequenced the exome of patients with unexplained PDAC syndrome and performed functional validation of candidate variants. RESULTS: We identified bi-allelic variants in WNT7B in fetuses with PDAC syndrome from two unrelated families. In one family, the fetus was homozygous for the c.292C>T (p.(Arg98*)) variant whereas the fetuses from the other family were compound heterozygous for the variants c.225C>G (p.(Tyr75*)) and c.562G>A (p.(Gly188Ser)). Finally, a molecular autopsy by proxy in a consanguineous couple that lost two babies due to lung hypoplasia revealed that both parents carry the p.(Arg98*) variant. Using a WNT signalling canonical luciferase assay, we demonstrated that the identified variants are deleterious. In addition, we found that wnt7bb mutant zebrafish display a defect of the swimbladder, an air-filled organ that is a structural homolog of the mammalian lung, suggesting that the function of WNT7B has been conserved during evolution for the development of these structures. CONCLUSION: Our findings indicate that defective WNT7B function underlies a form of lung hypoplasia that is associated with the PDAC syndrome, and provide evidence for involvement of the WNT-ß-catenin pathway in human lung, tracheal, ocular, cardiac, and renal development.

Human IFT52 mutations uncover a novel role for the protein in microtubule dynamics and centrosome cohesion.

Mutations in genes encoding components of the intraflagellar transport (IFT) complexes have previously been associated with a spectrum of diseases collectively termed ciliopathies. Ciliopathies relate to defects in the formation or function of the cilium, a sensory or motile organelle present on the surface of most cell types. IFT52 is a key component of the IFT-B complex and ensures the interaction of the two subcomplexes, IFT-B1 and IFT-B2. Here, we report novel IFT52 biallelic mutations in cases with a short-rib thoracic dysplasia (SRTD) or a congenital anomaly of kidney and urinary tract (CAKUT). Combining in vitro and in vivo studies in zebrafish, we showed that SRTD-associated missense mutation impairs IFT-B complex assembly and IFT-B2 ciliary localization, resulting in decreased cilia length. In comparison, CAKUT-associated missense mutation has a mild pathogenicity, thus explaining the lack of skeletal defects in CAKUT case. In parallel, we demonstrated that the previously reported homozygous nonsense IFT52 mutation associated with Sensenbrenner syndrome [Girisha et al. (2016) A homozygous nonsense variant in IFT52 is associated with a human skeletal ciliopathy. Clin. Genet., 90, 536-539] leads to exon skipping and results in a partially functional protein. Finally, our work uncovered a novel role for IFT52 in microtubule network regulation. We showed that IFT52 interacts and partially co-localized with centrin at the distal end of centrioles where it is involved in its recruitment and/or maintenance. Alteration of this function likely contributes to centriole splitting observed in Ift52-/- cells. Altogether, our findings allow a better comprehensive genotype-phenotype correlation among IFT52-related cases and revealed a novel, extra-ciliary role for IFT52, i.e. disruption may contribute to pathophysiological mechanisms.

Loss-of-function mutations in KIF14 cause severe microcephaly and kidney development defects in humans and zebrafish.

Mutations in KIF14 have previously been associated with either severe, isolated or syndromic microcephaly with renal hypodysplasia (RHD). Syndromic microcephaly-RHD was strongly reminiscent of clinical ciliopathies, relating to defects of the primary cilium, a signalling organelle present on the surface of many quiescent cells. KIF14 encodes a mitotic kinesin, which plays a key role at the midbody during cytokinesis and has not previously been shown to be involved in cilia-related functions. Here, we analysed four families with fetuses presenting with the syndromic form and harbouring biallelic variants in KIF14. Our functional analyses showed that the identified variants severely impact the activity of KIF14 and likely correspond to loss-of-function mutations. Analysis in human fetal tissues further revealed the accumulation of KIF14-positive midbody remnants in the lumen of ureteric bud tips indicating a shared function of KIF14 during brain and kidney development. Subsequently, analysis of a kif14 mutant zebrafish line showed a conserved role for this mitotic kinesin. Interestingly, ciliopathy-associated phenotypes were also present in mutant embryos, supporting a potential direct or indirect role for KIF14 at cilia. However, our in vitro and in vivo analyses did not provide evidence of a direct role for KIF14 in ciliogenesis and suggested that loss of kif14 causes ciliopathy-like phenotypes through an accumulation of mitotic cells in ciliated tissues. Altogether, our results demonstrate that KIF14 mutations result in a severe syndrome associating microcephaly and RHD through its conserved function in cytokinesis during kidney and brain development.

Functional characterization of tektin-1 in motile cilia and evidence for TEKT1 as a new candidate gene for motile ciliopathies.

A child presenting with Mainzer-Saldino syndrome (MZSDS), characterized by renal, retinal and skeletal involvements, was also diagnosed with lung infections and airway ciliary dyskinesia. These manifestations suggested dysfunction of both primary and motile cilia, respectively. Targeted exome sequencing identified biallelic mutations in WDR19, encoding an IFT-A subunit previously associated with MZSDS-related chondrodysplasia, Jeune asphyxiating thoracic dysplasia and cranioectodermal dysplasia, linked to primary cilia dysfunction, and in TEKT1 which encodes tektin-1 an uncharacterized member of the tektin family, mutations of which may cause ciliary dyskinesia. Tektin-1 localizes at the centrosome in cycling cells, at basal bodies of both primary and motile cilia and to the axoneme of motile cilia in airway cells. The identified mutations impaired these localizations. In addition, airway cells from the affected individual showed severe motility defects without major ultrastructural changes. Knockdown of tekt1 in zebrafish resulted in phenotypes consistent with a function for tektin-1 in ciliary motility, which was confirmed by live imaging. Finally, experiments in the zebrafish also revealed a synergistic effect of tekt1 and wdr19. Altogether, our data show genetic interactions between WDR19 and TEKT1 likely contributing to the overall clinical phenotype observed in the affected individual and provide strong evidence for TEKT1 as a new candidate gene for primary ciliary dyskinesia.

Mutations in MAPKBP1 Cause Juvenile or Late-Onset Cilia-Independent Nephronophthisis.

Nephronophthisis (NPH), an autosomal-recessive tubulointerstitial nephritis, is the most common cause of hereditary end-stage renal disease in the first three decades of life. Since most NPH gene products (NPHP) function at the primary cilium, NPH is classified as a ciliopathy. We identified mutations in a candidate gene in eight individuals from five families presenting late-onset NPH with massive renal fibrosis. This gene encodes MAPKBP1, a poorly characterized scaffolding protein for JNK signaling. Immunofluorescence analyses showed that MAPKBP1 is not present at the primary cilium and that fibroblasts from affected individuals did not display ciliogenesis defects, indicating that MAPKBP1 may represent a new family of NPHP not involved in cilia-associated functions. Instead, MAPKBP1 is recruited to mitotic spindle poles (MSPs) during the early phases of mitosis where it colocalizes with its paralog WDR62, which plays a key role at MSP. Detected mutations compromise recruitment of MAPKBP1 to the MSP and/or its interaction with JNK2 or WDR62. Additionally, we show increased DNA damage response signaling in fibroblasts from affected individuals and upon knockdown of Mapkbp1 in murine cell lines, a phenotype previously associated with NPH. In conclusion, we identified mutations in MAPKBP1 as a genetic cause of juvenile or late-onset and cilia-independent NPH.

Recessive and Dominant De Novo ITPR1 Mutations Cause Gillespie Syndrome.

Gillespie syndrome (GS) is a rare variant form of aniridia characterized by non-progressive cerebellar ataxia, intellectual disability, and iris hypoplasia. Unlike the more common dominant and sporadic forms of aniridia, there has been no significant association with PAX6 mutations in individuals with GS and the mode of inheritance of the disease had long been regarded as uncertain. Using a combination of trio-based whole-exome sequencing and Sanger sequencing in five simplex GS-affected families, we found homozygous or compound heterozygous truncating mutations (c.4672C>T [p.Gln1558(∗)], c.2182C>T [p.Arg728(∗)], c.6366+3A>T [p.Gly2102Valfs5(∗)], and c.6664+5G>T [p.Ala2221Valfs23(∗)]) and de novo heterozygous mutations (c.7687_7689del [p.Lys2563del] and c.7659T>G [p.Phe2553Leu]) in the inositol 1,4,5-trisphosphate receptor type 1 gene (ITPR1). ITPR1 encodes one of the three members of the IP3-receptors family that form Ca(2+) release channels localized predominantly in membranes of endoplasmic reticulum Ca(2+) stores. The truncation mutants, which encompass the IP3-binding domain and varying lengths of the modulatory domain, did not form functional channels when produced in a heterologous cell system. Furthermore, ITPR1 p.Lys2563del mutant did not form IP3-induced Ca(2+) channels but exerted a negative effect when co-produced with wild-type ITPR1 channel activity. In total, these results demonstrate biallelic and monoallelic ITPR1 mutations as the underlying genetic defects for Gillespie syndrome, further extending the spectrum of ITPR1-related diseases.

Novel NEK8 Mutations Cause Severe Syndromic Renal Cystic Dysplasia through YAP Dysregulation.

Ciliopathies are a group of genetic multi-systemic disorders related to dysfunction of the primary cilium, a sensory organelle present at the cell surface that regulates key signaling pathways during development and tissue homeostasis. In order to identify novel genes whose mutations would cause severe developmental ciliopathies, >500 patients/fetuses were analyzed by a targeted high throughput sequencing approach allowing exome sequencing of >1200 ciliary genes. NEK8/NPHP9 mutations were identified in five cases with severe overlapping phenotypes including renal cystic dysplasia/hypodysplasia, situs inversus, cardiopathy with hypertrophic septum and bile duct paucity. These cases highlight a genotype-phenotype correlation, with missense and nonsense mutations associated with hypodysplasia and enlarged cystic organs, respectively. Functional analyses of NEK8 mutations in patient fibroblasts and mIMCD3 cells showed that these mutations differentially affect ciliogenesis, proliferation/apoptosis/DNA damage response, as well as epithelial morphogenesis. Notably, missense mutations exacerbated some of the defects due to NEK8 loss of function, highlighting their likely gain-of-function effect. We also showed that NEK8 missense and loss-of-function mutations differentially affect the regulation of the main Hippo signaling effector, YAP, as well as the expression of its target genes in patient fibroblasts and renal cells. YAP imbalance was also observed in enlarged spheroids of Nek8-invalidated renal epithelial cells grown in 3D culture, as well as in cystic kidneys of Jck mice. Moreover, co-injection of nek8 MO with WT or mutated NEK8-GFP RNA in zebrafish embryos led to shortened dorsally curved body axis, similar to embryos injected with human YAP RNA. Finally, treatment with Verteporfin, an inhibitor of YAP transcriptional activity, partially rescued the 3D spheroid defects of Nek8-invalidated cells and the abnormalities of NEK8-overexpressing zebrafish embryos. Altogether, our study demonstrates that NEK8 human mutations cause major organ developmental defects due to altered ciliogenesis and cell differentiation/proliferation through deregulation of the Hippo pathway.

Hepatocyte growth factor signaling in intrapancreatic ductal cells drives pancreatic morphogenesis.

In a forward genetic screen for regulators of pancreas development in zebrafish, we identified donut(s908) , a mutant which exhibits failed outgrowth of the exocrine pancreas. The s908 mutation leads to a leucine to arginine substitution in the ectodomain of the hepatocyte growth factor (HGF) tyrosine kinase receptor, Met. This missense mutation impedes the proteolytic maturation of the receptor, its trafficking to the plasma membrane, and diminishes the phospho-activation of its kinase domain. Interestingly, during pancreatogenesis, met and its hgf ligands are expressed in pancreatic epithelia and mesenchyme, respectively. Although Met signaling elicits mitogenic and migratory responses in varied contexts, normal proliferation rates in donut mutant pancreata together with dysmorphic, mislocalized ductal cells suggest that met primarily functions motogenically in pancreatic tail formation. Treatment with PI3K and STAT3 inhibitors, but not with MAPK inhibitors, phenocopies the donut pancreatic defect, further indicating that Met signals through migratory pathways during pancreas development. Chimera analyses showed that Met-deficient cells were excluded from the duct, but not acinar, compartment in the pancreatic tail. Conversely, wild-type intrapancreatic duct and "tip cells" at the leading edge of the growing pancreas rescued the donut phenotype. Altogether, these results reveal a novel and essential role for HGF signaling in the intrapancreatic ducts during exocrine morphogenesis.

The ciliary gene RPGRIP1L is mutated in cerebello-oculo-renal syndrome (Joubert syndrome type B) and Meckel syndrome.

Cerebello-oculo-renal syndrome (CORS), also called Joubert syndrome type B, and Meckel (MKS) syndrome belong to the group of developmental autosomal recessive disorders that are associated with primary cilium dysfunction. Using SNP mapping, we identified missense and truncating mutations in RPGRIP1L (KIAA1005) in both CORS and MKS, and we show that inactivation of the mouse ortholog Rpgrip1l (Ftm) recapitulates the cerebral, renal and hepatic defects of CORS and MKS. In addition, we show that RPGRIP1L colocalizes at the basal body and centrosomes with the protein products of both NPHP6 and NPHP4, known genes associated with MKS, CORS and nephronophthisis (a related renal disorder and ciliopathy). In addition, the RPGRIP1L missense mutations found in CORS individuals diminishes the interaction between RPGRIP1L and nephrocystin-4. Our findings show that mutations in RPGRIP1L can cause the multiorgan phenotypic abnormalities found in CORS or MKS, which therefore represent a continuum of the same underlying disorder.

Sox9b is a key regulator of pancreaticobiliary ductal system development.

The pancreaticobiliary ductal system connects the liver and pancreas to the intestine. It is composed of the hepatopancreatic ductal (HPD) system as well as the intrahepatic biliary ducts and the intrapancreatic ducts. Despite its physiological importance, the development of the pancreaticobiliary ductal system remains poorly understood. The SRY-related transcription factor SOX9 is expressed in the mammalian pancreaticobiliary ductal system, but the perinatal lethality of Sox9 heterozygous mice makes loss-of-function analyses challenging. We turned to the zebrafish to assess the role of SOX9 in pancreaticobiliary ductal system development. We first show that zebrafish sox9b recapitulates the expression pattern of mouse Sox9 in the pancreaticobiliary ductal system and use a nonsense allele of sox9b, sox9b(fh313), to dissect its function in the morphogenesis of this structure. Strikingly, sox9b(fh313) homozygous mutants survive to adulthood and exhibit cholestasis associated with hepatic and pancreatic duct proliferation, cyst formation, and fibrosis. Analysis of sox9b(fh313) mutant embryos and larvae reveals that the HPD cells appear to mis-differentiate towards hepatic and/or pancreatic fates, resulting in a dysmorphic structure. The intrahepatic biliary cells are specified but fail to assemble into a functional network. Similarly, intrapancreatic duct formation is severely impaired in sox9b(fh313) mutants, while the embryonic endocrine and acinar compartments appear unaffected. The defects in the intrahepatic and intrapancreatic ducts of sox9b(fh313) mutants worsen during larval and juvenile stages, prompting the adult phenotype. We further show that Sox9b interacts with Notch signaling to regulate intrahepatic biliary network formation: sox9b expression is positively regulated by Notch signaling, while Sox9b function is required to maintain Notch signaling in the intrahepatic biliary cells. Together, these data reveal key roles for SOX9 in the morphogenesis of the pancreaticobiliary ductal system, and they cast human Sox9 as a candidate gene for pancreaticobiliary duct malformation-related pathologies.

Fluid shear stress triggers cholesterol biosynthesis and uptake in inner medullary collecting duct cells, independently of nephrocystin-1 and nephrocystin-4.

Renal epithelial cells are subjected to fluid shear stress of urine flow. Several cellular structures act as mechanosensors-the primary cilium, microvilli and cell adhesion complexes-that directly relay signals to the cytoskeleton to regulate various processes including cell differentiation and renal cell functions. Nephronophthisis (NPH) is an autosomal recessive tubulointerstitial nephropathy leading to end-stage kidney failure before adulthood. NPHP1 and NPHP4 are the major genes which code for proteins that form a complex at the transition zone of the primary cilium, a crucial region required for the maintenance of the ciliary composition integrity. These two proteins also interact with signaling components and proteins associated with the actin cytoskeleton at cell junctions. Due to their specific subcellular localization, we wondered whether NPHP1 and NPHP4 could ensure mechanosensory functions. Using a microfluidic set up, we showed that murine inner medullary collecting ductal cells invalidated for Nphp1 or Nphp4 are more responsive to immediate shear exposure with a fast calcium influx, and upon a prolonged shear condition, an inability to properly regulate cilium length and actin cytoskeleton remodeling. Following a transcriptomic study highlighting shear stress-induced gene expression changes, we showed that prolonged shear triggers both cholesterol biosynthesis pathway and uptake, processes that do not seem to involve neither NPHP1 nor NPHP4. To conclude, our study allowed us to determine a moderate role of NPHP1 and NPHP4 in flow sensation, and to highlight a new signaling pathway induced by shear stress, the cholesterol biosynthesis and uptake pathways, which would allow cells to cope with mechanical stress by strengthening their plasma membrane through the supply of cholesterol.

PHGDH-related microcephalic dwarfism in two fetuses: Expanding the phenotypical spectrum of L-serine biosynthesis defect.

Defects in L-serine biosynthesis are a group of autosomal recessive diseases resulting in a wide phenotypic spectrum ranging from viable to lethal presentations and caused by variants in the three genes encoding the L-serine biosynthesis enzymes, PHGDH, PSAT1, and PSPH. Neu-Laxova syndrome (NLS) is the fetal form of this group, characterized by multiple congenital anomalies including severe intrauterine growth retardation, cutaneous lesions extending from ichthyosis to severe restrictive dermopathy with ectropion and eclabion, edema, microcephaly, central nervous system abnormalities, and flexion contractures. Here we report on two unrelated fetuses with an attenuated phenotype of NLS, that initially evoked Taybi-Linder syndrome. They carry biallelic pathogenic variants in the PHGDH gene. These observations expand the phenotypic continuum of L-serine biosynthesis defects, and illustrate the phenotypic overlap between NLS and microcephalic primordial dwarfism.

A missense mutation in the proprotein convertase gene furinb causes hepatic cystogenesis during liver development in zebrafish.

Hepatic cysts are fluid-filled lesions in the liver that are estimated to occur in 5% of the population. They may cause hepatomegaly and abdominal pain. Progression to secondary fibrosis, cirrhosis, or cholangiocarcinoma can lead to morbidity and mortality. Previous studies of patients and rodent models have associated hepatic cyst formation with increased proliferation and fluid secretion in cholangiocytes, which are partially due to impaired primary cilia. Congenital hepatic cysts are thought to originate from faulty bile duct development, but the underlying mechanisms are not fully understood. In a forward genetic screen, we identified a zebrafish mutant that developed hepatic cysts during larval stages. The cyst formation was not due to changes in biliary cell proliferation, bile secretion, or impairment of primary cilia. Instead, time-lapse live imaging data showed that the mutant biliary cells failed to form interconnecting bile ducts because of defects in motility and protrusive activity. Accordingly, immunostaining revealed a disorganized actin and microtubule cytoskeleton in the mutant biliary cells. By whole-genome sequencing, we determined that the cystic phenotype in the mutant was caused by a missense mutation in the furinb gene, which encodes a proprotein convertase. The mutation altered Furinb localization and caused endoplasmic reticulum (ER) stress. The cystic phenotype could be suppressed by treatment with the ER stress inhibitor 4-phenylbutyric acid and exacerbated by treatment with the ER stress inducer tunicamycin. The mutant liver also exhibited increased mammalian target of rapamycin (mTOR) signaling. Treatment with mTOR inhibitors halted cyst formation at least partially through reducing ER stress. Conclusion: Our study has established a vertebrate model for studying hepatic cystogenesis and illustrated the contribution of ER stress in the disease pathogenesis.

Nephrocystin-1 and nephrocystin-4 are required for epithelial morphogenesis and associate with PALS1/PATJ and Par6.

Nephronophthisis (NPH) is an autosomal recessive disorder characterized by renal fibrosis, tubular basement membrane disruption and corticomedullary cyst formation leading to end-stage renal failure. The disease is caused by mutations in NPHP1-9 genes, which encode the nephrocystins, proteins localized to cell-cell junctions and centrosome/primary cilia. Here, we show that nephrocystin mRNA expression is dramatically increased during cell polarization, and shRNA-mediated knockdown of either NPHP1 or NPHP4 in MDCK cells resulted in delayed tight junction (TJ) formation, abnormal cilia formation and disorganized multi-lumen structures when grown in a three-dimensional collagen matrix. Some of these phenotypes are similar to those reported for cells depleted of the TJ proteins PALS1 or Par3, and interestingly, we demonstrate a physical interaction between these nephrocystins and PALS1 as well as their partners PATJ and Par6 and show their partial co-localization in human renal tubules. Taken together, these results demonstrate that the nephrocystins play an essential role in epithelial cell organization, suggesting a plausible mechanism by which the in vivo histopathologic features of NPH might develop.