A splice site mutation in the TSEN2 causes a new syndrome with craniofacial and central nervous system malformations, and atypical hemolytic uremic syndrome.

Recessive mutations in the genes encoding the four subunits of the tRNA splicing endonuclease complex (TSEN54, TSEN34, TSEN15, and TSEN2) cause various forms of pontocerebellar hypoplasia, a disorder characterized by hypoplasia of the cerebellum and the pons, microcephaly, dysmorphisms, and other variable clinical features. Here, we report an intronic recessive founder variant in the gene TSEN2 that results in abnormal splicing of the mRNA of this gene, in six individuals from four consanguineous families affected with microcephaly, multiple craniofacial malformations, radiological abnormalities of the central nervous system, and cognitive retardation of variable severity. Remarkably, unlike patients with previously described mutations in the components of the TSEN complex, all the individuals that we report developed atypical hemolytic uremic syndrome (aHUS) with thrombotic microangiopathy, microangiopathic hemolytic anemia, thrombocytopenia, proteinuria, severe hypertension, and end-stage kidney disease (ESKD) early in life. Bulk RNA sequencing of peripheral blood cells of four affected individuals revealed abnormal tRNA transcripts, indicating an alteration of the tRNA biogenesis. Morpholino-mediated skipping of exon 10 of tsen2 in zebrafish produced phenotypes similar to human patients. Thus, we have identified a novel syndrome accompanied by aHUS suggesting the existence of a link between tRNA biology and vascular endothelium homeostasis, which we propose to name with the acronym TRACK syndrome (TSEN2 Related Atypical hemolytic uremic syndrome, Craniofacial malformations, Kidney failure).

High-throughput behavioral assay to investigate seizure sensitivity in zebrafish implicates ZFHX3 in epilepsy.

Epilepsy, which affects ∼1% of the population, is caused by abnormal synchronous neural activity in the central nervous system (CNS). While there is a significant genetic contribution to epilepsy, the underlying causes for the majority of genetic cases remain unknown. The NIH Undiagnosed Diseases Project (UDP) utilized exome sequencing to identify genetic variants in patients affected by various conditions with undefined etiology, including epilepsy. Confirming the functional relevance of the candidate genes identified by exome sequencing in a timely manner is crucial to translating exome data into clinically useful information. To this end, we developed a high throughput version of a seizure-sensitivity assay in zebrafish (Danio rerio) to rapidly evaluate candidate genes found by exome sequencing. We developed open access software, Studying Epilepsy In Zebrafish using R (SEIZR), to efficiently analyze the data. SEIZR was validated by disrupting function of a known epilepsy gene, prickle 1. Next, using SEIZR, we analyzed a candidate gene from the UDP screen (Zinc Finger Homeobox 3, ZFHX3), and showed that reduced ZFHX3 function in zebrafish results in a significant hyperactive response to the convulsant drug pentylenetetrazol (PTZ). We find that ZFHX3 shows strong expression in the CNS during neurogenesis including in the pallium, thalamus, tegmentum, reticular formation, and medulla oblongata - all regions which have roles in motor control and coordination. Our findings in the zebrafish confirm human ZFHX3 is a strong candidate for further neurological studies. We offer SEIZR to other researchers as a tool to rapidly and efficiently analyze large behavioral data sets.

The Nkd EF-hand domain modulates divergent wnt signaling outputs in zebrafish.

Wnt proteins regulate diverse biological responses by initiating two general outcomes: ß-catenin-dependent transcription and ß-catenin-independent activation of signaling cascades, the latter including modulation of calcium and regulation of cytoskeletal dynamics (Planar Cell Polarity, PCP). It has been difficult to elucidate the mechanisms by which Wnt signals are directed to effect one or the other outcome due to shared signaling proteins between the ß-catenin-dependent and -independent pathways, such as the Dishevelled binding protein Naked. While all Naked paralogs contain a putative calcium-binding domain, the EF-Hand, Drosophila Naked does not bind calcium. Here we find a lineage-specific evolutionary change within the Drosophila Naked EF-hand that is not shared with other insects or vertebrates. We demonstrate the necessary role of the EF-hand for Nkd localization changes in calcium fluxing cells and using in vivo assays, we identify a role for the zebrafish Naked EF-hand in PCP but not in ß-catenin antagonism. In contrast, Drosophila-like Nkd does not function in PCP, but is a robust antagonist of Wnt/ß-catenin signaling. This work reveals that the zebrafish Nkd1 EF-hand is essential to balance Wnt signaling inputs and modulate the appropriate outputs, while the Drosophila-like EF-Hand primarily functions in ß-catenin signaling.

Hypomorphic mutations in TRNT1 cause retinitis pigmentosa with erythrocytic microcytosis.

Retinitis pigmentosa (RP) is a highly heterogeneous group of disorders characterized by degeneration of the retinal photoreceptor cells and progressive loss of vision. While hundreds of mutations in more than 100 genes have been reported to cause RP, discovering the causative mutations in many patients remains a significant challenge. Exome sequencing in an individual affected with non-syndromic RP revealed two plausibly disease-causing variants in TRNT1, a gene encoding a nucleotidyltransferase critical for tRNA processing. A total of 727 additional unrelated individuals with molecularly uncharacterized RP were completely screened for TRNT1 coding sequence variants, and a second family was identified with two members who exhibited a phenotype that was remarkably similar to the index patient. Inactivating mutations in TRNT1 have been previously shown to cause a severe congenital syndrome of sideroblastic anemia, B-cell immunodeficiency, recurrent fevers and developmental delay (SIFD). Complete blood counts of all three of our patients revealed red blood cell microcytosis and anisocytosis with only mild anemia. Characterization of TRNT1 in patient-derived cell lines revealed reduced but detectable TRNT1 protein, consistent with partial function. Suppression of trnt1 expression in zebrafish recapitulated several features of the human SIFD syndrome, including anemia and sensory organ defects. When levels of trnt1 were titrated, visual dysfunction was found in the absence of other phenotypes. The visual defects in the trnt1-knockdown zebrafish were ameliorated by the addition of exogenous human TRNT1 RNA. Our findings indicate that hypomorphic TRNT1 mutations can cause a recessive disease that is almost entirely limited to the retina.

Functional characterization of Prickle2 and BBS7 identify overlapping phenotypes yet distinct mechanisms.

Ciliopathies are genetic disorders that are caused by dysfunctional cilia and affect multiple organs. One type of ciliopathy, Bardet-Biedl syndrome, is a rare disorder characterized by obesity, retinitis pigmentosa, polydactyly, mental retardation and susceptibility to cardiovascular diseases. The Wnt/Planar cell polarity (PCP) has been associated with cilia function and ciliogenesis in directing the orientation of cilia and basal bodies. Yet the exact relationship between PCP and ciliopathy is not well understood. Here, we examine interactions between a core PCP component, Prickle2 (Pk2), and a central BBS gene, Bbs7, using gene knockdown in the zebrafish. pk2 and bbs7 knockdown both disrupt the formation of a ciliated organ, the Kupffer׳s vesicle (KV), but do not display a synergistic interaction. By measuring cell polarity in the neural tube, we find that bbs7 activity is not required for Pk asymmetric localization. Moreover, BBS protein complex formation is preserved in the Pk2-deficient (Pk2(-/-)) mouse. Previously we reported an intracellular melanosome transport delay as a cardinal feature of reduced bbs gene activity. We find that pk2 knockdown suppresses bbs7-related retrograde transport delay. Similarly, knockdown of ift22, an anterograde intraflagellar transport component, also suppresses the bbs7-related retrograde delay. Notably, we find that pk2 knockdown larvae show a delay in anterograde transport. These data suggest a novel role for Pk2 in directional intracellular transport and our analyses show that PCP and BBS function independently, yet result in overlapping phenotypes when knocked down in zebrafish.

Wnt5b-Ryk pathway provides directional signals to regulate gastrulation movement.

Noncanonical Wnts are largely believed to act as permissive cues for vertebrate cell movement via Frizzled (Fz). In addition to Fz, Wnt ligands are known to regulate neurite outgrowth through an alternative receptor related to tyrosine kinase (Ryk). However, Wnt-Ryk signaling during embryogenesis is less well characterized. In this study, we report a role for Wnt5b as an instructive cue to regulate gastrulation movements through Ryk. In zebrafish, Ryk deficiency impairs Wnt5b-induced Ca(2+) activity and directional cell movement. Wnt5b-Ryk signaling promotes polarized cell protrusions. Upon Wnt5b stimulation, Fz2 but not Ryk recruits Dishevelled to the cell membrane, suggesting that Fz2 and Ryk mediate separate pathways. Using co-culture assays to generate directional Wnt5b cues, we demonstrate that Ryk-expressing cells migrate away from the Wnt5b source. We conclude that full-length Ryk conveys Wnt5b signals in a directional manner during gastrulation.

Genetic interaction between Bardet-Biedl syndrome genes and implications for limb patterning.

Bardet-Biedl syndrome (BBS) is a pleiotropic, genetically heterogeneous disorder characterized by obesity, retinopathy, polydactyly, cognitive impairment, renal and cardiac anomalies, as well as hypertension and diabetes. Multiple genes are known to independently cause BBS. These genes do not appear to code for the same functional category of proteins; yet, mutation of each results in a similar phenotype. Gene knockdown of different BBS genes in zebrafish shows strikingly overlapping phenotypes including defective melanosome transport and disruption of the ciliated Kupffer's vesicle. Here, we demonstrate that individual knockdown of bbs1 and bbs3 results in the same prototypical phenotypes as reported previously for other BBS genes. We utilize the zebrafish system to comprehensively determine whether simultaneous pair-wise knockdown of BBS genes reveals genetic interactions between BBS genes. Using this approach, we demonstrate eight genetic interactions between a subset of BBS genes. The synergistic relationships between distinct combinations are not due to functional redundancy but indicate specific interactions within a multi-subunit BBS complex. In addition, we utilize the zebrafish model system to investigate limb development. Human polydactyly is a cardinal feature of BBS not reproduced in BBS-mouse models. We evaluated zebrafish fin bud patterning and observed altered Sonic hedgehog (shh) expression and subsequent changes to fin skeletal elements. The SHH fin bud phenotype was also used to confirm specific genetic interactions between BBS genes. This study reveals an in vivo requirement for BBS function in limb bud patterning. Our results provide important new insights into the mechanism and biological significance of BBS.

Calcium dynamics integrated into signalling pathways that influence vertebrate axial patterning.

Many aspects of animal development including fertilization as well as organ formation and function are dependent upon the dynamic release of calcium (Ca(2+)) ions. Although the controlled release and/or accumulation of Ca(2+) ions has been extensively studied, how the release dynamics produce a specific biological output in embryonic development is less clear. We will briefly summarize Ca(2+) sources, highlight data on endogenous Ca(2+) release in vertebrate embryos relevant to body plan formation and cell movement, and integrate pharmacological and molecular-genetic studies to lend insight into the signalling pathways involved. Finally, based on in vivo imaging in zebrafish genetic mutants, we will put forward the model that distinct Ca(2+) release dynamics lead to antagonism of the developmentally important Wnt/beta-catenin signalling pathway, while sustained Ca(2+) release modulates cell polarization or directed migration.

hecate, a zebrafish maternal effect gene, affects dorsal organizer induction and intracellular calcium transient frequency.

A zebrafish maternal effect mutation, in the gene hecate, results in embryos that have defects in the formation of dorsoanterior structures and altered calcium release. hecate mutant embryos lack nuclear accumulation of beta-catenin and have reduced expression of genes specific to the dorsal organizer. We found that hecate mutant embryos exhibit a nearly 10-fold increase in the frequency of intracellular Ca2+ transients normally present in the enveloping layer during the blastula stages. Inhibition of Ca2+ release leads to ectopic expression of dorsal genes in mutant embryos suggesting that Ca2+ transients are important in mediating dorsal gene expression. Inhibition of Ca2+ release also results in the expression of dorsal-specific genes in the enveloping layer in a beta-catenin-independent manner, which suggests an additional function for the Ca2+ transients in this cellular layer. The mutant phenotype can be reversed by the expression of factors that activate Wnt/beta-catenin signaling, suggesting that the Wnt/beta-catenin pathway, at least as activated by an exogenous Wnt ligand, is intact in hec mutant embryos. Our results are consistent with a role for the hecate gene in the regulation of Ca2+ release during the cleavage stages, which in turn influences dorsal gene expression in both marginal cells along the dorsoventral axis and in the enveloping layer.

Wnt-5/pipetail functions in vertebrate axis formation as a negative regulator of Wnt/beta-catenin activity.

We provide genetic evidence defining a role for noncanonical Wnt function in vertebrate axis formation. In zebrafish, misexpression of Wnt-4, -5, and -11 stimulates calcium (Ca2+) release, defining the Wnt/Ca2+ class. We describe genetic interaction between two Wnt/Ca2+ members, Wnt-5 (pipetail) and Wnt-11 (silberblick), and a reduction of Ca2+ release in Wnt-5/pipetail. Embryos genetically depleted of both maternal and zygotic Wnt-5 product exhibit cell movement defects as well as hyperdorsalization and axis-duplication phenotypes. The dorsalized phenotypes result from increased beta-catenin accumulation and activation of downstream genes. The Wnt-5 loss-of-function defect is consistent with Ca2+ modulation having an antagonistic interaction with Wnt/beta-catenin signaling.

Requirement for intracellular calcium modulation in zebrafish dorsal-ventral patterning.

The phosphoinositide (PI) cycle is an important signal transduction pathway that, upon activation, generates intracellular second messengers and leads to calcium release. To determine whether PI cycle-mediated intracellular calcium release is required for body plan formation, we systematically dissect PI cycle function in the zebrafish (Danio rerio). We inhibit PI cycle function at three different steps and deplete internal calcium stores, demonstrating an impact on endogenous calcium release and Wnt/beta-catenin signaling. Inhibition of endogenous calcium modulation induces hyperdorsalized phenotypes in a dose-dependent manner. Ectopic dorsal-signaling centers are generated in PI cycle-inhibited embryos as demonstrated by altered beta-catenin subcellular localization and ectopic expression of beta-catenin target genes. These results provide evidence that modulation of calcium release is critical for early embryonic patterning and acts by influencing the stabilization of beta-catenin protein.