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Studies of human genetic disorders have traditionally followed a reductionist paradigm. Traits are defined as Mendelian or complex based on family pedigree and population data, whereas alleles are deemed rare, common, benign, or deleterious based on their population frequencies. The availability of exome and genome data, as well as gene and allele discovery for various conditions, is beginning to challenge classic definitions of genetic causality. Here, I discuss recent advances in our understanding of the overlap between rare and complex diseases and the context-dependent effect of both rare and common alleles that underscores the need for revising the traditional categorizations of genetic traits.

Key Points Clinical molecular genetic testing is transforming personalized medicine and is appropriate for a range of applications, such as rare disease diagnostics and predictive testing for common disorders. Whole-exome and whole-genome sequencing may become a first-line clinical test for some naive diagnostic cases, but classic genetic tests will continue to be used for the high analytical sensitivity of specific defects and for the confirmation of genome findings. There remains no single test to detect the wide array of genetic defects that may be inherited or arise de novo ; clinical diagnostics requires multiple approaches to determine a causal genetic defect. Although genome sequencing may transform diagnostic approaches in large academic medical centres, access to expensive and sophisticated tests are not universal. Genetic testing must be available globally through validated simple technologies for molecular diagnostics (such as direct PCR, linkage analysis or multiplex ligation-dependent probe amplification). The greatest challenge to clinical genomics is the reliable interpretation of the multiple and novel variants found through genome sequencing. Pathogenicity of genetic variants can be examined with bioinformatics prediction approaches, protein stability studies, transcriptional activity studies and allele- and/or gene-specific animal models. As broader genomic information becomes available to providers and patients, partnerships will develop to convey patient-centred data, including incidental findings. The regulatory environment must adapt to the coming volume of genomic information to maximize benefit to patients and health-care systems and to match the expectations of the patient population with regard to these technologies. The authors review current technologies for clinical genetic testing. Moves are being made towards whole-genome and whole-exome sequencing in the clinic, although other technologies will continue to be of value. Genomic technologies are reaching the point of being able to detect genetic variation in patients at high accuracy and reduced cost, offering the promise of fundamentally altering medicine. Still, although scientists and policy advisers grapple with how to interpret and how to handle the onslaught and ambiguity of genome-wide data, established and well-validated molecular technologies continue to have an important role, especially in regions of the world that have more limited access to next-generation sequencing capabilities. Here we review the range of methods currently available in a clinical setting as well as emerging approaches in clinical molecular diagnostics. In parallel, we outline implementation challenges that will be necessary to address to ensure the future of genetic medicine.

Genome-wide association studies (GWAS) have identified over 100 risk loci for schizophrenia, but the causal mechanisms remain largely unknown. We performed a transcriptome-wide association study (TWAS) integrating a schizophrenia GWAS of 79,845 individuals from the Psychiatric Genomics Consortium with expression data from brain, blood, and adipose tissues across 3,693 primarily control individuals. We identified 157 TWAS-significant genes, of which 35 did not overlap a known GWAS locus. Of these 157 genes, 42 were associated with specific chromatin features measured in independent samples, thus highlighting potential regulatory targets for follow-up. Suppression of one identified susceptibility gene, mapk3 , in zebrafish showed a significant effect on neurodevelopmental phenotypes. Expression and splicing from the brain captured most of the TWAS effect across all genes. This large-scale connection of associations to target genes, tissues, and regulatory features is an essential step in moving toward a mechanistic understanding of GWAS. A transcriptome-wide association study integrating genome-wide association data with expression data from brain, blood and adipose tissues identifies new candidate susceptibility genes for schizophrenia, providing a step toward understanding the underlying biology.

Modern biomedical research and preclinical pharmaceutical development rely heavily on the phenotyping of small vertebrate models for various diseases prior to human testing. In this article, we demonstrate an acoustofluidic rotational tweezing platform that enables contactless, high-speed, 3D multispectral imaging and digital reconstruction of zebrafish larvae for quantitative phenotypic analysis. The acoustic-induced polarized vortex streaming achieves contactless and rapid (~1 s/rotation) rotation of zebrafish larvae. This enables multispectral imaging of the zebrafish body and internal organs from different viewing perspectives. Moreover, we develop a 3D reconstruction pipeline that yields accurate 3D models based on the multi-view images for quantitative evaluation of basic morphological characteristics and advanced combinations of metrics. With its contactless nature and advantages in speed and automation, our acoustofluidic rotational tweezing system has the potential to be a valuable asset in numerous fields, especially for developmental biology, small molecule screening in biochemistry, and pre-clinical drug development in pharmacology. Existing methods of zebrafish phenotyping rely on contact-based processes. Here the authors report on an acoustofluidic-based platform which performs contactless specimen rotation, that results in multispectral images for rapid morphological phenotyping of zebrafish larvae.

Signaling through the store-operated Ca2+ release-activated Ca2+ (CRAC) channel regulates critical cellular functions, including gene expression, cell growth and differentiation, and Ca2+ homeostasis. Loss-of-function mutations in the CRAC channel pore-forming protein ORAI1 or the Ca2+ sensing protein stromal interaction molecule 1 (STIM1) result in severe immune dysfunction and nonprogressive myopathy. Here, we identify gain-of-function mutations in the cytoplasmic domain of STIM1 (p.R304W) associated with thrombocytopenia, bleeding diathesis, miosis, and tubular myopathy in patients with Stormorken syndrome, and in ORAI1 (p.P245L), associated with a Stormorken-like syndrome of congenital miosis and tubular aggregate myopathy but without hematological abnormalities. Heterologous expression of STIM1 p.R304W results in constitutive activation of the CRAC channel in vitro, and spontaneous bleeding accompanied by reduced numbers of thrombocytes in zebrafish embryos, recapitulating key aspects of Stormorken syndrome. p.P245L in ORAI1 does not make a constitutively active CRAC channel, but suppresses the slow Ca2+-dependent inactivation of the CRAC channel, thus also functioning as a gain-of-function mutation. These data expand our understanding of the phenotypic spectrum of dysregulated CRAC channel signaling, advance our knowledge of the molecular function of the CRAC channel, and suggest new therapies aiming at attenuating store-operated Ca2+ entry in the treatment of patients with Stormorken syndrome and related pathologic conditions.

Overexpression of all 29 human transcripts of a region of the 16p11.2 chromosome in zebrafish embryos identifies KCTD13 as the message inducing the microcephaly phenotype associated with 16p11.2 duplication, whereas its suppression yields the macrocephalic phenotype associated with the reciprocal deletion, suggesting that KCTD13 is a major driver for the neurodevelopmental phenotypes associated with the 16p11.2 copy number variants. Gene dosage in psychiatric disease Copy number variants (CNVs) make an important contribution to genetic disorders, and some CNVs have been shown to have reciprocal phenotypic effects. For instance, duplication of chromosomal region 16p11.2 has been linked to autism, schizophrenia and microcephaly, and reciprocal deletion to autism, obesity and macrocephaly. By manipulating levels of expression — in pairwise combination — of zebrafish orthologues in this genomic interval, Nicholas Katsanis and colleagues identified KCTD13 as the locus that can recapitulate the macro- and microcephalic phenotype, which they show is underpinned by a proliferative defect. Together with further human genetic data, these results suggest that KCTD13 is a major driver for the neurodevelopmental phenotypes associated with 16p11.2 duplication/deletion. The approach used here also offers a way of identifying other dosage-sensitive loci. Copy number variants (CNVs) are major contributors to genetic disorders 1 . We have dissected a region of the 16p11.2 chromosome—which encompasses 29 genes—that confers susceptibility to neurocognitive defects when deleted or duplicated 2 , 3 . Overexpression of each human transcript in zebrafish embryos identified KCTD13 as the sole message capable of inducing the microcephaly phenotype associated with the 16p11.2 duplication 2 , 3 , 4 , 5 , whereas suppression of the same locus yielded the macrocephalic phenotype associated with the 16p11.2 deletion 5 , 6 , capturing the mirror phenotypes of humans. Analyses of zebrafish and mouse embryos suggest that microcephaly is caused by decreased proliferation of neuronal progenitors with concomitant increase in apoptosis in the developing brain, whereas macrocephaly arises by increased proliferation and no changes in apoptosis. A role for KCTD13 dosage changes is consistent with autism in both a recently reported family with a reduced 16p11.2 deletion and a subject reported here with a complex 16p11.2 rearrangement involving de novo structural alteration of KCTD13 . Our data suggest that KCTD13 is a major driver for the neurodevelopmental phenotypes associated with the 16p11.2 CNV, reinforce the idea that one or a small number of transcripts within a CNV can underpin clinical phenotypes, and offer an efficient route to identifying dosage-sensitive loci.

The intestinal epithelium forms a barrier protecting the organism from microbes and other proinflammatory stimuli. The integrity of this barrier and the proper response to infection requires precise regulation of powerful immune homing signals such as tumor necrosis factor (TNF). Dysregulation of TNF leads to inflammatory bowel diseases (IBD), but the mechanism controlling the expression of this potent cytokine and the events that trigger the onset of chronic inflammation are unknown. Here, we show that loss of function of the epigenetic regulator ubiquitin-like protein containing PHD and RING finger domains 1 ( uhrf1 ) in zebrafish leads to a reduction in tnfa promoter methylation and the induction of tnfa expression in intestinal epithelial cells (IECs). The increase in IEC tnfa levels is microbe-dependent and results in IEC shedding and apoptosis, immune cell recruitment, and barrier dysfunction, consistent with chronic inflammation. Importantly, tnfa knockdown in uhrf1 mutants restores IEC morphology, reduces cell shedding, and improves barrier function. We propose that loss of epigenetic repression and TNF induction in the intestinal epithelium can lead to IBD onset. Significance Inflammatory bowel diseases (IBD), including Crohn’s disease and ulcerative colitis, are intestinal disorders of poorly understood origin and are associated with significant morbidity and mortality. A crucial factor associated with IBD onset is the presence of elevated levels of the proinflammatory cytokine tumor necrosis factor (TNF) in the intestine, signified by the use of anti-TNF therapy to treat patients with Crohn’s disease. Despite its pathogenic relevance, the mechanisms regulating TNF expression and IBD onset remain largely unknown. Here, we show that loss of epigenetic regulation results in the induction of TNF in the intestinal epithelium, leading to a loss of intestinal barrier function and inflammation. Our results suggest that mutations in genes controlling epigenetic regulators can lead to IBD onset.

Jeremy Reiter and colleagues show that Tctn1 is a component of a transition zone complex that regulates ciliogenesis and ciliary membrane composition. They also identify a likely causal mutation in TCTN1 in two siblings with Joubert syndrome. Mutations affecting ciliary components cause ciliopathies. As described here, we investigated Tectonic1 (Tctn1), a regulator of mouse Hedgehog signaling, and found that it is essential for ciliogenesis in some, but not all, tissues. Cell types that do not require Tctn1 for ciliogenesis require it to localize select membrane-associated proteins to the cilium, including Arl13b, AC3, Smoothened and Pkd2. Tctn1 forms a complex with multiple ciliopathy proteins associated with Meckel and Joubert syndromes, including Mks1, Tmem216, Tmem67, Cep290, B9d1, Tctn2 and Cc2d2a. Components of this complex co-localize at the transition zone, a region between the basal body and ciliary axoneme. Like Tctn1, loss of Tctn2, Tmem67 or Cc2d2a causes tissue-specific defects in ciliogenesis and ciliary membrane composition. Consistent with a shared function for complex components, we identified a mutation in TCTN1 that causes Joubert syndrome. Thus, a transition zone complex of Meckel and Joubert syndrome proteins regulates ciliary assembly and trafficking, suggesting that transition zone dysfunction is the cause of these ciliopathies.

Retinoic acid (RA) is a potent morphogen required for embryonic development. RA is formed in a multistep process from vitamin A (retinol); RA acts in a paracrine fashion to shape the developing eye and is essential for normal optic vesicle and anterior segment formation. Perturbation in RA-signaling can result in severe ocular developmental diseases—including microphthalmia, anophthalmia, and coloboma. RA-signaling is also essential for embryonic development and life, as indicated by the significant consequences of mutations in genes involved in RA-signaling. The requirement of RA-signaling for normal development is further supported by the manifestation of severe pathologies in animal models of RA deficiency—such as ventral lens rotation, failure of optic cup formation, and embryonic and postnatal lethality. In this review, we summarize RA-signaling, recent advances in our understanding of this pathway in eye development, and the requirement of RA-signaling for embryonic development (e.g., organogenesis and limb bud development) and life.