Recessive PRDM13 mutations cause fatal perinatal brainstem dysfunction with cerebellar hypoplasia and disrupt Purkinje cell differentiation.

Pontocerebellar hypoplasias (PCHs) are congenital disorders characterized by hypoplasia or early atrophy of the cerebellum and brainstem, leading to a very limited motor and cognitive development. Although over 20 genes have been shown to be mutated in PCHs, a large proportion of affected individuals remains undiagnosed. We describe four families with children presenting with severe neonatal brainstem dysfunction and pronounced deficits in cognitive and motor development associated with four different bi-allelic mutations in PRDM13, including homozygous truncating variants in the most severely affected individuals. Brain MRI and fetopathological examination revealed a PCH-like phenotype, associated with major hypoplasia of inferior olive nuclei and dysplasia of the dentate nucleus. Notably, histopathological examinations highlighted a sparse and disorganized Purkinje cell layer in the cerebellum. PRDM13 encodes a transcriptional repressor known to be critical for neuronal subtypes specification in the mouse retina and spinal cord but had not been implicated, so far, in hindbrain development. snRNA-seq data mining and in situ hybridization in humans show that PRDM13 is expressed at early stages in the progenitors of the cerebellar ventricular zone, which gives rise to cerebellar GABAergic neurons, including Purkinje cells. We also show that loss of function of prdm13 in zebrafish leads to a reduction in Purkinje cells numbers and a complete absence of the inferior olive nuclei. Altogether our data identified bi-allelic mutations in PRDM13 as causing a olivopontocerebellar hypoplasia syndrome and suggest that early deregulations of the transcriptional control of neuronal fate specification could contribute to a significant number of cases.

Neuropathological hallmarks of antenatal mitochondrial diseases with a corpus callosum defect.

Corpus callosum defects are frequent congenital cerebral disorders caused by mutations in more than 300 genes. These include genes implicated in corpus callosum development or function, as well as genes essential for mitochondrial physiology. However, in utero corpus callosum anomalies rarely raise a suspicion of mitochondrial disease and are characterized by a very large clinical heterogeneity. Here, we report a detailed pathological and neuro-histopathological investigation of nine foetuses from four unrelated families with prenatal onset of corpus callosum anomalies, sometimes associated with other cerebral or extra-cerebral defects. Next generation sequencing allowed the identification of novel pathogenic variants in three different nuclear genes previously reported in mitochondrial diseases: TIMMDC1, encoding a Complex I assembly factor never involved before in corpus callosum defect; MRPS22, a protein of the small mitoribosomal subunit; and EARS2, the mitochondrial tRNA-glutamyl synthetase. The present report describes the antenatal histopathological findings in mitochondrial diseases and expands the genetic spectrum of antenatal corpus callosum anomalies establishing OXPHOS function as an important factor for corpus callosum biogenesis. We propose that, when observed, antenatal corpus callosum anomalies should raise suspicion of mitochondrial disease and prenatal genetic counselling should be considered.

A wave of deep intronic mutations in X-linked Alport syndrome.

X-linked Alport syndrome (XLAS) is an inherited kidney disease caused exclusively by pathogenic variants in the COL4A5 gene. In 10-20% of cases, DNA sequencing of COL4A5 exons or flanking regions cannot identify molecular causes. Here, our objective was to use a transcriptomic approach to identify causative events in a group of 19 patients with XLAS without identified mutation by Alport gene panel sequencing. Bulk RNAseq and/or targeted RNAseq using a capture panel of kidney genes was performed. Alternative splicing events were compared to those of 15 controls by a developed bioinformatic score. When using targeted RNAseq, COL4A5 coverage was found to be 23-fold higher than with bulk RNASeq and revealed 30 significant alternative splicing events in 17 of the 19 patients. After computational scoring, a pathogenic transcript was found in all patients. A causative variant affecting COL4A5 splicing and absent in the general population was identified in all cases. Altogether, we developed a simple and robust method for identification of aberrant transcripts due to pathogenic deep-intronic COL4A5 variants. Thus, these variants, potentially targetable by specific antisense oligonucleotide therapies, were found in a high percentage of patients with XLAS in whom pathogenic variants were missed by conventional DNA sequencing.

Bi-allelic Variations of SMO in Humans Cause a Broad Spectrum of Developmental Anomalies Due to Abnormal Hedgehog Signaling.

The evolutionarily conserved hedgehog (Hh) pathway is essential for organogenesis and plays critical roles in postnatal tissue maintenance and renewal. A unique feature of the vertebrate Hh pathway is that signal transduction requires the primary cilium (PC) where major pathway components are dynamically enriched. These factors include smoothened (SMO) and patched, which constitute the core reception system for sonic hedgehog (SHH) as well as GLI transcription factors, the key mediators of the pathway. Here, we report bi-allelic loss-of-function variations in SMO in seven individuals from five independent families; these variations cause a wide phenotypic spectrum of developmental anomalies affecting the brain (hypothalamic hamartoma and microcephaly), heart (atrioventricular septal defect), skeleton (postaxial polydactyly, narrow chest, and shortening of long bones), and enteric nervous system (aganglionosis). Cells derived from affected individuals showed normal ciliogenesis but severely altered Hh-signal transduction as a result of either altered PC trafficking or abnormal activation of the pathway downstream of SMO. In addition, Hh-independent GLI2 accumulation at the PC tip in cells from the affected individuals suggests a potential function of SMO in regulating basal ciliary trafficking of GLI2 when the pathway is off. Thus, loss of SMO function results in abnormal PC dynamics of key components of the Hh signaling pathway and leads to a large continuum of malformations in humans.

A shared pattern of altered gene expression in human embryos affected by mitochondrial diseases.

STUDY QUESTION: Does mitochondrial deficiency affect human embryonic preimplantation development? SUMMARY ANSWER: The presence of a pathogenic mitochondrial variant triggers changes in the gene expression of preimplantation human embryos, compromising their development, cell differentiation, and survival. WHAT IS KNOWN ALREADY: Quantitative and qualitative anomalies of mitochondrial DNA (mtDNA) are reportedly associated with impaired human embryonic development, but the underlying mechanisms remain unexplained. STUDY DESIGN, SIZE, DURATION: Taking advantage of the preimplantation genetic testing for mitochondrial disorders in at-risk couples, we have compared gene expression of 9 human embryos carrying pathogenic variants in either mtDNA genes or nuclear genes encoding mitochondrial protein to 33 age-matched control embryos. PARTICIPANTS/MATERIALS, SETTING, METHODS: Single-embryo transcriptomic analysis was performed on whole human blastocyst embryos donated to research. MAIN RESULTS AND THE ROLE OF CHANCE: Specific pathogenic mitochondrial variants downregulate gene expression in preimplantation human embryos [566 genes in oxidative phosphorylation (OXPHOS)-deficient embryos], impacting transcriptional regulators, differentiation factors, and nuclear genes encoding mitochondrial proteins. These changes in gene expression primarily alter OXPHOS and cell survival pathways. LIMITATIONS, REASONS FOR CAUTION: The number of OXPHOS-deficient embryos available for the study was limited owing to the rarity of this material. However, the molecular signature shared by all these embryos supports the relevance of the findings. WIDER IMPLICATIONS OF THE FINDINGS: While identification of reliable markers of normal embryonic development is urgently needed in ART, our study prompts us to consider under-expression of the targeted genes reported here, as predictive biomarkers of mitochondrial dysfunction during preimplantation development. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the 'Association Française contre les Myopathies (AFM-Téléthon)' and the 'La Fondation Maladies Rares'. No competing interests to declare. TRIAL REGISTRATION NUMBER: N/A.

CNVxplorer: a web tool to assist clinical interpretation of CNVs in rare disease patients.

Copy Number Variants (CNVs) are an important cause of rare diseases. Array-based Comparative Genomic Hybridization tests yield a ∼12% diagnostic rate, with ∼8% of patients presenting CNVs of unknown significance. CNVs interpretation is particularly challenging on genomic regions outside of those overlapping with previously reported structural variants or disease-associated genes. Recent studies showed that a more comprehensive evaluation of CNV features, leveraging both coding and non-coding impacts, can significantly improve diagnostic rates. However, currently available CNV interpretation tools are mostly gene-centric or provide only non-interactive annotations difficult to assess in the clinical practice. Here, we present CNVxplorer, a web server suited for the functional assessment of CNVs in a clinical diagnostic setting. CNVxplorer mines a comprehensive set of clinical, genomic, and epigenomic features associated with CNVs. It provides sequence constraint metrics, impact on regulatory elements and topologically associating domains, as well as expression patterns. Analyses offered cover (a) agreement with patient phenotypes; (b) visualizations of associations among genes, regulatory elements and transcription factors; (c) enrichment on functional and pathway annotations and (d) co-occurrence of terms across PubMed publications related to the query CNVs. A flexible evaluation workflow allows dynamic re-interrogation in clinical sessions. CNVxplorer is publicly available at http://cnvxplorer.com.

Functional and genetic testing in adults with HLH reveals an inflammatory profile rather than a cytotoxicity defect.

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory condition. Primary HLH occurs early in life as a result of monogenic biallelic mutations affecting lymphocyte cytotoxicity. Secondary HLH occurs mostly in adults secondary to infection, lymphoma, or rheumatic disease. In this latter setting, lymphocyte cytotoxicity status is not known. We conducted a systematic evaluation of natural killer (NK) cell cytotoxicity in adult patients with secondary HLH. Adult patients with secondary HLH were prospectively studied ex vivo for total lymphocyte count and subtype, NK cell phenotype, perforin expression and degranulation, and natural or antibody-dependent cell cytotoxicity, in comparison with patients affected by the same underlying disease without HLH (disease controls [DCs]) and with healthy controls (HCs). Screening for variants of cytotoxity genes was systematically performed. 68 patients were included in the HLH group and 34 each in the DC and HC groups. In HLH patients, severe and transient lymphopenia, activated NK cell phenotype (eg, increased CD69, ICAM-1, HLADR, and CCR5 expression), and decreased capacity of interferon γ production were observed; mean perforin expression was normal; and degranulation tests and NK cell cytotoxicity were not different from those in DCs. A monoallelic variant of uncertain significance affecting a lymphocyte cytotoxicity gene or the perforin variant A91V was observed in almost 50% of the patients. We detected no major intrinsic cytotoxicity dysfunction in secondary HLH patients compared with DCs and no predicted pathogenic gene variant. The activated NK phenotype profile associated with decreased interferon γ production seems similar to those of other hyperinflammatory diseases such as sepsis or systemic juvenile idiopathic arthritis.

Lack of the multidrug transporter MRP4/ABCC4 defines the PEL-negative blood group and impairs platelet aggregation.

The rare PEL-negative phenotype is one of the last blood groups with an unknown genetic basis. By combining whole-exome sequencing and comparative global proteomic investigations, we found a large deletion in the ABCC4/MRP4 gene encoding an ATP-binding cassette (ABC) transporter in PEL-negative individuals. The loss of PEL expression on ABCC4-CRISPR-Cas9 K562 cells and its overexpression in ABCC4-transfected cells provided evidence that ABCC4 is the gene underlying the PEL blood group antigen. Although ABCC4 is an important cyclic nucleotide exporter, red blood cells from ABCC4null/PEL-negative individuals exhibited a normal guanosine 3',5'-cyclic monophosphate level, suggesting a compensatory mechanism by other erythroid ABC transporters. Interestingly, PEL-negative individuals showed an impaired platelet aggregation, confirming a role for ABCC4 in platelet function. Finally, we showed that loss-of-function mutations in the ABCC4 gene, associated with leukemia outcome, altered the expression of the PEL antigen. In addition to ABCC4 genotyping, PEL phenotyping could open a new way toward drug dose adjustment for leukemia treatment.

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.

Impaired Transferrin Receptor Palmitoylation and Recycling in Neurodegeneration with Brain Iron Accumulation.

Neurodegeneration with brain iron accumulation (NBIA) is a genetically heterogeneous condition characterized by progressive dystonia with iron accumulation in the basal ganglia. How NBIA-associated mutations trigger iron overload remains poorly understood. After studying fibroblast cell lines from subjects carrying both known and unreported biallelic mutations in CRAT and REPS1, we ascribe iron overload to the abnormal recycling of transferrin receptor (TfR1) and the reduction of TfR1 palmitoylation in NBIA. Moreover, we describe palmitoylation as a hitherto unreported level of post-translational TfR1 regulation. A widely used antimalarial agent, artesunate, rescued abnormal TfR1 palmitoylation in cultured fibroblasts of NBIA subjects. These observations suggest therapeutic strategies aimed at targeting impaired TfR1 recycling and palmitoylation in NBIA.

EFL1 mutations impair eIF6 release to cause Shwachman-Diamond syndrome.

Shwachman-Diamond syndrome (SDS) is a recessive disorder typified by bone marrow failure and predisposition to hematological malignancies. SDS is predominantly caused by deficiency of the allosteric regulator Shwachman-Bodian-Diamond syndrome that cooperates with elongation factor-like GTPase 1 (EFL1) to catalyze release of the ribosome antiassociation factor eIF6 and activate translation. Here, we report biallelic mutations in EFL1 in 3 unrelated individuals with clinical features of SDS. Cellular defects in these individuals include impaired ribosomal subunit joining and attenuated global protein translation as a consequence of defective eIF6 eviction. In mice, Efl1 deficiency recapitulates key aspects of the SDS phenotype. By identifying biallelic EFL1 mutations in SDS, we define this leukemia predisposition disorder as a ribosomopathy that is caused by corruption of a fundamental, conserved mechanism, which licenses entry of the large ribosomal subunit into translation.

Correction: A homozygous KAT2B variant modulates the clinical phenotype of ADD3 deficiency in humans and flies.

[This corrects the article DOI: 10.1371/journal.pgen.1007386.].

A homozygous KAT2B variant modulates the clinical phenotype of ADD3 deficiency in humans and flies.

Recent evidence suggests that the presence of more than one pathogenic mutation in a single patient is more common than previously anticipated. One of the challenges hereby is to dissect the contribution of each gene mutation, for which animal models such as Drosophila can provide a valuable aid. Here, we identified three families with mutations in ADD3, encoding for adducin-γ, with intellectual disability, microcephaly, cataracts and skeletal defects. In one of the families with additional cardiomyopathy and steroid-resistant nephrotic syndrome (SRNS), we found a homozygous variant in KAT2B, encoding the lysine acetyltransferase 2B, with impact on KAT2B protein levels in patient fibroblasts, suggesting that this second mutation might contribute to the increased disease spectrum. In order to define the contribution of ADD3 and KAT2B mutations for the patient phenotype, we performed functional experiments in the Drosophila model. We found that both mutations were unable to fully rescue the viability of the respective null mutants of the Drosophila homologs, hts and Gcn5, suggesting that they are indeed pathogenic in flies. While the KAT2B/Gcn5 mutation additionally showed a significantly reduced ability to rescue morphological and functional defects of cardiomyocytes and nephrocytes (podocyte-like cells), this was not the case for the ADD3 mutant rescue. Yet, the simultaneous knockdown of KAT2B and ADD3 synergistically impaired kidney and heart function in flies as well as the adhesion and migration capacity of cultured human podocytes, indicating that mutations in both genes may be required for the full clinical manifestation. Altogether, our studies describe the expansion of the phenotypic spectrum in ADD3 deficiency associated with a homozygous likely pathogenic KAT2B variant and thereby identify KAT2B as a susceptibility gene for kidney and heart disease in ADD3-associated disorders.

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.

FDXR Mutations Cause Sensorial Neuropathies and Expand the Spectrum of Mitochondrial Fe-S-Synthesis Diseases.

Hearing loss and visual impairment in childhood have mostly genetic origins, some of them being related to sensorial neuronal defects. Here, we report on eight subjects from four independent families affected by auditory neuropathy and optic atrophy. Whole-exome sequencing revealed biallelic mutations in FDXR in affected subjects of each family. FDXR encodes the mitochondrial ferredoxin reductase, the sole human ferredoxin reductase implicated in the biosynthesis of iron-sulfur clusters (ISCs) and in heme formation. ISC proteins are involved in enzymatic catalysis, gene expression, and DNA replication and repair. We observed deregulated iron homeostasis in FDXR mutant fibroblasts and indirect evidence of mitochondrial iron overload. Functional complementation in a yeast strain in which ARH1, the human FDXR ortholog, was deleted established the pathogenicity of these mutations. These data highlight the wide clinical heterogeneity of mitochondrial disorders related to ISC synthesis.

Biallelic Mutations in LIPT2 Cause a Mitochondrial Lipoylation Defect Associated with Severe Neonatal Encephalopathy.

Lipoate serves as a cofactor for the glycine cleavage system (GCS) and four 2-oxoacid dehydrogenases functioning in energy metabolism (α-oxoglutarate dehydrogenase [α-KGDHc] and pyruvate dehydrogenase [PDHc]), or amino acid metabolism (branched-chain oxoacid dehydrogenase, 2-oxoadipate dehydrogenase). Mitochondrial lipoate synthesis involves three enzymatic steps catalyzed sequentially by lipoyl(octanoyl) transferase 2 (LIPT2), lipoic acid synthetase (LIAS), and lipoyltransferase 1 (LIPT1). Mutations in LIAS have been associated with nonketotic hyperglycinemia-like early-onset convulsions and encephalopathy combined with a defect in mitochondrial energy metabolism. LIPT1 deficiency spares GCS deficiency and has been associated with a biochemical signature of combined 2-oxoacid dehydrogenase deficiency leading to early death or Leigh-like encephalopathy. We report on the identification of biallelic LIPT2 mutations in three affected individuals from two families with severe neonatal encephalopathy. Brain MRI showed major cortical atrophy with white matter abnormalities and cysts. Plasma glycine was mildly increased. Affected individuals' fibroblasts showed reduced oxygen consumption rates, PDHc, α-KGDHc activities, leucine catabolic flux, and decreased protein lipoylation. A normalization of lipoylation was observed after expression of wild-type LIPT2, arguing for LIPT2 requirement in intramitochondrial lipoate synthesis. Lipoic acid supplementation did not improve clinical condition nor activities of PDHc, α-KGDHc, or leucine metabolism in fibroblasts and was ineffective in yeast deleted for the orthologous LIP2.

Mutations in GREB1L Cause Bilateral Kidney Agenesis in Humans and Mice.

Congenital anomalies of the kidney and urinary tract (CAKUT) constitute a major cause of chronic kidney disease in children and 20% of prenatally detected anomalies. CAKUT encompass a spectrum of developmental kidney defects, including renal agenesis, hypoplasia, and cystic and non-cystic dysplasia. More than 50 genes have been reported as mutated in CAKUT-affected case subjects. However, the pathophysiological mechanisms leading to bilateral kidney agenesis (BKA) remain largely elusive. Whole-exome or targeted exome sequencing of 183 unrelated familial and/or severe CAKUT-affected case subjects, including 54 fetuses with BKA, led to the identification of 16 heterozygous variants in GREB1L (growth regulation by estrogen in breast cancer 1-like), a gene reported as a target of retinoic acid signaling. Four loss-of-function and 12 damaging missense variants, 14 being absent from GnomAD, were identified. Twelve of them were present in familial or simplex BKA-affected case subjects. Female BKA-affected fetuses also displayed uterus agenesis. We demonstrated a significant association between GREB1L variants and BKA. By in situ hybridization, we showed expression of Greb1l in the nephrogenic zone in developing mouse kidney. We generated a Greb1l knock-out mouse model by CRISPR-Cas9. Analysis at E13.5 revealed lack of kidneys and genital tract anomalies in male and female Greb1l-/- embryos and a slight decrease in ureteric bud branching in Greb1l+/- embryos. We showed that Greb1l invalidation in mIMCD3 cells affected tubulomorphogenesis in 3D-collagen culture, a phenotype rescued by expression of the wild-type human protein. This demonstrates that GREB1L plays a major role in early metanephros and genital development in mice and humans.

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.

Mutations in the Spliceosome Component CWC27 Cause Retinal Degeneration with or without Additional Developmental Anomalies.

Pre-mRNA splicing factors play a fundamental role in regulating transcript diversity both temporally and spatially. Genetic defects in several spliceosome components have been linked to a set of non-overlapping spliceosomopathy phenotypes in humans, among which skeletal developmental defects and non-syndromic retinitis pigmentosa (RP) are frequent findings. Here we report that defects in spliceosome-associated protein CWC27 are associated with a spectrum of disease phenotypes ranging from isolated RP to severe syndromic forms. By whole-exome sequencing, recessive protein-truncating mutations in CWC27 were found in seven unrelated families that show a range of clinical phenotypes, including retinal degeneration, brachydactyly, craniofacial abnormalities, short stature, and neurological defects. Remarkably, variable expressivity of the human phenotype can be recapitulated in Cwc27 mutant mouse models, with significant embryonic lethality and severe phenotypes in the complete knockout mice while mice with a partial loss-of-function allele mimic the isolated retinal degeneration phenotype. Our study describes a retinal dystrophy-related phenotype spectrum as well as its genetic etiology and highlights the complexity of the spliceosomal gene network.

A Recurrent De Novo Nonsense Variant in ZSWIM6 Results in Severe Intellectual Disability without Frontonasal or Limb Malformations.

A recurrent de novo missense variant within the C-terminal Sin3-like domain of ZSWIM6 was previously reported to cause acromelic frontonasal dysostosis (AFND), an autosomal-dominant severe frontonasal and limb malformation syndrome, associated with neurocognitive and motor delay, via a proposed gain-of-function effect. We present detailed phenotypic information on seven unrelated individuals with a recurrent de novo nonsense variant (c.2737C>T [p.Arg913Ter]) in the penultimate exon of ZSWIM6 who have severe-profound intellectual disability and additional central and peripheral nervous system symptoms but an absence of frontonasal or limb malformations. We show that the c.2737C>T variant does not trigger nonsense-mediated decay of the ZSWIM6 mRNA in affected individual-derived cells. This finding supports the existence of a truncated ZSWIM6 protein lacking the Sin3-like domain, which could have a dominant-negative effect. This study builds support for a key role for ZSWIM6 in neuronal development and function, in addition to its putative roles in limb and craniofacial development, and provides a striking example of different variants in the same gene leading to distinct phenotypes.