Further delineation of the phenotypic and metabolomic profile of ALDH1L2-related neurodevelopmental disorder

ALDH1L2, a mitochondrial enzyme in folate metabolism, converts 10-formyl-THF (10-formyltetrahydrofolate) to THF (tetrahydrofolate) and CO2. At the cellular level, deficiency of this NADP+-dependent reaction results in marked reduction in NADPH/NADP+ ratio and reduced mitochondrial ATP. Thus far, a single patient with biallelic ALDH1L2 variants and the phenotype of a neurodevelopmental disorder has been reported. Here, we describe another patient with a neurodevelopmental disorder associated with a novel homozygous missense variant in ALDH1L2, Pro133His. The variant caused marked reduction in the ALDH1L2 enzyme activity in skin fibroblasts derived from the patient as probed by 10-FDDF, a stable synthetic analog of 10-formyl-THF. Additional associated abnormalities in these fibroblasts include reduced NADPH/NADP+ ratio and pool of mitochondrial ATP, upregulated autophagy and dramatically altered metabolomic profile. Overall, our study further supports a link between ALDH1L2 deficiency and abnormal neurodevelopment in humans.

Biallelic NAA60 variants with impaired n-terminal acetylation capacity cause autosomal recessive primary familial brain calcifications

Primary familial brain calcification (PFBC) is characterized by calcium deposition in the brain, causing progressive movement disorders, psychiatric symptoms, and cognitive decline. PFBC is a heterogeneous disorder currently linked to variants in six different genes, but most patients remain genetically undiagnosed. Here, we identify biallelic NAA60 variants in ten individuals from seven families with autosomal recessive PFBC. The NAA60 variants lead to loss-of-function with lack of protein N-terminal (Nt)-acetylation activity. We show that the phosphate importer SLC20A2 is a substrate of NAA60 in vitro. In cells, loss of NAA60 caused reduced surface levels of SLC20A2 and a reduction in extracellular phosphate uptake. This study establishes NAA60 as a causal gene for PFBC, provides a possible biochemical explanation of its disease-causing mechanisms and underscores NAA60-mediated Nt-acetylation of transmembrane proteins as a fundamental process for healthy neurobiological functioning.

Genomic analysis of presumed perinatal stroke in Saudi Arabia reveals a strong monogenic contribution

Perinatal stroke is associated with significant short- and long-term morbidity and has been recognized as the most common cause of cerebral palsy in term infants. The diagnosis of presumed perinatal stroke (PPS) is made in children who present with neurological deficit and/or seizures attributable to focal chronic infarction on neuroimaging and have uneventful neonatal history. The underlying mechanism of presumed perinatal stroke remains unknown and thorough investigation of potential monogenic causes has not been conducted to date. Here, we describe the use of untargeted exome sequencing to investigate a cohort of eight patients from six families with PPS. A likely deleterious variant was identified in four families. These include the well-established risk genes COL4A2 and JAM3. In addition, we report the first independent confirmation of the recently described link between ESAM and perinatal stroke. Our data also highlight NID1 as a candidate gene for the condition. This study suggests that monogenic disorders are important contributors to the pathogenesis of PPS and should be investigated by untargeted sequencing especially when traditional risk factors are excluded.

Beyond the exome: utility of long-read whole genome sequencing in exome-negative autosomal recessive diseases

BACKGROUND

Long-read whole genome sequencing (lrWGS) has the potential to address the technical limitations of exome sequencing in ways not possible by short-read WGS. However, its utility in autosomal recessive Mendelian diseases is largely unknown.

METHODS

In a cohort of 34 families in which the suspected autosomal recessive diseases remained undiagnosed by exome sequencing, lrWGS was performed on the Pacific Bioscience Sequel IIe platform.

RESULTS

Likely causal variants were identified in 13 (38%) of the cohort. These include (1) a homozygous splicing SV in TYMS as a novel candidate gene for lethal neonatal lactic acidosis, (2) a homozygous non-coding SV that we propose impacts STK25 expression and causes a novel neurodevelopmental disorder, (3) a compound heterozygous SV in RP1L1 with complex inheritance pattern in a family with inherited retinal disease, (4) homozygous deep intronic variants in LEMD2 and SNAP91 as novel candidate genes for neurodevelopmental disorders in two families, and (5) a promoter SNV in SLC4A4 causing non-syndromic band keratopathy. Surprisingly, we also encountered causal variants that could have been identified by short-read exome sequencing in 7 families. The latter highlight scenarios that are especially challenging at the interpretation level.

CONCLUSIONS

Our data highlight the continued need to address the interpretation challenges in parallel with efforts to improve the sequencing technology itself. We propose a path forward for the implementation of lrWGS sequencing in the setting of autosomal recessive diseases in a way that maximizes its utility.

SLC4A10 mutation causes a neurological disorder associated with impaired GABAergic transmission

SLC4A10 is a plasma-membrane bound transporter that utilizes the Na+ gradient to drive cellular HCO3- uptake, thus mediating acid extrusion. In the mammalian brain, SLC4A10 is expressed in principal neurons and interneurons, as well as in epithelial cells of the choroid plexus, the organ regulating the production of CSF. Using next generation sequencing on samples from five unrelated families encompassing nine affected individuals, we show that biallelic SLC4A10 loss-of-function variants cause a clinically recognizable neurodevelopmental disorder in humans. The cardinal clinical features of the condition include hypotonia in infancy, delayed psychomotor development across all domains and intellectual impairment. Affected individuals commonly display traits associated with autistic spectrum disorder including anxiety, hyperactivity and stereotyped movements. In two cases isolated episodes of seizures were reported in the first few years of life, and a further affected child displayed bitemporal epileptogenic discharges on EEG without overt clinical seizures. While occipitofrontal circumference was reported to be normal at birth, progressive postnatal microcephaly evolved in 7 out of 10 affected individuals. Neuroradiological features included a relative preservation of brain volume compared to occipitofrontal circumference, characteristic narrow sometimes 'slit-like' lateral ventricles and corpus callosum abnormalities. Slc4a10 -/- mice, deficient for SLC4A10, also display small lateral brain ventricles and mild behavioural abnormalities including delayed habituation and alterations in the two-object novel object recognition task. Collapsed brain ventricles in both Slc4a10-/- mice and affected individuals suggest an important role of SLC4A10 in the production of the CSF. However, it is notable that despite diverse roles of the CSF in the developing and adult brain, the cortex of Slc4a10-/- mice appears grossly intact. Co-staining with synaptic markers revealed that in neurons, SLC4A10 localizes to inhibitory, but not excitatory, presynapses. These findings are supported by our functional studies, which show the release of the inhibitory neurotransmitter GABA is compromised in Slc4a10-/- mice, while the release of the excitatory neurotransmitter glutamate is preserved. Manipulation of intracellular pH partially rescues GABA release. Together our studies define a novel neurodevelopmental disorder associated with biallelic pathogenic variants in SLC4A10 and highlight the importance of further analyses of the consequences of SLC4A10 loss-of-function for brain development, synaptic transmission and network properties.

A founder DBR1 variant causes a lethal form of congenital ichthyosis

DBR1 encodes the only known human lariat debranching enzyme and its deficiency has been found to cause an autosomal recessive inborn error of immunity characterized by pediatric brainstem viral-induced encephalitis (MIM 619441). We describe a distinct allelic disorder caused by a founder recessive DBR1 variant in four families (DBR1(NM_016216.4):c.200A > G (p.Tyr67Cys)). Consistent features include prematurity, severe intrauterine growth deficiency, congenital ichthyosis-like presentation (collodion membrane, severe skin peeling and xerosis), and death before the first year of life. Patient-derived fibroblasts displayed the characteristic accumulation of intron lariats in their RNA as revealed by targeted and untargeted analysis, in addition to a marked reduction of DBR1 on immunoblot analysis. We propose a novel DBR1-related developmental disorder that is distinct from DBR1-related encephalitis susceptibility and highlight the apparent lack of correlation with the degree of DBR1 deficiency.

Diagnostic implications of pitfalls in causal variant identification based on 4577 molecularly characterized families

Despite large sequencing and data sharing efforts, previously characterized pathogenic variants only account for a fraction of Mendelian disease patients, which highlights the need for accurate identification and interpretation of novel variants. In a large Mendelian cohort of 4577 molecularly characterized families, numerous scenarios in which variant identification and interpretation can be challenging are encountered. We describe categories of challenges that cover the phenotype (e.g. novel allelic disorders), pedigree structure (e.g. imprinting disorders masquerading as autosomal recessive phenotypes), positional mapping (e.g. double recombination events abrogating candidate autozygous intervals), gene (e.g. novel gene-disease assertion) and variant (e.g. complex compound inheritance). Overall, we estimate a probability of 34.3% for encountering at least one of these challenges. Importantly, our data show that by only addressing non-sequencing-based challenges, around 71% increase in the diagnostic yield can be expected. Indeed, by applying these lessons to a cohort of 314 cases with negative clinical exome or genome reports, we could identify the likely causal variant in 54.5%. Our work highlights the need to have a thorough approach to undiagnosed diseases by considering a wide range of challenges rather than a narrow focus on sequencing technologies. It is hoped that by sharing this experience, the yield of undiagnosed disease programs globally can be improved.

PRSS8, encoding prostasin, is mutated in patients with autosomal recessive ichthyosis

Ichthyosis is a genetically heterogeneous genodermatosis characterized by severely rough, dry and scaly skin. We report two consanguineous families with congenital ichthyosis. Combined positional mapping and exome sequencing of the two families revealed novel homozygous likely deleterious variants in PRSS8 (encoding prostasin) within a linkage locus on chromosome 16. One variant involved a canonical splice site and was associated with reduced abundance of the normal transcript, while the other was a missense variant that altered a highly conserved residue. The phenotype of Prss8 knockout mouse bears a striking resemblance to the one we describe in human patients, including the skin histopathology. Our data suggest a novel PRSS8-related ichthyosis disorder.

Lethal variants in humans: lessons learned from a large molecular autopsy cohort

BACKGROUND

Molecular autopsy refers to DNA-based identification of the cause of death. Despite recent attempts to broaden its scope, the term remains typically reserved to sudden unexplained death in young adults. In this study, we aim to showcase the utility of molecular autopsy in defining lethal variants in humans.

METHODS

We describe our experience with a cohort of 481 cases in whom the cause of premature death was investigated using DNA from the index or relatives (molecular autopsy by proxy). Molecular autopsy tool was typically exome sequencing although some were investigated using targeted approaches in the earlier stages of the study; these include positional mapping, targeted gene sequencing, chromosomal microarray, and gene panels.

RESULTS

The study includes 449 cases from consanguineous families and 141 lacked family history (simplex). The age range was embryos to 18 years. A likely causal variant (pathogenic/likely pathogenic) was identified in 63.8% (307/481), a much higher yield compared to the general diagnostic yield (43%) from the same population. The predominance of recessive lethal alleles allowed us to implement molecular autopsy by proxy in 55 couples, and the yield was similarly high (63.6%). We also note the occurrence of biallelic lethal forms of typically non-lethal dominant disorders, sometimes representing a novel bona fide biallelic recessive disease trait. Forty-six disease genes with no OMIM phenotype were identified in the course of this study. The presented data support the candidacy of two other previously reported novel disease genes (FAAH2 and MSN). The focus on lethal phenotypes revealed many examples of interesting phenotypic expansion as well as remarkable variability in clinical presentation. Furthermore, important insights into population genetics and variant interpretation are highlighted based on the results.

CONCLUSIONS

Molecular autopsy, broadly defined, proved to be a helpful clinical approach that provides unique insights into lethal variants and the clinical annotation of the human genome.

PLXNA2 as a candidate gene in patients with intellectual disability

Intellectual disability (ID) is one of the most common disabilities in humans. In an effort to contribute to the expanding genetic landscape of ID, we describe a novel autosomal recessive ID candidate gene. Combined autozygome/exome analysis was performed in two unrelated consanguineous families with ID. Each of the two families had a novel homozygous likely deleterious variant in PLXNA2 and displayed the core phenotype of ID. PLXNA2 belongs to a family of transmembrane proteins that function as semaphorin receptors. Sema5A-PlexinA2 is known to regulate brain development in mouse, and Plxna2-/- mice display defective associative learning, sociability, and sensorimotor gating. We note the existence of variability in the phenotype among the three patients, including the existence of variable degree of ID, ranging from borderline intellectual functioning to moderate-severe ID, and the presence of cardiac anomalies in only one of the patients. We propose incomplete penetrance as a possible explanation of the observed difference in phenotypes. Future cases will be needed to support the proposed link between PLXNA2 and ID in humans.

Exploiting the Autozygome to Support Previously Published Mendelian Gene-Disease Associations: An Update

There is a growing interest in standardizing gene-disease associations for the purpose of facilitating the proper classification of variants in the context of Mendelian diseases. One key line of evidence is the independent observation of pathogenic variants in unrelated individuals with similar phenotypes. Here, we expand on our previous effort to exploit the power of autozygosity to produce homozygous pathogenic variants that are otherwise very difficult to encounter in the homozygous state due to their rarity. The identification of such variants in genes with only tentative associations to Mendelian diseases can add to the existing evidence when observed in the context of compatible phenotypes. In this study, we report 20 homozygous variants in 18 genes (ADAMTS18, ARNT2, ASTN1, C3, DMBX1, DUT, GABRB3, GM2A, KIF12, LOXL3, NUP160, PTRHD1, RAP1GDS1, RHOBTB2, SIGMAR1, SPAST, TENM3, and WASHC5) that satisfy the ACMG classification for pathogenic/likely pathogenic if the involved genes had confirmed rather than tentative links to diseases. These variants were selected because they were truncating, founder with compelling segregation or supported by robust functional assays as with the DUT variant that we present its validation using yeast model. Our findings support the previously reported disease associations for these genes and represent a step toward their confirmation.

An exome-first approach to aid in the diagnosis of primary ciliary dyskinesia

Unlike disorders of primary cilium, primary ciliary dyskinesia (PCD) has a much narrower clinical spectrum consistent with the limited tissue distribution of motile cilia. Nonetheless, PCD diagnosis can be challenging due to the overlapping features with other disorders and the requirement for sophisticated tests that are only available in specialized centers. We performed exome sequencing on all patients with a clinical suspicion of PCD but for whom no nasal nitric oxide test or ciliary functional assessment could be ordered. Among 81 patients (56 families), in whom PCD was suspected, 68% had pathogenic or likely pathogenic variants in established PCD-related genes that fully explain the phenotype (20 variants in 11 genes). The major clinical presentations were sinopulmonary infections (SPI) (n = 58), neonatal respiratory distress (NRD) (n = 2), laterality defect (LD) (n = 6), and combined LD/SPI (n = 15). Biallelic likely deleterious variants were also encountered in AKNA and GOLGA3, which we propose as novel candidates in a lung phenotype that overlaps clinically with PCD. We also encountered a PCD phenocopy caused by a pathogenic variant in ITCH, and a pathogenic variant in CEP164 causing Bardet-Biedl syndrome and PCD presentation as a very rare example of the dual presentation of these two disorders of the primary and motile cilia. Exome sequencing is a powerful tool that can help "democratize" the diagnosis of PCD, which is currently limited to highly specialized centers.

Analysis of transcript-deleterious variants in Mendelian disorders: implications for RNA-based diagnostics

Background

At least 50% of patients with suspected Mendelian disorders remain undiagnosed after whole-exome sequencing (WES), and the extent to which non-coding variants that are not captured by WES contribute to this fraction is unclear. Whole transcriptome sequencing is a promising supplement to WES, although empirical data on the contribution of RNA analysis to the diagnosis of Mendelian diseases on a large scale are scarce.

Results

Here, we describe our experience with transcript-deleterious variants (TDVs) based on a cohort of 5647 families with suspected Mendelian diseases. We first interrogate all families for which the respective Mendelian phenotype could be mapped to a single locus to obtain an unbiased estimate of the contribution of TDVs at 18.9%. We examine the entire cohort and find that TDVs account for 15% of all “solved” cases. We compare the results of RT-PCR to in silico prediction. Definitive results from RT-PCR are obtained from blood-derived RNA for the overwhelming majority of variants (84.1%), and only a small minority (2.6%) fail analysis on all available RNA sources (blood-, skin fibroblast-, and urine renal epithelial cells-derived), which has important implications for the clinical application of RNA-seq. We also show that RNA analysis can establish the diagnosis in 13.5% of 155 patients who had received “negative” clinical WES reports. Finally, our data suggest a role for TDVs in modulating penetrance even in otherwise highly penetrant Mendelian disorders.

Conclusions

Our results provide much needed empirical data for the impending implementation of diagnostic RNA-seq in conjunction with genome sequencing.

Biallelic Mutations in Tetratricopeptide Repeat Domain 26 (Intraflagellar Transport 56) Cause Severe Biliary Ciliopathy in Humans

BACKGROUND AND AIMS

The clinical consequences of defective primary cilium (ciliopathies) are characterized by marked phenotypic and genetic heterogeneity. Although fibrocystic liver disease is an established ciliopathy phenotype, severe neonatal cholestasis is rarely recognized as such.

APPROACH AND RESULTS

We describe seven individuals from seven families with syndromic ciliopathy clinical features, including severe neonatal cholestasis (lethal in one and necessitating liver transplant in two). Positional mapping revealed a single critical locus on chromosome 7. Whole-exome sequencing revealed three different homozygous variants in Tetratricopeptide Repeat Domain 26 (TTC26) that fully segregated with the phenotype. TTC26 (intraflagellar transport [IFT] 56/DYF13) is an atypical component of IFT-B complex, and deficiency of its highly conserved orthologs has been consistently shown to cause defective ciliary function in several model organisms. We show that cilia in TTC26-mutated patient cells display variable length and impaired function, as indicated by dysregulated sonic hedgehog signaling, abnormal staining for IFT-B components, and transcriptomic clustering with cells derived from individuals with closely related ciliopathies. We also demonstrate a strong expression of Ttc26 in the embryonic mouse liver in a pattern consistent with its proposed role in the normal development of the intrahepatic biliary system.

CONCLUSIONS

In addition to establishing a TTC26-related ciliopathy phenotype in humans, our results highlight the importance of considering ciliopathies in the differential diagnosis of severe neonatal cholestasis even in the absence of more typical features.

The morbid genome of ciliopathies: an update

PURPOSE

Ciliopathies are highly heterogeneous clinical disorders of the primary cilium. We aim to characterize a large cohort of ciliopathies phenotypically and molecularly.

METHODS

Detailed phenotypic and genomic analysis of patients with ciliopathies, and functional characterization of novel candidate genes.

RESULTS

In this study, we describe 125 families with ciliopathies and show that deleterious variants in previously reported genes, including cryptic splicing variants, account for 87% of cases. Additionally, we further support a number of previously reported candidate genes (BBIP1, MAPKBP1, PDE6D, and WDPCP), and propose nine novel candidate genes (CCDC67, CCDC96, CCDC172, CEP295, FAM166B, LRRC34, TMEM17, TTC6, and TTC23), three of which (LRRC34, TTC6, and TTC23) are supported by functional assays that we performed on available patient-derived fibroblasts. From a phenotypic perspective, we expand the phenomenon of allelism that characterizes ciliopathies by describing novel associations including WDR19-related Stargardt disease and SCLT1- and CEP164-related Bardet-Biedl syndrome.

CONCLUSION

In this cohort of phenotypically and molecularly characterized ciliopathies, we draw important lessons that inform the clinical management and the diagnostics of this class of disorders as well as their basic biology.

Bi-allelic Variants in RALGAPA1 Cause Profound Neurodevelopmental Disability, Muscular Hypotonia, Infantile Spasms, and Feeding Abnormalities

Ral (Ras-like) GTPases play an important role in the control of cell migration and have been implicated in Ras-mediated tumorigenicity. Recently, variants in RALA were also described as a cause of intellectual disability and developmental delay, indicating the relevance of this pathway to neuropediatric diseases. Here, we report the identification of bi-allelic variants in RALGAPA1 (encoding Ral GTPase activating protein catalytic alpha subunit 1) in four unrelated individuals with profound neurodevelopmental disability, muscular hypotonia, feeding abnormalities, recurrent fever episodes, and infantile spasms . Dysplasia of corpus callosum with focal thinning of the posterior part and characteristic facial features appeared to be unifying findings. RalGAPA1 was absent in the fibroblasts derived from two affected individuals suggesting a loss-of-function effect of the RALGAPA1 variants. Consequently, RalA activity was increased in these cell lines, which is in keeping with the idea that RalGAPA1 deficiency causes a constitutive activation of RalA. Additionally, levels of RalGAPB, a scaffolding subunit of the RalGAP complex, were dramatically reduced, indicating a dysfunctional RalGAP complex. Moreover, RalGAPA1 deficiency clearly increased cell-surface levels of lipid raft components in detached fibroblasts, which might indicate that anchorage-dependence of cell growth signaling is disturbed. Our findings indicate that the dysregulation of the RalA pathway has an important impact on neuronal function and brain development. In light of the partially overlapping phenotype between RALA- and RALGAPA1-associated diseases, it appears likely that dysregulation of the RalA signaling pathway leads to a distinct group of genetic syndromes that we suggest could be named RALopathies.

Author Correction: A human ciliopathy reveals essential functions for NEK10 in airway mucociliary clearance

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Confirming the recessive inheritance of PERP-related erythrokeratoderma

Erythrokeratoderma (EK) is heterogeneous clinical entity characterized by excessive scaling with resulting erythrokeratotic plaques. Several genes have been linked to EK and they encode a number of proteins that are important for the integrity of the keratinocyte layer of the epidermis. PERP is a transcription factor that is activated by both p53 and p63. However, its deficiency in a mouse model appears to only recapitulate p63-mediated role in skin development and organization. We report an extended multiplex consanguineous family in which an EK phenotype with a striking similarity to that observed in Perp^-/- mice, is mapped to an autozygous region on chromosome 6 that spans PERP. Whole-exome sequencing revealed a novel variant in PERP that fully segregated with the phenotype. Functional analysis of patient- and control-derived keratinocytes revealed a deleterious effect of the identified variant on the intracellular localization of PERP. A previous report showed that PERP mutation causes a dominant form of keratoderma but a single patient in that report with a homozygous variant in PERP suggests that recessive inheritance is also possible. Our results, therefore, support the establishment of an autosomal recessive PERP-related EK phenotype in humans.

Multiple Family Members With Delayed Cord Separtion and Combined Immunodeficiency With Novel Mutation in IKBKB

Background: Inhibitor of kappa kinase 2 (IKK2) deficiency is a recently described combined immunodeficiency. It undermines the nuclear factor-kappa B (NF-κB) activation pathway. Methods: The clinical and immunological data of four patients diagnosed with combined immunodeficiency (CID) from two related Saudi families were collected. Autozygosity mapping of all available members and whole exome sequencing of the index case were performed to define the genetic etiology. Results: The patients had early onset (2-4 months of age) severe infections caused by viruses, bacteria, mycobacteria, and fungi. They all had hypogammaglobulinemia and low absolute lymphocyte count. Their lymphocytes failed to respond to PHA mitogen stimulation. A novel homozygous non-sense mutation in the IKBKB gene, c.850C>T (p. Arg284*) was identified in the index patient and segregated with the disease in the rest of the family. He underwent hematopoietic stem cell transplantation (HSCT) from a fully matched sibling with no conditioning. The other three patients succumbed to their disease. Interestingly, all patients had delayed umbilical cord separation. Conclusion: IKK2 deficiency causes CID with high mortality. Immune reconstitution with HSCT should be considered as early as possible. Delayed umbilical cord separation in CID patients may be a clue to IKK2 deficiency.

MDH1 deficiency is a metabolic disorder of the malate-aspartate shuttle associated with early onset severe encephalopathy

The reversible oxidation of L-malate to oxaloacetate is catalyzed by NAD(H)-dependent malate dehydrogenase (MDH). MDH plays essential roles in the malate-aspartate shuttle and the tricarboxylic acid cycle. These metabolic processes are important in mitochondrial NADH supply for oxidative phosphorylation. Recently, bi-allelic mutations in mitochondrial MDH2 were identified in patients with global developmental delay, epilepsy and lactic acidosis. We now report two patients from an extended consanguineous family with a deleterious variant in the cytosolic isoenzyme of MDH (MDH1). The homozygous missense variant in the NAD⁺-binding domain of MDH1 led to severely diminished MDH protein expression. The patients presented with global developmental delay, epilepsy and progressive microcephaly. Both patients had normal concentrations of plasma amino acids, acylcarnitines, lactate, and urine organic acids. To identify the metabolic consequences of MDH1 deficiency, untargeted metabolomics was performed on dried blood spots (DBS) from the patients and in MDH1 knockout HEK293 cells that were generated by Crispr/Cas9. Increased levels of glutamate and glycerol-3-phosphate were found in DBS of both patients. In MDH1 KO HEK293 cells, increased levels of glycerol-3-phosphate were also observed, as well as increased levels of aspartate and decreased levels of fumarate. The consistent finding of increased concentrations of glycerol-3-phosphate may represent a compensatory mechanism to enhance cytosolic oxidation of NADH by the glycerol-P-shuttle. In conclusion, MDH1 deficiency is a new metabolic defect in the malate-aspartate shuttle characterized by a severe neurodevelopmental phenotype with elevated concentrations of glycerol-3-phosphate as a potential biomarker.

Immunodeficiency and EBV-induced lymphoproliferation caused by 4-1BB deficiency

BACKGROUND

The tumor TNF receptor family member 4-1BB (CD137) is encoded by TNFRSF9 and expressed on activated T cells. 4-1BB provides a costimulatory signal that enhances CD8⁺ T-cell survival, cytotoxicity, and mitochondrial activity, thereby promoting immunity against viruses and tumors. The ligand for 4-1BB is expressed on antigen-presenting cells and EBV-transformed B cells.

OBJECTIVE

We investigated the genetic basis of recurrent sinopulmonary infections, persistent EBV viremia, and EBV-induced lymphoproliferation in 2 unrelated patients.

METHODS

Whole-exome sequencing, immunoblotting, immunophenotyping, and in vitro assays of lymphocyte and mitochondrial function were performed.

RESULTS

The 2 patients shared a homozygous G109S missense mutation in 4-1BB that abolished protein expression and ligand binding. The patients' CD8⁺ T cells had reduced proliferation, impaired expression of IFN-γ and perforin, and diminished cytotoxicity against allogeneic and HLA-matched EBV-B cells. Mitochondrial biogenesis, membrane potential, and function were significantly reduced in the patients' activated T cells. An inhibitory antibody against 4-1BB recapitulated the patients' defective CD8⁺ T-cell activation and cytotoxicity against EBV-infected B cells in vitro.

CONCLUSION

This novel immunodeficiency demonstrates the critical role of 4-1BB costimulation in host immunity against EBV infection.

Immunologic reconstitution following hematopoietic stem cell transplantation despite lymph node paucity in NF-κB-inducing kinase deficiency

This case demonstrates successful immune reconstitution following hematopoietic stem cell transplantation in NIK deficiency.

NUP214 deficiency causes severe encephalopathy and microcephaly in humans

Nuclear pore complex (NPC) is a fundamental component of the nuclear envelope and is key to the nucleocytoplasmic transport. Mutations in several NUP genes that encode individual components of NPC known as nucleoporins have been identified in recent years among patients with static encephalopathies characterized by developmental delay and microcephaly. We describe a multiplex consanguineous family in which four affected members presented with severe neonatal hypotonia, profound global developmental delay, progressive microcephaly and early death. Autozygome and linkage analysis revealed that this phenotype is linked to a founder disease haplotype (chr9:127,113,732-135,288,807) in which whole exome sequencing revealed the presence of a novel homozygous missense variant in NUP214. Functional analysis of patient-derived fibroblasts recapitulated the dysmorphic phenotype of nuclei that was previously described in NUP214 knockdown cells. In addition, the typical rim staining of NUP214 is largely displaced, further supporting the deleterious effect of the variant. Our data expand the list of NUP genes that are mutated in encephalopathy disorders in humans.

The many faces of peroxisomal disorders: Lessons from a large Arab cohort

Defects in the peroxisomes biogenesis and/or function result in peroxisomal disorders. In this study, we describe the largest Arab cohort to date (72 families) of clinically, biochemically and molecularly characterized patients with peroxisomal disorders. At the molecular level, we identified 43 disease-causing variants, half of which are novel. The founder nature of many of the variants allowed us to calculate the minimum disease burden for these disorders in our population ~1:30 000, which is much higher than previous estimates in other populations. Clinically, we found an interesting trend toward genotype/phenotype correlation in terms of long-term survival. Nearly half (40/75) of our peroxisomal disorders patients had documented survival beyond 1 year of age. Most unusual among the long-term survivors was a multiplex family in which the affected members presented as adults with non-specific intellectual disability and epilepsy. Other unusual presentations included the very recently described peroxisomal fatty acyl-CoA reductase 1 disorder as well as CRD, spastic paraparesis, white matter (CRSPW) syndrome. We conclude that peroxisomal disorders are highly heterogeneous in their clinical presentation. Our data also confirm the demonstration that milder forms of Zellweger spectrum disorders cannot be ruled out by the "gold standard" very long chain fatty acids assay, which highlights the value of a genomics-first approach in these cases.

Autozygome and high throughput confirmation of disease genes candidacy

Purpose

Establishing links between Mendelian phenotypes and genes enables the proper interpretation of variants therein. Autozygome, a rich source of homozygous variants, has been successfully utilized for the high throughput identification of novel autosomal recessive disease genes. Here, we highlight the utility of the autozygome for the high throughput confirmation of previously published tentative links to diseases.

Methods

Autozygome and exome analysis of patients with suspected Mendelian phenotypes. All variants were classified according to the American College of Medical Genetics and Genomics guidelines.

Results

We highlight 30 published candidate genes (ACTL6B, ADAM22, AGTPBP1, APC, C12orf4, C3orf17 (NEPRO), CENPF, CNPY3, COL27A1, DMBX1, FUT8, GOLGA2, KIAA0556, LENG8, MCIDAS, MTMR9, MYH11, QRSL1, RUBCN, SLC25A42, SLC9A1, TBXT, TFG, THUMPD1, TRAF3IP2, UFC1, UFM1, WDR81, XRCC2, ZAK) in which we identified homozygous likely deleterious variants in patients with compatible phenotypes. We also identified homozygous likely deleterious variants in 18 published candidate genes (ABCA2, ARL6IP1, ATP8A2, CDK9, CNKSR1, DGAT1, DMXL2, GEMIN4, HCN2, HCRT, MYO9A, PARS2, PLOD3, PREPL, SCLT1, STX3, TXNRD2, WIPI2) although the associated phenotypes are sufficiently different from the original reports that they represent phenotypic expansion or potentially distinct allelic disorders.

Conclusions

Our results should facilitate the timely relabeling of these candidate disease genes in relevant databases to improve the yield of clinical genomic sequencing.

Expanding the phenome and variome of skeletal dysplasia

PURPOSE

To describe our experience with a large cohort (411 patients from 288 families) of various forms of skeletal dysplasia who were molecularly characterized.

METHODS

Detailed phenotyping and next-generation sequencing (panel and exome).

RESULTS

Our analysis revealed 224 pathogenic/likely pathogenic variants (54 (24%) of which are novel) in 123 genes with established or tentative links to skeletal dysplasia. In addition, we propose 5 genes as candidate disease genes with suggestive biological links (WNT3A, SUCO, RIN1, DIP2C, and PAN2). Phenotypically, we note that our cohort spans 36 established phenotypic categories by the International Skeletal Dysplasia Nosology, as well as 18 novel skeletal dysplasia phenotypes that could not be classified under these categories, e.g., the novel C3orf17-related skeletal dysplasia. We also describe novel phenotypic aspects of well-known disease genes, e.g., PGAP3-related Toriello-Carey syndrome-like phenotype. We note a strong founder effect for many genes in our cohort, which allowed us to calculate a minimum disease burden for the autosomal recessive forms of skeletal dysplasia in our population (7.16E-04), which is much higher than the global average.

CONCLUSION

By expanding the phenotypic, allelic, and locus heterogeneity of skeletal dysplasia in humans, we hope our study will improve the diagnostic rate of patients with these conditions.

Expanding the phenotype of SLC25A42-associated mitochondrial encephalomyopathy

SLC25A42 gene encodes an inner mitochondrial membrane protein that imports Coenzyme A into the mitochondrial matrix. A mutation in this gene was recently reported in a subject born to consanguineous parents who presented with mitochondrial myopathy with muscle weakness and lactic acidosis. In this report, we present 12 additional individuals with the same founder mutation who presented with variable manifestations ranging from asymptomatic lactic acidosis to a severe phenotype characterized by developmental regression and epilepsy. Our report confirms the link between SLC25A42 and mitochondrial disease in humans, and suggests that pathogenic variants in SLC25A42 should be interpreted with the understanding that the associated phenotype may be highly variable.

KIAA1109 Variants Are Associated with a Severe Disorder of Brain Development and Arthrogryposis

Whole-exome and targeted sequencing of 13 individuals from 10 unrelated families with overlapping clinical manifestations identified loss-of-function and missense variants in KIAA1109 allowing delineation of an autosomal-recessive multi-system syndrome, which we suggest to name Alkuraya-Kučinskas syndrome (MIM 617822). Shared phenotypic features representing the cardinal characteristics of this syndrome combine brain atrophy with clubfoot and arthrogryposis. Affected individuals present with cerebral parenchymal underdevelopment, ranging from major cerebral parenchymal thinning with lissencephalic aspect to moderate parenchymal rarefaction, severe to mild ventriculomegaly, cerebellar hypoplasia with brainstem dysgenesis, and cardiac and ophthalmologic anomalies, such as microphthalmia and cataract. Severe loss-of-function cases were incompatible with life, whereas those individuals with milder missense variants presented with severe global developmental delay, syndactyly of 2^nd and 3^rd toes, and severe muscle hypotonia resulting in incapacity to stand without support. Consistent with a causative role for KIAA1109 loss-of-function/hypomorphic variants in this syndrome, knockdowns of the zebrafish orthologous gene resulted in embryos with hydrocephaly and abnormally curved notochords and overall body shape, whereas published knockouts of the fruit fly and mouse orthologous genes resulted in lethality or severe neurological defects reminiscent of the probands’ features.

Correction to: Expanding the genetic heterogeneity of intellectual disability

Variant nomenclature discrepancy was identified in the article.

A null mutation in MICU2 causes abnormal mitochondrial calcium homeostasis and a severe neurodevelopmental disorder

Mitochondrial calcium homeostasis is a tightly controlled process that is required for a variety of cellular functions. The mitochondrial calcium uniporter complex plays a critical role in this process. MICU2 is a major component of the mitochondrial calcium uniporter complex and its deficiency has been shown to impair mitochondrial calcium [Ca2+]m homeostasis although the exact mechanism remains unclear. We used exome sequencing, positional mapping, and functional characterization of MICU2 deficiency to investigate the role of MICU2 in calcium homeostasis. Using combined autozygome/exome analysis, a homozygous truncating mutation in MICU2 was found to fully segregate with a neurodevelopmental disorder in the form of severe cognitive impairment, spasticity, and white matter involvement in a multiplex consanguineous family. Patient-derived MICU2-deficient cells displayed impaired [Ca2+]m homeostasis, with associated increase in mitochondrial sensitivity to oxidative stress, and abnormal regulation of inner mitochondrial membrane potential. This is the first demonstration of MICU2 deficiency in humans, which we suggest causes a distinct neurodevelopmental phenotype secondary to impaired mitochondrial calcium uniporter-mediated regulation of intracellular calcium homeostasis.

Expanding the genetic heterogeneity of intellectual disability

Intellectual disability (ID) is a common morbid condition with a wide range of etiologies. The list of monogenic forms of ID has increased rapidly in recent years thanks to the implementation of genomic sequencing techniques. In this study, we describe the phenotypic and genetic findings of 68 families (105 patients) all with novel ID-related variants. In addition to established ID genes, including ones for which we describe unusual mutational mechanism, some of these variants represent the first confirmatory disease-gene links following previous reports (TRAK1, GTF3C3, SPTBN4 and NKX6-2), some of which were based on single families. Furthermore, we describe novel variants in 14 genes that we propose as novel candidates (ANKHD1, ASTN2, ATP13A1, FMO4, MADD, MFSD11, NCKAP1, NFASC, PCDHGA10, PPP1R21, SLC12A2, SLK, STK32C and ZFAT). We highlight MADD and PCDHGA10 as particularly compelling candidates in which we identified biallelic likely deleterious variants in two independent ID families each. We also highlight NCKAP1 as another compelling candidate in a large family with autosomal dominant mild intellectual disability that fully segregates with a heterozygous truncating variant. The candidacy of NCKAP1 is further supported by its biological function, and our demonstration of relevant expression in human brain. Our study expands the locus and allelic heterogeneity of ID and demonstrates the power of positional mapping to reveal unusual mutational mechanisms.

WDR45B-related intellectual disability, spastic quadriplegia, epilepsy, and cerebral hypoplasia: A consistent neurodevelopmental syndrome

The advancement in genomic sequencing has greatly improved the diagnostic yield for neurodevelopmental disorders and led to the discovery of large number of novel genes associated with these disorders. WDR45B has been identified as a potential intellectual disability gene through genomic sequencing of 2 large cohorts of affected individuals. In this report we present 6 individuals from 3 unrelated families with homozygous pathogenic variants in WDR45B: c.799C>T (p.Q267*) in 1 family and c.673C>T (p.R225*) in 2 families. These individuals shared a similar phenotype including profound development delay, early-onset refractory epilepsy, progressive spastic quadriplegia and contractures, and brain malformations. Neuroimaging showed ventriculomegaly, reduced cerebral white matter volume, and thinning of cerebral gray matter. The consistency in the phenotype strongly supports that WDR45B is associated with this disease.

Molecular autopsy in maternal-fetal medicine

PurposeThe application of genomic sequencing to investigate unexplained death during early human development, a form of lethality likely enriched for severe Mendelian disorders, has been limited.MethodsIn this study, we employed exome sequencing as a molecular autopsy tool in a cohort of 44 families with at least one death or lethal fetal malformation at any stage of in utero development. Where no DNA was available from the fetus, we performed molecular autopsy by proxy, i.e., through parental testing.ResultsPathogenic or likely pathogenic variants were identified in 22 families (50%), and variants of unknown significance were identified in further 15 families (34%). These variants were in genes known to cause embryonic or perinatal lethality (ALPL, GUSB, SLC17A5, MRPS16, THSD1, PIEZO1, and CTSA), genes known to cause Mendelian phenotypes that do not typically include embryonic lethality (INVS, FKTN, MYBPC3, COL11A2, KRIT1, ASCC1, NEB, LZTR1, TTC21B, AGT, KLHL41, GFPT1, and WDR81) and genes with no established links to human disease that we propose as novel candidates supported by embryonic lethality of their orthologs or other lines of evidence (MS4A7, SERPINA11, FCRL4, MYBPHL, PRPF19, VPS13D, KIAA1109, MOCS3, SVOPL, FEN1, HSPB11, KIF19, and EXOC3L2).ConclusionOur results suggest that molecular autopsy in pregnancy losses is a practical and high-yield alternative to traditional autopsy, and an opportunity for bringing precision medicine to the clinical practice of perinatology.

GZF1 Mutations Expand the Genetic Heterogeneity of Larsen Syndrome

Larsen syndrome is characterized by the dislocation of large joints and other less consistent clinical findings. Heterozygous FLNB mutations account for the majority of Larsen syndrome cases, but biallelic mutations in CHST3 and B4GALT7 have been more recently described, thus confirming the existence of recessive forms of the disease. In a multiplex consanguineous Saudi family affected by severe and recurrent large joint dislocation and severe myopia, we identified a homozygous truncating variant in GZF1 through a combined autozygome and exome approach. Independently, the same approach identified a second homozygous truncating GZF1 variant in another multiplex consanguineous family affected by severe myopia, retinal detachment, and milder skeletal involvement. GZF1 encodes GDNF-inducible zinc finger protein 1, a transcription factor of unknown developmental function, which we found to be expressed in the eyes and limbs of developing mice. Global transcriptional profiling of cells from affected individuals revealed a shared pattern of gene dysregulation and significant enrichment of genes encoding matrix proteins, including P3H2, which hints at a potential disease mechanism. Our results suggest that GZF1 mutations cause a phenotype of severe myopia and significant articular involvement not previously described in Larsen syndrome.

Clinical genomics expands the morbid genome of intellectual disability and offers a high diagnostic yield

Intellectual disability (ID) is a measurable phenotypic consequence of genetic and environmental factors. In this study, we prospectively assessed the diagnostic yield of genomic tools (molecular karyotyping, multi-gene panel and exome sequencing) in a cohort of 337 ID subjects as a first-tier test and compared it with a standard clinical evaluation performed in parallel. Standard clinical evaluation suggested a diagnosis in 16% of cases (54/337) but only 70% of these (38/54) were subsequently confirmed. On the other hand, the genomic approach revealed a likely diagnosis in 58% (n=196). These included copy number variants in 14% (n=54, 15% are novel), and point mutations revealed by multi-gene panel and exome sequencing in the remaining 43% (1% were found to have Fragile-X). The identified point mutations were mostly recessive (n=117, 81%), consistent with the high consanguinity of the study cohort, but also X-linked (n=8, 6%) and de novo dominant (n=19, 13%). When applied directly on all cases with negative molecular karyotyping, the diagnostic yield of exome sequencing was 60% (77/129). Exome sequencing also identified likely pathogenic variants in three novel candidate genes (DENND5A, NEMF and DNHD1) each of which harbored independent homozygous mutations in patients with overlapping phenotypes. In addition, exome sequencing revealed de novo and recessive variants in 32 genes (MAMDC2, TUBAL3, CPNE6, KLHL24, USP2, PIP5K1A, UBE4A, TP53TG5, ATOH1, C16ORF90, SLC39A14, TRERF1, RGL1, CDH11, SYDE2, HIRA, FEZF2, PROCA1, PIANP, PLK2, QRFPR, AP3B2, NUDT2, UFC1, BTN3A2, TADA1, ARFGEF3, FAM160B1, ZMYM5, SLC45A1, ARHGAP33 and CAPS2), which we highlight as potential candidates on the basis of several lines of evidence, and one of these genes (SLC39A14) was biallelically inactivated in a potentially treatable form of hypermanganesemia and neurodegeneration. Finally, likely causal variants in previously published candidate genes were identified (ASTN1, HELZ, THOC6, WDR45B, ADRA2B and CLIP1), thus supporting their involvement in ID pathogenesis. Our results expand the morbid genome of ID and support the adoption of genomics as a first-tier test for individuals with ID.

GLI3-related polydactyly: a review

GLI3 mutations are known to be associated with nine syndromes/conditions in which polydactyly is a feature. In this review, the embryology, pathogenesis, and animal models of GLI3-related polydactyly are discussed first. This is followed by a detailed review of the genotype-phenotype correlations. Based on our review of the literature and our clinical experiences, we recommend viewing GLI3-related syndromes/conditions as four separate entities; each characterized by a specific pattern of polydactyly. These four entities are: the preaxial polydactyly type IV-Greig-acrocallosal spectrum, postaxial polydactyly types A/B, Pallister-Hall syndrome (PHS), and oral-facial-digital overlap syndrome. We also provide illustrative clinical examples from our practice including a family with a novel GLI3 mutation causing PHS. The review also introduces the term 'Forme Fruste' preaxial polydactyly and gives several conclusions/recommendations including the recommendation to revise the current criteria for the clinical diagnosis of PHS.

A lethal neonatal phenotype of mitochondrial short-chain enoyl-CoA hydratase-1 deficiency

Short-chain enoyl-CoA hydratase (SCEH) is a mitochondrial enzyme involved in the oxidation of fatty acids and the catabolic pathway of valine and, to a lesser extent, isoleucine. Deficiency of this enzyme was recently shown to cause an early childhood Leigh syndrome phenotype. The few reported patients were compound heterozygotes for two missense or missense with truncating variants in ECHS1 that encodes SCEH. We describe two siblings with severe refractory lactic acidosis and death within the first 2 days of life. Following negative clinical whole-exome and whole-genome sequencing, we resorted to autozygome/exome analysis on research basis and identified a homozygous splice site mutation (c.88+5G>A) in the two cases. Analysis of cDNA confirmed complete replacement of the normal transcript with an aberrant transcript (r.88_89ins 88+1_88+11) predicting premature truncation of the protein [p.(Ala31Glufs*23)]. Furthermore, quantitative reverse transcriptase polymerase chain reaction (RTPCR) showed marked reduction in ECHS1, most likely nonsense-mediated decay (NMD)-mediated. This is the first report of homozygosity for a truncating mutation in ECHS1, which may explain the severe phenotype. Our report highlights the need to consider SCEH deficiency in patients with lethal neonatal lactic acidosis, and the potentially limited sensitivity of untargeted genomic sequencing towards non-canonical splicing mutations, which may explain at least some of the 'negative' cases on clinical exome/genome sequencing.

KDF1, encoding keratinocyte differentiation factor 1, is mutated in a multigenerational family with ectodermal dysplasia

Ectodermal dysplasia is a highly heterogeneous group of disorders that variably affect the derivatives of the ectoderm, primarily skin, hair, nails and teeth. TP63, itself mutated in ectodermal dysplasia, links many other ectodermal dysplasia disease genes through a regulatory network that maintains the balance between proliferation and differentiation of the epidermis and other ectodermal derivatives. The ectodermal knockout phenotype of five mouse genes that regulate and/or are regulated by TP63 (Irf6, Ikkα, Ripk4, Stratifin, and Kdf1) is strikingly similar and involves abnormal balance towards proliferation at the expense of differentiation, but only the first three have corresponding ectodermal phenotypes in humans. We describe a multigenerational Saudi family with an autosomal dominant form of hypohidrotic ectodermal dysplasia in which positional mapping and exome sequencing identified a novel variant in KDF1 that fully segregates with the phenotype. The recapitulation of the phenotype we observe in this family by the Kdf1-/- mouse suggests a causal role played by the KDF1 variant.

Increasing the sensitivity of clinical exome sequencing through improved filtration strategy

BACKGROUND

Clinical exome sequencing (CES) has greatly improved the diagnostic process for individuals with suspected genetic disorders. However, the majority remains undiagnosed after CES. Although understanding potential reasons for this limited sensitivity is critical for improving the delivery of clinical genomics, research in this area has been limited.

MATERIALS AND METHODS

We first calculated the theoretical maximum sensitivity of CES by analyzing >100 families in whom a Mendelian phenotype is mapped to a single locus. We then tested the hypothesis that positional mapping can limit the search space and thereby facilitate variant interpretation by reanalyzing 33 families with "negative" CES and applying positional mapping.

RESULTS

We found that >95% of families who map to a single locus harbored genic (as opposed to intergenic) variants that are potentially identifiable by CES. Our reanalysis of "negative" CES revealed likely causal variants in the majority (88%). Several of these solved cases have undergone negative whole-genome sequencing.

CONCLUSION

The discrepancy between the theoretical maximum and the actual clinical sensitivity of CES is primarily in the variant filtration rather than the variant capture and sequencing phase. The solution to negative CES is not necessarily in expanding the coverage but rather in devising approaches that improve variant filtration. We suggest that positional mapping is one such approach.Genet Med advance online publication 06 October 2016.

GNB5 mutation causes a novel neuropsychiatric disorder featuring attention deficit hyperactivity disorder, severely impaired language development and normal cognition

Background

Neuropsychiatric disorders are common forms of disability in humans. Despite recent progress in deciphering the genetics of these disorders, their phenotypic complexity continues to be a major challenge. Mendelian neuropsychiatric disorders are rare but their study has the potential to unravel novel mechanisms that are relevant to their complex counterparts.

Results

In an extended consanguineous family, we identified a novel neuropsychiatric phenotype characterized by severe speech impairment, variable expressivity of attention deficit hyperactivity disorder (ADHD), and motor delay. We identified the disease locus through linkage analysis on 15q21.2, and exome sequencing revealed a novel missense variant in GNB5. GNB5 encodes an atypical β subunit of the heterotrimeric GTP-binding proteins (Gβ5). Gβ5 is enriched in the central nervous system where it forms constitutive complexes with members of the regulator of G protein signaling family of proteins to modulate neurotransmitter signaling that affects a number of neurobehavioral outcomes. Here, we show that the S81L mutant form of Gβ5 has significantly impaired activity in terminating responses that are elicited by dopamine.

Conclusions

We demonstrate that these deficits originate from the impaired expression of the mutant Gβ5 protein, resulting in the decreased ability to stabilize regulator of G protein signaling complexes. Our data suggest that this novel neuropsychiatric phenotype is the human equivalent of Gnb5 deficiency in mice, which manifest motor deficits and hyperactivity, and highlight a critical role of Gβ5 in normal behavior as well as language and motor development in humans.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-1061-6) contains supplementary material, which is available to authorized users.

A lethal phenotype associated with tissue plasminogen deficiency in humans

The role of plasminogen in preventing thrombosis requires activation by tissue plasminogen activator (t-PA) encoded by PLAT. While case-control associations have been pursued for common variants in PLAT, no disease-causing mutations have been reported. We describe a consanguineous family with two children who died shortly after birth due to complications related to severe hydranencephaly and diaphragmatic hernia. A combined exome/autozygome analysis was carried out with informed consent. We identified a homozygous null mutation in PLAT that abrogated t-PA level in patient cells. This is the first reported human knockout mutation of PLAT. The apparent association with hydranencephaly, diaphragmatic hernia and postnatal lethality requires further validation.

A null mutation in TNIK defines a novel locus for intellectual disability

Intellectual disability (ID) is one of the most common disabilities and, although many genes have been implicated in its etiology, the genetic heterogeneity of ID continues to expand. The purpose of the study was to describe a novel autosomal recessive non-syndromic ID locus. Autozygome and linkage analysis, and exome sequencing followed by RNA and protein analysis of the candidate disease gene were performed. We describe two multiplex consanguineous families with non-syndromic ID phenotype, which maps to a critical linkage locus on 3q26. Exome sequencing of the index in each family revealed the same homozygous truncating mutation in TNIK that results in complete loss of the protein. TNIK is a kinase with a well-established role in dendrite development and synaptic transmission. The phenotype we observe in human patients who lack TNIK is consistent with the previously published Tnik (-/-) phenotype in the murine model. Our data strongly implicate TNIK deficiency in the causation of ID in humans.

Novel copy number variants and major limb reduction malformation: Report of three cases

Limb reduction malformations are highly heterogeneous in their clinical presentation and so, predicting the underlying mutation on a clinical basis can be challenging. Molecular karyotyping is a powerful genomic tool that has quickly become the mainstay for the study of children with malformation syndromes. We describe three patients with major limb reduction anomalies in whom pathogenic copy number variants were identified on molecular karyotyping. These include a patient with hypoplastic phalanges and absent hallux bilaterally with de novo deletion of 11.9 Mb on 7p21.1-22.1 spanning 63 genes including RAC1, another patient with severe Holt-Oram syndrome and a large de novo deletion 2.2 Mb on 12q24.13-24.21 spanning 20 genes including TBX3 and TBX5, and a third patient with acheiropodia who had a nullizygous deletion of 102 kb on 7q36.3 spanning LMBR1. We discuss the potential of these novel genomic rearrangements to improve our understanding of limb development in humans.

GOLGA2, encoding a master regulator of golgi apparatus, is mutated in a patient with a neuromuscular disorder

Golgi apparatus (GA) is a membrane-bound organelle that serves a multitude of critical cellular functions including protein secretion and sorting, and cellular polarity. Many Mendelian diseases are caused by mutations in genes encoding various components of GA. GOLGA2 encodes GM130, a necessary component for the assembly of GA as a single complex, and its deficiency has been found to result in severe cellular phenotypes. We describe the first human patient with a homozygous apparently loss of function mutation in GOLGA2. The phenotype is a neuromuscular disorder characterized by developmental delay, seizures, progressive microcephaly, and muscular dystrophy. Knockdown of golga2 in zebrafish resulted in severe skeletal muscle disorganization and microcephaly recapitulating loss of function human phenotype. Our data suggest an important developmental role of GM130 in humans and zebrafish.

Mutation of the mitochondrial carrier SLC25A42 causes a novel form of mitochondrial myopathy in humans

Myopathies are heterogeneous disorders characterized clinically by weakness and hypotonia, usually in the absence of gross dystrophic changes. Mitochondrial dysfunction is a frequent cause of myopathy. We report a simplex case born to consanguineous parents who presented with muscle weakness, lactic acidosis, and muscle changes suggestive of mitochondrial dysfunction. Combined autozygome and exome analysis revealed a missense variant in the SLC25A42 gene, which encodes an inner mitochondrial membrane protein that imports coenzyme A into the mitochondrial matrix. Zebrafish slc25a42 knockdown morphants display severe muscle disorganization and weakness. Importantly, these features are rescued by normal human SLC25A42 RNA, but not by RNA harboring the patient's variant. Our data support a potentially causal link between SLC25A42 mutation and mitochondrial myopathy in humans.

Identification of embryonic lethal genes in humans by autozygosity mapping and exome sequencing in consanguineous families

Background

Identifying genetic variants that lead to discernible phenotypes is the core of Mendelian genetics. An approach that considers embryonic lethality as a bona fide Mendelian phenotype has the potential to reveal novel genetic causes, which will further our understanding of early human development at a molecular level. Consanguineous families in which embryonic lethality segregates as a recessive Mendelian phenotype offer a unique opportunity for high throughput novel gene discovery as has been established for other recessive postnatal phenotypes.

Results

We have studied 24 eligible families using autozygosity mapping and whole-exome sequencing. In addition to revealing mutations in genes previously linked to embryonic lethality in severe cases, our approach revealed seven novel candidate genes (THSD1, PIGC, UBN1, MYOM1, DNAH14, GALNT14, and FZD6). A founder mutation in one of these genes, THSD1, which has been linked to vascular permeability, accounted for embryonic lethality in three of the study families. Unlike the other six candidate genes, we were able to identify a second mutation in THSD1 in a family with a less severe phenotype consisting of hydrops fetalis and persistent postnatal edema, which provides further support for the proposed link between this gene and embryonic lethality.

Conclusions

Our study represents an important step towards the systematic analysis of “embryonic lethal genes” in humans.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0681-6) contains supplementary material, which is available to authorized users.