Genetic convergence of developmental and epileptic encephalopathies and intellectual disability.

AIM: To determine whether genes that cause developmental and epileptic encephalopathies (DEEs) are more commonly implicated in intellectual disability with epilepsy as a comorbid feature than in intellectual disability only. METHOD: We performed targeted resequencing of 18 genes commonly implicated in DEEs in a cohort of 830 patients with intellectual disability (59% male) and 393 patients with DEEs (52% male). RESULTS: We observed a significant enrichment of pathogenic/likely pathogenic variants in patients with epilepsy and intellectual disability (16 out of 159 in seven genes) compared with intellectual disability only (2 out of 671) (p<1.86×10-10 , odds ratio 37.22, 95% confidence interval 8.60-337.0). INTERPRETATION: We identified seven genes that are more likely to cause epilepsy and intellectual disability than intellectual disability only. Conversely, two genes, GRIN2B and SCN2A, can be implicated in intellectual disability without epilepsy; in these instances intellectual disability is not a secondary consequence of ongoing seizures but rather a primary cause. What this paper adds A subset of genes are more commonly implicated in epilepsy than other neurodevelopmental disorders. GRIN2B and SCN2A are implicated in intellectual disability and epilepsy independently.

De Novo Truncating Mutations in the Last and Penultimate Exons of PPM1D Cause an Intellectual Disability Syndrome.

Intellectual disability (ID) is a highly heterogeneous disorder involving at least 600 genes, yet a genetic diagnosis remains elusive in ∼35%-40% of individuals with moderate to severe ID. Recent meta-analyses statistically analyzing de novo mutations in >7,000 individuals with neurodevelopmental disorders highlighted mutations in PPM1D as a possible cause of ID. PPM1D is a type 2C phosphatase that functions as a negative regulator of cellular stress-response pathways by mediating a feedback loop of p38-p53 signaling, thereby contributing to growth inhibition and suppression of stress-induced apoptosis. We identified 14 individuals with mild to severe ID and/or developmental delay and de novo truncating PPM1D mutations. Additionally, deep phenotyping revealed overlapping behavioral problems (ASD, ADHD, and anxiety disorders), hypotonia, broad-based gait, facial dysmorphisms, and periods of fever and vomiting. PPM1D is expressed during fetal brain development and in the adult brain. All mutations were located in the last or penultimate exon, suggesting escape from nonsense-mediated mRNA decay. Both PPM1D expression analysis and cDNA sequencing in EBV LCLs of individuals support the presence of a stable truncated transcript, consistent with this hypothesis. Exposure of cells derived from individuals with PPM1D truncating mutations to ionizing radiation resulted in normal p53 activation, suggesting that p53 signaling is unaffected. However, a cell-growth disadvantage was observed, suggesting a possible effect on the stress-response pathway. Thus, we show that de novo truncating PPM1D mutations in the last and penultimate exons cause syndromic ID, which provides additional insight into the role of cell-cycle checkpoint genes in neurodevelopmental disorders.

TRIO loss of function is associated with mild intellectual disability and affects dendritic branching and synapse function.

Recently, we marked TRIO for the first time as a candidate gene for intellectual disability (ID). Across diverse vertebrate species, TRIO is a well-conserved Rho GTPase regulator that is highly expressed in the developing brain. However, little is known about the specific events regulated by TRIO during brain development and its clinical impact in humans when mutated. Routine clinical diagnostic testing identified an intragenic de novo deletion of TRIO in a boy with ID. Targeted sequencing of this gene in over 2300 individuals with ID, identified three additional truncating mutations. All index cases had mild to borderline ID combined with behavioral problems consisting of autistic, hyperactive and/or aggressive behavior. Studies in dissociated rat hippocampal neurons demonstrated the enhancement of dendritic formation by suppressing endogenous TRIO, and similarly decreasing endogenous TRIO in organotypic hippocampal brain slices significantly increased synaptic strength by increasing functional synapses. Together, our findings provide new mechanistic insight into how genetic deficits in TRIO can lead to early neuronal network formation by directly affecting both neurite outgrowth and synapse development.

Validation and application of a novel integrated genetic screening method to a cohort of 1,112 men with idiopathic azoospermia or severe oligozoospermia.

Microdeletions of the Y chromosome (YCMs), Klinefelter syndrome (47,XXY), and CFTR mutations are known genetic causes of severe male infertility, but the majority of cases remain idiopathic. Here, we describe a novel method using single molecule Molecular Inversion Probes (smMIPs), to screen infertile men for mutations and copy number variations affecting known disease genes. We designed a set of 4,525 smMIPs targeting the coding regions of causal (n = 6) and candidate (n = 101) male infertility genes. After extensive validation, we screened 1,112 idiopathic infertile men with non-obstructive azoospermia or severe oligozoospermia. In addition to five chromosome YCMs and six other sex chromosomal anomalies, we identified five patients with rare recessive mutations in CFTR as well as a patient with a rare heterozygous frameshift mutation in SYCP3 that may be of clinical relevance. This results in a genetic diagnosis in 11-17 patients (1%-1.5%), a yield that may increase significantly when more genes are confidently linked to male infertility. In conclusion, we developed a flexible and scalable method to reliably detect genetic causes of male infertility. The assay consolidates the detection of different types of genetic variation while increasing the diagnostic yield and detection precision at the same or lower price compared with currently used methods.

Mutations affecting the SAND domain of DEAF1 cause intellectual disability with severe speech impairment and behavioral problems.

Recently, we identified in two individuals with intellectual disability (ID) different de novo mutations in DEAF1, which encodes a transcription factor with an important role in embryonic development. To ascertain whether these mutations in DEAF1 are causative for the ID phenotype, we performed targeted resequencing of DEAF1 in an additional cohort of over 2,300 individuals with unexplained ID and identified two additional individuals with de novo mutations in this gene. All four individuals had severe ID with severely affected speech development, and three showed severe behavioral problems. DEAF1 is highly expressed in the CNS, especially during early embryonic development. All four mutations were missense mutations affecting the SAND domain of DEAF1. Altered DEAF1 harboring any of the four amino acid changes showed impaired transcriptional regulation of the DEAF1 promoter. Moreover, behavioral studies in mice with a conditional knockout of Deaf1 in the brain showed memory deficits and increased anxiety-like behavior. Our results demonstrate that mutations in DEAF1 cause ID and behavioral problems, most likely as a result of impaired transcriptional regulation by DEAF1.

Cantú syndrome is caused by mutations in ABCC9.

Cantú syndrome is a rare disorder characterized by congenital hypertrichosis, neonatal macrosomia, a distinct osteochondrodysplasia, and cardiomegaly. Using an exome-sequencing approach applied to one proband-parent trio and three unrelated single cases, we identified heterozygous mutations in ABCC9 in all probands. With the inclusion of the remaining cohort of ten individuals with Cantú syndrome, a total of eleven mutations in ABCC9 were found. The de novo occurrence in all six simplex cases in our cohort substantiates the presence of a dominant disease mechanism. All mutations were missense, and several mutations affect Arg1154. This mutation hot spot lies within the second type 1 transmembrane region of this ATP-binding cassette transporter protein, which may suggest an activating mutation. ABCC9 encodes the sulfonylurea receptor (SUR) that forms ATP-sensitive potassium channels (K(ATP) channels) originally shown in cardiac, skeletal, and smooth muscle. Previously, loss-of-function mutations in this gene have been associated with idiopathic dilated cardiomyopathy type 10 (CMD10). These findings identify the genetic basis of Cantú syndrome and suggest that this is a new member of the potassium channelopathies.

Diagnostic exome sequencing in persons with severe intellectual disability.

BACKGROUND: The causes of intellectual disability remain largely unknown because of extensive clinical and genetic heterogeneity. METHODS: We evaluated patients with intellectual disability to exclude known causes of the disorder. We then sequenced the coding regions of more than 21,000 genes obtained from 100 patients with an IQ below 50 and their unaffected parents. A data-analysis procedure was developed to identify and classify de novo, autosomal recessive, and X-linked mutations. In addition, we used high-throughput resequencing to confirm new candidate genes in 765 persons with intellectual disability (a confirmation series). All mutations were evaluated by molecular geneticists and clinicians in the context of the patients' clinical presentation. RESULTS: We identified 79 de novo mutations in 53 of 100 patients. A total of 10 de novo mutations and 3 X-linked (maternally inherited) mutations that had been previously predicted to compromise the function of known intellectual-disability genes were found in 13 patients. Potentially causative de novo mutations in novel candidate genes were detected in 22 patients. Additional de novo mutations in 3 of these candidate genes were identified in patients with similar phenotypes in the confirmation series, providing support for mutations in these genes as the cause of intellectual disability. We detected no causative autosomal recessive inherited mutations in the discovery series. Thus, the total diagnostic yield was 16%, mostly involving de novo mutations. CONCLUSIONS: De novo mutations represent an important cause of intellectual disability; exome sequencing was used as an effective diagnostic strategy for their detection. (Funded by the European Union and others.).

Exome sequencing identifies truncating mutations in human SERPINF1 in autosomal-recessive osteogenesis imperfecta.

Osteogenesis imperfecta (OI) is a heterogeneous genetic disorder characterized by bone fragility and susceptibility to fractures after minimal trauma. After mutations in all known OI genes had been excluded by Sanger sequencing, we applied next-generation sequencing to analyze the exome of a single individual who has a severe form of the disease and whose parents are second cousins. A total of 26,922 variations from the human reference genome sequence were subjected to several filtering steps. In addition, we extracted the genotypes of all dbSNP130-annotated SNPs from the exome sequencing data and used these 299,494 genotypes as markers for the genome-wide identification of homozygous regions. A single homozygous truncating mutation, affecting SERPINF1 on chromosome 17p13.3, that was embedded into a homozygous stretch of 2.99 Mb remained. The mutation was also homozygous in the affected brother of the index patient. Subsequently, we identified homozygosity for two different truncating SERPINF1 mutations in two unrelated patients with OI and parental consanguinity. All four individuals with SERPINF1 mutations have severe OI. Fractures of long bones and severe vertebral compression fractures with resulting deformities were observed as early as the first year of life in these individuals. Collagen analyses with cultured dermal fibroblasts displayed no evidence for impaired collagen folding, posttranslational modification, or secretion. SERPINF1 encodes pigment epithelium-derived factor (PEDF), a secreted glycoprotein of the serpin superfamily. PEDF is a multifunctional protein and one of the strongest inhibitors of angiogenesis currently known in humans. Our data provide genetic evidence for PEDF involvement in human bone homeostasis.

Massively parallel sequencing of ataxia genes after array-based enrichment.

Massively parallel sequencing has tremendous diagnostic potential but requires enriched templates for sequencing. Here we report the validation of an array-based sequence capture method in genetically heterogeneous disorders. The model disorder selected was AR ataxia, using five subjects with known mutations and two unaffected controls. The genomic sequences of seven disease genes, together with two control loci were targeted on a 2-Mb sequence-capture array. After enrichment, the patients' DNA samples were analyzed using one-quarter Roche GS FLX Titanium sequencing run, resulting in an average of 65 Mb of sequence data per patient. This was sufficient for an average 25-fold coverage/base in all targeted regions. Enrichment showed high specificity; on average, 80% of uniquely mapped reads were on target. Importantly, this approach enabled automated detection of deletions and hetero- and homozygous point mutations for 6/7 mutant alleles, and greater than 99% accuracy for known SNP variants. Our results also clearly show reduced coverage for sequences in repeat-rich regions, which significantly impacts the reliable detection of genomic variants. Based on these findings we recommend a minimal coverage of 15-fold for diagnostic implementation of this technology. We conclude that massive parallel sequencing of enriched samples enables personalized diagnosis of heterogeneous genetic disorders and qualifies for rapid diagnostic implementation.

Overrepresentation of genetic variation in the AnkyrinG interactome is related to a range of neurodevelopmental disorders.

Upon the discovery of numerous genes involved in the pathogenesis of neurodevelopmental disorders, several studies showed that a significant proportion of these genes converge on common pathways and protein networks. Here, we used a reversed approach, by screening the AnkyrinG protein-protein interaction network for genetic variation in a large cohort of 1009 cases with neurodevelopmental disorders. We identified a significant enrichment of de novo potentially disease-causing variants in this network, confirming that this protein network plays an important role in the emergence of several neurodevelopmental disorders.

De novo variants in FBXO11 cause a syndromic form of intellectual disability with behavioral problems and dysmorphisms.

Determining pathogenicity of genomic variation identified by next-generation sequencing techniques can be supported by recurrent disruptive variants in the same gene in phenotypically similar individuals. However, interpretation of novel variants in a specific gene in individuals with mild-moderate intellectual disability (ID) without recognizable syndromic features can be challenging and reverse phenotyping is often required. We describe 24 individuals with a de novo disease-causing variant in, or partial deletion of, the F-box only protein 11 gene (FBXO11, also known as VIT1 and PRMT9). FBXO11 is part of the SCF (SKP1-cullin-F-box) complex, a multi-protein E3 ubiquitin-ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation. Twenty-two variants were identified by next-generation sequencing, comprising 2 in-frame deletions, 11 missense variants, 1 canonical splice site variant, and 8 nonsense or frameshift variants leading to a truncated protein or degraded transcript. The remaining two variants were identified by array-comparative genomic hybridization and consisted of a partial deletion of FBXO11. All individuals had borderline to severe ID and behavioral problems (autism spectrum disorder, attention-deficit/hyperactivity disorder, anxiety, aggression) were observed in most of them. The most relevant common facial features included a thin upper lip and a broad prominent space between the paramedian peaks of the upper lip. Other features were hypotonia and hyperlaxity of the joints. We show that de novo variants in FBXO11 cause a syndromic form of ID. The current series show the power of reverse phenotyping in the interpretation of novel genetic variances in individuals who initially did not appear to have a clear recognizable phenotype.

Meta-analysis of 2,104 trios provides support for 10 new genes for intellectual disability.

To identify candidate genes for intellectual disability, we performed a meta-analysis on 2,637 de novo mutations, identified from the exomes of 2,104 patient-parent trios. Statistical analyses identified 10 new candidate ID genes: DLG4, PPM1D, RAC1, SMAD6, SON, SOX5, SYNCRIP, TCF20, TLK2 and TRIP12. In addition, we show that these genes are intolerant to nonsynonymous variation and that mutations in these genes are associated with specific clinical ID phenotypes.

De novo loss-of-function mutations in WAC cause a recognizable intellectual disability syndrome and learning deficits in Drosophila.

Recently WAC was reported as a candidate gene for intellectual disability (ID) based on the identification of a de novo mutation in an individual with severe ID. WAC regulates transcription-coupled histone H2B ubiquitination and has previously been implicated in the 10p12p11 contiguous gene deletion syndrome. In this study, we report on 10 individuals with de novo WAC mutations which we identified through routine (diagnostic) exome sequencing and targeted resequencing of WAC in 2326 individuals with unexplained ID. All but one mutation was expected to lead to a loss-of-function of WAC. Clinical evaluation of all individuals revealed phenotypic overlap for mild ID, hypotonia, behavioral problems and distinctive facial dysmorphisms, including a square-shaped face, deep set eyes, long palpebral fissures, and a broad mouth and chin. These clinical features were also previously reported in individuals with 10p12p11 microdeletion syndrome. To investigate the role of WAC in ID, we studied the importance of the Drosophila WAC orthologue (CG8949) in habituation, a non-associative learning paradigm. Neuronal knockdown of Drosophila CG8949 resulted in impaired learning, suggesting that WAC is required in neurons for normal cognitive performance. In conclusion, we defined a clinically recognizable ID syndrome, caused by de novo loss-of-function mutations in WAC. Independent functional evidence in Drosophila further supported the role of WAC in ID. On the basis of our data WAC can be added to the list of ID genes with a role in transcription regulation through histone modification.

Mobster: accurate detection of mobile element insertions in next generation sequencing data.

Mobile elements are major drivers in changing genomic architecture and can cause disease. The detection of mobile elements is hindered due to the low mappability of their highly repetitive sequences. We have developed an algorithm, called Mobster, to detect non-reference mobile element insertions in next generation sequencing data from both whole genome and whole exome studies. Mobster uses discordant read pairs and clipped reads in combination with consensus sequences of known active mobile elements. Mobster has a low false discovery rateand high recall rate for both L1 and Alu elements. Mobster is available at http://sourceforge.net/projects/mobster.

Mutations in the chromatin modifier gene KANSL1 cause the 17q21.31 microdeletion syndrome.

We show that haploinsufficiency of KANSL1 is sufficient to cause the 17q21.31 microdeletion syndrome, a multisystem disorder characterized by intellectual disability, hypotonia and distinctive facial features. The KANSL1 protein is an evolutionarily conserved regulator of the chromatin modifier KAT8, which influences gene expression through histone H4 lysine 16 (H4K16) acetylation. RNA sequencing studies in cell lines derived from affected individuals and the presence of learning deficits in Drosophila melanogaster mutants suggest a role for KANSL1 in neuronal processes.

De novo nonsense mutations in ASXL1 cause Bohring-Opitz syndrome.

Bohring-Opitz syndrome is characterized by severe intellectual disability, distinctive facial features and multiple congenital malformations. We sequenced the exomes of three individuals with Bohring-Opitz syndrome and in each identified heterozygous de novo nonsense mutations in ASXL1, which is required for maintenance of both activation and silencing of Hox genes. In total, 7 out of 13 subjects with a Bohring-Opitz phenotype had de novo ASXL1 mutations, suggesting that the syndrome is genetically heterogeneous.

A de novo paradigm for mental retardation.

The per-generation mutation rate in humans is high. De novo mutations may compensate for allele loss due to severely reduced fecundity in common neurodevelopmental and psychiatric diseases, explaining a major paradox in evolutionary genetic theory. Here we used a family based exome sequencing approach to test this de novo mutation hypothesis in ten individuals with unexplained mental retardation. We identified and validated unique non-synonymous de novo mutations in nine genes. Six of these, identified in six different individuals, are likely to be pathogenic based on gene function, evolutionary conservation and mutation impact. Our findings provide strong experimental support for a de novo paradigm for mental retardation. Together with de novo copy number variation, de novo point mutations of large effect could explain the majority of all mental retardation cases in the population.

De novo mutations of SETBP1 cause Schinzel-Giedion syndrome.

Schinzel-Giedion syndrome is characterized by severe mental retardation, distinctive facial features and multiple congenital malformations; most affected individuals die before the age of ten. We sequenced the exomes of four affected individuals (cases) and found heterozygous de novo variants in SETBP1 in all four. We also identified SETBP1 mutations in eight additional cases using Sanger sequencing. All mutations clustered to a highly conserved 11-bp exonic region, suggesting a dominant-negative or gain-of-function effect.