BOD1 Is Required for Cognitive Function in Humans and Drosophila.

Here we report a stop-mutation in the BOD1 (Biorientation Defective 1) gene, which co-segregates with intellectual disability in a large consanguineous family, where individuals that are homozygous for the mutation have no detectable BOD1 mRNA or protein. The BOD1 protein is required for proper chromosome segregation, regulating phosphorylation of PLK1 substrates by modulating Protein Phosphatase 2A (PP2A) activity during mitosis. We report that fibroblast cell lines derived from homozygous BOD1 mutation carriers show aberrant localisation of the cell cycle kinase PLK1 and its phosphatase PP2A at mitotic kinetochores. However, in contrast to the mitotic arrest observed in BOD1-siRNA treated HeLa cells, patient-derived cells progressed through mitosis with no apparent segregation defects but at an accelerated rate compared to controls. The relatively normal cell cycle progression observed in cultured cells is in line with the absence of gross structural brain abnormalities in the affected individuals. Moreover, we found that in normal adult brain tissues BOD1 expression is maintained at considerable levels, in contrast to PLK1 expression, and provide evidence for synaptic localization of Bod1 in murine neurons. These observations suggest that BOD1 plays a cell cycle-independent role in the nervous system. To address this possibility, we established two Drosophila models, where neuron-specific knockdown of BOD1 caused pronounced learning deficits and significant abnormalities in synapse morphology. Together our results reveal novel postmitotic functions of BOD1 as well as pathogenic mechanisms that strongly support a causative role of BOD1 deficiency in the aetiology of intellectual disability. Moreover, by demonstrating its requirement for cognitive function in humans and Drosophila we provide evidence for a conserved role of BOD1 in the development and maintenance of cognitive features.

Penetrance of pathogenic mutations in haploinsufficient genes for intellectual disability and related disorders.

De novo loss of function (LOF) mutations in the ASXL3 gene cause Bainbridge-Ropers syndrome, a severe form of intellectual disability (ID) and developmental delay, but there is evidence that they also occur in healthy individuals. This has prompted us to look for non-pathogenic LOF variants in other ID genes. Heterozygous LOF mutations in ASXL1, a paralog of ASXL3, are known to cause Bohring-Opitz syndrome (BOS), and benign LOF mutations in this gene have not been published to date. Therefore, we were surprised to find 56 ASXL1 LOF variants in the ExAC database (http://exac.broadinstitute.org), comprising exomes from 60,706 individuals who had been selected to exclude severe genetic childhood disorders. 4 of these variants have been described as disease-causing in patients with BOS, which rules out the possibility that pathogenic and clinically neutral LOF variants in this gene are functionally distinct. Apparently benign LOF variants were also detected in several other genes for ID and related disorders, including CDH15, KATNAL2, DEPDC5, ARID1B and AUTS2, both in the ExAC database and in the 6,500 exomes of the Exome Variant Server (http://evs.gs.washington.edu/EVS/). These observations argue for low penetrance of LOF mutations in ASXL1 and other genes for ID and related disorders, which could have far-reaching implications for genetic counseling and research.

The Role of a Novel TRMT1 Gene Mutation and Rare GRM1 Gene Defect in Intellectual Disability in Two Azeri Families.

Cognitive impairment or intellectual disability (ID) is a widespread neurodevelopmental disorder characterized by low IQ (below 70). ID is genetically heterogeneous and is estimated to affect 1-3% of the world's population. In affected children from consanguineous families, autosomal recessive inheritance is common, and identifying the underlying genetic cause is an important issue in clinical genetics. In the framework of a larger project, aimed at identifying candidate genes for autosomal recessive intellectual disorder (ARID), we recently carried out single nucleotide polymorphism-based genome-wide linkage analysis in several families from Ardabil province in Iran. The identification of homozygosity-by-descent loci in these families, in combination with whole exome sequencing, led us to identify possible causative homozygous changes in two families. In the first family, a missense variant was found in GRM1 gene, while in the second family, a frameshift alteration was identified in TRMT1, both of which were found to co-segregate with the disease. GRM1, a known causal gene for autosomal recessive spinocerebellar ataxia (SCAR13, MIM#614831), encodes the metabotropic glutamate receptor1 (mGluR1). This gene plays an important role in synaptic plasticity and cerebellar development. Conversely, the TRMT1 gene encodes a tRNA methyltransferase that dimethylates a single guanine residue at position 26 of most tRNAs using S-adenosyl methionine as the methyl group donor. We recently presented TRMT1 as a candidate gene for ARID in a consanguineous Iranian family (Najmabadi et al., 2011). We believe that this second Iranian family with a biallelic loss-of-function mutation in TRMT1 gene supports the idea that this gene likely has function in development of the disorder.

Intelligence: shared genetic basis between Mendelian disorders and a polygenic trait.

Multiple inquiries into the genetic etiology of human traits indicated an overlap between genes underlying monogenic disorders (eg, skeletal growth defects) and those affecting continuous variability of related quantitative traits (eg, height). Extending the idea of a shared genetic basis between a Mendelian disorder and a classic polygenic trait, we performed an association study to examine the effect of 43 genes implicated in autosomal recessive cognitive disorders on intelligence in an unselected Dutch population (N=1316). Using both single-nucleotide polymorphism (SNP)- and gene-based association testing, we detected an association between intelligence and the genes of interest, with genes ELP2, TMEM135, PRMT10, and RGS7 showing the strongest associations. This is a demonstration of the relevance of genes implicated in monogenic disorders of intelligence to normal-range intelligence, and a corroboration of the utility of employing knowledge on monogenic disorders in identifying the genetic variability underlying complex traits.

A defect in the CLIP1 gene (CLIP-170) can cause autosomal recessive intellectual disability.

In the context of a comprehensive research project, investigating novel autosomal recessive intellectual disability (ARID) genes, linkage analysis based on autozygosity mapping helped identify an intellectual disability locus on Chr.12q24, in an Iranian family (LOD score = 3.7). Next-generation sequencing (NGS) following exon enrichment in this novel interval, detected a nonsense mutation (p.Q1010*) in the CLIP1 gene. CLIP1 encodes a member of microtubule (MT) plus-end tracking proteins, which specifically associates with the ends of growing MTs. These proteins regulate MT dynamic behavior and are important for MT-mediated transport over the length of axons and dendrites. As such, CLIP1 may have a role in neuronal development. We studied lymphoblastoid and skin fibroblast cell lines established from healthy and affected patients. RT-PCR and western blot analyses showed the absence of CLIP1 transcript and protein in lymphoblastoid cells derived from affected patients. Furthermore, immunofluorescence analyses showed MT plus-end staining only in fibroblasts containing the wild-type (and not the mutant) CLIP1 protein. Collectively, our data suggest that defects in CLIP1 may lead to ARID.

Integrated sequence analysis pipeline provides one-stop solution for identifying disease-causing mutations.

Next-generation sequencing has greatly accelerated the search for disease-causing defects, but even for experts the data analysis can be a major challenge. To facilitate the data processing in a clinical setting, we have developed a novel medical resequencing analysis pipeline (MERAP). MERAP assesses the quality of sequencing, and has optimized capacity for calling variants, including single-nucleotide variants, insertions and deletions, copy-number variation, and other structural variants. MERAP identifies polymorphic and known causal variants by filtering against public domain databases, and flags nonsynonymous and splice-site changes. MERAP uses a logistic model to estimate the causal likelihood of a given missense variant. MERAP considers the relevant information such as phenotype and interaction with known disease-causing genes. MERAP compares favorably with GATK, one of the widely used tools, because of its higher sensitivity for detecting indels, its easy installation, and its economical use of computational resources. Upon testing more than 1,200 individuals with mutations in known and novel disease genes, MERAP proved highly reliable, as illustrated here for five families with disease-causing variants. We believe that the clinical implementation of MERAP will expedite the diagnostic process of many disease-causing defects.

Genetics of recessive cognitive disorders.

Most severe forms of intellectual disability (ID) have specific genetic causes. Numerous X chromosome gene defects and disease-causing copy-number variants have been linked to ID and related disorders, and recent studies have revealed that sporadic cases are often due to dominant de novo mutations with low recurrence risk. For autosomal recessive ID (ARID) the recurrence risk is high and, in populations with frequent parental consanguinity, ARID is the most common form of ID. Even so, its elucidation has lagged behind. Here we review recent progress in this field, show that ARID is not rare even in outbred Western populations, and discuss the prospects for improving its diagnosis and prevention.

De novo truncating mutations in ASXL3 are associated with a novel clinical phenotype with similarities to Bohring-Opitz syndrome.

BACKGROUND: Molecular diagnostics can resolve locus heterogeneity underlying clinical phenotypes that may otherwise be co-assigned as a specific syndrome based on shared clinical features, and can associate phenotypically diverse diseases to a single locus through allelic affinity. Here we describe an apparently novel syndrome, likely caused by de novo truncating mutations in ASXL3, which shares characteristics with Bohring-Opitz syndrome, a disease associated with de novo truncating mutations in ASXL1. METHODS: We used whole-genome and whole-exome sequencing to interrogate the genomes of four subjects with an undiagnosed syndrome. RESULTS: Using genome-wide sequencing, we identified heterozygous, de novo truncating mutations in ASXL3, a transcriptional repressor related to ASXL1, in four unrelated probands. We found that these probands shared similar phenotypes, including severe feeding difficulties, failure to thrive, and neurologic abnormalities with significant developmental delay. Further, they showed less phenotypic overlap with patients who had de novo truncating mutations in ASXL1. CONCLUSION: We have identified truncating mutations in ASXL3 as the likely cause of a novel syndrome with phenotypic overlap with Bohring-Opitz syndrome.

Mutations in NSUN2 cause autosomal-recessive intellectual disability.

With a prevalence between 1 and 3%, hereditary forms of intellectual disability (ID) are among the most important problems in health care. Particularly, autosomal-recessive forms of the disorder have a very heterogeneous molecular basis, and genes with an increased number of disease-causing mutations are not common. Here, we report on three different mutations (two nonsense mutations, c.679C>T [p.Gln227(∗)] and c.1114C>T [p.Gln372(∗)], as well as one splicing mutation, g.6622224A>C [p.Ile179Argfs(∗)192]) that cause a loss of the tRNA-methyltransferase-encoding NSUN2 main transcript in homozygotes. We identified the mutations by sequencing exons and exon-intron boundaries within the genomic region where the linkage intervals of three independent consanguineous families of Iranian and Kurdish origin overlapped with the previously described MRT5 locus. In order to gain further evidence concerning the effect of a loss of NSUN2 on memory and learning, we constructed a Drosophila model by deleting the NSUN2 ortholog, CG6133, and investigated the mutants by using molecular and behavioral approaches. When the Drosophila melanogaster NSUN2 ortholog was deleted, severe short-term-memory (STM) deficits were observed; STM could be rescued by re-expression of the wild-type protein in the nervous system. The humans homozygous for NSUN2 mutations showed an overlapping phenotype consisting of moderate to severe ID and facial dysmorphism (which includes a long face, characteristic eyebrows, a long nose, and a small chin), suggesting that mutations in this gene might even induce a syndromic form of ID. Moreover, our observations from the Drosophila model point toward an evolutionarily conserved role of RNA methylation in normal cognitive development.

Deep sequencing reveals 50 novel genes for recessive cognitive disorders.

Common diseases are often complex because they are genetically heterogeneous, with many different genetic defects giving rise to clinically indistinguishable phenotypes. This has been amply documented for early-onset cognitive impairment, or intellectual disability, one of the most complex disorders known and a very important health care problem worldwide. More than 90 different gene defects have been identified for X-chromosome-linked intellectual disability alone, but research into the more frequent autosomal forms of intellectual disability is still in its infancy. To expedite the molecular elucidation of autosomal-recessive intellectual disability, we have now performed homozygosity mapping, exon enrichment and next-generation sequencing in 136 consanguineous families with autosomal-recessive intellectual disability from Iran and elsewhere. This study, the largest published so far, has revealed additional mutations in 23 genes previously implicated in intellectual disability or related neurological disorders, as well as single, probably disease-causing variants in 50 novel candidate genes. Proteins encoded by several of these genes interact directly with products of known intellectual disability genes, and many are involved in fundamental cellular processes such as transcription and translation, cell-cycle control, energy metabolism and fatty-acid synthesis, which seem to be pivotal for normal brain development and function.

ST3GAL3 mutations impair the development of higher cognitive functions.

The genetic variants leading to impairment of intellectual performance are highly diverse and are still poorly understood. ST3GAL3 encodes the Golgi enzyme ß-galactoside-α2,3-sialyltransferase-III that in humans predominantly forms the sialyl Lewis a epitope on proteins. ST3GAL3 resides on chromosome 1 within the MRT4 locus previously identified to associate with nonsyndromic autosomal recessive intellectual disability. We searched for the disease-causing mutations in the MRT4 family and a second independent consanguineous Iranian family by using a combination of chromosome sorting and next-generation sequencing. Two different missense changes in ST3GAL3 cosegregate with the disease but were absent in more than 1000 control chromosomes. In cellular and biochemical test systems, these mutations were shown to cause ER retention of the Golgi enzyme and drastically impair ST3Gal-III functionality. Our data provide conclusive evidence that glycotopes formed by ST3Gal-III are prerequisite for attaining and/or maintaining higher cognitive functions.

Next generation sequencing in a family with autosomal recessive Kahrizi syndrome (OMIM 612713) reveals a homozygous frameshift mutation in SRD5A3.

As part of a large-scale, systematic effort to unravel the molecular causes of autosomal recessive mental retardation, we have previously described a novel syndrome consisting of mental retardation, coloboma, cataract and kyphosis (Kahrizi syndrome, OMIM 612713) and mapped the underlying gene to a 10.4-Mb interval near the centromere on chromosome 4. By combining array-based exon enrichment and next generation sequencing, we have now identified a homozygous frameshift mutation (c.203dupC; p.Phe69LeufsX2) in the gene for steroid 5α-reductase type 3 (SRD5A3) as the disease-causing change in this interval. Recent evidence indicates that this enzyme is required for the conversion of polyprenol to dolichol, a step that is essential for N-linked protein glycosylation. Independently, another group has recently observed SRD5A3 mutations in several families with a type 1 congenital disorder of glycosylation (CDG type Ix, OMIM 212067), mental retardation, cerebellar ataxia and eye disorders. Our results show that Kahrizi syndrome and this CDG Ix subtype are allelic disorders, and they illustrate the potential of next-generation sequencing strategies for the elucidation of single gene defects.

Identification of mutations in TRAPPC9, which encodes the NIK- and IKK-beta-binding protein, in nonsyndromic autosomal-recessive mental retardation.

Mental retardation/intellectual disability is a devastating neurodevelopmental disorder with serious impact on affected individuals and their families, as well as on health and social services. It occurs with a prevalence of approximately 2%, is an etiologically heterogeneous condition, and is frequently the result of genetic aberrations. Autosomal-recessive forms of nonsyndromic MR (NS-ARMR) are believed to be common, yet only five genes have been identified. We have used homozygosity mapping to search for the gene responsible for NS-ARMR in a large Pakistani pedigree. Using Affymetrix 5.0 single nucleotide polymorphism (SNP) microarrays, we identified a 3.2 Mb region on 8q24 with a continuous run of 606 homozygous SNPs shared among all affected members of the family. Additional genotype data from microsatellite markers verified this, allowing us to calculate a two-point LOD score of 5.18. Within this region, we identified a truncating homozygous mutation, R475X, in exon 7 of the gene TRAPPC9. In a second large NS-ARMR/ID family, previously linked to 8q24 in a study of Iranian families, we identified a 4 bp deletion within exon 14 of TRAPPC9, also segregating with the phenotype and truncating the protein. This gene encodes NIK- and IKK-beta-binding protein (NIBP), which is involved in the NF-kappaB signaling pathway and directly interacts with IKK-beta and MAP3K14. Brain magnetic resonance imaging of affected individuals indicates the presence of mild cerebral white matter hypoplasia. Microcephaly is present in some but not all affected individuals. Thus, to our knowledge, this is the sixth gene for NS-ARMR to be discovered.

Genetics of intellectual disability.

Early onset intellectual disability (ID) is one of the largest unsolved problems of health care. Yet, it has received very little public attention in the past because many health care professionals do not perceive it as a health condition but as a social or educational issue. In severe ID, cytogenetically visible chromosomal abnormalities like trisomy 21 continue to be common, but since the introduction of array CGH, it is becoming clear that submicroscopic deletions and duplications are equally frequent, yet previously overlooked causes of ID. Until recently, the search for gene defects causing ID has focused on the X-chromosome. So far, >80 genes have been implicated in X-linked ID, largely owing to coordinated efforts of international consortia, and mutations in these genes account for >50% of the families with this condition. Autosomal forms, either due to dominant de novo mutations or to recessive gene defects, are presumably (far) more common than X-linked ones, and their molecular elucidation is a new challenge for research in this field. As recently shown, autosomal recessive ID (ARID) is extremely heterogeneous, and common forms are unlikely to exist. Ongoing studies into the function of ID genes are shedding more light on the pathogenesis of this disorder, and there is reason to believe that at least some genetic forms of ID may be amenable to drug treatment.

Mapping translocation breakpoints by next-generation sequencing.

Balanced chromosome rearrangements (BCRs) can cause genetic diseases by disrupting or inactivating specific genes, and the characterization of breakpoints in disease-associated BCRs has been instrumental in the molecular elucidation of a wide variety of genetic disorders. However, mapping chromosome breakpoints using traditional methods, such as in situ hybridization with fluorescent dye-labeled bacterial artificial chromosome clones (BAC-FISH), is rather laborious and time-consuming. In addition, the resolution of BAC-FISH is often insufficient to unequivocally identify the disrupted gene. To overcome these limitations, we have performed shotgun sequencing of flow-sorted derivative chromosomes using "next-generation" (Illumina/Solexa) multiplex sequencing-by-synthesis technology. As shown here for three different disease-associated BCRs, the coverage attained by this platform is sufficient to bridge the breakpoints by PCR amplification, and this procedure allows the determination of their exact nucleotide positions within a few weeks. Its implementation will greatly facilitate large-scale breakpoint mapping and gene finding in patients with disease-associated balanced translocations.

Recurrent reciprocal genomic rearrangements of 17q12 are associated with renal disease, diabetes, and epilepsy.

Most studies of genomic disorders have focused on patients with cognitive disability and/or peripheral nervous system defects. In an effort to broaden the phenotypic spectrum of this disease model, we assessed 155 autopsy samples from fetuses with well-defined developmental pathologies in regions predisposed to recurrent rearrangement, by array-based comparative genomic hybridization. We found that 6% of fetal material showed evidence of microdeletion or microduplication, including three independent events that likely resulted from unequal crossing-over between segmental duplications. One of the microdeletions, identified in a fetus with multicystic dysplastic kidneys, encompasses the TCF2 gene on 17q12, previously shown to be mutated in maturity-onset diabetes, as well as in a subset of pediatric renal abnormalities. Fine-scale mapping of the breakpoints in different patient cohorts revealed a recurrent 1.5-Mb de novo deletion in individuals with phenotypes that ranged from congenital renal abnormalities to maturity-onset diabetes of the young type 5. We also identified the reciprocal duplication, which appears to be enriched in samples from patients with epilepsy. We describe the first example of a recurrent genomic disorder associated with diabetes.

Array CGH identifies reciprocal 16p13.1 duplications and deletions that predispose to autism and/or mental retardation.

Autism and mental retardation (MR) are often associated, suggesting that these conditions are etiologically related. Recently, array-based comparative genomic hybridization (array CGH) has identified submicroscopic deletions and duplications as a common cause of MR, prompting us to search for such genomic imbalances in autism. Here we describe a 1.5-Mb duplication on chromosome 16p13.1 that was found by high-resolution array CGH in four severe autistic male patients from three unrelated families. The same duplication was identified in several variably affected and unaffected relatives. A deletion of the same interval was detected in three unrelated patients with MR and other clinical abnormalities. In one patient we revealed a further rearrangement of the 16p13 imbalance that was not present in his unaffected mother. Duplications and deletions of this 1.5-Mb interval have not been described as copy number variants in the Database of Genomic Variants and have not been identified in >600 individuals from other cohorts examined by high-resolution array CGH in our laboratory. Thus we conclude that these aberrations represent recurrent genomic imbalances which predispose to autism and/or MR.

Homozygosity mapping in consanguineous families reveals extreme heterogeneity of non-syndromic autosomal recessive mental retardation and identifies 8 novel gene loci.

Autosomal recessive gene defects are arguably the most important, but least studied genetic causes of severe cognitive dysfunction. Homozygosity mapping in 78 consanguineous Iranian families with nonsyndromic autosomal recessive mental retardation (NS-ARMR) has enabled us to determine the chromosomal localization of at least 8 novel gene loci for this condition. Our data suggest that in the Iranian population NS-ARMR is very heterogeneous, and they argue against the existence of frequent gene defects that account for more than a few percent of the cases.

Mutation screening of brain-expressed X-chromosomal miRNA genes in 464 patients with nonsyndromic X-linked mental retardation.

MiRNAs are small noncoding RNAs that control the expression of target genes at the post-transcriptional level and have been reported to modulate various biological processes. Their function as regulatory factors in gene expression renders them attractive candidates for harbouring genetic variants with subtle effects on IQ. In an attempt to investigate the potential role of miRNAs in the aetiology of X-linked mental retardation, we have examined all 13 known, brain-expressed X-chromosomal miRNAs in a cohort of 464 patients with non-syndromic X-linked MR and found four nucleotide changes in three different pre-miRNA hairpins. All the observed changes appear to be functionally neutral which, taken together with the rarity of detected nucleotide changes in miRNA genes, may reflect strong selection and thus underline the functional importance of miRNAs.

SNP array-based homozygosity mapping reveals MCPH1 deletion in family with autosomal recessive mental retardation and mild microcephaly.

Very little is known about the molecular basis of autosomal recessive MR (ARMR) because in developed countries, small family sizes preclude mapping and identification of the relevant gene defects. We therefore chose to investigate genetic causes of ARMR in large consanguineous Iranian families. This study reports on a family with six mentally retarded members. Array-based homozygosity mapping and high-resolution microarray-based comparative genomic hybridization (array CGH) revealed a deletion of approximately 150-200 kb, encompassing the promoter and the first six exons of the MCPH1 gene, one out of four genes that have been previously implicated in ARMR with microcephaly. Reexamination of affected individuals revealed a high proportion of prematurely condensed chromosomes, which is a hallmark of this condition, but in spite of the severity of the mutation, all patients showed only borderline to mild microcephaly. Therefore the phenotypic spectrum of MCPH1 mutations may be wider than previously assumed, with ARMR being the only consistent clinical finding.