Haploinsufficiency of MBD5 associated with a syndrome involving microcephaly, intellectual disabilities, severe speech impairment, and seizures.
ABSTRACT: Microdeletion of chromosome 2q23.1 results in a novel syndrome previously reported in five individuals. Many of the del(2)(q23.1) cases were thought to have other syndromes such as Angelman, Prader-Willi, or Smith-Magenis because of certain overlapping clinical features. We report two new cases of the 2q23.1 microdeletion syndrome, describe the syndrome phenotype, define the minimal critical region, and analyze the expression of critical region genes toward identification of the causative gene(s) for the disorder. Individuals with del(2)(q23.1) have severe developmental and cognitive delays, minimal speech, seizures, microcephaly, mild craniofacial dysmorphism, behavioral disorders, and short stature. The deletions encompassing 2q23.1 range from >4 Mb to <200 kb, as identified by oligonucleotide and BAC whole-genome array comparative hybridization. The minimal critical region includes a single gene, MBD5, deleted in all cases, whereas all but one case also include deletion of EPC2. Quantitative real-time PCR of patient lymphoblasts/lymphocytes showed an approximately 50% reduced expression of MBD5 and EPC2 compared with controls. With similar phenotypes among the 2q23.1 deletion patients, the idea of one or more common genes causing the pathological defect seen in these patients becomes evident. As all five previous cases and the two cases in this report share one common gene, MBD5, we strongly suspect that haploinsufficiency of MBD5 causes most of the features observed in this syndrome.
Project description:Copy number variations associated with abnormal gene dosage have an important role in the genetic etiology of many neurodevelopmental disorders, including intellectual disability (ID) and autism. We hypothesize that the chromosome 2q23.1 region encompassing MBD5 is a dosage-dependent region, wherein deletion or duplication results in altered gene dosage. We previously established the 2q23.1 microdeletion syndrome and report herein 23 individuals with 2q23.1 duplications, thus establishing a complementary duplication syndrome. The observed phenotype includes ID, language impairments, infantile hypotonia and gross motor delay, behavioral problems, autistic features, dysmorphic facial features (pinnae anomalies, arched eyebrows, prominent nose, small chin, thin upper lip), and minor digital anomalies (fifth finger clinodactyly and large broad first toe). The microduplication size varies among all cases and ranges from 68 kb to 53.7 Mb, encompassing a region that includes MBD5, an important factor in methylation patterning and epigenetic regulation. We previously reported that haploinsufficiency of MBD5 is the primary causal factor in 2q23.1 microdeletion syndrome and that mutations in MBD5 are associated with autism. In this study, we demonstrate that MBD5 is the only gene in common among all duplication cases and that overexpression of MBD5 is likely responsible for the core clinical features present in 2q23.1 microduplication syndrome. Phenotypic analyses suggest that 2q23.1 duplication results in a slightly less severe phenotype than the reciprocal deletion. The features associated with a deletion, mutation or duplication of MBD5 and the gene expression changes observed support MBD5 as a dosage-sensitive gene critical for normal development.
Project description:2q23.1 microdeletion syndrome is characterized by intellectual disability, motor delay, autistic-like behaviors, and a distinctive craniofacial phenotype. All patients carry a partial or total deletion of methyl-CpG-binding domain protein 5 (MBD5), suggesting that haploinsufficiency of this gene is responsible for the phenotype. To confirm this hypothesis and to examine the role of MBD5 in vivo, we have generated and characterized an Mbd5 gene-trap mouse model. Our study indicates that the Mbd5(+/) (GT) mouse model recapitulates most of the hallmark phenotypes observed in 2q23.1 deletion carriers including abnormal social behavior, cognitive impairment, and motor and craniofacial abnormalities. In addition, neuronal cultures uncovered a deficiency in neurite outgrowth. These findings support a causal role of MBD5 in 2q23.1 microdeletion syndrome and suggest a role for MBD5 in neuronal processes. The Mbd5(+/) (GT) mouse model will advance our understanding of the abnormal brain development underlying the emergence of 2q23.1 deletion-associated behavioral and cognitive symptoms.
Project description:2Iq23.1 microdeletion syndrome is a recently described rare disease that includes intellectual disability, motor delay, autistic-like behaviors, and craniofacial abnormalities. Dosage insufficiency of the methyl-CpG-binding domain protein 5 (MBD5) gene was suggested as the genetic cause, since all the described patients carry a partial or total heterozygous deletion of MBD5. We reported the generation and characterization of a mouse model with haploinsufficiency for Mbd5 that confirmed this hypothesis. As in human 2q23.1 microdeletion syndrome, the MBD5 (+/GT) mouse model exhibited abnormal social behavior, cognitive impairment, and motor and craniofacial abnormalities, supporting a causal role for MBD5 in 2q23.1 microdeletion syndrome. The use of mouse neuronal cultures uncovered a deficiency in neurite outgrowth, suggesting the participation of MBD5 in neuronal processes. The study of the MBD5 (+/GT) mouse advanced our understanding of the abnormal brain development associated with behavioral and cognitive symptoms.
Project description:Individuals with autism spectrum disorders (ASD) who have an identifiable single-gene neurodevelopmental disorder (NDD), such as fragile X syndrome (FXS, FMR1), Smith-Magenis syndrome (SMS, RAI1), or 2q23.1 deletion syndrome (del 2q23.1, MBD5) share phenotypic features, including a high prevalence of sleep disturbance. We describe the circadian deficits in del 2q23.1 through caregiver surveys in which we identify several frequent sleep anomalies, including night/early awakenings, coughing/snoring loudly, and difficulty falling asleep. We couple these findings with studies on the molecular analysis of the circadian deficits associated with haploinsufficiency of MBD5 in which circadian gene mRNA levels of NR1D2, PER1, PER2, and PER3 were altered in del 2q23.1 lymphoblastoid cell lines (LCLs), signifying that haploinsufficiency of MBD5 can result in dysregulation of circadian rhythm gene expression. These findings were further supported by expression microarrays of MBD5 siRNA knockdown cells that showed significantly altered expression of additional circadian rhythm signaling pathway genes. Based on the common sleep phenotypes observed in del 2q23.1, SMS, and FXS patients, we explored the possibility that MBD5, RAI1, and FMR1 function in overlapping circadian rhythm pathways. Bioinformatic analysis identified conserved putative E boxes in MBD5 and RAI1, and expression levels of NR1D2 and CRY2 were significantly reduced in patient LCLs. Circadian and mTOR signaling pathways, both associated with sleep disturbance, were altered in both MBD5 and RAI1 knockdown microarray data, overlapping with findings associated with FMR1. These data support phenotypic and molecular overlaps across these syndromes that may be exploited to provide therapeutic intervention for multiple disorders.
Project description:Six submicroscopic deletions comprising chromosome band 2q23.1 in patients with severe mental retardation (MR), short stature, microcephaly and epilepsy have been reported, suggesting that haploinsufficiency of one or more genes in the 2q23.1 region might be responsible for the common phenotypic features in these patients. In this study, we report the molecular and clinical characterisation of nine new 2q23.1 deletion patients and a clinical update on two previously reported patients. All patients were mentally retarded with pronounced speech delay and additional abnormalities including short stature, seizures, microcephaly and coarse facies. The majority of cases presented with stereotypic repetitive behaviour, a disturbed sleep pattern and a broad-based gait. These features led to the initial clinical impression of Angelman, Rett or Smith-Magenis syndromes in several patients. The overlapping 2q23.1 deletion region in all 15 patients comprises only one gene, namely, MBD5. Interestingly, MBD5 is a member of the methyl CpG-binding domain protein family, which also comprises MECP2, mutated in Rett's syndrome. Another gene in the 2q23.1 region, EPC2, was deleted in 12 patients who had a broader phenotype than those with a deletion of MBD5 only. EPC2 is a member of the polycomb protein family, involved in heterochromatin formation and might be involved in causing MR. Patients with a 2q23.1 microdeletion present with a variable phenotype and the diagnosis should be considered in mentally retarded children with coarse facies, seizures, disturbed sleeping patterns and additional specific behavioural problems.
Project description:Intellectual disability (ID) is a clinical sign reflecting diverse neurodevelopmental disorders that are genetically and phenotypically heterogeneous. Just recently, partial or complete deletion of methyl-CpG-binding domain 5 (MBD5) gene has been implicated as causative in the phenotype associated with 2q23.1 microdeletion syndrome. In the course of systematic whole-genome screening of individuals with unexplained ID by array-based comparative genomic hybridization, we identified de novo intragenic deletions of MBD5 in three patients leading, as previously documented, to haploinsufficiency of MBD5. In addition, we described a patient with an unreported de novo MBD5 intragenic duplication. Reverse transcriptase-PCR and sequencing analyses showed the presence of numerous aberrant transcripts leading to premature termination codon. To further elucidate the involvement of MBD5 in ID, we sequenced ten coding, five non-coding exons and an evolutionary conserved region in intron 2, in a selected cohort of 78 subjects with a phenotype reminiscent of 2q23.1 microdeletion syndrome. Besides variants most often inherited from an healthy parent, we identified for the first time a de novo nonsense mutation associated with a much more damaging phenotype. Taken together, these results extend the mutation spectrum in MBD5 gene and contribute to refine the associated phenotype of neurodevelopmental disorder.
Project description:We report on two patients with developmental delay, hypotonia, and autistic features associated with duplications of chromosome region 2q23.1-2q23.2 detected by chromosome microarray analysis. The duplications include one OMIM Morbid Map gene, MBD5, as well as seven known RefSeq genes (ACVR2A, ORC4L, EPC2, KIF5C, MIR1978, LYPD6B, and LYPD6). MBD5 lies in the minimum area of overlap of the 2q23.1 microdeletion syndrome. This report provides the first detailed clinical examination of two individuals with a duplication of this region and suggests that brain development and cognitive function may be affected by an increased dosage of the genes involved.
Project description:2q23.1 deletion syndrome is characterized by intellectual disability, speech impairment, seizures, disturbed sleep pattern, behavioral problems, and hypotonia. Core features of this syndrome are due to haploinsufficiency of MBD5. Deletions that include coding and noncoding exons show reduced MBD5 mRNA expression. We report a patient with a neurological and behavioral phenotype similar to 2q23.1 deletion syndrome with an inherited intronic deletion in the 5-prime untranslated region of MBD5. Our data show that this patient has normal MBD5 mRNA expression; therefore, this deletion is likely not causative for 2q23.1 deletion syndrome. Overall, it is important to validate intronic deletions for pathogenicity.
Project description:MBD5-associated neurodevelopmental disorder (MAND) is an umbrella term that describes a group of disorders, 2q23.1 deletion syndrome, 2q23.1 duplication syndrome, and MBD5 variants, that affect the function of methyl-binding domain 5 (MBD5) and share a common set of neurodevelopmental, cognitive, and behavioral impairments. This review provides a comprehensive clinical and molecular synopsis of 2q23.1 deletion syndrome. Approaches to diagnosis, genetic counseling, and up-to-date management are summarized, followed by a discussion of the molecular and functional role of MBD5. Finally, we also include a brief summary of MBD5 variants that affect function of MBD5 and 2q23.1 duplication syndrome.
Project description:Persons with neurodevelopmental disorders or autism spectrum disorder (ASD) often harbor chromosomal microdeletions, yet the individual genetic contributors within these regions have not been systematically evaluated. We established a consortium of clinical diagnostic and research laboratories to accumulate a large cohort with genetic alterations of chromosomal region 2q23.1 and acquired 65 subjects with microdeletion or translocation. We sequenced translocation breakpoints; aligned microdeletions to determine the critical region; assessed effects on mRNA expression; and examined medical records, photos, and clinical evaluations. We identified a single gene, methyl-CpG-binding domain 5 (MBD5), as the only locus that defined the critical region. Partial or complete deletion of MBD5 was associated with haploinsufficiency of mRNA expression, intellectual disability, epilepsy, and autistic features. Fourteen alterations, including partial deletions of noncoding regions not typically captured or considered pathogenic by current diagnostic screening, disrupted MBD5 alone. Expression profiles and clinical characteristics were largely indistinguishable between MBD5-specific alteration and deletion of the entire 2q23.1 interval. No copy-number alterations of MBD5 were observed in 7878 controls, suggesting MBD5 alterations are highly penetrant. We surveyed MBD5 coding variations among 747 ASD subjects compared to 2043 non-ASD subjects analyzed by whole-exome sequencing and detected an association with a highly conserved methyl-CpG-binding domain missense variant, p.79Gly>Glu (c.236G>A) (p = 0.012). These results suggest that genetic alterations of MBD5 cause features of 2q23.1 microdeletion syndrome and that this epigenetic regulator significantly contributes to ASD risk, warranting further consideration in research and clinical diagnostic screening and highlighting the importance of chromatin remodeling in the etiology of these complex disorders.