Inherited 2q23.1 microdeletions involving the MBD5 locus

Oligogenic inheritance in severe adult obesity

BACKGROUND/OBJECTIVE

The genetic architecture of extreme non-syndromic obesity in adults remains to be elucidated. A range of genes are known to cause monogenic obesity, but even when pathogenic mutations are present, there may be variable penetrance.

METHODS

Whole-exome sequencing (WES) was carried out on a 15-year-old male proband of Pakistani ancestry who had severe obesity. This was followed by family segregation analysis, using Sanger sequencing. We also undertook re-analysis of WES data from 91 unrelated adults with severe obesity (86% white European ancestry) from the Personalised Medicine for Morbid Obesity (PMMO) cohort, recruited from the UK National Health Service.

RESULTS

We identified an oligogenic mode of inheritance of obesity in the proband's family-this provided the impetus to reanalyze existing sequence data in a separate dataset. Analysis of PMMO participant data revealed two further patients who carried more than one rare, predicted-deleterious mutation in a known monogenic obesity gene. In all three cases, the genes involved had known autosomal dominant inheritance, with incomplete penetrance.

CONCLUSION

Oligogenic inheritance may explain some of the variable penetrance in Mendelian forms of obesity. We caution clinicians and researchers to avoid confining sequence analysis to individual genes and, in particular, not to stop looking when the first potentially-causative mutation is found.

Systematic analysis of copy number variants of uncertain significance partially overlapping with the haploinsufficient or triplosensitive genes in clinical testing

Background: Copy number variants of uncertain significance (VUS) has brought much distress for patients and great counselling challenges for clinicians. Of these, a special type of VUS (HT-VUS), harbouring one or both breakpoints within the established haploinsufficient or triplosensitive genes, were considered to be more likely to cause clinical effects compared with other types of VUS.Methods: We retrospectively evaluated the properties and clinical significance of those HT-VUS samples in clinical testing for chromosome microarray analysis (CMA).Results: A total of 7150 samples were selected for HT-VUS screening, and 75 (1.05%) subjects with 75 HT-VUS were found. The majority of these HT-VUS were heterozygous duplications and chromosome X had the most HT-VUS. The prevalence of HT-VUS was 0.90% (28/3116) for prenatal low-risk samples, 1.18% (26/2196) for prenatal high-risk samples, 1.37% (10/728) for postnatal samples and 0.99% (11/1110) for early pregnancy loss samples. However, the incidence of HT-VUS was not statistically different between different groups.Conclusions: HT-VUS (deletions or duplications) involving introns and HT-VUS (duplications) including terminal coding exons (either the first or last exons) might be clinically neutral. Our study will be helpful for both interpretation and genetic counselling in the future.

A Novel Genetic Variant in MBD5 Associated with Severe Epilepsy and Intellectual Disability: Potential Implications on Neural Primary Cilia

Disruptions in the MBD5 gene have been linked with an array of clinical features such as global developmental delay, intellectual disability, autistic-like symptoms, and seizures, through unclear mechanisms. MBD5 haploinsufficiency has been associated with the disruption of primary cilium-related processes during early cortical development, and this has been reported in many neurodevelopmental disorders. In this study, we describe the clinical history of a 12-year-old child harboring a novel MBD5 rare variant and presenting psychomotor delay and seizures. To investigate the impact of MBD5 haploinsufficiency on neural primary cilia, we established a novel patient-derived cell line and used CRISPR-Cas9 technology to create an isogenic control. The patient-derived neural progenitor cells revealed a decrease in the length of primary cilia and in the total number of ciliated cells. This study paves the way to understanding the impact of MBD5 haploinsufficiency in brain development through its potential impact on neural primary cilia.

Germline mosaicism in a family with MBD5 haploinsufficiency

Haploinsufficiency of the methyl-CpG-binding domain protein 5 (MBD5) gene causes a neurodevelopmental disorder that includes intellectual disability, developmental delay, speech impairment, seizures, sleep disturbances, and behavioral difficulties. Microdeletion of 2q23.1 is the most common cause of haploinsufficiency, although MBD5 haploinsufficiency may also cause this genetic disorder. We report a family harboring a heterozygous loss-of-function variant in MBD5 (NM_018328.5:c.728delC; p.Pro243Hisfs*26), which includes three affected siblings with varying phenotypic features. Both parents were phenotypically normal but deep coverage sequencing of the parents showed germline mosaicism in the mother.

Phenotypic Spectrum of Seizure Disorders in MBD5-Associated Neurodevelopmental Disorder

Objective

To describe the phenotypic spectrum in patients with MBD5-associated neurodevelopmental disorder (MAND) and seizures; features of MAND include intellectual disability, epilepsy, psychiatric features of aggression and hyperactivity, and dysmorphic features including short stature and microcephaly, sleep disturbance, and ataxia.

Methods

We performed phenotyping on patients with MBD5 deletions, duplications, or point mutations and a history of seizures.

Results

Twenty-three patients with MAND and seizures were included. Median seizure onset age was 2.9 years (range 3 days–13 years). The most common seizure type was generalized tonic-clonic; focal, atypical absence, tonic, drop attacks, and myoclonic seizures occurred frequently. Seven children had convulsive status epilepticus and 3 nonconvulsive status epilepticus. Fever, viral illnesses, and hot weather provoked seizures. EEG studies in 17/21 patients were abnormal, typically showing slow generalized spike-wave and background slowing. Nine had drug-resistant epilepsy, although 3 eventually became seizure-free. All but one had moderate-to-severe developmental impairment. Epilepsy syndromes included Lennox-Gastaut syndrome, myoclonic-atonic epilepsy, and infantile spasms syndrome. Behavioral problems in 20/23 included aggression, self-injurious behavior, and sleep disturbance.

Conclusions

MBD5 disruption may be associated with severe early childhood-onset developmental and epileptic encephalopathy. Because neuropsychiatric dysfunction is common and severe, it should be an important focus of clinical management.

Inherited 2q23.1 microdeletions involving the MBD5 locus

Background

Microdeletions of 2q23.1 disrupting MBD5 and loss of function mutations of MBD5 cause MBD5‐Associated Neurodevelopmental disorders (MAND). Nearly all reported patients have been isolated cases of de novo origin.

Methods

This study investigates three families with inherited MBD5 mutations from three different Regional Genetics Centres in the UK.

Results

Two of the parents in the study had MBD5 deletions in a mosaic form. The parent with an MBD5 deletion in an apparently nonmosaic form has a psychiatric disorder in the absence of developmental delay or dysmorphism.

Conclusions

Inherited forms of MBD5 deletions are rare, but do occur, especially in a mosaic form. The phenotypic spectrum of MAND may be wider than previously thought.