CHRNE compound heterozygous mutations in congenital myasthenic syndrome: A case report

Structural switch in acetylcholine receptors in developing muscle

During development, motor neurons originating in the brainstem and spinal cord form elaborate synapses with skeletal muscle fibres1. These neurons release acetylcholine (ACh), which binds to nicotinic ACh receptors (AChRs) on the muscle, initiating contraction. Two types of AChR are present in developing muscle cells, and their differential expression serves as a hallmark of neuromuscular synapse maturation2-4. The structural principles underlying the switch from fetal to adult muscle receptors are unknown. Here, we present high-resolution structures of both fetal and adult muscle nicotinic AChRs, isolated from bovine skeletal muscle in developmental transition. These structures, obtained in the absence and presence of ACh, provide a structural context for understanding how fetal versus adult receptor isoforms are tuned for synapse development versus the all-or-none signalling required for high-fidelity skeletal muscle contraction. We find that ACh affinity differences are driven by binding site access, channel conductance is tuned by widespread surface electrostatics and open duration changes result from intrasubunit interactions and structural flexibility. The structures further reveal pathogenic mechanisms underlying congenital myasthenic syndromes.

Rare disease research workflow using multilayer networks elucidates the molecular determinants of severity in Congenital Myasthenic Syndromes

Exploring the molecular basis of disease severity in rare disease scenarios is a challenging task provided the limitations on data availability. Causative genes have been described for Congenital Myasthenic Syndromes (CMS), a group of diverse minority neuromuscular junction (NMJ) disorders; yet a molecular explanation for the phenotypic severity differences remains unclear. Here, we present a workflow to explore the functional relationships between CMS causal genes and altered genes from each patient, based on multilayer network community detection analysis of complementary biomedical information provided by relevant data sources, namely protein-protein interactions, pathways and metabolomics. Our results show that CMS severity can be ascribed to the personalized impairment of extracellular matrix components and postsynaptic modulators of acetylcholine receptor (AChR) clustering. This work showcases how coupling multilayer network analysis with personalized -omics information provides molecular explanations to the varying severity of rare diseases; paving the way for sorting out similar cases in other rare diseases.

Homozygous Duplication in the CHRNE in a Family with Congenital Myasthenic Syndrome 4C: 18-Year Follow Up

BACKGROUND AND OBJECTIVES

Congenital myasthenic syndromes (CMSs) are rare inherited diseases characterized by muscle weakness and fatigability on exertion resulting from defects in the neuromuscular junctions. Mutations in 32 genes have been reported as the underlying causes of CMS, with mutations in the cholinergic receptor nicotinic epsilon subunit (CHRNE) being the most common cause of the disease. Methodology and Materials: This study investigated a large consanguineous family with multiple individuals suffering from abnormal fatigue and muscle weakness in the ocular and limb regions. Moreover, the affected individuals were followed up for 18 years to observe the clinical course of the disease.

RESULTS

High-quality exome sequencing followed by bidirectional Sanger sequencing revealed a homozygous duplication variant (NM_000080.4: c.1220-8_1227dup) in the splice acceptor site of exon 11 of the CHRNE gene. This variant is predicted to cause frameshift and premature termination (p.Cys410ProfsTer51). Both parents had heterozygous duplication variants with no clinical symptoms. The personalized treatment of the affected individuals resulted in a marked improvement in the clinical symptoms. More than 80% of the disease symptoms in the affected individuals subsided after the use of pyridostigmine and salbutamol (4 mg).

CONCLUSIONS

This is the first report of long-term follow up of cases with homozygous insertion (c.1220-8_1227dup) in the CHRNE gene. Furthermore, this report expands the phenotypic symptoms associated with the CHRNE mutation.

Two heterozygous mutations in the calcium/calmodulin-dependent serine protein kinase gene (CASK) in cases with developmental disorders

BACKGROUND

The calcium/calmodulin-dependent serine protein kinase gene (CASK) is an essential gene in mammals, critical for neurodevelopment. The purpose of this study is to expand the understanding of the diagnosis of CASK-linked disorders.

MATERIALS/METHODS

From clinical and genetic mutational analyses, relevant data in 2 Han Chinese patients were collected and analyzed. Real-time quantitative PCR (RT-qPCR) was performed to investigate the CASK expression levels in the patients. The X-chromosome inactivation (XCI) patterns of the patients and their nuclear families were tested by quantitation of methylation of the polymorphic human androgen receptor (HUMARA) locus.

RESULTS

Two Han Chinese patients both presented with intellectual disability (ID), microcephaly with pontine and cerebellar hypoplasia (MICPCH). Two de novo mutations of c.82C>T (p.Arg28*) and c.846C>G (p.Tyr282*) in CASK have been investigated and predicted to be deleterious, which have produced truncated proteins. The functional protein association network of STRING (http://string-db.org) generated three-dimensional (3D) atomic models based on protein sequences in CASK and two Arg28 and Tyr282 residues were marked. RT-qPCR showed lower copy numbers of CASK expression in the patients than in their parents, as well as the sex- and age- matched control groups. Patient 1 showed a skewed XCI pattern, while no related changes noted in patient 2.

CONCLUSIONS

Patients carrying different nonsense variants may have different degrees of different clinical phenotypes. This study expands the spectrum of genotype and phenotype correlations of CASK-linked disorders in the Han Chinese ethnicity and provides new insights into the molecular mechanism.

Whole genome variation in 27 Mexican indigenous populations, demographic and biomedical insights

There has been limited study of Native American whole genome diversity to date, which impairs effective implementation of personalized medicine and a detailed description of its demographic history. Here we report high coverage whole genome sequencing of 76 unrelated individuals, from 27 indigenous groups across Mexico, with more than 97% average Native American ancestry. On average, each individual has 3.26 million Single Nucleotide Variants and short indels, that together comprise a catalog of 9,737,152 variants, 44,118 of which are novel. We report 497 common Single Nucleotide Variants (with allele frequency > 5%) mapped to drug responses and 316,577 in enhancer or promoter elements; interestingly we found some of these enhancer variants in PPARG, a nuclear receptor involved in highly prevalent health problems in Mexican population, such as obesity, diabetes, and insulin resistance. By detecting signals of positive selection we report 24 enriched key pathways under selection, most of them related to immune mechanisms. No missense variants in ACE2, the receptor responsible for the entry of the SARS CoV-2 virus, were found in any individual. Population genomics and phylogenetic analyses demonstrated stratification in a Northern-Central-Southern axis, with major substructure in the Central region. The Seri, a northern group with the most genetic divergence in our study, showed a distinctive genomic context with the most novel variants, and the most population specific genotypes. Genome-wide analysis showed that the average haplotype blocks are longer in Native Mexicans than in other world populations. With this dataset we describe previously undetected population level variation in Native Mexicans, helping to reduce the gap in genomic data representation of such groups.