IGHMBP2 -related clinical and genetic features in a cohort of Chinese Charcot–Marie–Tooth disease type 2 patients

Novel biallelic nonsense mutation in IGHMBP2 gene linked to neuropathy (CMT2S): A comprehensive clinical, genetic and bioinformatic analysis of a Turkish patient with literature review

BACKGROUND

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S) typically present before age 10. Genetic factors account for up to 50 % of neuropathies, which often display varied symptoms. Mutations in the IGHMBP2 gene are associated with both CMT2S and SMARD1, resulting in a rare clinical condition marked by axonal neuropathy, spinal muscular atrophy, respiratory distress, and muscle weakness.

METHOD

Detailed family histories and medical data were collected. Segregation analysis was performed using Sanger sequencing and whole exome sequencing. Additionally, a review of molecularly confirmed patients was conducted. Protein tertiary structures expressed in the IGHMBP2 gene were tested for topological and conformational changes using modeling programs and in-silico tools.

RESULTS

We identified a novel homozygous nonsense mutation (c.2568_2569del p.Gly857Alafs*27) in a family with a member showing neuropathy. This report details the clinical and genetic findings of the affected individuals, including a Turkish patient with neuropathy, and compares them with literature cases.

CONCLUSION

Understanding the clinical impact of the (c.2568_2569del p.Gly857Alafs*27) mutation will enhance our knowledge of IGHMBP2 gene defects role in neuropathy. This study aims to highlight this severe recessive disease caused by pathogenic IGHMBP2 gene mutations and to examine the mutation spectrum and phenotype differences.

Ighmbp2 mutations and disease pathology: Defining differences that differentiate SMARD1 and CMT2S

Mutations in the Immunoglobulin mu DNA binding protein 2 (IGHMBP2) gene result in two distinct diseases, SMA with Respiratory Distress Type I (SMARD1) and Charcot Marie Tooth Type 2S (CMT2S). To understand the phenotypic and molecular differences between SMARD1 and CMT2S, and the role of IGHMBP2 in disease development, we generated mouse models based on six IGHMBP2 patient mutations. Previously, we reported the development and characterization of Ighmbp2D564N/D564N mice and in this manuscript, we examine two mutations: D565N (D564N in mice) and H924Y (H922Y in mice) in the Ighmbp2H922Y/H922Y and Ighmbp2D564N/H922Y contexts. We found significant differences between these mouse models, providing critical insight into the role of IGHMBP2 in the pathogenesis of SMARD1 and CMT2S. Importantly, these studies also demonstrate how disease pathogenesis is significantly altered in the context of Ighmbp2 D564N and H922Y homozygous recessive and compound heterozygous mutations. Notably, there were short-lived and long-lived lifespan cohorts within Ighmbp2D564N/H922Y mice with early (P12/P16) respiratory pathology serving as a key predictor of lifespan. Despite differences in lifespan, motor function deficits initiated early and progressively worsened in all Ighmbp2D564N/H922Y mice. There was decreased limb skeletal muscle fiber area and increased neuromuscular junction (NMJ) denervation in Ighmbp2D564N/H922Y mice. Consistent with CMT2S, Ighmbp2H922Y/H922Y mice did not have altered lifespans nor respiratory pathology. Interestingly, Ighmbp2H922Y/H922Y limb muscle fibers demonstrated an increase in muscle fiber area followed by a reduction while changes in NMJ innervation were minimal even at P180. This is the first study that demonstrates differences associated with IGHMBP2 function within respiration with those within limb motor function. Significant to our understanding of IGHMBP2 function, we demonstrate that there is a direct correlation between disease pathogenesis associated with these IGHMBP2 patient mutations and IGHMBP2 biochemical activity. Importantly, these studies reveal the dynamic differences that are presented when either a single mutant protein is present (IGHMBP2-D564N or IGHMBP2-H922Y) or two mutant proteins are present (IGHMBP2-D564N and IGHMBP2-H922Y) within cells.

Expanding the genetic spectrum of hereditary motor sensory neuropathies in Pakistan

BACKGROUND

Hereditary motor and sensory neuropathy (HMSN) refers to a group of inherited progressive peripheral neuropathies characterized by reduced nerve conduction velocity with chronic segmental demyelination and/or axonal degeneration. HMSN is highly clinically and genetically heterogeneous with multiple inheritance patterns and phenotypic overlap with other inherited neuropathies and neurodegenerative diseases. Due to this high complexity and genetic heterogeneity, this study aimed to elucidate the genetic causes of HMSN in Pakistani families using Whole Exome Sequencing (WES) for variant identification and Sanger sequencing for validation and segregation analysis, facilitating accurate clinical diagnosis.

METHODS

Families from Khyber Pakhtunkhwa with at least two members showing HMSN symptoms, who had not previously undergone genetic analysis, were included. Referrals for genetic investigations were based on clinical features suggestive of HMSN by local neurologists. WES was performed on affected individuals from each family, with Sanger sequencing used to validate and analyze the segregation of identified variants among family members. Clinical data including age of onset were assessed for variability among affected individuals, and the success rate of genetic diagnosis was compared with existing literature using proportional differences and Cohen's h.

RESULTS

WES identified homozygous pathogenic variants in GDAP1 (c.310 + 4 A > G, p.?), SETX (c.5948_5949del, p.(Asn1984Profs*30), IGHMBP2 (c.1591 C > A, p.(Pro531Thr) and NARS1 (c.1633 C > T, p.(Arg545Cys) as causative for HMSN in five out of nine families, consistent with an autosomal recessive inheritance pattern. Additionally, in families with HMSN, a SETX variant was found to cause cerebellar ataxia, while a NARS1 variant was linked to intellectual disability. Based on American College of Medical Genetics and Genomics criteria, the GDAP1 variant is classified as a variant of uncertain significance, while variants in SETX and IGHMBP2 are classified as pathogenic, and the NARS1 variant is classified as likely pathogenic. The age of onset ranged from 1 to 15 years (Mean = 5.13, SD = 3.61), and a genetic diagnosis was achieved in 55.56% of families with HMSN, with small effect sizes compared to previous studies.

CONCLUSIONS

This study expands the molecular genetic spectrum of HMSN and HMSN plus type neuropathies in Pakistan and facilitates accurate diagnosis, genetic counseling, and clinical management for affected families.

Clinical and Genetic Profiles of 5q- and Non-5q-Spinal Muscular Atrophy Diseases in Pediatric Patients

BACKGROUND

Spinal muscular atrophy (SMA) is a genetic disease characterized by loss of motor neurons in the spinal cord and lower brainstem. The term "SMA" usually refers to the most common form, 5q-SMA, which is caused by biallelic mutations in SMN1 (located on chromosome 5q13). However, long before the discovery of SMN1, it was known that other forms of SMA existed. Therefore, SMA is currently divided into two groups: 5q-SMA and non-5q-SMA. This is a simple and practical classification, and therapeutic drugs have only been developed for 5q-SMA (nusinersen, onasemnogene abeparvovec, risdiplam) and not for non-5q-SMA disease.

METHODS

We conducted a non-systematic critical review to identify the characteristics of each SMA disease.

RESULTS

Many of the non-5q-SMA diseases have similar symptoms, making DNA analysis of patients essential for accurate diagnosis. Currently, genetic analysis technology using next-generation sequencers is rapidly advancing, opening up the possibility of elucidating the pathology and treating non-5q-SMA.

CONCLUSION

Based on accurate diagnosis and a deeper understanding of the pathology of each disease, treatments for non-5q-SMA diseases may be developed in the near future.

Genotype-phenotype correlations of AR-CMT2S in a cohort of axonal Charcot-Marie-Tooth patients from Central South China

BACKGROUND AND AIMS

This study aimed to report nine Charcot-Marie-Tooth disease (CMT) families with six novel IGHMBP2 mutations in our CMT2 cohort and to summarize the genetic and clinical features of all AR-CMT2S patients reported worldwide.

METHODS

General information, clinical and neurophysiological data of 275 axonal CMT families were collected. Genetic screening was performed by inherited peripheral neuropathy related genes panel or whole exome sequencing. The published papers reporting AR-CMT2S from 2014 to 2023 were searched in Pubmed and Wanfang databases.

RESULTS

In our CMT2 cohort, we detected 17 AR-CMT2S families carrying IGHMBP2 mutations and eight were published previously. Among these, c.743 T > A (p.Val248Glu), c.884A > G (p.Asp295Gly), c.1256C > A (p.Ser419*), c.2598_2599delGA (p.Lys868Sfs*16), c.1694_1696delATG (p.Asp565del) and c.2509A > T (p.Arg837*) were firstly reported. These patients prominently presented with early-onset typical axonal neuropathy and without respiratory dysfunction. So far, 56 AR-CMT2S patients and 57 different mutations coming from 43 families have been reported in the world. Twenty-nine of 32 missense mutations were clustered in helicase domain and ATPase region. The age at onset ranged from 0.11to 20 years (Mean ± SD: 3.43 ± 3.88 years) and the majority was infantile-onset (<2 years). The initial symptoms included weakness of limbs (19, 29.7%), delayed milestones (12, 18.8%), gait disturbance (11, 17.2%), feet deformity (8, 12.5%), feet drop (8, 12.5%), etc. INTERPRETATION: AR-CMT2S accounted for 6.2% in our CMT2 cohort. We firstly reported six novel IGHMBP2 mutations which expanded the genotypic spectrum of AR-CMT2S. Furthermore, 17 AR-CMT2S families could provide more resources for natural history study, drug research and development.

Disease Mechanisms and Therapeutic Approaches in SMARD1-Insights from Animal Models and Cell Models

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal childhood motoneuron disease caused by mutations in the IGHMBP2 gene. It is characterized by muscle weakness, initially affecting the distal extremities due to the degeneration of spinal α-motoneurons, and respiratory distress, due to the paralysis of the diaphragm. Infantile forms with a severe course of the disease can be distinguished from juvenile forms with a milder course. Mutations in the IGHMBP2 gene have also been found in patients with peripheral neuropathy Charcot-Marie-Tooth type 2S (CMT2S). IGHMBP2 is an ATP-dependent 5'→3' RNA helicase thought to be involved in translational mechanisms. In recent years, several animal models representing both SMARD1 forms and CMT2S have been generated to initially study disease mechanisms. Later, the models showed very well that both stem cell therapies and the delivery of the human IGHMBP2 cDNA by AAV9 approaches (AAV9-IGHMBP2) can lead to significant improvements in disease symptoms. Therefore, the SMARD1 animal models, in addition to the cellular models, provide an inexhaustible source for obtaining knowledge of disease mechanisms, disease progression at the cellular level, and deeper insights into the development of therapies against SMARD1.

The contribution and therapeutic implications of IGHMBP2 mutations on IGHMBP2 biochemical activity and ABT1 association

Mutations within immunoglobulin mu DNA binding protein (IGHMBP2), an RNA-DNA helicase, result in SMA with respiratory distress type I (SMARD1) and Charcot Marie Tooth type 2S (CMT2S). The underlying biochemical mechanism of IGHMBP2 is unknown as well as the functional significance of IGHMBP2 mutations in disease severity. Here we report the biochemical mechanisms of IGHMBP2 disease-causing mutations D565N and H924Y, and their potential impact on therapeutic strategies. The IGHMBP2-D565N mutation has been identified in SMARD1 patients, while the IGHMBP2-H924Y mutation has been identified in CMT2S patients. For the first time, we demonstrate a correlation between the altered IGHMBP2 biochemical activity associated with the D565N and H924Y mutations and disease severity and pathology in patients and our Ighmbp2 mouse models. We show that IGHMBP2 mutations that alter the association with activator of basal transcription (ABT1) impact the ATPase and helicase activities of IGHMBP2 and the association with the 47S pre-rRNA 5' external transcribed spacer. We demonstrate that the D565N mutation impairs IGHMBP2 ATPase and helicase activities consistent with disease pathology. The H924Y mutation alters IGHMBP2 activity to a lesser extent while maintaining association with ABT1. In the context of the compound heterozygous patient, we demonstrate that the total biochemical activity associated with IGHMBP2-D565N and IGHMBP2-H924Y proteins is improved over IGHMBP2-D565N alone. Importantly, we demonstrate that the efficacy of therapeutic applications may vary based on the underlying IGHMBP2 mutations and the relative biochemical activity of the mutant IGHMBP2 protein.

Exploring the relationship between IGHMBP2 gene mutations and spinal muscular atrophy with respiratory distress type 1 and Charcot-Marie-Tooth disease type 2S: a systematic review

Background

IGHMBP2 is a crucial gene for the development and maintenance of the nervous system, especially in the survival of motor neurons. Mutations in this gene have been associated with spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth disease type 2S (CMT2S).

Methods

We conducted a systematic literature search using the PubMed database to identify studies published up to April 1st, 2023, that investigated the association between IGHMBP2 mutations and SMARD1 or CMT2S. We compared the non-truncating mutations and truncating mutations of the IGHMBP2 gene and selected high-frequency mutations of the IGHMBP2 gene.

Results

We identified 52 articles that investigated the association between IGHMBP2 mutations and SMARD1/CMT2S. We found 6 hotspot mutations of the IGHMBP2 gene. The truncating mutations in trans were all associated with SMARD1.

Conclusion

This study provides evidence that the complete LOF mechanism of the IGHMBP2 gene defect may be an important cause of SMARD1.

Validation of the Pathogenic Effect of IGHMBP2 Gene Mutations Based on Yeast S. cerevisiae Model

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a heritable neurodegenerative disease characterized by rapid respiratory failure within the first months of life and progressive muscle weakness and wasting. Although the causative gene, IGHMBP2, is well defined, information on IGHMBP2 mutations is not always sufficient to diagnose particular patients, as the gene is highly polymorphic and the pathogenicity of many gene variants is unknown. In this study, we generated a simple yeast model to establish the significance of IGHMBP2 variants for disease development, especially those that are missense mutations. We have shown that cDNA of the human gene encodes protein which is functional in yeast cells and different pathogenic mutations affect this functionality. Furthermore, there is a correlation between the phenotype estimated in in vitro studies and our results, indicating that our model may be used to quickly and simply distinguish between pathogenic and non-pathogenic mutations identified in IGHMBP2 in patients.

Models for IGHMBP2-associated diseases: an overview and a roadmap for the future

Models are practical tools with which to establish the basic aspects of a diseases. They allow systematic research into the significance of mutations, of cellular and molecular pathomechanisms, of therapeutic options and of functions of diseases associated proteins. Thus, disease models are an integral part of the study of enigmatic proteins such as immunoglobulin mu-binding protein 2 (IGHMBP2). IGHMBP2 has been well defined as a helicase, however there is little known about its role in cellular processes. Notably, it is unclear why changes in such an abundant protein lead to specific neuronal disorders including spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S). SMARD1 is caused by a loss of motor neurons in the spinal cord that results in muscle atrophy and is accompanied by rapid respiratory failure. In contrast, CMT2S manifests as a severe neuropathy, but typically without critical breathing problems. Here, we present the clinical manifestation of IGHMBP2 mutations, function of protein and models that may be used for the study of IGHMBP2-associated disorders. We highlight the strengths and weaknesses of specific models and discuss the orthologs of IGHMBP2 that are found in different systems with regard to their similarity to human IGHMBP2.

Molecular analysis and clinical diversity of distal hereditary motor neuropathy

BACKGROUND AND PURPOSE

Distal hereditary motor neuropathies (dHMNs) are a clinically and genetically heterogeneous group of disorders. The purpose of this study was to identify the genetic distribution of dHMNs in a large cohort of Chinese patients and provide insight into the underlying common pathophysiology of dHMNs.

METHODS

Multi-gene panel testing or whole-exome sequencing was performed in 70 index patients with clinically diagnosed dHMN between January 2007 and December 2018. The clinical features, Charcot-Marie-Tooth (CMT) neuropathy scores and electrophysiological data at diagnosis were recorded.

RESULTS

Twenty-four causative mutations were identified in 70 index patients with dHMN (34.3%). Mutation in the HSPB1 gene was the most common cause of dHMN. Some CMT genes (MPZ, SH3TC2, GDAP1) were found to be related to dHMN with minor sensory involvement. Patients with a dHMN-plus phenotype (distal motor neuropathy and additional neurological deficits) carried variants in genes related to hereditary spastic paraplegia, amyotrophic lateral sclerosis and spinal muscular atrophy (FUS, KIF5A, KIF1B, ZFYVE26, DNAJB2).

CONCLUSIONS

Comprehensive genetic testing of dHMN patients allows for identification of the pathogenic mutation in one-third of cases. Pure motor neuropathies and motor neuropathies with minor sensory involvement share many genes with CMT disease. Causes for dHMN-plus phenotypes overlap with motor neuron disease.

Diagnostic value of whole-exome sequencing in Chinese pediatric-onset neuromuscular patients

BACKGROUND

Neuromuscular disorders (NMDs) comprise a group of heterogeneous genetic diseases with a broad spectrum of overlapping the clinical presentations that makes diagnosis challenging. Notably, the recent introduction of whole-exome sequencing (WES) is introducing rapid changes on the genetic diagnosis of NMDs. We aimed to investigate the diagnostic value of WES for pediatric-onset NMDs.

METHODS

We applied integrated diagnostic approach and performed WES in 50 Chinese subjects (30 males, 20 females) with undiagnosed pediatric-onset NMDs despite previous specific tests. The patients were categorized in four subgroups according to phenotyping and investigation findings. Variants on NMDs gene list and open exome analysis for those with initial negative findings were identified.

RESULTS

WES identified causative variants in ACTA1 (n = 2), POMT1, COL6A1 (n = 2), MTMR2, LMNA, SELENON, DNM2, TGFB1, MPZ, IGHMBP2, and LAMA2 in 13 patients. Two subjects have variants of uncertain significance (VUSs) in TTN and SCN11A, unlikely to be pathogenic due to incompatible phenotypes. The mean interval time from symptom onset to genetic diagnosis was 10.4 years (range from 1 month to 33 years). The overall diagnostic yield of WES in our cohort was 26%. Open exome analysis was necessary to identify the pathogenic variant in TGFB1 that caused skeletal dysplasia with neuromuscular presentation.

CONCLUSION

Our study shows a clear role of WES in the pathway of integrated diagnostic approach to shorten the diagnostic odyssey in patients with rare NMDs.

Whole genome sequencing reveals novel IGHMBP2 variant leading to unique cryptic splice-site and Charcot-Marie-Tooth phenotype with early onset symptoms

BACKGROUND

Rare variants (RV) in immunoglobulin mu-binding protein 2 (IGHMBP2) [OMIM 600502] can cause an autosomal recessive type of Charcot-Marie-Tooth (CMT) disease [OMIM 616155], an inherited peripheral neuropathy. Over 40 different genes are associated with CMT, with different possible inheritance patterns.

METHODS AND RESULTS

An 11-year-old female with motor delays was found to have distal atrophy, weakness, and areflexia without bulbar or sensory findings. Her clinical evaluation was unrevealing. Whole exome sequencing (WES) revealed a maternally inherited IGHMBP2 RV (c.1730T>C) predicted to be pathogenic, but no variant on the other allele was identified. Deletion and duplication analysis was negative. She was referred to the Undiagnosed Disease Network (UDN) for further evaluation. Whole genome sequencing (WGS) confirmed the previously identified IGHMBP2 RV and identified a paternally inherited non-coding IGHMBP2 RV. This was predicted to activate a cryptic splice site perturbing IGHMBP2 splicing. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis was consistent with activation of the cryptic splice site. The abnormal transcript was shown to undergo nonsense-mediated decay (NMD), resulting in halpoinsufficiency.

CONCLUSION

This case demonstrates the deficiencies of WES and traditional molecular analyses and highlights the advantages of utilization of WGS and functional studies.

Charcot Marie Tooth disease type 2S with late onset diaphragmatic weakness: An atypical case

Immunoglobulin-helicase-μ-binding protein 2 (IGHMBP2) mutations are associated with partial continuum between two extremes of rapidly lethal disorder of spinal muscular atrophy with respiratory distress type 1 (SMARD1), with infantile axonal neuropathy, diaphragmatic weakness and commonly death before 1 year of age, and Charcot-Marie-Tooth disease (CMT) type 2S with slowly progressive weakness and sensory loss but no significant respiratory compromise. We present an atypical case of CMT2S. A 9 month old boy presented with bilateral feet deformities and axonal neuropathy. Genetic testing revealed two heterozygous variants in the IGHMBP2 gene: c.1156 T > C p.(Trp386Arg) in exon 8 and c.2747G > A p.(Cys916Tyr) in exon 14, that were inherited from his father and mother respectively. At 9 years, he developed diaphragmatic weakness, following which he was established on non-invasive ventilation. Our case emphasizes the importance of life long respiratory surveillance for patients with CMT2S and expands the phenotype of this condition.

UBA1/GARS-dependent pathways drive sensory-motor connectivity defects in spinal muscular atrophy

Deafferentation of motor neurons as a result of defective sensory-motor connectivity is a critical early event in the pathogenesis of spinal muscular atrophy, but the underlying molecular pathways remain unknown. We show that restoration of ubiquitin-like modifier-activating enzyme 1 (UBA1) was sufficient to correct sensory-motor connectivity in the spinal cord of mice with spinal muscular atrophy. Aminoacyl-tRNA synthetases, including GARS, were identified as downstream targets of UBA1. Regulation of GARS by UBA1 occurred via a non-canonical pathway independent of ubiquitylation. Dysregulation of UBA1/GARS pathways in spinal muscular atrophy mice disrupted sensory neuron fate, phenocopying GARS-dependent defects associated with Charcot-Marie-Tooth disease. Sensory neuron fate was corrected following restoration of UBA1 expression and UBA1/GARS pathways in spinal muscular atrophy mice. We conclude that defective sensory motor connectivity in spinal muscular atrophy results from perturbations in a UBA1/GARS pathway that modulates sensory neuron fate, thereby highlighting significant molecular and phenotypic overlap between spinal muscular atrophy and Charcot-Marie-Tooth disease.