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

Spatiotemporal deciphering of dynamic the FUS interactome during liquid-liquid phase separation in living cells

Liquid-liquid phase separations (LLPS) are membraneless organelles driven by biomolecule assembly and are implicated in cellular physiological activities. However, spatiotemporal deciphering of the dynamic proteome in living cells during LLPS formation remains challenging. Here, we introduce the Composition of LLPS proteome Assembly by Proximity labeling-assisted Mass spectrometry (CLAPM). We demonstrate that CLAPM can instantaneously label and monitor the FUS interactome shifts within intracellular droplets undergoing spatiotemporal LLPS. We report 129, 182 and 822 proteins specifically present in the LLPS droplets of HeLa, HEK 293 T and neuronal cells respectively. CLAPM further categorizes spatiotemporal dynamic proteome in droplets for living neuronal cells and identifies 596 LLPS-aboriginal proteins, 226 LLPS-dependent proteins and 58 LLPS-sensitive proteins. For validation, we uncover 11 previously unknown LLPS proteins in vivo. CLAPM provides a versatile tool to decipher proteins involved in LLPS and enables the accurate characterization of dynamic proteome in living cells.

A cross-sectional survey on the health status of patients with Charcot-Marie-Tooth disease in a Chinese national patient group

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is a rare inherited peripheral neuropathy, and the health status of CMT patients in China is not well understood without a national disease registry system. We aimed to obtain the related epidemiological data to support effective work on CMT.

METHODS

The online cross-sectional study included patients definitively diagnosed with CMT nationwide. Descriptive analyses were conducted on CMT's disease characteristics, diagnostic results, walking condition, rehabilitation status, comorbidities, family history, etc. RESULTS: CMT1A, CMT2A, CMTX1, CMT2S, CMT1E, and CMT1B were the top six types accounting for 64.4% of the 523 eligible patients. PMP22, MFN2, GJB1, MPZ, GDAP1, and IGHMBP2 ranked as the top six genes among the collected 44 pathogenic genes. The median ages of symptom onset and diagnosis were 7.3 and 18.7 years, respectively, with a median interval of 3.8 years between symptom onset and genetic confirmation. Only 8.3% exhibited unaffected walking speed and balance, the remaining experienced varying degrees of motor impairment, and 42.1% employed rehabilitation. Moreover, 26.8% experienced initial misdiagnosis, and 47.0% were estimated to suffer from depression. Of comorbidities complained by the 94 patients, gastrointestinal was most common (17/94) followed by hypertension (13/94), and hiatal hernia (2/94) was first reported. Family history was documented in 35.2% of the surveyed patients.

CONCLUSION

Chinese patients with CMT were in complicated and poor health status with predominant disease types and pathogenic genes generally as anticipated. A national CMT registry system is highly wanted to collect comprehensive information to guide further research and improve patients' health status.

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.

The molecular mechanisms that underlie IGHMBP2-related diseases

Immunoglobulin Mu-binding protein 2 (IGHMBP2) pathogenic variants result in the fatal, neurodegenerative disease spinal muscular atrophy with respiratory distress type 1 (SMARD1) and the milder, Charcot-Marie-Tooth (CMT) type 2S (CMT2S) neuropathy. More than 20 years after the link between IGHMBP2 and SMARD1 was revealed, and 10 years after the discovery of the association between IGHMBP2 and CMT2S, the pathogenic mechanism of these diseases is still not well defined. The discovery that IGHMBP2 functions as an RNA/DNA helicase was an important step, but it did not reveal the pathogenic mechanism. Helicases are enzymes that use ATP hydrolysis to catalyse the separation of nucleic acid strands. They are involved in numerous cellular processes, including DNA repair and transcription; RNA splicing, transport, editing and degradation; ribosome biogenesis; translation; telomere maintenance; and homologous recombination. IGHMBP2 appears to be a multifunctional factor involved in several cellular processes that regulate gene expression. It is difficult to determine which processes, when dysregulated, lead to pathology. Here, we summarise our current knowledge of the clinical presentation of IGHMBP2-related diseases. We also overview the available models, including yeast, mice and cells, which are used to study the function of IGHMBP2 and the pathogenesis of the related diseases. Further, we discuss the structure of the IGHMBP2 protein and its postulated roles in cellular functioning. Finally, we present potential anomalies that may result in the neurodegeneration observed in IGHMBP2-related disease and highlight the most prominent ones.

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.

Yeast Models and Molecular Mechanisms of Neurodegenerative Diseases 2.0

One of the goals of human genetics is to discover the variants that contribute to human diseases [...].

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.