Mutations in the Immunoglobulin Mu DNA Binding Protein 2 (IGHMBP2) gene cause Spinal Muscular Atrophy with Respiratory Distress type 1 (SMARD1), a rare, infantile, and fatal motor neuron disease, as well as the milder Charcot-Marie-Tooth disease type 2S (CMT2S). Gene therapy has emerged as a promising approach to correcting IGHMBP2 loss in SMARD1 models, but critical challenges remain. In this study, we compared the efficacy of two novel, optimized adeno-associated virus 9 (AAV9)-IGHMBP2 vectors, utilizing either the Chicken β-Actin (CBA) or a truncated form of the methyl-CpG-binding protein 2 (MeCP2) promoter (P546), in the SMARD1 murine model via intracerebroventricular delivery. Treated mice survival, histopathological and molecular profile were analyzed. Corroborating previous findings, both constructs effectively rescued the pathological phenotype, significantly improving survival, body weight, and motor function while preserving motor neurons and neuromuscular junctions. Notably, histopathological and RNA sequencing analyses revealed, for the first time, inflammatory marker alterations in the SMARD1 spinal cord, which resolved following treatment. A comparative analysis of the two vectors demonstrated superior long-term efficacy of the P546-promoter construct. ICV gene therapy approach can effectively rescue SMARD1 pathological hallmarks, including astrogliosis and microgliosis. Moreover, P546-promoter construct is superior in terms of safety profile and long-term therapeutic efficacy.

IGHMBP2-related disorders comprise a clinical spectrum from spinal muscular atrophy with respiratory distress type 1 (SMARD1) to Charcot-Marie-Tooth disease type 2S, with increasingly recognized atypical and overlapping phenotypes. We report four pediatric cases from three unrelated families with biallelic pathogenic variants in IGHMBP2. Case 1, a premature infant represents SMARD1. Case 2 had infantile-onset neuropathy without respiratory symptoms. Case 3 and 4, two siblings, presented with a Guillain-Barré syndrome-like phenotype, cauda equina enhancement on spinal neuroimaging, elevated cerebrospinal fluid protein, and electromyography revealing acute motor and sensory axonal neuropathy. Despite an initial response to intravenous immunoglobulin, previous symptoms in Case 3 led to consideration of an immune-mediated neuropathy superimposed on a genetic background. Genetic analysis identified a homozygous nonsense variant in Cases 1 and 2, and novel compound heterozygous missense variants in Case 3 and 4. Thus, the list of overlapping genetic and acquired neuropathies now also includes IGHMBP2-related CMT2S.

Immunoglobulin mu-binding protein 2 (IGHMBP2) pathogenic variants lead to a spectrum of disorders characterized by alpha-motor neuron degeneration. We describe a compound heterozygous patient diagnosed with Charcot-Marie-Tooth disease type 2S with variants in IGHMBP2: a pathogenic missense variant acting in trans with a confirmed intronic cryptic splice site variant. This variant was shown to result in the creation of a new splice acceptor site, loss of reading frame and nonsense-mediated decay. We designed a 19-mer antisense oligonucleotide targeting this cryptic intronic variant to restore IGHMBP2 levels. ASO treatment of patient fibroblasts significantly increased the ratio of restored wild-type transcript to cryptic exon-containing transcript and resulted in over a 50% increase in IGHMBP2 protein levels. Neuromuscular junction analyses revealed high fatigue and chaotic tetanus formulation in untreated patient cells. We demonstrate rescue of NMJ function following ASO treatment, captured by a reduction in fatigue and chaotic tetanus responses. Furthermore, toxicity testing revealed that intrathecal administration of the ASO to wild-type Sprague-Dawley rats over 3 months was well tolerated. Our preclinical data support this ASO as a potential CMT2S treatment by rescuing IGHMBP2. N-of-1 ASO-based therapeutics may prove instrumental in the design of treatments for this diverse genetic disorder.

Pathogenic variants in IGHMBP2 have been associated with spinal muscular atrophy with respiratory distress type 1 (SMARD1) and Autosomal Recessive Charcot-Marie-Tooth disease type 2S (AR-CMT2S), as well as a relatively wide spectrum of rare, atypical phenotypes. We describe clinical and molecular features of five patients who have diverse clinical findings associated with known and novel IGHMBP2 pathogenic variants. Genotype-phenotype correlations are evident, highlighting the association of specific variants with SMARD1 or AR-CMT2S. This study expands the spectrum of the IGHMBP2-related disease and highlights the necessity to study diverse populations to enhance diagnostic accuracy and refine genotype-phenotype correlations.

Spinal Muscular Atrophy with Respiratory Distress (SMARD1) is a lethal infantile disease, characterized by the loss of motor neurons leading to muscular atrophy, diaphragmatic paralysis, and weakness in the trunk and limbs. Mutations in IGHMBP2, a ubiquitously expressed DNA/RNA helicase, have been shown to cause a wide spectrum of motor neuron disease. Though mutations in IGHMBP2 are mostly associated with SMARD1, milder alleles cause the axonal neuropathy, Charcot-Marie-Tooth disease type 2S (CMT2S), and some null alleles are potentially a risk factor for sudden infant death syndrome (SIDS). Variant heterogeneity studied using an allelic series can be informative in order to create a broad spectrum of models that better exhibit the human variation. We previously identified the nmd2J mouse model of SMARD1, as well as two milder CMT2S mouse models. Here, we used CRISPR-Cas9 genome editing to create three new, more severe Ighmbp2 mouse models of SMARD1, including a null allele, a deletion of C495 (C495del) and a deletion of L362 (L362del). Phenotypic characterization of the IGHMBP2L362del homozygous mutants and IGHMBP2C495del homozygous mutants respectively show a more severe disease presentation than the previous nmd2J model. The IGHMBP2L362del mutants lack a clear denervation in the diaphragm while the IGHMBP2C495del mutants display a neurogenic diaphragmatic phenotype as observed in SMARD1 patients. Characterization of the Ighmbp2-null model indicated neo-natal lethality (median lifespan = 0.5 days). These novel strains expand the spectrum of SMARD1 models to better reflect the clinical continuum observed in the human patients with various IGHMBP2 recessive mutations.