RNA helicase IGHMBP2 regulates THO complex to ensure cellular mRNA homeostasis.

RNA helicases constitute a large protein family implicated in cellular RNA homeostasis and disease development. Here, we show that the RNA helicase IGHMBP2, linked to the neuromuscular disorder spinal muscular atrophy with respiratory distress type 1 (SMARD1), associates with polysomes and impacts translation of mRNAs containing short, GC-rich, and structured 5' UTRs. The absence of IGHMBP2 causes ribosome stalling at the start codon of target mRNAs, leading to reduced translation efficiency. The main mRNA targets of IGHMBP2-mediated regulation encode for components of the THO complex (THOC), linking IGHMBP2 to mRNA production and nuclear export. Accordingly, failure of IGHMBP2 regulation of THOC causes perturbations of the transcriptome and its encoded proteome, and ablation of THOC subunits phenocopies these changes. Thus, IGHMBP2 is an upstream regulator of THOC. Of note, IGHMBP2-dependent regulation of THOC is also observed in astrocytes derived from patients with SMARD1 disease, suggesting that deregulated mRNA metabolism contributes to SMARD1 etiology and may enable alternative therapeutic avenues.

In Vitro Modeling as a Tool for Testing Therapeutics for Spinal Muscular Atrophy and IGHMBP2-Related Disorders.

Spinal Muscular Atrophy (SMA) is the leading genetic cause of infant mortality. The most common form of SMA is caused by mutations in the SMN1 gene, located on 5q (SMA). On the other hand, mutations in IGHMBP2 lead to a large disease spectrum with no clear genotype-phenotype correlation, which includes Spinal Muscular Atrophy with Muscular Distress type 1 (SMARD1), an extremely rare form of SMA, and Charcot-Marie-Tooth 2S (CMT2S). We optimized a patient-derived in vitro model system that allows us to expand research on disease pathogenesis and gene function, as well as test the response to the AAV gene therapies we have translated to the clinic. We generated and characterized induced neurons (iN) from SMA and SMARD1/CMT2S patient cell lines. After establishing the lines, we treated the generated neurons with AAV9-mediated gene therapy (AAV9.SMN (Zolgensma) for SMA and AAV9.IGHMBP2 for IGHMBP2 disorders (NCT05152823)) to evaluate the response to treatment. The iNs of both diseases show a characteristic short neurite length and defects in neuronal conversion, which have been reported in the literature before with iPSC modeling. SMA iNs respond to treatment with AAV9.SMN in vitro, showing a partial rescue of the morphology phenotype. For SMARD1/CMT2S iNs, we were able to observe an improvement in the neurite length of neurons after the restoration of IGHMBP2 in all disease cell lines, albeit to a variable extent, with some lines showing better responses to treatment than others. Moreover, this protocol allowed us to classify a variant of uncertain significance on IGHMBP2 on a suspected SMARD1/CMT2S patient. This study will further the understanding of SMA, and SMARD1/CMT2S disease in particular, in the context of variable patient mutations, and might further the development of new treatments, which are urgently needed.