Beyond SMN1 mutation: environmental predispositions and epigenetic contributors to phenotypic variability and disease severity in spinal muscular atrophy.

Spinal Muscular Atrophy (SMA) is an autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in the survival motor neuron 1 gene (SMN1), and with SMN2 being a known principal modifier yet significant phenotypic variability exists even among individuals sharing identical genetic backgrounds, including siblings, suggesting that factors beyond SMN2 copy number contribute substantially to clinical heterogeneity. Eventhough with only lesser direct relevances, emerging evidences implicate environmental exposures and epigenetic mechanisms as key modulators of this variability. Hypoxia accelerates disease progression by promoting SMN2 exon 7 skipping through upregulation of the splicing repressors hnRNP A1 and Sam68, while metabolic dysfunctions including hepatic steatosis and dyslipidemia, independent of motor neuron degeneration, have been reported in approximately one third of SMA patients at autopsy. Differential DNA methylation at CpG sites upstream of the SMN2 promoter correlates with disease severity, identifying a potential epigenetic prognostic biomarker. The three currently approved disease-modifying therapies, nusinersen, risdiplam, and onasemnogene abeparvovec, have fundamentally transformed SMA management, yet their efficacy may be modulated by environmental factors including nutritional status, respiratory function, infection burden, and pre-existing immunity to viral vectors, underscoring the need for integrated environmental and epigenetic assessment in SMA clinical care and research. This review integrates SMA-specific evidence with mechanistically plausible hypotheses drawn from related neurodegenerative conditions, clearly distinguishing established findings from emerging concepts. We propose that integrating environmental monitoring, nutritional optimization, and epigenetic modulation with genetic therapies may enable precision medicine approaches that maximize therapeutic benefit across the SMA clinical spectrum.