Kentaro Okamoto
Spinal muscular atrophy (SMA) results from biallelic loss of functional survival motor neuron 1 (SMN1), reducing production of full-length survival motor neuron (SMN) protein and leaving endogenous SMN supply largely dependent on inefficient survival motor neuron 2 (SMN2) splicing. This review conceptualizes SMA as a disorder in which available SMN supply fails to meet tissue- and stage-specific SMN requirement, defined as the minimum effective SMN level needed to maintain normal cellular or tissue function. Disease-modifying therapies (DMTs) have transformed SMA outcomes, particularly when initiated presymptomatically or very early after symptom onset. However, treatment responses remain heterogeneous, especially among infants with two SMN2 copies and patients treated after symptomatic progression. These observations suggest that therapeutic response is shaped not only by SMN restoration but also by timing, residual SMN reserve, and tissue vulnerability already present at treatment initiation. This review argues that SMN requirement varies across tissues, developmental stages, and biological states. It proposes a three-component framework: developmental mismatch, post-developmental dynamic mismatch, and tissue-state transitions. Developmental mismatch is the most directly supported component and highlights tissue- and stage-specific vulnerability before and shortly after birth, particularly in the motor unit; early SMN insufficiency may leave latent developmental vulnerability even after early DMT. Post-developmental dynamic mismatch is presented as an evidence-informed, hypothesis-generating extension in which constrained SMN supply may be sufficient at baseline but inadequate during maintenance, remodeling, repair, intercurrent illness, or catabolic stress. Tissue-state transitions distinguish preserved tissue, functionally impaired but structurally retained tissue, and structurally degenerated tissue with limited reversibility. Together, this framework may help interpret residual deficits after early treatment, incomplete recovery after delayed treatment, and the need for multimodal biomarkers and longitudinal monitoring strategies that distinguish potentially reversible functional impairment from structural degeneration with limited reversibility.