Survival of motor neuron 2, centromeric (SMN2)

SMN2 (Survival of motor neuron 2, centromeric) is a protein-coding gene located on chromosome 5q13 as part of a 500 kb inverted duplication region. It is nearly identical to its paralog SMN1, differing by only a single critical nucleotide (C to T transition) in exon 7 that functions as an exon splice enhancer. This single nucleotide difference has profound functional consequences: while SMN1 produces predominantly full-length functional SMN protein, SMN2 generates approximately 80-90% truncated, unstable protein (SMNΔ7) that lacks exon 7 and is rapidly degraded, with only 10-20% of transcripts producing full-length functional protein. The SMN protein encoded by SMN2 has the chemical formula C₁₃₉₄H₂₁₅₄O₄₃₈N₃₈₄S₁₃ and localizes to both cytoplasm and nucleus, where it concentrates in subnuclear structures called gems near Cajal bodies. The protein contains GEMIN2-binding, Tudor, and YG-Box domains and forms the core of the SMN complex, which serves as the primary cellular machinery for assembling small nuclear ribonucleoproteins (snRNPs), the essential building blocks of the spliceosome. While mutations in SMN2 itself do not cause disease (unlike SMN1 mutations which cause spinal muscular atrophy), SMN2 functions as a critical genetic modifier of SMA severity. Humans typically have 1-2 copies of SMN2, though copy numbers can range up to 8, and higher copy numbers correlate with increased SMN protein production and milder disease phenotypes. This relationship makes SMN2 an important therapeutic target, as strategies to increase functional SMN protein production from SMN2 can compensate for SMN1 deficiency. The gene produces multiple transcript variants, but only the full-length isoform d is functionally stable, while other variants are quickly degraded.

Other names

Survival Motor Neuron 2SMN2 geneCentromeric copy of SMNSurvival of motor neuron 2

Splice modulation - therapies aim to increase the proportion of full-length SMN protein produced from SMN2 by modifying exon 7 splicing. Gene compensation - SMN2 acts as a natural backup gene producing approximately 10-15% of functional SMN protein. Copy number-dependent protein production - higher SMN2 copy numbers correlate with increased SMN protein levels and less severe disease.