SMN2

SMN2 is a protein-coding gene in the 5q13.2 region of chromosome 5 and is a paralog of SMN1. The two genes differ at only a small number of nucleotides, but a synonymous C-to-T change in exon 7 has a major functional effect.¹³ This change disrupts efficient recognition of exon 7 during pre-mRNA splicing, so most SMN2 transcripts omit that exon and encode an unstable, truncated protein.

A minority of SMN2 transcripts include exon 7 and produce functional full-length SMN protein. SMN participates in assembly of spliceosomal small nuclear ribonucleoproteins and supports RNA processing in many cell types; motor neurons appear especially vulnerable when its level is substantially reduced.² SMN2 therefore does not simply act as a duplicate gene: its unusually inefficient splicing makes it a dosage-sensitive reserve for SMN production.

SMN2 modifies, but does not by itself determine, the clinical expression of spinal muscular atrophy (SMA). SMA usually results from biallelic pathogenic loss-of-function variants in SMN1; SMN2 copy number can partly compensate by supplying additional full-length SMN transcripts.² In broad population patterns, fewer copies are associated with earlier and more severe disease, whereas more copies tend to be associated with later onset and milder weakness.

The relationship is probabilistic rather than absolute. Individuals with the same SMN2 copy number can have different phenotypes because of modifier variants, residual SMN1 function, differences in disease ascertainment, and other biological factors.² Copy-number testing is consequently useful for prognosis and trial stratification, but it cannot replace clinical assessment. Rare SMN2 alleles that favor exon 7 inclusion can also produce more functional protein than ordinary copies.

Modern SMA treatments target the disease mechanism by increasing functional SMN or supplying an external SMN1 copy. Nusinersen is an antisense oligonucleotide that binds a regulatory sequence in SMN2 intron 7 and promotes inclusion of exon 7 in the mature transcript.⁴ Risdiplam is an orally administered small molecule that modifies SMN2 pre-mRNA splicing, increasing production of full-length SMN protein throughout the body.

These approaches illustrate why SMN2 is a therapeutic target rather than merely a genetic marker. Treatment is generally more effective when begun before substantial motor-neuron loss, which has made newborn screening and rapid confirmatory testing important in many health systems.² Gene-replacement therapy with onasemnogene abeparvovec acts mainly through delivery of an SMN1-derived transgene, so it complements rather than directly corrects SMN2 splicing.

SMN2 is unusually variable in copy number because the surrounding 5q13 region contains duplicated, structurally unstable DNA. This makes dosage analysis technically more demanding than a simple single-site variant test; laboratories commonly use methods capable of distinguishing SMN1 from SMN2 and counting copies.¹²

The exon 7 defect is synonymous at the protein-coding level, yet it changes RNA processing, making SMN2 a classic example of a disease-relevant splicing variant that does not alter an amino acid. Researchers have also identified regulatory sequences and splicing factors that influence the proportion of exon-containing transcripts, helping explain why copy number alone is an imperfect predictor. SMN2-related research has consequently contributed to the wider development of antisense and small-molecule therapies for inherited disorders of pre-mRNA splicing.³⁴

Paralog

A gene related to another gene by duplication within a genome.

Pre-mRNA splicing

The RNA-processing step that removes introns and joins selected exons before translation.

Full-length SMN

The functional survival motor neuron protein produced when exon 7 is retained in the transcript.

SMN2 copy number

The number of SMN2 gene copies detected in an individual; it is a modifier of SMA phenotype, not a complete predictor.