The impact of three gene copies on clinical characteristics and effect of disease-modifying treatment in patients with spinal muscular atrophy: a systematic literature review.

OBJECTIVE

To review the clinical characteristics and effect of treatment in patients with spinal muscular atrophy (SMA) and three copies of the gene.

METHODS

We conducted a literature search in October 2022 to identify English-language clinical research on SMA that included copy number according to PRISMA guidelines.

RESULTS

Our search identified 44 studies examining the impact of three copies on clinical characteristics (21 on phenotype, 13 on natural history, and 15 on functional status and other signs/symptoms). In children with type I SMA or presymptomatic infants with an deletion, three copies was associated with later symptom onset, slower decline in motor function and longer survival compared with two copies. In patients with SMA type II or III, three copies is associated with earlier symptom onset, loss of ambulation, and ventilator dependence compared with four copies. Eleven studies examined treatment effects with nusinersen (nine studies), onasemnogene abeparvovec (one study), and a range of treatments (one study) in patients with three copies. In presymptomatic infants, early treatment delayed the onset of symptoms and maintained motor function in those with three copies. The impact of copy number on treatment response in symptomatic patients is still unclear.

CONCLUSION

copy number is strongly correlated with SMA phenotype in patients with deletion, while no correlation was found in patients with an mutation. Patients with three copies show a highly variable clinical phenotype. Early initiation of treatment is highly effective in presymptomatic patients with three copies.