Diagnosis of Challenging Spinal Muscular Atrophy Cases with Long-Read Sequencing

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder primarily caused by the deletion or mutation of the survival motor neuron 1 ( SMN1 ) gene. This study assesses the diagnostic potential of long -read sequencing (LRS) in three patients with SMA. For Patient 1, who has a heterozygous SMN1 deletion, LRS unveiled a missense mutation in SMN1 exon 5. In Patient 2, an Alu/Alu -mediated rearrangement covering the SMN1 promoter and exon 1 was identi fi ed through a blend of multiplex ligation -dependent probe ampli fi cation, LRS, and PCR across the breakpoint. The third patient, born to a consanguineous family, bore four copies of hybrid SMN genes. LRS determined the genomic structures, indicating two distinct hybrids of SMN2 exon 7 and SMN1 exon 8. However, a discrepancy was found between the SMN1 / SMN2 ratio interpretations by LRS (0:2) and multiplex ligation -dependent probe ampli fi cation (0:4), which suggested a limitation of LRS in SMA diagnosis. In conclusion, this newly adapted long PCR-based third -generation sequencing introduces an additional avenue for SMA diagnosis. (J Mol Diagn 2024, 26: 364 - 373; https://doi.org/10.1016/j.jmoldx.2024.02.004)