Analytical Study Showing a False-Negative Limitation of Deletion-Based Newborn Screening for Spinal Muscular Atrophy Using a Compound Heterozygous SMN1 Control

Abstract

Newborn screening (NBS) facilitates the presymptomatic diagnosis and early treatment of spinal muscular atrophy (SMA). An analytical study was conducted to evaluate the performance and limitations of a real-time polymerase chain reaction-based SMA-NBS assay. Dried blood spot specimens collected from 21 newborns and three patients with genetically confirmed SMA were analyzed. The SMA controls included two patients with homozygous SMN1 deletion and one with compound heterozygous SMN1 variants. SMN1 exon 7 copy numbers were quantified and interpreted. All newborn participants had SMN1 copy numbers above the screening cutoff value of 668 copies/µL and were classified as screening-negative. The two SMA controls with homozygous SMN1 deletion showed no detectable SMN1 amplification, which is consistent with positive screening results. In contrast, the compound heterozygous SMA control exhibited a mean SMN1 value of 7330 copies/µL, which was more than tenfold above the screening cutoff and would therefore have been classified as screening-negative; this value also fell within the range observed in the newborn cohort. This patient had a negative NBS result at birth despite subsequently developing genetically confirmed SMA. The SMA-NBS assay accurately detected patients with homozygous SMN1 deletion. However, it did not identify patients with compound heterozygous SMN1 variants. In conclusion, deletion-based SMA-NBS has an inherent false-negative limitation, and a negative screening result does not completely exclude an SMA diagnosis.