Key result
The presence of 4 copies of the SMN2 gene compensated for the homozygous deletion of the SMN1 gene, resulting in a milder disease phenotype despite the absence of the NAIP gene.
Case Report (n=1)
The presence of 4 copies of the SMN2 gene may compensate for SMN1 deletion and determine disease severity more critically than the NAIP genotype in spinal muscular atrophy.
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Hypothesis-generating in SMA; leaves open whether SMN2 copy number reliably predicts milder phenotypes independent of NAIP.
The patient was a 57-year-old Japanese man who gradually developed muscle atrophy and weakness in the trunk and limbs since the age of 20 years and was wheelchair bound at the age of 56 years. The gene copy number assay confirmed the combined homozygous deletion of the survival motor neuron (SMN) 1 and neuronal apoptosis inhibitory protein (NAIP) genes and showed the presence of 4 copies of the SMN2 gene. In this patient, the significant increase in the number of the SMN2 gene copies should compensate for the homozygous deletion of the SMN1 gene and make his disease milder despite the absence of the NAIP gene. Taken together with our previous data, we may reasonably hypothesize that the SMN2 gene copy number is more critical in determining the severity of the disease compared to the NAIP genotype.
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Yamashita et al. (2004) conducted a case report in Adult-Onset Type III Spinal Muscular Atrophy (n=1). 4 copies of the SMN2 gene was evaluated on Disease severity. The presence of 4 copies of the SMN2 gene compensated for the homozygous deletion of the SMN1 gene, resulting in a milder disease phenotype despite the absence of the NAIP gene.
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