We performed a qualitative and quantitative SMN1/SMN2 gene analysis in 13 unrelated HirD patients. The phenotype–genotype correlation was investigated, paying particular attention to the effect of the SMN1/SMN2 copy number on the disease's phenotype.
No patient had a homozygous deletion of the SMN1 or SMN2. No differences were found when comparing the SMN1 and SMN2 copy number distributions of the healthy population and HirD patients, and they do not therefore appear to be a susceptibility factor. There was also no correlation found between the number of copies of the SMN1 and SMN2 and the severity of the resulting phenotype.
Our results suggest that SMN1 and SMN2 are not predisposing factors for HirD and therefore support a lack of association between these genes and the resulting phenotype.