Apparent SMA I unlinked to 5q

A proband with a clinical picture indistinguishable from SMA type I is described. The parents are second cousins. On DNA analysis it appeared that the proband and his healthy 2 year old sib had inherited the same haplotypes for DNA markers flanking the SMA locus on 5q. This supports non-linkage of SMA to chromosome 5q in this family. The consanguinity of the parents raises the possibility of a second locus for autosomal recessive SMA type I outside the 5q12-13 region. This may have implications for genetic counselling after prenatal diagnosis in consanguineous families. Furthermore, this case illustrates the importance of the inclusion of all healthy sibs in prenatal DNA studies for SMA type I.