A single strand conformation polymorphism-based carrier test for spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disorder with a new born prevalence of 1 in 10,000, and a carrier frequency of 1 in 40-60 indiv iduals. The SMA locus has been mapped to chromosome 5q11.2-13, The disease is caused by a deletion of the SMN gene, often encompassing other genes and microsatellite markers. The SMN gene is present in two highly homologous c opies, SMN1 and SMN2, differing at five nucleotide positions. Only homozygo us SMN1 mutations cause the disease. The sequence similarity between the SM N1 and SMN2 genes can make molecular diagnosis and carrier identification d ifficult, We developed a sensitive and reliable molecular test for SMNI car rier identification, by setting up a nonradioactive single strand conformat ion polymorphism (SSCP)-based method, which allows for the quantification o f the amount of the SMNI gene product with respect to a control gene. The a ssay was validated in 56 obligate (ascertained) carriers and 20 (ascertaine d) noncarriers, The sensitivity of the test is 96.4%, and its specificity, 98%. In addition, 6 of 7 SMA patients without homozygous deletions presente d with a heterozygous deletion, suggesting a concomitant undetected point m utation on the nondeleted SMNI allele, Therefore, the present test is effec tive for detecting compound hemizygote patients, for testing carriers in SM A families, and for screening for SMA heterozygotes in the general populati on.