[Prenatal diagnosis of 5 fetuses with high risk of developing spinal muscular atrophy]

To perform prenatal diagnosis for 5 pregnant women who had given birth to children with spinal muscular atrophy (SMA).

Thirty to forty mililiters of amniotic fluid was obtained by amniocentesis under ultrasonic monitoring. DNA was extracted directly from sediment of amniotic fluid. Short tandem repeat (STR) profiling was carried out to evaluate the contamination of amniotic DNA by maternal genomic DNA. Two methods, PCR-restriction fragment length polymorphism (PCR-RFLP) and allele-specific PCR, were used to analyze exon 7 of SMN gene from amniotic DNA.

Comparing the 16 STR sites of each fetus with those of his/her parents, there was no or little contamination of amniotic DNA by maternal genomic DNA. In conventional PCR-RFLP, part of the PCR product (189 bp) from amniotic DNA of fetus A, C, or D remained intact after digestion with Dra I, while the PCR product from amniotic DNA of fetus B or E was completely digested by Dra I. In allele-specific PCR, exon 7 of both SMN1 and SMN2 gene could be seen when amniotic DNA of fetuses A, C, or D was analyzed, while only exon 7 of SMN2 could be seen when amniotic DNA of fetuses B or E was analyzed.

Homozygous deletion of SMN1 is not detected in fetuses A, C, and D, predicting that the risk of developing SMA after birth would be extremely low. Homozygous deletion of SMN1 was present in fetuses B and E suggesting high risk of developing SMA after birth.