Prenatal diagnosis of spinal muscular atrophy in Chinese by genetic analysis of fetal cells

Background Spinal muscular atrophy （SMA） is an autosomal recessive disease characterized by degeneration of anterior horn cells of the spinal cord. The survival motor neuron gene is SMA-determining gene deleted in approximately 95% of SMA patients. This study was undertaken to predict prenatal SMA efficiently and rapidly in families with previously affected child. Methods Prenatal diagnosis was made in 8 fetuses with a family history of SMA. Polymerase （PCR） and restriction fragment length polymorphism （RFLP） were used for the detection of the neuron gene. Results The survival motor neuron fetuses were detected positive and the gene was not found in 6 fetuses, ruling out the diagnosis of SMA. Two fetuses were detected positive and the pregnancies were terminated. Conclusion Our method is effective and convenient in prenatal diagnosis of SMA.

Deletion analysis of SMN1 and NAIP genes in southern Chinese children with spinal muscular atrophy

Spinal muscular atrophy （SMA） is a disorder characterized by degeneration of lower motor neurons and occasionally bulbar motor neurons leading to progressive limb and trunk paralysis as well as muscular atrophy. Three types of SMA are recognized depending on the age of onset, the maximum muscular activity achieved, and survivorship： SMA1, SMA2, and SMA3. The survival of motor neuron （SMN） gene has been identified as an SMA determining gene, whereas the neuronal apoptosis inhibitory protein （NAlP） gene is considered to be a modifying factor of the severity of SMA. The main objective of this study was to analyze the deletion of SMN1 and NAIP genes in southern Chinese children with SMA. Here, polymerase chain reaction （PCR） combined with restriction fragment length polymorphism （RFLP） was performed to detect the deletion of both exon 7 and exon 8 of SMN1 and exon 5 of NAIP in 62 southern Chinese children with strongly suspected clinical symptoms of SMA. All the 32 SMA1 patients and 76% （13/17） of SMA2 patients showed homozygous deletions for exon 7 and exon 8, and all the 13 SMA3 patients showed single deletion of SMNI exon 7 along with 24% （4/17） of SMA2 patients. Eleven out of 32 （34%） SMA1 patients showed NAIP deletion, and none of SMA2 and SMA3 patients was found to have NA1P deletion. The findings of homozygous deletions ofexon 7 and/or exon 8 ofSMN1 gene confirmed the diagnosis of SMA, and suggested that the deletion ofSMN1 exon 7 is a major cause of SMA in southern Chinese children, and that the NAIP gene may be a modifying factor for disease severity of SMAI. The molecular diagnosis system based on PCR-RFLP analysis can conveniently be applied in the clinical testing, genetic counseling, prenatal diagnosis and preimplantation genetic diagnosis of SMA.

Compound heterozygous mutation in two unrelated cases of Chinese spinal muscular atrophy patients

Background Infantile proximal spinal muscular atrophy （SMA） is a common autosomal recessive neuromuscular disorder. Approximately 90-95% cases of SMA result from homozygous deletion of survival motor neuron gene 1（SMN1） and 5% cases are caused by compound heterozygous mutation （a SMN1 deletion on one allele and a subtle mutation on the other allele）.Methods In this research, two unrelated patients were clinically diagnosed according to the criteria of proximal SMA. Genetic diagnosis was performed to detect the homozygous deletion of exon 7 of SMN1 by PCR-restriction fragment length polymorphism （RFLP） and genomic sequencing. Multiplex ligation-dependent probe amplification （MLPA） analysis was carried out to measure copy numbers of SMN1, SMN2 and neuronal apoptosis inhibitor protein （NAIP） in the patients. Further sequencing of SMN1allele-specific PCR （AS-PCR） and SMN1 clones were also performed to analyze the point mutation of SMN1 gene. Additionally,the pedigree analysis of these two families was carried out to identify the transmission of the mutation.Results The inconsistent results using PCR-RFLP and genomic sequencing showed homozygous deletion of exon 7 of SMN1 and heterozygous deletion accompanied with a suspicious mutation in SMN1 gene, respectively. MLPA analysis of these two cases exhibited one SMN1 copy deletion. One identical c.863G〉T （p. Arg288Met） mutation was found in two cases by sequencing the SMN1 clones, which confirmed that both cases were SMA compound heterozygotes. One case showed partial conversion to form hybrid SMN （SMN2 17/SMN1 E8） identified by clones sequencing and another case carrying 3 SMN2 implied complete conversion from SMN1 to SMN2.Conclusion p. Arg288Met is more a disease-causing mutation than a polymorphism variation, and children with this mutation may have more severe phenotypes.