Prenatal prediction of spinal muscular atrophy in Chinese

Copy number analysis of survival motor neuron genes by multiplex ligation-dependent probe amplification

PURPOSE

To determine the copy number of survival motor genes using multiplex ligation-dependent probe amplification.

METHODS

Three hundred seventy-three subjects were recruited and divided into three groups. Group 1 included 310 subjects without a history of muscular atrophy, Group 2 consisted of 18 patients and 45 carriers of spinal muscular atrophy, and Group 3 included 20 subjects who were previously tested with denatured high-performance liquid chromatography. The copy number of survival motor neuron 1 and survival motor neuron 2 genes was determined with a commercially available multiplex ligation-dependent probe amplification kit.

RESULTS

Twenty-one genotypes of the survival motor neuron genes could be clearly defined in this series. The whole process of genotyping took <48 hours. In Group 1, 2:2 (survival motor neuron 1:survival motor neuron 2) was most common (52.90%), followed by 2:1 (30.32%); six (1.94%) subjects were found to be carriers of 1:2 or 1:3. In Group 2, all 18 patients had zero copies of the survival motor neuron 1 gene and variable copies of the survival motor neuron 2 gene. In Group 3, three subjects who had been told they were carriers of spinal muscular atrophy turned out to be noncarriers by multiplex ligation-dependent probe amplification. All 51 carriers from Groups 1 and 2 had one copy of the survival motor neuron 1 gene and one to four copies of the survival motor neuron 2 gene.

CONCLUSION

Multiplex ligation-dependent probe amplification is a simple and efficient method for copy number analysis of survival motor neuron genes. It can be used to detect the homozygous and heterozygous survival motor neuron deletion of spinal muscular atrophy.

Analysis of the mRNA transcripts of the survival motor neuron (SMN) gene in the tissue of an SMA fetus and the peripheral blood mononuclear cells of normals, carriers and SMA patients

Spinal muscular atrophy (SMA) is a disorder characterized by degeneration of the anterior horn cells of the spinal cord. The gene most highly associated with SMA is the survival motor neuron (SMN) gene. In this study, we present an analysis of messenger RNA (mRNA) expression of the SMN gene in peripheral blood mononuclear cells in normal subjects, SMA carriers and patients from 20 SMA families. We found at least 6-8 different transcripts of SMN gene formed by alternative splicing involving exons 3, 5 and 7. We compared transcripts from the different types of SMA and found no definite differences in transcript patterns and amounts. Normal subjects with the telomeric SMN (SMN(T)) gene only had variable splicing resulting in several transcripts, the most dominant being a transcript containing all coding regions. However, SMA patients with the centromeric SMN (SMN(C)) gene only had a higher degree of splice variation and tended to show little or no exon 7. These results demonstrate that SMN(T) and SMN(C) genes participate in alternative splicing phenomena. The different splicing patterns support the view that the SMN(T) gene is responsible for SMA disease. We also analyzed the transcripts from several tissues of an SMA fetus who had a homozygous SMN(T) gene deletion. Different splicing patterns were also found in these tissues, and were similar to the splicing pattern of leukocytes. We compared the major transcripts from exons 4 to 8 of both the SMN(T) and SMN(C) genes and found that the relative proportion varied among normal subjects, SMA carriers and patients. This approach could be used as a novel diagnostic method. We suggest that analyzing the mRNA expression of the SMN gene in peripheral blood mononuclear cells offers an apparently reliable technique for separating SMA patients, carriers, and normal individuals.