Decreased expression of full‐length mRNA for ^c BCD541 does not correlate with spinal muscular atrophy phenotype severity

Abstract

Spinal muscular atrophy (SMA) is characterized by degeneration of spinal cord anterior horn cells and muscular atrophy and has three phenotypes based on clinical severity and age of onset. One of the responsible genes for SMA is the survival motor neuron (SMN) gene, which is homozygously absent or interrupted in more than 90% of SMA patients. The cBCD541 (BCD) gene is a highly homologous copy of the SMN gene, which has a single synonymous transition in the coding region and may compensate for the loss of the SMN gene. To evaluate the effects of the BCD gene expression on the phenotypes of SMA, we examined lymphocyte mRNA from 9 SMA patients lacking the SMN gene, 10 asymptomatic parents, and 15 control subjects. We amplified mRNA fragments containing exon 7 of the SMN or BCD genes using reverse transcription-polymerase chain reaction since the transcript lacking exon 7 encodes a putative protein with a different C-terminal end. We used glyceraldehyde-3-phosphate dehydrogenase (GAPDH) transcript as an internal control, and the relative expression level of the SMN or BCD gene was shown as the ratio of SMN or BCD transcript to GAPDH transcript (S/G ratio). The mean S/G ratios of the patients were significantly lower than that of the parents and controls. However, among the patients examined in this study, there was no relationship between the S/G ratios and phenotypes of SMA. The results showed that the BCD gene expression was not related to the phenotypes of SMA. Furthermore, there was an overlap between the S/G ratios in patients and controls. As our discrimination study showed that the S/G ratio reflected the expression of the BCD transcripts in patients and the SMN transcripts in controls, this finding suggested that the BCD gene expression per se does not compensate for the loss of the SMN gene.

Correlation between SMN2 copy number and clinical phenotype of spinal muscular atrophy: three SMN2 copies fail to rescue some patients from the disease severity

“…It has been shown in studies using lymphocytes and lymphoblastoid cells from SMA patients that there is no significant difference in the amount of full-length mRNA species among patients with type I-II and III SMA [20,41]. Gavrilov et al [20] have observed that the ratio of the truncated mRNA species to full-length mRNA species was significantly higher in type I-II than in type III SMA, and that type I-II patients appeared to produce significantly more truncated mRNA species than type III patients.…”

“…s Messenger RNA analysis Total RNA was extracted from leukocytes using the acid guanidium thiocyanate-phenol-chloroform method, and SMN1 or SMN2 mRNA species were amplified by reverse transcriptase (RT)-PCR according to the method described previously [41]. The primer ex1-F (5'-GCT ATG GCG ATG AGC A GC GGC-3') was newly designed based on the sequences of exon 1.…”

Clinical features and skewed X-chromosome inactivation in female carriers of X-linked recessive spinal and bulbar muscular atrophy

“…The signal intensity of the bands on the gel was measured by a charge-coupled device imaging system, the Densitograph AE-6900-F (Atto, Tokyo, Japan) [15]. For each carrier, the peak intensity ratio between two alleles using undigested PCR products was calculated and used as a correction factor.…”

Identification of a candidate modifying gene for spinal muscular atrophy by comparative genomics

“…SMN2 is identical in amino acid composition to SMN1 and remains intact in SMA patients 4 . As both genes produce SMN protein 8,9 , an increase in SMN2 copy number could raise SMN protein levels and result in a milder phenotype; however, efforts to demonstrate a correlation between SMN2 copy number and disease severity have produced conflicting results [19][20][21][22] . NAIP, a gene that lies 16.5 kb downstream of SMN, has also been suggested to be an SMA modifier due to its proximity to SMN and its homology to apoptosis inhibitory proteins 23 , but nearly one-third of type I SMA patients fail to show deletions or mutations of NAIP (refs 19,23).…”