Kashima Tsuyoshi, Manley James L
Department of Biological Sciences, Columbia University, New York, New York 10027, USA.
Nat Genet. 2003 Aug;34(4):460-3. doi: 10.1038/ng1207.
Spinal muscular atrophy (SMA) is a relatively common neurodegenerative disease caused by homozygous loss of the survival motor neuron 1 (SMN1) gene. Humans possess a linked, nearly identical gene, SMN2, which produces a functional SMN protein but at levels insufficient to compensate for loss of SMN1 (refs. 1,2). A C/T transition at position +6 in exon 7 is all that differentiates the two genes, but this is sufficient to prevent efficient exon 7 splicing in SMN2 (refs. 2,3). Here we show that the C/T transition functions not to disrupt an exonic splicing enhancer (ESE) in SMN1 (ref. 4), as previously suggested, but rather to create an exonic splicing silencer (ESS) in SMN2. We show that this ESS functions as a binding site for a known repressor protein, hnRNP A1, which binds to SMN2 but not SMN1 exon 7 RNA. We establish the physiological importance of these results by using small interfering RNAs to reduce hnRNP A protein levels in living cells and show that this results in efficient SMN2 exon 7 splicing. Our findings not only define a new mechanism underlying the inefficient splicing of SMN2 exon 7 but also illustrate more generally the remarkable sensitivity and precision that characterizes control of mRNA splicing.
脊髓性肌萎缩症（SMA）是一种相对常见的神经退行性疾病，由生存运动神经元1（SMN1）基因的纯合缺失引起。人类拥有一个与之连锁且几乎相同的基因SMN2，它能产生功能性的SMN蛋白，但产生水平不足以弥补SMN1的缺失（参考文献1,2）。外显子7中第+6位的C/T转换是区分这两个基因的唯一差异，但这足以阻止SMN2中外显子7的有效剪接（参考文献2,3）。我们在此表明，C/T转换并非如先前推测的那样破坏SMN1中的外显子剪接增强子（ESE），而是在SMN2中产生一个外显子剪接沉默子（ESS）。我们发现这个ESS作为一种已知阻遏蛋白hnRNP A1的结合位点，hnRNP A1能与SMN2外显子7的RNA结合，但不与SMN1外显子7的RNA结合。我们通过使用小干扰RNA降低活细胞中hnRNP A蛋白水平，证实了这些结果的生理重要性，并表明这会导致SMN2外显子7的有效剪接。我们的发现不仅定义了SMN2外显子7剪接效率低下的一种新机制，还更普遍地说明了mRNA剪接控制所具有的显著敏感性和精确性。
