The survival motor neuron gene <i>smn-1</i> interacts with the U2AF large subunit gene <i>uaf-1</i> to regulate <i>Caenorhabditis elegans</i> lifespan and motor functions

Spinal muscular atrophy (SMA), the most frequent human congenital motor neuron degenerative disease, is caused by loss-of-function mutations in the highly conserved survival motor neuron gene <i>SMN1</i>. Mutations in <i>SMN</i> could affect several molecular processes, among which aberrant pre-mRNA splicing caused by defective snRNP biogenesis is hypothesized as a major cause of SMA. To date little is known about the interactions of <i>SMN</i> with other splicing factor genes and how SMN affects splicing <i>in vivo</i>. The nematode <i>Caenorhabditis elegans</i> carries a single ortholog of <i>SMN</i>, <i>smn-1</i>, and has been used as a model for studying the molecular functions of SMN. We analyzed RNA splicing of reporter genes in an <i>smn-1</i> deletion mutant and found that <i>smn-1</i> is required for efficient splicing at weak 3′ splice sites. Genetic studies indicate that the defective lifespan and motor functions of the <i>smn-1</i> deletion mutants could be significantly improved by mutations of the splicing factor U2AF large subunit gene <i>uaf-1</i>. In <i>smn-1</i> mutants we detected a reduced expression of U1 and U5 snRNAs and an increased expression of U2, U4 and U6 snRNAs. Our study verifies an essential role of <i>smn-1</i> for RNA splicing <i>in vivo</i>, identifies the <i>uaf-1</i> gene as a potential genetic modifier of <i>smn-1</i> mutants, and suggests that SMN-1 has multifaceted effects on the expression of spliceosomal snRNAs.

脊髓性肌萎缩症（Spinal muscular atrophy, SMA）是人类最常见的先天性运动神经元退行性疾病，由高度保守的运动神经元生存基因1（SMN1）的功能丧失性突变引发。SMN基因的突变可影响多种分子进程，其中由核小核糖核蛋白颗粒（small nuclear ribonucleoprotein particle, snRNP）生物发生缺陷所导致的异常前信使RNA（pre-mRNA）剪接，被假设为SMA的主要致病原因。迄今为止，学界对SMN与其他剪接因子基因的相互作用，以及SMN如何在体内（in vivo）调控剪接过程仍知之甚少。秀丽隐杆线虫（Caenorhabditis elegans）仅携带一个SMN的同源基因smn-1，已被广泛用作研究SMN分子功能的模式生物。我们对smn-1缺失突变体中的报告基因RNA剪接进行了系统分析，发现smn-1对于弱3'剪接位点的高效剪接是必需的。遗传学研究表明，通过剪接因子U2小核核糖核蛋白辅助因子（U2 small nuclear ribonucleoprotein auxiliary factor, U2AF）大亚基基因uaf-1的突变，可显著改善smn-1缺失突变体的寿命缩短与运动功能缺陷。在smn-1突变体中，我们检测到U1、U5小核RNA（small nuclear RNA, snRNA）的表达水平降低，而U2、U4、U6 snRNA的表达水平显著升高。本研究证实了smn-1在体内RNA剪接中的关键作用，确定uaf-1基因可作为smn-1突变体的潜在遗传修饰因子，并表明SMN-1对剪接体小核RNA的表达具有多方面调控作用。