Interleukin 7 receptor α-chain is crucial for the development and maintenance of T cells and genetically associated with autoimmune disorders including multiple sclerosis (MS). Exon 6 of IL7R encodes for its transmembrane domain and regulated by alternative splicing (AS): Inclusion or skipping of IL7R exon 6 results in membrane-bound or soluble IL7R isoforms, respectively. We previously identified SNP rs6897932 in IL7R exon 6, associated with MS risk, and showed that the risk allele (C) results in increased exon skipping and elevated sIL7R. Elevated levels of sIL7R have been shown to exacerbate the disease in the experimental autoimmune encephalomyelitis mouse model of MS. Here we report two mechanisms by which IL7R exon 6 is controlled. A competition between PTBP1 and U2AF2 at the polypyrimidine tract (PPT) of intron 5, and an unexpected U2AF2-mediated assembly of splicing factors in the exon. We noted the presence of a branchpoint sequence (BPS) (TACTAAT or TACTAAC) within exon 6, which is stronger with the C allele. Importantly, the BPS is followed by a PPT and we conjectured that silencing could be mediated by binding of U2AF2 to that tract. Here, we show that evolutionary conservation of the exonic PPT correlates well with the degree of AS of exon 6 in two nonhuman primate species and that U2AF2 binding to this PPT recruits U2 snRNP components to the exon. These observations provide the first explanation for the stronger silencing of IL7R exon 6 with the disease-associated C allele at rs6897932.

中文翻译：

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U2AF2 异位结合 IL7R 外显子 6 并抑制其包含


白细胞介素 7 受体 α 链对于 T 细胞的发育和维持至关重要，并且与包括多发性硬化症 (MS) 在内的自身免疫性疾病存在遗传相关性。 IL7R的外显子 6 编码其跨膜结构域并受选择性剪接 (AS) 调节：包含或跳过IL7R外显子 6 分别导致膜结合或可溶性 IL7R 亚型。我们之前在IL7R外显子 6 中发现了与 MS 风险相关的 SNP rs6897932，并表明风险等位基因 (C) 导致外显子跳跃增加和 sIL7R 升高。在多发性硬化症的实验性自身免疫性脑脊髓炎小鼠模型中，sIL7R 水平升高已被证明会加剧疾病。在这里，我们报道了控制IL7R外显子 6 的两种机制。 PTBP1 和 U2AF2 在内含子 5 的多嘧啶束 (PPT) 上发生竞争，以及意外的 U2AF2 介导的外显子剪接因子组装。我们注意到外显子 6 内存在分支点序列 (BPS)（TACTAAT 或 TACTAAC），该序列对于 C 等位基因更强。重要的是，BPS 之后是 PPT，我们推测沉默可能是通过 U2AF2 与该区域的结合来介导的。在这里，我们表明外显子 PPT 的进化保守性与两种非人灵长类动物中外显子 6 的 AS 程度密切相关，并且 U2AF2 与该 PPT 结合将 U2 snRNP 成分招募到外显子。这些观察结果为IL7R外显子 6 与疾病相关 C 等位基因 rs6897932 处的更强沉默提供了第一个解释。