DROSOPHILA FUS MUTANT PHENOTYPES ARE MEDIATED BY INCREASED XRP1 EXPRESSION LEADING TO GENE EXPRESSION DYSREGULATION

Several RNA-binding proteins (RBPs) are implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), including the FET proteins FUS, TAF15 and EWSR1. Cabeza (caz) is the single Drosophila FET ortholog. Here, we identified Xrp1, a poorly characterized DNA-binding protein, as a key modifier of caz mutant phenotypes. Xrp1 expression was strongly upregulated in caz mutants, and Xrp1 heterozygosity rescued their motor defects and life span. Interestingly, selective neuronal Xrp1 knock-down was sufficient to rescue, and neuronal Xrp1 overexpression phenocopied caz mutant phenotypes. The caz/Xrp1 genetic interaction depended on the functionality of the AT-hook DNA-binding domain in Xrp1. Consistently, caz mutants displayed gene expression dysregulation, which was mitigated by Xrp1 heterozygosity. Finally, Xrp1 knock-down substantially rescued the motor deficits and life span of flies expressing ALS-mutant FUS in motor neurons. Taken together, caz mutant phenotypes are mediated by increased neuronal Xrp1 levels, leading to gene expression dysregulation and neuronal dysfunction.

多项RNA结合蛋白（RNA-binding proteins, RBPs）与肌萎缩侧索硬化症（amyotrophic lateral sclerosis, ALS）及额颞叶痴呆（frontotemporal dementia, FTD）的发病机制密切相关，其中包括FET蛋白家族成员FUS、TAF15及EWSR1。Cabeza（简称caz）是果蝇体内唯一的FET同源基因。本研究鉴定出Xrp1——一种功能尚未被充分阐释的DNA结合蛋白——作为caz突变表型的关键修饰因子。研究发现，caz突变体中Xrp1的表达水平显著上调；而Xrp1基因杂合缺失可挽救其运动缺陷与寿命缩短表型。值得注意的是，仅在神经元中特异性敲低Xrp1即可实现表型挽救，而过表达神经元内的Xrp1则可模拟caz突变的表型。caz与Xrp1的遗传互作依赖于Xrp1中AT钩DNA结合结构域的功能。一致性实验结果显示，caz突变体存在基因表达失调现象，而Xrp1杂合缺失可缓解该失调。最后，敲低Xrp1可显著挽救运动神经元中表达ALS致病突变型FUS的果蝇的运动缺陷与寿命缩短问题。综上，caz突变表型由神经元中Xrp1水平升高所介导，该过程最终导致基因表达失调与神经元功能异常。