Dual RNA 3’ end processing of H2A.X messenger RNA maintains DNA damage repair throughout the cell cycle

Replication-dependent (RD) histone mRNA are the only non-polyadenylated protein coding transcripts in humans. However, a subset of RD histone genes also expresses polyadenylated mRNA upon terminal differentiation to maintain histone proteins. We now show that even in cycling cells, RD histone genes express some poly(A) mRNA. The H2AFX gene, encoding the DNA damage signalling histone variant H2A.X, expresses both an S-phase-specific SL mRNA ending in the typical stem-loop (SL) structure and substantial amounts of an extended poly(A) mRNA. We show, by the selective removal of either mRNA isoform, that the S-phase-specific H2A.X SL mRNA can generate sufficient histone for deposition onto DNA damage responsive (DDR) chromatin in some cell types. In contrast, when cells have evolved to make predominantly H2A.X poly(A) mRNA they then require de novo H2A.X for efficient DDR. This highlights the importance of differential H2AFX mRNA 3’ end processing for the maintenance of effective DDR

依赖复制（Replication-dependent, RD）的组蛋白mRNA是人类体内唯一一类非多聚腺苷酸化的编码蛋白转录本。然而，部分RD组蛋白基因在细胞终末分化时会表达多聚腺苷酸化mRNA，以维持组蛋白的稳态供应。本研究证实，即便在增殖细胞中，RD组蛋白基因也会产生少量poly(A) mRNA。编码DNA损伤信号通路组蛋白变体H2A.X的H2AFX基因，既可表达S期特异性的茎环（stem-loop, SL）型mRNA（该转录本以典型的茎环结构作为3'末端），也会合成大量延长型的poly(A) mRNA。通过选择性敲除这两种mRNA异构体，我们发现：在部分细胞类型中，S期特异性的H2A.X茎环型mRNA足以生成足量的组蛋白，以沉积到DNA损伤应答（DNA damage responsive, DDR）染色质上。与之相反，当细胞进化为主要表达H2A.X的poly(A) mRNA时，则需要从头合成H2A.X以实现高效的DNA损伤应答。这一结果凸显了差异化的H2AFX基因mRNA 3'端加工过程，对于维持有效DNA损伤应答的重要性。