DNA損傷耐受機制 (DNA damage tolerance pathway, DDT) 是細胞在DNA複製遇到損傷或障礙時，為了避免複製叉因為複製中斷而瓦解的修復機制。在DDT中跨過損傷的方法有兩種，分別是利用低忠實性聚合酶(low fidelity DNA polymerases) 的translesion synthesis (TLS) 修復途徑，以及利用互補股當模板的template switching (TS) 修復途徑。
實驗室先前的研究發現PARP1會與TS路徑中的HLTF結合，並且參與姊妹染色體交換(sister chromatid exchange，SCE)的機制，此一機制與同源重組修復機制 (homologous recombination, HR)有關。PARP1是一個酵素，會在DNA損傷處對周遭蛋白，包含自身蛋白，產生poly(ADP-ribosyl)ation (PAR) 的修飾，此修飾是DNA損傷反應機制 (DNA damage response, DDR)中最先啟動的反應之一。ALC1包含macrodomain，會與 PAR修飾的蛋白結合，因此我的研究重點是探討ALC1是否也參與在DNA損傷耐受機制。我們的實驗是利用抗藥性鼻咽癌細胞進行探討，在缺乏ALC1的狀況下抗藥性細胞對化療藥物 (Cisplatin) 的敏感性顯著提升，且在預處理Cisplatin與照射UV兩種條件的DNA fiber實驗中發現，缺乏ALC1的細胞，在受到DNA損壞時，其DNA複製的效率降低，同時增加DNA複製叉停滯的現象，另外也發現缺乏ALC1的細胞其代表同源重組修復的姊妹染色體交換率也會下降，本篇結果顯示在缺乏ALC1的細胞，其DNA損傷耐受機制功能顯著下降，進而影響鼻咽癌細胞的抗藥性。

To prevent the stalling of DNA replication, the DNA damage tolerance (DDT) pathway is evolved to bypass DNA lesions during DNA replication. There are two mechanisms in the DDT pathway. One is the translesion synthesis (TLS) pathway and the other is the template-switching (TS) pathway. While the TLS pathway is extensive studied, the TS pathway is largely unclear. The current model suggests that the TS pathway bypasses DNA lesions through homologous recombination (HR). However, it is still unclear how HR is incorporated into the TS pathway. Previous studies in our laboratory have demonstrated that chronic treatment of cisplatin can induce chemoresistant phenotype of nasopharyngeal carcinoma cells (NPC) through the enhanced TS pathway. Interestingly, we found that the E3 ligase HLTF can interact with PARP1. The depletion of HLTF and PARP1 sensitizes the chemoresistant NPC cells to cisplatin, significantly increases the number of stalled forks, and reduce sister-chromatid exchange (SCE). In this study, we further extend our understanding of the TS pathway by showing that ALC1 is involved in the TS pathway through the interaction between ALC1 and PARP1. We demonstrate that the depletion of ALC1 sensitizes the chemoresistant NPC cells to cisplatin, significantly increases the number of stalled forks, and reduces the progression of DNA replication in response to DNA lesions. Significantly, the ALC1 deficient cells show reduced SCE, suggesting an impairing of the TS pathway. Taken together, our results suggest that ALC1 is involved in the TS pathway, through the assembly of HLTF, PARP1, and ALC1 complex.

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