DNA聚合酶θ：易错的多功能DNA末端修复分子

DNA聚合酶θ (DNA polymerase theta,Polθ)是一种广泛存在于动植物中的DNA修复酶。它在选择性末端连接（alternative end-joining,Alt-EJ）途径中发挥着关键作用,常参与DNA双链断裂（DNA double-strand breaks,DSB）损伤修复。在正常生理状态下,Polθ主要调控基因组稳定性。然而,在恶性肿瘤发生时,Polθ表现出异常高表达水平,并参与调控肿瘤细胞的恶性转变过程。研究表明,抑制Polθ活性可导致同源重组（homologous recombination,HR）缺陷的肿瘤细胞发生合成致死（synthetic lethality,SL）。因此,已经开发出多种针对Polθ的小分子抑制剂,可与其他化疗药物联合使用以抑制恶性肿瘤的发展。此外,敲除或抑制Polθ活性还能增加HR修复效率,从而提高外源基因靶向整合效果。本文综述了Polθ及其介导的Alt-EJ修复机制在生物学功能方面的最新研究进展,为靶向Polθ在肿瘤治疗和基因编辑方面的应用提供理论基础。

DNA Polymerase θ: a Multifunctional and Error-prone DNA End Repair Enzyme

DNA polymerase theta (Polθ), also known as DNA polymerase θ, is the member of the DNA polymerase A family and plays a crucial role in the repair of DNA double-strand breaks (DSB). Polθ has 3 distinct structural domains: the N-terminal helicase-like domain with a conserved sequence, the C-terminal polymerase domain, and the central domain, which is a disordered sequence connecting these two regions. Notably, Polθ is the only known polymerase in eukaryotes that possesses helicase activity. However, it is also an error-prone polymerase. When DNA DSBs occur, a specialized network consisting of at least 4 pathways, including classical-non homologous end joining (C-NHEJ), homologous recombination (HR), single-strand annealing (SSA), and alternative-end joining (Alt-EJ), is responsible for repairing DNA damage caused by DSBs. In the absence of major DNA repair pathways like HR, cells rely on Alt-EJ pathway mediated by Polθ to repair damaged DNA and maintain genomic stability. Nevertheless, due to the low fidelity of Polθ, Alt-EJ repair often leads to errors. Depletion of Polθ has shown to increases DSB formation and compromise genomic stability. Conversely, overexpression of Polθ has been associated with increases DNA damage markers and impairs cell cycle progression. As a result, the impact of Polθ on genome stability remains controversial. Furthermore, overexpression of Polθ is frequently observed in cancer and is associated with a characteristic mutational signature and poor prognosis. Depleting Polθ in an HR-deficient background has been shown to impair cell viability, suggesting a synthetic lethal (SL) relationship between Polθ and HR factors. In recent years, targeted chemotherapy drugs that inhibit tumor growth have gained significant attention. However, off-target effects and drug resistance pose challenges for clinical application, particularly with poly-ADP-ribose polymerase inhibitor (PARPi). Blocking Polθ activity in HR-deficient tumor cells has been found to reverse PARPi resistance, making Polθ a very promising therapeutic target in cancer treatment. The availability of crystal structures for both helicase and polymerase domain has facilitated the design of potent inhibitors of Polθ. Currently, several highly specific and effective small molecule inhibitors targeting Polθ, such as Novobiocin, RP-6685, and ART558, have been reported to effectively block various cancers with HR deficiency. The initial success of these inhibitors points to new directions for treating BRCA1/2-mutated tumors. Additionally, reducing the Alt-EJ repair pathway mediated by Polθ can improve HR repair efficiency and increase the chance of exogenous gene target integration (TI), suggesting potential new applications for Polθ inhibitors. This article reviews the recent research progress on the molecular function of Polθ and its involvement in the Alt-EJ pathway modification mechanism, providing insights for a deeper understanding of this field.