Elevated intracellular levels of dNTPs have been shown to be a biochemical marker of cancer cells. Recently, a series of mutations in the multifunctional dNTP triphosphohydrolase (dNTPase), sterile alpha motif and histidine-aspartate domain-containing protein 1 (SAMHD1), have been reported in various cancers. Here, we investigated the structure and functions of SAMHD1 R366C/H mutants, found in colon cancer and leukemia. Unlike many other cancer-specific mutations, the SAMHD1 R366 mutations do not alter cellular protein levels of the enzyme. However, R366C/H mutant proteins exhibit a loss of dNTPase activity, and their X-ray structures demonstrate the absence of dGTP substrate in their active site, likely because of a loss of interaction with the γ-phosphate of the substrate. The R366C/H mutants failed to reduce intracellular dNTP levels and restrict HIV-1 replication, functions of SAMHD1 that are dependent on the ability of the enzyme to hydrolyze dNTPs. However, these mutants retain dNTPase-independent functions, including mediating dsDNA break repair, interacting with CtIP and cyclin A2, and suppressing innate immune responses. Finally, SAMHD1 degradation in human primary-activated/dividing CD4+ T cells further elevates cellular dNTP levels. This study suggests that the loss of SAMHD1 dNTPase activity induced by R366 mutations can mechanistically contribute to the elevated dNTP levels commonly found in cancer cells.

中文翻译：

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结构和功能表征解释了癌症特异性 R366C/H 突变体 SAMHD1 蛋白 dNTPase 活性丧失的原因。


细胞内 dNTP 水平升高已被证明是癌细胞的生化标志物。最近，在多种癌症中报道了多功能 dNTP 三磷酸水解酶 (dNTPase)、无菌 α 基序和含组氨酸天冬氨酸结构域的蛋白 1 (SAMHD1) 的一系列突变。在这里，我们研究了结肠癌和白血病中发现的 SAMHD1 R366C/H 突变体的结构和功能。与许多其他癌症特异性突变不同，SAMHD1 R366 突变不会改变该酶的细胞蛋白水平。然而，R366C/H 突变蛋白表现出 dNTPase 活性的丧失，并且它们的 X 射线结构表明其活性位点不存在 dGTP 底物，这可能是因为与底物的 γ-磷酸盐相互作用的丧失。 R366C/H 突变体未能降低细胞内 dNTP 水平并限制 HIV-1 复制，而 SAMHD1 的功能依赖于酶水解 dNTP 的能力。然而，这些突变体保留了不依赖于 dNTPase 的功能，包括介导 dsDNA 断裂修复、与 CtIP 和细胞周期蛋白 A2 相互作用以及抑制先天免疫反应。最后，人类初级激活/分裂 CD4+ T 细胞中 SAMHD1 的降解进一步提高了细胞 dNTP 水平。这项研究表明，R366 突变引起的 SAMHD1 dNTPase 活性丧失可能在机制上导致癌细胞中常见的 dNTP 水平升高。