Structural and functional characterization explains loss of dNTPase activity of the cancer-specific R366C/H mutant SAMHD1 proteins

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 mu-tations, the SAMHD1 R366 mutations do not alter cellular protein levels of the enzyme. However, R366C/H mutant pro-teins 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 gamma- 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 sup-pressing innate immune responses. Finally, SAMHD1 degra-dation 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 mu-tations can mechanistically contribute to the elevated dNTP levels commonly found in cancer cells.

Automated summary

Mutations in SAMHD1, specifically the R366C/H mutants found in colon cancer and leukemia, lead to loss of dNTPase activity which may contribute to the elevated dNTP levels commonly observed in cancer cells. These mutants retain other functions but fail to reduce dNTP levels and restrict HIV-1 replication.