The structural basis for cancer drug interactions with the catalytic and allosteric sites of SAMHD1

Abstract

Abstract:SAMHD1 is a deoxynucleoside triphosphate triphosphohydrolase (dNTPase) that depletes cellular dNTPs in non-cycling cells to promote genome stability and to inhibit retroviral and herpes viral replication. In addition to being substrates, cellular nucleotides also allosterically regulate SAMHD1 activity. Recently, it was shown that high expression levels of SAMHD1 are also correlated with significantly worse patient responses to nucleotide analogue drugs important for treating a variety of cancers, including Acute Myeloid Leukemia (AML). In this study, we used biochemical, structural, and cellular methods to examine the interactions of various cancer drugs with SAMHD1. We found that both the catalytic and the allosteric sites of SAMHD1 are sensitive to sugar modifications of the nucleotide analogs, with the allosteric site being significantly more restrictive. We crystallized cladribine-TP, clofarabine-TP, fludarabine-TP, vidarabine-TP, cytarabine-TP, and gemcitabine-TP in the catalytic pocket of SAMHD1. We find that all of these drugs are substrates of SAMHD1 and that the efficacy of most of these drugs is affected by SAMHD1 activity. Of the nucleotide analogues tested, only cladribine-TP with a deoxyribose sugar efficiently induced the catalytically active SAMHD1 tetramer.Together, these results establish a detailed framework for understanding the substrate specificity and allosteric activation of SAMHD1 with regards to nucleotide analogues, which can be used to improve current cancer and antiviral therapies. SignificanceNucleoside analogue drugs are widely used to treat a variety of cancers and viral infections. With an essential role in regulating the nucleotide pool in the cell by degrading cellular nucleotides, SAMHD1 has the potential to decrease the cellular concentration of frequently prescribed nucleotide analogues and thereby decrease their clinical efficacy in cancer therapy. To improve future nucleotide analogue treatments, it is important to understand SAMHD1 interactions with these drugs. Our work thoroughly examines the extent to which nucleotide analogues interact with the catalytic and allosteric sites of SAMHD1. This work contributes to the assessment of SAMHD1 as a potential therapeutic target for cancer therapy and the future design of SAMHD1 modulators that might improve the efficacy of existing therapies.

Ribonucleotide reductase inhibitors suppress SAMHD 1 ara‐ CTP ase activity enhancing cytarabine efficacy

“…Purine nucleoside analogues are clinically combined with ara‐C. Some have also been shown to be substrates/activators of SAMHD1 (Arnold et al , ; Herold et al , ; Hollenbaugh et al , ; Knecht et al , ) and, importantly, are documented to allosterically inhibit RNR as part of their cytotoxic mechanism (Aye & Stubbe, ; Wisitpitthaya et al , ). We thus tested clofarabine (Cl‐F‐ara‐A), fludarabine (2‐F‐ara‐A) and cladribine (2‐CdA) for their ability to synergise with ara‐C.…”