Quantitative in vitro and in vivo characterization of the human P32T mutant ITPase

Human ITPase, encoded by the ITPA gene, and its orthologs (RdgB in Escherichia coli and HAM1 in Saccharomyces cerevisiae) exclude noncanonical nucleoside triphosphates (NTPs) from NTP pools. Deoxyinosine triphosphate (dITP) and 2′-deoxy-N-6-hydroxylaminopurine triphosphate are both hydrolyzed by ITPase to yield the corresponding deoxynucleoside monophosphate and pyrophosphate. In addition, metabolites of thiopurine drugs such as azathioprine have been shown to be substrates for ITPase. The ITPA 94C>A [P32T] variant is one of two polymorphisms associated with decreased ITPase activity. Furthermore, the ITPA 94C>A [P32T] variant is associated with an increased risk of adverse drug reactions for patients treated with azathioprine. The nature of the observed phenotypes for ITPA 94C>A [P32T] variant individuals is currently unclear. Our biochemical assays indicate the P32T ITPase has 55% activity with dITP compared to wild-type ITPase. Complementation experiments at 37 °C show that N-6-hydroxylaminopurine sensitivity of E. coli rdgB mutants is reduced with a plasmid bearing the ITPA 94C>A [P32T] gene approximately 50% less than with a plasmid bearing the wild-type ITPA gene. The reduction in sensitivity is less at 42 °C. Experiments with synthetic lethal E. coli recA(ts) rdgB mutants show that the ITPA 94C>A [P32T] gene also complements the recA(ts) rdgB growth deficiency at 42 °C approximately 40% lower than wild-type ITPA gene. Western blot analysis indicates that the expression level of P32T ITPase is reduced in these cells relative to wild type. Our data support the idea that P32T ITPase is a functional protein, albeit with a reduced rate of noncanonical NTP pyrophosphohydrolase activity and reduced protein stability.