Activity and frequency of the ITPA P32T variant among colorectal patients

Abstract

Inosine Triphosphate Pyrophosphatase (ITPase) is an NTPase responsible for preserving genomic integrity by maintaining low cellular dITP and dXTP levels. Of the many variants of ITPA, the P32T variant is of interest due to high allelic frequency and pharmacological significance of its phenotype. ITPA variants P32T, F31Q and P32T/F31Q were tested for in vivo activity against HAP mutagenesis in E.coli and for in vitro activity by PiPer assay with dITP and dXTP substrates. All variants were successful in protecting against HAP mutagenesis. Both single mutant variants (P32T and F31Q) showed enzymatic activity against both substrates, results for the P32T/F31Q variant were inconclusive. In addition, the frequency of the P32T allele among colorectal cancer patients was tested by HPY166II digest. Out of 53 samples tested, 8 are heterozygous for the P32T allele as confirmed by DNA sequencing, giving an allelic frequency of 0.075. This frequency is comparable to frequency among Caucasian Americans of 0.06 (Cao and Hegele, 2002) and therefore the P32T allele does not appear to be enriched amongst colorectal cancer patients. Activity of the variants is comparable to the wild type and the low levels of activity in P32T homozygous individuals does not appear to be due to a lack of enzymatic function of the protein. This publication funded by NIH grant 5P20RR016469 and NIGMS grant 8P20GM103427.