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Page 262 Franca et al. Cancer Drug Resist 2019;2:256-70 I http://dx.doi.org/10.20517/cdr.2019.004

activity (*1/*1), intermediate activity (*1/*2, *1/*3, *1/*4 and *1/*5), and low activity (*2/*3, *3/*3 and *3/*5).
Loss-of-function NUDT15 diplotypes were consistently associated with thiopurine intolerance. In vitro
studies on human lymphoid cell line showed an increased level of active TGTP in the NUDT15 knock-down
cells compared to control cells, with the elevation of intracellular incorporated DNA thionucleotides and
[42]
consequently higher thiopurines induced apoptosis .

A meta-analysis study of NUDT15 variants investigating 1,752 patients from 7 independent cohorts (both
ALL and IBD) showed a significant association between NUDT15 rs116855232 T allele and myelotoxicity:
indeed, the presence of the T allele increases ~8 times the probability to have leukopenia in comparison
to the C allele (P < 0.00001, OR = 7.86, 95% CI: 6.13, 10.08). Analysing 2745 patients from 13 cohorts, the
[43]
authors also found that T carriers tolerated lower mean daily thiopurines dose (P < 0.00001) . A second
meta-analysis, including 16 studies on rs116855232 (p.Arg139Cys), confirmed its role as clinically relevant
[44]
predictor of thiopurine-induced leukopenia . This SNP is the most common NUDT15 genetic variant,
particularly frequent in East Asians (10.4%) and Hispanics (7.1%), but almost absent in Europeans (0.46%)
[45]
and Africans . Distribution of other NUDT15 genetic variants are also ethnic specific: rs147390019 is
common only in Hispanics (1.7%) and rs186364861 in Asians (1.6%) . Recently, a NUDT15 variant with
[43]
[43]
a significant prevalence in Caucasian patients was discovered in a cohort of patients with IBD, rs746071566,
[46]
(p.Gly17_Val18del) and was associated with myelosuppression during thiopurine therapy . The relevance of
this variant in Caucasian patients with ALL has still to be fully understood.
International guidelines of thiopurines treatment suggest to consider NUDT15 together with TPMT
genotypes before drug administration. Patients who present the above-mentioned genetic variants need to
adjust drug dosage. CPIC suggests to use the normal starting dose for NUDT15 normal metabolizers (e.g.,
2
NUDT15 *1/*1, MP 75 mg/m /day in ALL) and a reduced one in intermediate metabolizers (e.g., NUDT15
2
*1/*3, MP 30%-80% normal starting dose) and poor metabolizers (e.g., NUDT15 *3/*3; MP 10 mg/m /day in
ALL) .
[36]
Moriyama et al. evaluated the effects of NUDT15 on thiopurines metabolism and identified DNA-
[47]
incorporated thioguanine (DNA-TG) and TGN as pharmacologic markers for NUDT15 genotype-guided
thiopurines dosing. They analyzed the level of these metabolites in a cohort of 55 Japanese ALL pediatric
patients: those with NUDT15 deficiencies accumulated DNA-TG more efficiently than patients without
NUDT15 variants (P = 4.0 × 10 ). They also showed that cytosolic TGN and nuclear DNA-TG were positively
-9
-4
correlated with each other (P = 6.5 × 10 ), but the ratio of DNA-TG to TGN was significantly higher in
-9
NUDT15 deficient patients (P = 3.6 × 10 ), suggesting that DNA-TG is a more relevant MP metabolite than
[47]
TGN to inform NUDT15 genotype-guided dose adjustments .
ITPA
Human inosine triphosphate pyrophosphatase (ITPA, EC 3.6.1.19) is a ~21 kDa enzyme that is ubiquitously
[48]
expressed in the cytoplasm of cells and acts in homodimer form . ITPA catalyses the hydrolysis of the
triphosphate moieties from noncanonical (deoxy-) purine triphosphate, such as inosine triphosphate, deoxy-
inosine triphosphate, xanthosine triphosphate, deoxy-xantosine triphosphate (ITP, dITP, XTP and dXTP
respectively), thus recovering the monophosphate derivatives and releasing the pyrophosphate group. IMP/
dIMP are fundamental intermediates in purine metabolism and can be converted into canonical adenosine
monophosphate (AMP/dAMP, and therefore ATP/dATP) and guanosine monophosphate (GMP/dGMP,
hence GTP/dGTP). It has been estimated that nearly one third of the human population has a genetically
determined decreased ITPase activity . The resulting unusual accumulation of noncanonical purine
[49]
triphosphate can be toxic for the cells because of their interference with polymerase enzymes, and their
incorporation into DNA or RNA. The 19 kb ITPA gene is located on the short arm of chromosome 20 and
[50]
includes 8 exons . Genetic knock-out (Itpa-/-) mice present abnormal features of growth retardation and