Journal of Personalized Medicine Review Importance of NUDT15 Polymorphisms in Thiopurine Treatments Yoichi Tanaka * and Yoshiro Saito Division of Medicinal Safety Science, National Institute of Health Sciences, Kawasaki City 210-9501, Kanagawa, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-44-270-6600 Abstract: Thiopurines, mercaptopurine, and azathioprine are used as immunosuppressants in the treatments of inflammatory bowel disease, rheumatoid arthritis, and organ transplantation and as chemotherapeutic drugs for the treatment of acute leukemia and chronic myeloid leukemia. This drug class sometimes causes severe adverse reactions, including bone marrow suppression and hair loss. Genetic polymorphisms of the metabolizing enzyme thiopurine S-methyltransferase have been used for predicting these reactions in Caucasians, but these allele frequencies are less frequently observed in Asian populations. Recently, nudix hydrolase 15 (NUDT15) polymorphisms have been shown to play an important role in thiopurine-induced adverse reactions in Asians. In this review, we summarize the NUDT15 studies, mainly in Asian countries, and their implementation in several countries. Keywords: NUDT15; thiopurine; mercaptopurine; azathioprine; Asian; adverse effect; implementation 1. Introduction Thiopurines, 6-mercaptopurine (6-MP) and azathioprine (AZA) are used as immuno- Citation: Tanaka, Y.; Saito, Y. suppressive and cytotoxic agents. AZA is used to maintain remission of inflammatory Importance of NUDT15 bowel disease (IBD) [1,2] and rheumatoid arthritis [3] and to inhibit immunological rejection Polymorphisms in Thiopurine of transplanted organs [4]. In hematological malignancies, 6-MP is used as maintenance Treatments. J. Pers. Med. 2021, 11, 778. therapy for 2–3 years [5]. Although thiopurine is the main component of these therapies, https://doi.org/10.3390/jpm11080778 severe adverse effects of grade 3 or higher frequently occur. The typical adverse effects of thiopurines include myelosuppression, hair loss, hepatotoxicity, and pancreatitis. Academic Editor: Su-Jun Lee To date, many studies have used a genetic approach to predict thiopurine-induced severe toxicities, especially myelosuppression. Thiopurine S-methyltransferase (TPMT) is Received: 5 July 2021 an enzyme involved in thiopurine metabolism (Figure1), and its genetic polymorphisms Accepted: 8 August 2021 are known to be risk factors for intolerance to thiopurines [6]. The TPMT variants of Published: 10 August 2021 c.238G>C, c.460G>A, and c.719G>A induce protein instability and decrease the TPMT enzyme activity [7]. A low TPMT activity results in the accumulation of active metabolites, Publisher’s Note: MDPI stays neutral thiopurine nucleosides, in the cells, leading to cytotoxicity. The frequencies of variants with regard to jurisdictional claims in with low TPMT activity differ among ethnic groups [6]. Caucasians and Africans have a published maps and institutional affil- high frequency of these polymorphisms, but Asians have lower frequencies of these TPMT iations. nonfunctional variants; nonetheless, the frequencies of severe toxicities are similar among these races. In addition to TPMT polymorphisms, nudix hydrolase 15 (NUDT15) c.415C>T (p.R139C) was associated with thiopurine intolerance in Asian populations, as reported in a previous Copyright: © 2021 by the authors. genome-wide association study (GWAS) [8,9]. To date, NUDT15 is known to have 26 alleles, Licensee MDPI, Basel, Switzerland. and NUDT15 *2 (p.V18_V19insGV and c.415C>T), *3 (c.415C>T), and *9 (c.50delGAGTCG) This article is an open access article are recognized as loss-of-function variants (Table1)[ 10,11]; however, other variants have distributed under the terms and been uncertain about the functional significance, referring to the Pharmacogene Variation conditions of the Creative Commons Consortium (PharmVar, www.pharmvar.org). The NUDT15 enzyme dephosphorylates Attribution (CC BY) license (https:// thiopurine triphosphate, which is the active metabolite incorporated into the DNA, to its creativecommons.org/licenses/by/ monophosphate (Figure1). The NUDT15 loss-of-function variants induce the increased 4.0/). J. Pers. Med. 2021, 11, 778. https://doi.org/10.3390/jpm11080778 https://www.mdpi.com/journal/jpm J. Pers. Med. 2021, 11, x FOR PEER REVIEW 2 of 11 other variants have been uncertain about the functional significance, referring to the Phar- J. Pers. Med. 2021, 11, 778 macogene Variation Consortium (PharmVar, www.pharmvar.org). The NUDT15 enzyme2 of 10 dephosphorylates thiopurine triphosphate, which is the active metabolite incorporated into the DNA, to its monophosphate (Figure 1). The NUDT15 loss-of-function variants induce the increased levels of thiopurine triphosphate and thus higher incorporation of levelsthioguanine of thiopurine nucleotides triphosphate into the DNA and thus and RNA. higher As incorporation a result, patients of thioguaninewho inherited nucleotides these intovariants the DNA experienced and RNA. severe As thiopurine-induced a result, patients whocytotoxicities, inherited such these as myelosuppression variants experienced severeand alopecia. thiopurine-induced NUDT15 c.415C>T cytotoxicities, (p.R139C) such is commonly as myelosuppression detected in Asians and alopecia. but is rarelyNUDT15 c.415C>Tfound in (p.R139C) Caucasians is and commonly Africans, detected with allele in Asiansfrequencies but isof rarely0.12 and found <0.01, in respectively Caucasians and Africans,[6]. In this with review, allele we frequencies aimed to provide of 0.12 anda summary <0.01, respectively of the associations [6]. In of this NUDT15 review, alleles we aimed toand provide their aclinical summary significance, of the associations mainly in Asians, of NUDT15 as well asalleles their andimplementation their clinical in significance,several countries. mainly in Asians, as well as their implementation in several countries. FigureFigure 1. Thiopurine 1. Thiopurine metabolism metabolism pathway. pathway. ABCC,ABCC, ATP-binding ca cassette,ssette, subfamily subfamily C; C;GDP, GDP, guanosine guanosine diphosphate; diphosphate; GMP,GMP, guanosine guanosine monophosphate; monophosphate; GMPS, GMPS, guanine guanine monophosphatemonophosphate synthetase; synthetase; GTP, GTP, guanos guanosineine triphosphate; triphosphate; HGPRT, HGPRT, hypoxanthine guanine phosphoribosyltransferase; IMP, inosine monophosphate; ITP, inosine triphosphate; IMPDH, ino- hypoxanthinesine monophosphate guanine phosphoribosyltransferase; dehydrogenase; NUDT15, nudix IMP, inosinehydrolase monophosphate; 15; TPMT, thiopurine ITP, inosine S-methyltransferase. triphosphate; IMPDH, inosine monophosphate dehydrogenase; NUDT15, nudix hydrolase 15; TPMT, thiopurine S-methyltransferase. Table 1. Clinical impact of NUDT15 alleles in Asians. Table 1. Clinical impact of NUDT15 alleles in Asians. Estimated Allele Variation Amino Acid Disease Clinical Impact Reference EnzymeEstimated Activity Allele Variation Amino Acid Disease Clinical Impact Reference Enzyme Activity AZA dose reduction [12] 55_56insGAGTCG V18_V19insGV IBD LeukopeniaAZA dose and reduction hair loss [12] *2 55_56insGAGTCG V18_V19insGV Low IBD [13] *2 415C>T R139C Low ALL Leukopenia,Leukopenia 6-MP and hairdose loss reduc- [13] 415C>T R139C ALL [14,15] tionLeukopenia, 6-MP dose reduction [14,15] AZAAZA dose dose reduction reduction [12] [12] IBDIBD *3 *3 415C>T415C>T R139CR139C LowLow LeukopeniaLeukopenia and and hair hair loss loss [13] [13] ALLALL Leukopenia,Leukopenia, dose dose reduction reduction [10,14,16][10,14 ,16] *4 *4 416G>A416G>A R139HR139H IntermediateIntermediate IBD IBD Leukopenia Leukopenia and and hair hair loss loss (homo) (homo) [13,17] [13 ,17] IBDIBD LeukopeniaLeukopenia [13] [13] *5 *5 52G>A52G>A V18IV18I IntermediateIntermediate ALLALL 6-MP6-MP dose dose reduction reduction (homo) (homo) [14] [14] *6 55_56insGAGTCG V18_V19insGV Intermediate *7 101G>C R34T Low ALL Dose reduction [11] *8 103A>G K35E Intermediate ALL Dose reduction (patient with *2/*8) [11] *9 50delGAGTCG del17_18GV Low IBD, inflammatory bowel disease; ALL, acute lymphoblastic leukemia; AZA, azathioprine; 6-MP, 6-mercaptopurine. J. Pers. Med. 2021, 11, 778 3 of 10 2. NUDT15 Variants and Toxicities in Thiopurine Metabolism 2.1. Thiopurine for Inflammatory Bowel Disease Thiopurine is administered for maintenance of remission and is used as a steroid- sparing treatment for IBD. Approximately 25% of patients experience thiopurine-induced toxicities, especially myelosuppression [18]. The first study that reported the association between NUDT15 polymorphisms and 6-MP-induced toxicities was the GWAS conducted in Korean patients with Crohn’s disease. Genotyping was performed in 978 patients us- ing the immunochip array, while genotype imputation was carried out using the Asian reference panel. This study identified that the NUDT15 c.415C>T (p.R139C) variant was strongly associated with thiopurine-induced early leukopenia (white blood cell [WBC] count <3000/mm3) (odds ratio [OR] = 35.6; P [combined] = 4.88 × 10−94)[8]. The associa- tion between the c.415C>T variant and with early leukopenia was replicated, and severe hair loss occurred within 8 weeks in 142 Japanese IBD patients with homozygous T variants treated with thiopurine [19]. The NUDT15 enzyme with 139C showed deficient activity and was significantly associated with thiopurine-induced leukopenia and alopecia. Some Asian study groups have validated the association between p.R139C or other variants of NUDT15 and thiopurine-induced