Thiopurine Drugs in the Treatment of Ulcerative Colitis: Identification of a Novel Deleterious Mutation in TPMT

Abstract

Chronic inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis. Both are characterized by inflammation of part of the digestive tract lining. Azathioprine (AZA) is a well-known immunosuppressant that has been known for many years for its ability to provide long-term disease remission in IBDs, but has important side effects, most of which are related to a single nucleotide polymorphism in the gene for thiopurine methyltransferase (TPMT), which ensures the degradation and efficacy of AZA. Since a direct correlation between TPMT gene polymorphisms and the haematological toxicity of the AZA treatment has been widely demonstrated, TPMT genotyping has been made necessary prior to any introduction of AZA. The monitoring of thiopurine metabolites presents one of the factors that limit wide adaptation of these thiopurines in clinical practice. Thus, identifying patients with asymmetric metabolism could help clinicians provide an ideal treatment recommendation to improve response and reduce adverse effects. Here, we review the role of AZA in the treatment of IBD and discuss the usefulness of TPMT genotyping to guide clinical decision-making. In addition, we report the identification of a new molecular alteration, never described, TPMT mutation affecting the TPMT activity and responsible for deleterious side effects in a clinical case of a 20-year-old woman patient.

Keywords: TPMT; azathioprine; mutation; targeted therapies; ulcerative colitis.

Figure 1

Involvement of TPMT on the thiopurine metabolic pathway. AZA (azathioprine); AOX1 (aldehyde oxidase type 1); FTO (fat mass and obesity associated); GST (gluthation s-transferase); GMPS (guanosine monophosphate synthetase); HGPRT (hypoxanthine guanine phosphoribosyl transferase); IMPDH1 (inosine 5-monophosphate deshydrogenase type 1); IPTA (inosine triphosphate pyrophosphatase); MOCOS (molybdenum cofactor sulfurase); NDPK (nucleotide diphosphate kinase); NUD15 (nucleoside diphosphate linked moiety X type 15); RNR (ribonucleotide reductase); TPMT (thiopurine s-methyltransferase); XDH (synonym-Xanthine oxidase); 6-MP (6-mercaptopurine); 6-MeMP (6-methyl-mercaptopurine); 6-MeMPR (6-methyl-mercaptopurine ribonucleotide); 6-MeTGMP (6-methyl-thioguanine monophosphate); 6-MeTIMP (6-methyl-thioinosine monophosphate); 6-deoxyTGMP (6-deoxy-thioguanine monophosphate); 6-deoxyTGDP (6-deoxy-thioguanine diphosphate); 6-deoxyTGTP (6-deoxy-thioguanine triphosphate); 6-TGMP (6- thioguanine monophosphate); 6-TGDP (6-thioguanine diphosphate); 6-TGTP (6-thioguanine triphosphate); 6-TIMP (6-thioinosine monophosphate); 6-TITP (6-thionosine triphosphate); 6-TXMP (6-thioxanthine monophsphate); 6-TU (thiouric acid); 6-TX (6-thioxanthine).

Figure 2

Allelic variants of the human TPMT locus. The TPMT gene is located on 6p22.3 (NM_000367.2, OMIM 187680) and comprises 10 exons and nine introns. Boxes depict exons in the human TPMT gene and spaces between boxes are introns. White boxes show untranslated exonic regions and black boxes represent exons in the open reading frame (ORF). Grey boxes correspond to different exons with molecular alterations inducing changes in amino acids. The positions of the most frequent SNPs are indicated by vertical arrows. The red box shows exon 7 with the c.483_484del, p.Asp162Serfs * 26 mutation inducing a TPMT * 16 mutation (c.488G > A,p.Arg163His).

Figure 3

Electropherogram obtained with the SeqScape v2.5v software ( APPLIED Biosystem, Foster City, CA, USA) representing part of the sequence of TPMT exon 7. The black arrow shows a silencing mutation. The red arrow shows a novel c.483_484del, p.Asp162Serfs * 26 mutation that generates a premature stop codon in exon 8.