P651. Identification of new genetic variants related to thiopurine-induced myelotoxicity in inflammatory bowel disease (IBD) patients with normal thiopurines-methyltransferase (TPMT): a genome-wide association study
M. Chaparro1, A. González-Neira2, M. Román3, G. Pita2, T. Cabaleiro4, D. Herrero2, B. Herráez2, R. Alonso2, C. Taxonera5, P. López-Serrano6, P. Martínez-Montiel7, I. Vera8, F. Bermejo9, A. López-San Román10, F. Abad-Santos4, J.P. Gisbert1, 1Hospital Universitario de La Princesa, IP and CIBERehd, Gastroenterology Unit, Madrid, Spain, 2Spanish Narional Cancer Research Institute, Madrid, Spain, 3Hospital Universitario de La Princesa and IP, Farmacy Unit, Madrid, Spain, 4Hospital Universitario de La Princesa and IP, Clinical Pharmacology Unit, Madrid, Spain, 5Hospital Clínico San Carlos, Gastroenterology Unit, Madrid, Spain, 6Hospital de Alcorcón, Gastroenterology Unit, Madrid, Spain, 7Hospital Doce de Octubre, Gastroenterology Unit, Madrid, Spain, 8Hospital Puerta de Hierro, Gastroenterology Unit, Madrid, Spain, 9Hospital de Fuenlabrada, Gastroenterology Unit, Madrid, Spain, 10Hospital Ramón y Cajal, Gastroenterology Unit, Madrid, Spain
Only a minority of patients with thiopurine-induced myelotoxicity are carriers of a mutant TPMT allele.
Aim: To identify genetic variants associated with thiopurine-induced myelotoxicity in IBD patients with TPMT wild-type alleles and normal activity of this enzyme.
93 IBD patients with normal TPMT activity and wild-type genotype treated with thiopurines were included. Case group: patients with thiopurine-induced myelotoxicity and without other thiopurine-induced adverse effect. Control group: patients without thiopurine-induced side effects consecutively included. TPMT activity over 13.7 UI/mL was considered normal. DNA was extracted from peripheral blood nucleated cells. TPMT genotype was determined by sequencing (TPMT*2, *3A, *3B, *3C, *3D, *4, *5, *6, *7, *9, *10, *15, *16, *19 and *22 alleles). Myelotoxicity was defined as <3,000/ml leucocytes, <1,500/ml neutrophils, or <100,000/ml platelets. Genomic DNA was analysed using Illumina OmniExpress Exome BeadChip genotyping array. This array interrogates a total of 951,117 single nucleotide polymorphisms (SNPs) (660K common and 200k rare coding variants). Genotype calls were generated using Illumina GenomeStudio. After standard QC control three samples were excluded. Logistic regression analysis for each SNP using PLINK package and gene-based analysis using SKAT software was performed.
The distribution of gender, type of IBD, mean age, mean TPMT activity and mean dose of mercaptopurine were similar between cases and controls (34 cases and 56 controls). The percentage of patients with mercaptopurine was higher among cases (32.4 vs. 12.5%, p = 0.02), while mean azathioprine dose was slightly higher among controls (2.2 vs. 2.5 mg/kg, p = 0.03). Five SNPs with p < 10−4 were found: SNP1 (p = 6.2×10−5, OR = 0.1); SNP2 (p = 7.6×10−5, OR = 10.2); SNP3 (p = 8×10−5, OR = 7.3), SNP4 (p = 9.4×10−5, OR = 4.2) and SNP5 (p = 7.8×10−5, OR = 6.5). The last SNP is located close to a gene that encodes a relevant enzyme of the thiopurine metabolic pathway. Through gene-based analysis a gene, GENE1, that encodes an enzyme that catabolizes a mediator of inflammation present in the human bone marrow, was associated with myelotoxicity development (p = 1.7×10−4).
Using a genome-wide approach in patients without TPMT deficiency 5 new SNPs and genetic variants in a gene have been found to be associated with thiopurine-induced myelotoxicity. The genetic variants in this gene could alter the levels of a mediator of leukocyte functions, platelet aggregation and inflammation, that could explain the susceptibility differences to suffer myelotoxicity. These promising results need to be validated.
- Posted in: Poster presentations: Genetics (2014)