Genetic Variation in the MTHFR Gene Influences Thiopurine Methyltransferase Activity (Technical Briefs‪)‬

The immunosuppressive drug 6-mercaptopurine (6-MP) and its prodrug azathioprine are used in the treatment of inflammatory bowel disease and other disorders of immune regulation (1). Thiopurine methyltransferase (TPMT) inactivates 6-MP by methylation. The genetic variants TPMT*2 to *19 are associated with decreased TPMT activity (2), and TPMT*3A, *3C, and *2 are the most common deficiency-associated variants (1). A heterozygous TPMT genotype (1 in 10 individuals from the general population) is associated with an increased risk of myelosuppression with standard-dose azathioprine therapy (3) and a favorable response to reduced-dose thiopurine therapy (1). Patients with complete TPMT deficiency (1 in 300 individuals from the general population) are at high risk for myelosuppression (4). The erythrocyte TPMT activity distribution is continuous, and concordance between genotype and phenotype in the carrier range varies, depending on where the cutoff is established between the ranges for carriers and noncarriers. We propose that genetic variation in folate metabolism influences TPMT activity and contributes to the lack of concordance between genotype and phenotype in the carrier range.