Azathioprine is a purine analogue that interferes with DNA synthesis and inhibits the proliferation of quickly growing cells, especially cells of the immune system. It is used as an immunosuppressant in patients undergoing organ transplantation, and its metabolite 6-mercaptopurine is used in the treatment of autoimmune diseases and acute lymphoblastic leukemia.
During metabolism, hypoxanthine-guanine phosphoribosyltransferase (HGPRT) converts 6-mercaptopurine to cytotoxic 6-thioguanine nucleotide analogues, while thiopurine methyltransferase (TPMT) inactivates 6-mercaptopurine through methylation to form 6-methylmercaptopurine.
Approximately 11% of the population has reduced TPMT activity and 0.3% of the population has true deficiency of TPMT.  In these patients, active 6-mercaptopurine accumulates, and a larger proportion of 6-mercaptopurine is converted to the cytotoxic 6-thioguanine nucleotide analogues, which can lead to bone marrow toxicity and myelosuppression. [2, 3]
Twenty-eight variant alleles of TPMT have been identified, most of which have been associated with decreased activity in vitro, but only a few of which have known clinical effects.  TPMT*3A, the most common variant (5% frequency in Caucasians), includes two nonsynonymous coding changes: an exon 7 A154T variant and an exon 10 Y240C variant. The second most frequent variant, TPMT*3C, contains only the exon 10 variant and is more common in Asian populations (2% frequency). TPMT*3B occurs rarely and contains only the exon 7 variant.  TPMT*2 is a rare nonsynonymous variant (A80P) with reduced catalytic activity.  Other variants, such as TPMT*8 occur more commonly in African populations (2% frequency). 
Clinical Implications of the Genetic Mutation
Patients with TPMT deficiency treated with standard doses of azathioprine or 6-mercaptopurine are at significantly increased risk of myelosuppression, bleeding, infection, and death associated with increased levels of cytotoxic 6-thioguanine nucleotide levels in the red blood cells. [8, 9] Therefore, it is not recommended that patients with known TPMT deficiency be treated with azathioprine or 6-mercaptopurine; if treatment with these agents is deemed necessary, a low-dosage regimen should be used along with extremely careful hematological monitoring.
Intermediate TPMT activity is associated with an increased risk of developing leukopenia, but there is significant clinical variability due to other modifying factors.  Nevertheless, lower azathioprine dosages have been suggested in some cases for patients with intermediate TPMT activity.  At a minimum, the drug should be administered with caution in these patients.
The US Food and Drug Administration (FDA) recommends but does not require genetic testing for TPMT. As noted on the product label for azathioprine and 6-mercaptopurine, TPMT genotype testing is recommended for the most common nonfunctional alleles, TPMT*2, *3A, and *3C, which account for the vast majority of patients with low or intermediate enzyme activity. Patients heterozygous for a nonfunctional TPMT allele are recommended azathioprine with a reduced dose. A drastically reduced dose, or alternative nonthiopurine immunosuppressant therapy, is recommended for individuals homozygous for nonfunctional TPMT alleles.
Phenotyping quantitates TPMT enzyme activity in erythrocytes, and based on the result, patients are classified as having normal, intermediate, or low TPMT activity. 
According to one study, more than 20% of patients with inflammatory bowel disease (IBD) discontinue thiopurine therapy because of severe adverse drug reactions (ADRs), with leukopenia being one of the most serious ADRs. This prospective study was performed to determine whether genotype analysis of TPMT before thiopurine treatment (and dose selection based on the results) would affect the outcomes of patients with IBD. According to the authors, screening for variants in TPMT did not reduce the proportions of patients with hematologic ADRs during thiopurine treatment for IBD, but there was a 10-fold reduction in hematologic ADRs in variant carriers who were identified and received a dose reduction, compared with variant carriers who did not, without differences in treatment efficacy. [12, 13]
Testing for the Genetic Mutation
Enzymatic activity of TPMT in blood can be determined by a variety of methods including HPLC, and radiochemical assays. Care must be taken to ensure assays measure activity, not concentration, since the common genetic polymorphisms effect enzymatic activity but not necessarily enzymatic levels. Note that TPMT testing cannot substitute for complete blood count monitoring.