P343. Biochemical assay to determine thiopurine S‑methyltransferase (TPMT) activity should be used in the Jewish population, rather than genotyping
Y. Kasirer1, R. Segel2, P. Renbaum2, N. Algur3, R. Mevorach1, E. Rachman1, D. Turner1
1Shaare Zedak Medical Center, Pediatric Gastroenterology Unit, Jerusalem, Israel; 2Shaare Zedek Medical Center, Medical Genetics Institute, Jerusalem, Israel; 3Shaare Zedek Medical Center, Biochemistry Laboratory, Jerusalem, Israel
Background: TPMT is a key enzyme that deactivates thiopurines into their inactive metabolite, 6‑methyl-mercaptopurine (6-MMP).
TPMT inheritance is co-dominant, which in Caucasian population results in a tri-modal activity pattern. Low TPMT activity can be found in 11% of Caucasians and very low activity in 0.3%, often resulting in severe leukopenia. To date, more than 23 alleles associated with decreased TPMT activity have been described, with wide variability between nationalities. Despite the high prevalence of inflammatory bowel disease (IBD) among Jews, the mutational spectrum in this population has not been previously described. The aim of this study was to determine TPMT activity and TPMT mutations among Jews in order to develop evidence-based pharmacogenetic assay for this population.
Methods: This was a 3‑phase study. First, TPMT activity was determined in 213 healthy Jewish volunteers using an enzymatic reaction with S‑adenosylmethionine (SAM) as a methyl donor; 6‑MMP, the end product of TPMT, was quantified by high performance liquid chromatography (HPLC). Next, phenotype–genotype correlation was determined through restriction enzyme analysis for the common variant alleles (TPMT*2 (G 238C), TPMT *3A (G 460A and A 719G), TPMT* 3B (G 460A) and TPMT*3C (A 719G)). Finally, discordant cases between phenotype and genotype were fully sequenced to identify any novel mutations.
Results: TPMT activity in the Jewish population was 16±4 U/ml RBC (range 6–34) and exhibited a unimodal pattern. Multivariate regression analysis showed no association between TPMT activity and age, gender and parental origin (r2 = 0.21). 15/213 (7%) samples showed low TPMT activity (<10 U/ml Packed RBC), of which only four (4/15, 26%) were positive for one of the frequent alleles. Sequencing the remainder 11 discordant cases did not reveal any novel mutations.
Conclusions: The scattering of TPMT activity is unimodal in the Jewish population unlike the European and North American Caucasians. Restriction enzyme analysis for the common SNP's failed to detect most of the low activity samples. This suggest that genetic testing for the common alleles is of limited value among Jews and therefore a biochemical assay to determine TPMT enzymatic activity may be superior in this population.