Polymorphisms at the thiopurine S-methyltransferase coding gene (TPMT) determine enzyme activity and as a consequence the development of toxicity to thiopurines
Materials and methods
A total of 108 DNA samples from volunteer donors and 39 from patients with acute lymphoblastic leukemia (ALL) were analyzed. Genomic DNA from peripheral blood leukocytes was isolated by standard methods. Known (wild type and polymorphic) sequenced PCR fragments of the TPMT gene were used as controls. TPMT gene fragments were amplified for exons 5, 7 and 10. PCR products were then analyzed by denaturating high performance liquid chromatography (DHPLC) for the most frequent mutant TPMT alleles, according to the method developed by Schaeffeler et al. on an analysis system from Transgenomics.
No elution profiles at the DHPLC analysis were different to those already reported. The frequency of gene polymorphisms was 53.7% in healthy and 38.4% in the ALL population. However, only 17.6% of all polymorphisms found are considered as functional, being the most frequent the *3A (n = 13, 8.8%), followed by *3B (n = 5, 3.47%), *3C (n = 5, 4.6%) and *2 (n = 3, 2. 7%).
DHPLC is a highly sensitive, rapid and efficient method to identify relevant TPMT gene mutations which allows the screening for genetic variability in the TPMT gene. This is the first analysis of the polymorphisms at this gene in a Mestizos population. The frequency of known silent polymorphisms was higher than those reported in other regions worldwide, but although the frequency of functional polymorphism in healthy population is within the range found in other reports, the frequency of such polymorphism was higher in the patients. All together, the routine typing of TMPT polymorphisms in the patients with ALL is being set in our Institution.
The donation of DNA's controls from Dr. Margaret Fischer-Bosch Institute of Clinical Pharmacology, is greatly appreciated.
This project was supported by CONACYT (SALUD 2004-01-05/A-I), and Psicofarma S.A. De C.V., Mexico.