mRNA expression levels of methotrexate resistance-related proteins in childhood leukemia as determined by a standardized competitive template-based RT-PCR method

Drug resistance of leukemic blasts is correlated to event-free survival and might be predicted by mRNA expression of drug resistance-related proteins. Methotrexate (MTX) is an important component in the treatment of childhood leukemia. Mechanisms of MTX resistance include (1) decreased transport via the reduced folate carrier (RFC), (2) altered levels of target enzymes, eg dihydrofolate reductase (DHFR) and thymidylate synthase (TS), (3) decrease d ratio of folylpolyglutamate synthetase (FPGS)/falylpolyglutemate hydrolas e (FPGH). We designed competitive templates for each of these genes to meas ure mRNA expression by quantitative RT-PCR and normalized the expression to that of p-actin. T-lineage acute lymphoblastic leukemia (T-ALL), relativel y MTX resistant compared to common/preB-ALL, displayed higher mRNA levels o f DHFR and TS (three- and four-fold higher, respectively; P < 0.001), while FPGS expression was lower (three-fold, P = 0.006) compared to common/preB- ALL. The ratio of (DHFR x FPGH)I(RFC x FPGS) was more discriminating betwee n T-ALL and c/preB-ALL (eight-fold higher; P < 0.001) than either target in dependently. Acute myeloid leukemia (AML) cells, considered MTX resistant, expressed two-fold lower levels of FPGS mRNA compared to c/preB-ALL (P = 0. 04). The ratio of FPGH/FPGS was more discriminating between AML and c/preB- ALL (fourfold higher; P = 0.001) than either target independently. For the total group of 79 leukemic samples, mRNA expression of DHFR varied 549-fold and paralleled TS mRNA expression (r = 0.80; P < 0.001). Although variatio ns in mRNA expression resembled variations in functional activity, no direc t correlations were found for RFC (58-fold variation in mRNA expression), F PGS (95-fold) and FPGH (178-fold). In conclusion, differences in mRNA expre ssion of MTX resistance parameters between leukemic subtypes as detected by competitive RT-PCR are in line with known differences in MTX resistance.