FOLATE GENE VARIANTS AFFECT METHOTREXATE-RELATED TOXICITY IN ADULT ACUTE LYMPHOBLASTIC LEUKEMIA PATIENTS

Background. Methotrexate (MTX), an antifolate, is an important component of maintenance therapy of acute lymphoblastic leukaemia (ALL). The major MTX activity is the impairment of folate metabolism with consequent antiproliferative effects. MTX exerts a competitive inhibition of dihydrofolate reductase (DHFR), the enzyme which reduces the dihydrofolate (DHF) into tetrahydrofolate (THF), resulting in the lack of folate pool and impairment of purine and pyrimidine synthesis. Another crucial folate metabolism enzyme is the 5,10-methylenetetrahydrofolate reductase (MTHFR). It catalyzes the conversion of 5,10-methyleneTHF into 5-methylTHF, a major circulating form of folate. MTHFR gene variants, which reduce the enzyme activity, have been involved in the risk and in the outcome of patients affected by haematological malignancies. Specifically, MTHFR C677T variant reduces adult ALL susceptibility and increases drug-related toxicity in non-Hodgkin lymphoma when folate antagonists are used [1]. Recently, a new 19 bp deletion in intron 1 of DHFR gene has been described [2] and the deleted allele has been associated with increased gene expression. Since MTHFR and DHFR are key players in folate metabolism, differences in their activity or gene expression, due to gene variants, might modulate therapy-associated toxicity to antifolate chemotherapeutic agents. Aim. To investigate the impact of MTHFR C677T and DHFR 19 bp deletion on therapy-related toxicity in adult ALL patients. Methods. We analyzed 93 ALL patients (46 male, 47 female; age at onset 40  16 years), treated with MTX in maintenance therapy (15 mg/ m2 weekly for 2 years). Haematological (anaemia, thrombocytic, lymphocytic), and non-haematological (mucositis and hepatic) toxicities were graded according to WHO criteria. Patients were genotyped by polymerase chain reaction –restriction fragment length polymorphism analysis. Genotypes stratified by MTHFR and DHFR variants, were scored by WHO toxicity and the associations were statistically analyzed. Results. When we compared patients without toxicity with patients with any grade of toxicity, the MTHFR 677TT genotype was significantly associated with an increased risk of hepatic (OR 4.46, 95%CI 1.05-19.02; P 0.03) and lymphocytic (OR 3.50, 95%CI 0.84-14.56; P 0.07) toxicity, respect to wild-type genotype. By considering the same comparison, patients carrying the 19 bp deleted allele of DHFR gene (heterozygous + homozygous genotypes) were associated with an increased risk of hepatic (OR 2.71, 95%CI 0.99-7.44; P 0.04) and thrombocytic (OR 5.20, 95%CI 1.08-25.16; P 0.02) toxicity. Notably, the combined analysis for MTHFR and DHFR gene variants showed that patients with a polymorphic allele in both genes had a further increased risk of hepatic toxicity (OR 6.00, 95%CI 1.14-31.53; P 0.05) compared with the risks for each polymorphism. Conclusions. Our data suggest that specific polymorphisms (MTHFR C677T and DHFR 19bp deletion) of the folate cycle play a critical role in ALL therapy-related toxicity, possibly by interference with MTX action. Genotyping of folate pathway gene variants might be useful to reduce chemotherapy toxicity by means of dose adjustments or the choice of alternative treatments.