Cell lines and animal model in the analysis of pharmacogenomics markers in childhood acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most frequent malignancy of childhood. It is the principal cause of cancer–related mortality in children due to a persistent group of patients who does not respond to standard anti-cancer treatment. Susceptible patients may also suffer from number of toxicities associated with intensive chemotherapy treatment. Pharmacogenetic studies of our group, showed that particular genetic variants of the folate dependent enzymes, particularly, dihydrofolate reductase (DHFR) and thymidylate synthase (TS), major targets of methotrexate (MTX), correlate both individually and combined with  increased risk of relapse in patients with childhood ALL. Furthermore, variations of ATF5 gene involved in asparagine synthetase (ASNS) regulation and of ASNS gene were associated with higher risk of ALL relapse or with ASNase related toxicity in patients who received E.coli asparaginase (ASNase). The major goal of my doctoral research project was to further understand from the functional point of view the role of genetic variations underlying therapeutic responses of childhood ALL, by focusing on two major components of ALL treatment, MTX and ASNase. My specific goal was to analyze associations found in clinical setting using cellular proliferation assay in lymphoblastoid cell lines (LCLs, n=93) and xenograft mice model of ALL. Genetic variation in TS polymorphism (homozygosity for triple repeat allele, 3R) and of DFHR haplotype *1b (defined by particular allelic combination derived from six polymorphic sites in the major and minor promoter of DHFR), on MTX sensitivity was assessed using cellular proliferation assay. Similar in vitro assay in response to E.coli ASNase was used to access the T1562C variation in the ATF5 5’UTR and particular haplotypes of ASNS gene (defined by two genetic variation and arbitrarily named haplotype *1).  Xenograft mouse model was used to access the effect of TS 3R3R genotype.  Analysis of additional polymorphisms in ASNS gene revealed diversification of haplotype *1 of ASNS gene in 5 subtypes, two polymorphisms (rs10486009 and rs6971012,) defining particular subtypes correlated with in vitro sensitivity to ASNase and one of them (rs10486009) seems particularly important for reducing sensitivity to ASNase in vitro, possibly providing mechanistic explanation for lower sensitivity of haplotype *1 observed in clinical setting. No association between ATF5 T1562C variation and cellular proliferation assay in response to E.coli ASNase was found.             We did not observe genotype-related association when in vitro sensitivity to MTX in LCLs was analyzed.  In contrast, in vivo results using xenograft mouse model demonstrated the relationship between the TS 3R/3R genotype and the resistance to MTX treatment in dose-dependent manner. Obtained results provided function explanation for the significantly higher risk of relapse seen in 3R/3R ALL patients and suggest that these patients might benefit from increase dose of MTX. Through these experiments we also showed that xenogeneic mice model can serve as a preclinical tool to explore individualized treatment options.  In conclusion, the knowledge acquired through my doctoral work confirmed and/or identified some functional variants in MTX and ASNase action pathway which may facilitate dose individualization strategies, allowing for optimal treatment selection or tailoring childhood ALL therapy based on individual genetics.

Keyphrases: Dihydrofolate reductase, Thymidilate synthase, Asparaginase, Pharmacogenomics, Childhood leukemia, Animal model