Exploring the Significance of TP53, ABCB1 and GST Polymorphisms on Susceptibility and Pharmacogenetics in Argentinian Patients with Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is specifically associated with the t(9;22)(q34;q11) reciprocal translocation giving rise to the Philadelphia chromosome and the subsequent formation of the BCR/ABL1 fusion gene, encoding a constitutively active tyrosine kinase. Advances in targeted therapies in chronic phase CML, notably the use of tyrosine kinase inhibitors (TKIs) such as imatinib mesylate (IM), have achieved successful treatment outcomes. However, some patients fail to achieve optimal response, and a substantial proportion of patients develop resistance to IM, which is frequently associated with mutations in the ABL kinase domain. Although BCR/ABL1 fusion oncogene is a key molecular marker involved in the pathogenesis and the clinical course of CML, molecular or cellular events that initiate leukemogenesis or drive translocation of the BCR/ABL1 genes are incompletely understood and little is known about individual susceptibility to this disease. Moreover, it is still unclear whether BCR/ABL1 oncoprotein alone is sufficient to explain the full range of clinical responses to ITKs. There is mounting evidence that genetic factors may play an important role in susceptibility to CML and variability in drug responsiveness. Polymorphic variants of several genes are linked to variations in expression, function, drug disposition and drug response and are potential factors accounting for susceptibility to complex diseases or drug resistance as they can uniquely influence the quality and quantity of gene product. Association studies have been performed to identify genetic variants associated with CML susceptibility and progression, but data are lacking for Argentina. In the present study, we determined the distribution of polymorphisms on TP53 tumor suppressor gene, drug transporter (ABCB1) and drug-metabolizing (glutathione-S-transferases, GSTs) genes to identify markers of susceptibility and pharmacogenetic response in Argentinian patients with CML.

Genomic DNA samples from peripheral blood of 141 patients (69 females/72 males, median age 50.8 ± 1.3 years,) treated with IM and 141 age and sex matched healthy controls were evaluated. IM therapy failure was defined by cytogenetics and qRT-PCR in 2 consecutive studies, finding 76 cases that fail treatment. All individuals provided their informed consent according to institutional guidelines and the study was approved by the Ethics Committee of our Institution. GSTM1 and GSTT1 gene deletion polymorphisms and single nucleotide polymorphisms (SNPs) in GSTP1 (313A> G), TP53 (215C> G) and ABCB1 (3435C> T, 1236 C> T and 2677G>T/A) were determined using PCR-based methods. BCR/ABL1 transcript level was analyzed using RT-PCR and ABL1 mutations were identified by RT-PCR and sequencing. Comparison of genotypes between patients and controls as well as regarding clinical parameters was performed by logistic regression. The Kaplan-Meier curves were analyzed using the log-rank test. The level of significance was p <0.05. The case/control association analysis highlighted that carriers of TP53 CC+CG genotypes influence CML susceptibility (p=0.007; OR: 4.5; CI: 1.5-13.3), but no other markers were significantly involved. Next, the pharmacogenetic analysis was performed associating different genotypes with molecular response and treatment failure. We determined that patients carrying GSTM1 -present genotype did not reached major molecular response (p=0.038; OR: 2.9; CI: 1.16-7.4). In addition, average time to treatment change was significantly lower for carriers of the following genotypes: ABCB1 1236TT (23 months), ABCB1 3435TT (13.6 months) and GSTM1 -present (53 months), respect to patients with other variants (p=0.017; p=0.0046 and p=0.04, respectively). Finally, patients with TP53 CC+CG polymorphisms had an increased risk of progression (p=0.039) and lower overall survival (p=0.017). These results indicate that TP53 genotype may represent a genetic cofactor that influences susceptibility to CML and genotypic variations of TP53, ABCB1 and GSTM1 may modulate the response to imatinib.