Genetic susceptibility to therapy-related leukemia after Hodgkin lymphoma or non-Hodgkin lymphoma: role of drug metabolism, apoptosis and DNA repair

Therapy-related myelodysplasia or acute myeloid leukemia (t-MDS/AML) is a major cause of non-relapse mortality in patients treated for Hodgkin lymphoma (HL) or non-Hodgkin lymphoma (NHL). t-MDS/AML is associated with exposure to alkylating agents and topoisomerase II inhibitors. The DNA-damaging events caused by these agents initiate apoptosis required for antineoplastic activity; occasionally, imperfect repair of chromosomal damage results in chromosomal aberrations, leading to leukemogenesis. The inter-individual variability in the risk of t-MDS/AML for any given exposure to genotoxic agents suggests the role of genetic susceptibility. In addition, t-MDS/AML shares morphological and genetic similarity with de novo MDS/AML, suggesting that therapy-related and de novo MDS/AML may share genetic susceptibility loci. Previous studies have been largely inconclusive, primarily because of the focus on single genes. In the few studies where multiple genes were examined simultaneously, individuals with more than one risk variant were at higher risk. We hypothesized that genetic variations encoded in key genes in the pathways of drug metabolism, apoptosis, DNA synthesis, methylation and repair, as well as genes involved in de novo AML, could potentially contribute to the risk of t-MDS/AML. Using both genotype and gene expression analyses, we investigated whether individual genetic variability in these pathways modify the risk of t-MDS/AML in patients with HL or NHL exposed to genotoxic agents (Figure 1). We also tested for synergy between apoptosis and other hypothesized pathways. Patients treated with conventional therapy or autologous hematopoietic cell transplantation (aHCT) for HL or NHL formed the sampling frame for this case-control study. Cases (n1⁄4 49) consisted of patients who subsequently developed t-MDS/AML. Controls (n1⁄4 49) were drawn from the same sampling frame, did not have t-MDS/AML and were matched to cases using the following criteria: primary diagnosis (HL or NHL), age at primary diagnosis and year of primary diagnosis, length of follow-up from diagnosis (longer for controls) and genetic ancestry. To further refine matching on genetic ancestry, we used STRUCTURE 2.0 8) to estimate ancestry composition of study subjects based on 51 informative markers (AIM) (Supplementary Methods and Supplementary Table 1). Gene expression patterns were studied in hematopoietic stem/progenitor cells (HSC) from peripheral blood stem cell (PBSC) autografts from a subset of 12 NHL cases and 22 matched controls. The Human Subjects Protection Committee at City of Hope approved the protocol. Informed consent was provided according to the Declaration of Helsinki. For each study participant, cumulative therapeutic exposures were calculated, as detailed in the Supplementary Methods. Demographic and clinical characteristics of the study subjects are summarized in Supplementary Table 2. We genotyped 29 SNPs and 2 deletion polymorphisms for 23 candidate genes (Supplementary Table 3). Association between individual polymorphisms and t-MDS/AML was analyzed using exact conditional logistic regression, adjusted for gender, treatment modality (conventional therapy vs aHCT) and cumulative exposure to alkylating agents and topoisomerase II inhibitors. We observed a higher risk for t-MDS/AML among patients with deletion of GSTM1, the Pro allele of P72R in TP53, the T allele of CYP1A1*2A, and the T allele of rs6030469 of PTPRT (Table 1). None of these associations withstood Bonferroni adjustment for multiple comparisons. TP53 modulates DNA repair and apoptosis upon DNA damage. A common germline polymorphism of TP53, P72R, produces a proline to arginine change that enhances apoptotic activity 15-fold. We used likelihood ratio tests with permutations implemented in UNPHASED to explore unadjusted gene--gene interaction between P72R in TP53 and all polymorphisms in other candidate genes. A significant interaction between P72R and C677T, a coding SNP in MTHFR, was detected and remained significant after correction for multiple testing using 10 000 permutations (adjusted Pinteraction1⁄4 0.048). This interaction was confirmed after adjustment for therapeutic exposures (Table 1). The homozygous T allele of C677T increased the risk 71-fold (P1⁄4 0.0059) when combined with the Pro carrier of P72R (conferring decreased apoptotic activity) compared with its combination with homozygous Arg. We also detected an interaction between P72R and another coding SNP, A1298C, in MTHFR (Table 1). The homozygous A allele of A1298C increased the risk 33-fold (P1⁄4 0.0005) when combined with the Pro carrier of P72R compared with its combination with homozygous Arg in the adjusted analysis. Figure 1. Candidate genes in the biological pathways implicated in the pathogenesis of t-MDS/AML. Citation: Blood Cancer Journal (2012) 2, e58 doi:10.1038/bcj.2012.4 & 2012 Macmillan Publishers Limited All rights reserved 2044-5385/12