In this study, we analyzed the frequency of a functional promoter polymorphism (SNP 309) in the gene MDM2 in a large group of children with AML treated on a cooperative group study. Our findings indicate that the presence of this genetic variant modifies susceptibility to leukemia. Children with the G/G genotype have an increased susceptibility to AML. We initially hypothesized, based on in vitro
data that this variant modifies sensitivity to topoisomerase II inhibitors, that the homozygous variant genotype would decrease susceptibility to AML. Therapeutic topoisomerase II inhibitors such as etoposide are a potent cause of secondary AML. In addition, natural topoisomerase inhibitors exist in the environment, particularly in fruits and vegetables which contain quercetin, beverages such as tea, cocoa and red wine which contain catechins, and soy, which is a sources of genistein [8
]. Increasing levels of maternal consumption of dietary topoisomerase II inhibitors are associated with an increased risk of de novo infant AML with MLL rearrangement [8
]. Therefore, we speculated that the homozygous variant G/G genotype would offer a protective effect against natural topoisomerase inhibitors due to its resistance to topoisomerase II inhibitors and decrease susceptibility to AML. However, our data do not support this hypothesis, but rather demonstrate that the homozygous variant genotype increases susceptibility to AML. The variant G allele of MDM2 has increased binding of the transcription factor Sp1, leading to increased expression of MDM2, and reduced expression of p53, reducing susceptibility of the cell to p53 induced apoptosis. Increased risk of AML in children with the homozygous variant genotype may be due to a failure of apoptosis in cells that have received DNA damage, so preserving potential oncogenic mutations. Our finding of increased risk of malignancy in association with the homozygous variant genotype is in agreement with a number of prior studies. Studies of solid tumors including renal cell carcinoma, colorectal cancer, pancreatic cancer, gastric carcinoma, lung cancer, soft tissue sarcoma and neuroblastoma have reported an increased risk of malignancy in persons with a homozygous variant genotype, with odds ratios typically between 1.5 and 2.0 [16
]. More recently, this genotype was found to be associated increased susceptibility to AML in an older cohort of patients (median age 35) [24
We also hypothesized that the presence of the SNP 309 variant allele might modify response to therapy, as the variant has been shown to modulate sensitivity to topoisomerase II inhibitors, and the homozygous variant genotype has been associated with inferior survival in a number of solid tumors [2
]. Our data showed no differences in survival according to MDM2 genotype. These data do not support our original hypothesis, as we expected an inferior response to therapy in persons with the variant genotype. We saw some minor increase in toxicity in patients with the G/G genotype, but caution is needed in interpreting these data also as data regarding 37 specific toxicities were collected, and these may also be chance findings. It is notable that our study of outcome is the first to include a group of patients all treated with chemotherapy on a uniform clinical trial, allowing us a better opportunity to directly examine the impact of genotype on chemotherapy response, rather than perhaps other biological features of solid tumors that modify survival, as many adult patients with solid tumors do not receive chemotherapy.
Our data contrast with a report of a very significant effect of SNP 309 genotype on outcome of therapy in adults with CLL [7
]. In CLL, deletion or reduced function of p53 is associated with more aggressive disease and reduced survival [7
]. The SNP 309 variant G allele causes over-expression of MDM2 protein and reduction in production of p53 in CLL, leading to a more aggressive disease course, similar to that seen in patients with CLL with deleted or mutated p53. The difference between this study and our results may indicate lesser importance for p53 status in response to chemotherapy in children with AML compared with CLL.
Multiple comparisons were made in the analysis of outcomes and toxicities, and the majority of the observations were negative. Great caution must be used in interpreting the somewhat limited positive findings of increased toxicity in this analysis as they may represent the consequence of multiple comparisons alone. The significant finding of increased susceptibility to AML was based on a simple analysis of genotype frequency in cases vs. controls. These observations regarding the influence of genotype and susceptibility to AML, were also based on a priori hypothesis that susceptibility would be modified and can be regarded with more confidence. However, in common with all association studies, these observations need to be repeated in a independent dataset for confirmation