Abstract

Background

The tumor suppressor p53 is frequently mutated in human cancer, including acute myeloid leukemia (AML), particularly in cases with high-risk cytogenetics. It has been shown that p53 stabilization, which frequently occurs when the protein is mutated, can compromise its function. We have shown that p53 stabilization, regardless of the presence of mutations, suggesting alterations of other components in the p53 pathway.

Methodology

p53 expression was determined using high-throughput reverse phase protein array (RPPA) technology in 719 samples from 511 pts. Eleven CD34+ bone marrow (BM) and 10 normal peripheral blood (PB) lymphocyte samples were used as controls. Samples were printed as 5 serial 1:2 dilutions in duplicate using an Aushon 2470 Arrayer. Mutational status of p53 alleles was assessed by Sanger sequencing of exons 5 through 9. Expression of components of the p53 pathway was determined using standard immunohistochemical techniques. Nutlin-3a was used in in vitro culture experiments.

Results

Paired PB- and BM-derived AML samples expressed similar p53 levels (p=0.25). A trend towards higher p53 expression at relapsed was observed among 47 paired diagnosis/relapse samples (p=0.07). p53 expression correlated directly with CD34 (p=0.001) and inversely correlated with WBC (p=0.007), PB and BM blast burden (p=0.0001), and survival (p=0.01). High p53 (p53high) expression was more associated with unfavorable cytogenetics, particularly -5 (p=0.00001). p53high resulted in lower complete remission (CR) rates (51% vs 56%; p=??), higher relapsed rates (82% vs 62%; p=??), and shorter median overall survival (OS; 29.8 vs. 51 wks, p=0.009) compared to p53low pts. Most cases with p53high had unfavorable cytogenetics. We next correlated p53 stabilization with the presence of p53 mutations in 68 pts. p53 mutations were detected in 20/54 (37%) p53high pts and in 0/14 (0%) pts with p53low. p53high, either in the presence (29 wks) or in the absence (24 wks) of p53 mutations (p=1.0), was associated with significantly shorter OS compared with p53low pts (56 wks; p=0.05). Multivariate analysis revealed p53 expression to be an independent risk factor for survival in AML (p=0.02). p53high was positively correlated with p53pSER15 (p=0.00001), Rbp807p811 (p=0.0002), BAD (p=0.0001), cleaved PARP (p=0.002), and cleaved PARP (p=0.01), and negatively with p21 (p=0.01), and MDM2 (p=0.001).Given the similar OS in p53high pts carrying mutant or wild-type p53, we scored the immunohistochemical expression of MDM2, MDM4, and p21 in 30 p53high pts (9 p53 mutated, 21 wild-type p53). Overexpression of MDM2 was observed in 44% vs 48% pts with mutant vs wild-type p53, respectively, whereas rates were 67% vs 62% for MDM4, and 0% vs 19% for p21, for each respective genotype. Overall, of the 21 p53high pts carrying wild-type p53, 15 (71%) had overexpression of MDM2 and/or MDM4, whereas 81% had no p21 expression, indicating deficient activation of the p53 pathway similar to those cases carrying mutant p53. We are currently assessing response to nutlin-3a therapy in 24 primary AML samples (4 mutant p53, 20 wild-type p53). Results showing the impact of p53 mutation and/or stabilization, and expression levels of MDM2, MDM4, and p21 on nutlin-3a therapy will be presented.

Conclusions

p53 stabilization (p53high) is a powerful predictive and prognostic factor in AML, which is independent of the presence of mutant p53 alleles. Poor outcomes in pts with p53high lacking p53 mutations are very frequently associated with overexpression of negative regulators of p53 such as MDM2 and/or MDM4 and p21 downregulation, indicating a functionally altered p53 pathway. These findings may have implications for therapies targeting the MDM2/p53 axis in AML.

Disclosures:

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.