Cellular expression of MDM2 and p53 in childhood leukemias with poor prognosis

Previous studies have suggested that altered expression or dysfunction of the tumor suppressor gene p53 or the oncogene MDM2 could indicate disease progression in children with leukemia who would fail to achieve complete remission or who would relapse. While these studies mainly have described aberrations of MDM2 and p53 function at the DNA and mRNA-level, we have examined p53 and MDM2 expression at the protein level. Mutation of the p53 tumor suppressor gene may result in cellular accumulation of the p53 protein, due to prolonged half-life of the abnormal protein. The p53 protein can also be rendered nonfunctional by overexpression of proteins that bind to p53, such as MDM2. Both pathways have been proposed to disrupt cell cycle regulation in humans. Recent studies have shown that increased expressions of MDM2 as well as of p53 can be detected at the protein level in the absence of gene amplification.

Forty-three bone marrow samples were analyzed immunohistochemically for p53 and MDM2. Twenty-nine bone marrow samples were obtained in children with active, prognostically unfavorable leukemia and MDS. Fourteen bone marrow samples were from children with non-malignant hematological disorders.

p53 protein was expressed in 12 patients and MDM2 in 17 patients with leukemia. In the control group MDM2 expression was detected in one child, while p53 was not found in any of the samples.

Our findings of p53 or MDM2 positive cells in a majority of children with unfavorable prognostic features suggests that dysfunction of the p53-dependent cell growth control have a role in the development of high risk leukemias.