Altered conformation of the p53 protein in myeloid leukemia cells and mitogen-stimulated normal blood cells

Expression of the normal p53 gene promotes cell differentiation, maturation and apoptosis. The mutant p53 gene, which does not function normally, is frequently expressed at elevated levels in tumor cells [for review see Lane, D.P. & Benchimol, S. (1990). Genes Dev., 4, 1-8]. We have analysed the expression of and mutational change in the p53 gene in the peripheral blood cells of 49 primary acute myeloid leukemia (AML) patients. The p53 protein levels were elevated in 37 patients (75%) when measured by immunoprecipitation with antibodies PAb1801 and PAb421, which recognize both normal and mutant forms of the protein. The p53 protein from 32 of these 37 patients was immunoprecipitated by PAb240, which recognizes a conformation of p53 protein associated with point mutations. However, point mutations were detected by single-stranded conformation polymorphism (SSCP) assay and direct sequencing in only three patients at codons 178, 245, 273 and 290. Growth stimulation of normal lymphocytes also generated p53 which was immunoprecipitable by PAb240. Thus, alteration of p53 conformation, rather than acquisition of point mutations, could be the mechanism underlying the increased proliferation of myeloid cells in most AML patients.