HEMIZYGOSITY OR HOMOZYGOSITY - A REQUIREMENT FOR SOME BUT NOT OTHER P53 MUTANT PROTEINS TO ACCUMULATE AND EXERT A PATHOGENETIC EFFECT

Activating mutations within the p53 gene cause stabilization and there fore increased steady-state levels of the p53 protein; some, but not a ll, also result in the generation of an epitope recognized by the anti body pAB240. We have shown here that in 70% of Burkitt lymphoma cell l ines, but not in normal EBV transformed B cell lines, p53 protein is r eadily detectable by Western blot analysis using either an antibody di rected against the 240 epitope or an antibody against wild-type p53. G enomic analysis of these BL cell lines demonstrated the presence of mu tations within the p53 gene in all cell lines with detectable p53 prot ein. We have also shown that in the cell lines ST486, Raji, and TE 110 , which are heterozygous for a neutral sequence codon polymorphism (Ar g/Pro) that causes altered migration of an otherwise normal protein an d also contain a heterozygous mutation, only the protein derived from the mutated allele is stabilized. Thus the dominant effect of the muta tions present in these cell lines apparently does not result from sequ estering of the normal protein by the abnormal protein, and therefore presumably is a consequence of a gain-of-function resulting from the m utation. Although all cell lines with stabilized p53 also contained p5 3 mutations, three lymphoid tumors (two cell lines and one fresh B-CLL ) with a heterozygous mutation at codon 248 did not express elevated l evels of p53. In contrast, p53 was readily detectable in Western blot analysis from cell lines KK124, Namalwa, and CA46, which had homo- or hemizygous mutations at codon 248, and from PP1084, a cell line with a codon 273 mutation and a carboxyl-terminal truncation in the other al lele. These results suggest that mutations at codon 248, unlike those in cell lines ST486 and TE110, are stabilizing only in the absence of the wild-type p53. Heterozygous mutations at codons 248 have been desc ribed in the germline of individuals belonging to cancer-prone familie s described by Li and Fraumeni (see ref 18), but tumors detected in su ch individuals are homozygous, i.e., contain only mutated p53. This is consistent with the possibility that such mutations exert a pathogene tic effect only in the absence of the wild-type protein, and are coupl ed to our findings that stabilization of p53 is a necessary component of the oncogenic pathways relevant to p53. However, whereas some mutat ions are stabilizing in the presence of the normal p53 protein, others are stabilizing only in the homozygous state.