The p53 and mdm-2 genes in human testicular germ-cell tumors

Mutations in the p53 gene are common in many cancers. They have been documented to occur in about 55% of all cancers of 51 different cell and tissue types. These mutations are accompanied by overexpression of the p53 protein in the nucleus of the cell, and this protein has lost its tumor suppressor function. In this study, 25 testicular germ-cell (TGC) tumors were tested for p53 mutations and the level of p53 protein expression. While 67% of the tumors overproduced the p53 protein in the nucleus of 10-60% of their cells, in all cases the DNA sequence of exons 4-9 of the p53 gene was wild type. In this tumor type, there was apparently no selection pressure for p53 mutations. The mdm-2 gene residues on chromosome 12 (12q13-q14), a chromosome often altered in TGC tumors. mdm-2 gene amplification (2.5- to 10-fold) was detected in three (12%) of these TGC tumors. These three tumors, and eight additional TGC tumors, overexpressed mdm-2 mRNA. There was a good correlation between overexpression of p53 protein and overexpression of mdm-2 mRNA (P = 0.01). This may well result from the fact that the level of mdm-2 mRNA is regulated by the p53 level. These studies demonstrate that TGC tumors fail to be selected for p53 mutations but nonetheless frequently expressed high levels of wild-type p53 protein in the cell nucleus. Perhaps this produces the excellent response to radiation and chemotherapy of these tumors, which generally have a good prognosis. Wild-type p53 may mediate apoptosis in these cells in response to the DNA damage.