Oncogene amplification in urothelial cancers with p53 gene mutation or MDM2 amplification

Previously, p53 (also known as TP53) gene mutations have been shown to be frequently detected in highly malignant urothelial cancers. Evidence has been accumulating that the disruption of the normal function of p53 may lead to genomic instability, including predisposition to gene amplification. Furthermore, the normal function of p53 may be abrogated by MDM2 (murine double minute-2) gene amplification in some human tumors.

Our purpose was to investigate the relationship between protooncogene amplification and p53 alteration in urothelial cancers by examining the existence of amplification of MDM2 and 14 other protooncogenes in 50 urothelial tumors in which p53 gene status was known.

We analyzed gene amplification by Southern-blot analysis in 50 urothelial cancer specimens. These tumors were previously examined for p53 mutations by polymerase chain reaction-single-strand conformation analysis, and 17 tumors contained p53 mutations.

Two high-grade advanced tumors (4%) without p53 mutation harbored MDM2 amplification with concurrent int-2 gene amplification. As for other genes, amplification was detected for int-2 (also known as WNT2) (seven [14%] of 50), erbB-2 (also known as ERBB2) (three [6%] of 50), N-ras (also known as NRAS) (one [2%] of 50), L-myc (also known as MYCL1) (one [2%] of 50), and raf-1 (also known as RAF1) (one [2%] of 50). The amplification of at least one gene examined was observed in 11 (22%) of 50 tumors. The presence of p53 mutations was not significantly associated with the occurrence of gene amplification, since the amplification was detected in six (35%) of 17 tumors with p53 mutations and in five (15%) of 33 tumors without p53 mutations. However, eight (73%) of 11 tumors with proto-oncogene amplification harbored p53 mutations or MDM2 amplification.

A subset of advanced urothelial cancers without p53 mutations may harbor MDM2 amplification. This finding should be taken into account when adopting p53 alteration as a marker of aggressiveness in urothelial cancers. Although the abrogation of normal p53 function may be one of the key steps to protooncogene amplification, the data further indicate that the predisposition to gene amplification in urothelial cancers was not determined by the presence of p53 alteration alone.