Chromosomal instability at a mutational hotspot in polyoma middle T-antigen affects its ability to activate the ARF-p53 tumor suppressor pathway.

We have isolated spontaneous mutants of polyoma virus middle T-antigen (PyMT) that do not activate the ARF-p53 pathway based on their inability to block REF52 cell division. The REF52 cells containing these mutants have a flat untransformed morphological phenotype and do not express the ARF protein. The PyMT mutations in the different cell isolates so far analysed occur at a mutational hotspot in the PyMT sequence between nucleotides 1241 and 1249, which contains nine consecutive cytosines. In one set of mutants a single cytosine was deleted, while in another mutant set an additional cytosine was inserted. Both these mutations result in frameshifts, generating altered PyMT proteins containing amino-acid sequences derived from each of the two other alternative reading frames of the polyoma virus early region. Both types of mutations result in the loss of the C-terminal PyMT region containing the membrane-binding hydrophobic region and result is mislocalization of the PyMT mutant proteins. Revertant wild-type PyMT (containing nine cytosines) was easily detected in transformants generated after infection of REF52 cells expressing high amounts of dominant negative p53 with retroviruses containing either mutation. We demonstrate that wild-type PyMT revertants are derived from mutations in the hotspot sequence of the integrated mutant PyMT sequences.