The oncogenic potential of Pax genes.

Our results demonstrate that murine paired domain-containing genes (Pax) can promote oncogenesis in tissue culture cells and in mice, and should thus be classified as a novel group of proto-oncogenes. The induction of tumor formation in mice was dependent on a functional paired domain, but did not require the presence of a homeodomain. Consequently, not only the Pax-3 and Pax-6 proteins, which in addition to paired domains contain intact homeodomains, but also Pax-2 and Pax-8, containing only residual homeodomains, and Pax-1, completely lacking a homeodomain, were able to induce transformation of cell cultures and tumor formation in mice. The oncogenic potential of the Pax proteins is dependent on the DNA binding function of the paired motif, as the Un-Pax-1 protein, which carries a point mutation in this domain that impairs DNA binding, is also defective in tumor formation. Therefore, the Pax gene products are not only involved in controlling embryogenesis, but they can, if deregulated, also induce tumorigenesis.

The oncogenic potential of deregulated homeobox genes.

Homeobox genes encode DNA-binding proteins that regulate gene expression during embryonic development. Deregulation of homeobox gene expression has been invoked as the molecular basis of a number of leukemias. We now demonstrate, using in vitro and in vivo transformation assays, that the overexpression of homeobox genes is the basis for transformation and tumorigenesis. Thus, clustered homeobox genes related to Drosophila Hom genes (Hoxa-7, Hoxa-5, Hoxa-1, Hoxb-7, and Hoxc-8) as well as the homeobox genes Evx-1 and Cdx-1 constitute a new family of nuclear protooncogenes. This finding suggests that many homeobox genes regulating embryogenesis can--if deregulated--also contribute to tumorigenesis.

A deletion in the simian virus 40 large T antigen impairs lytic replication in monkey cells in vivo but enhances DNA replication in vitro: new complementation function of T antigen.

We describe a new complementation function within the simian virus 40 (SV40) A gene. This function is required for viral DNA replication and virus production in vivo but, surprisingly, does not affect any of the intrinsic enzymatic functions of T antigen directly required for in vitro DNA replication. Other well-characterized SV40 T-antigen mutants, whether expressed stably from integrated genomes or in cotransfection experiments, complement these mutants for in vivo DNA replication and plaque formation. These new SV40 mutants were isolated and cloned from human cells which stably carry the viral DNA. The alteration in the large-T-antigen gene was shown by marker rescue and nucleotide sequence analysis to be a deletion of 322 bp spanning the splice-donor site of the first exon, creating a 14-amino-acid deletion in the large T antigen. The mutant gene was expressed in H293 human cells from an adenovirus vector, and the protein was purified by immunoaffinity chromatography. The mutant protein directs greater levels of DNA replication in vitro than does the wild-type protein. Moreover, the mutant protein reduces the lag time for in vitro DNA synthesis and can be diluted to lower levels than wild-type T antigen and still promote good replication, which is in clear contrast to the in vivo situation. These biochemical features of the protein are independent of the source of the cellular replication factors (i.e., HeLa, H293, COS 7, or CV1 cells) and the cells from which the T antigens were purified. The mutant T antigen does not transform Rat-2 cells. Several different models which might reconcile the differences observed in vivo and in vitro are outlined. We propose that the function of T antigen affected prepares cells for SV40 replication by activation of a limiting cellular replication factor. Furthermore, a link between the induction of a cellular replication factor and transformation by SV40 is discussed.