Ch 4: Cellular Oncogenes

Key Concepts:

• Unable to find tumor viruses in the majority of human cancers, researchers in the mid-1970s were left with one main theory of how most human cancers arise: the mutation by carcinogens of normal growth-controlling genes, converting them into oncogenes.

• To verify this model's prediction that transformed cells carry mutated genes functioning as oncogenes, a novel experimental strategy was devised: DNA from chemically transformed cells was introduced into normal cells--the procedure of transfection-and the recipient cells were then monitored to determine whether they too had become transformed.

• Cultures of NIH 3T3 cells that had been transfected with DNA from chemically transformed mouse cells yielded numerous transformants, which proved to be both anchorage-independent and tumorigenic; this indicated that the chemically transformed cells carried genes that could function as oncogenes and that oncogenes could arise in the genomes of cells independently of viral infections.

• Further experiments using human tumor donor cells transfected into murine cells showed that the oncogenes could act across species and tissue boundaries to induce cell transformation.

• The oncogenes detected in human tumor cells by transfection experiments and the oncogenes of transforming retroviruses were both found to derive from the same group of preexisting normal cellular genes. This meant that normal proto-oncogenes could be activated either by retrovirus-induced modifications or by somatic mutations.

• Proto-oncogenes were often found to be present in increased copy number in human tumor cell genomes, which suggested that gene amplification resulted in increased levels of protein products that favored the proliferation of cancer cells, an example being the amplification of the my gene in a variety of human cancer types.

• The activation of proto-oncogenes via deregulation by retro viral transcriptional promoters was well documented. However, the mechanism(s) by which normal human proto-oncogenes became converted into oncogenes in the absence of viruses was not evident until the discovery of a point mutation in the H-ras gene that yielded a structurally altered protein with aberrant behavior. This established a new mechanism for oncogene activation based on a change in the structure of an oncogene-encoded protein rather than the expression levels of such a protein.

• Both activation mechanisms--regulatory and structural-might collaborate to create an active oncogene.

• The myc oncogene was initially discovered in the genome of an avian retrovirus.

• This oncogene was found to be activated through three alternative mechanisms in cancer cells: provirus integration, gene amplification, and chromosomal translocation.

• Gene amplification occurs through preferential replication of a segment (an amplicon) of chromosomal DNA. The result may be repeating end-to-end linear arrays of the segment, which appear under the light microscope as homogeneously staining regions (HSRs) of a chromosome. Alternatively, the region carrying the amplified segment may break away from the chromosome and be seen as small, independently replicating extrachromosomal particles (double minutes, DMs).

• Translocation involves the fusion of a region from one chromosome to a non-homologous chromosome. Translocation can place a gene under the control of a foreign transcriptional promoter and lead to its over-expression, as is the case with the myc oncogenes in Burkitt's lymphomas. Translocation may also free an mRNA from inhibition by a microRNA by removing mRNA sequences normally recognized by the miRNA. Alternatively, a translocation can result in the fusion of two protein-coding sequences, leading to a hybrid protein that functions differently than the two normal proteins from which it arose, as is seen in the Bcr-Abl; protein encountered in chronic myelogenous leukemias.

• Besides the amino acid substitutions that activate signaling by the Ras oncoprotein and the fusion of protein domains, as is seen in the Bcr-Abl; protein, yet other changes in protein structure can lead to oncogene activation. For example, truncation of the EGF receptor leads this protein to emit growth-promoting signals in an unremitting fashion.