After many cancer-causing genes - oncogenes - were found in tumor viruses and their cellular counterparts - proto-oncogenes - identified in normal cells, it seemed possible that mutations in proto-oncogenes might be responsible for cancer. The idea was that these genes could become corrupted and, instead of performing their normal function, could cause proliferation to run amok. If so, all tumors, not only those that were induced by viruses, should bear such mutated genes.
The technique used to pick out oncogenes from among the thousands of genes in the genome appeared daunting. But a technique was soon developed that did the job (see figure). DNA was extracted from tumor cells, fragmented, and introduced into a line of mouse cells growing on a Petri plate. The recipient cells were carefully chosen. They had to grow well in culture and readily take up foreign DNA and integrate it into their chromosomes. If a cell happened to take up a piece of DNA carrying an oncogene, it would take on some of the characteristics of a cancer cell. Namely, it would change shape and its progeny would pile up on one another, forming little clumps among a flat background of normal cells.
But were the cells in these clumps really cancerous? To find out, scientists injected the cells into mice as Rous had done (albeit with chickens) decades previously. And, like Rous, they found that the injected cells did, in fact, produce tumors. With this information in hand, research now turned to far bigger and more difficult question: How do oncogenes turn normal cells into cancerous ones?
The answer to that question is complex. For example, a gene called myc, named for its discovery in an avian myelocytomatosis virus, was found to be a potent oncogene. It's cellular counterpart, c-myc, can become cancer-causing by several mechanisms. One way is by an increase in the number of genes though a process called gene amplification. Apparently, the presence of multiple copies of the myc gene results in an increase in the protein that myc specifies. Since the myc protein is a growth promoter, the result is an acceleration of cell proliferation and cancer. Gene amplification seems to be a general phenomenon. Amplification of a variety of oncogenes beside myc is known to be associated with at least 20 different malignancies including breast, colon, lung and pancreatic cancer.
Another way of turning myc into an oncogene is by modifying the sequences that control its transcription. Normally, genes are regulated by nearby DNA regions that manage the rate of RNA synthesis. In several cancers, myc comes under the control of sequences that increase the rate of transcription, thereby flooding the cell with growth promoters, causing unregulated proliferation.
Still another mechanism of perverting a proto-oncogene is via a change in the amino acid sequence of the protein that it specifies. The ras gene offers an instructive example. The ras gene, named for the rat sarcoma virus in which it was first found, plays an important role in normal cells. However, it can become a cellular oncogene and can be detected as such via the assay described above. DNA sequencing revealed that a single base change, a G to a T, was responsible for turning Dr. Jekyll into Mr. Hyde. The mutation results in a single amino acid change, a glycine into a valine, and that is sufficient to make a difference. The ras gene has been found to be so altered in 90% of pancreatic and 45% of colorectal cancers.
In the next post, I'll begin to delve into the intricacies of how cellular growth is controlled by signals from their surroundings.