Chapter 4 Chapter 4: One Misstep, Eternal Hate: Oncogenes Discovered in Human Tumors

In 1976, the announcement of the discovery of the proto-oncogene src pointed out the next direction for researchers.If the src gene does appear in the normal human genome, then, in human tumors, src is likely to exist as a mutated active oncogene.To confirm this, the mechanism used to create such an active src oncogene is quite different from that used by Rous sarcoma virus.By 1976, after six years of fruitless research, most researchers realized that virtually all human tumors harbored no retroviruses like RSV.Therefore, following this line of thinking, if sr' is activated as an oncogene in tumors, then as non-viral factors, chemicals must play an important role in the activation process.These chemicals mutate src while it is nestled in the cell's chromosomes.Every time a mutation occurs in a cell, the The C oncogene induces rapid cell growth, causing the mutant cells to produce a large number of offspring, and eventually form a tumor.So the researchers tried to find the mutated src oncogene in tumor DNA.But they found nothing, no trace of the mutated form of src.It appears that the src oncogene is just an accidental form of RSV.At this point, the idea of ​​simply treating proto-oncogenes as targets for carcinogens seems to have come to an end.

By 1979, another strategy was adopted in an attempt to uncover the true face of oncogenes.The new approach does not rely on existing knowledge about retroviruses, but instead uses the experimental technique of gene transfer.That is, DNA (that is, genes) is extracted from one cell and injected into another cell, which receives the genes that give it new traits or behaviors.Such a response shows that the information specifying the new trait originally belonged to the donor cell (the cell from which the DNA was extracted), and the information can be transferred to the recipient cell through the transplantation of DNA molecules.

Gene transplantation can be used to search for possible oncogenes in the genomes of mouse, rat, and human tumor cells.The tumors formed by these cells have nothing to do with viruses.Most of these tumors are known or suspected to be caused by chemical carcinogens. My laboratory initially conducted an experiment in which rat cells were exposed to a carcinogen called coal tar to turn them into cancer cells, and DNA extracted from them was then injected into normal cells.We hope that the cancerous information will appear in the DNA of chemically modified cells, and when this information is inserted into normal cells in the form of specific genes, the latter will also be transformed into cancerous cells accordingly.

Soon, we found that some cells that received the tumor cell's DNA began to change.The transformation of normal cells into cancerous cells proved that the cancerous information was carried by DNA, which came from chemically modified cells.Information about tumor growth can indeed be transferred from cell to cell via DNA molecules. Soon, it was discovered that the DNA of human tumors also contained oncogenes.Research in the respective labs of Jeffrey Cooper and Mikael Wiegler, and continued efforts by my research group, have expanded the range of genomes with information on tumor growth: 1 bladder cancer, 1 colon cancer , There is also a case of nerve cell tumors whose DNA has the ability to transform.In each case, normal mouse cells became cancerous because they were injected with cancerous DNA, even though normal cell DNA was not capable of transformation.

This portable genetic information behaves much like the oncogenes carried by oncoviruses.Most importantly, different fragments of cancer cell DNA injected into normal cells seemed to guide the metabolism of normal cells, prompting them to gradually become cancerous.These findings provide direct evidence that mutations in genes carried by normal cells can lead to cancer. The mutational process that creates oncogenes appears to be the same process that damages many other cellular genes: the sequence of bases changes while the gene is in its usual place on a chromosome.After the mutation, the genes are still in place, but they start sending very different instructions to the cell than before the mutation.

This scenario differs dramatically from that revealed by retrovirus research.All retroviruses that possess the ability to transform seem to have originated from copies of viruses that invade normal cells, hijack their genes, and turn them into active oncogenes.As for retroviruses, invading foreign enemies, powerful viral rulers, they destroy normal genes, drive genes far from their chromosomal nests, conquer normal cellular genes, and oncogenes are finally activated. However, these two different assumptions have a common thread: both cellular oncogenes and viral oncogenes originate from the pre-normal genes of cells—proto-oncogenes.But this commonality leads to an obvious question: what is the link, if any, between proto-oncogenes mutated by chemicals or radiation, and those hijacked and activated by retroviruses? Woolen cloth?

The answer came in 1982.At this time, using new techniques of gene cloning, people have been able to isolate certain human tumor oncogenes.The researchers compared an oncogene isolated from bladder cancer with a large collection of proto-oncogenes previously cloned by retrovirus researchers.The comparison revealed a striking link: the human bladder oncogene was virtually identical to the ras oncogene discovered by the retrovirus researchers. The different pieces of the cancer mystery came together at once.Here's how it went: When the rat cells were infected with the virus, the retrovirus acquired and activated the ras proto-oncogene just as the RSV (Rous sarcoma virus) precursor acquired the src gene.Harvey sarcoma virus (H8rVeyS. fCOm. Viflls) factor produces this activated. After the S oncogene, the latter can transform normal rodent cells into actively growing tumor cells.

Not surprisingly, a relative of the ras proto-oncogene exists in almost identical form in normal human DNA.By the early 1980s it was known that all proto-oncogenes shared the same face in the genomes of all mammals and birds. The ras proto-oncogene in human bladder cells has a different fate than its relative in the rat genome.After some chemical mutagens enter the bladder cells, the ras proto-oncogene is mutated and becomes an active oncogene.Once mutated, the ras oncogene drives the mutated cell and its direct descendants on a path to expansion, producing large numbers of bladder cancer cells with the mutated ras oncogene.

Thus, the proto-oncogene tricks uncovered by retrovirologists have a direct connection to the origins of human cancer.Normal genes captured and modified by retroviruses in animals can serve as targets for chemical mutagens in humans.These human genes were turned into potent oncogenes by mutagenized molecules, although they still did not change their position in the chromosomes of the target cells. The Myc gene is just a drop in the ocean. The myc gene was first known for its association with chicken myelocytoma; within a few months, mutations in the myC proto-oncogene were identified in human lymphoma and leukemia cases.Subsequently, in neuroblastoma, a proto-oncogene called N-mpc, a close relative of myc, was discovered; then, erbB, which was originally discovered because of its association with chicken erythroleukemia, and its variants were found in human stomach, breast, etc. , ovarian, and brain tumors.

Now the researchers can simplify the point of view.All vertebrate cells appear to carry a common set of proto-oncogenes.These genes can be turned into potent oncogenes by retroviral or non-viral mutagens.It appears that proto-oncogenes are the root cause of cancer.mutation The discovery of human tumor oncogenes and prenormal genes unifies the previously disjointed research, including the peripheral work on animal retroviruses.However, there are still loopholes in much of the evidence.How exactly does a nonviral mutagenesis process turn a normal proto-oncogene into a lethal oncogene? The first answer came later in 1982 when researchers compared the human bladder cancer gene to its predecessor, the normal human ras gene.They knew immediately that it would be difficult and time-consuming to find the mutation that separated the two genes.These two kinds of genes are similar in appearance, and both of them have a length of 5000 DNA bases, and also have the same sequence punctuation break gene on the base chain.This rules out one possible explanation for their difference—that proto-oncogenes turn oncogenes due to massive deletions or recombinations of DNA sequences.

However, the two classes of genes differ in some notable ways.Also injected into normal cells, the proto-oncogene has no obvious effect on the cells, while the oncogene quickly causes the cells to enter cancerous growth.Because the differences will be extremely subtle, it will be necessary to analyze the DNA sequence carefully, base by base. The end result was pleasantly surprised.Although both genes are 5,000 bases long, they are identical except for one gene.On a base chain, the normal gene sequence is GCCGGCGGT, while the corresponding base sequence in cancer genes is GCCGTCGGT.The G in the normal gene is replaced by a T in the bladder cancer cell gene.This small change in the sequence of bases -- called a point mutation -- is enough to change the meaning of an entire gene.It's as if the word "dear" was accidentally misprinted as "dead" and an entire chapter in the book would completely change the meaning. Now it is possible to put the various events leading up to bladder cancer in order.An old man who had smoked for 30 years had a tumor at the age of 55.Like all other smokers, his lungs were filled with potent mutagenic carcinogens, some of which were neutralized by the liver and some passed through the kidneys into the urine.Some of the potent carcinogens in the urine attack the cells lining the bladder, then enter the cells and randomly attack the cellular DNA.Inside one cell, a ras proto-oncogene was damaged by a base change from a G to a T.The ras gene that had undergone the base substitution was now an active oncogene that began to drive the cell to grow.Over a period of years, perhaps decades, the cell's descendants, all carrying the mutated ras oncogene, grow into deadly tumor masses. Soon, the mutation mechanisms of other human tumor oncogenes were revealed.Each oncogene goes through its own mutational history. Point mutations in the ras oncogene are the most subtle of these changes.In some human tumors, there are many copies of the Sichuan yc oncogene or its close relative N-mpc, sometimes as many as 10 or 20 copies in a cell, while there are only 2 copies in normal cells.As one of the results of "gene amplification", corresponding to the increase of gene copies, tumor cells are also subjected to a steady stream of growth stimulating signals. In some lymphocyte tumors of the immune system such as the famous Burkitt lymphoma, the mutation of the myC proto-oncogene is unique. The chromosome is broken and recombined, and the previously unconnected DNA fragments are fused together. One of the consequences is that, The normal myC gene sitting somewhere on the chromosome is fused with the gene that makes the antibody molecule. This unnatural combination alters the myC gene, forcing it to operate under the control of the antibody gene. The once methodical reading of mw gene information is now being Continuous high-speed driving makes the myC gene a powerful cancer gene. In a nutshell: each proto-oncogene turns into an oncogene according to its own unique mutation mechanism.Although the factors or forces that caused the mutation are still unclear, but in the not-too-distant future, people will be able to figure out everything.Apart from the reason, the result is now clear.After the cell acquires the activated oncogene, the normal growth of the cell will deviate from the normal track because the oncogene sends out a strong growth stimulating signal.Major pieces of the cancer mystery are becoming clearer.