Unraveling the Origins of Cancer
The initial clues about the pathogenesis of cancer came from both epidemiologic and laboratory observations. In 1775 Percival Pott, a surgeon at St. Bartholomew's Hospital noticed a marked increase in cases of scrotal cancer in his clinic. His patients were almost invariably chimney sweeps or "climbing boys" - poor indentured orphans who apprenticed as sweeps and were sent up chimneys to clean the flues of ash, often naked and covered in oil. He noted that they spent hours in contact with grime and ash and they had particles of soot embedded in their skin. In 1788 the Chimney Sweepers Act was passed in Parliament, preventing master sweeps from employing children under the age of 8. In 1834 the age was raised to 14, and in 1840 it was raised to 16. By 1865 the use of young climbing boys was forbidden. This simple observation was important, because it suggested that an environmental exposure could play a role in causing cancer.
German scientist Theodor Boveri proposed that cancer could develop from a single cell with a damaged chromosome.
Peyton Rous demonstrated injection of an extract containing a virus could induce a cancer, specifically a sarcoma, in live chickens.
In the 1920s, several US watch companies produced "glow in the dark" dials painted with radioluminescent paint that contained radioactive radium. The dials were painted by hand with small brushes, and in order to maintain a precise shape to the brush, the employees "pointed" the brushes on their tongues. The radium, ingested in small doses over time accumulated in bone and produced bone cancer in many of the employees (mostly young women).
As time went on there were similar observations suggesting a link between environmental agents and cancer. In 1950 Wynders and Graham conducted one of the earliest case-control studies suggesting a link between tobacco smoking and lung cancer. A similar conclusion was reached by Richard Doll and Bradford Hill. They argued that a chemical in tobacco smoke caused lung cancer, but they were unable to explain the mechanism.
From "The Emperor of All Maladies" by Siddhartha Mukherjee:
"By the early 1950s, cancer researchers had thus split into three feuding camps. The virologists, led by Rous, claimed that viruses caused cancer, although no such virus had been found in human studies. Epidemiologists, such as Doll and Hill, argued that exogenous chemical caused cancer, although they could not offer a mechanistic explanation for their theory or results. The third camp, of Theodor Boveri's successors, stood at the farthest periphery. They possessed weak, circumstantial evidence that genes internal to the cell might cause cancer, but had neither the powerful human data of the epidemiologists nor the exquisite experimental insights of the chicken virologists. Great science emerges out of great contradiction, and here was a gaping rift slicing its way through the center of cancer biology. Was human cancer caused by an infectious agent? Was it caused by an exogenous chemical? Was it caused by an internal gene? How could the three groups of scientists have examined the same elephant and returned with such radically variant opinions about its essential anatomy?"
"But the set of disparate observations - from Blumberg to Ames to Warren and Marshall - could not simply be stitched together into a coherent theory of carcinogenesis. How could DES, asbestos, radiation, hepatitis virus, and a stomach bacterium all converge on the same pathological state, although in different populations and in different organs?"
A virologist named Howard Temin added Rous sarcoma (RSV) virus to cells in culture and found that the cells began to divide uncontrollably. In addition, the viral genome had inserted itself into the DNA of the cells. This was particularly remarkable, since the genome of Rous sarcoma virus is contained in single-stranded RNA. Temin postulated that Rous sarcoma virus was able to convert its single-stranded RNA into a double-stranded DNA form that could be inserted into the DNA of the infected cells. Several years later Temin and Satoshi Mizutani isolated an enzyme, which is now called reverse transcriptase, which is able to create this transformation. Reverse transcriptase is the enzyme which HIV uses to insert itself into the genome of infected lymphocytes in humans.
In the aftermath of these findings many cancer biologists searched for evidence of retroviruses in human cancers, but none were found. Temin reasoned that the Rous sarcoma virus had caused cells to become cancerous by causing genetic alterations in infected cells. However, it was possible that the genetic alterations weren't necessarily caused just by a virus.
Src: A Protein Kinase
In the 1970s virologists began making mutants of RSV, which is a small virus with only 4 genes. Some of the mutants were able to replicate, but were unable to cause cancer, and with these experiments, they were able to identify which gene in RSV was responsible for causing cancer. The gene was called"Src", short for sarcoma. And because it cause a cancer, it was dubbed an "oncogene." Later studies revealed that Src encoded for a protein kinase. Protein kinases are a family of proteins that act as on-off switches by attaching a phosphate group to particularly enzymes. Attaching a phosphate to one enzyme might activate it, for example. Often a kinase activated another kinase, which in turn tagged another kinase with phosphate and activated it, and this in turn might activate another in a chain reaction such that the switching on became amplified in a powerful way. This sequence of events might then reconfigure a cell from a non-dividing state to a dividing state.
Mutant Src produced an abnormally hyperactive kinase that phosphorylated all kinds of kinases within the cell and therefore turned on many of the molecular switches, including those that controlled cell division. Normal cellular Src phosphorylated the same kinases, but at a slower, more normal rate that was carefully regulated. These observations suggested the possibility that this cancer causing viral Src was actually a normal cellular gene with had mutated. As a result, one could think of the normal precursor gene as the "proto-oncogene," which at some pointed mutated to give rise to the Src oncogene.
Scientists subsequently found that a family of similar genes was present in virtually all cells.
In the late 1950s Peter Nowell and David Hungerford found a small abnormality in the chromosomes of patients with chronic myelogenous leukemia (CML). At the time techniques for imaging chromosomes were still crude, but it appeared that a small portion of one copy of chromosome #22 was missing. They dubbed this the"Philadelphia" chromosome, since that is where they discovered it. This provided further weight to the idea that genetic alterations (mutations) could be involved in the occurrence of cancers.
A pathologist, Oscar Auerbach, published a study based on 1522 autopsies of smokers and non-smokers. A careful examination of the cells lining the airways of smokers showed a spectrum of pathological changes ranging from swelling and thickening, to cells with premalignant changes (atypia or dysplasia), to clusters of cells that were malignant and represented invasive carcinoma. This array of changes suggested to Auerbach that cancer evolved through a progression of changes from normal to cancerous over a long period of time.
Bruce Ames, a bacteriologist at Berkeley, was studying mutations in Salmonella and observed that mutations could enable or disable the growth of bacteria on a petri dish. A strain of Salmonella normally unable to grow on galactose could acquire a gene mutation that enabled it to do so. By counting the number of growth enabled colonies Ames could quantify the mutation rate. Bacteria could be exposed to a certain chemical and Ames could then measure the mutation rate. He also noticed that chemicals that were mutagens also tended to be carcinogens. Dye derivatives that were known to be potent human carcinogens caused hundreds of mutations in the bacteria, as did x-rays, benzene compounds, and nitrosoguanidine, all of which caused cancers in rats and mice. Some known carcinogens did not induce mutations in Ames's in vitro system, but his findings did provide evidence of a link between mutations and cancer.
Baruch Blumberg, a biologist, found evidence that viruses caused cancer. Patients with chronic hepatitis B infection had a 5-10 fold increase in risk of developing liver cancer. Blumberg concluded, "The inflammation induced by the virus in liver cells, and the associated cycle of death and repair, appeared to be responsible for the cancer...."
Herbst et al. published results of a case-control study indicating that the drug diethylstilbesterol caused vaginal cancer in females who had been exposed to the drug in utero. .
Alfred Knudson conducts an analysis that provides evidence that a mutation is responsible for retinoblastoma, a cancer of the eye that can occur in children. Retinoblastomas follows were known to occur in two distinct patterns: a familial form, which is seen with high frequency in some family lines, and a sporadic form. The inherited form occurs early and is typically diagnosed within the first 2-6 months after birth, while the sporadic form typically occurred 2-4 years after birth. Since humans have two copies of each gene, Knudson hypothesized that individuals with the familial form inherited one defective allele, which by itself was not enough to cause the cancer. With one inherited defect, however, he proposed that a mutation in the other allele could trigger the familial form of retinoblastoma. Individuals without an inherited defect would require two mutations, one in each of the two alleles in order to produce the sporadic form of the cancer. As a result, individuals with the sporadic form tended to develop retinoblastoma later in life. Knudson called this the two-hit hypothesis of cancer. The mutated gene that was responsible was dubbed "Rb."
Key Concept: Proto-Oncogenes and Anti-Oncogenes
The retinoblastoma gene (Rb) is an anti-oncogene (or tumor suppressor gene); it produces a protein whose normally function is to bind to several other proteins which prevents them from activating cell division. When a cell receives normal signals to divide, it inactivates Rb by tagging it with a phosphate group. Anti-oncogenes act like recessive genes; if one of the two genes is rendered non-functional by a mutation, there is no change in cell behavior, because the other gene is still functioning. However, if both copies of the tumor suppressor gene are knocked out, the "brakes" to cell proliferation no longer function. In the case of Rb, some persons are born with one defective copy, so they are predisposed. If the other copy mutates in a somatic cell, then they can develop a tumor.
In contrast, Src is a proto-oncogene, which normally serves to activate cell division when the cell receives an appropriate signal, whereas the mutant form of the gene (an oncogene) causes unrestrained activation.
Michael Bishop and Harold Varmus, demonstrated that precursors of oncogenes - proto-oncogenes - existed in all normal cells. Harold Varmus, Michael Bishop, and Alfred Knudson subsequently proposed that these two abnormalities, activated proto-oncogenes and inactivated tumor suppressors, represented the critical defects in a cancer cell.
Scientist Janet Rowley determines that the defect in patients with chronic myelogenous leukemia ("the Philadelphia chromosome") is actually a translocation in which the head of chromosome 22 is fused to the tail of chromosome 9 to create a novel gene.
Following up on the discovery of the Philadelphia chromosome, a team of Dutch researchers isolated the gene on chromosome 9 and called it "abl". Two years later the gene on chromosome 22 was isolated and called "Bcr". The oncogene created by their fusion was called Bcr-abl. In 1987 David Baltimore's lab in Boston created a transgenic mice with the fused gene, and they developed fatal leukemia. Follow up studies then determined that Bcr-abl encoded for a protein kinase (a protein that regulated other proteins by tagging them with a phosphate group.
Robert Weinberg, a virologist working at MIT, and Chiaho Shih, a graduate student in his lab, used gene transfer techniques to insert fragments of DNA from human bladder cancer cells into normal cells and identified clusters of cells that had begun to proliferate abnormally. Weinberg and two other labs, working independently published their findings regarding a cancer causing gene, called "ras" from human cancer cells.
From "The Emperor of All Maladies" by Siddhartha Mukherjee:
"Like src, ras was also a gene present in all cells. But like src again, the ras gene in normal cells was functionally different from the ras present in cancer cells. In normal cells, the ras gene encoded a tightly regulated protein that turned 'on' and 'off' like a carefully modulated switch. In cancer cells, the gene was mutated, just as Varmus and Bishop had predicted. Mutated ras encoded a berserk, perpetually hyperactive protein permanently locked 'on'. This mutant protein produced an unquenchable signal for a cell to divide - and to keep dividing. It was the long-sought 'native' human oncogene...."
A number of other oncogenes and tumor suppressor genes were found to be involved in human cancers.
Transgenic technology allowed scientists to introduce exogenous genes into early mouse embryos to create "transgenic mice" in which one or more genes were artificially and permanently modified. When Philip Leder's lab introduced the oncogene c-myc into mouse breast cells, it produced only small tumors, and typically the tumors only occurred after pregnancy, suggesting that hormonal influences played a role. Leder then created a transgenic line with two oncogenes: ras and myc. These grew multiple tumors, but Leder was puzzled because millions of breast cells had acquired ras and myc, but only a few dozen formed tumors.
Burt Auerbach and others had demonstrated the lung cancer, cervical cancer, and colon cancer evolved slowly over time progressing from hyperplasia to dysplasia to carcinoma in situ, and eventually to invasive carcinoma. Bert Vogelstein at Johns Hopkins Medical School collected specimens from patients who had different stages of colon cancer and tested them for the presence of four genes that coded for oncogenes or tumor suppressors. He found that the stages of cancer progression correlated with the activation of oncogenes and the inactivation of tumor suppressor genes.
Key Concept: Proto-oncogenes and tumor suppressor genes influence signaling pathways affecting the cell cycle.
Proto-oncogenes and tumor suppressor genes encode for proteins that ultimately have effects on the cell cycle. However, it is well known that regulatory proteins often have their effect by modifying another protein, which in turn modifies yet another protein, and so forth. This cascade of events is referred to as a signaling pathway.
"Proto-oncogenes and tumor suppressor genes, cancer biologists discovered, sit at the hubs of such signaling pathways. Ras, for instance, activates a protein called Mek. Mek in tern activates Erk, which through several intermediary steps, ultimately accelerates cell division. This cascade of steps, called the Ras-Mek-Erk pathway - is tightly regulated in normal cells, thereby ensuring tightly regulated cell division. In cancer cells, activated "Ras" chronically and permanently actives Mek, which permanently activates Erk, resulting in uncontrolled cell division - pathological mitosis." [Source: "The Emperor of All Maladies" by Siddhartha Mukherjee]
But the activated ras pathway (Ras-Mek-Erk) does not merely cause accelerated cell division; the pathway also intersects with other pathways to enable several other "behaviors" of cancer cells.
At Children's Hospital in Boston in the 1990s, the surgeon-scientist Judah Folkman demonstrated that certain activated signaling pathways within cancer cells, ras among them, could also induce neighboring blood vessels to grow. A tumor could thus "acquire" its own blood supply by insidiously inciting a network of blood vessels around itself and then growing, in grape like clusters, around those blood vessels, a phenomenon that Folkman called tumor angiogenesis. Folkman's Harvard colleague Stan Korsmeyer found other activated pathways in cancer cells, originating in mutated genes, that also blocked cell death, thus imbuing cancer cells with the capacity to resist signals. Other pathways allowed cancer cells to acquire motility, the capacity to move from one tissue to another - initiating metastasis. Yet other gene cascades increased cell survival in hostile environments, such that cancer cells traveling through the bloodstream could invade other organs and not be rejected or destroyed in environments not designed for survival."
The BRCA2 gene is discovery. It is a tumor suppressor which, if mutated, increase the risk of breast cancer.
The discovery of the Philadelphia chromosome and the subsequent discovery of Bcr-abl triggered a cascade of events along a signaling pathway with the cell. In normal cells the two separate genes were carefully regulated, but when they were fused, the result was a highly overactive kinase that signaled cells to divide continually. Humans normally make about 500 different kinases which regulate a wide variety of cellular pathways. By tagging specific proteins with a phosphate, they basically switch a pathway on or off. Scientists at the Ciba-Geigy pharmaceutical company in Basel, Switzerland began trying to synthesize drugs that could inhibit these kinases by binding to the protein and blocking its kinase activity. By the early 1990s they had created dozens of them. These compounds were also found to have specificity, meaning that one drug might block the "abl" kinase but not block the "src" kinase. An oncologist at Dana-Farber Cancer Center was able to obtain CGP57148, a kinase blocker that was specific for Bcr-abl. When the drug was added to chronic myelogenous leukemia (CML) cells growing in the lab, the cells died overnight. And the drug (Gleevec) was given to mice that had tumors from implanted CML, the tumors regressed in days, but normal cells were undamaged.
Licensing of Gleevec - Ciba-Geigy merged with Sandoz to form Novartis, but Novartis was reluctant to begin clinical trials for Gleevec, mainly out of concern that the cost of the trials would be prohibitive given the relatively small market for the drug. Eventually, they agreed to make a small amount of the drug and tested an initial group of 54 patients, 53 of whom responded with prompt remissions which were usually long lasting. The drug, now called Gleevec, has become the standard of care for patients with CML. It was licensed in 2001.
From The Emperor of All Maladies:
"Gleevec opened a new door for cancer therapeutics. The rational synthesis of a molecule to kill cancer cells - a drug designed to specifically inactivate an oncogene...."
Unfortunately, some CML patients treated with Gleevec eventually developed cancer cells that were resistant to the drug. These cells had acquired a mutation that altered the structure of the Bcr-abl in such a way that Gleevec would no longer bind to it, although it was still able to switch on cell division. However, in 2005 a second drug was developed that was able to bind to Gleevec-resistant Bcr-abl. The new drug is called dasatinib, and it was able to induce remission in Gleevec-resistant cases. Since then, many more cancer-targeted drugs have been developed. These drugs differ from the original chemotherapeutic drugs in that the newer drugs target cancer cells in a highly specific way that is custom-designed to neutralize the defect in the cancer cells. This highly specific attack means that normal cells are unaffected, resulting in far fewer side effects.
Declining Mortality from Cancer
A number of authors recognized that the mortality rates for some of the major forms of lung cancer, including lung, breast, colon, and prostate, had been slowly, but steadily declining over the past 15 years. The reasons for the decline differed among the major cancer types.
- Lung cancer was declining primarily because of prevention as a result of decreased rates of smoking.
- Colon cancer and cervical cancer declined primarily due to more widespread use of effective screening measures. Very early colon cancer or their predecessors (adenomas) were more frequently being diagnosed (and often treated) by colonoscopy. Cervical cancers were being identified at a very early stage by more widespread use of Pap smears.
- Leukemia, lymphoma, and testicular cancer deaths were declining mainly because of advances in chemotherapy.
- Breast cancer mortality was declining because of improvements in screening (mammography), surgery, and adjuvant chemotherapy.