Fundamental Understandings                                                                                                ...
count for much of the uncontrolled cell         when a mutation in one of its proto-on-      other cancer types secrete to...
Fundamental Understandings                              STIMULATORY                                                       ...
sponse to signals from TGF-ß, normallyshuts down the machinery that guides                       Some Genes Involved in Hu...
Fundamental Understandings                      ent cell finally divides in half to produce            of a variety of mole...
clin D, thus preventing the advance of                  Breast cancer cells often produce ex-            quently disabled,...
Fundamental Understandings                                                                                    I have so fa...
somes, known as telomeres, tally the          cells to replicate endlessly. The resulting   flicted with early-onset coloni...
Fundamental Understandingslignancy to develop will, by chance,           will enable us to predict which members        ci...
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Artículo how cancer

  1. 1. Fundamental Understandings become more and more aggressive over How Cancer Arises time, and they become lethal when they disrupt the tissues and organs needed for the survival of the organism as a whole. This much is not new. But over the An explosion of research is uncovering past 20 years, scientists have uncovered the long-hidden molecular underpinnings a set of basic principles that govern the development of cancer. We now know of cancer— and suggesting new therapies that the cells in a tumor descend from a common ancestral cell that at one point—usually decades before a tumor becomes palpable—initiated a program by Robert A. Weinberg of inappropriate reproduction. Further, the malignant transformation of a cell comes about through the accumulation of mutations in specific classes of the H ow cancer develops is no longer a mystery. During the past two decades, investiga- tors have made astonishing progress in identifying the deepest bases of the pro- pear to be quite similar. For that reason, I will refer in this article to “cancer” in generic terms, drawing on one or anoth- er type to illustrate the rules that seem to apply universally. genes within it. These genes provide the key to understanding the processes at the root of human cancer. Genes are carried in the DNA mole- cules of the chromosomes in the cell nu- cess—those at the molecular level. These The 30 trillion cells of the normal, cleus. A gene specifies a sequence of discoveries are robust: they will survive healthy body live in a complex, interde- amino acids that must be linked togeth- the scrutiny of future generations of re- pendent condominium, regulating one er to make a particular protein; the pro- searchers, and they will form the foun- another’s proliferation. Indeed, normal tein then carries out the work of the dation for revolutionary approaches to cells reproduce only when instructed to gene. When a gene is switched on, the treatment. No one can predict exactly do so by other cells in their vicinity. Such cell responds by synthesizing the encod- when therapies targeted to the molecu- unceasing collaboration ensures that ed protein. Mutations in a gene can per- lar alterations in cancer cells will find each tissue maintains a size and archi- turb a cell by changing the amounts or wide use, given that the translation of tecture appropriate to the body’s needs. the activities of the protein product. new understanding into clinical prac- Cancer cells, in stark contrast, violate Two gene classes, which together con- tice is complicated, slow and expensive. this scheme; they become deaf to the stitute only a small proportion of the full But the effort is now under way. usual controls on proliferation and fol- genetic set, play major roles in trigger- In truth, the term “cancer” refers to low their own internal agenda for re- ing cancer. In their normal configura- more than 100 forms of the disease. Al- production. They also possess an even tion, they choreograph the life cycle of most every tissue in the body can spawn more insidious property—the ability to the cell—the intricate sequence of events malignancies; some even yield several migrate from the site where they began, by which a cell enlarges and divides. types. What is more, each cancer has invading nearby tissues and forming Proto-oncogenes encourage such growth, unique features. Still, the basic processes masses at distant sites in the body. Tu- whereas tumor suppressor genes inhibit that produce these diverse tumors ap- mors composed of such malignant cells it. Collectively these two gene classes ac- Tumor Development Occurs in Stages T he creation of a malignant tumor in epithelial tissue is depicted schemat- ically below. Epithelial cancers are the most common malignancies and are called carcinomas. The mass seen here emerges as a result of mutations in four genes, but the number of genes involved in real tumors can vary.GENETICALLY ALTERED CELL HYPERPLASIA DYSPLASIA 2 The altered cell and its descendants 1 Tumor development begins when continue to look normal, but they re-DANA BURNS-PIZER some cell (orange ) within a normal produce too much—a condition 3 In addition to proliferating excessively, the off- population (beige) sustains a genet- termed hyperplasia. After years, one spring of this cell appear abnormal in shape and in ic mutation that increases its in a million of these cells (pink) suf- orientation; the tissue is now said to exhibit dys- propensity to proliferate when it fers another mutation that further plasia. Once again, after a time, a rare mutation would normally rest. loosens controls on cell growth. that alters cell behavior occurs (purple). 62 Scientific American September 1996 Copyright 1996 Scientific American, Inc. How Cancer Arises
  2. 2. count for much of the uncontrolled cell when a mutation in one of its proto-on- other cancer types secrete too much proliferation seen in human cancers. cogenes energizes a critical growth-stim- transforming growth factor alpha. These When mutated, proto-oncogenes can ulatory pathway, keeping it continu- factors act, as usual, on nearby cells, become carcinogenic oncogenes that ously active when it should be silent. but, more important, they may also drive excessive multiplication. The mu- These pathways within a cell receive turn back and drive proliferation of the tations may cause the proto-oncogene to and process growth-stimulatory signals same cells that just produced them. yield too much of its encoded growth- transmitted by other cells in a tissue. Researchers have also identified on- stimulatory protein or an overly active Such cell-to-cell signaling usually begins cogenic versions of receptor genes. The form of it. Tumor suppressor genes, in when one cell secretes growth factors. aberrant receptors specified by these on- contrast, contribute to cancer when they After release, these proteins move cogenes release a flood of proliferative are inactivated by mutations. The result- through the spaces between cells and signals into the cell cytoplasm even when ing loss of functional suppressor pro- bind to specific receptors—antennalike no growth factors are present to urge teins deprives the cell of crucial brakes molecules—on the surface of other cells the cell to replicate. For instance, breast that prevent inappropriate growth. nearby. Receptors span the outer mem- cancer cells often display Erb-B2 recep- For a cancerous tumor to develop, brane of the target cells, so that one end tor molecules that behave in this way. mutations must occur in half a dozen or protrudes into the extracellular space, Still other oncogenes in human tumors more of the founding cell’s growth-con- and the other end projects into the cell’s perturb parts of the signal cascade found trolling genes. Altered forms of yet oth- interior, its cytoplasm. When a growth- in the cytoplasm. The best understood er classes of genes may also participate stimulatory factor attaches to a recep- example comes from the ras family of in the creation of a malignancy, by spe- tor, the receptor conveys a proliferative oncogenes. The proteins encoded by cifically enabling a proliferating cell to signal to proteins in the cytoplasm. normal ras genes transmit stimulatory become invasive or capable of spread- These downstream proteins then emit signals from growth factor receptors to ing (metastasizing) throughout the body. stimulatory signals to a succession of other proteins farther down the line. other proteins, in a chain that ends in The proteins encoded by mutant ras Signaling Systems Go Awry the heart of the cell, its nucleus. Within genes, however, fire continuously, even the nucleus, proteins known as tran- when growth factor receptors are not V ital clues to how mutated proto- oncogenes and tumor suppressor genes contribute to cancer came from scription factors respond by activating a cohort of genes that help to usher the cell through its growth cycle. prompting them. Hyperactive Ras pro- teins are found in about a quarter of all human tumors, including carcinomas studying the roles played within the cell Some oncogenes force cells to over- of the colon, pancreas and lung. (Carci- by the normal counterparts of these produce growth factors. Sarcomas and nomas are by far the most common genes. After almost two decades of re- gliomas (cancers, respectively, of con- forms of cancer; they originate in epi- search, we now view the normal genet- nective tissues and nonneuronal brain thelial cells, which line the body cavities ic functions with unprecedented clarity cells) release excessive amounts of plate- and detail. let-derived growth factor. A number of Many proto-oncogenes code for pro- teins in molecular “bucket brigades” that relay growth-stimulating signals from INVASIVE CANCER outside the cell deep into its interior. The growth of a cell becomes deregulated IN SITU CANCER 5 If the genetic changes allow the tu-4 The affected cells become still more mor to begin invading underlying tis- abnormal in growth and appearance. If sue and to shed cells into the blood the tumor has not yet broken through or lymph, the mass is considered to any boundaries between tissues, it is have become malignant. The rene- BLOOD VESSEL called in situ cancer. This tumor may gade cells are likely to establish new remain contained indefinitely; however, tumors (metastases) throughout the some cells may eventually acquire ad- body; these may become lethal by ditional mutations (blue ). disrupting a vital organ. How Cancer Arises Copyright 1996 Scientific American, Inc. Scientific American September 1996 63
  3. 3. Fundamental Understandings STIMULATORY INHIBITORY PATHWAYS Normal cell PATHWAYS Growth factor Inhibitor (“go” signal) (“stop” signal) Neighboring Receptors Neighboring cells release at cell surface cells release growth-stimulatory growth-inhibitory factors factors No growth Cytoplasmic relay proteins factor attaches Receptor Transcription Relay factors molecule Nucleus is lost Receptor DNA fires Signaling anyway stops Proteins Proteins that Cell divides Cell divides that trigger inhibit cell when it DIMITRY SCHIDLOVSKY in the absence cell division division should not, of stimulation Cell cycle because by external clock decides inhibitory growth factors whether cell signal fails should to reach nucleus proliferateEXAMPLE OF EXAMPLE OFSTIMULATORY INHIBITORYABNORMALITY ABNORMALITY SIGNALING PATHWAYS in normal cells convey growth-con- convey “stop” signals. A stimulatory pathway will become hy- trolling messages from the outer surface deep into the nucleus. peractive if a mutation causes any component, such as a growth There a molecular apparatus known as the cell cycle clock col- factor receptor (box at left), to issue stimulatory messages au- lects the messages and decides whether the cell should divide. tonomously, without waiting for commands from upstream. Cancer cells often proliferate excessively because genetic muta- Conversely, inhibitory pathways will shut down when some tions cause stimulatory pathways ( green) to issue too many constituent, such as a cytoplasmic relay (box at right), is elimi- “go” signals or because inhibitory pathways (red) can no longer nated and thus breaks the signaling chain. and form the outer layer of the skin.) peutics. In an exciting turn of events, as to the nucleus much as stimulatory sig- Yet other oncogenes, such as those in many as half a dozen pharmaceutical nals do—via molecular bucket brigades. the myc family, alter the activity of tran- companies are working on drugs de- In cancer cells, these inhibitory brigades scription factors in the nucleus. Cells signed to shut down aberrantly firing may be disrupted, thereby enabling the normally manufacture Myc transcrip- growth factor receptors. At least three cell to ignore normally potent inhibitory tion factors only after they have been other companies are attempting to devel- signals at the surface. Critical compo- stimulated by growth factors impinging op compounds that block the synthesis nents of these brigades, which are speci- on the cell surface. Once made, Myc of aberrant Ras proteins. Both groups of fied by tumor suppressor genes, are ab- proteins activate genes that force cell agents halt excessive signaling in cultured sent or inactive in many types of cancer growth forward. But in many types of cancer cells, but their utility in blocking cells. cancer, especially malignancies of the the growth of tumors in animals and A secreted substance called transform- blood-forming tissues, Myc levels are humans remains to be demonstrated. ing growth factor beta (TGF-ß) can stop kept constantly high even in the ab- the growth of various kinds of normal sence of growth factors. Tumor Suppressors Stop Working cells. Some colon cancer cells become Discovery of trunk lines that carry oblivious to TGF-ß by inactivating a proliferative messages from the cell sur- face to its nucleus has been more than intellectually satisfying. Because these T o become malignant, cells must do more than overstimulate their growth-promoting machinery. They gene that encodes a surface receptor for this substance. Some pancreatic cancers inactivate the DPC4 gene, whose pro- pathways energize the multiplication of must also devise ways to evade or ig- tein product may operate downstream malignant cells, they constitute attrac- nore braking signals issued by their nor- of the growth factor receptor. And a va- tive targets for scientists intent on de- mal neighbors in the tissue. Inhibitory riety of cancers discard the p15 gene, veloping new types of anticancer thera- messages received by a normal cell flow which codes for a protein that, in re- 64 Scientific American September 1996 Copyright 1996 Scientific American, Inc. How Cancer Arises
  4. 4. sponse to signals from TGF-ß, normallyshuts down the machinery that guides Some Genes Involved in Human Cancersthe cell through its growth cycle. enes known as proto-oncogenes code for proteins that stimulate cell division; Tumor suppressor proteins can alsorestrain cell proliferation in other ways. G mutated forms, called oncogenes, can cause the stimulatory proteins to be over- active, with the result that cells proliferate excessively. Tumor suppressor genes codeSome, for example, block the flow of for proteins that inhibit cell division. Mutations can cause the proteins to be inacti-signals through growth-stimulatory cir- vated and may thus deprive cells of needed restraints on proliferation. Investigatorscuits. One such suppressor is the prod- are still trying to decipher the specific functions of many tumor suppressor genes.uct of the NF-1 gene. This cytoplasmicmolecule ambushes the Ras protein be- ONCOGENESfore it can emit its growth-promoting Genes for growth factors or their receptorsdirectives. Cells lacking NF-1, then, aremissing an important counterbalance PDGF Codes for platelet-derived growth factor. Involved in glioma (a brain cancer)to Ras and to unchecked proliferation. Various studies have shown that the erb-B Codes for the receptor for epidermal growth factor. Involved in glioblastoma (a brain cancer) and breast cancerintroduction of a tumor suppressor geneinto cancer cells that lack it can restore erb-B2 Also called HER-2 or neu. Codes for a growth factor receptor. Involveda degree of normalcy to the cells. This in breast, salivary gland and ovarian cancersresponse suggests a tantalizing way of RET Codes for a growth factor receptor. Involved in thyroid cancercombating cancer—by providing cancer Genes for cytoplasmic relays in stimulatory signaling pathwayscells with intact versions of tumor sup- Ki-ras Involved in lung, ovarian, colon and pancreatic cancerspressor genes they lost during tumor de-velopment. Although the concept is at- N-ras Involved in leukemiastractive, this strategy is held back by the Genes for transcription factors that activate growth-promoting genestechnical difficulties still encumbering c-myc Involved in leukemias and breast, stomach and lung cancersgene therapy for many diseases. Current N-myc Involved in neuroblastoma (a nerve cell cancer) and glioblastomaprocedures fail to deliver genes to a large L-myc Involved in lung cancerproportion of the cells in a tumor. Untilthis logistical obstacle is surmounted, Genes for other kinds of moleculesthe use of gene therapy to cure cancer Bcl-2 Codes for a protein that normally blocks cell suicide. Involvedwill remain a highly appealing but un- in follicular B cell lymphomafulfilled idea. Bcl-1 Also called PRAD1. Codes for cyclin D1, a stimulatory component of the cell cycle clock. Involved in breast, head and neck cancers The Clock Is Struck MDM2 Codes for an antagonist of the p53 tumor suppressor protein. Involved in sarcomas (connective tissue cancers) and other cancersO ver the past five years, impressive evidence has uncovered the desti-nation of stimulatory and inhibitory TUMOR SUPPRESSOR GENES Genes for proteins in the cytoplasmpathways in the cell. They converge on APC Involved in colon and stomach cancersa molecular apparatus in the cell nucle- DPC4 Codes for a relay molecule in a signaling pathway that inhibitsus that is often referred to as the cell cy- cell division. Involved in pancreatic cancercle clock. The clock is the executive de- NF-1 Codes for a protein that inhibits a stimulatory (Ras) protein. Involvedcision maker of the cell, and it appar- in neurofibroma and pheochromocytoma (cancers of the peripheralently runs amok in virtually all types of nervous system) and myeloid leukemiahuman cancer. In the normal cell, the NF-2 Involved in meningioma and ependymoma (brain cancers) andclock integrates the mixture of growth- schwannoma (affecting the wrapping around peripheral nerves)regulating signals received by the celland decides whether the cell should pass Genes for proteins in the nucleusthrough its life cycle. If the answer is MTS1 Codes for the p16 protein, a braking component of the cell cycle clock.positive, the clock leads the process. Involved in a wide range of cancers The cell cycle is composed of four RB Codes for the pRB protein, a master brake of the cell cycle. Involved instages. In the G 1 (gap 1) phase, the cell retinoblastoma and bone, bladder, small cell lung and breast cancerincreases in size and prepares to copy its p53 Codes for the p53 protein, which can halt cell division and induceDNA. This copying occurs in the next abnormal cells to kill themselves. Involved in a wide range of cancersstage, termed S (for synthesis), and en- WT1 Involved in Wilms’ tumor of the kidneyables the cell to duplicate precisely its Genes for proteins whose cellular location is not yet clearcomplement of chromosomes. After thechromosomes are replicated, a second BRCA1 Involved in breast and ovarian cancersgap period, termed G 2 , follows during BRCA2 Involved in breast cancerwhich the cell prepares itself for M (mi- VHL Involved in renal cell cancertosis)—the time when the enlarged par-How Cancer Arises Copyright 1996 Scientific American, Inc. Scientific American September 1996 65
  5. 5. Fundamental Understandings ent cell finally divides in half to produce of a variety of molecules. Its two essen- This action releases the braking effect its two daughters, each of which is en- tial components, cyclins and cyclin-de- of pRB and enables the cell to progress dowed with a complete set of chromo- pendent kinases (CDKs), associate with into late G 1 and thence into S (DNA somes. The new daughter cells immedi- one another and initiate entrance into synthesis) phase [see b in box below]. ately enter G 1 and may go through the the various stages of the cell cycle. In Various inhibitory proteins can re- full cycle again. Alternatively, they may G 1, for instance, D-type cyclins bind to strain forward movement through the stop cycling temporarily or permanently. CDKs 4 or 6, and the resulting complex- cycle. Among them are p15 (mentioned The cell cycle clock programs this es act on a powerful growth-inhibitory earlier) and p16, both of which block elaborate succession of events by means molecule—the protein known as pRB. the activity of the CDK partners of cy- The Cell Cycle Clock and Cancer driven to a large extent by rising levels of proteins called cyclins: first the D type, followed by E, A and then B. M ost, perhaps all, human cancers grow inappropriately not only because signaling pathways in cells are perturbed but also because the so-called cell cycle clock becomes deranged. A crucial step in the cycle occurs late in G1 at the restriction point (R), when the cell decides whether to commit itself to com- pleting the cycle. For the cell to pass through R and enter S, a The clock—composed of an assembly of interacting proteins in molecular “switch” must be flipped from “off” to “on.” The switch the nucleus—normally integrates messages from the stimulatory works as follows (b ): As levels of cyclin D and, later, cyclin E rise, and inhibitory pathways and, if the stimulatory messages win these proteins combine with and activate enzymes called cyclin- out, programs a cell’s advance through its cycle of growth and di- dependent kinases (1 ). The kinases (acting as part of cyclin-ki- vision. Progression through the four stages of the cell cycle (a ) is nase complexes) grab phosphate groups (2 ) from molecules of a c STAGES OF THE CELL CYCLE THE CELL CYCLE CLOCK IN ACTION Growth- Beginning promoting Cell of cycle divides signals (mitosis) issued by M Cell enlarges neighboring and makes cells Cell prepares new proteins to divide G G 2 1 Growth- Cell rests inhibitory G signals 0 issued by neighboring cellsDIMITRY SCHIDLOVSKY R Cell Restriction point: cell p27 Inactive replicates S decides whether pRB its DNA to commit itself to protein the complete cycle Cyclin D* b Cyclin D– CDK4/6 A MOLECULAR “SWITCH” complex Cyclin- 1 2 Inactive 3 Active dependent transcription transcription kinase 4* or 6 p15* ATP Active factor factor Cyclin D or E (CDK4/6) pRB (master brake) Proteins Transforming Phosphate needed growth Gene for cell’s factor beta advance (an inhibitor) Active through its complex cycle Cyclin- dependent Early G 1 kinase Inactive pRB PHASES OF CELL CYCLE 66 Scientific American September 1996 Copyright 1996 Scientific American, Inc. How Cancer Arises
  6. 6. clin D, thus preventing the advance of Breast cancer cells often produce ex- quently disabled, eliminating two of the the cell from G1 into S. Another inhibi- cesses of cyclin D and cyclin E. In many clock’s most vital restraints. The end re- tor of CDKs, termed p21, can act cases of melanoma, skin cells have lost sult in all these cases is that the clock throughout the cell cycle. P21 is under the gene encoding the braking protein begins to spin out of control, ignoring control of a tumor suppressor protein, p16. Half of all types of human tumors any external warnings to stop. If investi- p53, that monitors the health of the lack a functional p53 protein. And in gators can devise ways to impose clamps cell, the integrity of its chromosomal cervical cancers triggered by infection on the cyclins and CDKs active in the DNA and the successful completion of of cells with a human papillomavirus, cell cycle, they may be able to halt can- the different steps in the cycle. both the pRB and p53 proteins are fre- cer cells in their tracks. ATP (adenosine triphosphate) and transfer them to a protein called it releases the factors, freeing them to act on genes (3; top). The pRB, the master brake of the cell cycle clock. When pRB lacks liberated factors then spur production of various proteins required phosphates, it actively blocks cycling (and keeps the switch in for continued progression through the cell cycle. the “off” position) by sequestering other proteins termed tran- In figure c below, the switch is placed in the larger context of scription factors. But after the cyclin-kinase complexes add the many molecular interactions that regulate the cell cycle. Flip- enough phosphates to pRB, the brake stops working (3; bottom ); ping of the switch to “on” can be seen above the R point. Overac- tivity of the stimulatory proteins cyclin D, cyclin E and CDK4 have been implicated in certain human cancers. Inactivation of various inhibitory proteins has also been documented. The affected pro- DNA damage teins include p53 (lost or ineffective in more than half of all tumor or oxygen deprivation types), pRB, p16 and p15. The net effect of any of these changes is deregulation of the clock and, in turn, excessive proliferation of the cell. —R.A.W. Cell suicide p53* (apoptosis) MDM2 p21 Cyclin E* Proteins Cyclin E– DNA CDK2 complex involved in DNA synthesis synthesis CDK2 CDC25A Cell Liberated divisiontranscription factors Cyclin A Cyclin A–CDK1 Cyclin B complex Cyclin B– CDK1 complex Active pRB protein* CDK1 complexed with inactive transcription factors(master brake) green Activity that promotes red Activity that discourages Positive signal (increasing the amount cell division cell division or activity of the target molecule) p16* Main path leading to External signal that Negative signal (decreasing the amount cell division discourages cell division or activity of the target molecule) External signal that Feedback loop promotes cell division Mutation or deregulation of gene for this * protein has been found in human tumors Late G S G M 1 2 R How Cancer Arises Copyright 1996 Scientific American, Inc. Scientific American September 1996 67
  7. 7. Fundamental Understandings I have so far discussed two ways that whole: the potential dangers posed to evade apoptosis will be far less respon- our tissues normally hold down cell pro- the organism by carcinogenic mutations sive to treatment. By the same token, it liferation and avoid cancer. They pre- are far greater than the small price paid suggests that therapies able to restore a vent excess multiplication by depriving in the loss of a single cell. The tumors cell’s capacity for suicide could combat a cell of growth-stimulatory factors or, that emerge in our tissues, then, would cancer by improving the effectiveness of conversely, by showering it with antipro- seem to arise from the rare, genetically existing radiation and chemotherapeu- liferative factors. Still, as we have seen, disturbed cell that somehow succeeds tic treatment strategies. cells on their way to becoming cancerous in evading the apoptotic program hard- A second defense against runaway often circumvent these controls: they wired into its control circuitry. proliferation, quite distinct from theMERRYN MACVILLE AND THOMAS REID National Center for Human Genome Research, NIH stimulate themselves and turn a deaf ear Developing cancer cells devise several apoptotic program, is built into our cells NORMAL CELL 8 9 10 11 12 13 14 15 CANCER CELL 8q 8q 8 9 10 11 12 13 14 15 Truncated copies HUMAN CHROMOSOMES from a normal dividing cell (top) occur as identical as well. This mechanism counts and lim- pairs; those numbered 8 to 18 are shown. Chromosomes from a cervical cancer cell, in its the total number of times cells can contrast, display many abnormalities (bottom). Chromosome 8, for instance, exhibits three disturbances: gain of copy number; deletion of genetic material from individual reproduce themselves. copies; and breakage followed by joining of segments that do not belong together ( far right in 8 ). Copy loss, as in chromosome 13, is also common. These various changes Cells Become Immortal can favor tumor progression if they activate an oncogene, increase the copies of an oncogene or eliminate a tumor suppressor gene. The images were generated by spectral karyotyping, a new method for analyzing chromosomes. M uch of what is known about this safeguard has been learned from studies of cells cultured in a petri dish. When cells are taken from a mouse or to inhibitory signals. Prepared for such means of evading apoptosis. The p53 human embryo and grown in culture, eventualities, the human body equips protein, among its many functions, helps the population doubles every day or so. cells with certain backup systems that to trigger cell suicide; its inactivation by But after a predictable number of dou- guard against runaway division. But many tumor cells reduces the likelihood blings—50 to 60 in human cells—growth additional mutations in the cell’s genet- that genetically troubled cells will be stops, at which point the cells are said to ic repertoire can overcome even these eliminated. Cancer cells may also make be senescent. That, at least, is what hap- defenses and contribute to cancer. excessive amounts of the protein Bcl-2, pens when cells have intact RB and p53 which wards off apoptosis efficiently. genes. Cells that sustain inactivating mu- Fail-Safe Systems Fail Recently scientists have realized that tations in either of these genes continue this ability to escape apoptosis may en- to divide after their normal counterparts O ne such backup system, present in each human cell, provokes the cell to commit suicide (undergo “apopto- danger patients not only by contributing to the expansion of a tumor but also by making the resulting tumors resistant to enter senescence. Eventually, though, the survivors reach a second stage, termed crisis, in which they die in large num- sis”) if some of its essential components therapy. For years, it was assumed that bers. An occasional cell in this dying are damaged or if its control systems radiation therapy and many chemother- population, however, will escape crisis are deregulated. For example, injury to apeutic drugs killed malignant cells di- and become immortal: it and its descen- chromosomal DNA can trigger apopto- rectly, by wreaking widespread havoc dants will multiply indefinitely. sis. Further, recent work from a num- in their DNA. We now know that the These events imply the existence of a ber of laboratories indicates that crea- treatments often harm DNA to a rela- mechanism that counts the number of tion of an oncogene or the disabling of tively minor extent. Nevertheless, the doublings through which a cell popula- a tumor suppressor gene within a cell affected cells perceive that the inflicted tion has passed. During the past several can also induce this response. Destruc- damage cannot be repaired easily, and years, scientists have discovered the mo- tion of a damaged cell is bad for the cell they actively kill themselves. This dis- lecular device that does this counting. itself but makes sense for the body as a covery implies that cancer cells able to DNA segments at the ends of chromo- 68 Scientific American September 1996 Copyright 1996 Scientific American, Inc. How Cancer Arises
  8. 8. somes, known as telomeres, tally the cells to replicate endlessly. The resulting flicted with early-onset colonic tumors, number of replicative generations cell immortality can be troublesome in preordained by an inherited gene. In the through which cell populations pass a couple of ways. Obviously, it allows sporadic cases, a rare mutation silences and, at appropriate times, initiate senes- tumors to grow large. It also gives pre- a tumor suppressor gene called APC in cence and crisis. In so doing, they cir- cancerous or already cancerous cells an intestinal epithelial cell. The resulting cumscribe the ability of cell populations time to accumulate additional mutations proliferation of the mutant cell yields a to expand indefinitely [see “Telomeres, that will increase their ability to repli- benign polyp that may eventually pro- Telomerase and Cancer,” by Carol W. cate, invade and ultimately metastasize. gress to a malignant carcinoma. But de- Greider and Elizabeth H. Blackburn; From the point of view of a cancer fective forms of APC may pass from Scientific American, February]. cell, production of a single enzyme is a parents to children in certain families. clever way to topple the mortality bar- Members of these families develop hun- rier. Yet dependence on one enzyme dreds, even thousands of colonic polyps may represent an Achilles’ heel as well. during the first decades of life, some of If telomerase could be blocked in can- which are likely to become transformed cer cells, their telomeres would once into carcinomas. again shrink whenever they divided, The list of familial cancer syndromes pushing these cells into crisis and death. that are now traceable directly to inher-16 17 18 For that reason, a number of pharma- itance of mutant tumor suppressor genes ceutical firms are attempting to develop is growing. For instance, inherited defec- drugs that target telomerase. tive versions of the gene for pRB often lead to development of an eye cancer— Why Some Cancers Appear Early retinoblastoma—in children; later in life16 17 18 the mutations account for a greatly in- I t normally takes decades for an incip- ient tumor to collect all the muta- tions required for its malignant growth. creased risk of osteosarcomas (bone can- cers). Mutant inherited versions of the p53 tumor suppressor gene yield tumors In some individuals, however, the time at multiple sites, a condition known as Like the plastic tips on shoelaces, the for tumor development is clearly com- the Li-Fraumeni syndrome (named in telomere caps protect chromosomal ends pressed; they contract certain types of part for Frederick Li, co-author of from damage. In most human cells, telo- cancer decades before the typical age of “What Causes Cancer?”, page 80). meres shorten a bit every time chromo- onset of these cancers. How can tumor And the recently isolated BRCA1 and somes are replicated during the S phase formation be accelerated? BRCA2 genes seem to account for the of the cell cycle. Once the telomeres In many cases, this early onset is ex- bulk of familial breast cancers, encom- shrink below some threshold length, they plained by the inheritance from one or passing as many as 20 percent of all pre- sound an alarm that instructs cells to the other parent of a mutant cancer- menopausal breast cancers in this coun- enter senescence. If cells bypass senes- causing gene. As a fertilized egg begins try and a substantial proportion of fa- cence, further shrinkage of the telomere to divide and replicate, the set of genes milial ovarian cancers as well. will eventually trigger crisis: extreme provided by the sperm and egg is cop- Early onset of tumors is sometimes shortening of the telomeres will cause ied and distributed to all the body’s explained by inheritance of mutations the chromosomes in a cell to fuse with cells. Now a typically rare event—a mu- in another class of genes as well. As I one another or to break apart, creating tation in a critical growth-controlling implied earlier, most people avoid can- genetic chaos that is fatal to the cell. gene—becomes ubiquitous, because the cer until late in life or indefinitely be- If the telomere-based counting system mutation is implanted in all the body’s cause they enter the world with pristine operated properly in cancerous cells, cells, not merely in some randomly genes. During the course of a lifetime, their excessive proliferation would be stricken cell. In other words, the process however, our genes are attacked by car- aborted long before tumors became very of tumor formation leapfrogs over one cinogens imported into our bodies from large. Dangerous expansion would be of its early, slowly occurring steps, ac- the environment and also by chemicals stemmed by the senescence program or, celerating the process as a whole. As a produced in our own cells. And genetic if the cell evaded that blockade, by dis- consequence, tumor development, which errors may be introduced when the en- ruption of the chromosomal array at usually requires three or four decades zymes that replicate DNA during cell crisis. But this last defense is breached to reach completion, may culminate in cycling make copying mistakes. For the during the development of most cancer one or two. Because such mutant genes most part, such errors are rapidly cor- cells, overcome by activation of a gene can pass from generation to generation, rected by a repair system that operates that codes for the enzyme telomerase. many members of a family may be at in every cell. Should the repair system This enzyme, virtually absent from risk for the early development of cancer. slip up and fail to erase an error, the most healthy cell types but present in An inherited form of colon cancer pro- damage will become a permanent mu- almost all tumor cells, systematically re- vides a dramatic example. Most cases of tation in one of the cell’s genes and in places telomeric segments that are usu- colon cancer occur sporadically, the re- that same gene in all descendant cells. ally trimmed away during each cell cy- sults of random genetic events occurring The system’s high repair efficiency is cle. In so doing, it maintains the integri- during a person’s lifetime. In certain fam- one reason many decades can pass be- ty of the telomeres and thereby enables ilies, however, many individuals are af- fore all the mutations needed for a ma- How Cancer Arises Copyright 1996 Scientific American, Inc. Scientific American September 1996 69
  9. 9. Fundamental Understandingslignancy to develop will, by chance, will enable us to predict which members cinomas. Very likely, many of the solu-come together within a single cell. Cer- of cancer-prone families are at high risk tions to these mysteries will flow fromtain inherited defects, though, can ac- and which have, through good fortune, research in developmental biology (em-celerate tumor development through a inherited intact copies of these genes. bryology). After all, the genes that gov-particularly insidious means: they im- ern embryonic development are, muchpair the operation of proteins that re- Beyond Proliferation later, the sources of our malignancies.pair damaged DNA. As a result, muta- By any measure, the amount of infor-tions that would normally accumulateslowly will appear with alarming fre-quency throughout the DNA of cells. A lthough we have learned an enor- mous amount about the genetic basis of runaway cell proliferation, we mation gathered over the past two de- cades about the origins of cancer is with- out parallel in the history of biomedicalAmong the affected genes are inevitably still know rather little about the mutant research. Some of this knowledge hasthose controlling cell proliferation. genes that contribute to later stages of already been put to good use, to build Such is the case in another inherited tumor development, specifically those molecular tools for detecting and deter-colon cancer, hereditary nonpolyposis that allow tumor cells to attract blood mining the aggressiveness of certain typescolon cancer. Afflicted individuals make vessels for nourishment, to invade nearby of cancer, as David Sidransky discussesdefective versions of a protein responsi- tissues and to metastasize. But research in “Advances in Cancer Detection,” onble for repairing the copying mistakes in these areas is moving rapidly. ( Judah page 104. Still, despite so much insightmade by the DNA replication appara- Folkman describes the ingenuity of tu- into cause, new curative therapies havetus. Because of this impairment, colonic mor cells in generating their own blood so far remained elusive. One reason iscells cannot fix DNA damage efficiently; supply in “Fighting Cancer by Attack- that tumor cells differ only minimallythey therefore collect mutations rapidly, ing Its Blood Supply,” on page 150. from healthy ones; a minute fraction ofaccelerating cancer development by two Erkki Ruoslahti takes up metastasis in the tens of thousands of genes in a celldecades or more. People affected by an- “How Cancer Spreads” on page 72.) suffers damage during malignant trans-other familial cancer syndrome, xero- We are within striking distance of formation. Thus, normal friend andderma pigmentosum, have inherited a writing the detailed life histories of many malignant foe are woven of very similardefective copy of a gene that directs the human tumors from start to life-threat- cloth, and any fire directed against therepair of DNA damaged by ultraviolet ening finish. These biographies will be enemy may do as much damage to nor-rays. These patients are prone to sever- written in the language of genes and mal tissue as to the intended types of sunlight-induced skin cancer. molecules. Within a decade, we will Yet the course of the battle is chang- Similarly, cells of people born with a know with extraordinary precision the ing. The differences between normal anddefective ATM gene have difficulty rec- succession of events that constitute the cancer cells may be subtle, but they areognizing the presence of certain lesions complex evolution of normal cells into real. And the unique characteristics ofin the DNA and mobilizing the appro- highly malignant, invasive derivatives. tumors provide excellent targets for in-priate repair response. These people are By then, we may come to understand tervention by newly developed drugssusceptible to neurological degeneration, why certain localized masses never pro- [see the section “Therapies of the Fu-blood vessel malformation and a variety gress beyond their benign, noninvasive ture,” beginning on page 135]. The de-of tumors. Some researchers have pro- form to confront us with aggressive ma- velopment of targeted anticancer thera-posed that as many as 10 percent of in- lignancy. Such benign growths can be peutics is still in its infancy. This enter-herited breast cancers may arise in pa- found in almost every organ of the body. prise will soon move from hit-or-miss,tients with a defective copy of this gene. Perhaps we will also discern why certain serendipitous discovery to rational de- Over the next decade, the list of can- mutant genes contribute to the formation sign and accurate targeting. I suspectcer susceptibility genes will grow dra- of some types of cancer but not others. that the first decade of the new centurymatically, one of the fruits of the Human For example, mutant versions of the RB will reward us with cancer therapiesGenome Project (which seeks to identify tumor suppressor gene appear often in that earlier generations could not haveevery gene in the human cell). Together retinoblastoma, bladder carcinoma and dreamed possible. Then this nation’swith the increasingly powerful tools of small cell lung carcinoma but are seen long investment in basic cancer researchDNA analysis, knowledge of these genes only occasionally in breast and colon car- will begin to pay off handsomely. SA The Author Further Reading ROBERT A. WEINBERG is Member of the Whitehead Insti- Cancer: Science and Society. J. Cairns. W. H. Freeman, 1978.tute for Biomedical Research and a professor of biology at the Genes and the Biology of Cancer. H. Varmus and R. A. Wein-Massachusetts Institute of Technology, where he earned his doc- berg. Scientific American Library (distributed by W. H. Freeman),toral degree in biology in 1969. His laboratory was instrumen- 1993.tal in isolating the first human oncogene and the first human tu- The Multistep Nature of Cancer. B. Vogelstein and K. W. Kinzlermor suppressor gene. Weinberg, a member of the National Acad- in Trends in Genetics, Vol. 9, No. 4, pages 138–141; April 1993.emy of Sciences, has won many awards for his contributions to Cancer: The Rise of the Genetic Paradigm. J. M. Bishop in Genesthe understanding of cancer genetics, most recently the G.H.A. and Development, Vol. 9, No. 11, pages 1309–1315; June 1, 1995.Clowes Memorial Award of the American Association for Can- Oncogenes. Second edition. G. M. Cooper. Jones and Bartlett Pub-cer Research. This is his fourth article for Scientific American. lishers, Boston, 1995.70 Scientific American September 1996 Copyright 1996 Scientific American, Inc. How Cancer Arises