The Genetic Basis of Cancer

Patients stricken with cancer feelas if they have been invaded byan alien force. Yet malignanciesarise from our own tissue. In fact, theweight of evidence today indicates thatcancers generally derive from a singlecell that is changed dramatically by aseries of genetic alterations.

A healthy cell has a well-denedshape and ts neatly within the orderedarray of cells surrounding it. It respondsto the dictates of its environment, givingrise to daughter cells solely when thebalance of stimulatory and inhibitorysignals from the outside favors cell di-vision. But the process of replication, orgrowth, carries the constant hazard ofgenetic mutations: random changes thatcan impair the regulatory circuits of acell. If a single mutation occurs, thenewly damaged cell, which may looknormal and be slightly less responsiveto external messages, may occasionallyundergo unscheduled cell division.

Eventually, an accumulation of genet-ic damage can cause a daughter cell tobecome quite deaf to external messagesand to display the signs of malignancy.In particular, it loses its distinctive shapeand boundaries, ceases to respond togrowth-inhibiting signals and gains theability to replicate uncontrollably. Theresulting mass, in turn, can compress

and damage healthy tissue in its vicini-ty. What is worse, it can invade the bar-riers that separate one organ from an-other and can metastasize, establishingnew colonies at distant sites.

Studies carried out over the past 20years have begun to identify many ofthe genes that take part in this progres-sion from normalcy to cancer. The on-going research is conrming and ex-tending early proposals that cancer de-

velops primarily because cells suer ir-reversible damage to particular classesof genes. It is also creating opportunitiesfor improved diagnosis and therapy.

The emerging view of tumor progres-sion reects a convergence of severallines of research, the oldest of whichstill involves painstakingly looking atcells through a microscope. By 1914, forinstance, the German cytologist Theo-dor Boveri had concluded from such

72 SCIENTIFIC AMERICAN March 1995

WEBSTER K. CAVENEE and RAYMONDL. WHITE collaborated at the Universityof Utah in the early 1980s. Cavenee isdirector of the Ludwig Institute for Can-cer Research at the San Diego branch.He is also professor of medicine andmember of the Center for Molecular Ge-netics at the University of California,San Diego. Before accepting his currentposts, he had faculty appointments atthe University of Cincinnati and McGillUniversity. White, who has been on thefaculty of the University of Utah since1980, is director of the Huntsman Can-cer Institute and chairman of the depart-ment of oncological sciences. This is hissecond article for Scientic American.

The Genetic Basis of CancerAn accumulation of genetic defects can apparently

cause normal cells to become cancerous and cancerous cells to become increasingly dangerous

by Webster K. Cavenee and Raymond L. White

Copyright 1995 Scientific American, Inc.

observations that malignant cells hadabnormal chromosomes and that anyevent leading to such aberrancy wouldcause cancer.

Microscopic observations becameconsiderably more specic after 1970,when new staining techniques, togetherwith improved equipment, made it pos-sible to distinguish each of the 23 pairsof chromosomes that collectively con-tain all the genes forming the blueprintfor a human being. (All human cells, ex-cept for sperm and eggs, carry two setsof chromosomesone inherited fromthe mother and one from the father.)Each chromosome takes up the stain inspecic regions and thus becomesmarked by a characteristic series of lightand dark bands, a kind of bar codeidentifying the individual chromosome.

By comparing stained chromosomesfrom normal cells with those from tu-mors, investigators noted many dier-ent signs of genetic disarray in cancers.The chromosomes of tumors were oftenbroken, with some of the pieces joinedto other chromosomes. Individual chro-

mosomes were present in multiple cop-ies rather than the normal two. Wholechromosomes, or sometimes internalsegments, seemed to have disappearedentirely. Unfortunately, until the 1980sresearchers generally lacked the toolsthey needed to determine whether thechromosomal rearrangements wereamong the causes of cancer or were aby-product of its development.

Two Hits

Quite dierent evidence that geneshad a role to play came from ob-servations that some extended

families suffered an unusually high in-cidence of certain cancers. When par-ticular diseases run in families in pre-dictable patterns, an inherited defect isusually at fault.

Yet the discovery that some cancerscould apparently be inherited also raisedperplexing questions. A genetic defectpassed to a child through the sperm oregg should appear in every cell of thebody. Why, then, did people with inher-

ited disease typically acquire only oneor a few cancers and only at discretesites? Further, did the existence of fa-milial cancers necessarily mean thatsporadic (nonfamilial) disease, which ismuch more common, also had a genet-ic basis? Or did sporadic cancers ariseby completely dierent processes thaninherited ones?

A proposal put forward in 1971 byAlfred G. Knudson, Jr., now at the FoxChase Cancer Center in Philadelphia,seemed to oer an answer to both ques-tions, although it took about a decadefor his ideas to gain broad acceptance.Knudson had been puzzling over thecause of retinoblastoma, a rare child-hood disorder in which malignant tu-mors develop in the retina before theage of six. He noted that sometimes thedisease occurred in both eyes, but mostof the time it aected only one eye.Moreover, children who were aectedbilaterally often had close relatives af-icted with retinoblastoma.

A statistical analysis comparing theage at onset for each form of the dis-ease showed that the bilateral type wasusually diagnosed at an earlier age thanwas the unilateral type. Also, the shapeof the age distribution curves suggest-ed to Knudson that retinoblastoma re-sulted from two cellular defects arisingat separate times. In bilateral diseasethe rst defect was probably inheritedand present in all cells of the body fromthe moment of conception. In unilateraldisease the rst defect probably aroseduring development or later and per-haps exclusively in retinal cells. In bothcases, however, a tumor formed only ifthe rst defect in a retinal cell was lateraccompanied by a second, independentone. Knudsons two-hit theory, as it isfrequently called, turns out to be essen-tially correct for all cancers, not just ret-inoblastoma, although more than justtwo hits are often required.

The need for two hitsnow known toconstitute damage to genesexplainswhy patients in cancer-prone familiesare not riddled with tumors throughouttheir bodies: inheritance of just one ge-netic defect predisposes a person to

SCIENTIFIC AMERICAN March 1995 73

CANCER OF THE BRAIN progressed injust three months from being invisibleon a scan (left ) to covering a large areaof one hemisphere (blue area, right ).The patient, whose initial complaintwas an uncontrollable twitching in oneeye, died two months after the secondimage was made. Recent evidence indi-cates that brain tumors and other ma-lignancies arise when multiple geneticmutations combine to free a single cellfrom normal restraints on proliferation,invasiveness and movement.

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cancer but does not cause it directly; asecond event is required. Knudsons in-tuition that the causes of sporadic andfamilial cases can involve the same bio-chemical abnormalities has also beenconrmed. But even back in the 1970shis insights provided justication forthinking that research aimed at discov-ering genetic and other cellular aberra-tions in rare familial cancers could shedlight on the processes leading to spo-radic malignancies.

Oncogenes Take Center Stage

As various researchers focused on the genetics of familial malignan-cies, other workers convinced that geneswere at the root of cancer were takinga rather dierent approach to ndingcancer-related genes. It had been knownfor many years that viruses can causetumors in animals. That link hadspurred a great deal of research aimedat identifying the cancer-causing genescarried by the viruses and at nding thehost genes that were aected. Those ef-forts revealed, surprisingly, that thegenes implicated in malignant diseaseswere often altered forms of humangenes that the viruses had picked upduring their travels. Other times the vi-ruses activated host genes that wereusually quiescent.

The normal versions of the piratedand activated genesnow called proto-oncogenescarry codes specifying thecomposition of proteins that encouragecells to replicate. These growth-promot-ing genes come in many varieties. Somespecify the amino acid sequences of re-ceptors that protrude from the cell sur-face and bind to molecules known asgrowth factors. When bound by suchfactors, receptors issue an intracellularsignal that ultimately causes cells toreplicate. Others of the genes code forproteins that lie inside the cell and gov-ern the propagation of the intracellulargrowth signal. Still others encode pro-teins that control cell division.

Discovery that the viral genes hadhuman counterparts introduced the in-triguing possibility that human can-cersincluding the majority not causedby virusesmight stem from mutationsthat convert useful proto-oncogenesinto carcinogenic forms, or oncogenes.Consistent with this notion, studies in-dicated that alteration of just one copy,or allele, of these proto-oncogenes wasenough to transformrender cancer-oussome types of cells growing in cul-ture. Such dominant mutations causecells to overproduce a normal protein orto make an aberrant form that is over-active. In either case, the result is thatstimulatory signals increase within the

cell even when no such signals comefrom the outside.

Later studies supported a role for on-cogenesand also complicated matters.Notably, in 1982 and 1983, investigatorsin France and the U.S. conducted stud-ies similar to the original cell-culture ex-periments, but with an important dif-ference. Because normal cells would notgrow indenitely in a culture dish, thoseearlier studies had relied on rodent cellsthat were unusual in their ability to pro-liferate for a long time in culture. Toeliminate this possibly confounding in-uence, Franois Cuzin of the Universityof Nice, Robert A. Weinberg of the Mas-sachusetts Institute of Technology andH. Earl Ruley, then at Cold Spring Har-bor Laboratory in New York State, askedwhether single oncogenes could alsotransform normal rodent cells.

They found that mutations in at leasttwo proto-oncogenes had to be presentand that only certain combinations ofmutations led to malignancy. These re-sults suggested that individual onco-genes, though potentially quite power-ful, were not able to cause tumors bythemselves. A major eort was thenlaunched to see whether human tumorscarried oncogenic alterations of thetypes and combinations that were ableto transform cells in culture.

For a while it seemed that oncogenesmight explain most cases of cancer.This view was strengthened by discov-ery of more than a dozen of them inhuman tumors. The results were ulti-mately disappointing, however; a mere20 percent of human tumors turnedout to carry the expected alterationssingly, and none of them had the pairsof cooperative alterations found in cul-tured cells. At the time, it also appearedthat the inherited mutations responsi-ble for predisposing people to familialcancers were not oncogenes. Thesewere all strong hints that the full storywas yet to be told.

Enter Tumor Suppressor Genes

Even before those hints attractedmuch attention, the two of us werebeginning to suspect that damage to a dierent kind of gene might play apart in cancers. Such genes came to beknown as tumor suppressors becausemany of them code for proteins thatinhibit cell replication. In contrast tothe mutations that activate oncogenes,mutations of these genes, we believed,would be recessive: they would aectcell function only when both alleles weredamaged or lost. In testing this idea, werelied on new technology we had devel-oped for the more general purpose offollowing the inheritance of genes and

chromosomes through extended fami-lies [see Chromosome Mapping withDNA Markers, by Ray White and Jean-Marc Lalouel; SCIENTIFIC AMERICAN, Feb-ruary 1988].

In the early 1980s, while collaboratingat the University of Utah, we realizedthat our techniquewhich involvedtracking genetic markers ( identiablesegments of DNA) in tissuescould beused to determine whether segments ofchromosomes carried by normal cellswere missing in a tumor. For instance, ifa selected region of a chromosome wasdeleted in a tumor, we could spot thatloss by observing that a marker knownto travel with that region was alsomissing.

Our experiments were focused byearlier studies of Jorge J. Yunis of theUniversity of Minnesota and Uta Franckeof Yale University. That research indi-cated a gene on chromosome 13 mightbe involved in retinoblastoma. With ourDNA-marker technology, we were ableto demonstrate in 1983 that large seg-ments of chromosome 13 were missingin cells taken from sporadic as well asinherited retinoblastomas. This new ev-idence strongly supported the idea thatthe two hits hypothesized by Knudsoncould consist of the physical or func-tional loss of one allele of a gene fol-lowed by elimination of or damage tothe normal copy. The missing DNA onchromosome 13, now known as the RB(retinoblastoma) gene, was isolated byStephen H. Friend of Weinbergs labo-ratory in 1986 [see Finding the Anti-Oncogene, by Robert A. Weinberg; SCI-ENTIFIC AMERICAN, September 1988].

Subsequent studies have shown thatrecessive loss of the RB gene occurs inother cancers as well. What is more, in-activation or loss of DNA has now beenshown to be a major feature in the gen-esis of every solid cancer examined sofar. Breast cancer, prostate cancer, lungcancer, bladder cancer, pancreatic can-cer and many others are marked by thedisruption or elimination of multipletumor suppressor genes.

By the late 1980s, then, there wasgood evidence that mutations in bothproto-oncogenes and tumor suppres-sors could participate in causing can-cer. It seemed reasonable to guess thatsome kinds of cancer resulted from acombination of such mutations. But didthe mutations collect in the same cell ordid some aect one cell, and others, dif-ferent cells? A model of tumor progres-sion proposed in the 1950s by LeslieFoulds of the Chester Beatty ResearchInstitute in London and expanded inthe 1970s by Peter C. Nowell of theUniversity of Pennsylvania suggestedthat if both kinds of mutations were in-

74 SCIENTIFIC AMERICAN March 1995 Copyright 1995 Scientific American, Inc.

volved, they would accumulate in onecell and its direct descendants.

In this scheme, cancers are thoughtto arise and become more dangerousthrough a process known as clonal evo-lution. First, a single cell undergoes agenetic mutation that enables it to di-vide under conditions that cause normalcells to stop replicating. Because the in-appropriately dividing cells copy their

DNA and give identical sets to their o-spring, the next generation of cells car-ries the same changes and shows thesame inappropriate growth. Later, oneof these cells or their descendants un-dergoes a mutation that further enhanc-es its ability to escape normal regula-tion, perhaps allowing it to pass throughsurrounding tissue and enter the blood-stream. This mutation, too, is passed to

daughter cells. Repetition of the processenables one cell to accumulate the mu-tations it needs to metastasize and col-onize other organs.

If the theory were correct, it wouldmean the majority of cells in a tumorwould carry the same defects. That be-ing the case, therapy capable of coun-teracting one or more of those defectswould be eective against all, or a great


NORMAL CELL REPRODUCES ITSELF (sequence at top) in re-sponse to stimulation by external growth factors (green); itstops dividing in response to inhibitory factors (red, far right ).For either reaction to occur, messages from the factors mustbe relayed deep into the target cell (large panels). Many can-cer-causing genes are abnormal versions of ones that codefor proteins in stimulatory pathways (left panel ). The alteredgenes, called oncogenes, cause stimulatory proteins to be

overproduced or overactive. In one example, mutation of aparticular ras gene can lead to synthesis of a hyperactive rasprotein (inset at left ). Many other cancer-related genes codefor proteins in inhibitory pathways (right panel ) and are of-ten called tumor suppressors. Damage to these genes canpromote cancer if the defects prevent inhibitory proteinsfrom being made or functioning properlyas often occurswhen the p53 gene is mutated (inset at right ).

STIMULATORY PATHWAY INHIBITORY PATHWAY

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EFFECT OF MUTATIONIN P53 GENE

ABERRANTP53 PROTEINIS UNABLE TOACTIVATE GENES

PROTEIN NEEDED TO STALLCELL DIVISION IS NOT MADE

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majority, of the cancer cellsa featurethat is essential for eradicating any ma-lignancy. For this reason and others, weset out to see if we could nd evidencefor the clonal evolution of tumors. Oneof us (White) focused primarily on co-lon cancer, and the other of us (Cave-nee) on brain tumors. As part of thiswork, we had to identify many of thegenes involved in these cancers.

The Genetics of Colon Cancer

White turned to colon cancer in partbecause it usually emerges from awell-dened precursorthe colon pol-yp. If a cancer developed in a clonalfashion, mutations arising in an earlystage of tumor development would beexpected to be present in later stages,and each successive stage would bemarked by additional mutations. Totest this expectation experimentally, itis necessary to collect samples from the

successive stages and compare theirgenes. In colon disease, samples arefairly easy to obtain. As a polyp, whichis initially microscopic, becomes largerand more irregular, it becomes readilyaccessible to the gastroenterologist(who removes it for therapeutic purpos-es) and thus to the experimentalist.

Colon cancer also held appeal for ourpurpose because families that were ge-netically prone to a rare disease calledfamilial adenomatous polyposis hadbeen identied and were available forstudy. In aected individuals the colonbecomes carpeted with hundreds orthousands of polyps, one or more ofwhich is likely to become cancerous inmidlife. Clearly, an inherited defect insome genecalled APC (for adenoma-tous polyposis coli)was necessary forpolyp formation and, in turn, for thedevelopment of colon cancer in suchpatients. It also seemed possible thatappearance of a defect in the APC gene

was one of the earliest steps, if not therst step, leading to many cases of spo-radic colon cancer. If that gene couldbe isolated, these ideas could be tested,and investigators would have at leastone of the genes needed for evaluatingwhether colon cancer developed in aclonal manner.

In 1987 Mark Leppert in Whites lab-oratory at Utah and Walter F. Bodmerand his colleagues at the Imperial Can-cer Research Fund in London separatelydemonstrated, through use of the mark-er technology described earlier, that theAPC gene resided near the middle of thelong arm of chromosome 5. Intensivework, often collaborative, by Whiteslaboratory and those of two other in-vestigatorsYusuke Nakamura of theCancer Institute in Tokyo and Bert Vo-gelstein of Johns Hopkins Universityeventually revealed the precise locationof the gene. The research also identiedseveral inherited APC mutations thatappeared in sporadic as well as familialcolon tumors. This work thus dened arst step in the evolution of colon can-cer. It also provided additional conr-mation of the speculation that the samegenes are often mutated in both inher-ited and sporadic tumors.

The groups found, too, that all thecancer-related mutations in the APCgene led to production of an incompleteprotein. Evidently, cells could operaterelatively normally if they retained onenormal APC allele and thus made someamount of the full APC protein. But ifboth alleles became damaged, a neededbrake on replication disappeared. Theprecise function of the APC gene is un-clear, but now that the gene is in hand,its normal responsibilities and its rolein cancer should soon be dened.

Multiple Defects

The steps that follow immediatelyafter the APC gene is inactivated arestill obscure. In many cases, however,later mutation in a single allele of a par-ticular proto-oncogene seems to push apolyp toward malignancy. This gene, asManuel Perucho observed when he wasat Cold Spring Harbor Laboratory, is oneof several ras genes. The protein nor-mally made under the direction of thisgene sits under the cell membrane andrelays stimulatory messages fromgrowth factor receptors to other mole-cules in the cytoplasm. The mutant ver-sion does not wait for signals from theoutside but issues its own autonomousgrowth signals.

Vogelstein and his group have shownthat large polyps and colon cancers of-ten carry only mutated copies of twoadditional tumor suppressor genes. One


GENES ARE INHERITED IN MATCHING PAIRSone from the mother and one fromthe father (top). Sometimes mutation of a single copy pushes a cell toward cancer(left )such as when it leads to production of a protein that activates excessive celldivision. (Oncogenic mutations fall into that category.) Other times both copiesmust be alteredsuch as when a gene coding for a protein that stalls cell divisionis inactivated (right ). If only one copy of such a gene is aected (a), the other copycan still generate the needed protein. But if both copies are hobbled (b), an impor-tant brake on tumor development is lost.

ACTIVATING MUTATIONSMUTATED GENE

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is p53, which resides on chromosome17 and is now known to be involved inmany dierent cancers. The normal pro-tein product of this gene functions inseveral biochemical pathways, includ-ing those enabling a cell to repair dam-age to DNA. The other is a geneprob-ably DCC (for deleted in colorectal can-cer)that resides on chromosome 18.DCC codes for a protein that appearson the cell surface and helps colon cellsstick to one another.

The discovery that genetic changes inthe APC gene occur early and persist,whereas other changes appear only inlater stages, ts well with the theory ofclonal evolution. But that conclusionwas initially statistical and based on ex-amining tissues removed from manydierent patients. That approach couldnot demonstrate conclusively that mu-tations appearing in one generation ofcells are passed to later generations ofthose same cells. Another strategy, how-ever, provided more convincing results.

Sometimes the polyp from which acancer has emerged can be identied atthe edge of a cancer. By comparing theDNA in a polyp with that in its adjacentcancer, Vogelstein showed that everymutational hit found in a polyp also ap-peared in the corresponding cancer, aswould be expected if the tumor formedby clonal evolution. Further, the cancerinvariably included mutations that werenot found in the polyp, as would also

be expected if the added mutations ac-counted for the increased aggressive-ness of a cancer. For instance, somepolyps carried a ras mutation withouta p53 defect, but the cancers growingfrom the polyps had both mutations.As yet, there is no strong evidence thatmutation of ras, p53 and DCC genesmust happen in any particular order fora polyp to become cancerous, althoughthe ras mutation seems to come rstfairly often.

Brain Tumors Reveal Their Secrets

In spite of these encouraging ndings,study of colon cancer has a major an-alytical limitation. To truly demonstratethat a given clone of cells is undergoingprogressive changes in its genes, oneneeds to examine the same tumor overtime. In the case of colon cancer, tumorsare almost always removed at the earli-est stage of detection. Such practicemakes good clinical sense, but it pre-vents sequential observations. This con-sideration led Cavenee to seek out adisease in which removal of a tumor issometimes followed by the reappear-ance of the tumor in a more aggressiveform at the same site. In 1987, while hewas at the Ludwig Institute for CancerResearch at McGill University, he and hisco-workers settled on cancers known asastrocytomasthe most common tu-mors that originate in the brain.

Cancer of the brain is dened some-what dierently than it is in other tis-sues. In that organ, cells do not need toinvade connective tissue or metastasizein order to be lethal; sadly, proliferationat a site critical to survival can some-times be enough to kill a patient. Hence,most masses in the brain are called can-cers. Cavenees group examined pro-gression of astrocytomas from their lessmalignant to more malignant stages, asdetermined by the size and shape of thetumors and by the structure of theirconstituent cells.

When the investigators began thiswork in 1987, they did not have theblueprint of genetic change that wasemerging for colon cancer. They there-fore began by laying the groundworkfor future studies of individual patients.They obtained tumors from many dif-ferent patients, grouped them accord-ing to stages, or grades, of advancingdisease, and compared the genetic de-rangements found in each stage.

Over the next four years they madegood headway. They learned, for in-stance, that tumors of every grade hadinactivating alterations in chromosome17, in a gene they had not yet identied.Moreover, the proportion of tumors dis-playing the mutation in the lowest stagewas equal to that in all other stages;this pattern is a sign that the mutationcame early and was retained. If a muta-tion generally occurred later in disease,


EMERGENCE OF A CANCER CELL from a normal one (tan) is thought to occur through aprocess known as clonal evolution. First, one daughter cell (pink ) inherits or acquires acancer-promoting mutation and passes the defect to its progeny and all future genera-tions. At some point, one of the descendants (red) acquires a second mutation, and alater descendant (green) acquires a third, and so on. Eventually, some cell (purple) ac-cumulates enough mutations to cross the threshold to cancer.

NORMALCELL

FIRST MUTATION Cell seems normal but is predisposed to proliferate excessively

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Cell grows uncontrollablyand looks obviouslyderanged

Cell proliferates more rapidly; it also undergoes structural changes

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the frequency would rise in the laterstages. By the end of the 1980s Vogel-steins laboratory established that mu-tations in the p53 gene, on chromosome17, were among the most common al-terations in human cancer. Subsequentanalysis of Cavenees tissue samplesconrmed his growing suspicion thatthe chromosome 17 mutation was ac-tually a defect in the p53 gene.

Aware that a particular region ofchromosome 9 was deleted in otherkinds of brain tumors, C. David Jameson Cavenees team, in conjunction withV. Peter Collins of the Ludwig Institutein Stockholm, examined this chromo-

some as well. Middle- and late-stage as-trocytomas, but not early ones, oftenshowed a loss in both copies of thischromosome. Thus, the deletion proba-bly encouraged progression to middle-stage tumors from a lesser stage. Thelost region contains a cluster of genesthat code for proteins known as inter-ferons. Such proteins can draw the at-tention of the immune system to dis-eased cells, and so elimination of theirgenes presumably helps cancer cellsevade immune destruction. The missingregion may additionally include twonewly discovered genes, called multipletumor suppressors 1 and 2, whose pro-

tein products are involved in regulatingcell division. Disappearance of any ofthese genes could potentially contrib-ute to a variety of cancers.

The tissue studies also extended re-ports by Axel Ullrich of Genentech,Michael D. Watereld of the Ludwig In-stitute in London and Joseph Schlessin-ger of the Weizmann Institute of Sciencein Israel that chromosomes in astrocy-tomas often carry more than one copyof the gene specifying the receptor forepidermal growth factor. Because eachcopy can be used to make the protein,cells will carry extra receptors on theirsurface. That abundance, in turn, cancause cells to overreact to the presenceof the growth factor. This alterationseems to participate in bringing tumorsfrom a middle to a late stage of disease.

Finally, Cavenees group found thatvirtually all the end-stage tumors exam-ined were missing one copy of chromo-some 10 and that the loss was rare inearlier stages. This pattern says the lossis probably involved in advancement tothe most virulent stage. Regrettably,though, we do not yet know which geneor genes on the lost chromosome aremost important to the progression.

These results suggested by 1991 thatformation of brain tumors involves, ata minimum, inactivation of the p53gene, loss of a gene on chromosome 9,oncogenic amplication of the gene forthe epidermal growth factor receptorand, at a very late stage, loss of at leastone copy of chromosome 10. But strong-er proof that astrocytomas are causedby the accumulation of these, and pos-sibly other, defects in cells required ex-amining genetic changes in the cancerof single individuals over time.

At about that time Tom Mikkelsenjoined Cavenees laboratory and tookon the challenge of comparing the ge-netic makeup of original astrocytomaswith that of later recurrences arising atthe same sites. This task was impossi-ble earlier not only because the genesinvolved were not known but also be-


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GENETIC CHANGES indicated at the top are among thosethought to participate frequently in the development of co-

lon cancer (left ) or in the progression of a common brain can-cer (astrocytoma) from its mildest to its most aggressive

PHILADELPHIA CHROMOSOME (at right in inset ) was the rst chromosomal ab-normality ever linked to a specic cancer. In the 1960s Peter C. Nowell of the Uni-versity of Pennsylvania observed that the appearance of an unusually small chro-mosome in white blood cells was a hallmark of leukemia. It is now known that theaberrant structure forms when a normal version of chromosome 22 (at left in in-set ) swaps genetic material with another chromosome, in the process giving upmore than it receives. Unfortunately, the DNA gained by chromosome 22 com-bines with a preexisting gene to form a hybrid oncogene.

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yLOSS OF APC GENE(CHROMOSOME 5)

COLON CANCERONCOGENIC MUTATION

OF A RAS GENE(CHROMOSOME 12)

LOSS OF P53 GENE(CHROMOSOME 17)

LOSS OF A GENE (POSSIBLYDCC) ON CHROMOSOME 18

NORMAL TISSUE SMALL BENIGN POLYP LARGE BENIGN POLYP CANCER


cause matched pairs of tumors are hardto obtain. A patient seen initially at oneinstitution may be cared for elsewherewhen the cancer returns. Also, physi-cians do not remove tumors that reap-pear if it is thought that surgery is un-likely to extend survival. Luckily, howev-er, two distinguished cliniciansMark L.Rosenblum of the University of Califor-nia at San Francisco and Karl Schwech-heimer of Albert Ludwigs University inFreiberg, Germanyhad come forwardwith collections of frozen tissue thatincluded a few matched sets.

To Cavenees satisfaction and delight,the genetic analysis of these tissuesdone in collaboration with David Sid-ransky in Vogelsteins groupfullledthe predictions of the theory of clonalevolution. The initial tumors possessedfewer mutations than did the recurrenc-es. These alterations included one ormore of the genetic hits (such as dam-age to chromosome 17) that had beenidentied in the low-grade tumors ana-lyzed previously. And, most signicant,the corresponding high-grade versionspossessed each alteration found in theprimary tumor as well as additional de-fects (of the kinds identied in the ear-lier studies). For reasons that are notobvious, progression of astrocytomasseems to follow a more dened se-quence of genetic changes than is ap-parent in colon cancer.

Next on the Agenda

The collected results we have de-scribed oer strong support for theidea that cancer develops and becomesmore dangerous primarily because cellsin a single lineage accumulate defectsin genes that normally regulate cell pro-liferation. Changes in other kinds ofgenes, many of which have not yet beenidentied, presumably facilitate theability of tumors to grow, invade localtissue and establish distant metastases.Hormones and other factors in the en-vironment around the genetically al-

tered cells almost certainly enhancetheir genetically dened deregulation.

Questions remain. Why do cell typesdier in the mix of mutations they re-quire in order to become cancerous?And how is it possible for ve or moremutations to accumulate in cells? Afterall, the probability is actually quitesmall that any given cell bearing a per-manent mutation in a cancer-relatedgene will independently gain anothermutation in such a gene.

Newly discovered genetic aberrationsfound in a second form of inherited co-lon tumors (hereditary nonpolyposis co-lon cancer) may oer a partial answerto the last question. The aected genesspecify proteins responsible for identi-fying and repairing mistakes madewhen DNA in a replicating cell is cop-ied. If these repair genes themselvesare damaged, the number of mutationspassed to daughter cells will go up dra-matically. The daughter cells may thendeliver DNA carrying still more muta-tions to their progeny. Defects in repairgenes may thus play a role in makinglate-stage tumors highly aggressive.They may even account for the aston-ishingly fast rate at which some tumorsarise and become killers.

Mutations in certain genes can also beespecially devastating if the mutationshave multiple eects. As a case in point,damage to the p53 gene can apparentlydo more than release a brake on prolif-eration. Certain mutations seem to re-duce the ability of cells to limit bloodvessel formation. As extra vessels growin a tumor, they help to nourish themass and to serve as conduits throughwhich malignant cells can spread to dis-tant sites. In parallel, the abnormal pro-teins yielded by the altered gene mayaid tumor cells in resisting the destruc-tive eects of radiation.

As investigators gain clarity on thespecic groups of genetic changes thatlead to and exacerbate particular formsof cancer, their insights should point theway to practical benets for patients.

When the mutations follow in a fairlyset sequence, their identication in apatients tumor should be of value forclarifying the stage of disease and thusfor tailoring therapy to the individualsneeds. In addition, knowledge of thegenes that are mutated in a primary tu-mor may make it possible to detect re-currences of some cancers earlier thanis now possibleby spotting mutationsthat have occurred in tissues not yetdisplaying detectable masses.

Expanded understanding of the genet-ic bases of cancer can also be expectedto lead to the introduction of drugs thatwill counteract the eects of selectedmutations and thereby slow tumor de-velopment or halt it altogether. Someevidence suggests it may not be neces-sary to correct the eects of every mu-tation; doing so for one or two genesmay well prove to be sucient for tam-ing renegade cells.

The process by which normal cellsbecome cancerous and grow ever moredangerous is undoubtedly even morecomplicated than has been discoveredso far. But continued investigation ofthe genetic changes underlying speciccancers seems a rational way to teaseapart many of those complexitiesandto gain new leads for treatment.


FURTHER READING

THE CLONAL EVOLUTION OF TUMOR CELLPOPULATIONS. Peter C. Nowell in Science,Vol. 194, pages 2328; October 1, 1976.

GENETIC AND EPIGENETIC LOSSES OF HET-EROZYGOSITY IN CANCER PREDISPOSI-TION AND PROGRESSION. Heidi J. Scra-ble, Carmen Sapienza and Webster K.Cavenee in Advances in Cancer Re-search, Vol. 54, pages 2562; 1990.

A GENETIC MODEL FOR COLORECTAL TU-MORIGENESIS. Eric R. Fearon and BertVogelstein in Cell, Vol. 61, No. 5, pages759767; June 1, 1990.

TUMOR SUPPRESSOR GENES. Robert A.Weinberg in Science, Vol. 254, pages11381146; November 22, 1991.

form (right ). Other genes not listed here play roles as well.Unless otherwise indicated, the term gene loss indicates

that both copies of a tumor suppressor gene have been dam-aged or deleted. The images show magnied slices of tissue.

TOM

MIK

KELS

EN

LOSS OF P53 GENE

ASTROCYTOMA

LOSS OF A CLUSTER OF GENESON CHROMOSOME 9

MULTIPLICATION OF GENE FOREPIDERMAL GROWTH FACTORRECEPTOR (CHROMOSOME 7)

LOSS OF ONE COPY OFCHROMOSOME 10

NORMAL TISSUE LOW-GRADE TUMOR HIGHER-GRADE TUMOR MOST AGGRESSIVE FORM OF TUMOR


Documents

The Genetic Basis of Cancer