American Journal of Medical Genetics 111:96–102 (2002)

Research Review

Cancer Genetics

Alfred G. KnudsonInstitute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania

Cancer is a genetic disease of somatic cells.Tumor karyotypes are rarely normal, andmost show multiple abnormalities of bothnumber and structure. The first direct evi-dence for this concept of cancer came fromstudies of tumor-specific translocations inleukemias and lymphomas, revealing theimportance of oncogenes and the regulationof gene transcription in cancer. A secondmajor source of information about humancancer genes is hereditary cancer. Geneticpredisposition of the autosomal dominanttype imposes a high relative risk for one ormore kinds of cancer. In the past decade orso, more than 30 mutant genes for suchhereditary cancers have been cloned. Pene-trance depends upon additional, somatic,mutations. A few of the genes are oncogenesor DNA repair genes, but most are tumorsuppressor genes. Some tumor suppressorsregulate transcription, while others operatein signal transduction pathways that areinvolved in regulating processes of cellbirth, differentiation, and death. The knowl-edge gained is stimulating new approachesto the treatment and prevention of cancer.� 2002 Wiley-Liss, Inc.

KEY WORDS: mutational equilibrium; pe-netrance; somatic mutation;chromosomal translocation;oncogenes; DNA repair ge-nes; tumor suppressor genes;

transcription factors; signaltransduction; cell cycle; apo-ptosis; phakomatoses

INTRODUCTION

In the past three decades, considerable evidence hasbeen amassed in support of Boveri’s early hypothesisthat cancer is a somatic genetic disease. We also have arefined appreciation of the idea that cancer can arise asa consequence of spontaneous background mutation orof environmentally induced mutation, with or withoutan interaction between environment and genetic pre-disposition. However, investigation of the autosomaldominantly-inherited conditions that impose high rela-tive risks for cancer has been particularly fruitful inthat it has revealed genes that illuminate not only themechanism of predisposition, but also much about non-hereditary cancer, as well as normal regulation oftissue growth and differentiation. For the geneticist,it has been particularly interesting to learn how thetranslation of genotype into phenotype in a ‘‘hereditarydisease’’ can depend upon somatic mutations, therebyoffering the possibility of disease prevention by inter-ference with penetrance.

HEREDITARY PREDISPOSITIONTO CANCER

Incidence, Mutation, and Selection

Heritable predisposition is known for virtually everyform of cancer. The germline mutation never sufficesfor carcinogenesis; for each one that has been exam-ined, subsequent somatic mutation at one or more lociis required. Environmental factors, such as ionizingradiation, can increase the penetrance. Most of theentities produce some mortality before the end ofthe age of reproduction, in which case some germlinemutations are lost in every generation. Mutationalequilibrium is attained by a low rate of occurrence ofnew germline mutations, so the birth incidence ratesfor many such conditions are low, and similar in dif-ferent parts of the world. The highest known incidencewhere mutational equilibrium is determined by selec-tion against the heterozygote is that of approximatelyone per 3,000 births for neurofibromatosis type 1 (NF1),

This paper was presented at the International Symposium THEHUMAN GENOME, in Naples, September 6–8, 2000 on theoccasion of the Great Jubilee of the year 2000.

Correspondence to: Alfred G. Knudson, Institute for CancerResearch, Fox Chase Cancer Center, Philadelphia, PA 19111.E-mail: AG_Knudson@fccc.edu

Grant sponsor: National Institutes of Health; Grant number:CA06927; Grant sponsor: Commonwealth of Pennsylvania.

Received 2 February 2001; Accepted 29 December 2001

DOI 10.1002/ajmg.10320

� 2002 Wiley-Liss, Inc.

for which 50% of cases are the result of new mutation,at the very high rate (m) of approximately 8� 10�5 perlocus per generation. Of course, the previous familyhistory is negative in this situation. For hereditaryretinoblastoma (RB), with a birth incidence rate of2�10�5, about 80% of cases are newly mutant, andm¼0.8�10�5 per locus per generation. Incidences formost of the other conditions in this category lie betweenthese two, with mutation rates typically in the range of0.2–1.0�10�5 per locus per generation.

A few predispositions (hereditary breast cancer andhereditary non-polyposis colon cancer) have unusuallyhigh incidences of 0.1–1% (100–1,000� 10�5). Suchrates are totally incompatible with mutational equili-brium involving selection against the heterozygote, andnew mutations are very uncommon, perhaps at the levelof 1%. Furthermore, little mortality occurs before theend of the reproductive period, and this mortality, inthe case of breast cancer, may have been even lowerin previous centuries. Extensive pedigrees over manygenerations have been reported, and founder effects areoften observed, some extending over several centuries.In this situation, it seems that selection is operatingagainst the mutant homozygote, rather than the hete-rozygote. This idea is supported by the known prenatallethality for mice homozygous for Brca1 and Brca2[Ghebranious and Donehower, 1998]. With selectiononly against homozygotes and neutral effects forheterozygotes, a new mutation rate of even 0.1�10�5

loci per generation would yield, at equilibrium, a hete-rozygote incidence at birth of 200�10�5. New mutantswould constitute only 0.1% (0.2�10�5/200� 10�5) ofthe total.

Exceptional Cancers

Although heritable forms are well known for mostcancers, there are some interesting exceptions. Espe-cially notable are the very low incidences of dominantlyheritable carcinomas of the lung and cervix uteri, andrather low ones for leukemias, lymphomas, and sarco-mas. In the first two, environmental explanations seemto suffice: most cervical cancer is associated withinfection with human papilloma virus (HPV) and mostlung cancer is associated with cigarette smoking. Bothof these cancers would have low incidences in theabsence of these agents, but a heritable fraction wouldbe more apparent. In the leukemias, lymphomas, andsarcomas, a different explanation is required. Most ofthese cancers are characterized by specific transloca-tions, the first discovered one being the Philadelphiachromosome (Ph1) associated with chronic myelocyticleukemia (CML) [Rowley, 1973]. Why cancers in thehematopoietic system and connective tissues are domi-nated by this mechanism is not known. Does it signifythat other tissues are especially protected against it?Hereditary predisposition, when it does occur, neverinvolves inheritance of such an oncogenic translocation,but it sometimes entails inheritance of a predispositionto break chromosomes, as in the Bloom and Fanconisyndromes and in ataxia-telangiectasia (AT), all reces-sively inherited. It seems very likely that most germline

oncogenic translocations would be lethal, as is knownfor a few in the mouse.

Analysis of these translocations has led to the identi-fication of many genes whose resulting mutations areoncogenic, beginning with the discovery, in Burkittlymphoma, of the activation of the MYC oncogene onchromosome 8 by its juxtaposition to the heavy (H)chain immunoglobulin locus on chromosome 14 [Croceand Nowell, 1985]. In the case of CML, the Ph1 trans-location between chromosomes 9 and 22 activates theAbelson oncogene (ABL), but this chronic phase invari-ably progresses into an acute blastic phase in which oneof the main events has been characterized as a secondPh1, which evidently further increases the activity ofthe ABL oncogene. Oncogenic activation is so powerfulin these diseases that multiple events are apparentlynot necessary, and the karyotypes are often nearlynormal; general genomic instability is not typical[Gisselsson et al., 2000]. Only one or a few somatic gene-tic events seem to be necessary for oncogenesis. Amongthe genes activated by translocations are MYC andABL, both protooncogenes related to viral oncogenes;the mixed lineage leukemia (MLL) gene, which is homo-logous to the Trithorax gene of Drosophila melanoga-ster [Djabali et al., 1992; Gu et al., 1992; Tkachuk et al.,1992] and the B cell lymphoma 2 gene (BCL2), whichoperates upstream of p53 in the apoptotic signalingpathway and is homologous to the ced-9 gene inC. elegans [Yang and Korsmeyer, 1996].

We see then that the genetic events that play keyroles in human cancer can be divided into three groups:1) those that impose recessively inherited predisposi-tions to cancer, as in xeroderma pigmentosum; 2) thosethat are mutated somatically, but not germinally, aswith Burkitt lymphoma; and 3) those that can bemutated either germinally or somatically (Fig. 1). Thislast category embraces the dominantly heritable cancerpredispositions and much of nonhereditary cancer.

Cloned Heritable Cancer Genes

The mutant genes that impart dominantly inheritedsusceptibility to cancer have been cloned for approxi-mately 30 conditions. Most of them are tumor sup-pressors, but some are not. For example, three areoncogenes (RET, MET, KIT) that code for transmem-brane tyrosine kinase proteins. The first (RET) ismutant in the multiple endocrine neoplasia type 2(MEN2) syndrome [Donis-Keller et al., 1993; Mulliganet al., 1993], the second (MET) in hereditary papillaryrenal carcinoma (HPRC) [Schmidt et al., 1997], and thethird (KIT) in the hereditary gastrointestinal stromaltumor syndrome (GIST) [Nishida et al., 1998]. Theseoncogenes are not activated by translocation, however,but rather by point mutations that cause constitutivetyrosine kinase activity. Furthermore, the inheritedmutations are clearly not lethal to the fetus and are notsufficient for carcinogenesis. The most clearly under-stood case is that of MET in HPRC, wherein the tumorsare trisomic for chromosome 7, the site of MET, andtwo of the three copies are mutant [Zhuang et al.,1998]. Apparently one mutant in association with one

Cancer Genetics 97

wild-type chromosome is not sufficient for oncogenesis,but a ratio of 2:1 is sufficient, a situation resembling theacute phase of CML. The kidney is the site of multipletumors, each the result of a second, somatic, geneticevent.

Another surprise has been the discovery that somehereditary cancer genes are important for the repairof damage to DNA. In hereditary non-polyposis coloncancer, most of the cases involve one of two genes thatare homologous to yeast and bacterial genes known tooperate in DNA mismatch repair (MMR): MSH2, thehuman homologue of the MutS gene, and MLH1, that ofthe MutL gene [Fishel et al., 1993; Leach et al., 1993;Bronner et al., 1994; Papadopoulos et al., 1994]. Howcould the heterozygous state for a mutant repair genepredispose to cancer? The answer is that somatic muta-tion produces loss or mutation of the remaining wild-type allele in a cell, thereby rendering it defective forMMR. Subsequent somatic mutations are produced inthat cell at 1,000 times the normal rate [Ionov et al.,1993; Bhattacharyya et al., 1994], affecting especiallythe TGFBR2 gene [Markowitz et al., 1995], which con-tains a poly A tract of DNA that renders it vulnerable tomutation in the absence of normal MMR. Carcinomasoccur at high rates in several tissues, especially in

the colon, endometrium, and stomach, producing thehereditary non-polyposis colon cancer (HNPCC) syn-drome. Actually, polyps are found at a low rate but theyare apparently converted rapidly to carcinomas. Sincethe TGFBR2 gene is also a tumor suppressor gene, thepath to cancer through it and the APC gene involves aminimum of four genetic events, in addition to thesomatic mutation in one allele of the MMR gene. Anextra event is added by the latter, but passage throughthe other four happens at a very high rate. Thus, forexample, if the somatic mutation rate (m) per celldivision were 10�7, and the rate were 1,000 fold higherin the absence of normal MMR, then passage in anormal cell would be proportional to m4, and, in theHNPCC case, to m5(103)4. The ratio HNPCC/normalwould then be m(103)4¼10�7�1012¼ 105. However, fora two-event tumor, the ratio would be m(103)2, or 10�1,indicating that such a tumor should result lessfrequently from this mechanism than by the usualsequence of events. This probably explains the absenceof pediatric tumors in HNPCC.

TUMOR SUPPRESSOR GENES

Tumor suppressor (ts) genes constitute the largestgroup of cloned hereditary cancer genes. Many becameknown because they were cloned from patients withhereditary cancer; e.g., RB1, WT1, NF1, NF2, APC,VHL, TSC1, TSC2, BRCA1, and BRCA2 were all dis-covered in this way. Here I consider two ts genes whosepathways are operating abnormally in virtually allcancers, as well as ten genes mutated in a group ofdiseases known collectively as the phakomatoses.

Retinoblastoma Gene (RB1) and the Cell Cycle

The first ts gene to be cloned was RB1 [Friend et al.,1986]. Retinoblastoma was long known to occur in bothheritable and non-heritable forms. I had proposed thatthe two forms were related, in that tumors began ingenetically susceptible persons because a germinallymutated gene was directly causative of cancer, but thatan additional, somatic, mutation was necessary foroncogenesis, with this mutation occurring in the secondcopy of the gene; in non-hereditary cases both eventswere presumed to be somatic [Knudson, 1971, 1973].This accounted for the high incidence of bilateraltumors in cases with germline mutations and for theyounger ages at first appearance of tumor. Secondevents were visualized as intragenic mutation, dele-tion, loss of the wild-type chromosome, or somaticrecombination [Knudson, 1978]. Evidence for the two-hit idea and of these events came from subsequentstudies using linked polymorphic DNA markers [Cave-nee et al., 1983]. I had also proposed that this genewould be a candidate for early cloning because con-genital deletion cases localized the gene to a specificchromosomal band (13q14) [Knudson et al., 1976; Knud-son, 1973]. Indeed, intragenic deletion at this siteprovided the opportunity to clone RB1.

The incidence of the heritable form of this tumor, asnoted earlier, can be understood by recurring germlinemutations and selection against heterozygotes, at least

Fig. 1. Cancer-predisposing genes may be mutated in the germline andproduce recessively inherited diseases, such as the Bloom and Fanconisyndromes, AT, and xeroderma pigmentosum. Others, like RB1, TP53,NF1, and Wilms tumor 1 (WT1), may be mutant in the germline, impartingcancer susceptibility to heterozygotes, or be mutated somatically in non-hereditary cancer cases. A large number of somatic translocations areassociated with specific diseases, such as Burkitt lymphoma (BL), acutelymphocytic leukemia (ALL), CML, Ewing sarcoma (ES), and alveolarrhabdomyosarcoma (ARMS). None of these has been observed as a germlineevent. The t (11; ) translocation can involve any of a large number ofpartners to chromosome 11.

98 Knudson

in the past. The incidence of the non-hereditary formdepends upon a first somatic mutation in the devel-oping retina, growth of the mutant clone, and a subse-quent mutation in one cell of that clone (Fig. 2). Becausethere are more than 108 cells that are descended fromretinoblasts, there is ample opportunity for the firstmutation to occur; it may even be the case that mostindividuals have an eye with a clone of once-hit cellsthat differentiated before a second hit occurred. Inpersons with very large clones, the second hit wouldobviously be more probable. Retinoblastoma illustratesthe problem posed for an embryonal organ whose tissuestem cells are multiplying. However, because of normaldifferentiation, such cells cease to exist beyond earlylife, thus minimizing the time during which they are atrisk.

The continued growth of retinoblasts, and their fail-ure to differentiate normally in the presence of mutantRb protein (pRb), suggest that pRb may play a role inregulating DNA synthesis, either directly or indirectly.In fact, it is a key regulator of the cell cycle. The proteininteracts with the transcription factor E2F [Bagchiet al., 1991; Chellappan et al., 1991; Helin et al., 1992;Kaelin et al., 1992; Shirodkar et al., 1992], whichinterferes with the latter’s ability to activate transcrip-tion of some proteins important for DNA synthesis.Hyperphosphorylation of pRb in turn interferes with itsinteraction with E2F, so permitting passage of cellsthrough the cell cycle.

Other genes that are critical to pRb’s activity arecyclin D1 (CLND1), cyclin dependent kinase 4 (CDK4),and cyclin dependent kinase inhibitor 2 (CDKN2, whichencodes two products, p16 and p14ARF). Amplificationof CLND1 or CDK4, or loss of activity of CDKN2, canproduce the same effect as loss of RB1. It now appearsthat all, or virtually all, cancers have a defect in this‘‘RB pathway’’ [Sherr, 1996], which leads to an increasein the birth rate of cells.

TP53 and Apoptosis

The TP53 gene is the most frequently mutated of allcancer genes. It was initially discovered because itsprotein product was associated with transforming pro-

teins of certain DNA tumor viruses [Lane and Craw-ford, 1979; Linzer and Levine, 1979]. At first it wasthought to be the product of a cellular oncogene, butthen it was found to be an inhibitor of tumortransformation [Finlay et al., 1989] and defective insome human cancers [Baker et al., 1990]. Furthermore,pRb was also shown to associate with DNA viral pro-teins and it became clear that p53 and pRb were bothtumor suppressor proteins that could be inactivated byDNA viral transforming proteins [DeCaprio et al., 1988;Whyte et al., 1988].

TP53 is the gene mutated in many cases of one of thehereditary cancer syndromes, namely the Li-Fraumenisyndrome (LFS), which imposes especially high relativerisks to breast cancer, sarcomas, and adrenocorticaltumors [Malkin et al., 1990]. Of relevance, here is thefact that survivors of heritable retinoblastoma are alsoat increased risk of sarcomas, and that in non-heredi-tary sarcomas, both RB1 and TP53 may be mutant,although TP53 is not mutant in retinoblastoma. Forboth RB1 and TP53, the spectrum of tumors is notablefor those that arise in tissues like retina, bone, andbreast, that are growing in childhood and adolescence;with the exception of breast cancer, the carcinomas oftypical adult renewal tissues are not prominentlyfeatured in the syndromes. The sarcomas that areassociated with both heritable conditions are histologi-cally different from those that show specific transloca-tions as noted for some sarcomas, and their karyotypesare frequently very abnormal, showing marked chro-mosomal instability. Especially puzzling is the factthat TP53 is mutant in a majority of non-hereditarycarcinomas, yet only carcinoma of the breast is afeature of LFS, perhaps related to mammary growthin adolescence.

Cancers generally have not only an increased cellbirth rate, but also a decreased rate of apoptosis, theprincipal form of programmed cell death. A major factorin the activation of apoptosis is TP53, so the combina-tion of mutations in both RB1 and TP53 leads to in-creased cell birth and decreased cell death rates. It maybe that all cancers are defective in both pathways. Thesignals for activating this response of TP53 includedamage induced by ionizing radiation [Kastan et al.,1992], which leads to an increased concentration of p53and to arrest of the cell cycle. The outcome may beimpaired repair of DNA damage and cell recovery, orfailure of repair and apoptotic death. These responsescan also be elicited by cellular abnormality, such asoverexpression of the MYC oncogene. The first, but notthe second of these responses is mediated by a pathwaythat includes the products of the AT gene (ATM) andthe gene CHK2 [Matsuoka et al., 1998], which is homo-logous to the Rad53 gene in the yeast, S. cerevisiae.CHK2 is mutant in the germline in some cases of LFSthat do not show TP53 mutation [Bell et al., 1999]. Inthe absence of normal p53, the regulation of centrosomereplication is defective, leading to abnormalities inchromosome number [Fukasawa et al., 1996; Ghadimiet al., 2000]; repair of DNA double strand breaks is alsodefective, leading to structural aberrations in chromo-somes via the breakage-fusion-bridge cycle [Gisselsson

Fig. 2. Retinoblastoma-two mutations. Proliferating retinoblasts in thedeveloping retina. In the hereditary form, all cells are mutant for one RB1allele; in the non-hereditary form, a clone of once-hit cells can give rise to ahomozygously mutant tumor cell. Normally all retinoblasts ultimatelydifferentiate and become postmitotic.

Cancer Genetics 99

et al., 2000; Saunders et al., 2000] (Fig. 3). If a cellcontains numerous broken chromosomes, transloca-tions may occur.

Phakomatoses and Signal Transduction

The word phakomatosis was coined by Van der Hoeve[1932] and applied to a group of three diseases: neuro-fibromatosis (NF), tuberous sclerosis (TSC), and vonHippel-Lindau disease (VHL). All three conditionsmanifest scattered benign lesions (Greek phakos,mother spot) that occasionally become malignant. Thelist of phakomatoses has grown with the addition ofnevoid basal cell carcinoma syndrome (NBCCS, orGorlin syndrome), Cowden disease (CD), familialadenomatous polyposis (FAP), Peutz-Jegher (PJ) syn-drome, and juvenile polyposis (JP) [Knudson, 2000;Tucker et al., 2000]. The genes for all of them have beencloned, and all operate in signal transduction. Counter-parts of all of them except VHL are known inDrosophila[Rubin et al., 2000], and an increasing number of themare proving to be developmental lethals in the homo-zygous state in the mouse [Ghebranious and Done-hower, 1998]. Thus, the gene APC, whose mutation isresponsible for FAP, encodes a regulator of the wing-less/Wnt pathway, and the patched gene (PTCH) ofNBCCS operates in the hedgehog pathway [Hahn et al.,1996; Johnson et al., 1996]. The NF1 protein producthas GTPase activating activity that regulates RAS,which in its mutant form is a well known oncogeneproduct. All of the ten genes operate in a signal trans-duction network whose genes code for growth factorsor their receptors (including the MET, RET, and KIToncogenes), for links from them to the nucleus (e.g., theWnt pathway), and for proteins that operate in thecytoplasm, some of which connect DNA synthesis withthe mechanics of cell division (Fig. 4). Although muchhas been learned about their mechanisms of action,there remains a puzzle posed by the tissue specificityof tumor sites in individuals who carry germlinemutations in them.

The scattered benign lesions, some hamartomatousand some adenomatous, that characterize the phako-matoses become invasive and metastatic at a very lowrate. Thus, histologic examination of a kidney removed

for invasive renal carcinoma in VHL typically revealshundreds of small adenomas scattered throughout it. InFAP, the probability of carcinoma of the colon reachesvirtually 100% by the age of 40 years, but for each suchcarcinoma there are often more than a thousand benignadenomatous polyps. Carcinogenesis in this circum-stance is a slowly progressive process, usually occur-ring over many years. For FAP, it has been shown thatspecific genetic events are associated with phases ofthis process [Fearon and Vogelstein, 1990]. The earliestevent, and one that is apparently required for polypformation, is loss, mutation, or inactivation of the re-maining wild-type allele of APC; i.e., polyps are ‘‘two-hit’’ lesions. Penetrance of the polyp phenotype isuniversal and occurs early in life. Large dysplasticpolyps often show a mutation in one copy of the RASKoncogene, and even small carcinomas are usually mu-tant for TP53, suggesting that it may be an early lesionin malignant progression. The second event sets inmotion a series of further events, the pace of whichvaries greatly from tumor to tumor, a theme that seemsto apply to all of the phakomatoses (Fig. 5). Two hits arerate-limiting for precursor lesions, more hits are rate-limiting for the carcinomas.

As with RB and TP53, the same phakomatosis genesare regularly mutated somatically in the non-heredi-tary forms of the tumors that occur in persons with

Fig. 3. In the breakage-fusion-bridge cycle, a break gives rise to a lostacentric fragment and a chromosome with a sticky end, leading to fusedchromatids and dicentric chromosomes that break in new locations insubsequent cell divisions.

Fig. 4. Phakomatosis tumor suppressor proteins (in ellipses) and signaltransduction. These include TGFBR2 receptor; E-cadherin; b-catenin;smoothened (SMO); sonic hedgehog (SHH); MET, RET, and KIT tyrosinekinases; RAS oncoprotein; MAP kinase; phosphatidylinositol-3-kinase(PI3K); and the AKT oncoprotein, a serine-threonine kinase. Paths arestill in question for some gene products. RB1 and TP53 are key tumorsuppressors for the cell cycle and apoptosis.

100 Knudson

germline mutations. For example, VHL, PTCH, andAPC are mutated or inactive in the great majority ofsporadic clear cell carcinomas of the kidney, basal cellcarcinomas of the skin, and carcinomas of the colon,respectively. In each case, loss or inactivation of bothcopies of the gene is an early event. It may be that mostcarcinomas begin from ‘‘two-hit’’ benign lesions that arevery slowly progressive. This phenomenon leads to theidea that successful attempts at prevention or treat-ment of these genetically predisposed persons couldeven be translated to persons who are not so predis-posed. In contrast, for neoplasms that are malignantafter few events, which includes especially those thatarise in growing tissues, the opportunity for preventionis reduced, but treatment may be more effective.

CONCLUSIONS

Cancer is indeed a somatic genetic disease, and muchhas been learned, especially in the last decade of thispassing millennium, about specific genetic changes incancers. Some, perhaps even most, of the abnormalitiesobserved in many cancers may be non-specific, reflect-ing destabilization of the genome, and can be consid-ered the result of carcinogenesis. On the other hand,the uniquely specific aberrations revealed in trans-locations in leukemias, lymphomas, and sarcomas, andthose transmitted by persons genetically predisposed tocancer, are clearly significant in the carcinogenic pro-cess, and inviting targets of preventive and therapeuticmeasures.

ACKNOWLEDGMENTS

The author appreciates the constructive commentson the manuscript by Drs. Joseph R. Testa and AlfonsoBellacosa. He also regrets that many valuable publica-tions were not cited, owing to limitations of space.

REFERENCES

Bagchi S, Weinmann R, Raychaudhuri P. 1991. The retinoblastoma proteincopurifies with E2F-I, an E1A-regulated inhibitor of the transcriptionfactor E2F. Cell 65:1063–1072.

Baker SJ, Markowitz S, Fearon ER, Willson JK, Vogelstein B. 1990.Suppression of human colorectal carcinoma cell growth by wild-typep53. Science 249:912–915.

Bell DW, Varley JM, Szydlo TE, Kang DH, Wahrer DC, Shannon KE,Lubratovich M, Verselis SJ, Isselbacher KJ, Fraumeni JF, Birch JM,Li FP, Garber JE, Haber DA. 1999. Heterozygous germ line hCHK2mutations in Li-Fraumeni syndrome. Science 286:2528–2531.

Bhattacharyya NP, Skandalis A, Ganesh A, Groden J, Meuth M. 1994.Mutator phenotypes in human colorectal carcinoma cell lines. Proc NatlAcad Sci USA 91:6319–6323.

Bronner CE, Baker SM, Morrison PT, Warren G, Smith LG, Lescoe MK,Kane M, Earabino C, Lipford J, Lindblom A, Tannergard P, Bollag RJ,Godwin AR, Ward DC, Nordenskjøld M, Fishel R, Kolodner R, LiskayM. 1994. Mutation in the DNA mismatch repair gene homologuehMLH1 is associated with hereditary non-polyposis colon cancer.Nature 368:258–261.

Cavenee WK, Dryja TP, Phillips RA, Benedict WF, Godbout R, Gallie BL,Murphree AL, Strong LC, White RL. 1983. Expression of recessivealleles by chromosomal mechanisms in retinoblastoma. Nature 305:779–784.

Chellappan SP, Hiebert S, Mudryj M, Horowitz JM, Nevins JR. 1991. TheE2F transcription factor is a cellular target for the RB protein. Cell65:1053–1061.

Croce CM, Nowell PC. 1985. Molecular basis of human B cell neoplasia.Blood 65:1–7.

DeCaprio JA, Ludlow JW, Figge J, Shew JY, Huang CM, Lee WH, MarsilioE, Paucha E, Livingston DM. 1988. SV40 large tumor antigen forms aspecific complex with the product of the retinoblastoma susceptibilitygene. Cell 54:275–283.

Djabali M, Selleri L, Parry P, Bower M, Young BD, Evans GA. 1992. Atrithorax-like gene is interrupted by chromosome 11q23 translocationsin acute leukaemias [published erratum appears in Nat Genet1993;4:431]. Nat Genet 2:113–118.

Donis-Keller H, Dou S, Chi D, Carlson KM, Toshima K, Lairmore TC, HoweJR, Moley JF, Goodfellow P, Wells SA. 1993. Mutations in the RETproto-oncogene are associated with MEN 2A and FMTC. Hum MolGenet 2:851–856.

Fearon ER, Vogelstein B. 1990. A genetic model for colorectal tumorigen-esis. Cell 61:759–767.

Finlay CA, Hinds PW, Levine AJ. 1989. The p53 proto-oncogene can act as asuppressor of transformation. Cell 57:1083–1093.

Fishel R, Lescoe MK, Rao MR, Copeland NG, Jenkins NA, Garber J, KaneM, Kolodner R. 1993. The human mutator gene homolog MSH2 and itsassociation with hereditary nonpolyposis colon cancer. Cell 75:1027–1038.

Friend SH, Bernards R, Rogelj S, Weinberg RA, Rapaport JM, Albert DM,Dryja TP. 1986. A human DNA segment with properties of the gene thatpredisposes to retinoblastoma and osteosarcoma. Nature 323:643–646.

Fukasawa K, Choi T, Kuriyama R, Rulong S, Vande Woude GF. 1996.Abnormal centrosome amplification in the absence of p53. Science271:1744–1747.

Ghadimi BM, Sackett DL, Difilippantonio MJ, Schrock E, Neumann T,Jauho A, Auer G, Ried T. 2000. Centrosome amplification andinstability occurs exclusively in aneuploid, but not in diploid colorectalcancer cell lines, and correlates with numerical chromosomal aberra-tions. Genes Chromosomes Cancer 27:183–190.

Ghebranious N, Donehower LA. 1998. Mouse models in tumor suppression.Oncogene 17:3385–3400.

Gisselsson D, Pettersson L, Hoglund M, Heidenblad M, Gorunova L,Wiegant J, Mertens F, Dal Cin P, Mitelman F, Mandahl N. 2000.Chromosomal breakage-fusion-bridge events cause genetic intratumorheterogeneity. Proc Natl Acad Sci USA 97:5357–5362.

Gu Y, Nakamura T, Alder H, Prasad R, Canaani O, Cimino G, Croce CM,Canaani E. 1992. The t(4;11) chromosome translocation of human acuteleukemias fuses the ALL-1 gene, related to Drosophila trithorax, to theAF-4 gene. Cell 71:701–708.

Hahn H, Wicking C, Zaphiropoulous PG, Gailani MR, Shanley S,Chidambaram A, Vorechovsky I, Holmberg E, Unden AB, Gillies S,

Fig. 5. Renewal tissue stem cells undergo asymmetric cell divisionswith one daughter cell that continues as a tissue stem cell and one thatproduces differentiating cells that ultimately die and are replaced.Phakomatosis mutations may cause switches to some symmetric mitoses,leading to benign precursors of cancer.

Cancer Genetics 101

Negus K, Smyth I, Pressman C, Leffell DJ, Gerard B, Goldstein AM,Dean M, Toftgard R, Chenevix-Trench G, Wainwright B, Babe AE.1996. Mutations of the human homolog of Drosophila patched in thenevoid basal cell carcinoma syndrome. Cell 85:841–851.

Helin K, Lees JA, Vidal M, Dyson N, Harlow E, Fattaey A. 1992. A cDNAencoding a pRB-binding protein with properties of the transcriptionfactor E2F. Cell 70:337–350.

Ionov Y, Peinado MA, Malkhosyan S, Shibata D, Perucho M. 1993.Ubiquitous somatic mutations in simple repeated sequences reveal anew mechanism for colonic carcinogenesis. Nature 363:558–561.

Johnson RL, Rothman AL, Xie J, Goodrich LV, Bare JW, Bonifas JM,Quinn AG, Myers RM, Cox DR, Epstein EH, Scott MP. 1996. Humanhomolog of patched, a candidate gene for the basal cell nevus syndrome.Science 272:1668–1671.

Kaelin WG, Krek W, Sellers WR, Decaprio JA, Ajchenbaum F, Fuchs CS,Chittenden T, Li Y, Farnham PJ, Blanar MA, Livingston DM,Flemington EK. 1992. Expression cloning of a cDNA encoding aretinoblastoma-binding protein with E2F-like properties. Cell 70:351–364.

Kastan MB, Zhan Q, el-Deiry WS, Carrier F, Jacks T, Walsh WV, PlunkettBS, Vogelstein B, Fornace AJ. 1992. A mammalian cell cycle checkpointpathway utilizing p53 and GADD45 is defective in ataxia-telangiecta-sia. Cell 71:587–597.

Knudson AG. 1971. Mutation and cancer: statistical study of retinoblas-toma. Proc Natl Acad Sci USA 68:820–823.

Knudson AG. 1973. Mutation and human cancer. Adv Cancer Res 17:317–352.

Knudson AG. 1978. Retinoblastoma: a prototypic hereditary neoplasm.Semin Oncol 5:57–60.

Knudson AG. 2000. Chasing the cancer demon. Ann Rev Genet 34:1–19.

Knudson AG, Meadows AT, Nichols WW, Hill R. 1976. Chromosomaldeletion and retinoblastoma. N Engl J Med 295:1120–1123.

Lane DP, Crawford LV. 1979. T antigen is bound to a host protein in SV40-transformed cells. Nature 278:261–263.

Leach FS, Nicolaides NC, Papadopoulos N, Liu B, Jen J, Parsons R,Peltomaki P, Sistonen P, Aaltonen LA, Nystrom-Lahti M, Guan X-Y,Zhang J, Meltzer PS, Yu J-W, Kao F-T, Chen DJ, Cerosaletti KM,Fournier REK, Todd S, Lewis T, Leach RL, Naylor SL, Weissenbach J,Mecklin J-P, Jarvinen H, Petersen GM, Hamilton SR, Green J, Jass J,Watson P, Lynch HT, Trent JM, de la Chapelle A, Kinzler KW,Vogelstein B. 1993. Mutations of a mutS homolog in hereditarynonpolyposis colorectal cancer. Cell 75:1215–1225.

Linzer DI, Levine AJ. 1979. Characterization of a 54K dalton cellular SV40tumor antigen present in SV40-transformed cells and uninfectedembryonal carcinoma cells. Cell 17:43–52.

Malkin D, Li FP, Strong LC, Fraumeni JF, Nelson CE, Kim DH, Kassel J,Gryka MA, Bischoff FZ, Tainsky MA, Friend SH. 1990. Germ line p53mutations in a familial syndrome of breast cancer, sarcomas, and otherneoplasms. Science 250:1233–1238.

Markowitz S, Wang J, Myeroff L, Parsons R, Sun L, Lutterbaugh J, Fan RS,Zborowska E, Kinzler KW, Vogelstein B, Brattain M, Willson JKV.1995. Inactivation of the type II TGF-b receptor in colon cancer cellswith microsatellite instability. Science 268:1336–1338.

Matsuoka S, Huang M, Elledge SJ. 1998. Linkage of ATM to cellcycle regulation by the Chk2 protein kinase. Science 282:1893–1897.

Mulligan LM, Kwok JB, Healey CS, Elsdon MJ, Eng C, Gardner E, LoveDR, Mole SE, Moore JK, Papi L, Ponder MA, Telenlus H, Tunnacliffe A,Ponder BAJ. 1993. Germ-line mutations of the RET proto-oncogene inmultiple endocrine neoplasia type 2A. Nature 363:458–460.

Nishida T, Hirota S, Taniguchi M, Hashimoto K, Isozaki K, Nakamura H,Kanakura Y, Tanaka T, Takabayashi A, Matsuda H, Kitamura Y. 1998.Familial gastrointestinal stromal tumours with germline mutation ofthe KIT gene. Nat Genet 19:323–324.

Papadopoulos N, Nicolaides NC, Wei YF, Ruben SM, Carter KC, Rosen CA,Haseltine WA, Fleischmann RD, Fraser CM, Adams MD, Venter JC,Hamilton SR, Petersen GM, Watson P, Lynch HT, Peltomaki P,Mecklin JP, Delachapelle A, Kinzler KW, Vogelstein B. 1994. Mutationof a mutL homolog in hereditary colon cancer. Science 263:1625–1629.

Rowley JD. 1973. A new consistent chromosomal abnormality in chronicmyelogenous leukaemia identified by quinacrine fluorescence andGiemsa staining. Nature 243:290–293.

Rubin GM, Yandell MD, Wortman JR, Gabor Miklos GL, Nelson CR,Hariharan IK, Fortini ME, Li PW, Apweiler R, Fleischmann W, CherryJM, Henikoff S, Skupski MP, Misra S, Ashburner M, Birney E, BoguskiMS, Brody T, Brokstein P, Celniker SE, Chervitz SA, Coates D,Cravchik A, Gabrielian A, Galle RF, Gelbart WM. 2000. Comparativegenomics of the eukaryotes. Science 287:2204–2216.

Saunders WS, Shuster M, Huang X, Gharaibeh B, Enyenihi AH, Petersen I,Gollin SM. 2000. Chromosomal instability and cytoskeletal defects inoral cancer cells. Proc Natl Acad Sci USA 97:303–308.

Schmidt L, Duh FM, Chen F, Kishida T, Glenn G, Choyke P, Scherer SW,Zhuang ZP, Lubensky I, Dean M, Allikmets R, Chidambaram A,Bergerheim UR, Feltis JT, Casadevall C, Zamarron A, Bernues M,Richard S, Lips CJM, Walther MM, Tsui LC, Geil L, Orcutt M,Stackhouse T, Lipan J, Slife L, Brauch H, Decker J, Niehans G,Hughson M, Moch H, Storkel S, Lerman M, Linehan W, Zbar B. 1997.Germline and somatic mutations in the tyrosine kinase domain of theMET proto-oncogene in papillary renal carcinomas. Nat Genet 16:68–73.

Sherr CJ. 1996. Cancer cell cycles. Science 274:1672–1677.

Shirodkar S, Ewen M, DeCaprio JA, Morgan J, Livingston DM, ChittendenT. 1992. The transcription factor E2F interacts with the retinoblastomaproduct and a p107-cyclin A complex in a cell cycle-regulated manner.Cell 68:157–166.

Tkachuk DC, Kohler S, Cleary ML. 1992. Involvement of a homolog ofDrosophila trithorax by 11q23 chromosomal translocations in acuteleukemias. Cell 71:691–700.

Tucker M, Goldstein A, Dean M, Knudson A. 2000. National CancerInstitute Workshop Report: the phakomatoses revisited. J Natl CancerInst 92:530–533.

Van der Hoeve J. 1932. Eye symptoms in phakomatoses. Trans Ophth SocUK 52:380–401.

Whyte P, Buchkovich KJ, Horowitz JM, Friend SH, Raybuck M, WeinbergRA, Harlow E. 1988. Association between an oncogene and an anti-oncogene: the adenovirus E1A proteins bind to the retinoblastoma geneproduct. Nature 334:124–129.

Yang E, Korsmeyer SJ. 1996. Molecular thanatopsis: a discourse on theBCL2 family and cell death. Blood 88:386–401.

Zhuang Z, Park WS, Pack S, Schmidt L, Vortmeyer AO, Pak E, Pham T,Weil RJ, Candidus S, Lubensky IA, Lineham WM, Zbar B, Weirich G.1998. Trisomy 7-harboring non-random duplication of the mutant METallele in hereditary papillary renal carcinomas. Nat Genet 20:66–69.

102 Knudson