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Two genetic hits (more or less) to cancer
Author: Alfred G. Knudson
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"� 2001 Macmillan Magazines Ltd PERSPECTIVES 6. Borst, M. J. & Ingold, J. A. Metastatic patterns of invasive lobular versus invasive ductal carcinoma of the breast. Surgery 114, 637?641 (1993). 7. Elledge, R. M. et al. HER2 expression and response to tamoxifen in estrogen receptor-positive breast cancer: a Southwest Oncology Group Study. Clin. Cancer Res. 4, 7?12 (1998). 8. Elston, C. W. et al. Causes of inconsistency in diagnosing and classifying intraductal proliferations of the breast. Eur. J. Cancer 36, 1769?1772 (2000). 9. Jones, C. et al. Comparative genomic hybridization analysis of myoepithelial carcinoma of the breast. Lab. Invest. 80, 831?836 (2000). 10. Jones, C. et al. CGH analysis of ductal carcinoma of the breast with basaloid/myoepithelial cell differentiation. Br. J. Cancer 85, 422?427 (2001). 11. Tsuda, H. et al. Large, central acellular zones indicating myoepithelial tumor differentiation in high-grade invasive ductal carcinomas as markers of predisposition to lung and brain metastases. Am. J. Surg. Pathol. 24, 197?202 (2000). 12. Kitahara, O. et al. Altered gene expression during colorectal carcinogenesis revealed by cDNA microarrays after laser capture microdissection of tumour tissues and normal epithelia. Cancer Res. 61, 3544?3549 (2001). 13. Sgroi, D. C. et al. In vivo gene expression profile analysis of human breast cancer progression. Cancer Res. 59, 5656?5661 (1999). 14. Pollack, J. R. et al. Genome-wide analysis of DNA copy- number changes using cDNA microarrays. Nature Genet. 23, 41?46 (1999). 15. Simone, N. L. et al. Laser-capture microdissection: opening the microscopic frontier to molecular analysis. Trends Genet. 14, 272?276 (1998). 16. Sapolsky, R. J. et al. High-throughput polymorphism screening and genotyping with high-density oligonucleotide arrays. Genet. Anal. 14, 187?192 (1999). 17. Page, M. J. et al. Proteomic definition of normal human luminal and myoepithelial breast cells purified from reduction mammoplasties. Proc. Natl Acad. Sci. USA 96, 12589?12594 (1999). 18. Kononen, J. et al. Tissue microarrays for high-throughput molecular profiling of tumor specimens. Nature Med. 4, 844?847 (1998). 19. Camp, R. L. et al. Validation of tissue microarray technology in breast carcinoma. Lab. Invest. 80, 1943?1949 (2000). 20. Khan, J. et al. Classification and diagnostic prediction of cancers using gene expression profiling and artificial neural networks. Nature Med. 7, 673?679 (2001). 21. Hedenfalk, I. et al. Gene-expression profiles in hereditary breast cancer. N. Engl. J. Med. 344, 539?548 (2001). Online links DATABASES The following terms in this article are linked online to: CancerNet: http://cancernet.nci.nih.gov/ breast tumour | colorectal tumour | squamous cell carcinomas | thyroid tumours LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/ ABL | BCR | HER2/NEU | oestrogen receptor Medscape DrugInfo: http://promini.medscape.com/drugdb/search.asp Herceptin | Gleevec | Tamoxifen OMIM: http://www.ncbi.nlm.nih.gov/Omim/ multiple endocrine neoplasia FURTHER INFORMATION Breast Pathology Index: http://www- medlib.med.utah.edu/WebPath/BRESHTML/BRESTIDX.html Introduction to Microarray Analysis: http://www.nhgri.nih.gov/DIR/LCG/15K/HTML/ Laser Capture Microdissection: http://mecko.nichd.nih.gov/lcm/lcm.htm NHGRI Introduction to DNA Microarray Technology: http://industry.ebi.ac.uk/~alan/MicroArray/IntroMicroArrayTal k/index.htm Stanford University Microarray site: http://cmgm.stanford.edu/pbrown/array.html Tumour Pathology: http://www.tumorboard.com/ Access to this interactive links box is free online. NATURE REVIEWS | CANCER VOLUME 1 | NOVEMBER 2001 | 157 Two genetic hits (more or less) to cancer Alfred G. Knudson TIMELINE Most cancers have many chromosomal abnormalities, both in number and in structure, whereas some show only a single aberration. In the era before molecular biology, cancer researchers, studying both human and animal cancers, proposed that a small number of events was needed for carcinogenesis. Evidence from the recent molecular era also indicates that cancers can arise from small numbers of events that affect common cell birth and death processes. We are now very familiar with the concept that cancer occurs as a consequence of sev- eral somatic mutations, but how did this concept first arise? The idea that cancer is a genetic disease of somatic cells ? proposed by Theodor Boveri in 1914 (REF. 1) ? was prompted by previous observations of aber- rant mitoses by David von Hansemann 2 , and by Boveri?s own interest in centrosomes and their abnormalities during develop- ment (see TIMELINE). Boveri even suggested some consequences of abnormal chromo- some numbers, anticipating the contempo- rary era of tumour-suppressor genes and oncogenes (BOX 1) 3 . The term ?somatic muta- tion? was first applied to cancer by Ernest Tyzzer 4 , who observed that tumours sequen- tially transplanted in mice developed an ever-broadening host specificity among recipients from different inbred strains. Concrete support for the genetic concept came from Hermann J. Muller?s 5 discovery that ionizing radiation, already known to be carcinogenic, is mutagenic. The long latent period between exposure to such radiation and the appearance of most of the inducible cancers further indicated that more than one mutation per cell must be involved 6 . Subsequently, the high incidence of skin cancer in patients with xeroderma pigmen- tosum, a condition to which Boveri drew attention 1 , was shown to be a consequence of somatic mutations in the presence of a hereditary defect in the repair of ultraviolet- light-induced damage to DNA 7 . Chemical carcinogenesis also fitted into the mutation- al concept of cancer with the observations of ?initiation? by carcinogens and of ?promo- tion? by other kinds of chemicals 8 , the for- mer being an irreversible change, probably mutation, the latter, a reversible change affecting the growth kinetics of the target cells. But although some initiating chemical carcinogens were found to be mutagenic, others were not; this discrepancy was resolved when Bruce Ames and colleagues discovered that non-mutagenic initiators could be made mutagenic by metabolic acti- vation 9 . Most cancers came to be considered to be initiated by somatic mutation, either induced or spontaneous. The suggestion that more than one event seemed to be required for carcinogenesis then raised a question about their number. The number of ?hits? A conspicuous feature of the epidemiology of common cancers is that their incidence increases with age, so the notion of multiple mutations was invoked by way of explana- tion 10,11 .Ifr successive mutations occur in some cells at constant rates ? k 1 , k 2 ,?k r per unit time (t), if the size of the target-cell popu- lation remains constant, and if cells with an intermediate number of mutations have no growth advantage, the age-specific incidence (I) would be I = kt r?1 . Therefore, a log?log plot of the relationship would be ln I = ln k+(r?1) ln t ? a linear relationship in which the slope would yield r?1 (FIG. 1). Many cancers show this relationship, and r has been estimated for numerous cancers; for example, r = 6 for colon cancer 10,12 . This, of course, would be the num- ber of rate-limiting events that produce a rec- ognizable cancer. Subsequent events of biologi- cal importance for invasion and metastasis would not be included in this number if the sixth event gave a suddenly large growth advantage, producing an obvious tumour. A possible fallacy in the estimation of r is that the mutation rate might change with time. An obvious example is provided by lung cancer and smoking. Given the muta- genic effect of tobacco smoke, the mutation rate for a particular step in the process would be increased at the onset of smoking. Another case in which r can be incorrectly estimated is a biologically important event that occurs at a higher rate than is usual for mutations that are rate limiting and would � 2001 Macmillan Magazines Ltd 158 | NOVEMBER 2001 | VOLUME 1 www.nature.com/reviews/cancer PERSPECTIVES tumours. Without specific knowledge of these factors, however, the precise number of cru- cial events cannot be estimated. Are there, then, other means for discovering the number and nature of such events? Cytogenetics and ?one-hit? tumours One such means has been the cytogenetic examination of cancers with modern tech- niques. Although most cancers reveal exten- sive chromosomal instability, which is visible by karyotype analysis, a remarkably contrary discovery was made by Peter Nowell and David Hungerford in 1960 (REF. 19). In the chronic myelogenous leukaemia (CML) cells that they examined, they found the same cytogenetic change: chromosome 22 was too small. They named this chromosome the Philadelphia chromosome (Ph 1 ), and it was later shown by Janet Rowley to result from a reciprocal translocation between chromo- somes 9 and 22 (FIG. 2a) 20 . The subsequent discovery of cellular proto-oncogenes by Dominique Stehelin et al. 21 and the demon- stration by Robert Weinberg and colleagues 22 of in vitro transformation by DNA, paved the way to a mechanistic understanding of how translocations lead to cancer. Still later, fol- lowing the discovery that the typical 8;14 translocation in Burkitt?s lymphoma activat- ed the MYC oncogene 23,24 , the CML translo- cation was found to activate the Abelson (ABL) oncogene 25?27 . The resulting chimeric ABL protein seems to interfere with regula- tion of both the cell cycle (increasing cancer- cell birth rate) and apoptosis (decreasing cancer-cell death rate), via its activation of the AKT oncoprotein 28 . CML continues to provide excitement because the increased tyrosine kinase activity of the chimeric ABL gene product in the leukaemic cells can be that predisposes them to colorectal cancer. But because polyps already have a growth advantage, r for FAP is 3?4, showing how an intermediate growth advantage can affect the relationship to age 13 . For non-hereditary colon cancer the number of events should be 4?5, rather than 6. Two of these are accounted for by the mutation or loss of the two alleles of the APC gene that leads to polyp formation 14,15 . This confounding effect of intermediate growth advantage on the num- ber of hits deducible from incidence curves was already anticipated by Peter Armitage and Richard Doll 16 , contributors to the original interpretation 10 , in a second paper in which they fitted cancer-incidence curves to a two- mutation curve that took growth advantage into account. In a later model, this advantage was attributed to both an increase in cell birth rate and a decrease in cell death rate 17,18 .So, can we calculate the number of mutations necessary for a tumour to occur, simply from a log?log plot of age-specific incidence? We can conclude that the age-specific incidence for a cancer depends on the mitotic rate of target cells, mutation rates per mitosis, the number of mutational events on the path to detectable cancer and selective processes that occur at each step in the evolution of therefore not be counted; inactivation of gene expression by methylation might con- stitute such an event. Some cancers do not fit the mathematical model for other reasons. For example, the childhood cancers show a peak incidence in early life because the cells that give rise to tumours attain maximum numbers at that time. Similarly, osteosarcoma has an increas- ing incidence during adolescence, when the rate of growth of the long bones is highest. Breast-cancer incidence increases more slow- ly after the menopause, causing a downturn in the log?log plot of age-specific incidence. This is presumably due to a decrease in the number of dividing cells that could give rise to tumours after the menopause. The notion that there is no growth advan- tage in intermediate stages is also faulty for many cancers. For example, most colorectal carcinomas arise from adenomatous polyps, the cells of which clearly have a growth advantage that leads to a benign neoplasm. In theory, a log?log plot of age-specific colon cancer incidence in people with familial ade- nomatous polyposis (FAP) should show a slope that is compatible with one less somatic event (that is, r = 5), because these individuals have an inherited mutation (in the APC gene) David von Hansemann reports abnormal mitoses in cancer cells. Ernest Tyzzer applies the term ?somatic mutation? to cancer events. Theodor Boveri proposes that chromosomal abnormalities are fundamental to cancer. Hermann J. Muller discovers that ionizing radiation, already known to be carcinogenic, produces mutations. Isaac Berenblum and Philippe Shubik discover two stages of chemical carcinogenesis: initiation and promotion. C.O. Nordling uses age- specific incidence data of cancer to estimate the number of mutations. Hermann J. Muller proposes that cancer results when a single cell receives multiple mutations. James Cleaver finds that xeroderma pigmentosum involves a defect in repair of UV- induced damage to DNA. Bruce Ames and colleagues find that apparently non-mutagenic chemical carcinogens are metabolized into mutagenic compounds. Alfred Knudson discovers that two hits in retinoblastoma can relate hereditary and sporadic forms of cancer. Peter Nowell and David Hungerford discover the Philadelphia chromosome ? the first specific chromosomal aberration associated with a particular cancer (chronic myelogenous leukaemia). 1890 1914 1916 1927 1947 1951 1953 1960 1968 1971 1972 1976 1986 1990 Dominique Stehelin, Harold Varmus, Michael Bishop and Peter Vogt discover proto- oncogenes that are related to viral oncogenes. Stephen Friend, Robert Weinberg, Thaddeus Dryja and colleagues clone the tumour- suppressor gene RB1. Eric Fearon and Bert Vogelstein find specific sequential mutations in carcinoma. Timeline | Mutation and cancer Box 1 | Boveri?s prediction of oncogenes and tumour-suppressor genes ??in every normal cell there is a specific arrangement for inhibiting, which allows the process of division to begin only when the inhibition has been overcome by a special stimulus. To assume the presence of definite chromosomes which inhibit division, would harmonize best with my fundamental idea ? Cells of tumours with unlimited growth would arise if those ?inhibiting chromosomes? were eliminated ? On the other hand, the assumption of the existence of chromosomes which promote division, might satisfy this postulate ? cell-division would take place when the action of these chromatin parts ? should be strengthened by a stimulus ? If three or four such chromosomes meet, the whole number of chromosomes being otherwise normal, then the tendency to rapid proliferation would arise.? Boveri (1914) 1 . � 2001 Macmillan Magazines Ltd PERSPECTIVES I calculated that the numbers of tumours per heritable case followed a Poisson distribution, with a mean of three. From this, it can be inferred that 5% (e ?3 = 0.05) of carriers of the germ-line mutation would develop no tumour, which fits approximately with obser- vation 31 . The distribution of bilateral cases that have not yet been diagnosed (S) at differ- ent ages showed a linear decline on a semilog plot (that is, ln S = ?kt,where k is a constant that incorporates the mutation rate and t is time), as expected for a one-hit phenomenon (FIG. 3). From this, I predicated that hereditary retinoblastoma involves two mutations and, knowing that one of these had to be a germ- line mutation, I hypothesized that the other one would be somatic. The unilateral cases with no positive family history, only a minori- ty of whom carry a germ-line mutation, showed a distribution that is consistent with two mutations, so both of these ought to be somatic. The hereditary and nonhereditary forms of the tumour seemed to entail the same number of events ? a hypothesis that became known as the ?two-hit hypothesis?. These somatic mutations apparently occur at usual mutation rates. So, in the hereditary cases the somatic (second-event) mutations that would account for the Poisson mean of inhibited both in vitro and in vivo by a spe- cific chemical agent (STI-571; Gleevec) (REFS 29,30); the presumptively single abnormality is functionally crucial for the cancer. Although we cannot be sure that there are no other mutations in CML, below the resolution of cytogenetics, in its early and apparently ?one-hit? chronic phase, CML is strikingly different karyotypically from multihit carcinomas (FIG. 2b). When first diagnosed, CML is relatively benign but, after a few years, an acute blastic crisis ensues in which other chromosomal aberrations are observed; in some cases, the other aberration is acquisition of a second Ph 1 , so two copies of the activated chimeric gene produce a more serious effect than one. As many leukaemias, lymphomas and sarcomas are characterized by solitary, specific transloca- tions, an increasingly long list of activated oncogenes has emerged. Furthermore, many of these cancers also acquire other chromo- somal aberrations as they progress, so the whole group teaches us that a kind of genomic instability might occur after a can- cer has resulted from what seems to be a sin- gle event, a specific translocation. Retinoblastoma is a ?two-hit? tumour Another means for investigating cancer events is the study of hereditary cancers, exemplified here by retinoblastoma.Some cancers occur almost exclusively in children, reflecting their origin from a type of cell that normally differentiates into a different type and ceases to exist in its original form. There is no a priori need to hypothesize a large number of mutations in childhood cancers. In fact, some cases are apparent at birth, hardly enough time for many mutational events. Retinoblastoma is such a cancer, aris- ing from fetal retinoblasts that normally dif- ferentiate into post-mitotic retinal photore- ceptor cells and neurons. Differentiation fails to occur normally in the tumours, and the cells continue to cycle. Ultimately, they spread and metastasize. Predisposition to retinoblastoma is imparted by a germ-line mutation in approxi- mately 40% of cases in the United States 31 . I was interested in the fact that the germ-line mutation, which is a de novo mutation in 80% of the germ-line mutants, is not a sufficient condition for tumorigenesis ? some children with an affected parent do not develop a tumour, but later produce an affected child, indicating that they carry the germ-line muta- tion. Most affected children with an affected parent develop tumours bilaterally, but some do so unilaterally. Approximately 60% of all cases are unilateral in the United States and do not carry a predisposing germ-line mutation. NATURE REVIEWS | CANCER VOLUME 1 | NOVEMBER 2001 | 159 20 1000 United States Caucasian population 1940 United Kingdom 1939 France 1947 Norway 1941?1945 800 600 500 400 300 200 100 80 60 50 40 30 20 10 8 6 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80 De a t h ra t e Age (years) Figure 1 | Log?log plots of cancer death rates in males (per 100,000) versus age, showing a linear relationship that is consistent throughout the developed world. (Reproduced, with permission, from REF. 11 ?(1953) Harcourt, Inc.) a b 123 45 123 4 5 6 7 8 9 10 11 12 13 1514 16 17 18 19 20 21 22 XY 6 7 8 9 10 11 12 13 1514 16 17 18 19 20 21 Mar 22 X ? Figure 2 | A comparison of karyotypes. a | Chronic myelogenous leukaemia, showing the typical 9;22 translocation and an otherwise normal karyotype. b | Non-small-cell carcinoma of the lung, showing abnormalities of both number and structure. The arrows indicate aberrant chromosomes. � 2001 Macmillan Magazines Ltd 160 | NOVEMBER 2001 | VOLUME 1 www.nature.com/reviews/cancer PERSPECTIVES tumour-suppressor gene to be characterized. Its protein product is a key regulator of the cell cycle, and hence of the birth rate of cells. Loss of the protein is accompanied by failure of retinoblasts to differentiate normally. Incidentally, the cytogenetic discovery of germ-line aberrations, together with the use of RFLPs, led to the cloning not only of RB1, but also of several other hereditary cancer genes, including WT1, NF1, NF2 and APC. Back to more than two hits At about the same time, another gene, TP53, was found to have a principal role in controlling the death of tumour cells. Although discovered during the study of the mechanism of transformation by DNA tumour viruses 40,41 , it was later shown to be a tumour-suppressor gene 42 , and to be mutated 43 in the germ line of persons with Li?Fraumeni syndrome 44 ? an hereditary predisposition to several cancers, especially breast cancer. We now know that the gene?s protein product, p53, is a multifunctional protein that allows cells to respond appro- priately to stress by controlling the cell cycle, DNA repair and apoptosis, but most pertinent to this discussion is its function as an important mediator of apoptosis 45 . RB1 and TP53 ? or genes that function in their pathways ? are inactivated in most can- cers, thereby both increasing tumour-cell birth rate and decreasing death rate. Interestingly, loss of TP53 leads to defective The meaning of two hits What are the implications of two mutations in tumorigenesis? Are they dominant muta- tions in two different genes, or recessive mutations in the two alleles of one gene? David Comings and I both favoured the latter notion 33,34 . I later applied the name anti-onco- gene to such genes, but they are now known as tumour-suppressor genes, although both terms place them in opposition to oncogenes. I proposed that the second event could be caused by intragenic mutation, whole gene deletion, chromosomal loss by nondisjunc- tion or somatic recombination 35 , but evidence was not forthcoming until the application, by Webster Cavenee and colleagues, of DNA restriction fragment length polymorphisms (RFLPs) to the study of cancer 36 . Here again, cytogenetic analysis was vital in uncovering the mechanism behind the two hits in retinoblastoma: a few cases are associated with a germ-line (usually de novo) deletion of chromosomal band 13q14 (REFS 37,38). Heterozygosity for linked, but external, mark- ers on chromosomal 13 would be lost with deletion, chromosomal loss or recombina- tion, but not with intragenic mutations. The use of RFLPs supported the conclusion that any of these mechanisms can occur as second events in retinoblastoma. This work provided direct evidence for the identification of RB1 as a tumour-suppressor gene. This was subse- quently shown to be the case following the cloning of the gene 39 ; RB1 became the first three tumours per individual ? that is, one somatic mutation in each of three different cells, each leading to a different tumour ? are found against the background of the millions of mitoses that are necessary to generate dif- ferentiated retinal epithelium from fetal retinoblasts. Our model for retinoblastoma took into account this growth and required a mutation rate of 10 ?6 or less per locus per mitosis 32 . In the nonhereditary cases, the first somatic mutation might be expected to occur at a rate approximately equal to that of the second mutation in the hereditary cases, implying that the retinas of most people con- tain clones of cells that have sustained one hit, but differentiated before a second hit could occur. Second somatic events in these clones would be expected, at spontaneous mutation rates of 10 ?6 or so, to yield the observed inci- dence of the nonhereditary form of the tumour, which is 60% of the total birth inci- dence rate of about 5 � 10 ?5 ,or 3 � 10 ?5 .There is no need to invoke a high mutation rate for the origin of retinoblastoma, primarily because it arises in a rapidly expanding population of retinoblasts during fetal development of the eye (FIG. 4). Fraction of cases not yet diagnosed 0 0.01 0.02 0.04 0.06 0.08 0.1 0.2 0.4 0.6 0.8 1.0 10 20 30 40 50 60 Age (months) Unilateral cases (25) Bilateral cases (23) Two-hit curve One-hit curve Figure 3 | One-hit and two-hit curves for retinoblastoma. These semilog plots of the fraction of 23 bilateral (heritable) cases and 25 unilateral (most expected to be non-heritable) cases that were still not diagnosed at plotted ages (data were analysed retrospectively) show that the bilateral cases match the expected shape of a one-hit curve, whereas the unilateral cases match the shape of a two-hit curve. As the bilateral cases inherit one genetic hit, both heritable and spontaneous retinoblastoma are due to two hits. Hereditary Tumour Non-hereditary Tumour Figure 4 | Two-hit tumour formation in both hereditary and nonhereditary retinoblastoma. A ?one- hit? clone is a precursor to the tumour in nonhereditary retinoblastomas, whereas all retinoblasts (indeed, all cells) are one-hit clones in hereditary retinoblastoma. � 2001 Macmillan Magazines Ltd PERSPECTIVES HNPCC has not been determined. The cells with homozygous mutations in MMR genes clearly have a ?mutator phenotype?, even though they do not show CIN. What CIN and MIN seem to have in common is the ability to increase the rate of transit along the path to clinical cancer. The view ahead The genetics of cancer has passed from infan- cy to maturity in the past century and has brought us to a dazzling, often confusing, view. Cancer cells themselves experience birth, development and death (too often with the patient). In some, karyotypic changes are few, whereas in others there is a bewildering array of abnormalities. Consideration of can- cers from many perspectives raises the possi- bility that the crucial changes on the initiating path to all cancers are few, affect both birth and death processes, and are strongly selected for. In tumours with a single genetic defect, a solitary oncogenic translocation (as seems to be the case in chronic-phase CML), the prospect of developing a successfully targeted therapeutic agent promises to be the greatest. By contrast, developing therapies for the ?multi- hit? tumours will be more challenging, as one agent acting on one target might not be suffi- cient. On the other hand, the time intervals between multiple hits might be windows of opportunity for preventive agents, in which transition to the next step (such as the second hit in generating the colonic adenomatous polyp) could be delayed or prevented. Fox Chase Cancer Center, 7701 Burholme Avenue, Philadelphia, Pennsylvania 19111, USA. e-mail: ag_knudson@fccc.edu 1. Boveri, T. Zur Frage der Entstehung Maligner Tumoren (Gustav Fischer, Jena); English translation The Origin of Malignant Tumors by Boveri, M. (Williams and Wilkins, Baltimore, 1929, 1914). 2. von Hansemann, D. �ber asymmetrische Zellteilung in Epithelkrebsen und deren biologische Bedeutung. Virchow?s Arch. Path. Anat. 119, 299?326 (1890). 3. Balmain, A. Cancer genetics: from Boveri and Mendel to microarrays. Nature Rev. Cancer 1, 77?80 (2001). 4. Tyzzer, E. E. Tumor immunity. J. Cancer Res. 1, 125?156 (1916). 5. Muller, H. J. Artificial transmutation of the gene. Science 46, 84?87 (1927). 6. Muller, H. J. Radiation damage to the genetic material. Sci. Progress 7, 93?165, 481?493 (1951). 7. Cleaver, J. E. Defective repair replication of DNA in xeroderma pigmentosum. Nature 218, 652?656 (1968). 8. Berenblum, I. & Shubik, P. A new, quantitative, approach to the study of the stages of chemical carcinogenesis in the mouse?s skin. Br. J. Cancer 1, 383?391 (1947). 9. Ames, B. N., Sims, P. & Grover, P. L. Epoxides of carcinogenic polycyclic hydrocarbons are frameshift mutagens. Science 176, 47?49 (1972). 10. Armitage, P. & Doll, R. The age distribution of cancer and a multi-stage theory of carcinogenesis. Br. J. Cancer 8, 1?12 (1954). 11. Nordling, C. E. A new theory on the cancer-inducing mechanism. Br. J. Cancer 6, 68?72 (1953). 12. Ashley, D. J. B. The two ?hit? and multiple ?hit? theories of carcinogenesis. Br. J. Cancer 23, 313?328 (1969). 13. Ashley, D. J. B. Colonic cancer arising in polyposis coli. J. Med. Genet. 6, 376?378 (1969). centrosome replication and numerous chromosomal abnormalities 46 , the feature of cancer that first attracted the notice of von Hansemann and Boveri. Inactivation of several other cloned tumour-suppressor genes, including the APC gene of FAP, is associated with hereditary can- cer and with benign precursors of malignant tumours. These benign lesions, usually adeno- matous, are all ?two-hit? tumours that are found in large numbers in the target tissues, undergo malignant transformation at low fre- quency and require other mutations to do so. These genes, including APC, seem to inhibit passage through the cell cycle, so their loss or inactivation increases cell birth rate. In many cases, the transition to frank malignancy involves loss or inactivation of TP53 (REFS 47,48), thereby reducing cell death rate. Mutations and losses of these two genes could account for four events in the pathway to colon cancer. The well-known ?adenoma?carcinoma? sequence in colorectal cancer has made this disease a popular model for a multihit can- cer 47 . Events on the path to cancer include not only mutations in APC and TP53,but also in one copy of the RAS oncogene 47 .This path would seem to involve five mutational events, a number that is quite compatible with David Ashley?s estimate of four or five, which, as described earlier, was calculated from a comparison of log?log plots of age- specific colon cancer incidence in normal and FAP persons, long before we knew of the existence of oncogenes or tumour-sup- pressor genes (FIG. 5). This number of events could occur at normal spontaneous muta- tion rates ? given the number of cell divi- sions that occur in the colon over many years, and the clonal expansion that occurs because of selection for mutants that have increased growth rates and decreased death rates from apoptosis during cell turnover 49 . However, the transition from polyp to carci- noma has been reported to be associated with occult genomic instability 50,51 , as judged by changes in DNA that are not associated with visible karyotypic abnormalities. It seems that DNA lesions are normally repaired by processes, such as recombina- tional repair, that leave the chromosome intact. When the induction of this repair is compromised, apoptosis should ensue. This process fails in the presence of TP53 muta- tions, and florid karyotypic changes emerge abruptly. This is the state of chromosomal instability (CIN) 52 . Although it is true that some cancers show only one or a few chromosomal abnormalities, most are, like colon cancer, very abnormal at diagnosis. The continued growth of such cancers usually leads, in the absence of intervention, to invasion, metas- tasis and death over a relatively short time; for most cancers, these events are not rate limiting. The idea that a small number of events can lead to cancer might be correct, but at death there might be many more, some of which provide a further growth advantage subject to clonal selection. Centrosome abnormalities, the emergence of chromosomal breakages, fusions and bridges, and widespread heterologous translocations characterize this period in the life of most cancers. This state clearly repre- sents a ?mutator phenotype? 53 . A second kind of genomic instability ? mutational microsatellite instability (MIN) 52 ? is not associated with CIN. Tumours that occur in people with hereditary nonpolypo- sis colorectal cancer (HNPCC) have greatly elevated (~1,000- fold) rates of specific locus mutations 54 . The inherited mutation occurs in mismatch repair (MMR) genes, most fre- quently MSH2 or MLH1. A somatic muta- tion in, or loss of, the remaining normal allele renders the affected cell homozygously defective for MMR. Especially vulnerable is the TGFBR2 gene, which encodes a receptor in an important signal transduction pathway 55 . Mutations in this receptor are strongly selective for increased growth rate. The number of other events that are neces- sary for production of a carcinoma cell in NATURE REVIEWS | CANCER VOLUME 1 | NOVEMBER 2001 | 161 Normal colon cells: two APC mutations Adenomatous polyp: one RAS mutation Dysplastic polyp: two TP53 mutations Colon carcinoma: Other events; Chromosomal aberrations Metastatic carcinoma Figure 5 | A possible five-hit scenario for colorectal cancer, showing the mutational events that correlate with each step in the adenoma?carcinoma sequence. Based on a model from Fearon and Vogelstein (REF. 47). � 2001 Macmillan Magazines Ltd 162 | NOVEMBER 2001 | VOLUME 1 www.nature.com/reviews/cancer PERSPECTIVES 41. Linzer, D. I. & Levine, A. J. Characterization of a 54K dalton cellular SV40 tumor antigen present in SV40- transformed cells and uninfected embryonal carcinoma cells. Cell 17, 43?52 (1979). 42. Finlay, C. A., Hinds, P. W. & Levine, A. J. The p53 proto- oncogene can act as a suppressor of transformation. Cell 57, 1083?1093 (1989). 43. Malkin, D. et al. Germ line p53 mutations in a familial syndrome of breast cancer, sarcomas, and other neoplasms. Science 250, 1233?1238 (1990). 44. Li, F. P. & Fraumeni, J. F. Soft-tissue sarcomas, breast cancer, and other neoplasms. A familial syndrome? Ann. Intern. Med. 71, 747?752 (1969). 45. Yonish-Rouach, E. et al. Wild-type p53 induces apoptosis of myeloid leukaemic cells that is inhibited by interleukin-6. Nature 352, 345?347 (1991). 46. Fukasawa, K., Choi, T., Kuriyama, R., Rulong, S. & Vande Woude, G. F. Abnormal centrosome amplification in the absence of p53. Science 271, 1744?1747 (1996). 47. Fearon, E. R. & Vogelstein, B. A genetic model for colorectal tumorigenesis. 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Online links DATABASES The following terms in this article are linked online to: CancerNet: http://cancernet.nci.nih.gov/ breast cancer | Burkitt?s lymphoma | chronic myelogenous leukaemia | colorectal carcinomas | osteosarcoma | retinoblastoma LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/ ABL | AKT | APC | MLH1 | MSH2 | MYC | NF1 | NF2 | HRAS | RB1 | TGFBR2 | TP53 | WT1 Medscape DrugInfo: http://promini.medscape.com/drugdb/search.asp Gleevec OMIM: http://www.ncbi.nlm.nih.gov/Omim/ familial adenomatous polyposis | hereditary non-polyposis colorectal cancer | Li?Fraumeni syndrome Access to this interactive links box is free online. Actin? up: RHOB in cancer and apoptosis George C. Prendergast OPINION RHOB is a small GTPase that regulates actin organization and vesicle transport. It is required for signalling apoptosis in transformed cells that are exposed to farnesyltransferase inhibitors, DNA-damaging agents or taxol. Genetic analysis in mice indicates that RhoB is dispensable for normal cell physiology, but that it has a suppressor or negative modifier function in stress- associated processes, including cancer. RHO proteins are receiving increasing atten- tion from cancer researchers owing to evi- dence that they modulate the proliferation, survival, invasion and angiogenic capacity of cancer cells. This family of actin regulatory small GTPases (BOX 1) is not mutated in can- cer. However, their altered expression or activ- ity might be crucial to cancer progression and therapeutic responses. Recent advances indicate that RHOB is a specialized activator of apoptosis in transformed cells. Through a gain-of-func- tion mechanism, RHOB has an important role in mediating the cellular response to farnesyltransferase inhibitors (FTIs). These experimental therapeutics are widely known for their selective effects on neoplastically transformed cells. Although some questions remain about exactly how RHOB alteration fits into the FTI response, many of the bio- logical effects of FTI treatment have been linked to RHOB. Of particular interest, evi- dence indicates that RHOB is a crucial tar- get for FTI-induced apoptosis. Recently, this role was extended with the finding that RHOB is required for the apoptotic response of transformed cells to DNA dam- age or TAXOL. Genetic analysis in mice indi- cates that RhoB is dispensable for normal cell physiology, but that it limits cancer sus- ceptibility and modifies growth-factor and adhesion signalling in transformed cells. What are RHOB?s effector mechanisms, and how might they promote apoptosis? Unique features of RHOB RHO proteins, which are themselves a subset of the RAS superfamily of isoprenylated small GTPases, can be further divided into subgroups of RHO, RAC and CDC42 pro- teins. These regulate a number of cellular � 2001 Macmillan Magazines Ltd 170 | NOVEMBER 2001 | VOLUME 1 www.nature.com/reviews/cancer KNUDSON ONLINE Medscape DrugInfo Gleevec http://promini.medscape.com/drugdb/d rug_uses_dosage.asp?DrugCode=1%2 D22096&DrugName=GLEEVEC+OR AL&DrugType=1 OMIM Familial adenomatous polyposis http://www.ncbi.nlm.nih.gov/htbin- post/Omim/dispmim?175100 Hereditary non-polyposis col- orectal cancer http://www.ncbi.nlm.nih.gov/htbin- post/Omim/dispmim?120435 Li?Fraumeni syndrome http://www.ncbi.nlm.nih.gov/htbin- post/Omim/dispmim?151623 Biography Alfred G. Knudson obtained his M.D. from Columbia University, and his Ph.D. from Caltech. He is a native Californian, and is trained in paediatrics and has treated chil- dren with cancer at the City of Hope National Medical Center, where he wrote the book Genetics and Disease and became interested in viral and genetic theories of cancer. Later, at M. D. Anderson Cancer Center, he proposed his ?two-hit hypothesis? for retinoblastoma. At the Fox Chase Cancer Center, where he is a Senior Member, he has studied hereditary cancer in animals and currently pursues the investigation of prevention in hereditary can- cer in humans. He has also written on paedi- atric cancer with his wife, Dr Anna T. Meadows. LocRpt.cgi?l=25 AKT http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=207 APC http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=324 MLH1 http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=4292 MSH2 http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=4436 MYC http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=4609 NF1 http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=4763 NF2 http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=4771 RAS http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=3265 RB1 http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=5925 TGFBR2 http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=7048 TP53 http://www.ncbi.nlm.nih.gov/LocusLink/ LocRpt.cgi?l=7157 WT1 http://www.ncbi.nlm.nih.gov/LocusLin k/LocRpt.cgi?l=7490 Databases CancerNet Breast cancer http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_00013H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: Burkitt?s lymphoma http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_00066H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: Chronic myelogenous leukaemia http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_01031H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: Colorectal carcinomas http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_00008H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: Osteosarcoma http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_00008H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: retinoblastoma http://cancernet.nci.nih.gov/cgi- bin/srchcgi.exe?TYPE=search&ZUI=2 08_00993H&DBID=pdq&SFMT=pdq _statement/1/0/0&PASSTHRU=:ip:19 4.129.50.189::srchform:PDQ: LocusLink ABL http://www.ncbi.nlm.nih.gov/LocusLink/ "
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