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Introduction to

the Cellular and

Molecular Biology

of Cancer

Margaret A. Knowles

Peter J. Selby

OXFORD UNIVERSITY PRESS

Introduction to the Cellular and Molecular Biology of Cancer

This page intentionally left blank

Introduction to

the Cellular and

Molecular Biology

of Cancer

Margaret A. Knowles

Peter J. Selby

Cancer Research UK Clinical Centre, St James"s University Hospital, Leeds 1 1

Great Clarendon Street, Oxford OX2 6DP

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British Library Cataloguing in Publication Data

(Data available) Library of Congress Cataloging in Publication Data

Knowles, Margaret A.

Introduction to the cellular and molecular biology of cancer / Margaret A. Knowles, Peter J. Selby. p. cm. ISBN 0-19-852563-X (alk. paper) - ISBN 0-19-856853-3 (alk. paper) 1. Cancer-Molecular aspects.

2. Cancer cells. I. Selby, P. (Peter) II. Title.

RC268.5.K56 2005

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4071-dc22

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Typeset by Newgen Imaging Systems (P) Ltd., Chennai, India

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10987654321

Preface to the fourth edition

The first edition of this book, published in 1985 was a testimony to the dramatic molecular revolution that was taking place in biology and consequently in cancer research at that time. The book evolved from a series of introductory lectures developed to help new students and research fellows that came to work at the Imperial Cancer Research Fund

Laboratories in London to assimilate the rapidly

evolving body of knowledge on cancer. These popular talks were designed to give the non-expert a background to related areas of research and were given by experts from within the Imperial Cancer

Research Fund, many of whom subsequently con-

tributed chapters to the first edition of the book.

Twenty years later, the need for a comprehensive

introduction to this broad field is even more apparent and the introductory lectures at what is now the Cancer Research UK London Research

Institute continue and are as popular as ever.

Today, laboratory science has begun to have a real impact on clinical medicine and it is of utmost importance that scientists have not only a broad view of laboratory cancer research but also a good understanding of the most up to date treatment options. Similarly, it is essential that clinicians treating the various types of neoplastic disease are aware of developments in basic science and can apply these appropriately. It is our view that only when determined attempts to bridge the gap between the laboratory and clinic are made by both clinicians and scientists that rapid translation will take place. Our objective has been to facilitate acquisition of basic information on all aspects of cancer research to facilitate this process.

Inevitably over the years, many authors of this

book have changed, some topics have become less relevant and new topics have been added. How- ever, we are delighted that the initiator of the series and one of the editors of the first three editions of the book has given advice during the planning of

this fourth edition and has again contributed to thefirst chapter of the book. Sammy Franks was Ph.D.

supervisor to one of us (MK) and throughout his career has encouraged young scientists to look beyond the topic of their personal Ph.D. or post- doctoral project to encompass the wider picture. His care in selection of topics and authors for the earlier editions of the book generated a compre- hensive and readable text that has been used extensively. In preparing this new edition we have tried to keep his original goals in mind.

Our task in updating this has not been easy, not

least because of the unprecedented developments in many areas of biology. There are many more relevant and indispensable topics than before and this creates a conflict with the size limitations for a textbook of this kind. Perhaps the most difficult aspect of modern biology, however, is the com- plexity of current knowledge that seems to defy simplification to the level of the 'non-expert".

Inevitably, this is more apparent in some areas

than in others and we are aware that the factual content of the book has increased enormously. The modern cell or molecular biologist faces a challen- ging initiation into the field of cancer research.

Ultimately, however, the dramatic increase in

knowledge provides young scientists today with the power to understand and manipulate the fun- damental processes of life as never before. We believe, and hope, that the reader will find, that the obvious benefits in understanding complex biolo- gical problems far outweigh the effort required to assimilate the increased information content of this volume.

We have expanded the number of chapters from

22 to 30 to include chapters that cover some of the

new technologies such as global analyses of the genome, transcriptome, and proteome and more recent concepts and discoveries in cell biology such as the process of apoptosis, the rapid advances made in understanding the finite or infinite proliferative capacity of somatic cells and the epigenome. Huge strides have been made in our understandingofgenomicalterationsincancercells and these are reflected in an extensively updated chapter on molecular cytogenetics. All chapters with similar titles to the previous edition have been completely rewritten or extensively updated. On reviewing the final content of the book, one of the most striking changes is the general acceptance by authors of the identity of the key genes affecting the processes they seek to elucidate. No longer is identification of genes a critical issue but the (almost entire) sequence of the human genome now allows biologists to focus on biological processes rather than detective work designed to find genes. One of the striking observations is the diversity of types of genes involved in cancer development that is reflected in several chapters. Similarly, develop- ments in novel cancer therapies now draw on many areas of molecular biology and several are now represented as separate chapters. This is indeed aperiod of plenty in terms of what is known and what is possible and the scope for new scientists and clinicians to draw on this is unprecedented. Authorship for this edition continues to represent experts in each field of research but this now extends beyond the confines of a single organiza- tion to draw on expertise from around the world.

The assembly of such an impressive group of

experts in such a fast-moving area of research ensures that the content is as up-to-date as possible and we are indebted to all contributors for their efforts. Inevitably, there will be omissions and imbalances that will be felt more acutely by some readers than others and we encourage readers to comment and make suggestions for any future editions of the book.

Leeds M. A. K.

January 2005 P. J. S.

viPREFACE TO THE FOURTH EDITION

Preface to the third edition

Successive editions of this book have mirrored

developments in cancer research and we hope that this new edition will achieve our original objective of providing a relatively brief but comprehensive introduction to the initiation, development, and treatment of cancer. On this background we have tried to provide an introduction to the results and new developments in the field using the current techniques of cell and molecular biology. A fuller understanding of the detail in some chapters needs a basic knowledge of molecular biology which can be found in several textbooks (e.g. Lodish et al.,

1995) but the general principles in each chapter

should be comprehensible without this. This edi- tion has allowed us to bring up-to-date information in fields in which there has been great activity and even some achievement. In particular, the chapters concerned with epidemiology, genetic and chro- mosome changes, oncogenes, chemical and radia- tion carcinogenesis, growth factors, the biology of human leukaemia, and hormones and cancer, and the Glossary have been rewritten or extensively revised. Other chapters have been brought up-to- date and new chapters on cytokines and cancer, the molecular pathology of cancer, cancer prevention, and screening have been added.

Gene nomenclature may cause some confusion

since although there is now a standardized format it is not yet generally accepted by all workers in the field. Many of the genes and oncogenes described by some earlier workers have retained their origi- nal format for historical reasons. Some genes were discovered in mouse cells, others in humans, and still others in viruses, and different names were given to genes which are now known to be essen-

tially the same. Genes described for human cells arenow usually written in upper case, italic type and

their protein products in roman type. Mouse genes are often given in lower case italic type, their pro- ducts as for those of human genes; those from Drosophiliaare italicized with only the first letter capitalized. Specific oncogenes may be cited by a lower case first letter (c for cellular, v for viral), followed by a hyphen, and then the gene name in italic type. However, there may be further modifier terms. For the most part, we have tried to maintain some degree of consistency but in some chapters we have retained the original format if this is still used by many workers.

The apparently inevitable increase in girth that

seems to accompany middle age has had its effect on the book which is somewhat larger than its predecessors but we hope that the increase in information will compensate.

As one of the philosophers in The Crock of Gold

(Stephens 1931) commented 'Perfection is finality; finality is death. Nothing is perfect. There are lumps in it."* No doubt there are lumps, and errors, and omissions in this new edition. We should be pleased to have comments and suggestions for their correction.

References

Lodish, H., Baltimore, D., Berk, A., Zipursky, S. L., Matsudaira, P., Darnell, J. (1995).Molecular Cell Biology. Scientific American Books, W. H. Freeman, New York. Stephens, J. (1931).The Crock of Gold. Macmillan, London.

London L. M. F.

June 1996 N. M. T.

* He was complaining to his wife about his porridge. She hit him on the head.

Preface to the second edition

The second edition of this book-prepared sooner

than we had expected-has given us an opportu- nity to correct some of the faults and errors pointed out by our readers and reviewers, as well as allowing us to bring the book up-to-date in a number of areas in which there have been rapid developments. In particular the chapters on the genetic and chromosomal changes, growth factors, immunotherapy, and epidemiology have been expanded and more information on viral and che- mical carcinogenesis added to the appropriate sections. We have also clarified and added new information to most of the other chapters.

At some stage all authors and editors of intro-

ductory textbooks are faced with the awful choice of deciding what to leave out. When does com- pleteness conflict with comprehension? Is theomission of this and that piece of information really a mortal sin or could the distinguished reviewer who pointed it out just happen to have been told about it by a passing graduate student? In the end of course we did what all editors must do and made our own choice.

We hope that this second edition will continue to

be of use to its readers as an introduction to cancer studies and as a source of further information either in key references or in specialized reviews such asCancer Surveys.

We should still appreciate comments and sug-

gestions for further improvement.

London L. M. F.

January 1990 N. M. T.

Preface to the first edition

Cancer holds a strange place in modern mythology.

Although it is a common disease and it is true to

say that one person in five will die of cancer, it is equally true to say that four out of five die of some other disease. Heart disease, for example, a much more common cause of death, does not seem to carry with it the gloomy overtones, not always justifiable, of a diagnosis of cancer. This seems to stem largely from the fact that we had so little knowledge of the cause of a disease which seemed to appear almost at random and proceed inexorably. At the turn of the century, when the ICRF was founded (in 1902), the clinical behaviour and pathology of the more common tumours was known but little else. Over the years clinicians, laboratory scientists and epidemiologists estab- lished a firm database. The behaviour patterns of many tumours, and in some cases even the causal agents, were known but how these agents trans- formed normal cells and influenced tumour cell behaviour remained a mystery.

The development of molecular biology opened

up a major new approach to the molecular analysis of normal and tumour cells. We can now ask and begin to answer questions particularly about the genetic control of cell growth and behaviour that have a bearing on our understanding not only of the family of diseases that we know as cancer but of the whole process of life itself. It is this, as much as finding a cause and cure for the disease, that gives cancer research its importance.

The initiating event which ultimately led to the

publication of this book was the realization that many graduate students and research fellows who came to work in our Institute, although highly specialized in their own fields, had relatively little knowledge of cancer and there were few suitable textbooks to which they could be referred. Con- sequently, regular introductory courses were orga- nized for new staff members at which 'experts"

were asked to give a general introduction to theirparticular field of study. The talks were designed to

giveabackgroundforthenon-expert,asforexample, molecular biology for the morphologist or cell biol- ogyfortheproteinchemist.Thecoursesprovedtobe verypopular.Thisbookfollowsasimilarpatternand has many of the same contributors-hence the fact that most are, or have been, connected with the

Imperial Cancer Research Fund.

After a general introduction describing the

pathology and natural history of the disease, each section gives a more detailed, but nevertheless general, survey of its particular area. We have tried to present principles rather than a mass of infor- mation, but inevitably some chapters are more detailed than others. Each chapter gives a short list of recommended reading which provides a source for seekers of further knowledge.

The topics covered have been selected with some

care. Although some, particularly those concerned with treatment, may not at first glance appear to be directly related to cell and molecular biology, we feel that a knowledge of the methods used must give a wider understanding of the practical pro- blems which may ultimately prove to be solvable by the application of modern scientific technology.

On the other hand, knowledge of inherent cell

behaviour (e.g. radiosensitivity, cell cycling, development of drug resistance, etc.) is important for the design of novel therapeutic approaches that rely less on empirical considerations.

Despite differences in the levels of technical

details presented in some chapters, we hope that all are comprehensible. We have provided a fairly comprehensive glossary so that if some terms are not explained adequately in the text, do try the glossary. Finally, the editors would appreciate any comments, suggestions or corrections should a second edition prove desirable.

London L. M. F.

December 1985 N. M. T.

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Contents

Contributors xv

1 What is cancer? 1

Leonard M. Franks and Margaret A. Knowles

2 The causes of cancer 25

Naomi Allen, Robert Newton, Amy Berrington de Gonzalez,

Jane Green, Emily Banks, and Timothy J. Key.

3 Inherited susceptibility to cancer 45

D. Timothy Bishop

4 DNA repair and cancer 61

Beate Ko

¨berle, John P. Wittschieben,

and Richard D. Wood

5 Epigenetic events in cancer 78

Jonathan C. Cheng and Peter A. Jones

6 Molecular cytogenetics of cancer 95

Denise Sheer and Janet M. Shipley

7 Oncogenes 117

Margaret A. Knowles

8 Tumour suppressor genes 135

Sonia Laı

´n and David P. Lane

9 The cancer cell cycle 156

Chris J. Norbury

10 Cellular immortalization and telomerase activation

in cancer 170

Robert F. Newbold

11 Growth factors and their signalling pathways in cancer 186

Sally A. Prigent

12 Apoptosis: molecular physiology and significance for

cancer therapeutics 210

Dean A. Fennell

13 Mechanisms of viral carcinogenesis 229

Paul Farrell

14 Cytokines and cancer 242

Peter W. Szlosarek and Frances R. Balkwill

15 Hormones and cancer 257

Charlotte L. Bevan

16 The spread of tumours 278

Ian Hart

17 Tumour angiogenesis 289

Kiki Tahtis and Roy Bicknell

18 Stem cells, haemopoiesis, and leukaemia 305

Mel Greaves

19 Animal models of cancer 317

Jos Jonkers and Anton Berns

20 The immunology of cancer 337

Peter C. L. Beverley

21 The molecular pathology of cancer 356

Tatjana Crnogorac-Jurcevic, Richard Poulsom,

and Nicholas R. Lemoine

22 From transcriptome to proteome 369

Silvana Debernardi, Rachel A. Craven, Bryan D. Young, and Rosamonde E. Banks

23 Local treatment of cancer 390

Ian S. Fentiman

24 Chemotherapy 399

D. Ross Camidge and Duncan I. Jodrell

25 Radiotherapy and molecular radiotherapy 414

Anne Kiltie

26 Monoclonal antibodies and therapy 428

Tom Geldart, Martin J. Glennie, and Peter W. M. Johnson

27 Immunotherapy of cancer 443

Andrew M. Jackson and Joanne Porte

xiiCONTENTS

28 Cancer gene therapy 458

John D. Chester

29 Screening 480

Peter Sasieni and Jack Cuzick

30 Conclusions and prospects 503

Peter Selby and Margaret Knowles

Index 506

CONTENTSxiii

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Contributors

Naomi Allen, Cancer Research UK,

Epidemiology Unit, University of Oxford, Gibson

Building, Radcliffe Infirmary, Oxford OX2 6HE,

naomi.allen@cancer.org.uk

Frances Balkwill, Translational Oncology

Laboratory, Barts and the London, Queen

Mary"s School of Medicine and Dentistry,

The John Vane Science Centre,

Charterhouse Square, London EC1M 6BQ,

frances.balkwill@cancer.org.uk

Emily Banks, National Centre for

Epidemiology & Population Health,

Australian National University, Canberra,

ACT 0200, Australia, emily.banks@anu.edu.au

Rosamonde Banks, Cancer Research UK Clinical

Centre, St James"s University Hospital, Beckett

Street, Leeds LS9 7TF, r.banks@leeds.ac.uk

Amy Berrington de Gonzalez, Cancer Research

UKEpidemiologyUnit,UniversityofOxford,Oxford

OX26HE,amy.berringtondegonzalez@cancer.org.uk

Anton Berns, The Netherlands Cancer Institute,

Division of Molecular Genetics, Plesmanlaan 121,

1066 CX Amsterdam, The Netherlands,

a.berns@nki.nl

Charlotte Bevan, Department of Cancer

Medicine, Imperial College London, 5th Floor

Laboratories, MRC Cyclotron Building,

Du Cane Road, London W12 0NN,

charlotte.bevan@imperial.ac.uk

Peter Beverley, The Edward Jenner Institute for

Vaccine Research, Compton, Newbury, Berkshire

RG20 7NN, peter.beverley@jenner.ac.uk

Roy Bicknell, Cancer Research UK,

Angiogenesis Laboratory, Weatherall Institute

of Molecular Medicine, John Radcliffe Hospital,

Headley Way, Headington, Oxford OX3 9DS,

roy.bicknell@cancer.org.ukTim Bishop, Cancer Research UK, Genetic

Epidemiology, St. James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

t.bishop@cancer.org.uk

Ross Camidge, Edinburgh Cancer Centre,

Western General Hospital, Edinburgh EH4 2XU,

ross.camidge@ed.ac.uk

Jonathan Cheng, USC/Norris Cancer Center,

1441 Eastlake Avenue, Los Angeles, CA 9033,

USA, jonathcc@usc.edu

John Chester, Cancer Research UK Clinical

Centre, St James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

j.d.chester@cancermed.leeds.ac.uk

Rachel Craven, Cancer Research UK Clinical

Centre, St James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

r.craven@cancermed.leeds.ac.uk

Tatjana Crnogorac-Jurcevic, Barts and the

London, Queen Mary"s School of Medicine and

Dentistry, Charterhouse Square, London EC1M

6BQ, t.c.jurcevic@qmul.ac.uk

Jack Cuzick, Cancer Research UK, Centre for

Epidemiology, Mathematics and Statistics,

Wolfson Institute for Preventive Medicine,

Barts and the London, Queen Mary"s School

of Medicine and Dentistry, Charterhouse

Square, London EC1M 6BQ,

jack.cuzick@cancer.org.uk

Silvana Debernardi, Department of

Medical Oncology, Barts and The London,

Queen Mary"s School of Medicine and Dentistry,

Charterhouse Square, London

EC1M 6BQ, silvana.debernardi@cancer.org.uk

Paul Farrell, Ludwig Institute for Cancer

Research, Department of Virology, Imperial

College, St. Mary"s Campus, Norfolk Place,

London W2 1PG, p.farrell@imperial.ac.uk

Dean Fennell, Northern Ireland Thoracic

Oncology Research Group, Cancer Research

Centre, University Floor, Belfast City Hospital,

Lisburn Road, Belfast BT9 7AB, Northern Ireland,

d.fennel@qub.ac.uk

Ian Fentiman, Academic Oncology, 3rd Floor,

Thomas Guy House, Guy"s Hospital,

St. Thomas Street, London SE1 9RT,

ian.fentiman@cancer.org.uk

L. M. Franks, 13 Allingham Street,

London, N1 8NX,

sammyfranks@onetel.com

Tom Geldart, Cancer Research UK Oncology

Unit, Cancer Sciences Division, Southampton

University School of Medicine, Southampton

General Hospital, Southampton SO16 6YD,

trg@soton.ac.uk

Martin Glennie, Tenovus Laboratory, Cancer

Sciences Division, Southampton University School

of Medicine, Southampton General Hospital,

Southampton SO16 6YD,

m.j.glennie@soton.ac.uk

Mel Greaves, Institute of Cancer Research,

Chester Beatty Laboratories, 237 Fulham Road,

London SW3 6JB, mel.greaves@icr.ac.uk

Jane Green, Cancer Research UK Epidemiology

Unit, University of Oxford, Oxford OX2 6HE,

jane.green@cancer.org.uk

Ian Hart, Department of Tumour Biology, Barts

and The London, Queen Mary"s School of

Medicine and Dentistry, John Vane Science Centre,

Charterhouse Square, London EC1M 6BQ,

ian.hart@cancer.org.uk

Andrew Jackson, Genitourinary Cancer

Immunotherapy Program, Duke University

Medical Centre, Durham, NC2 7710, USA,

aj40@duke.edu

Duncan Jodrell, University of Edinburgh

Cancer Research Centre, Crewe Road South,

Edinburgh EH4 2XR,

duncan.jodrell@cancer.org.uk

Peter Johnson, Cancer Research UK Oncology

Unit, Cancer Sciences Division, Southampton

University School of Medicine, Southampton

General Hospital, Southampton SO16 6YD,

johnsonp@soton.ac.ukPeter Jones, USC/Norris Cancer Center, 1441

Eastlake Avenue, Los Angeles, CA 90033, USA,

jones_p@ccnt.hsc.usc.edu

Jos Jonkers, The Netherlands Cancer Institute,

Division of Molecular Biology, Plesmanlaan 121,

1066 CX Amsterdam, The Netherlands,

j.jonkers@nki.nl

Timothy Key, Cancer Research UK,

Epidemiology Unit, Gibson Building,

Radcliffe Infirmary, Oxford OX2 6HE,

timothy.key@cancer.org.uk

Anne Kiltie, Cancer Research UK Clinical

Centre, St James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

anne.kiltie@cancer.org.uk

Margaret Knowles, Cancer Research UK Clinical

Centre, St. James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

margaret.knowles@cancer.org.uk

Beate Ko

¨berle, University of Pittsburgh Cancer

Institute, Hillman Cancer Center, 5117 Centre

Avenue, Research Pavilion, Suite 2.6, Pittsburgh,

Pa, 15213-1863, USA, bmk27@pitt.edu

Sonia Laı

´n, Department of Surgery and

Molecular Oncology, Nine Wells Hospital Medical

School, University of Dundee, Dundee DD1 9SY,

s.lain@dundee.ac.uk

David Lane, Department of Surgery and

Molecular Oncology, Nine Wells Hospital Medical

School, University of Dundee, Dundee DD1 9SY,

d.p.lane@dundee.ac.uk

Nicholas Lemoine, Cancer Research UK,

Molecular Oncology Unit, Barts and the London,

Queen Mary"s School of Medicine and Dentistry,

Charterhouse Square, London EC1M 6BQ,

Nick.Lemoine@cancer.org.uk

Robert Newbold, Brunel Institute of Cancer

Genetics and Pharmacogenomics, Brunel

University, Uxbridge UB8 4SP,

robert.newbold@brunel.ac.uk

Robert Newton, Cancer Research UK

Epidemiology Unit, University of Oxford, Oxford

OX2 6HE, rob.newton@cancer.org.uk

Chris Norbury, Sir William Dunn School of

Pathology, University of Oxford,

South Parks Road, Oxford OX1 3RE,

chris.norbury@path.ox.ac.uk xviCONTRIBUTORS

Joanne Porte, Department of Reproductive

Endocrinology, University of North Carolina,

Chapel Hill, NC27599, USA

Richard Poulsom, In Situ Hybridisation Service

Histopathology Unit, Cancer Research UK,

44 Lincoln"s Inn Fields, London, WC2A 3PX,

richard.poulsom@cancer.org.uk

Sally Prigent, Department of Biochemistry,

Room 201E, Adrian Building, University of

Leicester, University Road, Leicester LE1 7RH

Sap8@leicester.ac.uk

Peter Sasieni, Cancer Research UK, Centre for

Epidemiology, Mathematics and Statistics,

Wolfson Institute for Preventive Medicine, Barts

and the London, Queen Mary"s School of Medicine and Dentistry, Charterhouse Square, London

EC1M 6BQ, peter.sasieni@cancer.org.uk

Peter Selby, Cancer Research UK Clinical

Centre, St. James"s University Hospital,

Beckett Street, Leeds LS9 7TF,

peter.selby@cancer.org.uk

Denise Sheer, Cancer Research UK London

Research Institute, Human Cytogenetics

Laboratory, Lincoln"s Inn Fields Laboratories,

44 Lincoln"s Inn Fields, London WC2A 3PX,

denise.sheer@cancer.org.uk

Janet Shipley, Molecular Cytogenetics, The

Institute of Cancer Research, 15 Cotswold Road,Belmont, Sutton, Surrey SM2 5NG, janet.shipley@icr.ac.uk

Peter Szlosarek, Translational Oncology

Laboratory, Cancer Research UK, Barts and The

London, Queen Mary"s School of Medicine and

Dentistry, Charterhouse Square, London EC1M

6BQ, peter.szlosarek@cancer.org.uk

Kiki Tahtis, Cancer Research UK,

Angiogenesis Laboratory,

Weatherall Institute of Molecular Medicine,

John Radcliffe Hospital, Headley Way,

Headington, Oxford OX3 9DS,

kiki.tahtis@cancer.org.uk

John Wittschieben, University of Pittsburgh

Cancer Institute, Hillman Cancer Center, 5117

Centre Avenue, Research Pavilion, Suite 2.6,

Pittsburgh, Pa, 15213-1863, USA,

jpw31@pitt.edu

Richard Wood, University of Pittsburgh

Cancer Institute, Hillman Cancer Center,

5117 Centre Avenue, Research Pavilion,

Suite 2.6, Pittsburgh, Pa, 15213-1863, USA,

rdwood@pitt.edu

Bryan Young, Department of Medical

Oncology, Barts and The London, Queen Mary"s

School of Medicine and Dentistry, Charterhouse

Square, London EC1M 6BQ,

bryan.young@cancer.org.uk

CONTRIBUTORSxvii

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CHAPTER 1

What is cancer?

Leonard M. Franks and Margaret A. Knowles

1.1 Introduction...................................................................... 1

1.2 Normal cells and tissues.............................................................. 2

1.3 Control of growth in normal tissues .................................................... 3

1.4 The cell cycle...................................................................... 4

1.5 Tumour growth or neoplasia.......................................................... 4

1.6 The process of carcinogenesis......................................................... 5

1.6.1 Genes involved in carcinogenesis . . .............................................. 6

1.7 Factors influencing the development of cancers........................................... 7

1.8 Genetic instability, clonal selection, and tumour evolution . . ................................. 9

1.8.1 Selection of altered clones .....................................................10

1.8.2 Tumour clonality. . ...........................................................12

1.9 Tumour diagnosis . .................................................................15

1.9.1 Benign tumours . . ...........................................................15

1.9.2 Malignant tumours...........................................................16

1.10 Tumour nomenclature...............................................................16

1.10.1 Tumours of epithelium........................................................17

1.10.2 Tumours of mesenchyme . .....................................................19

1.10.3 Tumours of the haemato-lymphoid system . ........................................19

1.10.4 Tumours of the nervous system.................................................19

1.10.5 Germ cell tumours ...........................................................20

1.10.6 Tumours showing divergent differentiation ........................................20

1.10.7 Tumour staging and the spread of tumours (metastasis)...............................21

1.11 How tumours present: some effects of tumours on the body. .................................21

1.12 How does cancer kill?...............................................................21

1.13 Treatment of cancer................................................................22

1.14 Cancer prevention and screening.......................................................22

1.15 Experimental methods in cancer research . . ..............................................23

1.16 Conclusions.......................................................................23

1.1 Introduction

Cancer has been known since human societies first

recorded their activities. It was well known to the ancient Egyptians and to succeeding civilizations but, as most cancers develop in the latter decades of life, until the expectation of life began to increase from the middle of the nineteenth century onwards, the number of people surviving to this age was relatively small. Now that the infectious diseases, the major causes of death in the past, have beencontrolled by improvements in public health and medical care, the proportion of the population at risk of cancer has increased dramatically. Although diseases of the heart and blood vessels are still the main cause of death in our ageing population, cancer is now a major problem. At least one in three will develop cancer and one in four men and one in five women will die from it. For this reason, cancer prevention and control are major health issues.

However, cancer research has wider significance.

1

Cancer is not confined to man and the higher

mammals but affects almost all multicellular organ- isms, plants as well as animals. Since it involves dis- turbances in cell proliferation, differentiation, and development, knowledge of the processes under- lying this disease help us to understand the very basic mechanisms of life.

About 140 years ago a German microscopist,

Johannes Mueller, showed that cancers were made

up of cells, a discovery which began the search for changes which would help to pinpoint the specific differences between normal and cancer cells. In the intervening period a huge amount of information has been acquired about the cancer cell. In the past two decades in particular, rapid technological progress has allowed us to begin to dissect the cancer genome, transcriptome, and proteome in unprecedented detail and today there seems no limit to the amount of information that can be obtained. However, this does not naturally answer all of the questions posed by those early cancer biologists. Some fundamental questions remain unanswered, despite our technical prowess and the availability of commercial 'kits" for most basic assays. Even the most advanced technology is of no value if it is not applied appropriately and it is still too early for the benefits of some recent technical advances to be clear. In the past, some of the major questions for the cancer biologist concerned what typesofexperimentswerepossibleandthedevelop- ment of new techniques to extend these possibil- ities formed a major part of the work done. Now that almost anything seems technically possible, the key issue for the twenty-first century biologist is to identify the right questions to ask. This can make the difference between a deluge of uninterpretable data and a real improvement in understanding.

This book does not aim to identify what these

'right" questions are but to provide an introduction to current understanding of cancer, its causes, biology, and treatment. However, we do indicate areas in which new and exciting discoveries are being made and those in which key questions remain unanswered.

Cancer is a disorder of cells and although it

usually appears as a tumour (a swelling) made up of a mass of cells, the visible tumour is the end result of a whole series of changes which may have taken many years to develop. In this chapter, we discuss in general terms what is known about the changes that take place during the process of tumour development, consider tumour diagnosisand nomenclature, and provide some definitions.

Succeeding chapters deal with specific aspects in

more detail.

1.2 Normal cells and tissues

The tissues of the body can be divided into four

main groups: the general supporting tissues col- lectively known as mesenchyme; the tissue-specific cells-epithelium; the 'defence" cells-the haemato- lymphoid system; and the nervous system. The mesenchyme consists of connective tissue- fibroblasts which make collagen fibres and asso- ciated proteins, bone, cartilage, muscle, blood vessels, and lymphatics. The epithelial cells are the specific, specialized cells of the different organs, for example, skin, intestine, liver, glands, etc. The haemato-lymphoid system consists of a wide group of cells, mostly derived from precursor cells in the bone marrow which give rise to all the red and white blood cells. In addition, some of these cells (lymphocytes and macrophages) are distributed throughout the body either as free cells or as fixed constituents of other organs, for example, in the liver, or as separate organs such as the spleen and lymph nodes. Lymph nodes are specialized nodules of lymphoid cells, which are distributed throughout the body and act as filters to remove cells, bacteria, and other foreign matter. The nerv- ous system is made up of the central nervous sys- tem (the brain and spinal cord and their coverings) and the peripheral nervous system, which is com- prised of nerves leading from these central struc- tures. Thus, each tissue has its own specific cells, usually several different types, which maintain the structure and function of the individual tissue. Bone, for example, has one group of cells respons- ible for bone formation and a second group responsible for bone resorption and remodelling when the need arises, as in the repair of fractures. The intestinal tract has many different epithelial cell types responsible for the different functions of the bowel, and so on.

The specific cells are grouped in organs which

have a standard pattern (Figure 1.1). There is a layer of epithelium, the tissue-specific cells, separated from the supporting mesenchyme by a semi- permeable basement membrane. The supporting tissues (or stroma) are made up of connective tissue (collagen fibres) and fibroblasts (which make collagen), which may be supported on a layer of muscle and/or bonedepending on the organ. Blood

2CELLULAR AND MOLECULAR BIOLOGY OF CANCER

vessels, lymphatic vessels, and nerves pass through the connective tissue and provide nutrients and nervous control among other things for the specific tissue cells. In some instances, for example, the skin and intestinal tract, the epithelium which may be one or more cells thick depending on the tissue, covers surfaces. In others it may form a system of tubes (e.g. in the lung or kidney), or solid cords (e.g. liver), but the basic pattern remains the same. Dif- ferent organsdifferinstructure onlyin thenatureof the specific cells and the arrangement and distri- bution of the supporting mesenchyme.

1.3 Control of growth in normal

tissues

The mechanism of control of cell growth and pro-

liferation is one of the most intensively studied areas in biology. It is important to make the dis- tinction between the terms 'growth" and 'prolifera- tion". Growth is used here to refer to an increase in size of a cell, organ, tissue, or tumour and prolif- eration to an increase in the number of cells by division. 'Growth" is often used as a loose term for both of these processes but the distinction is particularly important now that factors controlling both of these processes are becoming clear. In nor- mal development and growth there is a very precise mechanism that allows individual organs to reach a fixed size, which for all practical purposes, is never exceeded. If a tissue is injured, the surviving cells in most organs begin to divide to replace the damaged cells. When this has been completed, the process stops, that is, the normal control mechanismspersist throughout life. Although most cells in the embryo can proliferate, not all adult cells retain this ability. In most organs there are special reserve or stem cells, which are capable of dividing in response to a stimulus such as an injury to replace organ-specific cells. The more highly differentiated acellis,forexample,muscleornervecells,themore likely it is to have lost its capacity to divide. In some organs, particularly the brain, the most highly dif- ferentiated cells, the nerve cells, can only proliferate in the embryo, although the special supporting cells in the brain continue to be able to proliferate. A consequence of this, as we shall see later, is that tumours of nerve cells are only found in the very young and tumours of the brain in adults are derived from the supporting cells. In other tissues there is a rapid turnover of cells, particularly in the small intestine and the blood and immune system. A great deal of work has been done on the control of stem cell growth in the red and white cells (haemopoietic system), and the relationship of the factors involved in this process to tumour development (Chapter 18). For reasons that are still unclear, rapid cell division itself is not necessarily associated with an increased risk of tumour development, for example, tumours of the small intestine are very rare. In the embryo there is a range of stem cells, some cells capable of reproducing almost any type of cell and others with a limited potential for producing more specific cells, for example, liver or kidney. In the adult, there is now unequivocal evidence for the existence of stem cells capable of perpetuat- ing themselves through self-renewal to generate Nerve

Epithelium

Mesenchyme

Collagen fibres

Fibroblast

Differentiated cells

Stem cells

Blood vessels

MuscleBasement

membrane Figure 1.1A typical tissue showing epithelial and mesenchymal components.WHAT IS CANCER?3 specialized cells of particular tissues. Striking par- allels exist between the properties of stem cells and cancer cells. This, together with the potential for the use of human stem cells in various types of regen- erative medicine, makes this a very active area of research (Reya et al., 2001).

Control of organ or tissue size is achieved via

a fine balance between stimulatory and inhibitory stimuli. When the balance is shifted, for example, when the tissue is damaged and repair is needed, when a specific physiological stimulus is appl- ied, for example, hormonal stimulation or because extra work is required from an organ, the compon- ent cells may respond in one of two ways to achieve these objectives. This may be by hypertrophy, that is, an increase in size of individual components, usually of cells which do not normally divide. An example is the increase in size of particular muscles in athletes. The alternative is hyperplasia, that is, an increase in number of the cells. When the stimulus is removed, commonly the situation returns to thestatus quoas exemplified by the rapid loss of muscle mass in the lapsed athlete. Some of the stimuli that lead to these compensatory responses are well-known growth factors and hormones that are discussed in more detail in Chapters 11, 14, and 15. Recent work on the insulin/IGF (insulin- like growth factor) system, particularly in the fruit flyDrosophila,has demonstrated that this plays a pivotal role in the control of organ and organism size (Oldham and Hafen, 2003). It is of note that several molecules involved in these processes are known to act as oncogenes or to be dysregulated in cancer. For example, IGFs are commonly overexpressed and the phosphoinosi- tide 3-kinase (PI3K) pathway, which is activated by insulin/IGF signalling, is functionally disrupted in various ways in cancer cells (Vivanco and

Sawyers, 2002).

1.4 The cell cycle

The way in which cells increase in number is sim-

ilar for all somatic cells and involves the growth of all cell components (increase in cell mass) followed by division to generate two daughter cells.

Although the structural changes which take place

during this process, the cell cycle, have been known for many years, our current detailed knowledge of the molecular basis of the process has only been acquired in the past two decades. Four stagesare recognized: G1, S, G2, and M. Following a proliferative stimulus, G1 is a gap or pause after stimulation where little seems to be happening. However, if the cell is destined to divide, there is much biochemical activity in G1 in preparation for

DNA replication. S is the phase of DNA synthesis,

where the chromosomes are replicated and other cell components also increase. G2 is a second gap period following DNA synthesis and M is the stage of mitosis in which the nuclear membrane breaks down and the condensed chromosomes can be visualized as they pair and divide prior to division of the cytoplasm to generate two daughter cells. A further cell cycle phase is recognized, G0, which is a resting phase in which non-cycling cells rest with a G1 DNA content. Progression through the cell cycle is now known to be restricted at specific checkpoints, one in G1 and others in S and G2/M.

These provide an opportunity for cells to be

diverted out of the cycle or to programmed cell death (apoptosis) if, for example, there is DNA damage or inappropriate expression of oncogenic proteins. Disruption of these cell cycle checkpoints or alterations to key cell cycle proteins are found in many, if not all, cancers. A detailed discussion of the cell cycle, its regulation and disruption in cancer is given in Chapter 9.

1.5 Tumour growth or neoplasia

It is not possible to define a tumour cell in absolute terms. Tumours are usually recognized by the fact that the cells have shown abnormal proliferation, so that a reasonably acceptable definition is that tumour cells differ from normal cells in their lack of response to normal control mechanisms. Since there are almost certainly many different factors involved, the altered cells may still respond to some but not to others. A further complication is that some tumour cells, especially soon after the cells have been transformed from the normal, may not be dividing at all. In the present state of knowledge any definition must be 'operational". Given these qualifications we can classify tumours into three main groups: (1)Benign tumours may arise in any tissue, grow locally, and cause damage by local pressure or obstruction. However, the common feature is that they do not spread to distant sites. (2)In situtumours usually develop in epithelium and are usually but not invariably, small. The cells

4CELLULAR AND MOLECULAR BIOLOGY OF CANCER

have the morphological appearance of cancer cells but remain in the epithelial layer. They do not invade the basement membrane and supporting mesenchyme. Various degrees of dysplasia, that is, epithelial irregularity but not identifiable as cancer in situare recognized in some tissues and these may sometimes precede cancerin situ. Theoretic- ally, cancersin situmay arise also in mesenchymal, haemato-lymphoid, or nervous tissue but they have not been recognized. (3)Cancersarefullydeveloped(malignant)tumours with a specific capacity to invade and destroy the underlying mesenchyme (local invasion). The tumour cells need nutrients via the bloodstream and produce a range of proteins that stimulate the growth of blood vessels into the tumour, thus allowing continuous growth to occur (Chapter 17). The new vessels are not well formed and are easily damaged so that the invading tumour cells may penetrate these and lymphatic vessels. Tumour fragments may be carried in these vessels to local lymph nodes or to distant organs where they may produce secondary tumours (metastases) (Chapter 16). Cancers may arise in any tissue.

Although there may be a progression from benign

tomalignant,thisisfarfrominvariable.Manybenign tumours never become malignant. Some of these problems of definition may be more easily under- stood if we consider the whole process of tumour induction and development (carcinogenesis).

1.6 The process of carcinogenesis

Carcinogenesis (the process of cancer develop-

ment) is a multistage process (Figure 1.2). In an animal, the application of a cancer-producing agent (carcinogen)doesnotleadtotheimmediateproduc- tion of a tumour. Cancers arise after a long latent periodandmultiplecarcinogentreatmentsaremore effective than a single application. Experiments carriedoutonmouseskininthe1940sbyBerenblum and Shubik (reviewed by Yuspa, 1994) indicated that at least three major stages are involved. The first was termed initiation and was found to involve mutagenic effects of the carcinogen on skin stem cells. The second stage, which can be induced by a variety of agents that are not directly carcinogenic in their own right, was termed promotion. Fol- lowing chronic treatment of carcinogen-initiated mouse skin with promoting agents, papillomas (benign skin tumours) arise. The major effect of promoters seems to be their ability to promote clonal expansion of initiated cells. Finally in the third stage, progression, some of these benign tumours either spontaneously or following addi- tional treatment with carcinogens, progress to invasive tumours. The terms coined to describe this animalmodelarestillcommonlyappliedtodescribe the process of carcinogenesis in man.

The mouse skin model indicated that carcino-

genesis is a multistep process and clearly this is

Epithelium

Mesenchyme

Muscle

Metastasis possible

20 years0

Cain situTumour

invasionAccumulation of heritable changes

Clinical

tumour

Figure 1.2Tumour development showing progression from normal to invasive tumour via accumulation of heritable changes over a long

period of time. The rate of acquisition of these changes will be influenced by environmental exposures and host response.WHAT IS CANCER?5

also the case for human cancer. For example, most solid tumours of adults arise in the later decades of life, usually a long time after exposure to a specific carcinogenic insult or after a long period of con- tinuous exposure and this can be explained in terms of the requirement for several distinct herit- able changes. The nature of some of these changes is now known in detail and is discussed at length in several of the following chapters. These include genetic alterations to proto-oncogenes and tumour suppressor genes (Chapters 7 and 8) and epigenetic alterations (Chapter 5). Histopathological observa- tions also provide evidence for a long preneoplastic period, sometimes with morphologically identifi- able lesions such as benign tumours orin situ dysplasia, which may persist for many years and within which a malignant tumour eventually arises.

The latent period between initiation and the

appearance of tumours is great. In man, after exposure to industrial carcinogens, it may take over

20 years before tumours develop. Even in animals

given massive doses of carcinogens, it may take up to a quarter or more of the total lifespan before tumours appear. The requirement for acquisition of multiple events is the likely explanation for this. In the tumour that finally emerges, most of the genetic and epigenetic changes seen are clonal, that is they are present in the entire population of cells. It is likely that a series of selective phases of clonal expansion takes place in the tumour such that after each event, there is outgrowth of a clone of cells with a selective advantage. Evidence for this has come from studies on many tissues and particularly where areas of surrounding tissue or multiple related lesions can be sampled at surgery. In these circumstances, it is common to find several shared clonal events in different lesions and occasionally in the apparently 'normal" surrounding epithelium and additional events in the most histopathologi- cally advanced lesion (see Section 1.8.1).

1.6.1 Genes involved in carcinogenesis

Several types of genes are now known to contribute to the development of cancer. The discovery that the oncogenes of tumour-producing retroviruses are related to cellular genes (proto-oncogenes) (Chapter 7) has led to intensive research into the role of these genes in normal and tumour cell growth, proliferation, and differentiation. Many cellular genes can act as oncogenes when expressedinappropriately or mutated. These genes act in a dominant way at the cellular level to drive prolif- eration or prevent normal differentiation. This dominant mode of action makes oncogenes attractive potential targets for specific cancer ther- apies and there is currently a huge effort to inhibit the activity of specific oncogenes using a variety of approaches, some of which are already bearing fruit (Druker and Lydon, 2000). An interesting question in this regard concerns the role of such genes in the initiation, progression, and mainten- ance of tumours. In mouse skin, for example, mutational activation of arasoncogene is an initi- ating event and subsequent tumour progression and metastasis appear to depend on sequential incremental levels of expression of the gene which is clearly required for tumour maintenance. Sim- ilarly it has been shown in mouse models of mel- anoma that expression of arastransgene is required for tumour maintenance. However, in this model, examples of escape of tumours in whichrasgene expression had been switched off (Chin and DePinho, 2000), raises the possibility that not all genetic events required early in tumour develop- ment may be required later in the process, a fact that represents a caveat in the design of oncogene- targeted therapies.

Genes that provide negative regulatory signals in

the normal cell are also implicated in the develop- ment of cancer. If such a gene requires loss or inactivation to contribute to the transformation process, then it is likely that both copies of the gene must be altered and that such tumour suppressor genes would be genetically recessive at the cellular level. It was proposed by Knudson that two inde- pendent mutations are needed for the development of inherited cancers. In such cases of inherited (familial) tumour predisposition, the first mutation is present in the germ cells (sperm or ovum) and is therefore inherited by every cell in the body (Chapter 3). Only one further somatic mutation is required for complete gene inactivation in these cases. In the more common non-familial cancers, two somatic mutations in the gene are required and the chances of this happening in the same cell are much less. Many tumour suppressor genes have now been identified and many appear to conform to Knudson"s so-called 'Two-hit hypo- thesis" (reviewed in Knudson, 1996). Several mechanisms of inactivation of the two alleles have been described and these are discussed in detail in

Chapters 5 and 8. Not surprisingly however, there

6CELLULAR AND MOLECULAR BIOLOGY OF CANCER

are exceptions to this rule and there are several examples where loss of function of one allele of a tumour suppressor gene is sufficient to generate an altered cell phenotype that can contribute to transformation. This is termed haploinsufficiency and as discussed in Chapter 8, the levels of protein required for adequate function may vary from gene to gene, leading to the prediction that some genes will be more strongly haploinsufficient than others.

In normal cells, the requirement for efficient

repair mechanisms is clear. In the absence of such repair capacity, it is difficult to see how long-lived species such as man could survive daily exposure to environmental carcinogens without severe tox- icity and inevitably a high cancer rate. The mechanisms of repair of different types of DNA damage have now been elucidated in great detail both in lower organisms and in mammalian sys- tems. There are several familial syndromes in which components of the DNA repair machinery are mutated in the germline and these have pro- vided valuable tools for discovery of the mechan- isms of repair of different types of DNA lesions. One of the consequences of altered repair capacity is an increased risk of cancer. This class of cancer- causing genes can be referred to as 'mutator" genes as their altered function leads to an increased capacity for mutation of other genes. Any alteration in the function of such genes, however small, has the capacity to alter an individual"s lifetime risk of cancer. However, in contrast to the tumour sup- pressor genes, replacement of the function of these genes has no direct effect on tumour phenotype. There is currently much interest in identifying not only highly penetrant mutations in DNA repair and carcinogen-metabolizing genes but also the less penetrant polymorphisms that affect each indivi- dual"s response to environmental damage. DNA repair and its relationship to cancer development are discussed in detail in Chapter 4.

Other mutator genes include genes involved in

regulation of the mitotic apparatus which can also affect the rate of acquisition of other mutations. An example is aurora kinase A (AURKA) also known asSTK15, a gene whose product associates with the centrosome in S phase and appears to play a role in centrosome separation, duplication, and maturation. This gene is amplified and over- expressed in several types of cancer and this is associated with the generation of aneuploidy (deviation from the normal diploid number of chromosomes) (Dutertre et al., 2002). Other geneswhich also lead to aneuploidy when altered include the mitotic checkpoint genesBUB1and

BUBR1both of which can be classified as tumour

suppressor genes as inactivation is required for phenotypic effect.

This last example indicates that some subdivi-

sion within the large grouping of tumour sup- pressor genes is possible. This has led to invention of the terms 'gatekeeper" and 'caretaker" to describe these different suppressor roles (Kinzler and Vogelstein, 1998). Gatekeeper genes are defined as rate-limiting for a step in the pathway of tumour development. Thus, the adenomatous polyposis coli geneAPCis considered to be an initiation gatekeeper as its inactivation is required early in colorectal carcinogenesis. Caretaker genes include those which when functionally inactivated lead to defective DNA repair or other loss of function that leads to mutation, for example, some DNA repair genes,BUB1andBUBR1.

Finally, the ability of the immune system to

detect and destroy altered cells that are identified as 'non-self" (immune surveillance) may have an impact on cancer incidence. For some time it has been proposed that tumour cells expressing anti- gens that are recognized by the immune system will be eradicated at an early, preclinical stage and only those cells not eliciting such a response can survive to generate a clinically detectable tumour. This may be reflected in the difficulty in prompting a patient to mount a response against their tumour. However, it is still not clear how much impact this theoretical effect has on tumour incidence, nor whether specific defects in the immune system have a major impact (see Chapters 20 and 27).

1.7 Factors influencing the

development of cancers

Many factors are involved in the development of

cancer. These include both endogenous factors such as inherited predisposition and exogenous factors such as exposure to environmental carci- nogens and infectious agents. All of these factors are discussed in depth in Chapters 2, 3 and 13.

Another factor not discussed in detail elsewhere

in the book and which has a clear influence on the type of cancer which develops is age. In fact, age is the biggest risk factor for developing cancer (Figure 1.3). There is an age-associated, organ- specific tumour incidence. Most cancers in man

WHAT IS CANCER?7

and experimental animals can be divided into three main groups depending on their age-specific incidence: (1)Embryonic tumours, for example, neuroblasto- ma (tumours of embryonic nerve cells), embryonal tumours of kidney (Wilms" tumours), retinoblas- toma, etc. (2)Tumours found predominantly in the young, for example, some leukaemias, tumours of the bone, testis, etc. (3)Those with an increasing incidence with age, for example, tumours of prostate, colon, bladder, skin, breast, etc.

The juvenile onset of some cancers such as

some leukaemias and those described as embry- onal, is believed to reflect the requirement for only a limited number of alterations. Some of them such as familial retinoblastoma, originate in cells that already contain one inherited genetic defect and if only one or two further events is required for tumorigenicity of a target cell (e.g. inactivation of the second allele ofRB), tumour development becomes almost inevitable given the large number of essentially initiated cells present.

There are several possible molecular and physio-

logical explanations for the last group of age- associated tumours (group 3) which include the most common adult human cancers. First, in the normal individual there is continuous exposure throughout life to low levels of exogenous carci- nogens and it is likely that it is both the time required for accumulation of multiple genetic changes and for multiple phases of clonal selection that results in tumours only later in life. This isprobably a major factor in determining the age association of most epithelial tumours.

A second possibility is that with age there are

changes in the cellular environment that are more permissive for outgrowth of altered clones or which allow or encourage neoplastic change to take place, for example, alterations in the immune or hormonal systems or changes in the tissue microenviron- ment. The relationship of tumour development in endocrine-sensitive organs such as the breast or prostate to age-associated hormonal changes in the patient is still to be completely defined but seems likely to be involved in the rate of growth of these tumours. One of the roles of hormones is to stimu- latedivisionofhormone-sensitivecells,thatis,these may act as a promoting agent (Chapter 15). There is some debate about whether the relative decline in function of the immune system in old age plays a role and this is discussed in Chapter 20. Interest- ingly, there is also recent evidence that senescence- associated changes that occur in mesenchymal cells can affect adjacentepithelial cellsandthis mayhave a promoting effect. Senescent fibroblasts express several enzymes involved in extracellular matrix remodelling, for example, MMP-9 and stromelysin and it has been hypothesized that the ageing stroma may contribute to epithelial carcinogenesis in this way (Krtolica and Campisi, 2003). Some elegant experiments have been carried out by Cunha and colleagues which identify important effects of tumour stromal cells. They studied the effect of stromalcellsderivedfromprostatetumoursorfrom normal prostate on thein vivogrowth of pre- neoplastic prostate epithelial cells in tissue recombination experiments. In combination with 350
300
250
200
150

Rate per 100,000100

50
0

20- 25- 30- 35- 40- 45- 50-

A gegroup

55... 60... 65... 70... 75... 80...

Figure 1.3An example of age-specific cancer

incidence rate. Colon cancer incidence in men in England in 1999.8CELLULAR AND MOLECULAR BIOLOGY OF CANCER tumour stroma, the preneoplastic epithelium was able to form carcinomas, whereas when combined with normal stroma, only normal ductular struc- tures were formed indicating that, even though non-tumorigenic themselves, stromal cells derived from the tumour microenvironment had lost the ability to exert their normal inhibitory control over the epithelial component of the tissue. Thus, normal age-associated changes and acquired changes in the tumour stroma can contribute to epithelial carcinogenesis. Finally, it is possible that there are age-associated changes in some cells which increase their sus- ceptibility to neoplastic transformation. Age- associated decline in DNA repai