Biology of cancerKevin J Harrington

Abstract

Oncogenes are derived from mutated versions of normal

cellular genes (called proto-oncogenes) that control cell prolif-

eration, survival and spread. In normal cells, the expression of

proto-oncogenes is very tightly regulated to avoid uncontrolled

cell growth. In cancer, activating mutations of proto-oncogenes

are responsible for uncontrolled cell division, enhanced

survival (even in the face of anti-cancer treatment) and dissem-

ination. Oncogenes are described as being phenotypically domi-

nant e a single mutated copy of a proto-oncogene is sufficient to

promote cancer e and are generally not associated with inherited

cancer syndromes. Two exceptions to this rule are mutations in

the ret proto-oncogene that are associated with multiple endo-

crine neoplasia (MEN) syndromes (types 2A and 2B) and germ-

line mutations in H-ras that can cause Costellos syndrome (high

birth weight, cardiomyopathy and predisposition to cancers).

Oncogenes can be activated in three ways to cause cancers

(Figure 2).

Tumour suppressor genes (TSG) are normal cellular genes

whose function involves inhibition of cell proliferation and

survival. They are frequently involved in controlling cell cycle

progression and apoptosis. TSG are phenotypically recessive e the

function of both copies must be lost in order to promote cancer e

and are responsible for inherited cancer syndromes (see Genetic

predisposition to cancer in Medicine 2012; 40(1)).3

DNA consists of a double helix composed of a deoxyribose sugar-

phosphate backbone and four bases (adenine, guanine, thymine

and cytosine). The four bases form hydrogen bonds with specific

bases on the opposite strand. A binds with T and C binds with G.

deoxyR, deoxyribose; p, phosphate; a, adenine; t, thymine;

c, cytosine; g, guanine.

The structure of DNA

Therapies at The Institute of Cancer Research, Targeted Therapy

CANCER BIOLOGY AND IMAGINGLaboratory, Division of Cancer Biology, London, UK and a Consultant

Clinical Oncologist at The Royal Marsden Hospital, London. His

research interests include gene and virotherapy of cancer and targeted

radiation sensitisation of cancer.Introduction

Cancer is a genetic disease that occurs when the information in

cellular DNA becomes corrupted, leading to abnormal patterns of

gene expression. As a result, the effects of normal genes that

control normal cellular functions, such as growth, survival and

spread, are enhanced and those of genes that suppress these

effects are repressed. The main mechanism by which this

corruption of the genetic code occurs is through the accumula-

tion of mutations, although there is increasing recognition of the

role of non-mutational (epigenetic) changes in the process.

Aberrant gene expression leads to a number of key changes in

fundamental biological processes within cancer cells e the

so-called hallmarks and enabling characteristics of cancer1,2

(Figure 1).

Cancer genes

Cancer is driven by two classes of genes (oncogenes and tumour

suppressor genes), each of which provides an essential function

in normal cells.

Kevin J Harrington PhD FRCP FRCR is a Reader in Biological CancerCancer is caused by aberrant patterns of gene expression. Most common

cancers are caused by acquired mutations in somatic cells. In contrast,

specific germline mutations can account for rare hereditary cancer

syndromes. In general, the genes affected in cancers can be divided into

two groups: oncogenes and tumour suppressor genes. Oncogenes undergo

activation and are phenotypically dominant, while tumour suppressor genes

undergo inactivation and are phenotypically recessive. Oncogenic activation

can occur by specific point mutations within the sequence of a gene, by

amplification of the number of copies of the gene or by translocation of

DNA to a site where transcription is more active or where the formation of

a new fusion gene generates a protein with enhanced biological activity.

Tumour suppressor genes are inactivated bymutations that destroy the func-

tion of the protein encoded by the gene. The biological behaviour of cancer

can be considered in terms of eight specific hallmarks and two additional so-

called enabling characteristics. Improved understanding of the mechanistic

basis of these processes has resulted in rapid progress in diagnosis, treat-

ment and prognostication in cancer medicine.

Keywords angiogenesis; apoptosis; cancer; enabling characteristics;

hallmarks; metastasis; mutation; oncogene; tumour suppressor geneMEDICINE 39:12 689P

P

P

P

P

P

T

A

G

C

A

C

G

T

5

3

5

deoxyR

deoxyR

deoxyR

deoxyR

deoxyR

deoxyR

deoxyR

deoxyR

Figure 1 2011 Elsevier Ltd. All rights reserved.

a b cAbnormal

DNA

Normal DNA

Normal mRNA

Normal protein in

normal amount

Normal gene expression leads to formation of normal mRNA and

expression of a normal protein in normal amounts. a Specific mutations in the sequence of the DNA code lead to alterations in

the amino acid sequence of the protein, giving it enhanced activity.

b Increased numbers of normal copies of the gene (amplification) result in the formation of increased am ounts of normal protein.

c Translocation of part of the DNA from one chromosomal location to another can result in the generation of a fusion protein with

enhanced biological activity.

Oncogenic activation via three pathways

Intracellular

CANCER BIOLOGY AND IMAGINGHallmarks of cancer and enabling characteristics

In 2000, Hanahan and Weinberg described six key changes that

Mutant protein Fusion protein

mRNA

Protein

Amplified normal protein

Figure 2occur in cancer (growth factor independence, evading growth

suppressors, avoiding apoptosis, maintaining replicative poten-

tial, angiogenesis and invasion/metastasis); these can be seen as

largely responsible for driving malignant behaviour.1 Recently,

they have updated their description to include two additional

emerging hallmarks (re-programming energy metabolism and

evading immune destruction) and two enabling characteristics

(genomic instability and inflammation) (Table 1).2 The role

played by each of these processes will be reviewed briefly below.

as is

Hallmarks of cancer and enabling characteristics

Hallmarks of cancer

C Growth factor independence or self-sufficiency

C Insensitivity to anti-growth signals

C Avoidance of programmed cell death (apoptosis)

C Ability to recruit a dedicated blood supply

C Immortalization by reactivation of telomerase

C Ability to invade adjacent normal tissues and

metastasize to distant sites

C Reprogrammed energy metabolism

C Evading immune destruction

Enabling characteristics of cancer

C Genomic instability

C Inflammation

Table 1

MEDICINE 39:12 690activation of growth factor receptors is very tightly controlledeGrowth factor independence

Ageneral scheme for the functionof growth factor receptors and their

ligands in promoting cell growth (and other effects) is shown in

Figure 3. In this case, bindingof epidermal growth factor (the cognate

ligand) to its specific ligand-binding domain on the extracellular

component of the epidermal growth factor receptor (EGFR) leads to

a signal beingpassed from themembrane to thenucleusvia a cascade

of intermediary messengers, such that ligand binding on the cell

surface alters the behaviour of the cell. Under normal circumstances,

InvasionAngiogenesis

Proliferation

Gene transcriptionAnti-apoptosis

DNA repair

Phosphorylated

receptor activates

signal transduction

pathways

domain

Figure 3membraneP- -PEGF binds to

extracellular

domain

Intracellular domain

undergoes phosphorylation

Cell

Growth factor independence can lead to sustained signalling in

pathways that control essential biological functions, such as

growth, apoptosis, angiogenesis, invasion and DNA damage repair.

Growth factor independencethe synthesis and release of the ligands that stimulate them. Cancer

cells frequently usurp normal growth factor signalling pathways and

use them to promote unrestrained cell division.3

Cancer cells exploit three main strategies for achieving self-

sufficiency in growth factors: they manufacture and release

growth factors which stimulate their own receptors (autocrine

signalling) and those of their immediate neighbours (paracrine

signalling); they alter the number, structure or function of the

growth factor receptors on their surface, such that they are more

likely to send a growth signal to the nucleus (even in the absence of

the cognate ligand); and they deregulate the signalling pathway

downstream of the growth factor receptor so that it is permanently

turned on (constitutively active).

Insensitivity to anti-growth signals

Several normal anti-growth signals counteract the positively

acting growth signals described above. Anti-growth signals work

either by forcing cells into quiescence (G0 stage of the cell cycle)

or by inducing their terminal differentiation such that they are

permanently unable to re-enter the cell cycle. Anti-growth sig-

nalling is mediated by ligands (e.g. transforming growth factor

beta, TGF-b) that act on cellular receptors (e.g. TGF-b receptor)

and send signals to the nucleus via second messengers. These

pathways are mainly involved in controlling the cell cycle clock

and mediate their effects through proteins that include

2011 Elsevier Ltd. All rights reserved.

retinoblastoma protein (Rb), cyclins, cyclin-dependent kinases

(CDK) and their inhibitors (CDKi). Abnormalities in anti-growth

signalling pathways are extremely common in cancer and play

a role in helping cancer cells to progress through the cell cycle.

Therefore, loss of Rb and members of the CDKi family, and

overexpression of certain cyclins and CDK have been shown to

occur in a large number of tumour types.

Avoidance of apoptosis

Normal cells continually audit their viability by assessing the

balance of survival (anti-apoptotic) and death (pro-apoptotic)

signals that they receive. In normal cells, DNA damage leads to

a block in proliferation (cell cycle arrest) while the potential for

repair is assessed. If the level of damage exceeds the capacity for

repair, the balance of anti- and pro-apoptotic signals tips and the cell

undergoes programmed cell death (apoptosis). This prevents

maintenance ofDNAdamage and avoids the risk thatmutationswill

be passed to the progeny of cell division. As such, this mechanism

represents a very powerful barrier to the development of cancer.

Loss of normal apoptotic pathway signalling is an extremely

common event in cancer. Indeed, two of the best-known cancer-

associated genes (p53 (TSG) and bcl-2 (oncogene)) are intimately

involved in apoptosis. The two main mechanisms of apoptotic

signalling (intrinsic and extrinsic pathways) are illustrated in

a simplified form in Figure 4. Cancer cells are able to evade

apoptosis through an ability to ignore signals sent through the

extrinsic pathway, or by re-setting the balance of intracellular pro-

have switched off their apoptotic pathway are more likely to be

intrinsically resistant to anti-cancer treatments. In fact, the use of

these treatmentsmay promote the accumulation of othermutations

that may have a negative influence on the biology of the disease.

Sustained angiogenesis

In normal tissues, the growth of newblood vessels (angiogenesis) is

held very tightly in check by a balance between positive (pro-

angiogenic) and negative (anti-angiogenic) signals (see Table 2).

The growth of cancer deposits is intimately related to their ability

to secure a blood supply. A small cluster of cancer cells can grow to

60e100 mm by deriving a supply of oxygen and nutrients by direct

diffusion, but beyond this size the fledgling tumourmust acquire its

own dedicated blood supply. Cancers acquire the ability to grow

a new blood supply by subverting the balance between pro- and

anti-angiogenic factors. Essentially, cancers switch to an angio-

genic phenotype by upregulating production of pro-angiogenic

proteins, such as vascular endothelial growth factor (VEGF), and/

or by downregulating production of anti-angiogenic proteins, such

de in

the resynthesis of the DNA sequence of the telomere. Therefore,

sensor (p53)(cytochrome C)

CANCER BIOLOGY AND IMAGINGGenotoxicinsult

Common final pathwayto cell death

INTRINSIC PATHWAYCaspase 3

Figure 4Damagesignalling moleculeand anti-apoptoticmolecules in favour of inhibitionof apoptosis. By

circumventing apoptosis, cancer cells can sustain DNA damage

without it causing cell death (unless the damage is to a gene that is

absolutely necessary for cell survival). Therefore, cancer cells that

Cellmembrane

Pro-apoptoticsignal

Anti-apoptoticsignal

Death ligand

Death receptor

EXTRINSIC PATHWAY

Mitochondrion

Cells can undergo programmed cell death in response to activation

of either the intrinsic or extrinsic apoptotic pathway. Cancers

frequently subvert these pathways to allow them to survive signals

that would lead to the death of normal cells.

Normal apoptotic signalling pathways

Extrinsicpathwaysignalling molecule(caspase 8) Intrinsic pathwayMEDICINE 39:12 691C Thrombospondin-1 and -2 (TSP-1, TSP-2)

C Interleukins (IL-1b, IL-12, IL-18)

C Anti-thrombin III

Table 2C Angiostatin

C Endostatintumours that have reactivated the expression of telomerase are

Pro- and anti-angiogenic factors

Pro-angiogenic

C Vascular endothelial growth factor (VEGF)

C Basic fibroblast growth factor (bFGF)

C Acidic fibroblast growth factor (aFGF)

C Transforming growth factors a and b (TGF-a, TGF-b)

C Platelet-derived growth factor (PDGF)

C Tumour necrosis factor a (TNF-a)

Anti-angiogenicreverse transcriptase uses the hTR RNA template as a guias thrombospondin-1.

Cellular immortalization

Normal somatic cells can undergo only a finite number of cell

divisions (Hayflick limit) before they enter a period of permanent

growth arrest, known as replicative senescence. This process

occurs as a result of the cells inability to replicate the ends of

their chromosomes (the telomeres) fully at each division.

Therefore, over time the telomeres get progressively shorter,

effectively acting as molecular clocks that count down the cells

lifespan. In contrast, stem cells and malignant cells have

acquired immortality by maintaining the length of their telo-

meres. In most tumours, this occurs through upregulation of the

enzyme telomerase, but in 10e15% of cases a different mecha-

nism e the alternative lengthening of the telomeres (ALT) e is

responsible. Telomerase enzymatic activity involves a large

number of proteins but its two main components are an RNA

template (hTR) and a reverse transcriptase enzyme (hTERT); the 2011 Elsevier Ltd. All rights reserved.

with

a marked increase in specific cancers, especially those of viral

CANCER BIOLOGY AND IMAGINGFigure 5levels of cognate ligand(chemokine)

BrainTumour cell invasion,migration andintravasation

Tumour cell expressingspecific chemokine

receptor

Liver

Bone

Metastasis specifically totissues expressing high

Lymphnode

Patient withbreast cancer

Lung

Invasion and metastasis of cancer cells results from upregulated

expression of molecules that allow cells to digest the extracellular

matrix around them, migrate and intravasate into blood vessels

and then take up residence in distant organs. The sites of distant

metastasis can be determined by the expression of specific

chemokine receptors by cancer cells that allow them to home in

on suitable sites to establish secondary deposits.

Invasion and metastasis of cancer cells

Tumour cell invades lymphatic

or blood vesselable to re-build the parts of their telomeres that they lose with

each round of cell division and thereby avoid being sidelined into

replicative senescence.

Invasion and metastasis (Figure 5)

Distant metastases cause 90% of cancer deaths. Invasion and

metastasis involves careful orchestration of a series of complex

biological processes:

detachment from immediate neighbours and stroma at thelocal site

enzymatic digestion of the extracellular matrix followed byspecific directional motility

penetration (intravasation) of blood or lymphatic vesselsand tumour embolization

survival in the circulation until arrival at the metastaticsite, which may be chosen on the basis of provision of

a favourable supply of appropriate growth factors

adherence of the metastasis to the endothelium of bloodvessels at its destination and extravasation from the vessel

proliferation and invasion of the new location andrecruitment of a new blood supply.

One of the key processes underlying invasion and metastasis of

epithelial tumours is the epithelial-to-mesenchymal transition

(EMT). This multifaceted programme can be engaged transiently

or stably by invading cancer cells. The patterns of metastasis of

different cancers to specific organs (e.g. breast cancer to liver,

bone and brain; lung cancer to brain and adrenal gland) are not

random, but appear to be driven by expression of chemokine

receptors by tumour cells that allow them to seek a suitable

environment in which to establish a colony.4

e the

hallmarks of cancer. The second enabling characteristic describes

MEDICINE 39:12 6924 Muller A, Homey B, Soto H, et al. Involvement of chemokine rece

in breast cancer metastasis. Nature 2001; 410: 50e6.the common situation in which pre-malignant and frankly

malignant lesions excite an inflammatory state, through the

recruitment and activation of components of the immune system

that promote and support tumour growth and spread. A

FURTHER READING

1 HanahanD,WeinbergRA. Thehallmarksof cancer.Cell2000;100:57e70.

2 Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation.

Cell 2001; 144: 646e74.

3 Rogers SJ, Harrington KJ, Rhys Evans P, O-Charoenrat P, Eccles SA.

Biological significance of c-erbB family oncogenes in head and neck

cancer. Cancer Metastasis Rev 2005; 24: 47e69.

ptorschanges that progressively alter their biology and promotorigin. There is also evidence that the immune system presents

a significant barrier to non-virally induced cancers in immuno-

competent patients. Thus, the occurrence of tumours can be

perceived as a failure of the immune system to recognize, reject

and destroy tumour cells that express altered self antigens. As part

of this process, it is thought that selection of less immunogenic

cancer cells (through immuno-editing) and active recruitment of

immunosuppressive components of the immune system [e.g.

regulatory T cells (Treg) and myeloid-derived suppressor cells

(MDSCs)] to some cancers allows tumours to develop and spread

without becoming targets for immune clearance.

Enabling characteristics of cancer

As part of the biology underpinning cancer development, two

key enabling characteristics have recently been defined: genomic

instability and inflammation. The first relates to the state in

which cancer cells lose control of the integrity of their genetic

material and acquire an increasing repertoire of mutationalobservation that chronic immune suppression is associatedReprogrammed energy metabolism

In their updated review, Hanahan and Weinberg have designated

reprogrammed energy metabolism as an emerging hallmark

of cancer. This hallmark recognizes the fact that the chronic,

uncontrolled cell proliferation in cancer requires a reconfiguration

of the way in which cancer cells metabolize glucose. Normal cells

process glucose initially in the cytoplasm by glycolysis, to yield

pyruvate, and then in the mitochondria by oxidative phosphoryla-

tion, to generate carbon dioxide and water. In contrast, even under

oxygenated conditions, cancer cells tend to switch theirmetabolism

to preferential use of glycolysis with generation of lactate (the

so-calledWarburg effect). As yet, the reasons for this change are not

clear, but the fact that it is driven by mutations in key oncogenes

and tumour suppressor genes suggest that it is an important

underlying principle of cancer biology.

Evading immune destruction

An unresolved issue regarding tumour formation and mainte-

nance is the role of the immune system. According to the theory of

immune surveillance, the immune systemmounts a constant vigil

against the emergence of pre-malignant and frankly malignant

cells. The most often cited evidence for this effect comes from the 2011 Elsevier Ltd. All rights reserved.

Biology of cancer Introduction Cancer genes Hallmarks of cancer and enabling characteristics Growth factor independence Insensitivity to anti-growth signals Avoidance of apoptosis Sustained angiogenesis Cellular immortalization Invasion and metastasis (Figure 5) Reprogrammed energy metabolism Evading immune destruction Enabling characteristics of cancer

Further reading