Cancer Causes & Consequences - Genetic Causes

Content Overview (Genetic Causes)

• Initiation and promotion, multistage model.

• Causes of mutations:

  • Environmental factors.

  • Viruses.

  • Inherited mutations.

  • Spontaneous mutations (sporadic cancers).

  • Examples and relative importance.

• The concept of mutations; Knudsen 2-hit hypothesis, historic information.

• Tumour suppressors.

• Proto-oncogenes.

• Familial cancers.

• Epigenetics.

• Good Introductory Overviews:

  • M.R.S. Brothwell, G.C. Barnett. Cancer Genetics and Genomics - Part 1. Clinical Oncology 34 (2022) e254-e261

  • Cancer Genetics and Genomics e Part 2. Clinical Oncology 34 (2022) e262-e267

Neoplasm and Cancer Definitions

• Neoplasm Definition (Willis 1952): "A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of normal tissue, and persists in the same excessive manner after the cessation of the stimuli which evoked the change."

  • Note: Not all tumours are autonomous; some require hormonal support (e.g., breast, prostate).

• Cancer Definition: Fully developed tumour with the capacity to invade and destroy surrounding tissue (local invasion) and, in some cases, spread to distant sites (metastasis).

  • Examples given: Colon Cancer & Breast Cancer (Liver) (Brain)

Cancer Cells vs. Normal Cells

• Cancer Cells:

  • Loss of contact inhibition.

  • Increase in growth factor secretion.

  • Increase in oncogene expression.

  • Loss of tumour suppressor genes.

  • Neovascularization.

  • Frequent mitoses.

  • Abnormal heterogeneous cells.

• Normal Cells:

  • Oncogene expression is rare.

  • Intermittent or coordinated growth factor secretion.

  • Presence of tumour suppressor genes.

  • Few mitoses.

  • Normal cell.

Factors Contributing to Cancer

• Medical complications.

• Age.

• Endocrine/metabolic factors.

• Environment/lifestyle factors.

• Host-related factors.

• Therapy-related factors (Radiation, Chemotherapy).

• Emerging Genetics.

• Sex.

Cancer Statistics (UK)

• New cases of cancer (2014-2016 average): 363,484.

• Deaths from cancer (2015-2017): 164,901.

• Survival: 50% survive cancer for 10 or more years (2010-11, England and Wales).

• Preventable cases: 38% of cancer cases are preventable (UK, 2015).

• Common cancers: More than half of new cases are breast, prostate, lung, or bowel cancer (2014-2016 average, UK).

• Peak rate of cancer cases (2014-2016, UK) in age 85-89 years.

Male Cancer Statistics

• Adapted from Greenlee RT, et al. CA Cancer J Clin. 2000;50:16.

• Estimated Incidence:

  • Prostate: 29%

  • Lung & bronchus: 14%

  • Colon & rectum: 10%

  • Urinary bladder: 6%

  • Non-Hodgkin’s lymphoma: 5%

  • Melanoma of skin: 4%

  • Kidney & renal pelvis: 3%

  • Leukemia: 3%

  • Oral cavity & pharynx: 3%

  • Pancreas: 2%

  • All others: 19%

• Estimated Deaths:

  • Lung & bronchus: 31%

  • Prostate: 11%

  • Colon & rectum: 10%

  • Pancreas: 5%

  • Non-Hodgkin’s lymphoma: 5%

  • Leukemia: 4%

  • Esophagus: 3%

  • Liver & intrahepatic bile duct: 3%

  • Stomach: 3%

  • Urinary bladder: 3%

  • Melanoma of Skin: 3%

  • All others: 22%

Female Cancer Statistics

• Adapted from Greenlee RT, et al. CA Cancer J Clin. 2000;50:16.

• Estimated Incidence:

  • Breast: 25%

  • Lung & bronchus: 15%

  • Colon & rectum: 11%

  • Uterine corpus: 5%

  • Ovary: 5%

  • Non-Hodgkin’s lymphoma: 5%

  • Melanoma of Skin: 2%

  • Thyroid: 2%

  • Pancreas: 2%

  • Urinary bladder: 2%

  • All others: 21%

• Estimated Deaths:

  • Breast: 30%

  • Lung & bronchus: 12%

  • Colon & rectum: 11%

  • Uterine corpus: 6%

  • Ovary: 4%

  • Non-Hodgkin’s lymphoma: 4%

  • Pancreas: 2%

  • Brain & other nervous system: 3%

  • Stomach: 2%

  • Leukemia: 2%

  • Multiple myeloma: 2%

  • Melanoma of Skin: 3%

  • All others: 22%

Known Cancer Causes

• Occupational exposure.

• Lifestyle factors.

• Biologic agents.

• Physical agents.

• Reference: Trichopoulos D, et al. Cancer: Principles & Practice of Oncology. 5th ed. 1997;231-257. Lecture 1

Virus-Related Cancers (Table 4.6)

• EBV (Herpesviridae):

  • Cells infected: B cells, epithelial cells, lymphoid.

  • Human malignancy: Burkitt's lymphoma, oropharyngeal/nasopharyngeal carcinoma, Hodgkin's disease/lymphoma.

  • Transmission route: saliva

• HTLV-I (Retroviridae):

  • Cells infected: T cells.

  • Human malignancy: non-Hodgkin's lymphoma.

  • Transmission route: parenteral, venereal

• HHV-8 (Herpesviridae):

  • Cells infected: endothelial cells.

  • Human malignancy: Kaposi's sarcoma, body cavity lymphoma.

  • Transmission route: venereal

• HBV (Hepadnaviridae):

  • Cells infected: hepatocytes.

  • Human malignancy: hepatocellular carcinoma.

  • Transmission route: parenteral, venereal

• HCV (Flaviviridae):

  • Cells infected: hepatocytes.

  • Human malignancy: hepatocellular carcinoma.

  • Transmission route: parenteral

• HPV (Papovaviridae):

  • Cells infected: cervical epithelial.

  • Human malignancy: cervical carcinoma.

  • Transmission route: venereal

• JCV (Papovaviridae):

  • Cells infected: central nervous system.

  • Human malignancy: astrocytoma, glioblastoma.

  • Transmission route: ?

Bacterial-Related Cancers

• Adapted from Trichopoulos D, et al. Cancer: Principles & Practice of Oncology. 5th ed. 1997;249.

  • Helicobacter pylori: Stomach cancer.

  • Schistosoma haematobium: Urinary bladder cancer.

  • Opisthorchis viverrini: Liver cancer.

Physical Contributors to Cancer

• Adapted from Trichopoulos D, et al. Cancer: Principles & Practice of Oncology. 5th ed. 1997;231-257.

  • Ionizing radiation: Breast cancer, leukemia, etc.

Pharmaceuticals and Cancer

• Adapted from Trichopoulos D, et al. Cancer: Principles & Practice of Oncology. 5th ed. 1997;249.

  • Cancer chemotherapeutic drugs: Bone marrow.

  • Immunosuppressive drugs: Reticuloendothelial system.

  • Menopausal estrogens: Endometrium, breast.

  • Diethylstilbestrol: Vagina, cervix uteri.

  • Anabolic steroids: Liver.

  • Oral contraceptives: Liver.

  • Tamoxifen: Endometrium.

  • Phenacetin analgesics: Kidney, pelvis.

Environmental and Genetic Factors in Cancer

• Table 2.5: Geographic variation in cancer incidence and death rates.

• Lists countries with the highest and lowest incidence of specific cancer types.

• Includes relative risk (H/L).

  • Lip: Canada (Newfoundland) / Japan (155)

  • Nasopharynx: Hong Kong / Japan (151)

  • Skin (melanoma): Australia (Queensland) / United Kingdom (100)

  • Prostate: U.S. (African American) / China (70)

  • Liver: China (Shanghai) / Canada (Nova Scotia) (49)

  • Penis: Brazil / Israel (Ashkenazic) (42)

  • Cervix (uterus): Brazil / Israel (non-Jews) (28)

  • Stomach: Japan / Kuwait (29)

  • Lung: U.S. (Louisiana, African American) / India (Madras) (19)

  • Pancreas: U.S. (Los Angeles, Korean American) / India (11)

  • Ovary: New Zealand (Polynesian) / Kuwait (8)

Cumulative Cancer Rates by Age 75

• Graphical representation comparing cumulative cancer rates by age 75 for different cancer types and populations.

• Compares Osaka (1970-71 vs. 1988-92), Hawaiian Japanese (1988-92), and Hawaiian Caucasian (1968-72 vs. 1988-92).

• Cancer types: Prostate, Colon (M), Stomach (M), Breast (F).

• Grey v Red = Genetics, Blue v Grey = Environment.

Oncogenesis and Carcinogenesis

• Oncogenesis = carcinogenesis: the process of tumour induction and development.

• Carcinogenesis is a multistep process.

• Application of a carcinogen doesn't immediately produce a tumour.

• A series of changes takes place after the initiation step induced by the carcinogen, although the cells may be phenotypically normal.

• Subsequent changes – tumour promotion - may be produced by carcinogens or other substances which themselves do not produce tumours.

Initiators and Promoters

• Initiators: Chemical carcinogens, Viruses, Radiation, UV, Replication errors, Unknown factors.

• Promoters: Inflammation, Hormones, Normal growth promoters.

• Initiation is rapid, and once it has taken place, initiated cells may persist for long periods.

• Initiated cells are latent until acted upon by a promoter.

• This sequence of events is a consequence of genetic changes.

Historical Perspective on Carcinogenesis

• 1949 – Berenblum and Shublik: Carcinogenesis is at least a two-step process.

• 1954 – Armitage and Doll: It’s 6 or 7 stage process.

• 1957 – Foulds: Tumour progression occurs in a stepwise fashion, due to mutations, loss, or activation of genes. Discrete changes confer new biological activities and growth advantage, making the tumour more independent and aggressive.

• (1953 Watson & Crick publish structure of DNA)

• Rosalind Franklin and Maurice Wilkins

Stages of Tumour Development

• Normal cells -> initiation (genotoxic stress): cells still have normal phenotype.

• Promotion (changes in growth): cells pre-malignant.

• Promotion: expansion of pre-malignant clones.

• Progression: malignant cells.

• Expansion of malignant clones.

• Metastasis.

• Cells becoming increasingly unstable.

Hallmarks of Cancer: New Dimensions

• Hanahan D. Cancer Discov. 2022 Jan;12(1):31-46

• Including:

  • Sustaining proliferative signaling

  • Evading growth suppressors

  • Resisting cell death

  • Enabling replicative immortality

  • Inducing or accessing vasculature

  • Activating invasion & metastasis

  • Deregulating cellular metabolism

  • Avoiding immune destruction

  • Unlocking phenotypic plasticity

  • Nonmutational epigenetic reprogramming

  • Emerging hallmarks & enabling characteristics : Genome
    instability & mutation
    , Tumor-promoting
    inflammation, Senescent cells, Polymorphic microbiomes

Clonality of Tumours and Tumour Progression

• Tumours originating from multiple cells - polyclonal tumours - are not unknown (e.g. tumours induced by rapidly transforming RNA tumour viruses, which can spread from tumour cells to adjacent uninfected cells, are polyclonal. Other examples exist of tumours appearing to arise from multiple cells, either because of the presence of an infectious agent, or, as in certain hereditary cancer syndromes, because the initiating event is very common).

• Most spontaneous tumours are, however, monoclonal and originate from a single cell. BUT tumours are heterogenous – they continue to evolve!

• One of the central concepts of modern tumour biology is that most tumours are monoclonal: all the cells in a tumour descend from a single cell, so that the cells within a given tumour (or within metastases from that tumour) represent a clone.

• Until fairly recently it was generally assumed that cancer evolves through a linear stepwise selection of genetic changes.

• This thinking was radically altered in 2012 when Gerlinger et al., 2012 showed that clear-cell renal carcinoma evolved through branching tumor evolution, similar to Darwin's evolutionary tree.

• Many, if not most, solid cancers evolve through branching evolution.

• Unlike linear tumor evolution, branching tumor evolution creates extensive intra- and inter-tumor genetic heterogeneity and is the likely reason for the extensive heterogeneity seen.

Tumour Cell Properties and Genetic Alterations

• The properties of the tumour cells are clonally inherited because they result from genetic alterations within the cell.

• Stable changes in gene expression ("epigenetic changes") may also underlie some of the changes in malignant cells.

• The growth of tumours is thus a Darwinian process in which a succession of changes within the tumour cell population leads to the appearance and evolution of progressively more malignant cells.

• Before proto-oncogenes and tumour suppressor genes were identified the precise nature of the processes that are modified in human cancer remained poorly defined.

• Genetic changes in proto-oncogenes and tumour suppressor genes are primarily responsible for tumour initiation and progression.

Mutation Accumulation and Tumourigenesis

• The appearance of a human tumour is a result of the accumulation of mutations in a number of genes that directly, or indirectly, affect cell proliferation.

• The limiting step is the rate of mutation accumulation (somatic cells ~ $10^{-7}$/gene/cell generation).

• Tumours would never occur during a human lifetime (mathematically impossible).

• Tumourigenesis can occur if the genomes of pre-malignant cells are more mutable than those of their normal counterparts (e.g., HNPCC ® microsatellite instability).

• Mutations in DNA repair genes allow accumulation of mutations at a higher rate:

  • Defective mismatch repair, MMR (MLH1, MSH2, MSH6, PMS2) – Hereditary Non-Polyposis Colon Cancer, HNPCC – Lynch Syndrome.

  • Defective nucleotide excision repair, NER (XPA, XPB, XPC, ERCC2……) – Xeroderma Pigmentosum, XP (UV hypersensitivity).

Chromosomal Analysis and Aneuploidy

• Chromosomal analysis (Karyotyping and Multifluor FISH).

• Aneuploidy and LOH are very commonly observed; possibly due to improper allocation of chromatids during mitosis - normally tightly controlled via 'checkpoint'.

• Many tumours exhibit aneuploidy (genetic instability) due to a defective mitotic checkpoint.

• REVIEW ARTICLE: Aneuploidy as a promoter and suppressor of malignant growth. Vasudevan A, et al. Nat Rev Cancer. 2021. PMID: 33432169

Aneuploidy as a Promoter and Suppressor of Malignant Growth

• Anand Vasudevan et al. Nat Rev Cancer. 2021 Feb;21(2):89-103

• Most aneuploidies display antitumorigenic properties that frequently lead to cell death or senescence. Rarely, aneuploid karyotypes arise that exhibit cancer-favouring properties, including immune evasion, drug resistance and oncogene overexpression. These aneuploidies can be selected over time, thereby increasing in abundance.

• The level of chromosomal instability (CIN) influences tumorigenic potential: very high rates of CIN lead to cell death, but lower levels of CIN can be tolerated and produce favourable karyotypes. However, the ‘ideal’ degree of CIN is influenced by tissue identity.

• CIN leads to heterogeneous aneuploidy, which can lead to either tumour-inhibitory effects or tumour progression. Senescent cells can contribute to both tumour inhibition and tumour progression through mechanisms that are not yet fully understood.

• Diagram depicting how Down syndrome simultaneously leads to increased haematological cancer risk, and decreased cancer risk in most solid tissues. SASP, senescence-associated secretory phenotype.

Oncogenes and Tumour Suppressor Genes

• Introduction: Most human cancer genes are generated by mutations in normal (wild-type) cellular genes as a result of uncorrected errors in DNA replication or after exposure to physical or chemical carcinogens.

• Gene function is either activated or inactivated by these mutations.

• Normal genes whose cellular functions are activated or enhanced by mutation are called proto-oncogenes. Mutations in proto-oncogenes are referred to as activating or gain-in-function mutations because they enhance, or confer new properties on, the cellular functions of the encoded oncoproteins.

• Genes whose cellular functions are inactivated by mutation are called tumour suppressor genes. Examples include inactivation of the retinoblastoma and p53 genes. These mutations are referred to as inactivating or loss-of-function mutations because they inactivate the cellular functions of the wild- type proteins. This removes a natural constraint on cell proliferation, call adhesion or some other cellular functions that controls cell behaviour.

Proto-oncogenes vs. Tumour Suppressor Genes

• Protooncogenes:

  • Mutation leads to Oncogene activation, having a "+" effect

• Tumour suppressor genes:

  • Mutation leads to Tumour suppressor gene loss, having a "-" effect

Classes of Oncogenes and Suppressor Genes

• Growth factor receptors acting via tyrosine-specific protein kinase activity:

  • PDGFR, EGFR

  • M-CSFR, SCFR

• GTP binding Ras proteins.

  • c-Hras, c-Kras

  • c-Nras

• Growth factors

  • PDGF (sis)

  • FGF-3

  • FGF-4

  • EGF, M-CSF, SCF

• Serine/threonine specific protein kinases

  • c-mos

  • c-raf

• Membrane/cytoskeletal-associated tyrosine specific prot kinases

  • Src protein kinases

  • c-src, c-abl

• Nuclear transcription factors

  • c-myc, c-fos, c-jun

• Steroid-type growth factor receptors

  • Thyroid hormone receptor (c-erbA

Mutation Definitions

• Mutation: A heritable change in genetic material. Mutations (in the broadest sense) include any change, from a single base change in the DNA to substantial deletions or rearrangements, even involving major parts of a whole chromosome.

• Point mutation: The term ‘point mutation’ describes the substitution of one base pair of a DNA sequence by another.

• Translocation: In a translocation part of one chromosome is joined to another.

• Gene Amplification: Gene amplification is believed to contribute to oncogenesis by increasing the levels of mRNA that are transcribed from the gene, and as a consequence increasing the levels of protein that it encodes.

• Deletion: A wide range of DNA deletions occurs in tumour cells. At one extreme, a single base pair may be removed. Larger deletions may encompass part or all of a gene. Finally, a deletion may be large enough to be visible under the microscope by karyotype analysis or it may remove a whole chromosome.

Myc Amplification and Prognosis

• Demonstrates the impact of Myc amplification on event-free survival (EFS) probability.

• Compares EFS for patients with <10 copies of N-myc vs. >10 copies of N-myc.

Frequently Amplified Chromosomal Regions and Genes (Table 4.3)

• Name of oncogene / Human chromosomal location / Human cancers / Nature of protein

  • erbB1 - 7q12-13 - glioblastomas (50%); squamous cell carcinomas (10-20%) - RTK

  • cab1-erbB2-grb7 - 17q12 - gastric, ovarian, breast carcinomas (10-25%) - RTK, adaptor protein

  • k-sam - 7q26 - gastric, breast carcinomas (10-20%) - RTK

  • FGF-R1 - 8p12 - breast carcinomas (10%) - RTK

  • met - 7q31 - gastric carcinomas (20%) - RTK

  • K-ras - 6p12 - lung, ovarian, bladder carcinomas (5-10%) - small G protein

  • N-ras - 1p13 - head and neck cancers (30%) - TF

  • c-myc - 8q24 - various leukemias, carcinomas (10-50%) - TF

  • L-myc - 1p32 - lung carcinomas (10%) - TF

  • N-myc-DDX1 - 2p24-25 - neuroblastomas, lung carcinomas (30%) - TF

  • akt-1 - 14q32-33 - gastric cancers (20%) - ser/thr kinase

  • cyclin D1-exp1-hst1-ems1 (11q13) - breast and squamous cell carcinomas (40-50%) - G1 cyclin

  • cdk4-mdm2-sas-gli - 12q13 - sarcomas (40%) - CDK, p53 antagonist

  • cyclin E - 19q12 - gastric cancers (15%) - cyclin

  • akt2 - (19q13) - pancreatic, ovarian cancers (30%) - ser/thr kinase

  • AIB1, BTAK - (20q12-13) - breast cancers (15%) - receptor co-activator

  • cdk6 - (19q21-22) - gliomas (5%) - CDK

  • myb - 6q23-24 - colon carcinoma, leukemias - TF

  • ets-1 - 11q23 - lymphoma - TF

  • gli - 12q13 - glioblastomas - TF

  • FGFR2 - 10q26 - breast carcinomas - RTK

Chromosomal Translocations

• Reciprocal translocation, t(9;22)(q34;q11), giving the Philadelphia chromosome.

• This chromosome is unusually short - contains a fusion gene called BCR-ABL1.

• This gene is the ABL1 gene of chromosome 9 juxtaposed onto the breakpoint cluster region BCR gene of chromosome 22, coding for a hybrid protein: a tyrosine kinase signalling protein that is “always on”.

• CML - Gleevec

Cell Hybridisation Experiments: Oncogenes / Suppressor Genes

• Fusion of normal cell and cancer cell.

• Two possible outcomes:

  • Hybrid cell is tumorigenic: cancer alleles are dominant.

  • Hybrid cell is non-tumorigenic: cancer alleles are recessive.

• Can distinguish between viral-induced and spontaneous cancers.

Tumour Suppressor Genes: Knudson and Retinoblastoma

• Retinoblastoma – a childhood cancer of the eye, which occurs in two forms:

  • Hereditary (40%), usually in both eyes.

  • Sporadic (60%), occurs in one eye.

• Knudson (1971) studied age/incidence data, and concluded that disease arises from 2 sequential mutational events (2-hit hypothesis).

• Hereditary form: One mutation is inherited, is phenotypically silent or harmless. A second hit occurs in retinal cells, causing tumours to develop.

• Sporadic form: Both mutations occur in the somatic tissue; probability of 2 hits in same cells is low, hence disease is unilateral.

Knudson's Two-Hit Hypothesis

• Illustrates the difference between hereditary and sporadic retinoblastoma at the genetic level.

Confirmation of Knudson's Hypothesis and Tumour Suppressor Genes

• Knudson’s hypothesis has been confirmed by identification and characterisation of the genetic abnormality.

• In retinobastomas there is a loss of part of chromosome 13, and a mutation in the RB1 gene on the remaining copy of chromosome 13.

• This study is seminal, as it told us something about hereditary and sporadic tumours, but also provided strong evidence for the existence of TUMOUR SUPPRESSOR genes!

• What is RB?

The Role of RB Protein

• The RB1 gene encodes the RB protein.

• RB has a regulatory role in cell division, preventing the activation of a number of genes required for the S (synthesis) phase of the cell cycle.

• RB is phosphorylated in a cell-cycle specific manner by various cyclin/Cdks. When RB is phosphorylated, DNA synthesis and thus cell division proceeds.

• Loss of RB1 removes an important check on the cell cycle.

• RB1 is a tumour suppressor.

P53: Guardian of the Genome

• P53 is a transcription factor that has a number of targets (increasing and decreasing their transcription according to the stimulus, gene and cell type in question).

• It activates DNA repair genes, regulates genes controlling apoptosis and also genes encoding proteins with growth suppressing activity such as p21.

• P21 protein inhibits the kinase activity of cyclin/Cdk complexes. This prevents phosphorylation of RB and so the cell remains in G1 phase. In some instances the cell cycle does not resume and the cell dies by apoptosis.

• Levels of P53 rise rapidly after DNA damage, arresting the cell cycle and allowing time for DNA repair. If such repair is impossible, P53 signals the onset of apoptosis.

• Loss or mutation of P53 means that cells with damage may continue dividing in the presence of DNA damage or in adverse conditions such as hypoxia.

• TP53 is an important tumour suppressor.

Types of Genetic Mutations

• Frameshift mutation: A genetic mutation caused by a deletion or insertion in a DNA sequence that shifts the way the sequence is read.

• In-frame mutations: Occur when the number of deleted or inserted base pairs is a multiple of three. This results in a change in only a few amino acids; it may still be possible for the protein to function, even though its sequence may be slightly different.

• Missense mutation: A point mutation in which a single nucleotide change results in a codon that codes for a different amino acid.

• Nonsense mutation: A point mutation in a sequence of DNA that results in a premature stop codon, in the transcribed mRNA, and in a truncated, incomplete, and usually nonfunctional protein product.

• Silent mutation: A change in the sequence of nucleotide bases, without a subsequent change in the amino acid or the function of the overall protein.

• Splice site mutation: Inserts, deletes or changes a number of nucleotides in the specific site at which splicing takes place

Human Tumour Suppressor Genes (Table 7.1)

• Lists various human tumour suppressor genes, their chromosomal location, associated familial cancer syndromes (if any), their roles in sporadic cancers, and the function of the encoded protein.

Oncogenes

• An oncogene is a gene whose product can be shown to contribute to cellular “transformation”.

• It is normally closely related, or identical to, a normal gene (termed a “proto- oncogene”).

• Proto-oncogenes are likely to be involved in essential functions of the cell related to control of proliferation and differentiation.

• Cells are stimulated by growth factors, which bind to cell surface receptors, activating intracellular signalling pathways, leading to alterations in gene expression.

• Proto- oncogenes function at each of these steps.

• Mutations in any of these genes can give rise to oncogenes, whose products promote cell growth in the absence of external stimuli.

Oncogenes Related to Various Factors

• Oncogenes related to growth factors: Platelet-derived growth factor (sis), Epidermal growth factor, Fibroblast growth factor (int-2,hst,K-fgf), Insulin-like growth factor.

• Oncogenes related to growth factor receptors: EGF receptors (erbB, erbB2/HER2), Macrophage colony-stimulating factor (fms), Stem cell factor (kit).

• Oncogenes related to signal transducers: RAS, ABL, SRC, RAF etc.

• Oncogenes related to nuclear factors: MYC, MAX, AP-I (Jun/Fos).

Mechanisms of Oncogene Activation

• Production of abnormal protein product by:

  • Point mutation - (H-RAS, K-RAS) single activating mutation at codon 12, 13 or 61 causing GTPase activity to be constantly on (MAPK pathway).

  • Chromosomal translocation – In chronic myeloid leukaemia Chr 9 is tranlocated to Chr 22. This places the ABL gene next to the BCR region, giving rise to the BCR-ABL oncogene product, which has constitutively activated kinase activity.

  • Deletion of part of the protein - SRC

Oncogene Activation cont.

• (2) Gene amplification – this results in protein over production e.g. MYC and ERBB2/HER2

• (3) Loss of control mechanisms – insertional mutagenesis (retroviral); translocation e.g. MYC

Multistep Carcinogenesis (Colon Cancer)

• p53 is altered in >80% of CRC's but germline p53 mutations ® no polyposis ® no ­'d risk of CRC

• ras mutations in a normal colonic epithelial cell ® no CRC è not simply accumulation of mutations - ORDER of accumulation is important

• The Roles of Initiating Truncal Mutations in Human Cancers: The Order of Mutations and Tumor Cell Type Matters. Levine, Jenkins & Copeland (2019) Cancer Cell. 2019 Jan 14;35(1):10-15. doi: 10.1016/j.ccell.2018.11.009.

Vogelstein’s Model of Colorectal Carcinogenesis

• normal epithelium -> hyperproliferative epithelium -> early adenoma -> intermediate adenoma -> late adenoma -> carcinoma -> metastasis

• Chr 5q: APC mutation or loss

• Chr 12p: K-RAS mutation

• DNA hypomethylation

• Chr 18q: DCC loss

• Chr 17p: TP53 loss

• other alterations

Somatic Mutation and Epithelial Tissues

• Somatic Mutation: What Shapes the Mutational Landscape of Normal Epithelia? Joanna C. Fowler & Philip H. Jones. Cancer Discov (2022) 12 (7): 1642–1655.

• Somatic Mutations During Aging: Not All Mutations are Harmful

• There are progressive numbers of somatic mutant clones in all aging epithelial tissues. However, only a small fraction of these clones goes on to become tumors.

• Phil Jones and others have studied mutations in normal oesophagus, focusing on the effects of mutations in Notch1 and p53 genes. While quite common in normal esophageal tissue by middle age, clones carrying Notch1 mutations are much rarer in oesophageal cancer.

• In a mouse model, loss of Notch1 favored cell proliferation and clonal expansion, but did not promote tumor development. Notch1-mutant clones eliminated or slowed the growth of tumors that emerged after exposure to a carcinogen.

• By contrast, p53 mutations are common both in the normal esophagus and in cancer, and drive both clonal expansion and tumor development.

• Such observations challenge the assumption that a positively selected mutation that confers a proliferative advantage will also promote cancer, and support the notion that cell competition limits carcinogenesis.

• “The normal esophagus becomes a Darwinian battleground in which mutant clones fight for survival,”

• The overall mutational landscape determines cancer risk based on the number of “good mutations,” like Notch1, and “bad mutations,” like p53, one carries. If we can understand the phenotype of mutant clones, we might be able to devise strategies to neutralize the advantage of certain clones and curtail cancer risk,