Oncogenes, Tumor Markers, and Carcinogens

• The Neoplasia VIII – Carcinogenesis lecture was presented on Wednesday, 5th February 2025
• The PointSolutions code for the lecture is FFP2PathL12
• The lecture is for Year 1 Pathology class, and the lecturer is Sally McGrath

Learning Outcomes

• Define:
  • Hypoplasia
  • Hypertrophy
  • Agenesis
  • Metaplasia
  • Carcinogenesis
• Explain the cell cycle and the three different types of cells
• Enumerate tumor suppressor genes and proto-oncogenes and their roles in cancer
• Describe the interaction between environmental carcinogens, and genetic factors in carcinogenesis
• Describe the classes of genes in carcinogenesis and the mechanisms by which they cause uncontrolled cell proliferation
• List the classes of carcinogens with examples
• Explain the concept and uses of tumour markers in tumour management

Cell Proliferation

• Cell proliferation is key for development, maintaining a steady-state, and replacing dead cells.
• Cellular proliferation should be controlled
• Neoplasia is the result of uncontrolled cellular proliferation.

Cell Types

• Labile cells:
  • Have a rapid turnover, and continuously divide
  • Epithelial cells in the skin and GI tract are examples of labile cells
• Stable cells:
  • Have a low turnover.
  • Hepatocytes and renal tubular cells are examples of stable cells
• Permanent cells:
  • Have no turnover.
  • Neurons and cardiomyocytes are examples of permanent cells

Cell Cycle

• The four phases of the cell cycle are:
  • G1 (gap 1): a preparation phase.
  • S: the synthesis of DNA.
  • G2 (gap 2): assembly of the apparatus of the chromosome distribution.
  • M: mitosis.
• Quiescent cells in the resting phase G0 (gap 0) can re-enter the cell cycle
• Permanent cells cannot re-enter the cell cycle

Control of the Cell Cycle

• The cell cycle is tightly regulated.
• Cyclins and cyclin-dependent kinases (CDKs) control cell cycle progression.
• Cyclins and CDKs form complexes that activate transcription factors if no genetic defects are detected.
• Progression through the cell cycle is halted when DNA damage is detected

Checkpoints

• The two main checkpoints are:
  • G1-S checkpoint:
    • Senses DAN damage and prevents progression of the cell cycle.
    • It relies on restriction point Rb protein
  • G2-M restriction point:
    • Ensures accurate genetic replication before cell division

Abnormalities of Cell Proliferation

• Controlled cell proliferation:
  • Hyperplasia
  • Hypoplasia
• Uncontrolled cell proliferation:
  • Dysplasia
  • Neoplasia

Hyperplasia

• Involves an increase in the number of cells and, commonly, the size of the organ.
• It is restricted to labile or stable cells
• Hypertrophy is an increase in the size of the cells.
• Hypertrophy occurs in cells unable to divide

Hyperplasia Examples

• Breast:
  • Physiological example occurs during lactation
  • Pathological example includes hormonal Stimulation
• Endometrium:
  • Physiological example occurs during pregnancy
  • Pathological example includes hormonal Stimulation
• Thyroid:
  • Due to increased TSH
• Hyperplasia can raise the possibility of acquiring genetic aberrations since it creates “fertile soil” in which cancerous proliferations may eventually arise

Atrophy

• Atrophy involves a reduction in the size or number of cells which leads to a decrease in the size of the organ
• Examples:
  • Breasts experience atrophy as a result of loss of endocrine stimulation
  • Foetal development is a cause of atrophy. For example, thyroglossal duct atrophy
  • Atrophy of the brain

Hypoplasia and Agenesis

• Hypoplasia:
  • Failure of an organ to reach its anticipated size
• Agenesis:
  • Failure of an organ to develop

Metaplasia

• Metaplasia: -A change from one adult type to another adult type.
  • It acts as a protective mechanism
  • Involves genetic stem cell reprogramming
  • Squamous metaplasia of the bronchus from smoking.
  • Barrett's esophagus: changing from squamous epithelium to glandular epithelium

Dysplasia

• Dysplasia disordered growth of neoplastic epithelial cells
• Dysplasia does not invades the basement membrane
• Dysplasia precedes carcinoma
• Carcinoma in situ refers to the replacement of the full thickness of the epithelium

Carcinogenesis

• Is a sequence of events caused by uncontrolled cell proliferation
• Leads to the development of malignant neoplasm
• Cancer formation is initiated by DNA damage of stem cells
• DNA damage overcomes DNA repair mechanisms without killing cells, leading to an accumulation of mutations in genes that control growth and division

Epidemiology of Cancer

• Age: Most cancers occur in adults older than 55
  • Cancers occur due to long term carcinogen exposure causing multiple genetic alterations
  • Immunosuppression is linked to age
• Paediatric cancers:
  • More likely to be caused by inherited mutations like tumour suppressor genes
• Environmental factors and carcinogens:
  • Chemicals, Radiation, Infectious agents, Smoking, Alcohol, Diet and Obesity

Carcinogens

• Agents work at a variety of levels, involving:
  • Initiation: An event that alters the genome.
  • Promotion: An event causing the genetically altered cell to proliferate
  • Progression: The development of more genetic mutations

Chemical Carcinogens

• Aflatoxins:
  • Derived from Aspergillus contaminating rice and grains
  • Leads to Hepatocellular carcinoma
• Alkylating agents:
  • (chemotherapy) can cause leukaemia and lymphoma
• Alcohol:
  • can cause Squamous cell carcinoma of the oropharynx
• Aniline dye:
  • Is linked to bladder carcinoma
• Arsenic:
  • Is a by-product of metal smelting and leads to Squamous cell carcinoma of the skin/lung cancer
• Asbestos exposure:
  • Lung carcinoma and mesothelioma
• Cigarette smoke:
  • Can cause Carcinoma of the lung and oropharynx
• Nitrosamines in smoked food cause:
  • Stomach carcinoma
• Polycyclic hydrocarbons:
  • can cause Lung carcinoma
• Vinyl Chloride (occupational exposure):
  • can cause Angiosarcoma of the liver
• Silica/nickel (occupational exposure):
  • associated With lung carcinoma

Viruses that Cause Cancer

• Epstein Barr:
  • Can cause Nasopharyngeal carcinoma and Burkitt's lymphoma
• HPV (16,18) - High risk
• Can cause Squamous cell carcinoma of the vulva, vagina, anus, cervix and adenocarcinoma of the cervix and Oropharyngeal carcinoma
• Hepatitis B&C Viruses:
  • Can lead to Hepatocellular carcinoma
• Herpes type 8:
  • Leads to Kaposi's sarcoma

Radiation

• Ionizing radiation:
  • Can occur from nuclear reactor accidents
  • Example is Hiroshima and Chernobyl
  • Radiotherapy
  • Can lead to leukemia
  • Can lead to Papillary carcinoma of the thyroid
• Nonionizing (UVB):
  • Can lead to Basal and Squamous cell carcinoma and melanoma

Epidemiology of Cancer

• High levels of hormones:
  • Breast carcinoma
  • Can be due to Oestrogen (e.g. HRT)
  • Endometrial carcinoma - Oestrogen + Tamoxifen
  • Prostatic carcinoma - Androgens
• Acquired predisposition
  • Chronic inflammation causes:
    • Helicobacter pylori chronic gastritis which leads to gastric adenocarcinoma and gastric lymphoma
    • Chronic inflammatory bowel disease which leads to ulcerative colitis and colonic adenocarcinoma
    • Cholelithiasis which leads to carcinoma of the gallbladder
  • Precursors lesions:
    • endometrial hyperplasia leads to endometrial adenocarcinoma
    • Barrett's oesophagus leads to oesophageal adenocarcinoma.
    • Colonic adenoma leads to colonic adenoma
  • Immunodeficiency:
    • Renal transplant can cause carcinoma of the skin
    • AIDS can cause carcinomas and lymphoma
• Familial cancers:
  • Due to inherited germline mutations in a tumor suppressor gene
• The risk of cancer with BRCA1 & BRCA2 mutations is higher for females borne after 1940, change in reproductive history
• Genetic variation in enzymes that convert procarcinogens to active carcinogens can influence cancer risk
  • Polymorphism of P-450 gens

Genetic Predisposition

• Inherited germline mutations in a tumor suppressor gene
• Familial Adenomatous polyposis coli leads to colonic carcinoma
• Retinoblastomas occur in families
• Breast carcinoma (5%)

Molecular Basis of Cancer

• Caused by nonlethal genetic damage resulting from environmental exposure or spontaneous mutations
• Tumors form by the clonal expansion of a single precursor cell with genetic damage
• Carcinogenesis:
  • Results from accumulating mutations

Gene Abnormalities

• Two types include germline and somatic
• Germ line:
  • An inherited germ line loss of one allele and then loss of the second allele can cause cancers like retinoblastoma; all cells carry the abnormality; found in familial cancer syndromes
• Somatic:
  • Somatic gene loss is acquired, not inherited

Molecular Basis of Cancer

• The principal targets of cancer-causing mutations are:
  • Growth-promoting proto-oncogenes
  • Growth-inhibiting tumour suppressor genes
  • Genes that regulate apoptosis

Proto-oncogenes

• Are present in normal cells and they produce protein to promote cell division
• Oncogenes result from mutation or over-expression of proto-oncogenes. The resulting genes are usually activated regardless
• Mutation causes gain of function and can cause cancer despite the presence of a normal copy of the same gene.
• Oncogenes drive cellular proliferation, causing excess of cell growth

Physiologic Growth Factor-Induced Signaling

• Growth factor binds to its receptor
• Activation of the GF receptor activates cytoplasmic signal transducing proteins
• Signal transmitted to the nucleus
• DNA transcription is activated
• Genes expressed and promote entry and progression into the cell cycle and cell division

Classification of Some Proto-oncogenes

• Growth factors: HGF (overexpression in hepatocellular carcinoma)
• Growth factor receptors: HER2 (amplification in breast carcinoma)
• Signal transduction: KRAS (point mutation in colon cancer); BRAF(point mutations in melanoma and colon cancer)
• Nuclear regulation: MYC (translocation to the Burkitt lymphoma)
• Cell cycle regulators: cyclinD1 (translocation in mantle cell lymphoma); CDK4 (mutation in melanoma)

Tumour Suppressor Genes

• Encode proteins to inhibit cellular proliferation.
• Mutations in tumour suppressor genes lead to a loss of function, which causes the failure of growth inhibition.
• Both genes must be affected before a transformation occurs

Tumour Suppressor Gene Examples

Gatekeeper genes:

• They halt proliferation
• Rb (Retinoblastoma)
• P53
• APC
• Caretaker genes:
  • Responsible for repairing DNA damage
  • BRCA1 and BRCA2
  • DNA mismatch repair genes MSH2 MLH1 MLH6 PMS1 PMS2

Rb

• Is the governor of proliferation
• It is a negative regulator of the G1/S transition
• Sporadic or germ line mutations are possible, but need both copies to malfunction
• Germline mutations can be found in familial retinoblastomas that are autosomal dominant
  • Children inherit one bad copy of Rb and one normal copy and retinoblastoma will develop only when they inherit one mutated normal copy.

Knudson's Two-Hit Hypothesis

• Familial cases are when one defective copy of Rb is inherited with one normal copy
  • The defective copy is present in all somatic cells
  • Disease can be bilateral
• Sporadic cases is extremely rate but both hits must occur within a single retinal cell
• Sporadic occurs more frequently than Familial

P53

• is a “guardian of the genome."
• DNA damage is detected and the cycle is stopped so cell repair enzymes can be deployed.
• Normal P53 = wild type
• Mutated P53 is in a range of tumours
• Mutations can be sporadic or inherited (Li-Fraumeni syndrome)
• Li-Fraumeni syndrome inherited in an autosomal dominant pattern has a risk of sarcomas, breast carcinoma, and other types

Adenomatous Polyposis Coli Gene (APC)

• Sporadic or germline mutations occur
• APC mutations: associated with adenomatous polyposis coli syndrome which is autosomal dominant, resulting in formation of GIT polyps; both copies must be lost

BRCA1 and BRCA2

• Genes act as tumour suppressors
• Transmitted as autosomal dominant
  • Percentage of carriers who develop breast cancer: 30-90%
• Breast cancer:
  • Typically diagnosed early
  • Carriers more susceptible to other cancers (colon, prostate and pancreas
• BRCA 1:
  • Has a high risk for ovarian cancer (30%)
• BRCA 2:
  • Has a high risk for male breast cancer

Mismatch Repair Genes

• Encode for proteins for DNA proofreading. and recognize error.
• Mismatch genes function includes:
  • Repair erroneous insertion, deletion, and also mis-incorporation of bases
• Examples of mismatch repairs include:
  • MSH2
  • MLH1
  • MLH6
  • PMS1
  • PMS2

Mismatch Repair Genes

• Defects lead to micrometastability and accumulation of genetic mutations
• Hereditary nonpolyposis colorectal cancer (HNPCC =Lynch syndrome)
  • Is caused by defect in these genes -Autosomal dominant
• Lynch syndrome 1 increases risk of colon cancer
• Lynch syndrome 2 increases risk colon and non-Gl cancers

Anti Apoptotic Genes

• Cause mutations that lessen cell death and increases survival
• Follicular survival occurs due to BCL2 (anti-apoptotic) expression which allows them to evade cell death

Limitless Replicative Potential

• Cancer cells have limitless replicative potentials and are immortal
• Telomeres shorten in normal cells, which stops replication which kills cells
• Cancer cells express telomerase to prevent telomere shortening

Tumour Markers

• Are proteins produced by tumour cells that can be found in bodily fluids, such as blood, urine, or stool
• Types incude both circulating and tissue markers

Circulating Tumour Markers

• Hormones: -Human chorionic gonadotropin (HCG) – Choriocarcinoma -Calcitonin – Medullary carcinoma of thyroid
• Lineage specific proteins: -Prostatic specific antigen (PSA) -Prostatic carcinoma, may also be elevated in benign prostatic hyperplasia
• Ono fetal antigens: -Carcinoembryonic antigen (CEA): Colon, stomach, pancreas and breast carcinoma, also elevated in non-neoplastic conditions -Alpha-fetoprotein (AFP): Hepatocellular carcinoma, Yolk sac tumour and can be elevated in non-neoplastic conditions -Mucins: -CA-125: Ovarian cancer -CA 19.9: Pancreatic cancer
• Used in diagnostic assessment

Other Facts Tumour Markers

• Diagnosis happens with use with imaging (including biopsies)
• They lack specificity (non-cancerous causes conditions can cause elevations)
• Ex PSA in BPH
• They Lack sensitivity (not everyone with a particular type of cancer will have a marker)
• Can Estimates prognosis and for determination of tumour burden
• Utilised for determination of stage of cancer
• Can detect remaining cancer after treatment (residual disease) -Can detect recurrence after treatment -Monitors treatment and whether treatment has stopped working

Tumour Tissue Markers

• Can indicate which particular targeted therapy to use
• Oestrogen and progesterone receptors in breast cancer tissue can determine for hormone therapy,HER2 receptors to see if Herceptin can be used and PD-L1 to determine use for immunotherapy

Liquid Biopsies

• Tests a blood sample for biomarkers that can indicate cancer, such as circulating cancer cells or cell-free tumour DNA (cfDNA)
• Clinical applications: Is a minimally invasive strategy, can see if treatments are effective, can use on patients who cannot have surgery, to test for cancer cells in body early

Explore oncogenes, tumor suppressor genes, and tumor markers. Learn about the roles of these genes in cancer development and regulation of cell processes. Identify carcinogenic exposures and their association with different types of carcinoma.