Molecular Basis of Cancer Pathology Notes

Molecular Basis Of Cancer

The mechanism as to how a normal healthy cell is transformed into a precancerous and cancerous cell is quite complex. At different times, attempts have been made to unravel this mystery by various mechanisms. In the last few decades, a lot of research work has been done to explain the pathogenesis of cancer at the molecular level which is discussed here.

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Theories Of Carcinogenesis

A few mutually-interlinked theories which explain the driving forces behind cancer onset and proliferation are required to be understood first.

1. Monoclonal theory:

There is strong evidence to support that most human cancers arise by genetic transformation or mutation of a single clone of cells. For example:

• In a case of multiple myeloma (a malignant disorder of plasma cells), there is the production of a single type of immunoglobulin or its chain as seen by a monoclonal spike in serum electrophoresis.
• Due to inactivation of one of the two X-chromosomes in females (paternal or maternal derived), normal uterine myometrial cells are mosaic having two types of cell populations for glucose-6-phosphatase dehydrogenase (G6PD) isoenzyme A and B genotypes.

However, in leiomyomas (benign uterine tumour), it is observed that the tumour cells contain either A or B genotype of G6PD i.e. the tumour cells are derived from a single progenitor clone of cell.

2. Tissue organisation field theory:

According to this theory, carcinogenesis is primarily a problem of tissue organisation. Disorganisation of tissue architecture by carcinogenic agents is due to disruption in cell-to-cell signalling and cell-to-stromal cell interaction that alters or mutates the genome and initiates cancer.

The role of the stromal cells in the tumour microenvironment is discussed later. Thus, according to this theory, DNA mutations are the outcome, not the cause, of cellular events in cancer.

3. Somatic mutation theory:

According to this theory, cancer is derived from a single clone of somatic cells that has undergone multiple mutations in its DNA. These mutations occur in the genes that control cell proliferation and the cell cycle.

• The abnormalities in the genetic composition may be from inherited mutations or may be induced by etiologic carcinogenic agents (chemicals, viruses, radiation etc).
• Eventually, the mutated cells transmit their characters to the next progeny of cells and result in cancer. Thus, cancer is the result of DNA-level events.

4. Multi-step theory of cancer phenotype and progression:

The genetically transformed cells having phenotypic features of malignancy— excessive growth, invasiveness and distant metastasis, is preceded by multiple mutations:

• Driver mutations are those which transform the target cells into phenotypic malignant cells.

• Development of cancer phenotype is a multi-step gradual process involving generations of cells beginning with initiator mutation.
• The mutated and initiated cells remain in the host at the preclinical stage as cancer stem cells, most evident in the preclinical stage of acute leukaemias.
• Multiple driver mutations are also involved in the further progression of the tumour. The target cell is attacked by various etiologic agents one after another (multi-hit process).
• For example, in chemical carcinogenesis, there is an attack by initiator and promoter carcinogens in sequence.
• The evolution of colorectal cancer through adenoma-carcinoma sequence involving early APC mutation followed by loss of TP53 later supports the multi-step hypothesis of cancer.
• Another common type of genetic mutation in an early stage of malignancy, particularly in solid tumours, is loss-of-function mutation.
• The loss-of-function mutations cause genomic instability in the target cell which may transform it into a malignant phenotype due to driver mutations, or the target cell may carry passenger mutations without cancer phenotype but it commonly accumulates several acquired mutations for cancer development later.

5. Epigenetic theory:

In addition to genetic mutations in DNA, abnormalities in epigenetic phenomena in cancer have attracted attention in recent times, especially because it is possible to counteract epigenetic modifications by drugs.

• The basic concept of epigenetics in normal cell biology Errors in epigenetic processes which may appear in cancer are in DNA methylation and histone modification.
• These epigenetic changes are quite widespread; therefore for successful epigenetic therapy, drugs are being developed which have to target specifically abnormal cells.
• Above stated theories do not conflict with each other but instead come into confluence and complement each other into a unifying concept of carcinogenesis which is explained on a molecular basis below.

Basic Concept Of Molecular Carcinogenesis

In normal cell growth, regulatory genes control mitosis as well as cellular ageing, terminating in cell death by apoptosis.

In normal cell growth: There are 4 regulatory genes:

1. Proto-oncogenes are growth-promoting genes i.e. they encode for cell proliferation pathway.
2. Tumour-suppressor genes or antioncogenes are growth-inhibiting or negative regulators of cell proliferation.
3. Apoptosis regulatory genes control programmed cell death.
4. DNA repair genes are those normal genes which regulate the repair of DNA damage that has occurred during mitosis and also control the damage to proto-oncogenes and anti-oncogenes.

In cancer:

The transformed cells are produced by abnormal cell growth due to genetic damage to these normal controlling genes. Thus, corresponding abnormalities in these 4 cell regulatory genes are as under:

1. Activation of growth-promoting oncogenes: Causing transformation of the cell (a mutant form of normal proto-oncogene in cancer is termed oncogene).
   • Many of these cancer-associated genes (i.e. oncogenes) were first discovered in viruses and hence were named as v-onc.
   • Gene products of oncogenes are called oncoproteins. Oncogenes are considered dominant i.e. mutation of single-copy gene may transform the cell into a cancer cell.
2. Inactivation of tumour-suppressor genes: Inactivation of antioncogenes) permitting the cellular proliferation of transformed cells.
   • Tumour suppressor genes are active in recessive form i.e. loss of both alleles required for transformation of the cell to a neoplastic cell.
3. Abnormal apoptosis regulatory genes: These may act as oncogenes or tumour-suppressor genes. Accordingly, these genes may be active in the dominant or recessive form.
4. Failure of DNA repair genes: Thus, their inability to repair the DNA damage results in mutations.

Eventually, the evolution of genotypic features of malignancy in the cell shows the phenotypic appearance of the cancer cell, and by several cycles of proliferation, a mass or growth is formed.

• However, not all genotypic tumour cells survive to establish phenotypic growth. Many genotypic malignant cells die by apoptosis, due to deprivation of nutrition, or due to an unsuitable microenvironment.
• Thus, the tumour develops by malignant cells which have survived all odds i.e. ‘survival of the fittest’ holds for cancer cells contributing to form a tumour. The following discussion on molecular carcinogenesis is built on this basic concept.

Anti-Tumour Immune Responses

Although both cell-mediated and humoral immunity are incited against the tumour, significant anti-tumour effector mechanism is mainly cell-mediated.

• Cell-mediated mechanism:  This is the main mechanism of immune surveillance that involves following immune cells in targeting the tumour cells:
  • Specifically-sensitised cytotoxic T lymphocytes (CTL) i.e. CD8+ T-cells are directly cytotoxic to the target cell and require contact between them and tumour cells.
  • CTL are effective against virally-induced cancers, for example, Burkitt’s lymphoma (EBV-induced), and invasive squamous cell carcinoma of the cervix (HPV-induced).
  • Natural killer (NK) cells are lymphocytes which after activation by IL-2, destroy tumour cells without sensitisation, either directly or by antibody-dependent cellular cytotoxicity (ADCC).
  • NK cells together with T lymphocytes are the first line of defense against tumour cells and can lyse tumour cells.
  • Macrophages are activated by interferon-γ secreted by T-cells and NK-cells, and therefore there is close collaboration of these two subpopulations of lymphocytes and macrophages.
  • Activated macrophages mediate cytotoxicity by the production of reactive oxygen species or by tumour necrosis factor.

The effector cell-mediated mechanism against cancer cells involves the following 4 phases:

• 1. 1. Recognition of tumour cells by innate immune cells and their limited killing.
     2. Maturation and migration of antigen-presenting cells and cross-priming of T lymphocytes.
     3. Generation of tumour antigen-specific T lymphocytes and activation of cytotoxic mechanism.
     4. Homing of tumour antigen-specific T lymphocytes to the tumour site to eliminate tumour cells.
• Humoral mechanism: In vivo, anti-tumour humoral antibodies are quite ineffective against cancer cells. However, in vitro, humoral antibodies may kill tumour cells by complement activation or by antibody-dependent cytotoxicity.

The mechanisms of immune responses are schematically illustrated

Escape From Immune Surveillance

Most cancers grow relentlessly despite intact innate and adaptive host immunity. This is because the cancer cells escape the host defences by modulation of the immune system:

Cancer immunoediting:  This is the process in which the immune system shapes the malignant disease process in one of the following ways:

• • Eliminate cancer cells by the mechanism explained above.
  • Develop an equilibrium between growing cancer cells and immune competence to eliminate tumour cells. This includes the tumour-dormancy stage or slow-growth stage that may last for years.
  • The last stage is an escape of the cancer cells from the host immune attack and they continue to grow relentlessly

Escape mechanisms:  How cancer cells evade immune detection and elimination is explained by the following mechanisms:

• • Lack of tumour-antigen recognition by either alteration in the tumour cells or effector immune cells.
  • Resistance to cell death by immune hyporesponsiveness to reactive oxygen species and resist cell death.
  • Induction of immunologic ignorance and tolerance through immunosuppressive factors secreted by the tumour cells and the stromal cells.

Our greater understanding of host immune mechanisms against cancer has opened possibilities for therapeutic manipulation of the immune system.

• While there is no magic bullet against cancer, immunotherapy protocols are being developed and increasingly being used as treatment against cancer in combination with other therapies (surgery, radiation, chemotherapy).
• The above properties of cancer cells are schematically illustrated

Molecular Basis of Cancer:

• For understanding cancer at the molecular level, mutually interlinked theories form the basis of carcinogenesis. These are monoclonality, tissue organisation field theory, somatic mutation theory, multistep theory, and epigenetic theory.
• In cancer, normal regulatory genes of cellular growth undergo mutation in cancer. Major regulatory genes are growth-promoting oncogenes, tumour-suppressor genes, apoptosis regulatory genes, and DNA repair genes.

Seven molecular hallmarks of cancer have been categorised based on their action:

1. Growth and proliferation permissive components These are oncogenes and tumour suppressor genes.

• Oncogenes are mutated proto-oncogenes (by point mutation, or translocation, or gene amplification).
• There are several classes of oncogenes having different related cancers:
  • GFs (PDGF-β, TGF-α, FGF, HGF),
  • GF receptors (EGFR, cKIT, RET, FMS-like TKR, PSDFR, ALKR)
  • Cytoplasmic signal transduction proteins (RAS, BCR-ABL, JAK2)
  • Nuclear transcription proteins (MYC) and cell cycle regular proteins (CDK4, cyclin D and E).
• Tumour-suppressor genes (or antioncogenes having reverse function than oncogene) in mutated form result in the removal of the brakes for growth; common mutations are loss-of-function, deletion and point mutation.
• Different classes of mutated tumour-suppressor genes in different cancers are:
  • RB gene
  • p53,
  • TGF-β and its receptor
  • APC and β-catenin, others specific for certain tumours (BRCA 1 and 2, VHL, WT 1 and 2, NF 1 and 2, PTEN, PTCH1, CDKN2A).

2. Favouring overall cell survival by mechanisms of altered stress response Cancer cells faced with stresses of several types respond by subverting the process in favour of cancer growth and survival. These responses include:

• Avoiding DNA repair
• Escaping cell death by apoptosis
• Avoiding cell senescence, and
• Recycling intracellular components by autophagy.

3. Sustained perfusion of cancer by vascularisation Cancer cells thrive and metastasise due to vascularisation that provides them oxygen and nourishment. The most common mode is angiogenesis under the influence of VEGF.

4. Non-angiogenic mechanisms also exist and include: co-option of blood supply, development of split vessels and stem cell-like transdifferentiation of tumour cells to form endothelial cells.

The cancer spread by local invasion and distant metastasis Involves a series of events in cell biology called invasion metastasis cascade.

5. Growth-promoting metabolic changes (Warburg Effect) Metabolic alterations in growing cancer cells are:

• Higher acquisition of nutrients (oxygen, glucose, glutamate)
• Altered
• metabolic pathways, and
• Oncometabolites in promoting tumour.

6. Dynamic tumour microenvironment by stromal components Three types of stromal cells constitute TME having a role in growth promotion and in enabling other cancer hallmarks: angiogenic vascular cells, cancer-associated fibroblasts and infiltrating immune cells.

7. Evasion of immune destruction by immune modulation Host immune surveillance against cancer is exerted due to the presence of tumour antigens on cancer cells and anti-cancer responses by host immune cells (CD8+ T lymphocytes, NK cells, macrophages).

However, cancer cells evade immune destruction by cancer immunoediting and by various escape mechanisms