WEEK 3: Cellular and molecular basis of cancer

Question 1

Why man develops cancer?

Answer 1

• Genetic mutations are responsible for the generation of cancer cells and are thus present in all cancers.
• These mutations alter the quantity or function of protein products that regulate cell growth, division, DNA repair and also apoptosis.

Question 2

Outline the 4 classes of normal regulatory genes which are the principal targets of mutation.

Answer 2

• the growth promoting proto-oncogenes,
• the growth inhibiting tumor suppressor genes
• genes that regulate programmed cell death (apoptosis)
• the Genes involved in DNA repair

Question 3

Define proto-oncogenes.

Answer 3

Proto-oncogenes: normal cellular genes whose products promote normal cell proliferation.

Question 4

Mutations activate proto-oncogenes and transform them into oncogene.

Define an oncogene.

Answer 4

Mutated or overexpressed versions of proto-oncogenes that function autonomously, having lost dependence on normal growth promoting signals.

Question 5

What is an oncoprotein?

Answer 5

Oncoprotein: a protein encoded by an oncogene that drives increased cell proliferation through one of several mechanisms.

Question 6

Describes how cancers grow with the use of growth factors.

Answer 6

Oncogenes induce Constitutive expression of growth factors and their cognate growth factor receptors, setting up an autocrine cell signaling loop.

• Cancers may secrete their own growth factors or induce stromal cells to produce growth factors in the tumor microenvironment.
• Most soluble growth factors are made by one cell type and act on a neighboring cell to stimulate proliferation (paracrine action).
• Some cancer cells acquire growth self-sufficiency by acquiring the ability to synthesize the same growth factors to which they are responsive.
• For example, many glioblastomas secrete platelet-derived growth factor (PDGF) and express the PDGF receptor, and many sarcomas make both transforming growth factor-α (TGF-α) and its receptor.
• Similar autocrine loops are fairly common in many types of cancer.
• Some growth factor receptors have an intrinsic tyrosine kinase activity that is activated by growth factor binding, while others signal by
  stimulating the activity of downstream proteins.

Question 7

Name examples of oncogenes and cancers associated with them.

Answer 7

• HER2 (amplified in breast and gastric cancer and less
  commonly in lung cancer)
• BCR-ABL1 (a chimeric gene present in chronic myeloid
  leukemia and some B-cell acute lymphocytic leukemias)
• CMYC (Burkitt lymphoma)
• NMYC (small cell lung cancer, neuroblastoma)
• EGFR (adenocarcinoma of the lung)
  *CDK4: Sarcoma
  *BRAF: Melanoma
  *B catenin: Hepatoblastoma
  *ERB B1: SCC of the lung

Question 8

Outline 3 basic ways Oncogenes typically result from or are formed?

Answer 8

• Acquired somatic cell point mutations (e.g., due to chemical carcinogens)

Gene amplification (e.g., an increase in the number of copies of a normal gene)

• Translocations (in which pieces of different genes merge to form a unique sequence.

Question 9

The products of most tumor suppressor genes apply brakes to cell proliferation.
* abnormalities in these genes lead to failure of growth inhibition.
* Tumor suppressor proteins form a network of
checkpoints that prevent uncontrolled growth.

What is the general function of tumor suppressor genes?

Answer 9

• Many tumor suppressors, such as RB and p53, are part of a regulatory network that recognizes genotoxic stress from any source and responds by shutting down proliferation.

Question 10

What is the other name for p53?

Answer 10

• p53, Guardian of the Genome

Question 11

State the functions of p53.

Answer 11

p53 controls the expression and activity of proteins involved in:

*Cell cycle arrest, DNA repair, cellular senescence, and apoptosis.

• Active p53 upregulates the expression of proteins such as the cyclin-dependent kinase inhibitor p21, thereby causing cell-cycle arrest at the G1-S checkpoint.
• This pause allows cells to repair DNA damage.

Question 12

Describe the phases of the cell cycle.

Answer 12

• G1 phase. Metabolic changes prepare the cell for division.
• S phase. DNA synthesis replicates the genetic material.
• G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
• M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).

Question 13

What happens at checkpoints?

Answer 13

Checkpoints ascertain that individual stages of the cell cycle are completed correctly and ensure that incompletely replicated DNA is not passed onto daughter cells.

Question 14

• normal cells proliferation and progression through the cell cycle is strictly regulated by groups of proteins that interact with each other in a specific sequence of events.

What are the ‘master protein kinases’ that drive progression through the different phases of the cell cycle by phosphorylating and activating other
downstream kinases.

Answer 14

cyclin-dependent kinases (CDKs).

Question 15

Name the activating subunits that CDK activity is dependent on the presence of.

Answer 15

• CDK activity is dependent on the presence of activating subunits called cyclins.

Question 16

What tightly regulates Cyclin-CDK complexes?

Answer 16

• Cyclin-CDK complexes are further tightly regulated by CDK inhibitors.

Question 17

• CDKs are key regulatory enzymes involved in cell proliferation through regulating cell-cycle checkpoints and transcriptional events in response to extracellular and intracellular signals.

*What happens when there is dysregulation of CDKs?

Answer 17

• Dysregulation of CDKs is a hallmark of cancers, and inhibition of specific members is considered an attractive target in cancer therapy.

Question 18

Name a gene dubbed as the governor of the cell cycle.

Where was it discovered?

Which part of the cell cycle does this gene negatively regulate?

What else does this gene control?

• It exists in an active ______________state in quiescent cells and an inactive _______________state in cells passing through the G 1 /S cell cycle transition.

Answer 18

The governor of the cell cycle is the retinoblastoma gene discovered in an eye tumor called retinoblastoma.

• RB, a key negative regulator of the G 1 /S cell cycle transition, is directly or indirectly inactivated in most human cancers.
• RB also controls cellular differentiation.
• It exists in an active hypo-phosphorylated state in quiescent cells and an inactive hyper-phosphorylated state in cells passing through the G 1 /S cell cycle transition.

Question 19

Describe how the RB controls cellular differentiation.

Answer 19

• Hypo phosphorylated RB in complex with the E2F transcription factors inhibits the cell cycle.
• When RB is phosphorylated by the cyclin dependent kinases, it releases E2F.
• Then E2F activates transcription of S-phase genes.
• This in turn enhances mitosis.
• The phosphorylation of RB is inhibited by cyclin-dependent kinase inhibitors.

Question 20

• Hypo-phosphorylated, RB exerts antiproliferative effects by binding and inhibiting E2F transcription factors

The antiproliferative effect of RB is abrogated in cancers through a variety of mechanisms, including:

Answer 20

■ Loss-of-function mutations affecting RB
■ Gene amplifications of CDK4 and cyclin D genes
■ Loss of cyclin-dependent kinase inhibitors (p16/INK4a)
■ Viral oncoproteins that bind and inhibit RB (E7 protein of HPV)

Question 21

• Genes that regulate apoptosis primarily act by ________________, rather than stimulating proliferation per se.
• Genes of this class that protect against apoptosis are often _____________in cancer cells, whereas those that promote apoptosis tend to be
  _________________or functionally inactivated by mutations.

Answer 21

• Evasion of Programmed Cell Death (Apoptosis)
• Genes that regulate apoptosis primarily act by enhancing cell survival, rather than stimulating proliferation per se.
• Genes of this class that protect against apoptosis are often over-expressed in cancer cells, whereas those that promote apoptosis tend to be
  under-expressed or functionally inactivated by mutations.
• The pathways that induce timely cell death are disabled in many human cancers
• intrinsic or extrinsic pathways, both of which result in the activation of a proteolytic cascade of caspases that destroys the cell are inactivated,
  leading to immortalization of cells.
• Example:

85% of follicular B-cell lymphomas, the anti-apoptotic gene BCL2 is overexpressed due to a (14;18) translocation.

Question 22

What are telomeres?

What is the function of telomeres?

Describe how telomerase result in immortality in cancerous cells.

Answer 22

1. Telomeres are regions of repetitive DNA sequences at the end of chromosomes.
   A telomere is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes.
2. Telomeres protect the ends of chromosomes from becoming frayed or tangled.
3. Telomerase is an enzyme that adds genetic units onto the telomeres, which are the protective caps at the ends of chromosomes.

Telomeres shorten with each cell division, and when they become too short, the cell stops dividing or dies.

Telomerase is active in most cancer cells, but not in most normal cells. This allows cancer cells to maintain their telomeres and avoid senescence or cell death, essentially becoming immortal.

Question 23

Describe how mutation of DNA repair gene result in cancer.

Answer 23

• Loss-of-function mutations affecting DNA repair genes contribute to carcinogenesis indirectly by impairing the ability of the cell to recognize and repair nonlethal genetic damage in other genes.
• As a result, affected cells acquire mutations at an accelerated rate, a state referred to as a mutator phenotype that is marked by genomic instability

Question 24

What is multistep carcinogenesis?

Name the best characterized example supporting the theory of multi-step carcinogenesis.

Answer 24

1. Multistep carcinogenesis is the process of transforming a normal cell into a cancer cell through multiple stages involving genetic or epigenetic changes.

These stages include initiation, promotion, progression and malignancy.

2. The best characterized example supporting the theory of multi-step carcinogenesis is colorectal cancer.

• Genetic characterization of a large number of early, intermediate and late adenomas and frank carcinomas led to the establishment of a ‘preferred’
  sequence of genetic alterations during the adenoma-adenocarcinoma pathway of colorectal cancer.

Question 25

All cancers display eight fundamental changes in cell physiology, which are considered the hallmarks of cancer.

Outline them.

Answer 25

• Self-sufficiency in growth signals
• Insensitivity to growth-inhibitory signals
• Altered cellular metabolism
• Evasion of apoptosis
• Limitless replicative potential (immortality)
• Sustained angiogenesis
• Invasion and metastasis
• Evasion of immune surveillance