BIL 250 - Lecture 13

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The Genetics of Cancer
Regulation of cell number and division

Key Ideas

The cells of more derived eukaryotes contain mechanisms that control their survival and ability to proliferate.

These cells constantly evaluate their own condition via continuous communication among neighboring cells and tissues. Survival and proliferation controls are highly integrated and dependent on these inter-cellular communications.

A normal cell's proliferation is regulated at the level of the cell cycle (mitosis).

Apoptosis (from the Greek apo meaning "from, or away" and pto meaning "fall"), or programmed cell death, (The word is pronounced ah poh toh' sis, NOT a pop toh' sis; the second "p" is silent.) is a normal process by which cells are destroyed by intra- and extra-cellular mechanisms.

Cells may be triggered into an apoptotic cycle if they are damaged, dangerously abnormal, or needed only transiently during development.

Intercellular signaling systems allow organized cell proliferation and apotosis to proceed within any given population of cells.

In cancer cells, proliferation and apoptosis mechanisms have failed due to mutations in normal tumor-suppressing genes, preventing self-destruct mechanisms from operating. Cancer cells are immortal and highly proliferative.

Many of the genes in which mutations cause cancer are those which contribute either directly or indirectly to the normal control of growth and differentiation mechanisms in the cell.

Early detection and treatment of cancer is becoming more sophisticated with the application of functional genomics (i.e., discovering not only what genes are in the genome, but what they code for).

To understand what happens when a Cell Goes Bad, we must first understand the behavior of a Nice, Normal Cell....

Normal cell proliferation is necessary for

growth and development (stem cells are totipotent)
- totipotent stem cells can develop into an entire, new organism
- pluripotent stem cells can develop into the three germ layer cell types, but cannot give rise to an entire organism.
- multipotent stem cells can give rise to several cell types, but these are limited in variety. (e.g., haematopoietic blood stem cells)
replacement of destroyed cells

Cell death is necessary for

removal of cells not needed after a certain point in development
removal of potentially dangerous damaged cells

Cell proliferation and cell death balance one another, and when mechanisms controlling either or both go awry, neoplasia (from the Greek neo meaning "new" and plas meaning "form" or "shape") can result.

Cell Proliferation

Recall that mitosis consists of

M phase (active mitosis)
G1 phase (pre DNA synthesis)
S phase (DNA synthesis)
G2 phase (post DNA synthesis)

Of these, only the G1 phase is variable in length, mainly because of the variation in an optional resting phase known as G₀. The cell must pass through checkpoints--fail-safe mechanisms that won't allow the cell to proceed to one phase until all the parts of the previous phase are complete--during the cell cycle.

Enzymes involved in the proliferation process are

protein kinases - phosphorylate specific amino acid residues on target proteins
- cyclins
- cyclin-dependent protein kinases (CDK proteins)
protein phosphatases - remove phosphates from specific amino acid residues on target proteins
Cyclical variation in the phosphorylation/dephosphorylation of these key proteins determine which ones are active for each portion of the cell cycle.

Cell Death

In multicellular organisms, programmed cell death occurs primarily in somatic cells.

cell proliferation replaces somatic cells lost to cell death.

mechanisms have evolved to eliminate certain cells, and the process of such programmed cell death is known as apoptosis.

Enzymes & cells involved in apoptosis are

caspases - disrupt structural and functional systems of the target cell
scavenger cells - engulf and remove the "carcasses" of cells that have undergone apoptosis

The cell must respond to internal and external environmental cues to know when to proliferate and when to die. These consist of

intercellular chemical signals

receptors of those signals

transduction systems that relay the signal from receptor to other parts of the cell

Cyclins

exist in families of related enzymes, each of which is present only during a specific phase of the cell cycle.

are generated by the previous phase's specific cyclin-CDK complex, which acts as a transcription factor for its gene activation

don't last long in the cell. One type is rapidly degraded and replaced by the next via

quick inactivation of the transcription activator for a particular cyclin's gene (no transcription, no translation)
instability of cyclin mRNA (easily degraded by nucleases)
instability of cyclin protein itself

CDK

Definition: A kinase is an enzyme that phosphorylates proteins.
Definition: A cyclin-dependent kinase (CDK) is activated specifically by cyclin, which binds to it.
The cyclin component of a cyclin-CDK complex determines the target protein of that particular cyclin-CDK. (the cyclin component binds the protein, and the kinase component phosphorylates it.)
phosphorylation is transient and reversible

Variations in cyclin-CDK follow the cell cycle...

One example of a well-known transcription factor activation is the Rb/E2F protein regulation of the G1 to S phase transition in mammals cells...

Apoptosis

This is sometimes referred to as "programmed cell death" and it is

sequential destruction of the doomed cell via
- fragmentation of chromosomes
- organelle disruption
- fragmentation of cell
And it is driven by activity of caspases
- cysteine-containing aspartate-specific proteases
- these are normally present as inactive zymogen (harmless to cell)
- the zymogen form is activated by proteolysis of one part of the zymogen polypeptide, and the inactive zymogen becomes an active caspase.
- such active caspases target other proteins for destruction
- activator caspases respond by cleaving in response to protein signals from other cleaved proteins. These cleave the executioner caspases, activating them.

Proliferation vs. Apoptosis Controls

These are interrelated, and may induce apoptosis in cells that fail to successfully complete some phase of cell cycle.

Intracellular signals

cell cycle negative controls: inhibition of CDK-cyclin (see illustration below)

cell cycle positive controls: activation of CDK-cyclin

mitogens

receptor tyrosine kinases (RTK proteins)

apoptosis positive controls: leakage of cytochrome c from defective mitochondria acts as a trigger for apoptosis

apoptosis negative controls: proteins such as Bcl-2 and Bcl-x block the release of cytochrome c from mitochondria, possibly stabilizing the mitochondrial membrane and preventing its rupture). This maintains the apoptosis system in "off" mode

Extracellular signals

based on cell-cell communication

secreted molecules (paracrine signals act locally, are not sent via circulatory system)

direct cell-cell contact

An example of inhibitory control:

1. DNA is damaged by some mutagen during G1

2. This inhibits the activity of CDK-cyclin complexes

3. The protein responsible for this inhibition is named p53 (and the gene that encodes it, p53); it senses mismatches in the DNA strand.

4. In the presence of such mismatches, p53 protein activates another protein named p21 (encoded by a gene named p21).

5. When p21 is present in high concentration, it binds to CDK-cyclin complexes in the cell, inhibiting their kinase activity and preventing phosphorylation of proteins.

6. Without proper protein phosphorylation, the cell cycle cannot continue until DNA mismatches have been repaired.

7. Inhibitory processes are reversed once the DNA mismatches are repaired, as p53 levels drop in response to lowered levels of DNA mismatches.

8. As p53 levels drop, the binding capacity of p21 also drops.

9. As p21 levels drop, these proteins diffuse off the cyclin-CDK proteins, which then can resume their normal activity.

Mutations in the genes encoding any of these highly specific "tumor suppressor" genes can result in cancer.

Cancer cells are

immortal (do not undergo apoptosis)
highly proliferative
clonal (usually derived from a single aberrant Founder Cell)
malignant/invasive

Many different cell types can be altered to become cancerous. What are the common threads uniting them?

A cancer cell can be considered an aberrent cell with an accumulation of mutations that cause it to lose its proliferation and apoptotic controls. (In other words, a single mutation in a cell is not likely to cause it to become cancerous.) A cell that has a mutation preventing apoptosis will have more time to accumulate proliferation-promoting mutations that will cause it to become cancerous.

Some such mutations can be inherited via the germline (as in familial/heritable cancers)

Not all these genes have full penetrance: individuals having the mutations do not always develop the predicted cancers.

Others can arise de novoin the somatic cell lineage of a particular cell due to mutagenesis.

which suggests that the fewer carginogens to which an organism is subjected, the lower its likelihood of developing cancers

Two major types of mutations are associated with carcinogenesis. Mutations in

Oncogenes
Tumor-suppressor genes

Oncogenes

An oncogene is a dominant mutant gene that contributes to the formation of (animal) cancer.

The non-mutant form of an oncogene is known as a proto-oncogene.

These usually encode a protein active only when proper regulator signals activate them.
Often, these are proteins involved in positive control pathways.
Other proto-oncogenes encode negative controls of the apoptotic pathway.
The mutation of an oncogene uncouples the activity of a protein from its normal regulatory function.
This causes unregulated proliferation and no apoptosis of affected cells.

Oncogenes have been isolated from certain viruses known to have carcinogenic activity.

About 100 different oncogenes have thus far been identified.

Oncogenes can change due to

point mutations
loss of vital protein domain coding regions
gene fusion (as in the case of the Philadelphia chromosome. More on that in a moment.)
The carcinogencity of these genes is associated with the activation of certain proteins in cells containing at least one copy of a dominant allele.

Tumor Suppressor Genes

These are genes that encode an active form of protein that ordinarily functions to maintain normal proliferation (e.g., rb; a mutant form of this gene encodes a mutant, non-functional RB protein, and cells containing this mutation proliferate out of control.

Another form of retinoblastoma (HBR) is heritable. People who have inherited this form have the mutation in many cells, and suffer from tumors throughout the body, as well as both eyes. In this form, enucleation of the eye(s) is pointless, as tumors arise in many other locations.

mutant rb cells either carry two copies of a gene with the same mutation, or heterozygous for two different mutations of the same rb gene, meaning that the mutation is recessive.
Strangely enough, though, HBR is passed along as if it were autosomal dominant. How can this be?

If the mutation is inherited via the germline, mitotic crossing over (not uncommon during development) will result in at least some retinal cells will acquire two mutant copies of the gene, resulting in retinoblastomic cells.
Recall the activity of Rb protein, to see how this can be a problem!

Other tumor suppressor mutations involve problems with positive regulation of apoptosis (e.g., p53).

Mutations of p53 are associated with many different types of cancers.
p53 is thus considered a TUMOR SUPPRESSOR GENE
Normal P53 protein is a transcription factor activated in response to DNA damage.
- prevents progression of cell cycle in the presence of damaged DNA
- can induce apoptosis under some circumstances
Mutant p53 cannot induce apoptosis, and cannot halt the cell cycle in the presence of damaged DNA.
This results in an increase in the overal frequency of mutations
If some of those mutations occur in proteins regulating proliferation and apoptosis, CANCER will result.
carcinogenesis of p53 causes cancer indirectly, as it simply causes an elevated rate of retained mutation, increasing the chances of carcinogenic mutations.

Mutations of any of these genes result in inactive forms of the protein, allowing uncontrolled proliferation and/or lack of apoptosis.

Tumor suppressor mutations are generally recessive.

Tumor-promoting mutations were first identified by study of cells from cancer patients in single families showing the same type of cancer. These shared cancers variously showed

similar mutant sequences in specific genes
characteristic translocations or deletions of certain chromosomal regions

As you can see, a wide variety of mutations can potentially cause cancer, and cancer cannot be considered a single "disease." It is a failure of normal gene function.

As geneticists continue to work in the realm of FUNCTIONAL GENOMICS, determining not only the sequence of our genes, but their function, greater headway will be made in finally finding the answer to controlling this genetic disorder.

(Will you be the one to find the key?)

The Genetics of Cancer Regulation of cell number and division