Cancer Genetics pt.1

Question 1

What are cancer stem cell?

Answer 1

Subpopulations of cancer cells that can self renew, generate diverse cells in the tumor mass and sustain tumoriogenesis

Question 2

What is the theory on how tumors arise?

Answer 2

The theory is that they originate as a result of mutational hits on normal stem cells, by the transformation of restricted progenitor cells or even the differentiated cells that acquire self renewing capacity.

Question 3

What are proto oncogenes?

Answer 3

They normally promote normal cell growth and encode transcription factors which stimulate expression of other genes, signal transduction molecules that stimulate cell division and cell cycle regulatros that move the cell through the cell cycle. In normal cells, proto onco genes become quiscient

Question 4

What are Oncogenes?

Answer 4

These are cancer causing genes and in tumor cells they are often mutated or expressed at high levels

Question 5

Explain acutely transforming retrovirus steps

Answer 5

Cell is infected with retrovirus and proto oncogene is integrated

new host cell is infected

integration and expression of viral oncogene takes place

neoplastic transformation occurs

Question 6

Explain slow transforming retrovirus infection.

Answer 6

Infection of cell with retroviruse carrying promoter gene

integration of promoter gene adjacent to proto oncogene

conversion of proto oncogene to oncogene

neoplastic transformation takes placve

Question 7

What is a tumor suppressor gene

Answer 7

A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or reduction in its function

Question 8

What are the 2 general groups of tumor supressor with examples of each?

Answer 8

Governors of cell cycle: Retinoblastoma gene

Guradian” P53 gene

Question 9

Explain the role of the Governors like RB gene

Answer 9

They stop the cell cycle at the G1-S phase

They are classic tumor suppressor genes whos mutation leads to transformation of the cell by removing an important brake of cell proliferation

Its active form blocks the cell cyle at the G1-S phase but mutated or inactive forms result in loss of the break

It is recessive

Question 10

Explain sporadic retinoblastoma.

Answer 10

2 hits of mutations are acquired in both alleles after birth

Question 11

explain inherited/familial retinoblastoma

Answer 11

Children inherit one defective copy of RB gene while the other one is normal. During life one hit occurs leading to mutation of the 2nd allele

Question 12

Explain the role of Guardians such as p53 gene.

Answer 12

They are responsible for sensing genomic damage and stopping the cell cycle to prevent proliferation
They also stimulate DNA repair but if he damage is too great, they induce apoptosis by activating BAX and induce Permanent cell arrest

Activated p53 controls the expression and activity of genes involved in cell cycle, DNA repaur, cellular senescence and apoptosis

Question 13

Describe regulatory genes involved in apoptosis.

Answer 13

In normal cell, apoptosis is guided by
Pro apoptotic factors
anti apoptotic factors and cell death receptor CD95(fas)
Normally p53 activates pro apoptotic gene BAX

Question 14

What happens to apoptosis regulation when mutations occur in p53 gene?

Answer 14

p53 cannot activate BAX and over expression of BCL2(anti-apoptotic gene) occurs hence the transformed tumor cells escape from apoptosis.

CD95 receptors are depleted in hepatocellular carcinoma and escape apoptosis.

Question 15

Explain how DNA repair fails when mutation occurs.

Answer 15

During mitosis, normal cells suffer from minor DNA damage which is detected and repaired before mitosis by p53. However when this system becomes damaged, the defect is unrepaired and the defected DNA is passed to the next progeny of cells resulting in cancer

Question 16

Define Xeroderma pigmentosum.

Answer 16

An inherited disorder where there is a defect in DNA repair mechanism and upon exposure to sunlight, DNA damaged by UV radiation cannot be repaired. Such patients are more prone to skin cancer.

Question 17

Briefly describe the epidemiological studies of Breast cancer.

Answer 17

Reproductive and menstrual histories are important factors.
Women who have borne children are less likely to develop breast cancer and the lower her age when having her first pregnancy, the lower her risk of developing breast cancer.

The later the age at menarche, the lower the risk of breast cancer

Question 18

Breifly describe the epidemiological studies of Gastric cancer

Answer 18

People from lower socioeconomic groups have a greater risk of developing gastric cancer

Potential carcinogens include: Specific dietary irritants( salt and preservatives) or potential environmental agents (nitrates)

it is 8X more common in japanese and chinese population than wester europeans.

Question 19

What is chronic myeloid leukemia.

Answer 19

An acquired abnormality found in blood or bone marrow cells but not in other tissue.

Philadelphia chromosome (Ph1) long arm material has been reciprocally translocated to and from the long arm of Ch#9

Question 20

What is Epigenetics?

Answer 20

Refers to DNA and chromatin modification that persist from one cell division to the next despite a lack of change in the underlying DNA sequence

Question 21

Explain Lyonization.

Answer 21

The lyon hypothesis states that during early development, one of the X xhromosomes in a female gets turned off and this is maintained in all descendant cells of the clone

1) condensed X chromosome is genetically in active
2) Its inactivation occurs early in development when the embryo consists of 32 cells
3) at this stage, each of the 32 cells, one of the X chromosomes is randomly inactivated
4) Inactivation is mitotically stable
5) Net effect of this is to equalize phenotypes in males and females for genes that are carried on the X chromosome

N/B: inactivation is due to DNA methylation

Question 22

Explain X reactivation in females.

Answer 22

In the female fetus, future germ cells undergo lyonization along with somatic cells

X inactivation occurs in the early female embryo leading to mosaicism

The inactive X chromosome is the barr body, seen in female somatic cells

Following differentiation of female fetus, the inactivated X chromosomes are reactivated during female gametogenesis

When germ cels develope into oocytes and enter meiodis, their inactivated X chromosomes become reactivated so that every egg produced has an activated X chromosome prior to fertilization

Question 23

What are the fundamental epigenetic mechanisms?

Answer 23

DNA methylation
Chromatin remodelling
Histone modification
Non coding RNAs

Question 24

Explain DNA methylation

Answer 24

This is the addition of methyl groups to certain cytosine in DNA sequence resulting in 5-methylcytosine

Methylation of CpG islands results in transcriptional repression.

Methylation of promoter regions can block access of proteins needed for transcription and serve as binding sites for proteins that condesne chromatin

Question 25

What is chromatin remodelling?

Answer 25

This is the rearrangement of chromatin from a condensed state to a transcriptionally accessible state.

Nucleosomes can be repositioned or removed by chromatin remodelling complexes usually by using energy from ATP hydolysis

Question 26

Briefly explain acetylation

Answer 26

Unmodified lysine residues are positively charged but acetylation results in neutralization of the charge on histones, which reduces the interaction of histones and negatively charged DNA. The charge neutralization results in a weaker histone: DNA interaction, allows transcription factor binding and significantly increases gene expression

Histone acetylation is involved in cell cycle regulation, cell proliferation, and apoptosis and may play a vital role in regulating many other cellular processes, including cellular differentiation, DNA replication and repair, nuclear import and neuronal repression. An imbalance in the equilibrium of histone acetylation is associated with tumorigenesis and cancer progression.

Question 27

Explain Methylation

Answer 27

Methylation is added to the lysine or arginine residues of histones H3 and H4, with different impacts on transcription.

Methylation at H3-K9 and H3-K27 results in gene silencing

Question 28

What are non coding RNA?

Answer 28

Function RNA molecules that are transcribed from DNA but are not translated into proteins.

Their function is to regulate gene expression at the transcriptional and non transcriptional level by interacting with mRNAs

Question 29

Briefly explain RNA interference.

Answer 29

A large double stranded RNA molecule is diced into small double stranded interfering RNAs 21028 base pairs long

The small iRNAs assemble into ribonucleoprotein particles

The iRNA in a ribonucleoprotein particle is unwound to produce an RNA-induced silencing complex

The RISC targets a sequence in a messenger RNA that is complementary to the interfering RNA

The RISCs iRNA base pairs with its target in the mRNA

If perfectly base paired, the mRNA is cleaved and then degraded

if imperfectly base paired, translocation of the mRNA is arrested and polypeptide synthesis from the mRNA is repressed

Question 30

Describe in utero phase of epigenetic programming

Answer 30

In the preimplantation embryo (i.e., the blastocyst) shortly after fertilization
embryonic epigenetic patterns are re-established in a lineage-specific manner in the inner
cell mass of the blastocyst
Active, rapid (paternal) vs. passive (maternal) genome-wide demethylation

in the developing gametes of the fetus
rapid genome-wide demethylation in primordial germ cells (PGCs) to erase existing
parental methylation patterns
re-establishment of epigenetic signatures
(remethylation)
give rise to gametes with sex-specific epigenetic signatures.