Lecture 8- tumor suppressors

Question 1

1) Describe the technique of cell fusion: How does it work, what is a fusing agent and give an example of one. A fused cell with two nuclei is called _______. A fused cell where the nuclei have fused too into one is called ________. How do these multi nucleic cells develop single nucleic daughter cells? How did these experiments suggest that cancerous cells were recessive?
What was the one exception?

Answer 1

• place normal and tumorous cell on same plate, close enough that the cell membranes touch. Use a fusing agents such as PEG to cause cell membrane to degrade and for the two nuclei to be within one cell.
• syncytium
• polykaryon
• When they undergo mitosis, the nucleic membrane breaks and the chromosomes separate properly
• When the tumorogenic hybrid cells were produced, they always had normal cell phenotypes and didn’t cause tumor growth when injected into animals.
• Tumor viruses are the exception, they are dominant.

Question 2

2) Why are cancers caused by tumor suppressor genes more rare than cancers caused by oncogenes? What does loss of heterozygocity mean?

Answer 2

• need to lose both copies of the of a tumor suppressor gene to get cancerous activity

Question 3

3) Name the two types of Retinoblastoma, which one is more severe?

Answer 3

Sporatic, which is de novo, usually only in one eye(unilateral), not as severe.
- familial, both eyes (bilateral), parents had it at a young age and were treated, more severe, much higher rate of secondary tumors.

Question 4

4) Explain the genetic factors that have a role in the chance of a sporatic or familial Rb occurring. Use the terms one hit and two hit model as well as somatic mutation.

Answer 4

• in familial, you inherit one mutant Rb copy, one somatic mutation required to lose other remaining copy= one hit model, more likely
• sporatic, two somatic mutations, two hit model, less likely

Question 5

5) In molecular terms, how are some common ways the Rb gene may mutated? Name 4

Answer 5

• mitotic recombination :during mitosis, rare recombination events may occur where you may get daughter cells that no longer have any functioning copy of the Rb gene.
• Gene conversion: DNA polymerase begins the duplicate the normal template strand of the Rb gene but then jumps to the mutated template strand of the Rb gene and duplicates that, then jumps back. Misses the functional genes.
• non disjunction events occurring where a triploid daughter cells ejected the normal chromosome and now has two mutated ones or one that receives only one mutated chromosome.
• deletion on the long arm of chromosome 13 in the q12 and q14 positions.

Question 6

6) How are tumor suppressor genes found in the genome in cases where LOH events are too small to view by karyotyping. Name 3 techniques used

Answer 6

• with the use of the Esterase D marker, they could determine if the gene was present or not by RFLP, using restriction enzymes to cut at the sites of those markers, use probes that bind complementary to the sequence needed and appears on the gel
• compared the percentage of tumors existed with certain deletions on each arm or each chromosome
• overlapping probes and genetic sequencing

Question 7

7) Differentiate between the term “gatekeepers” and “care takers” in the context of cancer and genetics.

Answer 7

• gate keepers are referred to our tumor suppressor genes: they allow cells to grow properly through the cycles
• care takers are referred to DNA repair enzymes, they help maintain cellular genome integrity

Question 8

8) How do cancerous cells inhibit the ability of tumor suppressor genes?

Answer 8

• they inactivate them by methylation of the CpG islands near their promoters
• HDAC’s up-regulated in cancerous cells, increase the affinity for histones to bind to DNA, which causes hypermethylation of tumor suppressor genes in cancer cells. Meaning they don’t produce their proteins and don’t function.

Question 9

9) How are APC and PTEN classed as tumor suppressors? Think of their functions.

Answer 9

Apc induces the degration of beta-catenin which is a proto-oncogene.
PTEN degrades PIP3 which increases cell proliferation, and prevents apoptosis.

Question 10

10) What does DNA methyltranferase enzyme do?

Answer 10

Adds methyl groups to DNA, methylating them. Derp.

Question 11

11) What is Hypomethylation?

Answer 11

A rare case of over expression of genes, in a case where they are not controlled because of the lack of methylation. Classified as oncogenic activity.