Topics A58-62. Neoplasm 2: Oncogenes, Tumor Suppressor Genes, DNA Repair, Telomeres, Epigenetics, Genetic Cancers

Question 1

What are the 4 things that can be genetically altered which relate to cancer development?

Answer 1

1. Oncogenes
2. Tumor suppressor genes
3. DNA repair genes
4. Telomer and telomerase

Question 2

What is the difference between a protooncogene and an oncogene?

Answer 2

Protooncogene: normal gene that produces proliferatory signaling molecules to help the cell grow. The production is coordinated, can turn on and off

Oncogene: mutated form of a protooncogene that functions autonomously, overproducing the normal growth-promoting signals

Question 3

What are the 4 types of mutations to know related to oncogenes?

Answer 3

1. Point mutation / Deletion / Insertion
2. Translocation: Promoter Exchange
3. Translocation: Fusion gene. (Breaking in the exons -> fusion of 2 genes -> new hybrid gene -> new protein)
4. Gene amplification: Everything is fine, but just coding unusually high amount

Question 4

What is an example of point mutation creating an oncogene?

Answer 4

EGFR: Epidermal Growth Factor Receptor. Importantly controls cell growth and proliferation, allows MAPK, STAT, and AKT.

With oncogenic mutation, the intracytoplasmic domain mutates -> continuous cell prolferation, transcription, and cell cycle progression

Question 5

What is an example of promoter exchange in oncogene formation?

Answer 5

BCL2 protooncogene: BCL2 is normally an anti-apoptotic protein. BCL2 protein blocks apoptosis by forming heterodimer with BAX/BAK

Chromosome 18 is Ig heavy chain gene, and chromosome 14 where BCL2 is coded.. they break and form a hybrid where BCL2 is coordinated by enhancer/promoter of Ig heavy chain gene.

Question 6

What is an example of gene fusion in oncogene formation?

Answer 6

t(9:22) translocation: “Philadelphia chromosome” - Breakpoint Cluster Region (BCR) is on chromosome 22, fuses with 9 -> chronic myeloid leukemia

Fusion gene activates JAK-STAT, SRC, RAS etc all cell proliferation, plus suspends BCL2 activity in blocking apoptosis

Question 7

What is an example of gene amplification?

What are 2 ways that gene amplification may appear / be physically manifested?

Answer 7

HER2 is amplified in 30% of breast cancers. HER2 is an epidermal growth factor-like receptor (EGFR)

May see extended chromosome regions, or see small aggregates of extrachromosomal DNA called “double minutes”

Question 8

What is the double-hit theory (of Knudson)?

Answer 8

Because we have two alleles, just one mutating is not usually enough to be a problem. Have to have a “double hit” - both alleles mutated. Usually one allele is inactive from hereditary cause, and then the other one is mutated sporadically (point mutation etc). First mentioned for retinoblastoma.

Doesn’t apply to oncogenes, but it does apply to tumor suppressor genes

Question 9

What are tumor suppressor genes?

Answer 9

Genes that encode proteins that inhibit cellular proliferation, for example by regulating the cell cycle

Question 10

What are the 5 ways that tumor suppressor genes can be mutated/inactivated?

Answer 10

1. Loss of Heterozygosity (LOH): both alleles inactivated
2. Point mutation/deletion: causes stop codon, missense etc
3. Hypermethylation: epigenetic inactivation of promoter region
4. Transdominant gene: mutant gene complexes with non-mutant one, but silences the protein
5. Haplo-insufficiency: one allele is silenced, and the other is too weak / insufficient. Need product of both alleles to work.

Question 11

How does the retinoblastoma (RB) tumor suppressor gene work?

Which tumors is RB silencing implicated in?

Answer 11

RB binds to E2F nuclear transcription factor when it’s unphosphorylated, preventing transcription. If RB is deleted, then E2F is free to transcribe uncontrolled.

(This is in G1/S checkpoint regulation. CDK4,6 binds to cyclin D, the complex phosphorylates RB and frees E2F to promote transcription)

Retinoblastoma, osteosarcoma from hereditary neoplasms, but RB sporadic mutation involved in breast, colon, and lung cancer (remember additional mutations must also be present)

Question 12

What is p53 normally linked to?

What happens to p53 if there is a mutation detected in the cell? (3 things)

Answer 12

Normally linked to MDM2, without that it’s degraded quickly.

If there is a detected mutogenic effect, p53 is activated and then activates other genes:

1. Genes for DNA repair
2. Genes that stop cell cycle (like RB)
3. If it can’t be repaired, activates genes that initiate apoptosis (BAX, APAF1)

Question 13

Hereditary mutation of the APC gene is involved in which cancer?
What is the mechanism?

Answer 13

Adenomatosus polyposis coli

β-Catenin Pathway: APC and β-Catenin are together in the “destruction complex” - blocks the activation of cell proliferation signal transcription. If there is WNT signal, β-Catenin separates and forms new complex that promotes transcription.

Mutation of APC in adenomatosus polyposis coli: there is then no downregulation of β-Catenin, so it continues to promote transcription that activates the cell cycle -> colon cancer

Question 14

What are 3 types of DNA repair from lecture? (There are others though)

Answer 14

1. Mismatch repair: if opposing nucleotide is incorrect (not A-T, C-G)
2. Nucleotide Excision Repair: usually repair of thymidine dimers
3. Double-break DNA Repair

Question 15

What are the proteins that perform mismatch repair?

Answer 15

MSH2-MSH6 or MSH2-MSH3 proteins

Colon, GI tract cancers are common from mutations of these proteins

Question 16

When is nucleotide excision repair usually needed?

How is it performed? What are some important proteins involved in the process that, if mutated, cancer can develop?

Answer 16

Commonly needed after UV radiation that makes thymidine dimers that block transcription of DNA.

XPC-R23 (A xeroderma pigmentosa protein) detects the thymidine dimer. Proteins (complex with XPA, XPG, XPC etc) cut out the DNA of one strand then synthesize normal DNA based on complementary base pairs

Mutation of any of the XP_ proteins can lead to skin cancers.

Question 17

What usually causes double-break DNA damage?

What are some proteins involved in its repair?

Answer 17

Usually occurs after ionizing radiation

ATM protein recognizes, then a BRCA1/BRCA2 complex that helps repair.

Mutation of BRCA1 or BRCA2 is involved in breast, ovarian and prostate cancers
Mutation of ATM also involved in cancers.

Question 18

What are telomeres?

Answer 18

Repeating sequences of DNA at the end of the chromosome, which protect the genetic material.

Without telomerase activity (only active in germ, stem, and cancer cells) to renew the telomeres, some part of them is lost with each division. The progressive shortening of telomeres means that cells have a limited capacity to divide - the # of cell cycles is determined by # of telomeres, after which they go into sensecence

Question 19

If the telomeres of a cell are missing, how does the DNA repair system interpret this?

Answer 19

Loss of telomeres means the chromosome edges are interpreted as a double-stranded DNA break. Attempts homologous recombination, but it makes a decentric chromosome that’s large and fragile, breaks. Cycle will occur with breaking, fusion, breaking, fusion, and so on. Causes heterogenous genetic malformation. Every break can make activation of oncogene or inactivation of a tumor suppressor gene.

This cycle may also reactive telomerase. If cells regain telomerase activity, they will be immortalized neoplastic cells with complex genetic malformations

Question 20

Epigenetic changes: which enzyme performs DNA methylation? Which nucleotide does it methylate?

What is the effect of DNA methylation?

Answer 20

DNA methyltransferase

Methylates cytidine (technically deoxycytidine -> 5-methyl-cytidine)

DNA hypermethylation silences the genetic region because the DNA coils around histone more tightly

Question 21

What are some important genes that can be silenced by DNA hypermethylation, thus contributing to cancer development?

Answer 21

Promoters / tumor suppressor genes (RB, APC, ARF, BRCA1, hMLH1, etc)

Question 22

What are microRNAs (miRNA)?

Answer 22

Small DNA fragments that are non-coding but contribute to regulation by inhibiting gene expression.

A form of epigenetic regulation

Question 23

How do miRNAs regulate gene silencing?

Answer 23

miRNA bind to RISC: RNA-Induced Silencing Complex

RISC-miRNA complex finds the complementary sequence of messenger RNA (mRNA), then either blocks translation there, or they degrade the mRNA

Can either silence tumor suppressors, or poor expression of miRNA can allow proliferation of oncogenes.

Question 24

How many different gene mutations are usually necessary in a cascade that leads to cancer?

Answer 24

At least 15-20 mutations

Cancer requires many mistakes to really develop.

Question 25

What is epigenetic regulation?

card should have been in earlier but got deleted bc brainscape is glitchy

Answer 25

Reversible, heritable change that develops during lifetime, occurs without mutation.

Occurs via DNA methylation, histone acetylation, or miRNAs

Question 26

What are the 3 most important examples of autosomal dominant mutations associated with cancer?

Answer 26

1. Retinoblastoma mutation: RB1 mutation is first hit for retinoblastoma, osteosarcoma risk. Still need double-hit.
2. APC mutation: β-catenin etc. First hit. Second hit causes adenomatous polyposis coli
3. Li-Fraumenni Syndrome: p53 mutation. 50x risk of cancer by age of 50: many possible cancers (sarcomas, breast cancer, etc)

Question 27

How do autosomal recessive mutations related to cancer differ from autosomal dominant ones?

Answer 27

Autosomal recessives usually have lower penetrance, and are involved in mutations of repair mechanisms

Question 28

What are 3 examples of autosomal recessive

Answer 28

1. Xeroderma Pigmentosa: defects of XPA-XPD. Poor correction of thymidine dimers -> skin cancer
2. Ataxia Teleangiectasia: ATM is defective, homologous recombination repair defective. Related to breast cancer, lymphoma, hepatocellular carcinoma
3. Bloom Syndrome: BLM gene is defective, causing mutated DNA helicase and genomic instability. Related to leukemias, lymphomas, carcinomas.

Question 29

What is meant by “familial cancers?”

Answer 29

Cancer that occurs in families more often than would be expected by chance. These cancers often occur at an early age, and may indicate the presence of a gene mutation that increases the risk of cancer. They may also be a sign of shared environmental or lifestyle factors.

Familiar cancers of uncertain inheritance: no specific marker phenotypes or genes like BRCA1 has been found, yet higher incidence within the family. Virtually all common types of cancers that occur sporadically have also been reported to occur in familiar forms. Siblings have higher relative risk.

Question 30

What are 3 examples of familial cancers of uncertain inheritance? (from Robbins)

Answer 30

1. Breast cancer (not linked to BRCA1 or BRCA2)
2. Ovarian cancer
3. Pancreatic cancer