Tumor Suppressir Genes

Question 1

What are the cancer causing genes?

Answer 1

1. Oncogenes-normally stimulate growth
2. Tumor suppressor genes- which normally inhibit growth
3. DNA Repair genes- normally limit mutations

Question 2

What inherited predispositionsbto cancer are caused by tumor suppressor genes?

Answer 2

-Inherited autosomal dominant cancer syndrome (recessive at the cellular level, but with guaranteed loss of second copy)

-Inherited autosomal recessive cancer syndromes of defective DNA repair
Xeroma pigmentosum; Ataxia-telangiectasia; Bloom syndrome ; fanconibanemia

These above are associated with a single, susceptibility

Familial cancers-familial clustering; but role of inherited predisposition not clear for each individual: breast cancer; ovarian cancer; pancreatic cancer

-this one has a multifactorial cause

Question 3

What does wills tumor result from?

Answer 3

Results from loss of function in the WT1 gene on chromosome 11, which encodes a transcription factor important in the control of cell growth and differentiation (renal cancer)

Question 4

Why do tumor suppressor genes require two hits to get cancer?

Answer 4

Genes that cause cancer when they lose their function (‘loss of function’ mutation)

The normal function of these proteins is to stop cancer (control growth)

They are cell cycle control genes, apoptosis promoting genes and/or DNA repair genes

These genes encode for proteins that control cell cycle, proteins that promote apoptosis and DNA repair genes

Therefore, one good gene is enough fir cell cycle control, you need to lose the function of both to get cancer = two hits

Question 5

Explain the two hit hypothesis for sporadic cancers

Answer 5

• A mutation occurs in a tumor suppressor gene on 9ne homologs in one cell (1st hit)
• The second mutation occurs in the same gene (2nd copy) in the same cell
• loss of all tumor suppressor activity
• Cancer/malignant transformation

‘Two hits’ /two mutations are needed for tumor initiation

Question 6

Explain the two hit hypothesis for familial cancer

Answer 6

• first mutation is inherited, and therefore, present in every cell -1st hit
• Second mutation can occur in any cell (2nd hit)
• Loss of all tumor suppressor activity in that cell
• Cancer/malignant transformation

The 1st hit is present in every cell (inherit3d). Only ‘one more hit’ (2nd hit) needed fir tumor initiation

2nd hit usually results in loss of heterozygosity

Question 7

Explain; loss of heterozygosityin tumor suppressor mutations

Answer 7

Mom is homozygous fir normal allele of RB gene

• Dad. And child are heterozygous (contain a mutant allele abs normal allele)
• But the Rb tumor tissue has only a single allele (mutant allele; the normal allele is lost)
• In the tumor tissue, there is loss of normal allele
• Tumor tissue is homozygous fir the mutant allele
• LOH (loss of heterozygosity ) for the RB gene in the tumor (two hit hypothesis)

Question 8

State the mechanisms that produce a second hit?

Answer 8

Loss through non-disjunction

Mitotic recombination

Gene deletion

Point mutation

Question 9

How can epigenetics produce a 2nd hit?

Answer 9

Sometimes aberrant methylation of the gene results in gene silencing(epigenetic mechanism)

The gene is still present but mistakenly silenced (methylated)

Mutations in DNA silencing mechanisms can also result in tumors

Question 10

How does RB protein regulate G1/S phase transition?

Answer 10

At G1/S checkpoint

Cycklin/CDKs hyper phosphorylate RBC

• RBCs no longer represses E2F
• E2F activates S-phase genes

No Cyclin/Cdk

• Rb/E2F complex still binds DNA
• Recruits histone methylated and histone deacetylase
• Result in transcriptional block and G1 arrest

Question 11

What happens in Rb inactivation?

Answer 11

Inactivation of Rb is the critical final step in deriving G1-S transition (I.e. cell division)

Loss of Rb/mutant of Rb, does NOT bind to E2F —> increased transcription of S phase genes (unregulated cell division)

Question 12

Virtually all cancer cells show…

Answer 12

Dysregulation of the G1S checkpoint as a result of mutation in one in four genes that regulate the phosphorylation of RB

The four genes are RB, CDK4, Cyclin D gene, and CDKN2A(p16)

Question 13

What is retinoblastoma?

Answer 13

A childhood cancer

White pupillary reflex in the eye

Incidence of about 1 in 20,000 births

Mutation of Rb gene on chromosome 13

Question 14

Summar8ze retinoblastoma as a familial cancer

Answer 14

With familial cancers, the first hit is inherited (usually germline mutation)

Question 15

Differentiate between familial and sporadic cancer?

Answer 15

Familial form vs sporadic form

• Often multiple tumors and bilateral (familial)
• Familial form present at an earlier age than sporadic form
• Family history often present (one parent is usually affected in familial form )

Question 16

Summarize p53 as a tumor suppressor protein

Answer 16

Transcription factor activated by cell stress, particularly DNA damage and it activates the G1/S checkpoint

P53 is supposed to block cell cycle control/cell birth and activate apoptosis

Loss of p53 control results in net cell growth, and increased mutation frequency, which drives cancer progression.

Mutated in more than 50% of all cancers

Question 17

P53 is regarded as the…

Answer 17

Guardian of the genome

Question 18

How does p53 activate the intrinsic pathway?

Answer 18

p53 drives expression of Bax that promotes apoptosis

Increases expression of the following:

• increases expression of Bax(promotes apoptosis)
• Fas receptor (CD95)
• IGFBP-3 (sequesters cell survival proteins like IGF1/2 away from receptors)
• Activates intrinsic and extrinsic pathways for apoptosis.

Question 19

Explain the genetic basis of LinFraumeni syndrome

Answer 19

• Rare disorder that increases the risk of cancer at a young age
• Results in several kinds of cancer including breast, bone, brain and soft-tissue carcinomas
• 1st hit is inherited from Mom
• 2nd hit is somatic (responsible for LOH)

Families with Li-fragments syndrome have inherited mutations in the TP53 gene

Question 20

What are common inherited cancers?

Answer 20

Colorectal cancer
-familial adenomatous polyposis(FAP)

-Heriditary non-polyposis colon cancer (HNPCC): Lynch syndrome

Breast/ovarian

Question 21

What causes FAP?

Answer 21

Mutations in the APC (adenomatous polyposis coli) gene on chromosome 5 (5q21)

Question 22

Explain familial adenomatous polyposis

Answer 22

Multiple (over 100) adenomatous polyps develop throughout distal colon

• Very high penetrance
• about 1% of colorectal cancers
• tumor suppressor gene (affects B-catenin involved in growth control pathway)
• Autosomal dominance

Many different mutations of the APC gene are reported (allelic heterogeneity)

Question 23

What is the normal APC pathway?

Answer 23

APC is a component of the WNT signaling pathway

APC encodes a tumor suppressor whose role to. Down regulate growth promoting signals

Question 24

Describe the normal APC Pathway with WNT signal present

Answer 24

-APC is a component of the WNT signaling pathway

When WNT signal present

• Destruction complex inactivated
• B-catenin not degraded
• B-catenin moves to nucleus, forms complex with TCF-4
• Activates growth promoting genes

Question 25

Describe APC pathway without the WNT signal

Answer 25

• APC is a component of the WNTsignaling pathway
• NO WNT SIGNAL
• APC interacts with B-catenin
• Triggers phosphorylation of B-catenin
• Ubiquitination and B-catenin degradation
• Resulting in low B-catenin levels
• APC acts like a ‘gatekeeper’

Question 26

How is the APC pathway disrupted?

Answer 26

88% of mutations in FAP

• Mutation APC does NOT. interact with B-catenin in absence of WNT signal
• B-catenin is NOT phosphorylated and NOT degraded
• B catenin interacts with TCF-4 and activates GROWTH (in the absence of WNT signal)

Question 27

Describe progressive/ cumulative nature of cancer in familial adenomatous polyposis

Answer 27

APC mutation is an early ‘gatekeeper’ event. Loss of p53 is a late event, cells that gain oncogene signaling without loss of p53 eventually enter oncogene-induced senescience

Question 28

What causes hereditary non-polyposis colon cancer?

Answer 28

Defects. In mismatch repair

Question 29

explain hereditary non-polyposis colon cancer(HNPCC):Lynch cancer

Answer 29

Mutation of DNA mismatch repair (MMR) genes (caretaker genes)

At least five genes can be responsible (locus heterogeneity)

• MSH2 chromosome 2 - about 60% known mutations
• MLH1 chromosome 3 about 30% of known mutations
• Other MMR genes combined account for 5-8%

Genes/proteins not directly involved in control of cell division = mutator genes

Cells accumulate mutations at rates up to 1000 times higher than normal

Tumors also. Exhibit microsatellite instability (short repetitive sequences of DNA)

Question 30

Contrast FAP & HNPCC

Answer 30

FAP- APC mutation , accelerated tumor initiation, numerous polyp, normal tumor progesssion - gatekeeper

HNPCC- MSH2/ MLH1 mutation, normal tumor initiation, fewer polyps, accelerated tumor progression, microsatellite instability- caretaker

Question 31

Describe the incidence of breast cancer/ ovarian cancer

Answer 31

• Incidence 1/12 in western society
• 1/3 affected women go on to develop aggressive tumors
• up to 20% have familial history
• lifetime penetrance about 85% for breast and 55% (BRCA1) or 25% (BRCA2) for ovarian cancer
• Pancreatic and other cancers also are common
• Male breast and prostate cancer incidence is also increased
• Two significant mutant genes identified so far
  - BRCA1 & BRCA2 (locus heterogeneity)

Question 32

What is the function of BRCA1 & BRCA2?

Answer 32

Genes code for breast cancer type 1. And type 2 susceptibility protein

Found in the cells of breast and other tissue

• Involved in DNA repair or apoptosis when DNA can’t be repaired
• Loss of BRCA1/2, cells can duplicate with dna damage and predisposes to cancer
• Researchers have identified hundreds of mutations in the BRCA1 gene, many of which are associated with an increased predisposition to cancer(allelic heterogeneity)

Question 33

What. Is the effect of epigenetic in tumorigenesis?

Answer 33

• DNA methylation and histone modifications are frequently altered in tumor cells
• Tumor suppressor loci are frequently hypermethylated in cancer cells (silencing of tumor suppressor gene)

Question 34

What are the possible direct roles of epigenetics in tumorigenesis?

Answer 34

• Silencjng of tumor suppressor loci causing cell overgrowth
• Loss of imprinting causing activation of growth
• MicroRNAs

Question 35

What is the effect of MicroRNAs in tumorigenesis?

Answer 35

• miRNAs act to reduce the expression of genes by targeting specific miRNAs
• Aberrant expression/activity of miRNAs could be tumorigenesis
• miRNAs have been shown to undergo changes in expression in cancer cells with frequent amplifications and deletions of miRNA loci

Reduction in miRNA that could have inhibited oncogene RNA e.g. miRNA Upregulation of RAS in lung tumors and Myc in b-cell leukemia’s

Increase in miRNA that inhibits tumor suppressor RNA

Question 36

Explain the use of expression array analysis and cancer

Answer 36

Helps cancer classification

Useful to design appropriate therapy (HER 2+)

Requires molecular determination of genes being expressed in order to determine!
-Rate of proliferation

• capacity for invasion
• potential of metastasis

Determining changes in the expression of large numbers of genes between two groups is possible with microarray analysis 
    Tumors may be grouped as:
       -Tumor vs non-tumors
       -benign vs malignant 
       -primary vs metastatic manner