Cancer Bio Exam 2

Mechanisms for Loss of Heterozygosity

1. Mitotic Recombination: chromosome crossover in G2 and M phases (Frequency = 10^-4 per cell cycle division)

2. Chromosomal Non-disjunction

3. Gene Conversion: jumping of DNA polymerase between chromosomes

4. Epigenetic Mechanisms

Epigenetic Mechanisms

1. Histone Modification (acetylation, methylation, ubiquitination, phosphorylation)

2. DNA methylation at CpG nucleotide pairing

Acetylation

Prevents tight binding of heterochromatin to allow access of transcription factors

Histone Methylation

• Lys-4-Trimethylation = On-state

• Lys-9-trimethylation = Off-state

• Writers = methyltransferases

• Readers = bind to methylated groups for further molecule recruiting

Histone Modification + DNA Methylation Example

1. G9A methylates Lys-9 in histone H3

2. HP1 binds methylated H3K9 and recruits DMNT1

3. DMNT1 methylates the cytosine residues of CpG dinucleotides to block transcription factor binding

INK Family CDKIs

• Encoded for by CDKN2A in senescence pathway

• Inhibit CDK 4/6 - Cyclin D complex formation

• Act early in G1 phase

• Bind CDK in absence of Cyclin

Cip/Kip Family CDKIs

• Act on all other CDKs besides CDK4/6

• Bind in the cleft of CDKs to prevent ATP binging

• p21, p27, p57

CDK4/6-Cyclin D to CDK2-Cyclin E Transition

1. Mitogenic Signaling induces Cyclin D production (Ras pathway) and Cyclin D forms complex with CDK4/6

2. p21/p27 stabilize CDK4/6 complex

3. CDK4/6-Cyclin D complex hypophosphorylates Rb leading to partial access by transcription factors and E2F and Cyclin E transcription

4. Cyclin E then binds CDK2, but complexes are initially inhibited by p21/p27

5. Tyrosine Kinase (SRC) phosphorylation of p21/p27 allows for weak CDK2/Cyclin-E activity

6. Cyclin E-CDK2 complex phosphorylates p27 (Thr187) creating a phosphodegron.

7. Ubiquitin Ligase Complex SCF-SKP2-CKS1 recognizes and ubiquitinates phospho-p27.

8. As CKIs are degraded, CycE-CDK2 activity increases causing Rb hyperphosphorylation and strong E2F activity

9. Commitment to the S phase is complete

10. CDK4/6 activity declines due to loss of stability by p21/p27 and Cyclin D degredation

Results of Damage at the G1 Checkpoint

ATM and ATR Activation

1. ATM is activated and activates/stabilizes p53 and sequesters MDM2 → p53 transcribes p21 which inhibits cyclin A-CDK1 complex

2. p53 also transcribes 14-3-3 and activates Wee 1 which inhibits CDK1-Cyc B

3. ATR inhibits Cdc25A which inturn inhibits CDK1-A and CDK1-B

Intrinsic Apoptotic Pathway

1. BH3 only proteins interact with BAX/BAK at the mitochondrial membrane to cause oligimorization of the mitochondrial membrane and the releace of cytochrome c

2. Cytochrome c interacts with Apaf 1 to activate initiator caspases

3. Initiator caspases activate executioner caspases

Cene Expression Induced by p53

1. p21 Cip → inhibitor of CDKs

2. BAX,PUMA<NOXA → mitochondrial pore proteins

3. PTEN → reduces levels of anti-apoptotic PIP3

4. Represses expression of BCL-2

Associated Causes of Senescence

1. Oncogenic signaling (G12V Ras) (B-Raf melanocytes)

2. Age (senescence associated B-galactosidase)

3. DNA damage (BRCA 1 mutation in mice embryos resulted in increase B-galactosidase signaling)

4. Therapeutic pressure (induced by taxol chemo drugs that prevent the formation of the mitotic spindle)

Senescence Cell Anti-apoptotic Pathways (SCAP)

• BCL-2 proteins that sequester BH3-only proteins

• PI3K/AKT signaling

• p21 and p53 modulators

Extrinsic Mechanism of Senescence Associated Secretory Phenotype (SASP)

• VEGF → angiogenesis

• MMP → degrade ECM, promote invasion

• IL6, CCL5 → promote proliferation

• CCL2 → immune evasion

• Myeloid Derived Suppressor cells → produce PPL1 that interacts with PD1 on killer T-cells to prevent immune killing

Navitoclax

Targets BCL-2 and BCL-xL to prevent sequestering of BH3 only compounds

Dasatinib

Inhibits TYR kinase signaling crucial to survival signaling (SRC)

Quecertin

Inhibits AKT/PI3K pathway to increase oxidative stress

Fisetin

Reduces BCL-2 proteins to increase oxidative stress

Piperlongumine

Elevates reactive oxygen species selectively within senescent cells

Temsirolimus

mTOR pathway inhibitor