Neoplasia 4 Dr. Hiba Wazeer Al Zou’bi

4- Nuclear Transcription Factors: DNA transcription regulated by genes e.g. MYC, MYB, JUN, FOS, REL oncogenes  regulate the expression of growth promoting genes such as cyclins. MYC protein  Activation of Cyclin Dependant Kinases (CDKs), inhibition of CDK inhibitors (CDKI) MYC mutation  sustained activation Examples: – Dysregulation of MYC present in Burkitt lymphoma due to translocation t (8:14) – Amplified in Breast,colon, lung CA & neuroblastoma

Normal Cell Cycle

-Progression through the cell cycle is regulated by proteins called cyclins (D, E, A, and B), and associated enzymes, the cyclin dependent kinases (CDKs). - The activity of CDK–cyclin complexes is regulated by CDK inhibitors (CDKIs)

5- Cyclins & Cyclin Dependant – Kinases Mutations that disregulate activity of cyclins & CDKs → cell proliferation Examples: – Cyclin D is overexpressed in breast, liver, & esophageal cancers – Amplification of CDK4 gene present in melanoma, sarcomas, glioblastoma The CDKIs frequently are disabled by mutation or gene silencing in many human malignancies

HALLMARKS OF CANCER Self-sufficiency in growth signals Insensitivity to growth-inhibiting signals Evasion of Cell death Limitless replicative potential: Telomerase Development of sustained Angiogenesis Invasive ability Metastatic ability Reprogramming Energy metabolism Evasion of immune system Genomic instability Tumor promoting inflammation

2- Cancer Suppressor Genes:- Growth inhibitory pathway by: * Regulate cell cycle: Rb gene * Regulate cycle & apoptosis: TP 53 * Block GF signals: TGF-  * APC gene

1- RB gene: Governor of the Cell Cycle First studied in Retinoblastoma: - Autosomal dominant hereditary disease (40%), Sporadic (60%) -Two mutations (hits) are required to produce retinoblastoma.These involve the RB gene, which has been mapped to chromosomal locus 13q14. Both of the normal alleles of the RB locus must be inactivated. -Patients with familial retinoblastoma also are at greatly increased risk for development of osteosarcomas A fairly common feature of breast cancer, small cell cancer of the lung, and bladder cancer.

-The RB gene product is a DNA-binding protein -Exists in an active hypophosphorylated state and an inactive hyperphosphorylated state. -The importance of Rb lies in its regulation of the G1/S.

- The initiation of DNA replication (S phase) requires the activity of cyclin E/CDK2 complex -Expression of cyclin E is dependent on the E2F family of transcription factors. -Early in G1, Rb is in its hypophosphorylated active form, binds to and inhibits the E2F family, preventing transcription of cyclin E. - Growth factor signaling leads to activation of cyclin D–CDK4/6 complexes. These complexes phosphorylate Rb, inactivating the protein and releasing E2F to induce target genes such as cyclin E.

- During M phase the phosphate groups are removed from Rb by cellular phosphatases, regenerating the hypophosphorylated form of Rb. - Human papillomavirus (HPV) E7 protein binds to the hypophosphorylated form of Rb, preventing it from inhibiting the E2F transcription factors. Thus, Rb is functionally deleted, leading to uncontrolled growth.

Mode of action of RB gene:

2- TP 53: Guardian of the Genome OR Policeman P53 is activated by anoxia, inappropriate oncoprotein activity (RAS or MYC), and damage to integrity of DNA. Activated p53 → – Transcription of CDKI (p21, prevent RB phophorylation) → temporary cell cycle arrest (Quiescence) at G1 – Result in more time for repair, P53 induces expression of DNA repair genes  Normal Failed repair  Apoptosis or Senescence (permanent cell cycle arrest) P53 loss or mutation  Neoplasia

P53 (Short half life, 20 minutes) is inactivated by its negative regulator MDM2. P53 senses DNA damage through various sensors, like protein kinases e.g. Ataxia telangiectasia mutated (ATM) protein Upon DNA damage or other stresses, various pathways will lead to the dissociation of the p53 and MDM2 complex. P53 can activate transcription of certain repair genes, micro RNAs (inhibit cyclins and BCL2) Proteins encoded by oncogenic HPVs, hepatitis B virus (HBV), and possibly Epstein-Barr virus (EBV) can bind to normal p53 and nullify its protective function

Significance of TP53 mutation: 70% of tumors show homozygous loss of TP 53 (commonest tumor suppressor) Acquired Biallelic loss in P53 in many cancers e.g. colon, breast, lung, leukemia…etc Inherited mutation in one allele in germ line  Lose the second one in somatic tissue  Li - Fraumeni Syndrome  25 fold  malignancy: sarcoma, leukemia, breast carcinoma, brain tumors.

3- TGF-  Potent inhibitor of proliferation : - Transcriptional activation of CDKI - Repression of growth promoting genes such as MYC, CDK2 and 4, Cyclins A and E Mutational inactivation of TGF-  (may alter the type II TGF-β receptor or SMAD molecules that serve to transduce antiproliferative signals from the receptor to the nucleus) seen in 100% of pancreatic carcinoma & 83% of colonic CA

4- Contact Inhibition: When nontransformed cells are grown in culture, they proliferat until confluent monolayers are generated; cell–cell contacts formed in these monolayers suppress further cell proliferation “contact inhibition”. Contact inhibition” is abolished in cancer cells, allowing them to pile on top of one another. E-cadherin (E for epithelial) is atransmembrane protein mediates cell–cell contact. E- cadherins are reduced in many cancers. Mechanisms that sustains contact inhibition mediated by: 1-Tumor suppressor gene NF2.  NF2 gene product (merlin) 2- APC gene (Adenomatous polyposis coli)

Action of APC gene: APC gene exerts antiproliferative effects, and encodes a cytoplasmic protein that regulate the intracellular levels of β-catenin. β-catenin binds to the cytoplasmic portion of E-cadherin. β-catenin can translocate to the nucleus where it acts as a transcriptional activator in conjunction with another molecule, called TcF, and activate cell proliferation β-Catenin is an important component of the so-called WNT signaling pathway that regulates cell proliferation WNT is a soluble factor that can induce cellular proliferation by binding to its receptor and transmitting signals that prevent the degradation of β-catenin.

In quiescent cells, which are not exposed to WNT, cytoplasmic β-catenin is degraded by a destruction complex, of which APC is an integral part With loss of APC (in malignant cells), β-catenin degradation is prevented, and the WNT signaling response is inappropriately activated in the absence of WNT transcription of growth-promoting genes, such as cyclin D1 and MYC, as well as transcriptional regulators, such as TWIST and SLUG, that repress E-cadherin expression and thus reduce contact inhibition.