Neoplasia Book Flashcards
What are the four classes of normal regulatory genes?
Proto-oncogenes (promote growth)
Tumor suppressor genes (suppress growth)
Genes that regulate apoptosis
Genes involved in DNA repair
Oncogenes
Genes that induce a transformed phenotype when expressed in cells - most are mutated or overexpressed versions of normal cellular genes (proto-oncogenes)
Mutation of a single allele can lead to cellular transformation (DOMINANT)
Tumor Supressor Genes
Genes that normally prevent uncontrolled growth and when they are mutated or lost form a cell, allow the transformed phenotype to develop.
Both normal alleles of tumor suppressor genes must be damaged for transformation to occur.
What are the two groups of tumor suppressor genes
GOVERNERS (classic tumor suppressor genes) that when mutated lead to transformation by removing an important break in cellular proliferation (e.g. Rb)
GUARDIANS (responsible for sensing genomic damage and lead to cessation of proliferation or induction of apoptosis) e.g. TP53 (guaridan of genome) - mutation does NOT directly cause altered proliferation, it allows acquisition of mutations that can lead to cancer
what are the genetic lesions of cancer
point mutations in PROTO-ONCOGENES result in overactivity of protein.
point mutations in TUMOR SUPPRESSORS reduce or disable the function of encoded protein
what are the karyotypic changes in tumors
common types of nonrandom structural abnormalities in tumor cells are balanced translocations, deletions, and cytogenetic manifestations of gene amplification
Balanced Translocations can activate proto-oncogenes in what two ways
1) overexpression of proto-oncogenes by removing them from normal regulatory elements and placing them under control of an inappropriate, highly active promoter (e.g. translocation of MYC gene on chromosome 8 in Burkitt lymphoma or BCL2 on chromosome 18 in follicular B cell lymphoma under Ig heavy chain gene regulatory elements)
2) creation of fusion genes encoding novel chimeric proteins (e.g. philadelphia chromosome - balanced translocation of chromosome 22 and 9 - in CML)
what is the second most prevalent karyotypic abnormality in tumor cells?
DELETIONS - loss of particular tumor suppressor genes with deletion of other, nonmutated allele leading to loss of heterozygosity
proto-oncogenes may be converted to oncogenes by what process?
AMPLIFICATION - may produce several hundred copies of the proto-oncogene in the tumor cell (e.g. HER2/NEU amplification in 20% of breast cancers)
What two patterns are seen with amplification
DOUBLE MINUTES: multiple small, extrachromsomal structures
HOMOGENOUSLY STAINING REGIONS: derive from insertion of the amplified genes into new chromosomal locations, which may be distant from the normal location of the involved genes; because regions containing amplified genes lack a normal banding pattern, they appear homogenous in G-banded karyotype
MicroRNAs and CAncer
overexpression of miRNAs can contribute to cacinogenesis by reducing the expression of tumor suppressors, while deletion or loss of expression of miRNA can lead to overexpression of proto-oncogenes
Epigenic Changes
reversible, heritable changes in gene expression that occur without mutation. main mechanisms are DNA methylation and histone modification.
cancer cells are characterized by: global DNA hypomethylation and selective promoter localized hypermethylation
Categories of oncogenes include…
GF, GFR, signal transducers, nuclear regulators, and cell cycle regulators
Growth Factors
many cancer cells acquire GROWTH SELF-SUFFICIENCY by acquiring the ability to synthesize the same growth factors to which they are responsive
- glioblastomas secrete PDGF express receptor
- sarcomas secrete TGFa express receptor
GFR and Non-Tyrosine Kinases
Mutant receptor protein deliver continuous mitogenic signals to cells
More common is overexpression of growth factor receptors that can render cells hyperresponsive to growth factors that would not normally trigger proliferation
ERBB1 - EGF receptor is overexpressed in squamous cell carcinoma of lung
ERBB2 (HER2/NEU) - breast cancer
What are the two important genes that encode various components of the signaling pathway
RAS
ABL
RAS Mutation
MOST COMMONLY MUTATED PROTO-ONCOGENE IN HUMAN TUMORS (either point mutation in GTP-binding pocket or enzymatic region essential for GTP hydrolysis that prevent inactivation of RAS)
RAS Pathway
1) Ras is associated with GFRs in an inactive GDP-bound state
2) Receptor binding causes GDP to be replaced with GTP, activating Ras
3) Ras inactivates itself by cleaving GTP to GDP
Mutated Ras inhibits the activity of GTPase activating protein. This prolongs the activated state of ras, which stimulates growth and cell cycle progression through RAF/ERK/MAP kinase pathway or PI3K/AKT pathway
ABL
non-receptor-associated tyrosine kinase whose gene is translocated from chromosome 9 to chromosome 22 where it fuses with breakpoint of BCR gene
hybrid protein (BCR-ABL) maintains tyrosine kinase activity
what is the ultimate consequence of signaling oncoproteins such as RAS or ABL?
continuous stimulation of nuclear transcription factors that drive the expression of growth-promoting genes
MYC Protein
MYC protein can either activate or repress the transcription of other genes
- activates CDKs (products drive cells into cell cycle)
- represses CDK inhibitors (enforces cell cycle checkpoints)
Dysregulation of MYC promoters tumorigenesis by increasing the expression of genes that promote progression through the cell cycle or repressing genes that slow or prevent progression through the cell cycle
What other effect is MYC protein responsible for?
Warburg effect - upregulating genes that promote aerobic glycolysis and increased utilization of glutamine
What is the restriction point in the cell cycle?
G1 to S transition - cells committed to DNA replication
Normal Cell Cycle
1) Cyclins (D,E,A,B) help stimulate progression through the cell cycle
2) Cyclins bind CDKs and activate them, leading to phosphorylation of crucial target proteins (e.g. Rb) that drive cell cycle
3) CDK inhibitors enforce cell cycle checkpoints
4) if DNA damage is too severe to continue when checked at checkpoints, p53 eliminates cell through apoptosis or enters them in a non-replicative state (senescence)
Cell Cycle in Cancer
Mutation in CDKs and cyclins favor cell proliferation - ALL CANCERS have genetic lesions that disable the checkpoint
Increasing expression of CDK4 and cyclin D is common in neoplastic transformation. CDKIs are also frequently disabled by mutations or gene silencing.
What cyclins regulate the G1 to S transition and what do they do?
cyclin D-CDK4
cyclin D-CDK6
cyclin E-CDK2
regulate the G1-S transition by phosphorylating the Rb protein
What cyclins are active in S phase?
cyclin A-CDK2
cyclin A-CDK1
What cyclin is active in G2 to M transition?
cyclin B-CDK1
What are the two families of CDK inhibitors that can block activity of CDKs and progression through cell cycle?
1) INK4 INHIBITORS - act on cyclin D-CDK4 and cyclin D-CDK6 (p16, p15, p18, p19)
2) p21, p27, p57 can inhibit all CDKs
RB GENE is known as the…
GOVERNOR OF THE CELL CYCLE
Two-Hit Hypothesis
- 2 mutations (hits) are required to produce retinoblastoma.
- familial: children inherit one defective copy of the Rb gene in the germline. Retinoblastoma occurs when the other normal RB gene is lost as a result of somatic mutation (AD inheritance pattern)
- sporadic: both normal RB alleles lost by somatic mutation
Homozygous loss of RB gene is a common feature of which tumors?
breast, small cell cancer of lung, and bladder cancer
RB gene product is a DNA-binding protein that exists in which states?
active hypophosphorylated state
inactive hyperphoshporylated state
What does the RB gene regulate?
G1/S checkpoint !!!!!!!!
Loss of normal cell control is central to malignant transformation and at least one of four key regulators of the cell cycle is mutated in most cancers
CDKN2A
Cyclin D
CDK4
Rb
RB gene activity
1) Rb is in its hypophosphorylated active form where it inhibits E2F and prevents transcription of cyclin E
2) GF signaling (e.g. EGF, PDGF) leads to activation of cyclin D-CDK4/6 complex, which phosphorylates Rb
3) Rb releases E2F to induce target genes such as cyclin E
4) cyclin E expression stimulates DNA replication and progression through cell cycle
What are the two mechanisms that RB uses to inhibit E2F?
1) Rb sequesters E2F, preventing it from interacting with transcriptional activators
2) Rb recruits chromatin remodeling protein (e.g. histone deacetylases and histone methyltransferases), which bind to promoters of E2F-responsive genes such as cyclin E and modify chromatin at the promoters to make DAN insensitivie to transcription factors
What else can bind to Rb and render it nonfunctional
oncogenic DNA viruses such as HPV, which encodes proteins (e.g. E7) which can bind to Rb
TP53 Gene is known as the…
GUARDIAN OF THE GENOME
What are some stressors that trigger p53?
anoxia
inappropriate oncoprotein activity (e.g. MYC or RAS)
damage to integrity of DNA
In non-stressed cells, discuss p53
short half-life (20 min) because of its association with MDM2, which targets p53 for destruction
In stressed cells, discuss p53
sensors that include protein kinases such as ATM (ataxia telangiectasia mutated) are activated, which catalyze post-transcriptional modifications in p53 that release it from MDM2 and increase its half-life and ability to drive the transcription of target genes
p53 stops neoplastic transformation via what 3 mechanisms?
1) Quiescence (temporary cell cycle arrest)
2) Sensecence (permanent cell cycle arrest)
3) Apoptosis (ultimate protective mechanism against neoplastic transformation - mediated by several pro-apoptotic genes)
Discuss Quiescence
- Occurs late in G1 phase and is caused mainly by p53-dependent transcription of the CDKI gene CDKN1A (p21), which inhibits cyclin-CDK complexes and prevents phosphorylation of Rb, thereby arresting cell in G1 phase.
- Pause allows cells to repair DNA damage and p53 induces expression of damage repair genes.
More than 70% of human cancers have a defect in which gene?
p53!
Biallelic loss of TP53 gene is found in virtually every type of cancer, including carcinomas of the lung, colon and breast - three leading causes of cancer death
Li-Fraumeni Syndrome
Patient inherit a mutant TP53 allele
Can DNA viruses (HPV, HBV, EBV) render p53 nonfunctional?
YES, just like they render Rb nonfunctional
TGF-B Function?
potent inhibitor of proliferation by activation of growth-inhibiting genes such as CDKIs and suppression of growth-promoting genes such as MYC and those encoding cyclins
Is TGF-B function compromised is tumors?
YES by mutations in its receptor (colon, stomach, endometrium) or mutation inactivation of SMAD genes that transduce TGF-B signaling (pancreas)
In 100% of pancreatic cancers, and 83% of colon cancers, at least one component of the TGFB pathway is mutated.
In many late stage tumors, TGF-B signaling activates what?
eptihelial-to-mesenchymal transition (EMT), which promotes migration, invasion and metastasis
SNAIL and TWIST are transcription factors whose primary function is to promote EMT and are primary candidates for metastasis oncogenes.
In EMT, carcinoma cells do what?
downregulate certain epithelial markers (e.g. E-cadherin) and upregulate certain mesenchymal markers