Molecular Mechanisms of Cancer Flashcards
Trophic factor of primary cells
“Bad” - if trophic factor is removed - cell undergoes apoptosis
Cyclin and CDK control of cell division
E2F transcription factor
The cell cycle can be arrested due to DNA damage in what stage(s) of the cell cycle?
G1 and G2
Maintenance methylase
Ensures that the daughter DNA strand has the same methylation pattern as parent strand - so that same genes will be expressed in daughter cell
Burkitt’s lymphoma
How do transforming retroviruses produce cancers?
By carrying extraviral DNA that encodes and oncogene
How do DNA viruses cause transformation?
They are sometimes, accidentally, incorporated into the host cell genome
List common tumor supressor genes
Tumor supressor gene that functions as a transcription factor
p53
Series of mutations associated with colon cancer
- Loss of APC
- Activation of K-ras
- Loss of DCC
- Loss of p53
Function of Mdm2
Prevents p53 activity in the absencce of DNA damage
The majority of mutations in p53 are located in what region?
DNA binding domain
Caspases
Cysteine proteases that set apoptosis in motion
Induce apoptosis
Presence of trophic factor
PI-3 kinase activates Akt, which phosphorylates bad.
P-Bad is bound by a cytosolic protein, 14-3-3, preventing Bad from inhibiting the antiapoptotitc Bcl-2/Bcl-xl proteins.
Bcl-2/Bcl-xl inhibit Bax channel formation, so cytochrome C stays inside the mitochondria and caspases remain in the inactive (procaspase) forms.
Absence of trophic factor
Bad (soluble pro-apoptotic protein) binds to and inhibits anti-apoptotis proteins (Bcl-2 and Bcl-x) in the mitochondrial membrane.
This allows a membrane-bound pro-apoptotic protein, Bax, to form channels in the outer mitochondrial membrane leading ot the release of cytochrome C.
In the cytosol, cytochrome C interacts with adapter protein, Apaf-1, and promotes activation of the caspase cascade - leads to cell death
The cell cycle
Entry of cells into S and M phases
Strictly regulated
Dependent upon supplies to initiate DNA synthesis and complete replication of the genome
Restriction point
Major checkpoint late in G1
Mitotic cyclins
Clyclins A and B bind during the G2 to M transition
G1 cyclins
Cyclins D and E bind to CDK’s during G1 to S transition
Regulation of cyclin-CDK complexes
CDK inhibitor proteins (CKIs)
i.e. p21Cip, p27Kip, and p16Ink4
E2F
Activation is an early event in the transition from G1 to S phase - transcription factor that induces expression of genes needed in S phase
Retinoblastoma protein (Rb)
Inhibitor of cell proliferation - tumor supressor
Hypo-phosphorylated Rb binds to and inactivates E2F arresting cells in G1 phase
CDK in mid-late G1 (and during S) hyper-phosphorylate Rb causing it to resease E2F (promotes transition of cells from G1 to S
Cell arrest in G1 vs. G2
G1 - prevent copying of damaged bases during DNA synthesis
G2 - alows DNA double stranded breaks to be repaired
Sensors of DNA damage
Include ATR and ATM
ATR - ataxia talangiectasia and Rad3-related
ATM - ataxia telangiectasia mutated
Both ATM and ATR have kinase activity
P53
Transcirption factor necessary for cell cycle arrest due to damaged DNA
Normally p53 is unstable and degraded quickly, so levels are kept very low
DNA damage leads to phosphorylation of p53 - stabilizes p53 - allowing it to accumulate and stimulate transcription of a cyclin-CDK inhibitor gene (21CIP) p21 inhibits cyclin-Cdk activity to halt cell cycle progression
What allows for a small number of transcription factors to regulate a wide variety of genes, contributing to cellular diversty
Combinational gene control
Maintenance methylase
Methylates CG sequences on the duaghter strands which are paired to methylated CG sequences of the parental strand, so methylation pattern is inherited by daughter cell
What can impair fetal DNA methylation during embryogenesis?
Acetaldehyde from alcohol metabolism
Malignant tumors
High nucleus to cytoplasm ratio, prominent nucleoli, many mitoses, and relatively little specialized structure
Presence of invading cells in otherwise normal tissue section is the most diagnostic indication of malignancy
Cancer can result from the mutations in what classes of proteins?
- Growth factors and receptors
- Signal transduction proteins
- Transcription factors
- Cell-cycle control proteins
- Apoptosis proteins
- DNA-repair proteins
Gain of function mutations
pro-oncogenes to oncogenes
Act dominant, only one altered allele will produce and effect in the cell
Loss of function mutations
Mutations in tumor supressor genes
They act recessive (usually), both alleles have to be inactivated before the phenotypic effect is produced
Tumor supressor genes
Function to halt cell division, induce apoptosis, or repair damaged DNA
Coversion of proto-oncogene to an oncogene
Spontaneous or chemical induced GOF mutations that result in a constitutively active protein
Localized duplication (gene amplification) of a region of DNA containing a proto-oncogene - > too much gene expressed
Chromosomal translocations and DNA rearrangements that bring a frowth regulatory gene under control of a different promoter causing over expression of gene
Or produced an actively transcribed fusion gene (part of gene A is combined with part of gene B) resulting in a hyperactive fusion protein
Infection by tumor causing viruses
The neu receptor
aka ErbB-2 or HER2 (human epidermal growth factor receptor 2) can undergo mutation that alters a single amino acid in the transmemebrane region, resulting in an RTK with constitutive tyrosine kinase activity
Herceptin
mAb directed against HER-2 receptor
Loss of the EGF-binding domain of HER1
Produces a truncated protein with unregulated tyrosine kinase activity
These cells are stimulated to proliferate even at concentrations of EGF that do not stimulate normal cells
Cetuximab
mAb that targets HER1 and is used to treat certain cancers, including colorectal and head and neck cancers.
By inhibiting HER1, it helps to slow down or stop the growth of cancer cells.
Ras signaling pathway
Diagram
Mutations in Ras that reduce its GTPase activity
Ras stays active longer than normal - oncogenic
GOF mutation in guanine exchange factors (GEFs)
Result in hyperstimulation of Ras signaling
LOF mutations in GTPase activating proteins (GAPs)
Will prolong Ras signaling
GAPs are tumor supressors
NF1 encodes a GAP that accelerates GTP hydrolysis of Ras
Loss of NF1 function allows Ras to remain in its activated GTP-bound state longer than normal
Individuals with neurofibromatosis inherit one mutant NF1 allele, but do not develop neurofibromas until a second somatic mutation occurs in the other NF1 allele of a cell. The loss of both functional NF1 alleles in the same cell leads to uncontrolled cell growth and the formation of tumors, such a neurofibromas
Proto-oncogenes of the restriction point
Cyclin D1 becomes an oncogene when it is overexpressed or amplified, leading to excessive activation of the cell cycle. This drives uncontrolled cell division, contributing to cancer development by bypassing the normal regulatory mechanisms.
Retinoblastoma gene
RB1is a tumor suppressor, when RB is mutated, E2F is unchecked, leading to uncontrolled cell division and contributing to the development of retinoblastomas. RB is a tumor suppressor; loss of both alleles is needed in the cell for transformation
SV40 DNA tumor virus
Encodes a protein called large T antigen that inhibits two tumor suppressor proteins, Rb and p53, that normally halt cell proliferation.
Large T causes tumors in primates
Human Papillomavirus (HPV)
Expresses two separate proteins; E6 which binds p53, and E7 which binds Rb. If the E6/7 viral genes accidentally integrate into the host genome they can be overproduced in an unregulated fashion resulting in cancer
Acute transforming retroviruses
When a pro-virus integrated next to a proto-oncogene. As the viral genome was copied, part of the proto-oncogene was also copied due to read-through transcription.
The proto-oncogene then underwent a GOF mutation that converted it into an oncogene
Rous Sarcoma Virus (RSV)
Causes fibrosarcomas (in chickens, not humans). RSH was the first characterized retrovirus
Three structural genes (gag, pol, env, and v-src)
V-src is required for cancer induction
c-src
proto-oncogene nearly identical to v-src which is a protein-tyrosine kinase
v-Src is an oncogene - truncated or mutated at the C-terminus which removes regulatory domain in c-src
Human T-cell lymphotropic Virus type 1 (HTLV-1)
A retrovirus that cuases adult T-cell leukemia/lymphoma (ATLL). HTLV-1 contains a regulatory gene, tax, that is a transcriptional activator.
Tax disrupts a cellular tumor suppressor pathway and enhances the expression of proto-oncogenes contributing to transformation
Chromosomal transolacations that cause transformations
- Placing a proto-oncogene under control of a different gene promoter leading to over expression - Burkitt’s lymphoma
- Creating a fusion (frankenstein) protein - Chronic myelongeous leukemia
Burkitt’s lymphoma
B-cell non-Hodgkin lymphoma that results from t(8:14)
c-myc - proto-oncogene, on ch 8 is placed next to an immunoglobulin gene promoter on ch 14. Since immunoglobulins are highly expressed in B-cells, c-myc is now highly expressed. C-myc activateds expression of genes that drive cells through the G1-S transition.
Chronic myelogenous leukemia (CML)
t(9:22)
ch 9 carries thec-abl gene - tyrosine kinase
Flanking region on ch22 carriers the N-terminus of the BCR gene
BCR-Abl fusion protein has consitutive tyrosine kinase activity resulting in phosphorylation of proteins that are not normally substrates of Abl. These proteins are activated to drive excessive proliferation of a clone of hematopoeitic cells in bone marrow.
Classes of proteins that function as tumor supressors
Checkpoint control proteins that halt cell cycle progression (i.e. Rb and p53)
CDK inhibitor proteins (p16 and p21CIP)
Receptors for hormones the inhibit cell proliferation (i.e. TGF-beta receptors)
Pro-apoptotic proteins
DNA repair enzymes
Prominent Tumor Suppressors
Retinoblastoma - spontaneous vs hereditary
P53
Adenomatous polyposis coli (APC gene)
Neurofibromatosis type 1 (NF1 gene)
Genomic instability
When cells bypass senscence and telomeres become critically short
Exposed chromosome ends are seen as DNA breaks and the cell tries to repair these
Creates a highly mutagenic environment
PD-1 Receptor
Programmed Death-1
Receptor primarily expressed on activated T cells
PD-L1
Programmed death ligand-1
Expressed on various healthy/normal cell types
Many cancers upregulate PD-L1 to hide from immune system
What tumor supressor gene is negative-dominant?
p53
Familial adenomatous polyposis
- LOF of adenomatous polyposis coli (APC) gene (a series of mutations, including activation of Ras - specifically K-ras)
- Loss of another TS gene - deleted in colon carcinoma (DCC) gene
- Loss of p53
Murine mAbs
100% mouse proteins
High risk of hypersensitivity rx’ns
Suffix -omab
Chimeric mAbs
~65% human
suffix = -ximab
Humanized mAbs
~95% human
suffx = -zumab
Human mAbs
100% human Abs
suffix = -mumab
Produced in humans or transgenic mice
Imatinib mesylate (Gleevec)
1st cancer drug targeted to singal-transducing protein unique to cancer cells - (BCR-Abl)
Highly lethal to CML cells
2nd gen AbI kinase inhibitors - Dasatinib/Nilotinib are used against Gleevec-resistant tumors
Small molecule inhibitors of the Tyr kinase activity of the EGF receptor (EGFR/HER1)
Geftinib (Iressa)
Erlotinib (Tarveca)
mAb that binds human EGF receptor 2 (HER-2)
Trastuzumab (Herceptin)
mAbs that bind to the external portion of EGFR to prevent binding of growth signals?
Cetuximab (Erbitux)
Panitumumab (Vectibix)
What small molecule drug activates apoptosis of cancerous cells?
Bortezomib (Velcade)
Interferes with the proteasome - accumulates damaged proteins and triggers apoptosis
What drugs target angiogenesis to Tx cancer?
Bevacizumab (Avastin) - binds to the VEGF to prevent angiogenesis
Sorafenib (Nexavar) and sunitinib (Sutent) - small molecule inhibitors of Tyr kinases involved in VEGF signaling
Rituximab (Rituxan)
mAb that Tx’s certain B-cell nonHodgkin lymphomas
Recognizes CD20 on B cells, binds and triggers an immune response resulting in their destruction
mAbs that bind to PD-1 and PD-L1
PD-1 is Tx’d by nivolumab (Opdivo)
PD-L1 Tx’d by pembrolizumab (Keytruda)
The absence of growth factors (trophic factors) activates apoptosis by which of the following events?
A) Stimulation of Akt/PKB phosphorylation of Bad
B) Activating cleavage of proscaspases to mature caspases
C) Closing of Bax channels in the mitochondria
D) Increasing the binding of 14-3-3 pfogdin go Bad
HPV may cause cervical cancer when ________
A part of the HPV genome inadverntently integrates into the host cell DNA and is averrantly expressed
HPV is a DNA virus
Which protein functions to halt the cell cycle if damaged DNA is detected?
p53
Gain of function mutation in the Ras proto-oncogene
Loss of GTPase activity
What enzyme is responsible for preserving DNA methylation patterns during cell division in eukaryotes?
Maintenance methlase
Sorafenib (Nexavar) is a treatment for metastatic renal cancer. What is its mechanism of action?
Inhibits tyrosine kinase activity of some receptor tyrosine kinases as well as a member of the MAP kinase pathway (MAKKK, Raf).
Sorafenib
In the progression of familial adenomatous polyposis (FAP), what is the only gain of function mutation?
GOF of the K-Ras gene
A patient with unresectable, locally advanced, metastatic non–small cell lung cancer undergoes treatment with Bevacizumumab (Avastin). After 6 cycles, tumor growth has halted. The most likely reason for the absence of disease progression in the patient is?
Reduction of tumor-induced angiogenesis due to blockade of Vascular Endothelial Growth Factor (VEGF) signaling.
What familial cancer results from an inherited defect in the tumor suppressor gene, p53?
Li Fraumeni syndrome
Both familial adenomatous polyposis colon cancer and smoking-related lung cancer often involve inaction of what gene?
p53
