CANCER GENETICS :) Flashcards
Knudson’s ‘Two hit hypothesis’ of a single gene
two genetic alterations are required
- one germline & one somatic
- two somatic
BRCA1/2 seen what percent of breast cancer cases?
~5%
Haploinsufficiency of a single gene
loss of single copy of gene is sufficient
Some dominant cancer predisposition phenotypes
see elevated risk in heterozygotes
RB1 - inherited retinoblastoma
APC - familial adenomatous polyposis (FAP)
NF1, NF2
Tumor suppressor gene mechanisms
Regulates rate or cessation cell cycle
Regulate cell death/apoptosis
Repair DNA
Tumor suppressor genes
normal activity of gene is to suppress (or restrain) growth
when mutated and function is loss, growth (cell division) can occur uncontrollably
the number of identified tumor suppressors continues to expand
Mechanism of action: Tumor suppressor genes
- regulate rate or cessation of cell cycle
- Regulate cell death/apoptosis
- Repair DNA
cdk4/cyclin D
promote progression from G1 to S
cdk 2/cyclin E
commit to replication
cdk 2/cyclin A
initiate replication
cdk 1/cyclin B
promote mitosis
Retinoblastoma (Rb)
tumor suppressor, first one discovered
ubiquitously expressed
loss of finction = unscheduled cell proliferation
regulation of cell cycle progression via interaction with E2F to repress E2F dependent transcription of cell cycle genes
promotes cellular differentiation (cell cycle independent)
Rb heterozygotes have clear elevated cancer risk: Knudson’s Two hit hypothesis
Multiple phosphorylations of Rb
inhibits Rb-E2F complex formation and the ability of Rb to arrest the cell cycle
how can DNA viruses cause tumors?
via inactivation of Rb and p53
inhibiting Rb inhibits E2F leading to activation of genes for DNA replication
inhibiting p53 leads to inhibition of apoptosis
How can Rb be inactivated and what results?
results in molecular and functional consequences
occurs by:
-mutation (retinoblastoma, small cell lung cancer, osteosarcoma)
-phosphorylation (breast cancer, melanoma, colon cancer)
-viral oncoprotein (cervical cancer)
Deregulation of Rb pathway
is common in human cancer
some examples Glioma/blastoma, breast, lung pancreas, GI, endometrium, bone marrow, head and neck, liver
p53
tumor supressor
transcription factor induced by stress
promotes growth arrest, senescence and apoptosis
Mutant p53
highly expressed in cancer cells
wildtype is lacking
frequent in most of the common types of human cancer
over expression of wildtype p53
repressed oncogene (MYC, HRAS) transformation of cells
TP53
most frequently mutated gene in human cancer
about half of all tumor specimens studied have mutant p53
tumors that lack mutations in TP53 often have changes in upstream regulators that abrogate p53 function
Hallmarks of tumor suppressor gene
- absence of wildtype gene/protein in tumor cells
- Humans carrying germline mutations should exhibit increased cancer susceptibility (germline mutations in p53 are largely responsible for Li-Fraumeni syndrome)
- Loss of the gene in experimental animal model should confer a cancer prone phenotype - p53 null mice develop cancers (lymphomas) with a high penetrance, early onset
p53 mutant mice had
a short life span
confirms p53 has important role tumor suppression
Li-Fraumeni syndrome
hereditary syndrome that is characterized by early onset cancers of diverse types
-germline mutation in p53 usually the culprit
if MDM2 is overexpressed then
p53 is turned off
what sort of cellular responses does p53 mediate?
Through regulation of p53 target genes -metabolic homeostaiss -antioxidant defense -DNA repair -Growth arrest -Senescence Protein-protein interaction -apoptosis (severe stress)
what sort of stress would active p53 signaling?
oxidative stress, nitric oxide, hypoxia, ribonucleotide depletion, mitotic apparatus dysfunction, oncogene activation, DNA replication, double-strand breaks, telomere erosion
What is the role of MDM2?
inhibits p53
its transcription is also regulated by p53
Some examples of tumors where amplification of MDM2 is seen?
Esophagus
Nasopharynx
Brain and nervous system
Prostate
Phosphorylating p53
stabilizes it.
Mechanisms of mutant p53 function
- Mutant p53 binds DNA to alter gene expression
- Mutant p53 binds to transcription factors to enhance their function
- Mutant p53 forms a complex with transcription factors to prevent their function
- Mutant p53 interacts with proteins to change their function directly
Explain what happens when mutant p53 binds DNA
interacts with DNA directly using mutant p53 binding elements or other regions on the DNA, including MARs, to regulate transcription. some cofactors and other proteins are involved.
ex: EGR1
Explain mutant p53 binding to transcription factors to enhance their function
mutant p53 enhances transcription by forming a complex with TFs that can include transcriptional cofactors and other proteins
response to stimulus mutant p53 is recruited to a transcription regulatory complex that can include TFs, cofactors and other proteins. This mostly results in activation of target gene expression
Stimulated by: TPA, VIT D, DNA damage
Explain how mutant p53 forms a complex with transcription factors to prevent their function
mutant p53 decreases transcription by binding TFs and/or transcriptional cofactors and other proteins, sometimes preventing their binding to DNA. This activity can also involve aggregation of mutant p53 with other proteins
Explain how mutant p53 interacts with proteins to change their function directly
mutant p53 interacts with other proteins, not directly involved in transcriptional regulation, and enhances or blocks their function
level of p53 activity impacts
cancer susceptibility and cancer resistance
mutations cause the mutant p53 function to lie on a continuum where it is not or nothing when a point mutation occurs
what are some roles of p53 outside of cellular survival?
Reproduction Prolonged organismal life span Antioxidant functions Autophagy Glucose metabolism Mitochondrial respiration Embryonic stem cell pools
APC
adenomatous polyposis coli
tumor suppressor
functions in cell adhesion and signaling
loss of APC is among the earliest events in colorectal cancer
70-75% of colorectal tumors - one allele of APC is mutated
FAP - familial adenomatous polyposis, autosomal dominant disorder (APC is truncated) (almost 100% FAP develop cancer by age 60)
What does APC regulate?
Beta catenin in normal and cancer cells
When APC is mutated is no longer able to bind b-catenin for degradation, instead b-catenin is free to translocate to the nucleus and induce gene transcription
BRCA 1 and BRCA 2
Tumor suppressor genes
germline mutation in BRCA1 increases lifetime cancer risk to 85% for breast cancer and 50% ovarian
few sporadic cancers have mutation in BRCA 1
most mutations identified as result in truncated proteins
large nuclear proteins
potential role in repairing double stranded DNA breaks and transcriptional regulation
potential role in G1/S checkpoint control
BRCA is associated with
DNA repair
when inactivated or truncated will see cannot interact properly
failed repair of most genes - leads to cell cycle arrest or apoptosis through functioning p53 and p21
failed repair of p53 or other checkpoint genes - cell proliferation
Tissue type and context play a big role when considering functional loss of tumor suppressor genes (uterus v. prostate v. breast v. blood)
w/r/t PTEN
at ~ 80% of function will start to see dysplasia in the uterus and cancer in the breast and symptoms in blood but prostate remains normal
30% is when start to cancer in the prostate
With low to none PTEN, context becomes important. with Prostate cancer and p53 wildtype there is senescence but aggressive cancer with a mutated p53. Same seen with Blood when normal p53 cases see HSC exhaustion/BM failure while mutated p53 cases see Leukemia
Pt with colon adenocarcinoma. Characterization of tumor after surgical resection finds high levels of mutant p53. This most likely indicates
that the cellular response to stress was abnormal
Oncogenes
gain of function which is bad news
transition from regulated to unregulated cell growth
-chromosomal instability (defects in mitotic chkpts and chromo segregation)
-genomic instability (defective DNA repair and DNA damage chkpts)
-unscheduled proliferation (active mitogenic sensors, defecive mitogenic breaks, overcome oncogenic stress)
-aberrant mitogenic signaling
Proto-oncogene
proteins that are involved in the control of cell growth
becomes excessively active (gain of function) and excessive activity results in uncontrolled cell growth, and now is an oncogene
Five groups of oncogenes
- growth factors
- growth factor receptors
- signal transducers
- Transcription factors
- other -program cell death regulators, bcl2
Growth factors
PDGF, EGF, FGF, NGF
Growth factor receptors
typically receptor tyrosine kinases that consist of extracellular ligand binding, transmembrane, and kinase catalytic domains (inside the cell)
ex: erbB, erbB-2, fms, kit, met, ret, ros, trk
Signal transducers
nonreceptor protein kinases, GTP binding proteins
ex: src, abl, raf-1, H-ras, K-ras, N-ras, gsp
Transcription factors
nuclear proteins regulate expression of genes
ex: erbA, ets, fos, jun, myb, c-myc
How do proto-oncogenes become oncogenes?
gain of function from mutations (point mutations and deletions)
result in structural rearrangements of the protein so that the protein is continuously active
a frequent mechanism in tumors that increases actiivty of K-ras, H-ras and N-ras
Normal Ras signaling
Bound to GEF and GDP GDP exchanged for GTP this turns on Ras GAP removes Pi from GTP to make GDP on bound Ras this turns off Ras Inactive form - Ras-GDP
Ras signaling and cancer
point mutations (at G12 or Q61) convert Ras into GAP insensitive, GTP is constitutively bound and Ras is constitutively activated
when GTP is always bound then Ras is always on
Mutation in Ras results in inability of GAP to bind and cannot facilitate GTP -> GDP for inactivation of Ras
so this process leaves Ras more active
Ras mutations correlate highly with what type of cancers?
Pancreatic cancer - 90%
papillary thyroid cancer - 60%
Colon cancer 50%
Increases in what growth factors are seen with cancer?
EGFR overexpression - colorectal, pancreatic, lung cancer, non small cell lung
EGFR mutation - Glioblastoma
Ras mutation
B-Raf mutation - melanoma, papillary thyroid cancer, colon cancer
Why is a mutation in a down stream cytoplasmic signaling molecule so bad?
little regulation at this level with cytoplasmic proteins
Gene amplification
expansion of gene copy number within the cell (redundant replication of genominc DNA= increased expression of genes)
frequently seen with c-myc (transcription factor)
erbB-2 (HER2/neu) amplified in breast and ovarian cancers
Briefly describe the EGF receptor
extracellular domain - on this there is an area for mAb inhibition
transmembrane domain
Intracellular domain - has a kinase domain where TK inhibition can occur
HER2
orphan receptor (means do not know what binds and regulates)
with no extracellular ligand binding domain
so it is ALWAYS active
this can lead to breast cancer
HER3
Dead kinase - no functional intracellular domain
no catalytic domain
inactive - no ATP binding
HER2 overexpression
results in signaling abnormalities that contribute to tumorigenesis
Heterodimer of EGFR and HER2
increased EGFR
activates Ras, PI3k, PLC
this leads to invasion and cyclin D1/p27
G1/S deregulation
Homodimer of HER2
increased signaling
leads to PAR6 and aPKC activity
loss of polarity
Interaction of HER2, HER3 and src
increased HER3 signaling
activates PI3K which acts on Akt
Leads to Cyclin D1/p27 and g1/s deregulation
see proliferation, survival, metabolism and invasion
Redundant replication of genomic DNA
a section of chromosome that is chunked and duplicated
can amplify tumor ability depending on what genes are duplicated
which depends on proximity
ex: GRB7 seen in resistant HER2 tumors
Chromosomal rearrangement
common in hematologic malignancies
some solid tumors
Mechanisms by which chromosomal rearrangements produce malignancy
- transcriptional activation of proto-oncogenes
2. creation of fusion gene
Transcriptional activation of proto-oncogenes
increased gene expression
most freq. expression of proto-oncogene is controlled by T cell receptor gene or immunoglobin regulatory elements
Creation of fusion gene
portions of two different genes joined into single chimeric gene that has transforming activity
loss of normal regulation
Example of transcriptional activation: c-myc translocations in Burkitts lymphoma
85% cases of Burkitts lymphoma t(8;14)(q24;q32)
this moves c-myc to locus of immunoglobulin heavy-chain gene resulting in constant transcription of c-myc
some cases the light chain chromosome sections (chr 22 or 2) are moved into closer proximity of c-myc (instead of c-myc being inserted into those chains)
Fusion gene: Philadelphia chromosome (super famous-impt to know)
seen in CML - chronic myelogenous leukemia
t(9;22)(q34;q11)
c-abl gene fused to bcr gene which results in increased kinase activity and abnormal cellular localization
t(9;22) also found in 20% Acute lymphoblasitc leukemia (ALL)
there is a gain of function in Abl
How does Abl transform cells?
-Mainly through activating Ras-Raf-CDKs pathway to E2F to increase S phase promoting genes and c-myc
-DOK pathway to activate Ras-Raf-MAPK & PI3K-Akt to E2F
-activate Rac-ERK&JNK to active AP1- mediated gene transcription (serum response)
-Activate JAK-STAT & cytokine dep transcription
Downstream - all of this affects the cell cycle and cytokine production
Bcl-2 (B cell lymphoma-2)
Bcl-2 family has two subgroups
- Prosurvival
- Proapoptotic
