Genes that Predispose to Cancer Flashcards
What are the 2 basic properties of the HSC?
Can generate all lineages
When HSC divides, 1 daughter cell maintains the stem cell compartment
How do cell properties change as the cell lineage progresses?
Loss of replicative potential
Lineage-specific cell surface receptors appear
What enzymes control the cell cycle?
Series of cyclin-dependent kinases (CDKs)
What stimulates the 1st cyclin to activate the cell cycle?
An external stimulus - cytokine acts on cytokine receptor, tyrosine kinase activity stimulates signalling pathway which acts via Ras to activate a TF (e.g. Myc)
TF upregulates transcription of cyclins (e.g. cyclin D) which in turn activate CDKs (e.g. CDK4) to progress the cell cycle (via various other TFs and cyclins)
Give an example of a cytokine receptor involved in control of cell cycle that is implicated in some breast cancers
HER2
What is “R” in the cell cycle? What is its purpose?
Restriction point - check to see if there are adequate metabolic and energy requirements for the cell to progress to divide, and assess DNA for damage
What happens if DNA damage is detected during the R phase?
p53 arrests the cell cycle
What is the role of p53?
Arrests cell cycle in presence of DNA damage by inducing a CDK inhibitor (CDKI) called p16, which acts as a growth inhibitory signal to the cell (suppresses CDK4)
Induces apoptosis if DNA cannot be repaired
List 10 established and emerging hallmarks of cancer
Evasion of apoptosis Autonomous growth signalling Evading growth inhibitory signals Activating invasion and metastasis Immortality Angiogenesis Deregulating cellular energetics Avoiding immune destruction Genome instability and mutation Tumour-promoting inflammation
Give examples of mechanisms employed by 4 different cancers to achieve autonomous growth signalling
CML: Bcr-Abl
ALL: Bcr-Abl
MALTomas: NF-kB
Burkitt’s lymphoma: Myc
Give examples of mechanisms used by 2 different cancers to evade growth inhibitory signals
Burkitt’s lymphoma: Myc
H.pylori: Myc
Give examples of general mechanisms of invasion and metastatic capacity
Cells become GF/anchorage independent
Are able to enter and exit blood vessels
How do cancer cells achieve immortality?
Express telomerases that extend telomeres
Give an example of a mechanism used to evade apoptosis
CLL: Bcl-2, ABT-199
How are cancer cells though to deregulate cellular energetics to their advantage?
Warburg effect: cancer cells increase glucose metabolism using a high rate of glycolysis followed by lactic acid fermentation in the cytosol, rather than a comparatively low rate of glycolysis followed by oxidation of pyruvate in mitochondria as in most normal cells
What is the molecular basis of development of haemopoietic neoplasms? What is the clinical relevance of this?
Following the initial mutation, cells can take quite different routes to similar cancers, meaning that different combination of mutations are encountered in different patients
This confers different responses to treatment and the possibility of personalised management strategies/drug regimes
Give 3 examples of TFs commonly mutated in haemopoietic neoplasms
Myc
NF-kB
ZAP-70
Give 3 examples of TSGs commonly lost in haemopoietic neoplasms
Rb
APC
Ras-GAP
Give 4 examples of genes commonly mutated in haemopoietic neoplasms and which enable cytokine-independent cell signalling in their mutated form
Bcr-Abl
HER2
Ras
SOS (Ras-GEF)
Give an example of a pro-apoptotic family of proteins commonly mutated in haemopoietic neoplasms
Bcl-2
What are 3 common mechanisms for inducing DNA mutations?
Chromosomal translocation or other acquired changes
Point mutations or deletions within a gene
Viruses/bacteria
What are 3 examples of types of chromosomal translocations or other acquired DNA mutations which may lead to cancer? Give specific examples for each
Joining of 2 genes or portions of genes to generate proteins with altered function (e.g. Bcr-Abl)
Inappropriate placement of a gene under the control of a powerful enhancer sequence, resulting in excessive synthesis of the protein (e.g. translocation of Myc gene to control of Ig enhancer)
Deletion of a gene (e.g. loss of miRNA genes regulating Bcl-2)
What are 3 examples of types of point mutations or deletions which may lead to cancer? Give specific examples for each
Change in protein activity, including inappropriate constitutive activation of loss of the ability to inactive a protein (e.g. Ras, SOS, Ras-GAP, p53)
Inability to degrade a protein (e.g. Myc)
Truncation of a protein (e.g. APC, full or attenuated FAP)
What are 3 mechanisms whereby viruses/bacteria induce DNA mutations which may lead to cancer? Give specific example for each
Viral proteins blocking or activating cellular proteins (e.g. HPV E6/E7 blocks p53, Rb and p16)
Viral promoters activating normal genes/viral oncogenes affecting cell cycle
Chronic inflammation promoting mutations and translocation by promoting hypermutation and class switching in B cells (e.g. EBV, H. pylori)
What are 2 common characteristics of mutations/translocations seen in haemopoeitic neoplasms specifically?
Abrogate cytokine dependence for entering cell cycle
Allow avoidance of apoptosis
When are B cells prone to acquiring chromosomal translocations and point mutations? Why?
During antibody diversification, because of the enzyme AID which normally controls Ig class switching and somatic hypermutation (involves inserting point mutations) While this is advantageous for antibody diversity, it increases the number of mutations in non-Ig genes
What are the 3 categories of myeloid neoplasm and what does each involve?
Acute myeloid leukaemias: accumulation of immature blasts in bone marrow with suppression of normal haemopoiesis
Myelodysplastic syndromes: cytopaenia from disorderly proliferation in bone marrow, lack of mature cells in blood
Myeloproliferative disorders: increased production in 1 or 2 categories of mature cells (e.g. CML)
What are the 2 classes of lymphoid neoplasms and how do they differ?
Lymphomas: arise as discrete tissue mass in LNs or extranodal sites
Lymphocytic leukaemias: solid tissue neoplasms (e.g. LN, skin) but that present with involvement of bone marrow and generally blood (e.g. CLL)
Distinctions are blurred and 1 may progress to the other; generally refers to initial presentation
How does WHO classify lymphoid neoplasms?
Morphologic, genotypic, immunophenotypic and clinical characteristics
What is the origin of the majority (85-90%) of lymphomas and lymphocytic leukaemias?
B-cell origin
What kind of immune abnormalities are associated with lymphoid neoplasms?
Loss of protective immunity and breakdown of tolerance (autoimmunity)
Immunodeficiency (higher risk of acquiring lymphoid neoplasms, especially from oncogenic viruses)
What is the natural history of CML?
Slow progression with a median survival of 3 years
Patients enter an accelerated phase, accumulating mutations or cytogenetic abnormalities (e.g. trisomy 8, duplication of Ph Ch), followed by blast crisis (where cells may be of myeloid or pre-B origin)
What can be seen on a peripheral blood smear in CML?
Many mature neutrophils and less immature myelocytes and metamyelocytes
Why is CML thought to originate from a pluripotent stem cell?
Because cells can be of myeloid or pre-B origin in the blast crisis
What is Abl?
Abl is a cytoplasmic (non-receptor) TK, part of controlled cytoplasmic signalling pathways, with defined interactions within its pathway
List 6 downstream interactions of Bcr-Abl and the role of each of these interactions
Cytoplasmic signalling: Ras, MAP kinase, Hck, Fes (proliferation)
TFs: Jun, Myc, NF-kB, Rac (Ras-like; survival, proliferation, migration)
Jak/STAT (metabolism)
PI3 kinase-Akt/PKB (increased glucose metabolism, anti-apoptosis)
Where is the Abl gene normally located?
Ch 9
What is the Philadelphia Ch?
Fusion Ch formed by translocation of tip of Ch 9 (Abl) to breakpoint cluster region of Ch 22
What is the role of the Bcr moiety in the Bcr-Abl fusion gene?
Constitutively activates Abl kinase and contributes many new interaction domains into other signalling pathways
Which residue on the Bcr-Abl protein is commonly mutated when Gleevec becomes ineffective?
Threonine
What is Gleevec?
Imatinib mesylate: drug used to treat CML by binding specifically to Bcr-Abl to inhibit its activity
How is Gleevec able to be so specific for Bcr-Abl?
Most TKs have a very similar binding pocket and loop, but subtle variation in AAs make each binding site characteristic; by exploiting this, Gleevec remains specific for only 4 of the ~90 known TKs (including Bcr-Abl)
How can CML treatment be monitored for efficacy?
Blood smear: treatment efficacy is evident with decreasing numbers of neutrophils and improved representation of other lineages
PCR: used to amplify Bcr-Abl in blood, reduction is calculated as the difference between the number of PCR cycles achieving half maximal product
Why is not possible to cure CML with Gleevec?
Causative translocation occurs in an early progenitor cell, which does not cycle often and is therefore protected from Gleevec; these cells can repopulate if treatment is ceased
When does blast crisis occur in patients treated with Gleevec?
When resistance to Gleevec is acquired via new mutations in the Abl kinase domain
When the mutated Bcr-Abl gene is amplified, resulting in amplified protein (exceeds capacity of normal Gleevec treatment)
When additional mutations are accumulated in the original CML progenitor cell
What is the mechanism of Gleevec resistance in CML patients?
New point mutations in the Abl kinase domain which causes cells to lose the ability to bind Gleevec but retain the ability to phosphorylate substrates
What is the most common leukaemia in adults?
CLL
What is the median age of CLL diagnosis?
60
Is CLL more common in males or females?
Males (2:1)
What is the prognosis of CLL?
Survival is 4-10 years with relatively early diagnosis (Bcl-2 only; worse prognosis with ZAP-70)
What causes CLL?
Exact cause unknown but most cases present with high levels of Bcl-2 due to loss of regulatory miRNA 15a and 16-1 (loss of apoptosis)
Subsequent changes in ZAP-70 and p53 are also observed, and are associated with a poorer prognosis
What 4 main histological features are seen in a LN affected by CLL/SLL?
Loss of normal architecture
Infiltration of small lymphocytes with small nuclei and condensed chromatin (majority of tumour cells)
Aggregation of larger proliferating lymphocytes with mitotic nuclei in proliferation centres
Prolymphocyte (larger cell with centrally placed nucleolus)
What 4 main histological features can be seen on a peripheral blood smear in CLL?
Lots of small lymphocytes with condensed chromatin and scant cytoplasm
Smudge cells (ruptured tumour cells - affected lymphocytes are fragile and can be broken in the preparation process)
Spherocytes (hyperchromatic, round erythrocytes) present because of a concomitant AI haemolytic anaemia
Immature nucleated erythroid cell stemming from premature release of progenitors after marrow infiltration by tumour cells (leukoerythroblastosis)
What is the cell of origin in CLL?
Not completely clear (cells have mixed markers) but may be naive or post germinal centre/memory B cell
What markers are present on cancer cells in CLL?
Typical B cell: CD19, CD20, CD23
Typical T cell: CD5
Ig: IgM, IgD or single Ig light chain (post germinal centre B cell), IgVH (naive B cell)
Which cell marker corresponds with poorer prognosis in CLL?
IgVH
What chromosomal abnormalities are most common on presentation of CLL?
Deletion of Ch 13q14 (70% of cases)
What is the result of the deletion of Ch 13q seen in most cases of CLL?
Loss of miRNA 15a and 16-1
Overexpression of Bcl-2 due to loss of downregulatory miRNAs
What is the mechanism of action of ABT-119?
BH3-only mimetic, inhibits Bcl-2 (in phase I clinical trials to treat CLL)
What is ZAP-70 and what is its normal physiological role?
Cytoplasmic kinase
Activated by T cell binding and phosphorylates substrates involved in proliferation
How does loss of functional p53 in CLL occur on a chromosomal level? What is its effect on prognosis?
Deletion of Ch 17p
Rapidly progressive disease (poor prognosis)
What trisomy is seen in 15% of CLL cases?
Trisomy 12
What is the difference between the Bcl-2 protein and Bcl-2 family in terms of role?
Bcl-2 protein is a member of the Bcl-2 family, a family of pro-survival and pro-apoptotic proteins
Bcl-2 protein is a PRO-SURVIVAL protein
What is the effect of excess Bcl-2 protein, increased ZAP-70 activity and p53 loss on cell cycle control?
Excess Bcl-2 protein: cells accumulate as they cannot apoptose
High ZAP-70 activity: cells proliferate
Loss of p53: cells acquire many new mutations, cell cycle cannot be arrested
What is the mechanism of miRNA regulation of Bcl-2 expression?
miRNA genes are expressed and cropped into dsRNA in the nucleus
Further processing ("dicing") occurs in the cytoplasm, resulting in assembly of ss miRNA with Argonaute to form the RISC complex
miRNA guides RISC to target mRNAs; if binding is very homologous, the mRNA is cleaved ("slicing") by RISC, resulting in its rapid degradationWhat happens to an mRNA that does not bind extensively with the miRNA in the RISC complex?
It survives but translation is reduced
How does Bcl-2 act in apoptosis?
p53 stimulates activation of Bcl-2 family members in mitochondria, which in turn cause release of pro-apoptotic molecules (e.g. cytochrome c) by forming an open pore Cytochrome c (and others) act on initiator caspases to recruit executioner caspases Executioner caspases activate endonucleases and cytoskeleton breakdown to cause cell blebbing and the production of apoptotic bodies
What pro-survival proteins are contained in a closed pore? What is the role of these proteins?
Bcl-2, Bcl-XL
Prevent cytochrome c from leaving the mitochondrion
What pro-apoptotic proteins are contained in an open pore? What is the role of these proteins?
Bax, Bak
Allow cytochrome c to leave the mitochondrion and activate caspases
What pro-apoptotic proteins does p53 stimulate the synthesis of? What do they do?
BH3-only (Bad, Bid, Bim, Puma, Noxa)
Bind and inhibit Bcl-2 to allow the pore to open
Which BH domains aggregate to form an open pore following an apoptotic signal?
BH1,2,3 (e.g. in Bak, Bax)
What BH domains do Bcl-2 and Bcl-XL have that binds to BH123 proteins to prevent release of cytochrome c?
BH1,2,3,4
What is ABT-199 and how does it work?
BH3-only mimetic
Mimics action of Bad, Bim, Puma, etc. (binds to and inhibits Bcl-2 to enable apoptosis)
Give an example of a T cell mutation that causes SCID
A deficiency in ZAP-70 in T cells
How many CDK-cyclins are present at successive steps in the cell cycle?
~16
What is STF?
p53
How is entry into G1 phase of the cell cycle controlled?
Cell assessed for DNA damage
If damage is detected, p53 activates p16 (CDKI), which binds to the CDK4-cyclin D complex and inactivates it
What is the molecular mechanism of cell proliferation when p53 is lost?
Cell assessed for DNA damage
p53 absent, p16 (CDKI) not activated
CDK4-cyclin D phosphorylates Rb to release its inhibition of E2F
Active E2F induces S phase enzymes and cyclin E to continue the cell cycle
Why do mutations acquire rapidly following loss of p53? Give 3 reasons
Cannot arrest cell cycle to repair mutations
Cannot activate some repair processes
Cannot be directed to apoptosis
What must be considered when thinking about the effect of radiation in cancer treatment?
If p53 is lost, irreparable DNA damage may occur with radiation
In what 3 ways does EBV predispose to cancer?
LMP-1 protein activates cellular NF-kB and Jak/Stat pathway, which promotes autonomous B cell survival and proliferation
LMP-1 also activates Bcl-2 to prevent apoptosis
EBNA-2 transactivates cellular CyclinD and src kinases, resulting in increased proliferation
What kind of stimuli activate NF-kB?
Signalling via TNF-a receptor and IL-1R
What is the normal physiological role of NF-kB?
TF that, when activated, turns on 100s of genes that participate in stressful, inflammatory or immune responses (e.g. INF, pro-inflammatory cytokines and chemokines)
What is Burkitt’s lymphoma common in certain geographical regions?
Not all B-cells infected with EBV are killed by T cells (some downregulate surface Ag and survive) Chronic infection (e.g. with malaria) promotes somatic hypermutation and translocations associated with class switching of Ig, which can induce malignant changes
What is a common translocation seen in Burkitt’s lymphoma and what is its molecular action?
t(8;14)
Translocates Myc gene to the Ig promoter, resulting in upregulation of Myc (powerful TF directing entry into S phase without checkpoints)
