4. B cell cancers; origins and therapies Flashcards
What are the 6 hallmark capabilities of cancer?
- Sustaining proliferative signaling
- Evading growth suppressors
- Metastasis - dissemination
- Immortality of replication (senescence)
- Blood supply/inducing angiogenesis
- Blocking apoptosis
Molecules that promote B cell cancer must…
- promote cell division
- extend cell life span by enhancing survival
- enable DNA mutation
*B cells have special properties that predispose them to malignancy
What are the risks of being a B cell?
In CSR, SHM processes, DNA is deliberately damaged to enhance Ab affinity and diversity, and provide memory.
Most B cell lymphomas originate from GC B cells, as indicated by somatically mutated Ig genes and chromosomal translocations involving the Ig locus.
AID in GC
In GC, B cells undergo multiple rounds of proliferation, suppressing differentiation
AID expression is restricted to proliferating B cells in GC
AID triggers events that promote chromosomal rearrangement and gene mutation
- risk of chromosome translocation when undergoing CSR (double strand break & rejoin)
- mismatch, base-excision, error-prone repair
Detection of chromosomal translocations in B cell cancers
- cytogenetic techniques looking for banding patterns
- painting chromosomes to see c-myc oncogene (taken out of context)
- paired end cancer genome mapping
- cancer genome sequencing (take tumour and sequence)
- chromosome map
Recurrent chromosomal translocations in B cell cancers
Bcl6 = trascription factor Ccnd = cell cycle regulator Cdk6 = cell cycle G1/S Myc = cell growth Bcl2 = apoptosis, suppressor of cell death
B cells make Ig and ALWAYS make Ig
Translocation into Ig locus means translocated gene is constitutively activated
What is the translocation resulting in Burkitt lymphoma?
myc gene into IgH locus
What is the translocation resulting in DLBCL (diffuse large B cell lymphoma)?
Bcl6 gene translocation
increase in NFkB
What is the translocation resulting in LPL (lymphoplasmacytic lymphoma)?
PAX5 gene into IgH locus
What is the translocation resulting in Follicular lymphoma?
Bcl2 into IgH
Cyclins
Cell cycle regulator
-transiently expressed, activate CDK (cyclin dependent kinases) and drive the cell cycle
*several inhibitors (p21, p15, p18 etc) bind to specific CDKs and inhibit their activity which allows time to repair DNA damage
Myc
Transcription factor with wide influence
- drives proliferation by up-regulating cyclins & down-regulating cyclin inhibitors
- regulates cell growth by enhancing rRNA and protein synthesis
- inhibits differentiation
-Myc is a very strong proto-oncogene found to be abnormally upregulated in many types of cancers
Bcl2
Bc2-like proteins hold “death” signals in check, overexpression of Bcl2 blocks apoptosis.
Is a weak oncogene because it cannot promote cell proliferation. Blocking apoptosis is very common in cancer.
Frequency: Follicular 90% CLL 85% Large cell 45% Hodgekin 20% Burkitt 5%
BH3-only family
ARE KILLERS
Under stress, cells activate BH3-only, overwhelming Bcl2 and triggering apoptosis by perforating mt, releasing Cyt C and ultimately, activating caspases
Bcl6
Transcriptional repressor
- inhibits DNA damage response in GC B cells (represses ATR - DNA damage sensor)
- inhibits B cell differentiation during GC reaction to enable isotype switching and affinity maturation
- blocks cell cycle regulation (represses p53)
- expression is absolutely required for GC to form
- suppress apoptosis
Translocation & mutation can result in DLBCL where aberrant Bcl6 expression causes DNA damage to occur unchecked with inhibited differentiation, which is associated with cessation of proliferation.
*CD40 and BCR signalling in a normal cell block Bcl6
What mutations convert Bcl6 into an oncogene?
Translocations that lead to continued expression
Somatic mutations that stop transcription from being repressed
Mutations that stop Bcl6 protein from being degraded - increased stability
*AID, IRF4 wondering off target & mutating Bcl6 by accident
Lymphoma
=a malignancy of lymphocytes
Presents as a solid tumor of lymphoid cells, eg. enlargement of LN or spleen
2 basic categories: Hodgkin’s lymphoma (presence of Reed-Sternberg cell) & non-Hodgkin’s lymphoma (large, diverse group of B cell or T cell lymphomas)
Both have indolent (slow growing) and aggressive subtypes that behave and respond differently (to treatment).
What is the greatest risk factor for Non-Hodgekin’s Lymphomas?
.Age, as most cases arise in people 60+ years of age.
Leukaemia
Cancer that starts in blood-forming tissue such as bone marrow and cause large numbers of malignant cells to be produced and enter the bloodstream.
B cell, T cell or myeloid cell leukemias have a prevalence of 8-14/100,000
B cell types include:
- Acute lymphocytic leukaemia (ALL): a childhood peak incidence at 2-5 years and another in old age
- Chronic lymphocytic leukaemia (CLL): most common type of lymphoid leukemia, mostly adults, 50+ yo, males
Myeloma/Multiple myeloma
Cancer of plasma cells, prevalence 4-10/100,000
Often preceded by an asymptomatic, premalignant stage of clonal plasma cell proliferation: “monoclonal gammopathy of undetermined significance” (MGUS)
MGUS is present in >3% population over 50yo and progresses to myeloma at 1% per year.
No cure.
Classification of B cell malignancies historically has been based on…
Physical observations
- patient’s condition
- histology of tumour or involved tissues (inc. blood)
- karyotype of tumour cells: c’some number & structure (translocations?)
- abnormal accumulation of clones of cells
- flow cytometry using mAbs that recognise surface molecules (CDs) on malignant cells
What is a Reed-Sternberg cell?
Mysterious but diagnostic giant cells found in biopsies from individuals with Hodgkin’s lymphoma, with markers of both myeloid or lymphoid cells
Common pathogenesis of B cell cancers
Malignant cells crowd out normal cells in limited “niches”, such as bone marrow. This severely limits the function of normal haematopoietic cells such as RBC & WBC that are essential to life, thereby causing infection, bleeding problems and respiratory failure.
Diagnostic feature: Monoclonal serum Ig in MM
Normal serum Ig concentration: 10-15mg/mL
Myeloma patients: clonal “paraprotein” (all the same), in serum and urine often»_space;30mg/ml with free light chains “Bence Jones proteins”
*too much Ab produced, H & L chains don’t bind, therefore L chains pass through kidney into urine
What happens when myeloma cells bind to stromal cells in the bone marrow?
An imbalance between factors that control bone homeostasis:
- growth factor RANK-L (bone resorbing) is overproduced
- growth factor OGP (bone-forming; osteogenic growth peptide) is diminished
- results in osteoclastogenesis and osteolysis
*brittle bones
What is the diagnostic criteria for MM?
Clonal plasma cells >10% on bone marrow biopsy
A monoclonal Ab in either serum or urine
End organ damage
- bone lesions & fractures
- hypercalcemia
- renal insufficiency (too much protein prod.)
- anemia
- frequent severe infection
Current therapies for Non-Hodgkin Lymphoma
- Chemotherapy (systemic)
General - tonxins that kill rapidly dividing cells - CHOP - Antibody - Rituxan - R-CHOP
* targets all B cells for destruction by patient’s own immune system - Radiation - limited
- Bone marrow transplantation - replace immune system, can see cancer cells as ‘foreign’
Bortezimib for MM
A boronic acid dipeptide
Inhibits 26S proteosome - a large protease complex that collaborates with the ubiquitin system to mark proteins for degradation including key regulatory proteins & transcription factors (NFkB).
NFkB is req for expression of survival genes on which MM rely.
Plasma cells (& myeloma cells) are protein secreting factories which handle large amount of Ig, hence are especially sensitive to Bortezimib.
If this proteosome is inhibited, all the malformed proteins will build up, and cell will undergo apoptosis. Because of the inhibition, cell also cannot make NFkB.
Has many side effects as 26S proteosome is in every cell - hence dosage very important.
What additional therapies are used for MM to slow growth and relieve symptoms of fragile bones?
Bisphosphonates slow MM growth and block bone destruction.
Rituximab for B cell leukaemia and lymphoma
monoclonal anti-CD20 antibody
CD20 is a surface protein with no known ligand, function is unclear but may act as a Ca2+ channel
- all mature B cells express CD20 (not plasma cells)
- intravenous Rituximab depletes all CD20+ B cells transiently by coating surface, which imm sys recognises as something to destroy
- first trials in ‘97/’98 on “non responding” patients: ~50% partial or complete remission
-non toxic, highly efficacious, “blockbuster” drug
DLBCL therapy and types
Clinically heterogeneous: 40% patients respond well to current therapy with prolonged survival, while the remainder succumb to disease
Distinct types of diffuse large B cell lymphoma identified by gene expression profiling of many tumours –> personalised medicine in future
2 types:
GC B-like vs activated B like (ABC)
different pathways hyperactive in each type
GC is low risk - standard chemotherapy (+rituximab)
ABC is high risk - specific enzyme inhibitors targeting hyperactive pathways
Novitoclax (ABT-263)
Bcl2-like proteins hold “death” signals in check, whereas BH3 pro-survival antagonists initiate a cell death cascade.
ABT-263 blocks binding site of Bcl2, and displaces it therefore releasing death inducer BH3-only and triggering apoptosis.
Which patient group is good to treat with the new drug Novitoclax (ABT-263)?
Follicular and CLL B cell lymphomas.
CLL patient responses to treatment with BH3-mimetic-ABT-199 and ABT-737
ABT-199: induced almost complete disappearance of CLL cells in blood while leaving platelets unaffected
AB-737: less effective, kills platelets, leading to bleeding.
