Genetics of Chronic Myelogenous Leukemia (CML) Flashcards
What is the oncogenic mechanism and treatment for CML?
- Genomic translocation t(9;22) creates new gene encoding constitutively active tyrosine kinase
- Targeted molecular therapy based on mechanism - tyrosine kinase inhibitors - Imantinib (Gleevac)
What are important things to remember about how CML develops?
- CML arises from defects in normal neutrophil differentiation pathway
- BCR-ABL genomic translocation = genetic basis for defects
- Fundamental defect in CML, BCR-ABL fusion protein = exploited to create therapy
What are the most abundant white cells in circulation?
Neutrophils!
What do we need to regulate proliferation of neutrophils?
- It’s critical for fighting infection
- Short lived
- Generated continuously in the bone marrow
- Job of neutrophil lineage to generate neutrophils in a regulated fashion
What should you remember about stem cells?
- Undergoes self-renewal
- Pluripotent - capable of generating all lineages
- Capable of proliferation
What should you remember about progenitor cells?
- Capable of proliferation but not self renewal
- Multipotent - can generate more than one lineage
- Capacity becomes narrower as progresses down pathway
What should you remember about committed cells?
- Do not proliferate
- Only one fate, to generate the next step in the path to the neutrophil
Self renewal and proliferation are tightly regulated by. . .
. . .extracellular signaling from bone marrow!
How does differentiation of the neutrophil lineage occur?
-Bone marrow secretes factors specific for different lineages
-Factors bind receptors on progenitor cells
-SIgnal activates cell-type specific transcription factors
Ex: Progenitor cell - GMP
Cytokine - G-CSF
Transcription factor - CEBPalpha
Result - Neutrophil differentiation
What spurs the neutrophil lineage to go into CML chronic phase?
-BCR-ABL1 genomic translocation in hematopoietic stem cell
-Creates genetic chimera of parts of BCR (breakpoint cluster region) gene and ABL1 (ableson tyrosine kinase) gene
-BCR-ABL1 fusion protein is a constitutively active tyrosine kinase that activates proliferation and blocks apoptosis in absence of extracellular signals
-Mutation arises in hematopoietic stem cell but is passed down to all progeny
Expression fo BCR-ABL fusion protein from genomic translocation disrupts normal neutrophil differentiation.
What is special about BCR-ABL1 progenitor cells?
- Proliferate more than other cells, survive longer!
- Have more opportunity to acquire more mutations which can make cells more oncogenic
- DO NOT self renew
- Continue to differentiate
What is the CML chronic phase like?
- Expansion of progenitor and committed cells
- Mature cells still produced
- Disease relatively mild
- BCR-ABL1 translocation in hematopoietic stem cell –> Increased proliferation and survival of progenitor cells, opportunity to acquire more mutations
What is the blast phase like?
-GMP acquires ability tot self renew, acquires block to differentiation –> huge expansion of blasts, 30% extramedullary
What should you remember about CML accelerated and blast phases?
- BCR-ABL1 alone does not cause progression from chronic to blast phase
- Additional changes needed:
- –GMP acquire self renewal capacity - Wnt beta cattiness signaling is activated
- –Differentiation is blocked - translation of CEBPalpha is inhibited
What is the outcome of CML accelerated and blast phases?
- Extreme expansion of blasts
- Production of functional mature cells blocked
- Severe disease
What is the mechanism of the BCR-ABL1 translocation?
- Double stranded breaks occur in two chromosomes at specific breakpoints
- -Two most common:
- –p210 in CML, most common
- –p190 in some ALL, less common
- -Why these sites is not understood
- Mechanism used is non-homologous end joining (NHEJ)
- -Common DNA repair mechanisms
- -Doesn’t require extensive homology
- Ends of different chromosomes are joined
What is BCR?
- Key functional domain for normal protein - Ser/Thr kinase domain
- Key functional domains for oncogenic fusion protein coiled-coil domain tyrosine 177
- Normal function - role in inhibition of some inflammatory responses
What is ABL1?
-Key functional domain for both normal and oncogenic fusion protein is tyrosine kinase
-In normal protein, kinase is held inactive unless activated by external signals
-Long chain fatty acid myristate maintains inhibition
-Normal functions: role in DNA repair cytoskeletal organization
NEITHER BCR or ABL ALONE ARE ONCOGENIC!!
What is the difference between the ABL and BCR-ABL1 kinases?
ABL - require extracellular signaling - then protein opens up conformation and this makes it possible for the kinase to dimerize
BCR-ABL1 - dimerization can occur WITHOUT AN EXTRACELLULAR SIGNAL –> this causes phosphorylation of tyrosine 177 (occurs in CML cells)
ABL tyrosine kinase phosphorylates Y177 in CML. Phosphorylation causes:
- Dysregulated proliferation
- Protection against apoptosis
Is BCR a tyrosine kinase?
NO
How is CML treated?
- Targeting it BCR-ABL fusion protein
- CML is treated with BCR-ABL specific tyrosine kinase inhibitors
- Most BCR-ABL mutated cells can be eliminated
- Transition to blast phase can be blocked
What is Imatinib?
- Prototype of BCR-ABL specific tyrosine kinase inhibitors
- It specifically inactivates ABL1 kinase by blocking ATP binding
- Prevents selective advantage that comes from activated oncogenic signaling pathways
- Inactivation of BCR-ABL in mutant cells also causes apoptosis (cells became dependent on BCR-ABL for proliferation and anti-apoptosis signaling)
- Loss of BCR-ABL mutant cells allows normal cells to repopulate
How effective is Imatinib?
-5 yr survival for individuals with newly diagnosed chronic phase CML, treated with tyrosine kinase inhibitors - 85-90%
Natural history of disease w/out treatment:
—Chronic phase lasts
If BCR-ABL translocation arises in HSC, why does leukemia arise in the neutrophil lineage?
Specific signaling opportunities created by BCR-ABL protein benefit neutrophil progenitor more than others
- Some lymphocytic leukemias do arise from BCR-ABL translocation but he fusion is at a different site, so many initiate different signaling
- In CML, a subset of B cells do carry the BCR-ABL translocation but are not leukemic
What are the limitations of Imatinib?
- Many develop secondary resistance to Imatinib primarily due to mutations in BCR-ABL (trying to create 2nd and 3rd line tyrosine kinase inhibitors)
- BCR-ABL1 tyrosine kinase inhibitors not effective against blast phase disease
- CML stem cells are resistant to tyrosine kinase inhibitors (affects even successful treatment, even a few stem cells are sufficient to restart disease if treatment stops
- -> so Imatinib must be taken for life!!
