Chronic Myeloid Leukaemia Flashcards
What specific genetic abnormality is linked to CML?
Philadelphia chromosome
What is CML an example of?
Myeloproliferative neoplasm (MPN)
What diseases are classed as MPNs? (6)
- Chronic myeloid leukaemia (CML)
- Chronic neutrophilic leukaemia (CNL)
- Polycythaemia vera (PV)
- Essential thrombocythaemia (ET)
- Chronic eosinophilic leukaemia (CEL)
- Mastocytosis
What is CML characterised by? (3)
- Fusion of the BCR gene (22q) and the ABL1 gene (9q) in 100% of cases
- In 95% of patients fusion occurs via translocation event = Philadelphia chromosome
- In 5% of patients fusion occurs by a more complex rearrangement
What is the Philadelphia chromosome?
Abnormal chromosome 22 containing the BCR-ABL1 fusion oncogene
What was the first genetic abnormality to be linked to cancer?
Philadelphia chromosome
What is the prevalence of CML?
1 to 2 new cases diagnosed in the UK per 100 000 per year (rare in terms of cancer)
What are the symptoms of CML? (5)
- Weight loss
- Night sweats
- Fatigue
- Anaemia
- Splenomegaly
What is splenomegaly?
Enlarged spleen
How many phases does CML have?
3 (triphasic)
What are the phases of CML? (3)
- Chronic phase
- Acceleration phase
- Blast phase
What is blast phase?
Acute phase of the disease where blast cells account for more than 20% of peripheral blood
What is acceleration phase?
Intermediate stage which is often short-lived and therefore missed due to rapid progression of the disease
What is the effect of BCR-ABL1 fusion on the haematopoietic pathway? (2)
- Formation of BCR-ABL1 in a myeloid stem cell reprograms the cell with deregulated and increased proliferation
- Results in leukocytosis of the granulocyte lineage
How are cytosis and cytopenia linked? (3)
- Proliferative leukaemias such as CML result in massive cytosis of affected lineages
- This causes bone marrow hypercellularity and inhibits the ability of other stem cells to divide and differentiate resulting in cytopenia
- Cytopenia is also caused by genomic abnormalities which block differentiation into mature cell types
When else is BCR-ABL1 observed? (2)
- Acute lymphoblastic leukaemia (most common in children)
- BCR-ABL1 is diagnostic of CML if there is also evidence of abnormal proliferation of the myeloid lineages
What is observed in the chronic phase? (3)
- Cellularity of the bone marrow is increased
- Platelet count is high
- Leukocytosis of granulocytes
What is observed in the acceleration phase? (2)
- Can be seen in karyotype evaluation (clonal expansion and evolution - BCR-ABL1 plus evolution)
- Blast cells start to account for between 10 and 19% of cells in peripheral blood
What is observed in the blast phase? (4)
- Blast cells account for 20% or more of cells in peripheral blood
- Involvement of other lineages due to de-differentiation of malignant myeloid stem cells
- Further clonal expansion and evolution resulting in granulocytes being unable to mature so blood is full of cells with impaired function
- Persistence of chronic features (high platelets, leukocytosis)
How is CML diagnosed? (3)
- Mostly picked up by routine blood tests showing elevated wbc counts
- Karyotype showing reciprocal translocation between chromosomes 9 and 22 (Philadelphia)
- CML can only be diagnosed by the presence of BCR-ABL1 fusion oncogene
How is the BCR-ABL1 fusion gene generated by translocation event? (4)
- A proximal break in chromosome 9 between the centromere and ABL1
- A distal break in chromosome 22 between BCR and the telomere
- Exchange of the broken segments so that the ABL1 gene from 9 fuses to the BCR gene from 22 resulting in BCR-ABL1 fusion oncogene on abnormal 22 (Philadelphia chromosome)
- Generates an abnormally long 9q and an abnormally small and pale 22
What is a proximal break?
A break in the DNA that is between the centromere and the gene of interest
Where is the BCR gene located?
22q
Where is the ABL1 gene located?
9q
What is a distal break?
A break in the DNA that is between the gene of interest and the telomere
What is the formal nomenclature to describe the Philadelphia chromosome translocation?
t(9;22)(q34;q11.2)
How is clonal evolution detected in karyotyping?
Multiple cells need to be analysed to detect or exclude the presence of clonal evolution and to quantify prevalence of each clone
What is an example of an unusual rearrangement resulting in BCR-ABL1 fusion? (5)
- Three way translocation
- First 2 breaks result in fusion of ABL1 from 9 onto BCR in 22 = BCR-ABL1 oncogene
- Break in 4q: distal part of 22q fuses onto 4 resulting in an abnormal small 4
- 4q fuses onto 9q resulting in an abnormal long 9
- t(4;9;22)(q21;q34;q11.2)
How is molecular fusion of BCR and ABL1 confirmed after observation of rearrangement in karyotype? (4)
- Dual fusion FISH probes
- Probes are specific to BCR and ABL1 regions
- A typical translocation splits each probe signal in half so half of each signal is moved to the other derivative chromosome
- Abnormal result is 2 fusion signals (2 red and green foci)
Why is dual fusion FISH used to detect BCR-ABL1 fusion? (3)
- Reduce false positive results
- 2 signals are expected to co-localise by chance in 1% of human interphase nuclei which is determined by the size of the probe and the nucleus volume
- Dual fusion reduces the chance of colocalisation to 1 in 10 000
How are patients monitored after initial diagnosis of disease load? (3)
- FISH until only 1% of nuclei are abnormal
- Then qPCR of the transcript (more sensitive method to quantify expression rather than genomic copies of the gene)
- Called Minimal Residual Disease (MRD)
What tools are used for detection of relapse? (3)
- FISH and qPCR
- Both capable of rapid result turnaround
- qPCR is more sensitive and cheaper than FISH which allows frequent monitoring of patients to detect relapse early on
