15. Genetics of CML

Question 1

What type of cells proliferate in CML?

Answer 1

Granulocytes

But mutation originates high up in lineage (HSC in BM) so proliferation of entire myeloid lineage seen

Question 2

What are the clinical symptoms of CML?

Answer 2

Usually asymptomatic at diagnosis

Weight loss, fever, tiredness, muscle weakness, bone pain, swollen lymph nodes

Question 3

What characterises CML genetically?

Answer 3

BCR-ABL fusion gene

Reciprocal translocation between chr 9 and 22

t(9;22)

Question 4

What is the effect of BCR-ABL on cells?

Answer 4

Constitutively active tyrosine kinase

Activates singalling pathways that are normally tightly controlled and inactive

Uncontrolled cellular proliferation and inhibition of apoptosis

Question 5

How is CML tested for?

Answer 5

Diagnostic - FISH using dual fusion probe, confirmed by karyotype

Monitoring - detection of BCR-ABL transcripts with RT-PCR

Question 6

What are the 3 phases of disease progression in CML?

Answer 6

1. Chronic - expansion of myeloid cell compartment, cells can function and differentiate normally. Lasts 3-4 years
2. Accelerated - mutant stem cells persist in bone marrow and accumulate mutations, cell proliferation increases rapidly (>15% blasts in BM). Fever, hepatosplenomegaly
3. Blast crisis – immature myeloid blood cells dominate circulation (>30% blasts in BM). Severe anaemia, bleeding, increased infections

Question 7

What secondary abnormalities are seen in CML?

Answer 7

+8
+Ph
+19
Loss of MYC and TP53
Complex karyotype

Question 8

How is CML treated?

How do they work?

Answer 8

Tyrosine kinase inhibitors (TKIs) e.g. Imatinib

Inhibit activity of BCR-ABL fusion protein by binding to ATP binding site - prevents phosphorylation of tyrosine residues in target proteins – switches off activated signalling pathway

Question 9

When are secondary TKIs needed?

Answer 9

In the event of ABL kinase mutation that inhibits Imatinib from binding

Don’t work on T315I

Question 10

What are granulocytes?

Answer 10

Neutrophils, eosinophils, and basophils

Question 11

What downstream pathways are affected by the BCR-ABL oncoprotein?

Answer 11

JAK/STAT
RAS/MEK
PI3K/AKT

Question 12

What is the common TKI resistance mutation?

Answer 12

T315I

Question 13

What is the third generation TKI? When is it used?

Answer 13

Ponatinib

Used in patients with T315I TKI resistance mutation

Question 14

When is allogenic haematopoietic stem cell transplant considered?

Answer 14

Patients who are unresponsive to TKI treatment or have entered the accelerated or blast phases

Question 15

What are tyrosine kinases?

Answer 15

Family of enzymes that catalyse phosphorylation of tyrosine residues in target proteins using ATP

Question 16

What are additional chromosome abnormalities associated with in CML?

Answer 16

Poor response to treatment and higher risk of progression​ to acute phase

Question 17

What are the two common transcripts in BCR:ABL?

Answer 17

e13a2 and e14a2 - found in 98% of cases

Question 18

What are the 3 measures of response to treatment in CML?

Answer 18

1. Haematological response (HR) – normalisation of blood count and splenomegaly
2. Complete cytogenetic response (CCR) – No Ph chr in 20 metaphases
3. Major molecular response (MMR) - BCR-ABL1:ABL1 ratio of >0.1%

Deep MR - ratio of >0.01%

Question 19

What is a common secondary TKI?

Answer 19

Dasatinib