Tyrosine kinase in cancer Flashcards
How many TKs are there in the human genome?
~90
What are the two types of TKs?
Cytoplasmic and receptor TKs
what induces activation of RTKs?
oligomerisation (dimerisation etc)
What changes occur to an RTK following its activation
Following ligand binding, a conformational change induces the activation of the TK domain. This leads to transphosphorylation of the partner receptors cytoplasmic domain.
How do most cytoplasmic TKs associate with receptors that lack intrinsic TK activity
Non-covalently.
How do cytoplasmic TKs become activated.
Similar RTKs, oligomerisation induces transphosphorylation of TKs which then phosphorylate the receptor itself.
Give 5 ways in which tyrosine kinases get inappropriately activated in malignant disease
- Mutation resulting in constitutive activation or hypersensitivity
- By fusion to other proteins
- By receptor overexpression leading to reduced threshold for signalling
- By activation of upstream signalling pathways
- By autocrine production of growth factors
Give an example of a mutation resulting in constitutive activation or hypersensitivity
EGFR in lung cancer, FLT3 in AML, JAK2 in myeloproliferative neoplasms
Give an example of a fusion event to another protein that causes malignant disease
BCR-ABL in CML, EML-ALK in lung cancer
Give an example of receptor overexpression leading to a reduced threshold
EGFR, HER2 in various cancers
Give an example of upstream signalling activation pathways that lead to malignancy
B-cell receptor and subsequent BTK activation
Give an example of autocrine production of growth factors that leads to malignancy
IGF1 production by numerous tumours
How are chromosomal translocations detected?
Cytogenetics, FISH or PCR which spans the break point of the fusion.
How do chromosomal translocations induce malignancy?
Through inappropriate activation of target gene(s) or in the wrong cellular location (changes in protein localisation due to fusion may induce the activity of inappropriate pathways
Which hallmarks of cancer does TK activation mainly contribute to
Sustained proliferation and reduced sensitivity to cell death/antiproliferative signals.
What are the ideal characteristics of a TK inhibitor
- Target is activated by genomic alteration (tumour is more likely to be dependent on this type of lesion)
- Target is present in all the cells of the tumour, not just sub-clones
- The candidate has good drug-like properties (pharmacokinetics etc)
What is a small molecule
A a low weight chemical which can pass through the plasma membrane and get into the cell to give its effect
How do most small molecule inhibitors have their effect on TKs?
Compete for ATP binding, and less commonly or substrate binding.
How do monoclonal antibodies target RTKs
Prevent ligand binding and/or induce receptor downregulation/internalisation or trigger ADCC
How does chronic myeloid leukaemia present clinically?
Excessive production of mature, differentiated white blood cells (neutrophils). Enlargement of the spleen and liver
What is the pathological hallmark of CML?
The philadelphia chromosome
What is the Philadelphia chromosome?
An abnormal chromosome 22. This is a translocation between genes on chromosome 9 and 22. Sequences of the BCR gene runs into almost the entire coding region of ABL (a tyrosine kinase).
In what cell of haematopoiesis does the BCR-ABL fusion event occur, and what is the effect of this.
In the initial haematopoietic stem cell. Main effect is to cause excessive proliferation - biased to neutrophils
How does WT ABL kinase act? How does this change after fusion to BCR
Cytosolic kinase. Involved in DNA damage response. Highly regulated, quiescent kinase.
Complete disregulation, not confined to the nucleus so found throughout the cytosol and the ABL part phosphorylates multiple signalling pathway constantly. It also decreases apoptosis and increases susceptibility to DNA damage which increases likelyhood of developing mutations without the cell dying.
Which phase represents the ML stage in which only the BCR-ABL fusion mutant is present
Chronic ML
How does chronic (CML) progress into acute myeloin leukaemia (AML)
Amplify multiple chromosomes (Double Ph, Chr17, 19 abn)
How can ATP binding site targeted drugs be specific for a single kinase?
Whilst the ATP binding site is almost identical between each kinase. The entrance to the pocket is very different across kinase families.
What conformation suggests a kinase is active? how does this compare to inactive?
Inactive - DFG three amino acid sequence is inside the pocket when inactive. When active the DFG sequence flips outside the ATP binding pocket.
What 4 types of interaction between drugs and TKs are there?
1 - bind the inactive conformation
Type 2 - Binds the active confirmation
Type 3 - Binds the adjacent ATP pocket
Type 4 - Binds the substrate binding site or the allosteric site causing a conformational change to the ATP binding pocket
True or false- TKIs can be stopped after going into long term remission?
False - Only around 10-15% of CML patients can stop their therapy. In most cases once the treatment is stopped the amount of fusion protein present increases.
How does clinical resistance to CML drugs occur and what does this tell you about the BCR-ABL fusion gene
New mutations occur in BCR-ABL, which confirms that this oncogene is critical for the development of the tumour to be maintained.
Also, to a lesser extent some patients gain BCR-ABL amplification.
How do you tackle resistance to Imatinib
Increase the does of imatinib - especially in amplifcation
OR
Switch to a different tyrosine kinase inhibitor - eg dasatinib, nilotinib
What treatment can be offered if the initial methods of tackling resistance fail to work?
Use alternative modality eg allogenic stem cell transplant
OR
Reduce emergence of resistance by treatment with combination
What are the pros and cons for selectivity for a TK inhibitor
High specificity potentially validates the target and reduces off-target toxicity
Lack of selectivity may broaden the therapeutic indications, inhibit tumour growth by several mechanisms - may reveal novel pathophysiological mechanisms.