CML and Imatinib Flashcards
Describe how Ligand-Independent Activation is Oncogenic
- Normally functioning growth factor receptors emit cytoplasmic signals (red spikes) in response to binding ligand (blue, left).
- However, mutations in the genes encoding the receptor molecules (upper right) can cause subtle alterations in protein structure, such as amino acid substitutions (red dots), that cause ligand-independent firing.
- More drastic alterations in receptor structure, including truncation of the ectodomain , may also yield such deregulated signaling. In many human tumors, receptor proteins are overexpressed (lower right).
- Excessive numbers of normally structured receptor molecules can also drive ligand-independent receptor firing by causing these molecules to frequently collide and thereby spontaneously dimerize and release signals (red spikes)
What is leukaemia?
Excessive/uncontrolled proliferation of white blood cell precursors
What is leukaemia caused by?
Caused by clonal proliferation of haematopoietic stem or progenitor cells resulting in
1) diffuse replacement of normal bone marrow by leukaemic cells and accumulation in peripheral blood causing bone marrow failure (too few erythrocytes, neutrophils, platelets)
2) infiltration of organs such as liver, spleen etc.
What is the origin of Chronic Myeloid Leukaemia?
- Origin is in the haematopoietic stem cell.
- Why the myeloid and erythroid lineages are more strongly affected than lymphoid is unknown
Myeloid cells include neutrophils, eosinophils, basophils, megakaryocytes (from which platelets are derived), and monocytes
Give a simplified view of haematopoiesis
What are the Clinical Features of CML?
Incidence: 1:100,000
Risk factor: ionising radiation
Three Phases (Clonal Evolution):
- Chronic phase (4-5 years): expansion of myeloid compartment but cells differentiate normally.
Asymptomatic
- Accelerated phase (weeks-years): more immature cells in the blood
- Blast crisis (weeks-months): immature cells dominate; >30% of cells in bone marrow or blood
What is Molecular Origin of CML?
- CML results from a reciprocal translocation between chromosome 22 and chromosome 9.
- The resulting shorter version of chromosome 22 is called a Philadelphia chromosome (Ph).
- As a result of the translocation, a novel chimeric protein is generated, Bcr-Abl.
- Bcr-Abl is a constitutively active tyrosine kinase.
- BCR (breakpoint cluster region; function unknown) ABL (oncogene; cytoplasmic tyrosine kinase)
The Translocation
Responsible for Bcr-Abl
- The normal structures of chromosomes 9 and 22 are shown at the left.
- When a translocation occurs between them at the indicated site, the result is the abnormal pair at the right.
- The smaller of the two resulting abnormal chromosomes (22q–) is called the Philadelphia chromosome, after the city where the abnormality was first recorded
The chromosomal translocation results in a Bcr-Abl fusion protein
The chromosome translocation responsible joins the Bcr gene on chromosome 22 to the Abl gene from chromosome 9, thereby generating a Philadelphia chromosome (see Figure 20–5).
The resulting fusion protein has the N-terminus of the Bcr protein joined to the C-terminus of the Abl tyrosine protein kinase; in consequence, the Abl kinase domain becomes inappropriately active, driving excessive proliferation of a clone of hemopoietic cells in the bone marrow.
Bcr-Abl Is a Fusion Protein
Reciprocal chromosomal translocations between human Chromosomes 9 and 22, which carry the abl and bcr genes, respectively, result in the formation of fused, hybrid genes that encode hybrid Bcr–Abl proteins commonly found in chronic myelogenous leukemias (CML). Two other alternative breakpoint sites in the bcr gene are involved in bcr–abl translocations (not shown here) arising in certain other hematopoietic malignancies.
Different breakpoints in Bcr give rise to protein fusions of different lengths
The domain structures of the proteins encoded by the normal bcr and abl genes are shown here (above and middle). Each is a multidomain, multifunctional protein, as indicated by the labels attached to the colored areas.
The chromosomal translocation found in almost all cases of CML results in the fusion of most of the reading frame of the Bcr gene with most of the reading frame of the Abl gene (below). The fusion causes deregulated firing of the tyrosine kinase of the fusion protein, which is equivalent to the SH1 domain; this firing is responsible for most of the transforming effects of the newly formed oncoprotein
Activities of Bcr-Abl
Working Model: Bcr-Abl fails to bind the cis/trans inhibitor which normally regulates Abl activity; juxtaposition of Abl kinase domains in the dimer results in autophosphorylation.
- Increased proliferation (reduced cytokine dependence)
- Reduced apoptosis (too many survival signals?)
- Decreased adherence to stromal cells
- Genetic instability (inefficient DNA repair and too high a threshold for apoptosis?)
BCR-ABL is the causative agent and not just a disease marker
- Infecting the bone marrow of mice with a retrovirus expressing BCR-ABL results in a CML-like disease.
- The centrality of BCR-ABL to CML makes BCR-ABL an attractive drug target.
Describe the development of Imatinib
- In 1995, a high through-put screen for kinase inhibitors yielded compounds which inhibited PDGFR and Abl.
- Initial compounds were opitimised yielding: ST1571 (imatinib mesylate; sold as Gleevec or Glivec).
Dsecribe the MOA of Imatinib
(Imatinib Binding to Abl)
- Imatinib sits in the ATP-binding pocket of the tyrosine kinase domain of Bcr-Abl and thereby prevents Bcr-Abl from transferring a phosphate group from ATP onto a tyrosine residue in a substrate protein.
- This blocks transmission of a signal for cell proliferation and survival.
- The structure of the complex of imatinib (solid blue object) with the tyrosine kinase domain of the Abl protein (ribbon diagram), as determined by x-ray crystallography.
- The chemical structure of the drug. It can be given by mouth; it has side effects, but they are usually quite tolerable
Blocking Bcr-Abl with
Imatinib Reduces Leukemia
Specificity of Imatinib for Protein Kinases
(inhibition of substrate phosphorylation or cellular phosphorylation)
1 Micromolar Imatinib kills most BCR-ABL-positive cell lines
Imatinib Reduces the Load of Bcr-Abl mRNA
IRIS Trial Results Survival Rates at 10 Years in the Intention-to-Treat Population
Figure 2. Kaplan–Meier Estimated Overall Survival Rates at 10 Years in the Intention-to-Treat Population. Shown is the overall survival over time among patients assigned to each trial group.
For the curve for the group of patients who had been randomly assigned to receive interferon alfa plus cytarabine, data include survival among the 363 patients who crossed over to imatinib (65.6%).
These patients crossed over to imatinib after a median of 0.8 years of receiving interferon alfa plus cytarabine.
In patients with no reported death (whether because they were known to be alive or because their survival status was unknown), survival was censored (tick marks) at the date of last contact.
The stem cell may be the root of the cancer even though stem cells are rare
How stem cells produce transit amplifying cells.
How a small proportion of cancer stem cells can maintain a tumor.
- Suppose, for example, that each daughter of a cancer stem cell has a probability slightly greater than 50% of retaining stem-cell potential and a probability slightly less than 50% of becoming a transit amplifying cell that is committed to a program of cell divisions that stops after 10 division cycles.
- While the number of cancer stem cells will increase slowly but steadily to give a growing tumor, the non-stem cells that they give rise to will always outnumber the stem cells by a large factor-in this example, by a factor of about 1000. (If the cell-divisioncycle and survival times for the two classes of cells are equal.)
Bcr-abl is active in hematopoietic stem cells and can be inhibited by Imatinib
Stem Cells Can Use Cytokines to Compensate for Inactivation of Bcr-Abl by Imatinib
