Defining Pathways
Through our genetic screens we have identified suites of mutations that have similar effects - for example, Plt3, Plt4, Plt6, Plt8, Plt10, Plt11 and Plt15 all elevate the platelet count of Mpl-/- mice. Plt3 and Plt4 mice harbour independent mutations in the gene encoding the transcription factor c-Myb, while Plt6 mice contain a mutation in the gene for the histone acetyl transferase and c-Myb binding partner, p300. Consistent with the function of Myb and p300 in the same complex, Plt3, Plt4 and Plt6 mice show stem cell defects involving an increased commitment to the megakaryocyte lineage at the expense of red cell and B cell production. Plt8 and Plt10 in contrast contain a mutation in a gene that is a component of the polycomb repressive complex 2 (PRC2) which acts by methylating lysine 27 of Histone 3. Many interesting questions arise when analysing these mutants;
- Do each of the mutations exert their effect using the same biological mechanism - i.e. does mutation of Suz(12) or the mutations in the Plt11 and Plt15 mice, also bias differentiation down the megakaryocyte lineage?
- Do c-Myb, p300 and Suz(12) act in the same molecular pathway or separate pathways?
- As has occurred successfully in lower organisms, can we use epistasis analyses in which the phenotype of double mutants are compared to single mutants to infer pathway relationships.
- ENU-induced mutants provide finger-holds in biologically important pathways; can we combine genetics, genomics, proteomics and bioinformatic to flesh out other components of these pathways?
The haematopoietic hierarchy.
Stem cells shown at the left of the diagram are the ultimate source of all blood cells. The key property of stem cells, which distinguishes them from progenitors, is their capacity for self-renewal and their ability to produce all lineages of blood. Some more primitive progenitor cells retain the ability to form several lineages of cells. These are known as multipotent progenitors (CMP=common myeloid progenitor, MEP=megakaryocyte-erythroid progenitor, GMP=granulocyte-macrophage progenitor, CLP=common lymphoid progenitor) and they give rise to progenitors with restricted potential known as colony-forming cells or CFCs.