Regulation of the Kit (c-Kit, W) receptor in hematopoietic function
The development of HSCs is sub-divided into the long term self-renewing HSCs, short term self-renewing HSCs, and multipotent progenitors. The multipotent progenitors give rise to lymphoid and myeloid precursors, which differentiate into the cells for the lymphatic system and bone marrow. Because Kit mutations disrupt these processes, we know that the Kit gene is an essential regulatory element of HSC and erythroid progenitor cells. The myeloid system is of particular interest in studying Kit. The differentiated cells from this system include granulocytes, macrophages, platelets, and red blood cells. KIT expression has been identified in precursors of each of these cell types.
Several mutant Kit or W alleles have been identified with varying phenotypes in hematopoiesis, gametogenesis and melanogenesis. The degrees of defects in production of red blood cells (RBCs), melanocytes and gametes are not necessarily correlated, and there is no obvious link between these phenotypes and the molecular lesions observed with the KIT receptor. Several of the alleles are the result of various point mutations in the kinase domain. Most important are 3 mutations, each due to a single base-pair change in the intracellular region of c-Kit. The W-42 mutation has a far more severe effect on long-term HSC repopulation than the W-41 and W-v mutations. Therefore, W-42 identifies a location on the tyrosine kinase region of the c-Kit receptor that is vital for HSC differentiation. We focus on the cellular and molecular mechanisms affected by W-42 compared to the less severe mutations.
A novel assay, stage-specific competitive repopulation, compares proportions of marked donor and competitor cells in HSC and multipotent progenitor populations, identified by specific antigens. The c-Kit mutants (W-41, W-v, and W-42) have repopulating abilities that are widely disparate. The W-42 mutation has the greatest effect on long-term HSC repopulation. We are investigating how much of the functional defect of the W-42 mutation results from reduction of HSC function, relative to other cells types. Initial data suggest that each W mutation disrupts specific molecular interactions. We are searching for those interactions by testing differences in growth factors associated with c-Kit. We also will compare HSC gene expression in the mutants (Sharma et al., 2007).
An interesting hypothesis is that cancers may spread via their stem cells. Cancers of HSCs, as well as melanoblasts and germ cells, thus may be responsive to blocking of c-Kit at the extracellular receptor. If the c-Kit receptor is mutated, turning the pathway on, perhaps blockage would be effective later in the signal transduction pathway, possibly at the point of the nuclear binding factors.