Heterozygous Kit Mutants with Little or No Apparent Anemia Exhibit Large Defects in Overall Hematopoietic Stem Cell Function

OBJECTIVE: The evolutionarily conserved Kit receptor is vital for function of hematopoietic stem cells (HSC). Kit(W-41) (W-41) and Kit(W-42) (W-42) are single residue changes in the KIT intracellular phosphotransferase domain, while Kit(W-v) (W-v) is a single residue change in the ATP binding domain. This study tests how each mutation affects HSC function. METHODS: Cells in mutant and C57BL/6J +/+ blood and marrow were compared. Overall HSC function was measured by competitive repopulation. Functions of specific progenitor populations were tested with stage-specific competitive repopulation and standard colony forming unit assays. RESULTS: Bone marrow cells from these Kit mutants are severely defective at reconstituting peripheral blood lineages and bone marrow of irradiated recipients, when compared to +/+ control marrow. These defects increased with time. Marrow from W-41/+ and W-v/+ functions similarly but better than marrow from W-41/W-41 and W-42/+, to repopulate the erythroid and lymphoid lineages. Long term (LT) - and short term (ST)-HSC from W-v/+, W-41/W-41 and W-42/+ are more defective at reconstituting bone marrow than LT- and ST-HSC from W-41/+ and +/+. Common myeloid progenitor (CMP) cells from W-42/+ and W-41/W-41 are more defective at producing spleen colonies than CMP from W-v/+ and W-41/+. CONCLUSION: Heterozygous Kit mutants with little or no apparent anemia exhibit surprisingly large defects in overall HSC function. Multiplying the fractional defects in LT-HSC, ST-HSC and CMP can account for overall effects of W-v/+, but does not completely account for the defects observed with W-41/+, W-42/+ and W-41/W-41