Genetic modification of STAT5 levels reveals dose-dependent and isoform-specific requirement for cell lineage progression and differentiation.

Two highly conserved transcription factors STAT5A and STAT5B play an identical role in the intracellular signaling pathway upon cytokine stimulation, while gene deletion experiments have revealed separable and overlapping functions of STAT5. This questions whether the phenotypic differences in the organ development observed in the individual knockout mice result from isoform-specific functions or quantitative differences in the expression levels of each STAT5 isoform among tissues. To elucidate the redundancy and isoform-specificity of STAT5 for development at molecular levels, mice carrying only a single allele of either Stat5a or Stat5b were generated. Both of these mice overcame the lethal anemia observed in Stat5ab-null mice, indicating that development of erythroid cell lineage was totally dependent on the dosage of STAT5. The blocked progression of B cell lineage at the pre-pro B cell stage in Stat5ab-/- mice was rescued in the presence of a single allele of either Stat5a or Stat5b, while the number of total B220+ cells in bone marrow was smaller in Stat5abnull/Stat5b- mice than Stat5abnull/Stat5a- mice. The paucity of alveolar progenitor cells in the Stat5ab-null mammary epithelium was rescued by a single allele of either Stat5a but not Stat5b, suggesting cell-type dependent isoform-specific function. Genome-wide gene expression analyses revealed that different steps of cell lineage progression require different gene sets which expression requires the different isoform of STAT5 in a dose-dependent manner in the mammary epithelium. Taken together, this study demonstrates that dose-dependent isoform specificity of STAT5A and STAT5B controls progression and differentiation of each cell lineage.

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