TARGETED DISRUPTION OF THE MYC ANTAGONIST MAD1 INHIBITS CELL-CYCLE EXIT DURING GRANULOCYTE DIFFERENTIATION

The switch from transcriptionally activating MYC-MAX to transcriptiona lly repressing MAD1-MAX protein heterodimers has been correlated with the initiation of terminal differentiation in many cell types, To inve stigate the function of MAD1-MAX dimers during differentiation, we dis rupted the Mad1 gene by homologous recombination in mice, Analysis of hematopoietic differentiation in homozygous mutant animals revealed th at cell cycle exit of granulocytic precursors was inhibited following the colony-forming cell stage, resulting in increased proliferation an d delayed terminal differentiation of low proliferative potential clus ter-forming cells. Surprisingly, the numbers of terminally differentia ted bone marrow and peripheral blood granulocytes were essentially unc hanged in Mad1 null mice, This imbalance between the frequencies of pr ecursor and mature granulocytes was correlated with a compensatory dec rease in granulocytic cluster-forming cell survival under apoptosis-in ducing conditions, In addition, recovery of the peripheral granulocyte compartment following bone marrow ablation was significantly enhanced in Mad1 knockout mice, Two Mad1-related genes, Mxi1 and Mad3, were fo und to be expressed ectopically in adult spleen, indicating that funct ional redundancy and cross-regulation between MAD family members may a llow for apparently normal differentiation in the absence of MAD1. The se findings demonstrate that MAD1 regulates cell cycle withdrawal duri ng a late stage of granulocyte differentiation, and suggest that the r elative levels of MYC versus MAD1 mediate a balance between cell proli feration and terminal differentiation.