Dominant-negative HMGA1 blocks mu enhancer activation through a novel mechanism

The immunoglobulin mu intronic enhancer is a potent B cell-specific transcriptional activator. The enhancer is activated by the appropriate combination of transcription factors, amongst which are ets and bHLH proteins. HMGA1 (formerly HMG-I(Y)) is a demonstrated co-activator of the mu enhancer. HMGA1 functions through direct interaction with PU.1, one of the ets proteins critical for enhancer activation. New data demonstrates dominant negative HMGA1 dramatically decreases enhancer activity in B cells. EMSA analysis demonstrated that DN HMGA1 disrupts established PU.1/mu enhancer binding. Similarly, DN HMGA1 blocks mu enhancer binding by Ets-1. In sharp contrast, DN HMGA1 had no effect on binding activity of the ETS DNA binding domains of either PU.1 or Ets-1, or the bHLH-zip protein TFE3, suggesting specificity. Taken together, the data suggest that DN HMGA1 utilizes a novel mechanism to specifically block interaction between ets proteins and mu enhancer DNA, suggesting DN HMGA1 represents a new, highly specific means of regulating mu enhancer activity.