Functions of ZBRK1, a BRCA1-associated KRAB zinc-finger protein: A Paradigm for coordinating histone de-acetylation, methylation and heterochromatin formation
The Wistar Institute, Philadelphia, PA, United States
Gene regulation at the level of transcriptional initiation is critical for development and homeostasis in high vertebrates. A great emphasis has been placed on defining the exact molecular mechanisms whereby sets of genes are selected for coordinated activation or repression and how these, once initiated, are maintained. The past few years has seen an explosion of research on the regulation of chromatin structure as a primary mechanism of gene regulation. In particular, the post-transcriptional modification of core histone tails via phosphorylation, acetylation/de-acetylation, methylation and ubiquination has emerged as a key event. Multi-subunit macromolecular complexes have been isolated and shown to contain these activities while genetic studies have proven their requirement for promotor-specific gene regulation. However, much less clear is how these complexes are targeted to promotors in vivo, and even more enigmatic is how their enzymatic activities are spatially and temporarily coordinated. We have been studying ZBRK1, a zinc finger protein which binds to the BRCA1 protein and appears to target both activation and repression of transcription to specific genes. Repression is mediated by the transcriptional co-repressor protein, KAP-1, which has the capability to both bind to sequence-specific DNA binding subunits (KRAB domain-containing Zinc Finger proteins) and recruit complexes containing histone de-acetylase (NuRD), histone methylases (SETDB1) and heterochromatin proteins (HP1s). The ZBRK1-KAP1 complex co-repressor contains at least four distinct surfaces for associating with these activities and may function to sequentially recruit them to target promoters in vivo. A combination of CHIP, enzyme accessibility and Immuno-FISH assays have confirmed that endogenous genes silenced by the KAP-1 system display: 1) a closed chromatin confirmation 2) a compliment of core histones containing the predicted modifications, and 3) a direct association with the heterochromatin protein HP1 and condensed chromatin. These results form a paradigm for understanding the coordination of chromatin modifications that lead to stable silencing of gene expression. Interestingly, loss of HP1 expression is seen in advanced metastatic breast cancer suggesting that a global loss of silencing mechanisms contributes to tumor progression. Our current studies on the regulation of this system and its role in tumor progression will be presented.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in plenary session 703 (Oncogenic pathways).