M. Mitchell Smith, PhD
DNA damage, chromosome instability, altered gene expression, and cancer
DNA damage, chromosome instability, and aberrant gene expression play causative roles in neoplastic transformation in many malignancies. All of these processes depend on, and are influenced by, the state of chromatin structure and the activities of the histone proteins. The Smith lab is studying three key aspects of chromosome dynamics.
First, they have discovered a role for the acetylation of the N-terminal domains of the histones, particularly histone H4, in the maintenance of genome integrity. Mutations in these domains cause a marked increase in DNA damage, activate the DNA damage checkpoint pathway, and arrest division at the G2/M boundary. Recent work has shown that these mutants are defective in one or more pathways of DNA double strand break repair. The lab is following up with genetic and molecular experiments to investigate the mechanisms by which the histone acetyltransferases participate in this repair.
Second, the lab has identified the S. cerevisiae gene for histone H2A.Z, a member of a family of evolutionarily conserved histone H2A variants. Histone H2A.Z gene is essential in mammalian cells and is over-expressed in colorectal cancers of the microsatellite instability (MIN) class. By genetic dissection, they have found that histone H2A.Z regulates gene transcription and is functionally redundant with the nucleosome remodeling complex Swi/Snf. These genetic interactions have important implications for cell growth control since the SNF5 gene, encoding one of the subunits of the Swi/Snf complex, has been associated with human cancers. Functional genomic analysis of H2A.Z indicates it also is involved in the fidelity of mitotic chromosome transmission and mutants have strong genetic interactions with genes involved in microtubule, kinetochore, and spindle checkpoint function. They are continuing to pursue experiments to dissect the pathways of histone H2A.Z function in genome stability.
Third, the lab discovered a role for a human histone acetyltransferase complex, called HBO1, in DNA replication licensing. This story became even more intriguing when they discovered that HBO1 is part of a large molecular weight HAT complex that includes the tumor suppressor protein p53 as one of its components in vivo. Furthermore, purified HBO1 and p53 interact directly in vitro. The lab found that p53 negatively regulates the HAT activity of HBO1 both in vitro and in vivo, but that tumor-derived mutants of p53 are unable to inhibit activity despite retaining binding interactions. Current work is following up on these observations. Dr. Smith predicts that an HBO1-p53 regulatory loop exists whereby p53 negatively regulates HBO1 activity while HBO1 in turn activates p53 by acetylation.