Uncovering the Molecular Consequences of Cancer-Associated Histone Mutations Using Saccharomyces Cerevisiae as a Model Organism

Abstract

Histone proteins play key roles in regulating gene expression, DNA packaging, and genome integrity. Mutations in histone genes occur frequently in cancer patients and can disrupt these functions. By screening whole exome sequencing data from thousands of cancer patients we have identified thousands of unique histone missense mutations. Although most cancer-associated mutations remain unstudied, some affect histone post-translational modifications, nucleosome stability, and gene expression. Using Saccharomyces cerevisiae as a model system, my research aimed to expand our understanding of the molecular impacts of cancer-associated histone mutations. Previous work suggested a functional connection between the cancer histone mutations H2BG107R and H2BE79K, the latter being the most common cancer histone mutation affecting histone H2B. We found that these mutants shared phenotypic similarities and showed in silico that these mutations disrupt the same H2BE79-H4R92 salt bridge and surrounding salt bridge network. Despite this, only the H2BE79K mutation altered chromatin association, suggesting unique functional impacts for these mutations. Validating our in silico models, growth defects from the H2BG107R mutant could be rescued by a compensatory mutation (H4R67A) in the salt bridge network. However, the H4R67A mutation did not rescue the H2BE79K mutation, again indicating that H2BG107R and H2BE79K mutations alter histone function in different manners. Expanding our study to other cancer-associated histone mutations in the salt bridge network revealed that the H4R92T mutation produced similar growth defects to the H2BG107R and H2BE79K mutations. H4R92T also disrupts the H2BE79-H4R92 interaction, emphasizing the importance of this interaction for normal nucleosome function. Lastly, I developed a humanized yeast system to study H2B mutations on residues not identical between humans and yeast. Based on a pilot study of eight cancer-associated H2B mutations, I identified a subset that altered histone function. Most notable, the H2BT122A mutation produced growth advantages compared to the wild-type humanized control strain indicating that this mutation rendered the human histone more similar to the yeast version. Overall, this work expands our understanding of the impact of cancer histone mutations and describes a novel method to leverage the simplicity of the yeast model system to study mutations that affect residues not identical between humans and yeast.