Preserving Protein Homeostasis
Proteins are synthesized as long linear chains of amino acids that must properly “fold” into intricate three dimensional shapes to function. Protein folding is a delicate process that is readily derailed by a host of genetic and environmental cues frequently encountered by living cells. To preserve protein homeostasis, cells rely on the protein homeostasis system, an extensive network of specialized protein machines that assist in the folding and disposition of the rest of the cellular proteome. A major component of the protein homeostasis system is the abundant protein-folding chaperone heat shock protein 90 (HSP90) (see figure). We found that, in helping other proteins fold and function, HSP90 can “buffer” (that is - mitigate) the detrimental effects of human mutations, which allows the chaperone to alter the course of human disease (Karras Cell 2017). HSP90 achieves this powerful role by directly binding to partially folded proteins and salvaging them from misfolding and aggregation within human cells. We deemed this new class of human mutations, “HSP90-buffered”.
The beneficial ramifications of HSP90-buffering come at the cost of rendering the effects of mutations conditional upon seemingly benign changes in the cellular environment. Despite HSP90’s high abundance in eukaryotic cells, its availability is rapidly taxed by diverse proteotoxic stressors. As a result, highly HSP90-dependent proteins, especially those carrying HSP90-buffered mutations, lose function under proteotoxic conditions. Importantly, clinically relevant proteotoxic factors, such as fever, exacerbate the severity of HSP90-buffered mutations, some of which produce spectacular phenotypes, such as fever-induced loss of hair and qualitative phenotypic discordance in monozygotic twins. (Karras Cell 2017)
HSP90-buffered traits are widespread in nature, but have been classically overlooked. Our goal is to identify HSP90-buffered mutations across natural populations and to understand their roles in disease and evolution. To achieve this goal, we employ multidisciplinary systems approaches rooted in chemical biology, biochemistry, quantitative genetics and functional genomics. In one approach, we utilize the genetically tractable budding yeast Saccharomyces cerevisiae as a discovery platform, and subsequently extend the significance of our findings to mammalian models.
Safeguarding Genome Integrity
Compelling evidence from prokaryotic biology, vertebrate immunoglobulin diversification, and tumorigenesis has demonstrated that compromising genome integrity can have adaptive value. Yet, mutations tend to reduce the fitness of cells and organisms. We study how deranged tumor cells resolve this paradox, and tolerate a sea of detrimental mutations that fuels rapid tumor evolution. We have focused on genome maintenance processes that when misregulated can lead to premature aging and cancer. The FA/BRCA DNA repair pathway is defective in patients suffering from the cancer predisposition syndrome Fanconi Anemia (FA). Over 95% of individuals diagnosed with FA carry inactivating mutations in any of at least 22 genes (among which, BRCA1), all of which encode proteins involved in DNA repair (see figure).
Despite the strong genetic origins of FA, FA patients with the same mutation usually develop different manifestations of the disease. Notably, changes in the environment of the cell can reveal cryptic DNA repair vulnerabilities that are actionable. We characterize genetic and environmental perturbagens of DNA repair to dissect the wiring and post-translational regulation of genome maintenance and to uncover fundamental principles underlying the variable expressivity of DNA repair mutations and improve their usefulness for disease management.