Date Available

1-3-2025

Year of Publication

2024

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Medicine

Department/School/Program

Toxicology and Cancer Biology

Advisor

Eva Goellner

Co-Director of Graduate Studies

Isabel Mellon

Abstract

DNA mismatch repair (MMR) is a critical repair process necessary for not only repairing mispairs incorporated into the DNA during replication, but also for inducing apoptosis in response to certain types of DNA damage. MMR is required for maintaining genomic integrity and due to the importance of this pathway, any impairment of its function can lead to increased mutation frequency, which is well known to be a driving force of cancer development, progression, and resistance. Despite extensive studies involving the proteins involved in the MMR pathway, the regulation of those proteins remains relatively unknown. Regulation of MLH1, a protein necessary for MMR function and progression, is of particular interest due to many cancers being associated with its dysfunction. Our initial studies demonstrated a decrease in MLH1 protein expression after treatment with tyrosine kinase inhibitors (TKIs). Further, we found a novel regulatory mechanism of MLH1 involving phosphorylation-dependent stability by the TKI target, ABL1. In the absence of MLH1 tyrosine phosphorylation by ABL1, we show that MLH1 becomes unstable and is targeted by the Hsp70 chaperone protein where it is subsequently signaled for degradation through the lysosome.

The beforementioned results resulted in many more research directions, including identifying the MLH1 phosphorylation site and determining translational applications of this mechanism. We report an ABL1 tyrosine phosphorylation site located on the C-terminal of MLH1, which when disrupted, significantly impairs MLH1 activity and MMR function. Additionally, we confirmed this mechanism exists across cancerous cell lines, including melanoma. Due to the understanding that cancers with deficient MMR and a phenotype termed microsatellite instability (MSI) generally respond extremely well to immunotherapy because of increased neoantigen production, we hypothesized that long term TKI treatment would yield persistent impaired MMR function and an MSI phenotype, ultimately generating better immunotherapy response. TKIs are used very often in the clinic, so we also questioned whether patients treated with ABL1 kinase inhibitors for chronic myelogenous leukemia had an increased risk of developing secondary cancer. By using the SEERStat database, we observed a much higher secondary cancer risk in patients likely treated with ABL1 inhibitors when compared to the general population. Interestingly, when comparing the most common secondary cancers with cancers often associated with deficient MMR, there were similarities, specifically noting cancers of the GI tract and the respiratory system. Overall, results of this dissertation demonstrate a novel MMR regulatory mechanism by the ABL1 kinase and present the idea that ABL1 inhibitor treatment can be utilized to improve immunotherapy response in otherwise resistance cancers, while also noting the observational data indicating that ABL1 inhibitor treatment may be linked to increased secondary cancer risk, specifically cancers that are associated with deficient MMR.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.07

Funding Information

  • This study was supported by the National Institutes of Health (R00ES026653) from 2020-2022
  • This study was supported by the Markey Foundation Markey Women Strong Distinguished Researcher award in 2020
  • This study was supported by the UK-CARES Grant P30 ES026529 2020-2022
  • This study was supported by the National Institutes of Health (R03CA267326)

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Available for download on Friday, January 03, 2025

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