Graduation Date

Summer 8-9-2024

Document Type


Degree Name

Doctor of Philosophy (PhD)


Cancer Research

First Advisor

Robert E. Lewis


Tumor heterogeneity is an important contributor to therapy resistance and relapse. Within the tumor, there are subpopulations of tumorigenic, self-renewing cells with enhanced clonogenicity and chemoresistance, termed tumor-initiating cells (TICs). Despite their importance in treatment response, there are no effective therapies to target TICs in solid tumors.

This study identifies a novel role for Kinase Suppressor of Ras 1 (KSR1) in regulating TICs and therapy resistance in Ras-driven cancers. KSR1 is a molecular scaffold in the Raf/MEK/ERK kinase cascade downstream of Ras. KSR1 is necessary for Ras-mediated transformation, but dispensable for normal cell growth; further, ksr1-/- mice are phenotypically normal but resistant to tumor formation, making KSR1 an attractive therapeutic target. KSR1 was found to regulate key properties of TICs, including clonogenicity, expression of a key TIC marker, as well as in vitro and in vivo tumor formation by extreme limiting dilution analysis (ELDA) in colorectal and non-small cell lung cancer cells.

Recent studies indicate that targeted inhibitors can shift tumor cells into a more stem-like state. The discovery that KSR1 regulates TICs led to investigation of KSR1 role in this inhibitor-induced cell population shift, and its consequences on resistance to the MEK inhibitor, trametinib. KSR1 was found to be necessary for trametinib-mediated increase in TICs coinciding with rebound re-activation of ERK, and in development of trametinib-resistant cells. KSR1 KO was effective at reducing TICs and trametinib resistance across a range of Ras mutational alleles and co-mutational profiles, suggesting MEK inhibitor treatment combined with KSR1 inhibition as a therapeutic strategy could be applicable to many patients.

The role of KSR1 in regulating TICs and MEK inhibitor resistance supports its potential as a therapeutic target. As proof-of-concept for a protein degradation approach to targeting KSR1, the dTAG (degradation tag) system was applied to degrade KSR1 in CRC cells. KSR1 was successfully degraded with this system both in vitro and in vivo. Moreover, KSR1 targeting with dTAG inhibited anchorage-independent colony formation and ERK signaling rebound after trametinib treatment, consistent with data from KSR1 genomic KO models. These studies, collectively, provide novel insight into KSR1 as a therapeutic target in Ras-driven cancers.

Available for download on Friday, December 20, 2024