PROJECT SUMMARY
Breast cancer is a heterogeneous disease, which partly explains differences in prognosis, treatment response,
and metastasis between patients. Breast cancer is classified into histological subtypes (ESR1+;PGR+/-, HER2+,
and “triple-negative” (TNBC)), which are prognostic and predict responsiveness to hormonal and HER2-targeted
therapies. Of these subtypes, TNBC is associated with a worse prognosis and lacks a targeted therapy. The
lethality of TNBC is largely attributed to its aggressiveness and to resistance to traditional therapeutics, especially
as metastases. TNBC show substantial inter- and intratumoral heterogeneity, with phenotypically and
molecularly distinct tumor cell subpopulations existing within a single tumor. A rare subpopulation of cells known
to have intrinsic resistance to chemo- and targeted therapies are called tumor-initiating cells (TICs) (a.k.a. cancer
stem cells). TICs have the ability to self-renew and recapitulate clonally-derived cellular hierarchies upon
generation of a new tumor. Metastasis-initiating cells (MICs), thought to be derived from TICs, possess similar
phenotypic properties to TICs, but are also capable of seeding tumors at distant sites. Current chemotherapies
target the bulk of a lesion, but in many cases, do not effectively eliminate TICs resulting in metastatic recurrence
years after initial treatment. Cell surface markers and signaling reporters have been used to study TICs.
However, such markers are neither unique to TICs nor phenotypically stable, and there is no established method
to lineage trace TICs as they undergo cell state changes. As a result, studying TICs has been a significant
challenge. To address this issue, we have developed a novel Tamoxifen-inducible, Cre recombinase-dependent,
STAT3 signaling-specific lentiviral lineage-tracing (LT) system that will allow us to identify TICs in primary tumors,
to probe their behaviors and phenotypes, and to identify candidate genetic vulnerabilities. The central
hypothesis of this proposal is that a subset of STAT3 signaling TICs in some TNBC tumors represent
MICs, which possess a distinct transcriptional program that can be targeted to eliminate TICs and
improve response to chemotherapy. In Aim 1, we will clarify whether STAT3 signaling TICs represent the
MIC population. In Aim 2 we will determine whether STAT3 signaling TICs in the primary tumor express distinct
genes that can be targeted to prevent tumor progression. The results of this proposal will have a positive impact
on the field as it will uncover the role of STAT3 signaling TICs in metastasis and identify genetic vulnerabilities
that may be targeted to eliminate TICs and improve chemotherapy response. The identification of new
therapeutic targets that eliminate the TIC population can improve clinical outcomes for TNBC patients.