Breast Cancer Mutations HER2V777L and PIK3CAH1047R Activate the p21-CDK4/6-Cyclin D1 Axis to Drive Tumorigenesis and Drug Resistance

Metastatic progression of breast cancer along with decreased mitochondrial cell death priming of breast cancer cells: a case report

Metastatic breast cancer remains to be a major cause of cancer-related deaths in women. Exploring the molecular mechanisms to identify targetable alterations in progressing breast cancer and developing functional tools to predict therapy response in these patients are needed. In this report, we present a case of breast cancer patient who progressed following surgery and adjuvant endocrine therapy. Radiological and pathological analyses revealed metastasis to liver and brain. Paired liquid biopsies demonstrated acquired ERBB2 mutations in addition to TP53 and PIK3CA mutations, which were also present before progression. BH3 profiling test demonstrated decreased mitochondrial cell death priming in CTCs of the patient after progression. In conclusion, novel personalized treatment strategies are needed to monitor metastatic breast cancer patients for better clinical benefit.

Integrative modeling uncovers p21-driven drug resistance and prioritizes therapies for PIK3CA-mutant breast cancer

Utility of PI3Kα inhibitors like BYL719 is limited by the acquisition of genetic and non-genetic mechanisms of resistance which cause disease recurrence. Several combination therapies based on PI3K inhibition have been proposed, but a way to systematically prioritize them for breast cancer treatment is still missing. By integrating published and in-house studies, we have developed in silico models that quantitatively capture dynamics of PI3K signaling at the network-level under a BYL719-sensitive versus BYL719 resistant-cell state. Computational predictions show that signal rewiring to alternative components of the PI3K pathway promote resistance to BYL719 and identify PDK1 as the most effective co-target with PI3Kα rescuing sensitivity of resistant cells to BYL719. To explore whether PI3K pathway-independent mechanisms further contribute to BYL719 resistance, we performed phosphoproteomics and found that selection of high levels of the cell cycle regulator p21 unexpectedly promoted drug resistance in T47D cells. Functionally, high p21 levels favored repair of BYL719-induced DNA damage and bypass of the associated cellular senescence. Importantly, targeted inhibition of the check-point inhibitor CHK1 with MK-8776 effectively caused death of p21-high T47D cells, thus establishing a new vulnerability of BYL719-resistant breast cancer cells. Together, our integrated studies uncover hidden molecular mediators causing resistance to PI3Kα inhibition and provide a framework to prioritize combination therapies for PI3K-mutant breast cancer.