Triple‐negative breast cancer ( TNBC ) has poorer prognosis compared to other types of breast cancers due to the lack of effective therapies and markers for patient stratification. Loss of PTEN tumor suppressor gene expression is a frequent event in TNBC , resulting in over‐activation of the PI 3‐kinase ( PI 3K) pathway and sensitivity to its inhibition. However, PI 3K pathway inhibitors show limited efficacy as monotherapies on these tumors. We report a whole‐genome screen to identify targets whose inhibition enhanced the effects of different PI 3K pathway inhibitors on PTEN ‐null TNBC . This identified a signaling network that relies on both the G protein‐coupled receptor for thrombin ( PAR 1/F2R) and downstream G protein βγ subunits and also epidermal growth factor receptor ( EGFR ) for the activation of the PI 3K isoform p110β and AKT . Compensation mechanisms involving these two branches of the pathway could bypass PI 3K blockade, but combination targeting of both EGFR and PI 3Kβ suppressed ribosomal protein S6 phosphorylation and exerted anti‐tumor activity both in vitro and in vivo, suggesting a new potential therapeutic strategy for PTEN ‐null TNBC .

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Background: Oncogenic variants of GNAQ and GNA11 arising in embryonic or fetal development lead to a spectrum of congenital mosaic disorders including Sturge-Weber syndrome (SWS) and Phakomatosis Pigmentovascularis (PPV). Both SWS and PPV are characterised by vascular malformations in brain, skin and eye, with worsening neurological sequelae after birth suggesting a potential treatment window. Although the genetic causes are understood, and previous data in non-disease-specific cell lines have suggested MAPK activation, the functional effects of the mutations in vascular endothelium have not been studied. The characteristic finding of intracerebral intravascular calcification led us to hypothesise that intra-cellular calcium-signalling disturbances leading to localised imbalances in calcium homeostasis may be involved in disease pathogenesis. Methods: Stable cell lines were established to study cell signalling downstream of GNAQ c.548G>A, p.(R183Q) or GNA11 c.547C>T, p.(R183C) variants in endothelial cells. We re-expressed mutant GNAQ or GNA11 alleles in HEK293 cells in which both endogenous genes had been knocked out for validation purposes. Constitutive calcium signaling was evaluated by measuring inositol-monophosphate accumulation and by a NFAT-luciferase reporter assay, while ligand-stimulated calcium signaling was monitored over time following incubation with intra-cellular calcium probe Fluo-8. In a parallel complementary study, serum calcium and related indices were assayed in 35 patients with SWS or PPV, consented for research under appropriate approvals. Results: GNAQ and GNA11 variants led to marked constitutive calcium signalling in vascular endothelial cells, but not to MAPK activation. GNAQ-mutant endothelial cells also showed aberrant calcium signalling responses to GPCR ligand, which led in turn to sustained replenishment of intracellular calcium stores from the extracellular space. These calcium signalling defects could be rescued by a specific calcium channel inhibitor. Strikingly, and previously undescribed in these diseases, 43% of patients were found to have hypocalcaemia, and 20% hyperparathyroidism, currently presumed secondary but under further investigation. Conclusions: GNAQ mosaicism leads to constitutive and ligand-induced over-activation of intracellular calcium signalling, which increases influx of calcium from the extracellular to the intracellular space and could be the explanation for systemic hypocalcaemia in a substantial proportion of patients. These data have immediate implications for clinical management of these mosaic diseases, and shed light on the in vivo pathogenic actions of of GNAQ/GNA11 oncogenic variants. Citation Format: Davide Zecchin, Nicole Knoepfel, Anna Gluck, Mark Stevenson, Kate E. Lines, Satyamaanasa Polubothu, Noreen Muwanga-Nanyonjo, Sara Barberan-Martin, Fanourios Michailidis, Dale Bryant, Ulrike Loebel, Asuka Inoue, Robert Semple, Sarah Aylett, Rajesh V. Thakker, Veronica A. Kinsler. Functional dissection of GNAQ and GNA11 oncogenic mutations identifies potential targeted therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 851.

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