Abstract 2424: Screen for actionable ERBB3 mutations
Authors: Koivu Marika K. , Chakroborty Deepankar , Kurppa Kari J. , Elenius Klaus 
Published: July 2021

Abstract
Activating mutations and copy number variations in ERBB genes have been identified in several cancer types, and a number of cancer drugs targeting ERBB receptors have been approved for clinical use. These drugs include monoclonal antibodies (mAb) that selectively target either EGFR (ERBB1) or ERBB2 (HER2), and tyrosine kinase inhibitors (TKI) that block the kinase activity of all three kinase-competent ERBBs, i. e. EGFR, ERBB2, and ERBB4. ERBB3 is a unique member of the ERBB family in that it does not possess potent intrinsic kinase activity, and thus cannot be directly targeted with TKIs. However, it is still able to dimerize and activate the other three ERBB family members, and as a result, critically mediate oncogenic signaling as a member of heterodimeric complexes, such as the ERBB2/ERBB3 heterodimer. Thus, changes in ERBB3 structure and function may still affect responsiveness of cancer cells to ERBB TKIs. Moreover, several mAbs directly targeting ERBB3 in cancer are currently in preclinical and clinical development. To address whether ERBB3 mutations can affect the potential of ERBB3 to promote growth or affect sensitivity to ERBB inhibitors, we set up a screen for functional effects of all possible random ERBB3 missense or nonsense mutations in the context of ERBB2/ERBB3 heterodimers. To this end, the iSCREAM (in vitro screen of activating mutations) model of somatic evolution, recently developed in our laboratory, was chosen. This platform exploits randomly mutated cDNA libraries of the gene of interest created with error-prone PCR and allows the assessment of growth-advantage conferred by thousands of mutations in parallel. To set up the model for ERBB3 mutation screen, interleukin-3 (IL-3)-dependent Ba/F3 cells were engineered to express a homodimerization-incompetent ERBB2 V956R mutant together with ERBB3 constructs. The model was validated to serve as a readout for ERBB3's ability in activating ERBB2 kinase by demonstrating that cells expressing the two known transforming ERBB3 mutations, E928G and G284R, together with ERBB2 V956R survived and expanded in the absence of IL-3. However, control cells expressing ERBB2 V956R together with wild-type ERBB3 rapidly died in the absence of IL-3. The cell background with ERBB2 V956R expression will subsequently be used as a target for retroviral expression of cDNA library of randomly mutated ERBB constructs. After infection, the cell clones with activating ERBB3 mutations will be allowed to evolve and the activating mutations will be identified by ERBB3-targeted next generation sequencing. The goal is to identify activating ERBB3 mutations that are directly actionable or that provide predictive value for the use of drugs targeting ERBB signaling.
Citation Format: Marika K. Koivu, Deepankar Chakroborty, Kari J. Kurppa, Klaus Elenius. Screen for actionable ERBB3 mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2424.