Therapeutic targeting of class 2 & 3 BRAF mutated lung adenocarcinoma

Lung cancer ranks as the leading cause of cancer-related death. BRAF mutations account for approximately 5% of all lung adenocarcinoma (LUAD) cases, among which BRAFV600E mutation (class 1) is prevalent. This mutation leads to elevated kinase activity and is generally treated with a combination of BRAF and MEK inhibitors. Half of BRAF-mutated LUAD patients have non-V600 mutations, divided into class 2 and 3. Class 2 involves constitutively active RAS-independent dimers with high or intermediate BRAF kinase activity, while class 3 involves RAS-dependent dimers with impaired kinase activity, requiring additional upstream signaling. Patients with class 2 or 3 mutations generally show low or no response to combined BRAF and MEK inhibitors. This project aims to better characterize class 2 and 3 BRAF mutants and identify potential therapeutic targets.
In agreement with the existing literature, our first results highlighted that several class 2 and 3 BRAF mutated LUAD cell lines present various sensitivity profile towards BRAF, MEK and ERK inhibitors. The numerous co-existing alterations with BRAF mutations in these cell lines do not allow to draw any conclusions. Therefore, our first objective was to establish an isogenic model of BRAF mutants using the ATII KRAS-ER human lung cell line. These immortalized alveolar type II cells are engineered to express an active mutated KRAS upon tamoxifen treatment, and will allow the study of class 3 BRAF mutants which generally require a concomitant KRAS mutation. We are currently generating this isogenic models either with inducible exogenous mutated form of BRAF or with CRISPR-Cas9 systems. Our preliminary evaluation of expression and activation of some components of both the MAPK and PI3K signaling pathways shows that class 2 mutants have an intermediate kinase activity compared between class 1 and 3. In addition, KRAS activation upon tamoxifen treatment increases the phosphorylation of downstream MAPK pathway effectors in class 3 BRAF mutants.
In order to decipher potential vulnerabilities of these class 2 and 3 BRAF mutants, we are performing both whole genome CRISPR-Cas9 knockout and pharmaceutical screenings. The final objective is to identify new potential therapeutic targets for this patient population in order to achieve long lasting response.