Although targeting BRAF mutants with RAF inhibitors has achieved promising outcomes in cancer therapy, drug resistance remains a remarkable challenge, and underlying molecular mechanisms are not fully understood. Here, we characterized a previously unknown group of oncogenic BRAF mutants with in-frame insertions (LLRins506 or VLRins506) of αC-β4 loop. Using structure modeling and molecular dynamics simulation, we found that these insertions formed a large hydrophobic network that stabilizes R-spine and thus triggers the catalytic activity of BRAF. Furthermore, these insertions disrupted BRAF dimer interface and impaired dimerization. Unlike BRAF(V600E), these BRAF mutants with low dimer affinity were strongly resistant to all RAF inhibitors in clinic or clinical trials, which arises from their stabilized R-spines. As predicted by molecular docking, the stabilized R-spines in other BRAF mutants also conferred drug resistance. Together, our data indicated that the stability of R-spine but not dimer affinity determines the RAF inhibitor resistance of oncogenic BRAF mutants.

Hyperactive RAS/RAF/MEK/ERK signaling has a well-defined role in cancer biology. Targeting this pathway results in complete or partial regression of most cancers. In recent years, cancer genomic studies have revealed that genetic alterations that aberrantly activate the RAS/RAF/MEK/ERK signaling mainly occur on RAF or upstream, which motivated the extensive development of RAF inhibitors for cancer therapy. Currently, the firstgeneration RAF inhibitors have been approved for treating late-stage cancers with BRAF(V600E) mutations. Although these inhibitors have achieved promising outcomes in clinical treatments, their efficacy is abolished by quick-rising drug resistance. Moreover, cancers with hyperactive RAS exhibit intrinsic resistance to these drugs. To resolve these problems, the second-generation RAF inhibitors have been designed and are undergoing clinical evaluations. Here, we summarize the recent findings from mechanistic studies on RAF inhibitor resistance and discuss the critical issues in the development of next-generation RAF inhibitors with better therapeutic index, which may provide insights for improving targeted cancer therapy with RAF inhibitors.

Hairy cell leukemia (HCL) is a B-lymphoma induced by BRAF(V600E) mutation. However, introducing BRAF(V600E) in B-lymphocytes fails to induce hematological malignancy, suggesting that BRAF(V600E) needs concurrent mutations for driving HCL ontogeny. To resolve this issue, here we surveyed the human HCL genomic sequence data. Together with previous reports, we speculated that tumor suppressor, Trp53, P27, or pTEN restricted the oncogenicity of BRAF(V600E) in B-lymphocytes, whose loss-of-function facilitates the BRAF(V600E)-driven HCL ontogeny. Using genetic mice models, we demonstrated that indeed BRAF(V600E)KI together with Trp53KO or pTENKO in B-lymphocytes induced chronic lymphoma with pathological features of human HCL. To further understand cellular programs essential for HCL ontogeny, we next profiled the gene expression of leukemic cells isolated from BRAF(V600E)KI and Trp53KO or pTENKO mice, and found that these leukemic cells had similar but different gene expression signatures that resemble that of M2 or M1 macrophages. In addition, we examined the expression signature of transcription factors/regulators required for germinal center reaction and memory B cell versus plasma cell differentiation in these leukemic cells and found that most transcription factors/regulators essential for these programs were severely inhibited, which illustrates why hairy cells are arrested at a transitional stage between activated B cells and memory B cells. Together, our study has uncovered concurrent mutations for HCL ontogeny, revealed the B cell origination of hairy cells as well as molecular basis that underlies unique pathological features of this disease, and hence has important implications in both HCL research and clinic treatment. Citation Format: Jiajun Yap, Jimin Yuan, Bin Gao Chen, Wan Hwa Ng, Yuen Rong M. Sim, Kah Chun Goh, Jiancheng Hu. Braf(v600e) mutation together with loss of trp53 or pten drives the origination of hairy cell leukemia from b-lymphocyte [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 19.