Alternative genetic mechanisms of BRAF activation in Langerhans cell histiocytosis

• Published in: Blood Vol. 128; no. 21; pp. 2533 - 2537
• Main Authors: Chakraborty, Rikhia, Burke, Thomas M., Hampton, Oliver A., Zinn, Daniel J., Lim, Karen Phaik Har, Abhyankar, Harshal, Scull, Brooks, Kumar, Vijetha, Kakkar, Nipun, Wheeler, David A., Roy, Angshumoy, Poulikakos, Poulikos I., Merad, Miriam, McClain, Kenneth L., Parsons, D. Williams, Allen, Carl E.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 24.11.2016; American Society of Hematology
• Subjects: Adolescent; Adult; Aged; Brief Report; Child; Child, Preschool; Enzyme Activation - genetics; Female; Histiocytosis, Langerhans-Cell - enzymology; Histiocytosis, Langerhans-Cell - genetics; Humans; Infant; Male; Mutation; Myeloid Neoplasia; Oncogene Proteins, Fusion - genetics; Oncogene Proteins, Fusion - metabolism; Protein Domains; Proto-Oncogene Proteins B-raf - genetics; Proto-Oncogene Proteins B-raf - metabolism
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2016-08-733790

Langerhans cell histiocytosis (LCH) is characterized by inflammatory lesions containing pathologic CD207+ dendritic cells with constitutively activated ERK. Mutually exclusive somatic mutations in MAPK pathway genes have been identified in ∼75% of LCH cases, including recurrent BRAF-V600E and MAP2K1 mutations. To elucidate mechanisms of ERK activation in the remaining 25% of patients, we performed whole-exome sequencing (WES, n = 6), targeted BRAF sequencing (n = 19), and/or whole-transcriptome sequencing (RNA-seq, n = 6) on 24 LCH patient samples lacking BRAF-V600E or MAP2K1 mutations. WES and BRAF sequencing identified in-frame BRAF deletions in the β3-αC loop in 6 lesions. RNA-seq revealed one case with an in-frame FAM73A-BRAF fusion lacking the BRAF autoinhibitory regulatory domain but retaining an intact kinase domain. High levels of phospho-ERK were detected in vitro in cells overexpressing either BRAF fusion or deletion constructs and ex vivo in CD207+ cells from lesions. ERK activation was resistant to BRAF-V600E inhibition, but responsive to both a second-generation BRAF inhibitor and a MEK inhibitor. These results support an emerging model of universal ERK-activating genetic alterations driving pathogenesis in LCH. A personalized approach in which patient-specific alterations are identified may be necessary to maximize benefit from targeted therapies for patients with LCH. •A BRAF gene fusion and small in-frame BRAF deletions were found in a subset of LCH lesions lacking BRAF-V600E or MAP2K1 mutations.•In LCH model systems, responses to MAPK pathway inhibitors depend on the specific genetic alteration that drives ERK activation.