Alternative genetic mechanisms of BRAF activation in Langerhans cell histiocytosis

Abstract

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.

Recurrent BRAF Gene Rearrangements in Myxoinflammatory Fibroblastic Sarcomas, but Not Hemosiderotic Fibrolipomatous Tumors

“…15,18–22,24,25,35 Notably, pERK immunostain was also positive in tumors with TGFBR3-MGEA5 fusions, including MIFS, HFLT, and hybrid HFLT-MIFS. MAPK pathway activation may be a common event shared by MIFS with BRAF fusions and those with TGFBR3-MGEA5 fusions.…”

“…BRAF gene fusions have been identified as the driver genetic events in several different tumor types, including the majority of pilocytic astrocytomas, 18 a subset of pancreatic acinar cell carcinomas, 15 and smaller proportions of papillary thyroid carcinomas, 19,20 spitzoid melanocytic neoplasms, 21–23 rare cases of Langerhans cell histiocytosis, 24 prostate cancer, gastric cancer, 25 lung cancer, 26 etc. 27 In soft tissue tumors, only a chest wall mass of a 55 year-old woman diagnosed as a malignant spindle cell neoplasm has been reported to have KIAA1549–BRAF fusion to date, although its reported immunoprofile with co-expression of both S100 and CD34 are unsual.…”

Mutually exclusive recurrent KRAS and MAP2K1 mutations in Rosai–Dorfman disease

“…Interestingly, BRAF V600E and NRAS mutations, both being members of the MAPK/ERK pathway, appeared to be mutually exclusive; while PIK3CA mutation co-existed with BRAF mutation in 4 of 7 cases, supporting involvement of both the PI3K/AKT and MAPK/ERK signaling pathways in a subset of Erdheim-Chester disease patients (20). In another recent study using whole-exome sequencing, targeted BRAF sequencing, and/or whole-transcriptome sequencing (RNA seq), alternative genetic mechanisms of ERK activation were identified in a subset of BRAF V600E-unmutated Langerhans cell histiocytosis, namely in-frame BRAF deletions in the β3-αC loop and a FAM73A-BRAF fusion (23). Similarly, in non-Langerhans cell histiocytosis histiocytoses, MAPK-pathway-activating RNF11-BRAF, CLIP2-BRAF, KIF5B-ALK and LMNA-NTRK1 fusions were identified, the last two abnormalities leading to activation of related pathways, namely ALK, STAT3 and PI3K-AKT (21).…”

“…Mutations of PI3KCA in the PI3K-AKT-mTOR pathway, an alternative downstream effector of RAS signaling, have been shown in a subset of Erdheim-Chester disease cases (20–22). Furthermore, ERK activation has been found in a subset of BRAF - and MAP2K1 -wild type Langerhans cell histiocytosis and non- Langerhans cell histiocytosis cases (21, 23). These molecular discoveries have played an important role in the stratification of Langerhans cell histiocytosis and Erdheim-Chester disease from other histiocytoses in the recently revised classification of histiocytoses (15); conversely a very limited number of Rosai-Dorfman disease cases have been investigated for gene mutations and the results have been controversial.…”

High prevalence of myeloid neoplasms in adults with non–Langerhans cell histiocytosis

“…However, since 2010, a series of genomic studies have uncovered BRAFV600E mutations in 55% to 70% of ECD as well as Langerhans cell histiocytosis (LCH) patients, providing evidence that these diseases represent clonal disorders driven by activated MAPK signaling. 1,2 Subsequently, activating mutations in MAP2K1, [3][4][5][6] ARAF, 6,7 and fusions in kinases including BRAF 6,8 were found in the majority of BRAFV600-wild-type ECD and LCH patients. PI3KCA 9 and N/KRAS mutations are more frequent in ECD than in LCH.…”