CNS Langerhans cell histiocytosis: Common hematopoietic origin for LCH-associated neurodegeneration and mass lesions.

BACKGROUND: Central nervous system Langerhans cell histiocytosis (CNS-LCH) brain involvement may include mass lesions and/or a neurodegenerative disease (LCH-ND) of unknown etiology. The goal of this study was to define the mechanisms of pathogenesis that drive CNS-LCH. METHODS: Cerebrospinal fluid (CSF) biomarkers including CSF proteins and extracellular BRAFV600E DNA were analyzed in CSF from patients with CNS-LCH lesions compared with patients with brain tumors and other neurodegenerative conditions. Additionally, the presence of BRAFV600E was tested in peripheral mononuclear blood cells (PBMCs) as well as brain biopsies from LCH-ND patients, and the response to BRAF-V600E inhibitor was evaluated in 4 patients with progressive disease. RESULTS: Osteopontin was the only consistently elevated CSF protein in patients with CNS-LCH compared with patients with other brain pathologies. BRAFV600E DNA was detected in CSF of only 2/20 (10%) cases, both with LCH-ND and active lesions outside the CNS. However, BRAFV600E+ PBMCs were detected with significantly higher frequency at all stages of therapy in LCH patients who developed LCH-ND. Brain biopsies of patients with LCH-ND demonstrated diffuse perivascular infiltration by BRAFV600E+ cells with monocyte phenotype (CD14+ CD33+ CD163+ P2RY12- ) and associated osteopontin expression. Three of 4 patients with LCH-ND treated with BRAF-V600E inhibitor experienced significant clinical and radiologic improvement. CONCLUSION: In LCH-ND patients, BRAFV600E+ cells in PBMCs and infiltrating myeloid/monocytic cells in the brain is consistent with LCH-ND as an active demyelinating process arising from a mutated hematopoietic precursor from which LCH lesion CD207+ cells are also derived. Therapy directed against myeloid precursors with activated MAPK signaling may be effective for LCH-ND. Cancer 2018;124:2607-20. © 2018 American Cancer Society.

Alternative genetic mechanisms of BRAF activation in Langerhans cell histiocytosis.

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 ß3-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.

Mutually exclusive recurrent somatic mutations in MAP2K1 and BRAF support a central role for ERK activation in LCH pathogenesis.

Langerhans cell histiocytosis (LCH) is a myeloproliferative disorder characterized by lesions composed of pathological CD207(+) dendritic cells with an inflammatory infiltrate. BRAFV600E remains the only recurrent mutation reported in LCH. In order to evaluate the spectrum of somatic mutations in LCH, whole exome sequencing was performed on matched LCH and normal tissue samples obtained from 41 patients. Lesions from other histiocytic disorders, juvenile xanthogranuloma, Erdheim-Chester disease, and Rosai-Dorfman disease were also evaluated. All of the lesions from histiocytic disorders were characterized by an extremely low overall rate of somatic mutations. Notably, 33% (7/21) of LCH cases with wild-type BRAF and none (0/20) with BRAFV600E harbored somatic mutations in MAP2K1 (6 in-frame deletions and 1 missense mutation) that induced extracellular signal-regulated kinase (ERK) phosphorylation in vitro. Single cases of somatic mutations of the mitogen-activated protein kinase (MAPK) pathway genes ARAF and ERBB3 were also detected. The ability of MAPK pathway inhibitors to suppress MAPK kinase and ERK phosphorylation in cell culture and primary tumor models was dependent on the specific LCH mutation. The findings of this study support a model in which ERK activation is a universal end point in LCH arising from pathological activation of upstream signaling proteins.

Differentiating skin-limited and multisystem Langerhans cell histiocytosis.

OBJECTIVE: To identify features associated with multisystem involvement and therapeutic failure in patients with skin Langerhans cell histiocytosis (LCH). STUDY DESIGN: We reviewed medical records of 71 consecutive patients with LCH with skin involvement evaluated at Texas Children's Hospital and analyzed clinical features, laboratory results, and the presence of circulating cells with the BRAF-V600E mutation with respect to initial staging and clinical outcomes. RESULTS: Skin disease in patients older than 18 months of age at diagnosis was associated with the presence of multisystem disease (OR, 9.65; 95% CI, 1.17-79.4). Forty percent of patients referred for presumed skin-limited LCH had underlying multisystem involvement, one-half of these with risk-organ involvement. Patients with skin-limited LCH had a 3-year progression-free survival of 89% after initial therapy, and none developed multisystem disease. Patients with skin/multisystem involvement had a 3-year progression-free survival of 44% with vinblastine/prednisone therapy, and risk-organ involvement did not correlate with failure to achieve nonactive disease. Circulating cells with BRAF-V600E were detected at higher frequency in patients with multisystem involvement (8 of 11 skin/multisystem vs 1 of 13 skin-limited; P = .002). CONCLUSION: Skin-limited LCH necessitates infrequent therapeutic intervention and has a lower risk of progression relative to skin plus multisystem LCH. The less-aggressive clinical course and lack of circulating cells with the BRAF-V600E mutation in skin-limited LCH suggest a different mechanism of disease origin compared with multisystem or risk-organ disease.

Assessment of an automated approach for variant interpretation in screening for monogenic disorders: A single-center study.

BACKGROUND: Automation has been introduced into variant interpretation, but it is not known how automated variant interpretation performs on a stand-alone basis. The purpose of this study was to evaluate a fully automated computerized approach. METHOD: We reviewed all variants encountered in a set of carrier screening panels over a 1-year interval. Observed variants with high-confidence ClinVar interpretations were included in the analysis; those without high-confidence ClinVar entries were excluded. RESULTS: Discrepancy rates between automated interpretations and high-confidence ClinVar entries were analyzed. Of the variants interpreted as positive (likely pathogenic or pathogenic) based on ClinVar information, 22.6% were classified as negative (variants of uncertain significance, likely benign or benign) variants by the automated method. Of the ClinVar negative variants, 1.7% were classified as positive by the automated software. On a per-case basis, which accounts for variant frequency, 63.4% of cases with a ClinVar high-confidence positive variant were classified as negative by the automated method. CONCLUSION: While automation in genetic variant interpretation holds promise, there is still a need for manual review of the output. Additional validation of automated variant interpretation methods should be conducted.

Circulating CD1c+ myeloid dendritic cells are potential precursors to LCH lesion CD1a+CD207+ cells.

Langerhans cell histiocytosis (LCH) is a myeloproliferative disorder that is characterized by the inflammatory lesions with pathogenic CD1a+CD207+ dendritic cells (DCs). BRAFV600E and other somatic activating MAPK gene mutations have been identified in differentiating bone marrow and blood myeloid cells, but the origin of the LCH lesion CD1a+CD207+ DCs and mechanisms of lesion formation remain incompletely defined. To identify candidate LCH CD1a+CD207+ DC precursor populations, gene-expression profiles of LCH lesion CD1a+CD207+ DCs were first compared with established gene signatures from human myeloid cell subpopulations. Interestingly, the CD1c+ myeloid DC (mDC) gene signature was most enriched in the LCH CD1a+CD207+ DC transcriptome. Additionally, the BRAFV600E allele was not only localized to CD1a+CD207- DCs and CD1a+CD207+ DCs, but it was also identified in CD1c+ mDCs in LCH lesions. Transcriptomes of CD1a+CD207- DCs were nearly indistinguishable from CD1a+CD207+ DCs (both CD1a+CD207low and CD1a+CD207high subpopulations). Transcription profiles of LCH lesion CD1a+CD207+ DCs and peripheral blood CD1c+ mDCs from healthy donors were compared to identify potential LCH DC-specific biomarkers: HLA-DQB2 expression was significantly increased in LCH lesion CD1a+CD207+ DCs compared with circulating CD1c+ mDCs from healthy donors. HLA-DQB2 antigen was identified on LCH lesion CD1a+CD207- DCs and CD1a+CD207+ DCs as well as on CD1c+(CD1a+CD207-) mDCs, but it was not identified in any other lesion myeloid subpopulations. HLA-DQB2 expression was specific to peripheral blood of patients with BRAFV600E+ peripheral blood mononuclear cells, and HLA-DQB2+CD1c+ blood cells were highly enriched for the BRAFV600E in these patients. These data support a model in which blood CD1c+HLA-DQB2+ mDCs with activated ERK migrate to lesion sites where they differentiate into pathogenic CD1a+CD207+ DCs.

RAF/MEK/extracellular signal-related kinase pathway suppresses dendritic cell migration and traps dendritic cells in Langerhans cell histiocytosis lesions.

Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasia characterized by granulomatous lesions containing pathological CD207+ dendritic cells (DCs) with constitutively activated mitogen-activated protein kinase (MAPK) pathway signaling. Approximately 60% of LCH patients harbor somatic BRAFV600E mutations localizing to CD207+ DCs within lesions. However, the mechanisms driving BRAFV600E+ LCH cell accumulation in lesions remain unknown. Here we show that sustained extracellular signal-related kinase activity induced by BRAFV600E inhibits C-C motif chemokine receptor 7 (CCR7)-mediated DC migration, trapping DCs in tissue lesions. Additionally, BRAFV600E increases expression of BCL2-like protein 1 (BCL2L1) in DCs, resulting in resistance to apoptosis. Pharmacological MAPK inhibition restores migration and apoptosis potential in a mouse LCH model, as well as in primary human LCH cells. We also demonstrate that MEK inhibitor-loaded nanoparticles have the capacity to concentrate drug delivery to phagocytic cells, significantly reducing off-target toxicity. Collectively, our results indicate that MAPK tightly suppresses DC migration and augments DC survival, rendering DCs in LCH lesions trapped and resistant to cell death.

BRAF-V600E expression in precursor versus differentiated dendritic cells defines clinically distinct LCH risk groups.

Langerhans cell histiocytosis (LCH) is a clonal disorder with elusive etiology, characterized by the accumulation of CD207(+) dendritic cells (DCs) in inflammatory lesions. Recurrent BRAF-V600E mutations have been reported in LCH. In this study, lesions from 100 patients were genotyped, and 64% carried the BRAF-V600E mutation within infiltrating CD207(+) DCs. BRAF-V600E expression in tissue DCs did not define specific clinical risk groups but was associated with increased risk of recurrence. Strikingly, we found that patients with active, high-risk LCH also carried BRAF-V600E in circulating CD11c(+) and CD14(+) fractions and in bone marrow (BM) CD34(+) hematopoietic cell progenitors, whereas the mutation was restricted to lesional CD207(+) DC in low-risk LCH patients. Importantly, BRAF-V600E expression in DCs was sufficient to drive LCH-like disease in mice. Consistent with our findings in humans, expression of BRAF-V600E in BM DC progenitors recapitulated many features of the human high-risk LCH, whereas BRAF-V600E expression in differentiated DCs more closely resembled low-risk LCH. We therefore propose classification of LCH as a myeloid neoplasia and hypothesize that high-risk LCH arises from somatic mutation of a hematopoietic progenitor, whereas low-risk disease arises from somatic mutation of tissue-restricted precursor DCs.