Unraveling the Genetic Landscape of Langerhans Cell Histiocytosis in Korean Patients: Comprehensive Insights from Mutational Profiles and Clinical Correlations.

Purpose: This study aimed to conduct a comprehensive genetic analysis of patients with Langerhans cell histiocytosis (LCH), focusing on the frequency of MAPK pathway mutations, detailed mutation profiles of MAPK pathway genes, and their correlation with clinical features and prognosis in Korean LCH patients.

Materials and methods: We performed targeted next-generation sequencing, capable of capturing exons from 382 cancer-related genes, on genomic DNA extracted from formaldehyde-fixed and paraffin-embedded samples of 45 pathologically confirmed LCH patients.

Results: The majority of patients (91.1%) exhibited single-system disease, with bone being the most common location (84.4%). Initial treatments varied, and no patients died during a median follow-up of 6.8 years. Our genetic assays revealed that all patients had MAPK pathway alterations, including BRAF mutations in 51.2%, MAP2K1 mutations in 42.2%, RAF1 mutations in 4.4%, and a KRAS mutation in 2.2%. These mutations were mutually exclusive. Detailed mutation profiles indicated that among the BRAF mutations, there were 18 point mutations and 5 in-frame deletions, while most MAP2K1 mutations were in-frame deletions, with only one missense mutation. We detected previously unreported variations of point mutations in BRAF, MAP2K1, KRAS, and the first instance of a RAF1-KLC1 fusion in LCH. MAP2K1 mutations occurred more frequently in older patients, whereas BRAF V600 mutations were commonly associated with unifocal bone disease. Genetic mutations did not correlate with high-risk features or event-free survival.

Conclusion: This study identified mutually exclusive MAPK pathway mutations in every LCH patient through comprehensive genetic analysis, highlighting the importance of inclusive testing in understanding the disease's genetics.