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Langerhans cell histiocytosis (LCH) is a rare neoplastic disorder caused by somatic genetic alterations in hematopoietic precursor cells differentiating into CD1a+/CD207+ histiocytes. LCH clinical manifestation is highly heterogeneous. BRAF and MAP2K1 mutations account for approximately 80% of genetic driver alterations in neoplastic LCH-cells. However, their clinical associations remain incompletely understood. Here, we present an international clinicogenomic study of childhood LCH, investigating 377 patients genotyped for at least BRAFV600E. MAPK pathway gene alterations were detected in 300 (79.6%) patients, including 191 (50.7%) with BRAFV600E, 54 with MAP2K1 mutations, 39 with BRAF exon 12 mutations, 13 with rare BRAF alterations, and 3 with ARAF or KRAS mutations. Our results confirm that BRAFV600E associates with lower age at diagnosis and higher prevalence of multisystem LCH, high-risk disease, and skin involvement. Furthermore, BRAFV600E appeared to correlate with a higher prevalence of central nervous system (CNS)-risk bone lesions. In contrast, MAP2K1 mutations associated with a higher prevalence of SS-bone LCH, and BRAF exon 12 deletions seemed to correlate with more lung involvement. Although BRAFV600E correlated with reduced event-free survival (EFS) in the overall cohort, neither BRAF nor MAP2K1 mutations associated with EFS when patients were stratified by disease extent. Thus, the correlation of BRAFV600E with inferior clinical outcome is (primarily) driven by its association with disease extents known for high rates of progression or relapse, including multisystem LCH. These findings advance our understanding of factors underlying the remarkable clinical heterogeneity of LCH, but also question the independent prognostic value of lesional BRAFV600E status.

Childhood rhabdomyosarcoma (RMS) has two major histological subtypes: alveolar (aRMS) and embryonal. The aim of the study was to monitor minimal disseminated disease (MDD) using real-time quantitative reverse-transcription PCR (RQ-RT-PCR) of the PAX3-FKHR, PAX7-FKHR fusion genes and myoD1 gene. We prepared an assay using RQ-RT-PCR for a quantitative assessment of MDD in aRMS by using hydrolysis probe for quantification of PAX3-FKHR, PAX7-FKHR and myoD1 genes and beta-2-microglobulin housekeeping gene. Primary tumor samples (44), samples of local recurrences (26) from 48 patients with aRMS were examined by nested RT-PCR and RQ-RT-PCR techniques. Additionally, bone marrow samples (115), peripheral blood progenitor cell samples (27), and peripheral blood samples (25) from 33 aRMS patients were tested. PAX3/7-FKHR and myoD1 transcripts proved to be a sensitive tool for detection of MDD in RMS. We were able to identify 15/25 patients with bone marrow (BM) involvement at the time of presentation using RQ-RT-PCR. We analyzed PAX3-FKHR or PAX7-FKHR expression during the course of the disease. The RQ-RT-PCR results correlated well with nested RT-PCR results (p < 0.0001). The presence of metastases is the most adverse prognostic factor in RMS, and bone marrow is a frequent site of the tumor dissemination in RMS, especially in aRMS. Our results detecting the fusion transcripts or myoD1 transcript in the BM or peripheral blood suggest that patients with positive findings are at high risk of the tumor progression.

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