A European study on blood mutation monitoring in children with BRAFV600E-positive Langerhans Cell Histiocytosis.

Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasm driven by somatic variants in MAPK-encoding genes, most commonly BRAFV600E. The prognostic value of BRAFV600E in the peripheral blood of children with LCH has never been investigated in an international setting characterized by different approaches. To this aim, five centers from the European Consortium for Histiocytosis (ECHO) analyzed the peripheral blood of 226 children enrolled in, or managed according to, the LCH-IV protocol. We quantified BRAFV600E burden in different DNA sources at diagnosis and after 6 weeks of vinblastine/prednisone and evaluated its association with 2-year event-free survival (EFS). At diagnosis, BRAFV600E positivity could be equally determined through circulating cell-free (ccf) or peripheral blood mononuclear cell (PBMC)-DNA in patients with multi-system (MS) LCH. In single-system (SS) LCH, analyzing both substrates improved diagnostic sensitivity. For each clinical subgroup at diagnosis, we identified BRAFV600E levels associated with inferior 2-year EFS: ≥LOQ (0.1% for ccf-DNA, 0.01% for PBMC-DNA) in SS and MS risk-organ negative (RO-) groups; ≥1% (both ccf- and PBMC-DNA) in MS risk-organ positive (RO+) group. Notably, BRAFV600E burden was independently associated with inferior EFS in multivariable analysis (HR 2.05, 95%CI: 1.11-3.78 for BRAFV600E ≥LOQ, p=0.022; HR 4.53, 95%CI: 2.34-8.79 for BRAFV600E≥1%, p<0.001). After 6 weeks of vinblastine/prednisone, decreasing BRAFV600E burden (<0.1% in ccf-DNA or <0.01% in PBMC-DNA) was significantly associated with higher 2-year EFS (p<0.001 and p=0.009). These findings provide a framework for refining risk-stratification in pediatric LCH through the incorporation of peripheral blood BRAFV600E testing into international prospective clinical trials.