<i>BRAF</i> V600E-positive cells as molecular markers of bone marrow disease in pediatric Langerhans cell histiocytosis

Abstract

1] In order to clarify the clinical impact of measurable BMD in LCH, we investigated somatic mutations in paired tumor and BM samples using a sensitive detection system. We retrospectively performed mutational analyses of 59 LCH tumors by targeted amplicon sequencing using custom-designed primers and subsequently analyzed somatic mutations in 41 paired BM samples using allelespecific droplet digital polymerase chain reaction (ddPCR). BRAF V600E was identified in 25 of 41 (61%) tumor samples. Measurable BMD was detected in 21 of the 25 (84%) cases positive for BRAF V600E, but was not detected in cases positive for other mutations. High mutational loads in BM were significantly associated with multisystem LCH with risk organ involvement, younger age, and disease progression. BRAF V600E in BM was detectable in patients who were refractory to treatment, and the mutational load in BM cells was higher than those in peripheral blood mononuclear cells. A high mutational burden in the BM defines a distinct clinical phenotype of high-risk, young-age patients with multisystem LCH and potential alternative risk factors. Between 1996 and 2020, 65 Japanese pediatric patients diagnosed with LCH by a central or local pathology review were enrolled. The patients were treated with various protocols, including those of the Japan LCH Study Group -96 and -02 and the Japanese Pediatric Leukemia/Lymphoma Study Group LCH-12 in the following 13 domestic centers: