Oncogene-induced senescence distinguishes indolent from aggressive forms of pulmonary and non-pulmonary Langerhans cell histiocytosis

The clonal/neoplastic nature of Langerhans cell histiocytosis (LCH) has recently been demonstrated by a high prevalence of BRAF mutations, including pulmonary LCH (PLCH). We hypothesized that BRAF-induced senescence, as demonstrated in nevi and melanoma, is involved in the pathogenesis of LCH and PLCH. In a series of pulmonary (19 cases) and non-pulmonary LCH (19 cases), including five aggressive cases, we investigated occurrence of the BRAF V600E mutation by molecular analysis and/or immunohistochemistry using a validated antibody (VE1). The expression of cell-senescence markers p16(INK4a) and p21(CIP1/WAF1) was also immunohistochemically investigated. We demonstrated that 6/19 cases of LCH and 12/19 cases of PLCH were VE1 positive, matching with molecular analysis, and in all cases both p16(INK4a) and p21(CIP1/WAF1) were expressed, irrespective of BRAF mutation status. Interestingly, all the aggressive cases did not express p16(INK4a), thus suggesting that loss of senescence control could be related to clinical aggressiveness of LCH, as in melanoma.