Progress towards molecular-based management of childhood Langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is characterized by inflammatory lesions containing abundant CD1a+ CD207+ histiocytes that lead to the destruction of affected tissues. This disease has a remarkable pleiotropic clinical presentation and most commonly affects young children. Although the current mortality rate is very low for childhood LCH patients (<2%), reactivation frequently occurs after a long period of disease control and the rates of permanent complications and sequelae remain high. Advances in genomic sequencing technologies in this past decade have highlighted somatic molecular alterations responsible for the disease in around 80% of childhood LCH cases. More than half of these cases harbored the BRAF mutation, and most other mutations also concerned proteins involved in the MAPKinase pathway. In addition to improving what is known about the LCH pathology, this molecular knowledge provides opportunities to optimize patient management. The BRAF mutation is associated with more severe presentations of the disease, a high reactivation rate, and a high permanent complication rate; this mutation therefore paves the way for future stratified management approaches. These therapies may be based on the patient's molecular status as well as other clinical characteristics of the disease that are independently associated with undesired events. Moreover, as observed in patients with solid tumors, the BRAF allele can be detected in the circulating cell-free DNA of patients with severe BRAF-mutated LCH. Quantification of the plasmatic BRAF load for this group of patients can precisely monitor response to therapy. Finally, targeted therapies, such as BRAF inhibitors, are new therapeutic options especially designed for refractory multisystemic LCH involving risk organs. However, the long-term efficacy, long-term tolerance, optimal protocol scheme, and appropriate modalities of administration for these innovative therapies for children still need to be defined, a huge challenge.