The family of juvenile xanthogranuloma family neoplasms (JXG) with ERK-pathway mutations are now classified within the "L" (Langerhans) group, which includes Langerhans cell histiocytosis (LCH) and Erdheim Chester disease (ECD).
Recent molecular studies have revealed recurrent mutations involving genes in the MAPK/ERK pathway in Langerhans cell histiocytosis and Erdheim-Chester disease.
The recent discovery of somatic mutations in ARAF and in MAP2K1, which lead to activation of the RAS-RAF-MEK -ERK pathway in the setting of wild-type BRAF, as well as the finding that activating mutation in MAP2K1 are relatively insensitive to MEK inhibitors, suggest that a more detailed understanding of this pathway in LCH may be necessary for the development of more effective targeted therapies.
Increased levels of phospho-ERK in lesional tissue, activation of Ras/Raf/MEK/ERK signaling with these mutations in vitro, and the mutual exclusivity of these mutations in a given patient suggest a central role for activation of the Ras/Raf/MEK/ERK oncogenic pathway in LCH.
Studies have since reported that 100% of LCH cases show ERK phosphorylation, indicating that LCH is likely to be a clonally expanding myeloid neoplasm.
Langerhans cell histiocytosis (LCH) is the most common histiocytosis with constitutive activation of the RAS-RAF-MEK-ERK (MAPKinase) cell signaling pathway.
The demonstration of clonality of LCH cells, insufficient evidence alone for neoplasia, is now bolstered by finding driver somatic mutations in BRAF in up to 55% of patients with LCH, and activation of the RAS-RAF-MEK-ERK (where MEK and ERK are mitogen-activated protein kinase and extracellular signal-regulated kinase, respectively) pathway in nearly 100% of patients with LCH.