Molecular characterization of the histiocytoses: Neoplasia of dendritic cells and macrophages

The systemic histiocytoses encompass a clinically heterogeneous group of disorders leading to tissue damage secondary to the accumulation and infiltration of pathological cells thought to be derived from the dendritic or monocytic lineages with accompanying inflammation. For decades, whether or not the histiocytoses were inflammatory or neoplastic disorders was unclear, and their cellular origins have long been obscure and heavily debated. However, the rise of the molecular era led to the discovery of recurrent BRAF^V600E mutations in approximately 50% of patients with Langerhans cell and non-Langerhans cell histiocytoses, which provided the first convincing evidence that these are indeed histiocytic neoplasms. This also supplied a molecular biomarker for potentially mapping the cell(s)-of-origin of these neoplasms. The purpose of this review will be to highlight the barrage of recent molecular advancements in the histiocytic neoplasms and discuss the impact these insights have had on our understanding of the molecular pathophysiology and cellular origins of these rare, enigmatic diseases.