Pathophysiology
The pathogenesis of Langerhans cell histiocytosis (LCH) is a matter of debate. There are ongoing investigations to determine whether LCH is a reactive or neoplastic process. Arguments supporting the reactive nature of LCH include the occurrence of spontaneous remissions, the extensive secretion of multiple cytokines by dendritic cells and bystander-cells (a phenomenon known as cytokine storm) in the lesional tissue, favorable prognosis and relatively good survival rate in patients without organ dysfunction or risk organ involvement.]
On the other hand, the infiltration of organs by monoclonal population of pathologic cells, and the successful treatment of subset of disseminated disease using chemotherapeutic regimens are all consistent with a neoplastic process.] In addition, a demonstration, using X chromosome–linked DNA probes, of LCH as a monoclonal proliferation provided additional support for the neoplastic origin of this disease. While clonality is an important attribute of cancer, its presence does not prove that a proliferative process is neoplastic. Recurrent cytogenetic or genomic abnormalities would also be required to demonstrate convincingly that LCH is a malignancy.
Activating mutation of a protooncogen in the Raf family, the BRAF gene, was detected in 35 of 61 (57%) LCH biopsy samples with mutations being more common in patients younger than 10 years (76%) than in patients aged 10 years and older (44%). This study documented the first recurrent mutation in LCH samples. Two independent studies have confirmed this finding. ( Presence of this activating mutation could support the notion to characterize LCH as myelodysplastic disorder.
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