Immunohistochemical and molecular cytogenetic evaluation of potential targets for tyrosine kinase inhibitors in Langerhans cell histiocytosis.

Langerhans cell histiocytosis is a rare disorder of Langerhans cells, a component of the dendritic cell system, with an unknown pathogenesis. Conventional therapy for patients with Langerhans cell histiocytosis is usually effective, but some patients are refractory to treatment or develop toxicity. Thus, there is a need for innovative therapies. Recently, some cases of Langerhans cell histiocytosis were reported to express platelet-derived growth factor receptors α and β or c-KIT by immunohistochemistry, and some of these patients had a clinical response to imatinib mesylate. Other hematologic disorders with PDGFRα or PDGFRβ gene rearrangements also have responded to imatinib mesylate. The aim of this study was to evaluate immunohistochemical and molecular markers in Langerhans cell histiocytosis that would identify cases for possible treatment with tyrosine kinase inhibitors. We investigated formalin-fixed, paraffin-embedded tissue sections from 14 cases of Langerhans cell histiocytosis. As controls, we included cases of inflammatory dermatitis (n = 5) and dermatopathic lymphadenitis (n = 7). We performed immunohistochemistry for S100, CD1a, c-KIT, and platelet-derived growth factor receptors α and β. Fluorescence in situ hybridization analysis to detect rearrangements of the PDGFRα or PDGFRβ genes was also performed. Four (28.5%) of 14 cases of Langerhans cell histiocytosis were positive for platelet-derived growth factor receptor α, whereas absent/weak expression was seen in 10 cases and all controls. All cases were negative for platelet-derived growth factor receptor β and c-KIT. The fluorescence in situ hybridization studies were also negative in all 8 cases with adequate quality DNA. Our findings suggest that a subset of cases of Langerhans cell histiocytosis may be treated with tyrosine kinase inhibitors due to the expression of platelet-derived growth factor receptor α. Clinical trials that evaluate the use of tyrosine kinase inhibitors in Langerhans cell histiocytosis seem warranted and should evaluate these markers.