Deciphering the Epigenetic Landscape in Histiocytic Neoplasms

Background: Histiocytic neoplasms are driven by mutations activating the MAPK/ERK pathway; however, their epigenetic landscape remains poorly understood. MicroRNAs (miRNAs) are epigenetic regulators implicated in cancer development. Moreover, DNA methylation of miRNAs affects hematological malignancies development. Our previous study identified a unique miRNA expression signature in Erdheim-Chester Disease (ECD) patients. To further understand the biological mechanisms of ECD, we investigated the involvement of miRNAs, genes, and their methylation status in ECD, Langerhans cell histiocytosis (LCH), and Rosai-Dorfman disease (RDD).

Methods: Methylation profiles of 14 LCH, 6 ECD, and 10 RDD patients were determined from histyocitic infiltrated tissues (Illumina EPIC-methylation array). These were compared to controls, suture granulomas (n=4), localized inflammatory reactions characterized by non-neoplastic histiocytic infiltrates. miRNAome data was identified by NanoString analysis and qRT-PCR.

Results: We identified significant alterations in 23,322 methylation sites (FDR<0.05, |Δß|≧20%) between controls and the histiocytosis group, indicating differential epigenetic regulation. ECD and LCH exhibited a shared methylation signature distinct from RDD. Pathway analyses revealed that the differentially methylated genes (DMGs) were involved in inflammatory response, RAS, TNF, and PI3K-mTOR signaling pathways, linked to histiocytic neoplasms. Specifically, we found DMG involvement in regulation and generation of non-coding RNAs. Notably, we observed significant downregulation of the let7-miRNA family, particularly let7b, in ECD and LCH patients. Subsequent analysis revealed hypermethylation of specific CpG islands in the let7b DNA sequence, potentially silencing let7b expression in these diseases. Given the let7-miRNA family's role in regulating the MAPK pathway, this suggests a novel mechanism in the pathogenesis of these neoplasms.

Conclusions: We provide valuable insights into differential methylation patterns in histiocytosis neoplasms, highlighting the involvement of epigenetic modifications in their development and progression. This enhances our understanding of histiocytosis development and may contribute to the identification of novel diagnostic and therapeutic approaches for these patients.