The effect of methylation on the let-7-BCL2L1-BCL2 axis and the potential use of hypomethylating and BH3 mimetic drugs in histiocytic neoplasms

The histiocytoses [Erdheim-Chester Disease (ECD), Langerhans cell histiocytosis (LCH), and Rosai-Dorfman-Destombes disease (RDD)] are characterized by the infiltration of monocytes, macrophages, and dendritic cells in normal tissues, leading to significant morbidity due to disruption of critical organ systems. While their pathogenesis involves MAPK-ERK pathway mutations, their epigenetic landscape remains poorly understood. Epigenetic mechanisms such as DNA methylation, histone modification, and non-coding RNA are crucial in gene expression regulation and cancer development. We previously identified a distinct non-coding RNA profile in histiocytosis patients compared to controls. We analyzed the DNA methylation profile of 30 patients and found significant alterations in 23,322 methylation sites compared to controls, independent of age, gender, mutation type, or disease site. We observed a significant hypermethylation in the let-7b transcript, resulting in decreased let-7b expression. This led to the lack of regulation on the anti-apoptotic proteins BCL2L1, which was overexpressed in 21/21 (100%) patients' biopsies and BCL2 in 11/21 biopsies (52%). Treatment with the hypomethylating agent, 5-Azacytidine, restored let-7b expression, reduced BCL2L1 levels, and induced apoptosis in BRAFV600E mutant cells. Additionally, BCL2L1 and BCL2 inhibitors also induced apoptosis. This novel epigenetic mechanism suggests that hypomethylating agents or BH3 mimetic drugs may offer new therapeutic targets for histiocytosis. Methylation profiles of six ECD, 14 LCH, and 10 RDD patients were analyzed from diagnostically confirmed tumor biopsies by Illumina EPIC-methylation array according to the manufacturer protocol. These were compared to controls, suture granulomas (n=4), localized inflammatory reactions characterized by non-neoplastic histiocytic infiltrates.