Epigenetic upregulation of B-cell inappropriate genes induces extinction of B-cell program in classical Hodgkin lymphoma

A unique feature of the tumour cells (Hodgkin/Reed-Sternberg (HRS)) of classical Hodgkin lymphoma (cHL) is the loss of their B-cell phenotype despite their B-cell origin. Several lines of evidence suggest that epigenomic events, especially promoter DNA-methylation, are involved in this silencing of many B-cell associated genes. Here we show that DNA-demethylation alone or in conjunction with histone-acetylation is not able to reconstitute the B-cell gene expression program in cultured HRS cells. Instead, combined DNA-demethylation and histone-acetylation of B cells induce a nearly complete extinction of their B-cell expression program and a tremendous up-regulation of numerous cHL characteristic genes including key players such as Id2 known to be involved in the suppression of the B-cell phenotype. Since the up-regulation of cHL characteristic genes and the extinction of the B-cell expression program occurred simultaneously, epigenetic changes may also be responsible for the malignant transformation of cHL. The epigenetic up-regulation of cHL characteristic genes thus play - in addition to promoter DNA-hypermethylation of B-cell associated genes – a pivotal role for the reprogramming of HRS cells and explain why DNA-demethylation alone is unable to reconstitute the B-cell expression program in HRS cells. Keywords: Epigenetic modification 5-aza-dC treatment: The cHL derived cell lines L428, KMH2 and L1236 were treated with 5-aza-dC at a concentration of 5 µM for 5 days with drug and medium replacement after each 48 hours. Combined 5-aza-dC/TSA treatment: The Burkitt lymphoma derived cell lines (Daudi, Namalwa and Raji)were treated with 5-aza-dC at a concentration of 3 µM for 6 days. 5-aza-dC and medium was replaced at day 2 and 5. At the fifth day - in addition to 3 µM 5-aza-dC - cells were incubated for 24 hours with 625 nM TSA. The gene expression profiles of the untreated and treated cell lines were generated in duplicates.