Cross-talk between DNA methylation and active histone modifications regulates aberrant expression of ZAP70 in CLL

Zeta-associated protein of 70 kD (ZAP70) is a recognized adverse prognostic marker in chronic lymphocytic leukaemia (CLL) associated with enhanced B-cell receptor signalling, significantly more aggressive disease course and poor overall survival. Zeta-associated protein of 70 kD is ordinarily expressed in T cells where it has a crucial role in T-cell receptor signalling, whereas its aberrant expression in CLL leads to enhanced B-cell receptor signalling and significantly more aggressive disease course. Although much is known about the activation of ZAP70 following engagement of T-cell receptor, there are little data on the regulation of ZAP70 gene expression in normal T cells or CLL. To understand the molecular events underpinning epigenetic regulation of ZAP70 gene in CLL, we have defined ZAP70 promoter region and outlined the regions crucial in regulating the gene activity. Following a direct comparison of ZAP70+ and ZAP70- primary CLLs, we show ZAP70 promoter in expressing CLLs to be associated with a spectrum of active histone modifications, some of which are tightly linked to aberrant DNA methylation in CLL. Cross-talk between histone modifications and reduced DNA methylation culminates in transcriptional de-repression of ZAP70. Moreover, treatment with histone deacetylase (HDAC) and DNA methylation inhibitors results in recovery of ZAP70 expression, which provides a possible explanation for the failure of HDAC inhibitors in CLL treatment and might serve as a cautionary warning for their future use in treatment of this leukaemia.