CpG promoter methylation status is not a prognostic indicator of gene expression in beryllium challenge

Abstract

Individuals exposed to beryllium (Be) may develop Be sensitization (BeS) and progress to chronic beryllium disease (CBD). Recent studies with other metal antigens suggest epigenetic mechanisms may be involved in inflammatory disease processes, including granulomatous lung disorders and that a number of metal cations alter gene methylation. The objective of this study was to determine if Be can exert an epigenetic effect on gene expression by altering methylation in the promoter region of specific genes known to be involved in Be antigen-mediated gene expression. To investigate this objective, three macrophage tumor mouse cell lines known to differentially produce tumor necrosis factor (TNF)-α, but not interferon (IFN)-γ, in response to Be antigen were cultured with Be or controls. Following challenges, ELISA were performed to quantify induced TNFα and IFNγ expression. Bisulfate-converted DNA was evaluated by pyrosequencing to quantify CpG methylation within the promoters of TNFα and IFNγ. Be-challenged H36.12J cells expressed higher levels of TNFα compared to either H36.12E cells or P388D.1 cells. However, there were no variations in TNFα promoter CpG methylation levels between cell lines at the six CpG sites tested. H36.12J cell TNFα expression was shown to be metal-specific by the induction of significantly more TNFα when exposed to Be than when exposed to aluminum sulfate, or nickel (II) chloride, but not when exposed to cobalt (II) chloride. However, H36.12J cell methylation levels at the six CpG sites examined in the TNFα promoter did not correlate with cytokine expression differences. Nonetheless, all three cell lines had significantly more promoter methylation at the six CpG sites investigated within the IFNγ promoter (a gene that is not expressed) when compared to the six CpG sites investigated in the TNFα promoter, regardless of treatment condition (p < 1.17 × 10⁻⁹). These findings suggest that, in this cell system, promoter hypo-methylation may be necessary to allow expression of metal-induced TNFα and that promoter hyper-methylation in the IFNγ promoter may interfere with expression. Also, at the dozen CpG sites investigated in the promoter regions of both genes, beryllium had no impact on promoter methylation status, despite its ability to induce pro-inflammatory cytokine expression.

Keywords:

• Beryllium
• epigenetics
• chronic beryllium disease
• metal antigen
• CpG
• DNA methylation

Acknowledgements

The authors would like to thank Dr Ivana Yang and Miss Corinne Hennessy for their help in the design of pyrosequencing primers and Dr Ivana Yang and Dr David Schwartz for allowing us to utilize their pyrosequencing machine and laboratory space.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.