Background/Purpose: Systemic lupus erythematosus (SLE) is the prototypical autoimmune disease and is characterized by multi-systemic chronic inflammation. Epigenetic processes, such as posttranslational histone modifications, can regulate gene expression without altering the underlying genomic sequence and represent important disease mechanisms that have had little attention in SLE to date. We have previously reported that histone H4 acetylation (H4ac) is globally increased across the genome in monocytes of patients with SLE as compared to in monocytes of healthy controls using a tiling array approach. In order to further characterize H4ac to determine the pathologic process responsible for the hyperacetylation, we looked for an imbalance in histone acetylatransferases (HATs) and histone deacetylases (HDACs). We further utilized flow cytometry to identify specific lysine residues hyperacetylated in SLE.

Methods: Peripheral blood monocytes (PBMCs) were obtained from 7 controls and 7 SLE patients. The patients had low SLEDAI score (mean score < 2) and were on no immune suppressive medications at the time other than low-dose prednisone. Flow cytometry for different H4 lysine acetyl groups: K5, K8, K12 and K16 were run on a FacsCalibur instrument using appropriate isotype controls. H4 acetylation was defined on both T cells and monocytes. RNA-Seq studies were performed on purified monocytes from a different set of 8 controls and 8 SLE patients to examine the differential gene expression between the groups, in order to quantify mRNA for potential HAT and HDAC enzymes responsible for histone hyperacetylation in the patients with SLE.

Results: Analysis of gene expression of HATs found that PCAF-KAT2B expression was significantly increased in SLE monocytes as compared to controls; whereas, ATF2 expression was decreased significantly in the SLE group as compared to the control group. PCAF-KAT2B can associate with IRF1 and place H4K5, H4K8, and H4K16 acetylation marks. Further examination of the RNA-Seq data revealed increased expression of genes regulated by the IRF family of transcription factors. In addition, when compared to the control group, the SLE group had significantly decreased gene expression of HDAC3, which normally functions to deacetylate all H4 lysine acetyl groups, with a preference for acetylated H4K5 and H4K12. HDAC11 expression in SLE monocytes was also significantly reduced as compared to controls. HDAC11 has been shown to negatively regulate antigen-presenting cells’ production of IL-10, a cytokine that is known to have increased levels in SLE patients. Finally, using flow cytometry, we found H4K5, H4K8, and H4K16 acetylation clearly increased in SLE monocytes and H4K5 increased (but not significant statistically) in SLE T cells.

Conclusion: These data demonstrate that in addition to specific gene sets being dysregulated in SLE, global alterations to H4 acetylation occur as well. These findings parallel studies that examined CpG DNA methylation and discovered evidence of global gene demethylation. The identification of a candidate HAT provides for a potential therapeutic target.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/hyperacetylation-of-histone-h4-in-systemic-lupus-erythematosus/