Background/Purpose:

Systemic lupus erythematosus (SLE) is the classical systemic autoimmune disease. Epigenetic processes, such as posttranslational histone modifications, can regulate gene expression without altering the underlying genomic sequence; these disease mechanisms that have had little attention in SLE to date. We previously reported that histone H4 acetylation (H4ac) is globally increased across the genome in monocytes of SLE patients as compared to healthy controls using tiling array. Transcription factor motif analysis then found that 63% of genes with increased H4ac had potential interferon regulatory factor (IRF) 1 binding sites, associating this transcription factor in the dysregulated gene expression. In order to investigate how IRF1 interactions influence H4ac in SLE, we identified the specific hyperacetylated H4 lysine residues, looked for histone acetylatransferases (HATs) and histone deacetylases (HDACs) dysregulation that may lead to the pathological hyperacetylation pattern and examined IRF1 associations with HATs and HDACs.

Methods:

Flow cytometry for H4 lysine groups: K5, K8, K12 and K16 were ran using isotype controls. H4ac was defined in monocytes from 21 controls and 34 SLE patients. RNA-Seq studies were performed on monocytes from a different set of 8 controls and 8 SLE patients to look for an imbalance in HAT and HDAC expression; these imbalances were validated using real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR). IRF1 influences on H4ac were evaluated in vitro using D54MG cells overexpressed with IRF1. IRF1 interactions with HATs and HDACs were studied using co-immunoprecipitation assays.

Results:

Flow cytometry data showed that H4K5, H4K8, H4K12, and H4K16 acetylation were significantly increased in SLE monocytes. RNA-Seq results identified HDAC3 and HDAC11 with significantly decreased expression in SLE monocytes as compared to controls. HDAC3 can deacetylate all H4 lysine acetyl groups, preferentially acetylated H4K5 and H4K12. In contrast, the expression of PCAF, a HAT, was significantly increased in SLE monocytes as compared to controls. PCAF can place H4K5, H4K8, and H4K16 acetylation marks with a preference for H4K8. qRT-PCR data validated the HAT/HDAC expression patterns seen in the RNA-Seq studies. SLE monocytes had decreased gene expression levels of HDAC3 and HDAC11; PCAF had significantly higher gene expression in SLE than controls. IRF1-overexpressed in D54MG cells was associated with significantly increased H4K8 and H4K12 as compared to vector-only D54MG cells. While there was also some increase in acetylation at H4K5, no increase in acetylation was seen at H4K16 in IRF1-overexpressing D54MG cells. Co-immunoprecipitation studies using D54MG cells revealed IRF1 associations with PCAF, P300, CBP, GCN, ATF2, HDAC3 and SIRT1. 

Conclusion:

We hypothesize that IRF1 responds to alpha-IFN activation in SLE, and activated IRF1 recruits HATs, which then increases H4ac and leads to the chronic pathological gene expression in SLE. These studies have identified pivotal enzymes participating in the global hyper-acetylation in SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/irf1-influences-on-histone-h4-acetylation-in-systemic-lupus-erythematosus/