Circadian clock function does not require the histone methyltransferase MLL3

Abstract

AO_SCPLOWBSTRACTC_SCPLOWThe circadian clock controls the physiological function of tissues through the regulation of thousands of genes in a cell-type specific manner. The core cellular circadian clock is a transcription-translation negative feedback loop, which can recruit epigenetic regulators to facilitate temporal control of gene expression. Histone methyltransferase, mixed lineage leukemia gene 3 (MLL3) was reported to be required for maintenance of circadian oscillations in cultured cells. Here, we test the role of MLL3 in circadian organisation in whole animals. Using mice expressing catalytically inactive MLL3, we show that MLL3 methyltransferase activity is in fact not required for circadian oscillations in vitro in a range of tissues, nor for maintenance of circadian behavioural rhythms in vivo. In contrast to a previous report, loss of MLL3-dependent methylation did not affect global levels of H3K4 methylation in liver, indicating substantial compensation from other methyltransferases. Further, we found little evidence of genomic repositioning of H3K4me3 marks. We did, however, observe repositioning of H3K4me1 from intronic regions to intergenic regions and gene promoters, however there were no changes in H3K4me1 mark abundance around core circadian clock genes. Output functions of the circadian clock, such as control of inflammation, were largely intact in MLL3-methyltransferase deficient mice, although some gene specific changes were observed, with sexually dimorphic loss of circadian regulation of specific cytokines. Taken together, these observations call for a major reassessment of the inter-relationship between the circadian clock and MLL3-directed histone methylation, and a deeper examination of other epigenetic mechanisms which may facilitate circadian clock function. SO_SCPLOWIGNIFICANCEC_SCPLOWO_SCPCAP C_SCPCAPO_SCPLOWSTATEMENTC_SCPLOWA highly cited paper published in PNAS previously reported an essential role for the histone methyltransferase MLL3 in maintaining circadian oscillations in cultured cells. We tested the role of MLL3 in vivo and in primary tissues showing that MLL3 in fact plays no role in organising the core circadian clock, and has no functional impact on whole animal circadian behaviour. However, in further analysis, we newly discover a role for MLL3 in conferring circadian control to components of the inflammatory response, doing so in a sexually dimorphic manner. As the MLL family of histone methyltransferases are being targeted by pharmaceuticals for cancer, it is important to understand how methyltransferases may be driving circadian rhythms in gene expression.