Histone 3 lysine 4 (H3K4) methyltransferases MLL3 and MLL4 (MLL3/4) are required for enhancer activation during cell differentiation, though the mechanism is incompletely understood. We have attempted to address this issue by generating two mouse lines: one expressing H3.3K4M, a lysine-4-to-methionine (K4M) mutation of histone H3.3 that inhibits H3K4 methylation, and the other carrying conditional double knockout of MLL3/4 enzymatic SET domain. Expression of H3.3K4M in lineage-specific precursor cells depletes H3K4 methylation and impairs adipose tissue and muscle development. Mechanistically, H3.3K4M prevents enhancer activation in adipogenesis by destabilizing MLL3/4 proteins but not other Set1-like H3K4 methyltransferases MLL1, MLL2, SET1A and SET1B. Notably, deletion of the enzymatic SET domain in lineage-specific precursor cells mimics H3.3K4M expression, destabilizes MLL3/4 proteins, and prevents adipose tissue and muscle development. Interestingly, destabilization of MLL3/4 by H3.3K4M in adipocytes does not affect adipose tissue maintenance and thermogenic function. Together, our findings indicate that expression of H3.3K4M, or deletion of the enzymatic SET domain, destabilizes enhancer H3K4 methyltransferases MLL3/4 and impairs adipose tissue and muscle development.

Lysine demethylase 6A (KDM6A), also known as UTX, belongs to the KDM6 family of histone H3 lysine 27 (H3K27) demethylases that also includes UTY and KDM6B (JMJD3). The KDM6A protein contains six TPR domains and an enzymatic JmjC domain that catalyzes the removal of di- and trimethylation on H3K27. KDM6A physically associates with histone H3 lysine 4 mono-methyltransferases MLL3 (KMT2C) and MLL4 (KMT2D). Since its identification as a H3K27 demethylase in 2007, studies have reported KDM6A's critical roles in cell differentiation, development, and cancer. KDM6A is important for differentiation of embryonic stem cells and development of various tissues. Mutations of KDM6A cause Kabuki syndrome. KDM6A is frequently mutated in cancers and functions as a tumor suppressor. KDM6A is redundant with UTY and functions largely independently of its demethylase activity. It regulates gene expression likely through the associated transcription factors and MLL3/4 on enhancers. However, KDM6A enzymatic activity is required in certain cellular contexts. Functional redundancy between H3K27 demethylase activities of KDM6A and KDM6B in vivo has yet to be determined. Further understanding of KDM6A functions and working mechanisms will provide more insights into enhancer regulation and may help generate novel therapeutic approaches to treat KDM6A-related diseases.

The epigenetic mechanisms regulating adipose tissue development and function are poorly understood. In this study, we show that depletion of histone H3K36 methylation by H3.3K36M in preadipocytes inhibits adipogenesis by increasing H3K27me3 to prevent the induction of C/EBPα and other targets of the master adipogenic transcription factor peroxisome proliferator-activated receptor-γ (PPARγ). Depleting H3K36 methyltransferase Nsd2, but not Nsd1 or Setd2, phenocopies the effects of H3.3K36M on adipogenesis and PPARγ target expression. Consistently, expression of H3.3K36M in progenitor cells impairs brown adipose tissue (BAT) and muscle development in mice. In contrast, depletion of histone H3K36 methylation by H3.3K36M in adipocytes in vivo does not affect adipose tissue weight, but leads to profound whitening of BAT and insulin resistance in white adipose tissue (WAT). These mice are resistant to high fat diet-induced WAT expansion and show severe lipodystrophy. Together, these results suggest a critical role of Nsd2-mediated H3K36 methylation in adipose tissue development and function.

Highlights Black young adults smoked more cigarillos and blunts during the COVID-19 pandemic. Escalated smoking was attributed to pandemic-induced stress and anxiety. Changes in cigar smoking context signified addictive smoking patterns. Motivation to quit cigars was low even when perceiving higher COVID-19 risks.