MLL4 establishes enhancer-associated condensates to counterbalance Polycomb-mediated nuclear mechanical stress in Kabuki Syndrome [3'UTR RNA-seq]

The genetic elements required to tune gene expression are partitioned in active and repressive nuclear condensates. These chromatin compartments include enhancer-associated clusters whose dynamic establishment and functioning depends on multivalent interactions occurring among transcription factors, cofactors and basal transcriptional machinery. However how chromatin modifiers contribute to the assembly of enhancer-associated condensates have not been addressed. We herein defined the non-catalytic function of MLL4 in driving the formation active nuclear condensates and its interplay with Polycomb compartment in determining the nuclear architecture. By interrogating the role of haploinsufficiency of KMT2D in Kabuki Syndrome, we found that MLL4 guides the assembly of enhancer-associated condensates through liquid-liquid phase separation. The loss-of-function of MLL4 impaired the correct chromatin compartmentalization of Polycomb proteins, altering nuclear architecture. By releasing the nuclear mechanical stress through the inhibition of the mechano-sensor ATR, we re-established the mechano-signaling of MSCs and their commitments towards chondrocytes. This study supports the notion that MLL4 contributes to the functional partitioning of chromatin, which is required to determine the structure and the mechanical properties of the nucleus. Overall design: We address the impact of ATR inhibiton on gene expression in wildtype and MLL4 (Q4092X/wt) heterozygous hTERT-immortalized human mesenchymal stem cells (iMSCs). Two different cell types, wildtype and MLL4 (Q4092X/wt) heterozygous iMSCs, were treated with an ATR inhibitor for 8 hours and 24 hours or left untreated, respectively. 3'UTR RNA-seq was performed to analyse gene expression changes due to the inhibitor treatment.