1180Cardiomyocyte-specific deletion of Brd4 induces hypertrophic cardiomyopathy and suggests a novel function for the chromatin reader as co-repressor in the healthy heart
Authors: Lbik D , Buchholz E , Mohamed B , Fischer A , Hasenfuss G , Toischer K 
Published: October 2019

Abstract

Introduction
There is increasing evidence that epigenetic mechanisms such as histone modifications integrate stress stimuli into cellular responses during pathogenesis of heart failure (HF). The chromatin readers of the Bromo- and extraterminal domain (BET) family (BRD2, BRD3, BRD4) were reported to recognize acetylated histone tails and to promote transcription. Previously, BET inhibition by JQ1 was shown to displace BET readers from chromatin preventing pathologic remodelling after pressure overload (PO). This suggests a major role for BET proteins in HF development and demonstrates their therapeutic potential. However, JQ1 acts systemically and simultaneously inhibits all BET proteins making precise conclusions on functions of particular BET members in cardiac cells impossible. This work presents the first in vivo deletion of Brd4 in cardiomyocytes and its impact on the healthy and diseased murine heart.

Purpose
This study aims to analyse if deleting Brd4 in cardiomyocytes can protect the heart from pathologic remodelling after PO as observed with BET inhibition.

Methods
The αMHC-MerCreMer line and a conditional Brd4 allele were used to induce a knockout of Brd4 (Brd4 KO) in cardiomyocytes by tamoxifen application (i.p. 3x30 mg/kg/day) at postnatal week 5. Transverse aortic constriction (TAC) was used to induce PO in 8 weeks old mice. Global gene expression changes in Brd4 KO mice were analysed by mRNA sequencing.

Results
Adult Brd4 KO mice showed left ventricular (LV) wall thickening, increased LV mass, cardiomyocyte hypertrophy and mild interstitial fibrosis in comparison to control but lived for over 1 year and showed normal ejection fraction. Transcriptome analysis of Brd4 KO hearts revealed the induction of a pathologic gene program like the expression of fetal genes such as Myh7 or Acta1, Nppa, and Nppb and genes involved in extra cellular matrix organization. After TAC, Brd4 KO mice showed higher mortality with a median survival of 37 days in comparison to 132 days in the control group. However, Brd4 KO mice that survived the acute phase showed significantly higher ejection fraction than control 10 weeks after TAC. Despite the basal hypertrophy, Brd4 KO mice showed no further pathologic remodelling in response to PO and had significantly lower LV weights and diameters.

Conclusions
The development of concentric hypertrophy, expression of fetal genes, and fibrosis with preserved ejection fraction in Brd4 KO animals resemble typical characteristics of hypertrophic cardiomyopathy, and the increased mortality after TAC is likely due to diastolic dysfunction or arrythmias. However, surviving Brd4 KO mice show limited remodelling and partially preserved heart function suggesting cardio-protective potential as previously observed with BET inhibition. Together with relevant literature our findings suggest two distinct roles for Brd4, co-repression of hypertrophy genes in the healthy heart and their co-activation in response to stress.

Acknowledgement/Funding
German Research Foundation - Collaborative Research Center 1002