BET bromodomain inhibition attenuates cardiac phenotype in myocyte-specific lamin A/C–deficient mice

Abstract

Mutation in the LMNA gene, encoding lamin A/C, causes a diverse group of diseases called laminopathies. Cardiac involvement is the major cause of death and manifests as dilated cardiomyopathy, heart failure, arrhythmias, and sudden death. There is no specific therapy for LMNA-associated cardiomyopathy. We report that deletion of Lmna in cardiomyocytes in mice leads to severe cardiac dysfunction, conduction defect, ventricular arrhythmias, fibrosis, apoptosis, and premature death within 4 weeks. The phenotype is similar to LMNA-associated cardiomyopathy in humans. RNA sequencing, performed before the onset of cardiac dysfunction, led to identification of 2338 differentially expressed genes (DEGs) in Lmna-deleted cardiomyocytes. DEGs predicted activation of bromodomain-containing protein 4 (BRD4), a regulator of chromatin-associated proteins and transcription factors, which was confirmed by complementary approaches, including chromatin immunoprecipitation sequencing. Daily injection of JQ1, a specific BET bromodomain inhibitor, partially reversed the DEGs, including those encoding secretome; improved cardiac function; abrogated cardiac arrhythmias, fibrosis, and apoptosis; and prolonged the median survival time 2-fold in the myocyte-specific Lmna-deleted mice. The findings highlight the important role of LMNA in cardiomyocytes and identify BET bromodomain inhibition as a potential therapeutic target in LMNA-associated cardiomyopathy, for which there is no specific effective therapy.

BETs that cover the spread from acquired to heritable heart failure

“…Can BET bromodomain inhibition ever be a viable therapy for human HF? Observations in preclinical models of acquired and heritable HF support that BET bromodomain inhibition can have broad therapeutic benefit (3,(7)(8)(9)17), possibly via modulating gene expression in multiple cardiac cellular compartments. While BET bromodomain inhibitors are currently being tested in cancer clinical trials (18), their ontarget toxicities in extracardiac tissues may not permit a tractable therapeutic index for chronic disease applications, such as HF.…”

“…The researchers found that Lmna-cKO mice had 100% mortality by 4 weeks of age, preceded by severe cardiac dilation, contractile dysfunction, fibrosis, and cardiac rhythm disturbances that began after 2 weeks of age. Lmna haploinsufficiency caused a milder form of DCM, suggesting dosage sensitivity (3). These phenotypes are observed in patients with LMNA-associated DCM, suggesting cardiomyocyte-specific Lmna loss can trigger major cardiac manifestations of laminopathy.…”

“…Although several groups have developed mouse models harboring pathogenic germline Lmna mutations, the cardiomyocyte-specific role of LMNA in vivo is poorly understood. In this issue of the JCI, Auguste et al deleted Lmna in a cardiomyocytespecific manner in mice (Lmna-cKO) using the Myh6-Cre line, which principally drives gene deletion during postnatal cardiac maturation (3). The researchers found that Lmna-cKO mice had 100% mortality by 4 weeks of age, preceded by severe cardiac dilation, contractile dysfunction, fibrosis, and cardiac rhythm disturbances that began after 2 weeks of age.…”

“…To detect early effects of Lmna deletion and avoid secondary transcriptomic effects of advanced heart failure (HF), they profiled cardiomyocytes from two-week-old mice, a time point preceding onset of cardiac dysfunction. Analyses of differentially expressed genes revealed upregulation of profibrotic programs and predicted activation of specific signaling pathways, including activation of the bromodomain and extraterminal (BET) family of chromatin regulatory proteins (3)(4)(5). BETs are a highly conserved family of epigenetic "reader" proteins consisting of the ubiquitously expressed BRD2, BRD3, and BRD4 and the testis-specific BRDT.…”

“…The RNA-Seq data from isolated cardiomyo-cifically bind the bromodomains of all four BET proteins with high affinity, reversibly displacing these coactivators from their acetylated interaction partners and inhibiting transcription of specific gene programs (4). JQ1 improves cardiac function in mouse models of pressure overload and myocardial infarction-induced HF (7-9), prompting Auguste and colleagues (3) to test its effects in Lmna-cKO mice. They chronically treated two-week-old mice with JQ1 and observed substantial improvements in survival and cardiac function.…”