Translational control of cardiac fibrosis

Background Fibrosis is a common pathology in many cardiac disorders and is driven by the activation of resident fibroblasts. The global post-transcriptional mechanisms underlying fibroblast-to-myofibroblast conversion in the heart have not been explored. Methods Genome-wide changes of RNA transcription and translation during human cardiac fibroblast activation were monitored with RNA sequencing and ribosome profiling. We then used miRNA-and RNA-binding protein-based analyses to identify translational regulators of fibrogenic genes. To reveal post-transcriptional mechanisms in the human fibrotic heart, we then integrated our findings with cardiac ribosome occupancy levels of 30 dilated cardiomyopathy patients. Results We generated nucleotide-resolution translatome data during the TGFβ1-driven cellular transition of human cardiac fibroblasts to myofibroblasts. This identified dynamic changes of RNA transcription and translation at several time points during the fibrotic response, revealing transient and early-responder genes. Remarkably, about one-third of all changes in gene expression in activated fibroblasts are subject to translational regulation and dynamic variation in ribosome occupancy affects protein abundance independent of RNA levels. Targets of RNA-binding proteins were strongly enriched in post-transcriptionally regulated genes, suggesting genes such as MBNL2 can act as translational activators or repressors. Ribosome occupancy in the hearts of patients with dilated cardiomyopathy suggested an extensive post-transcriptional regulatory network underlying cardiac fibrosis. Key network hubs include RNA-binding proteins such as PUM2 and QKI that work in concert to regulate the translation of target transcripts in human diseased hearts. Conclusions We reveal widespread translational effects of TGFβ1 and define novel post-transcriptional events that control the fibroblast-to-myofibroblast transition. Regulatory networks that affect ribosome occupancy in fibroblasts are paralleled in human heart disease. Our findings show the central importance of translational control in fibrosis and highlight novel pathogenic mechanisms in heart failure

Cardiomyocyte BRAF and type 1 RAF inhibitors promote cardiomyocyte and cardiac hypertrophy in mice in vivo

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade promotes cardiomyocyte hypertrophy and is cardioprotective, with the three RAF kinases forming a node for signal integration. Our aims were to determine if BRAF is relevant for human heart failure, whether BRAF promotes cardiomyocyte hypertrophy, and if Type 1 RAF inhibitors developed for cancer (that paradoxically activate ERK1/2 at low concentrations: the “RAF paradox”) may have the same effect. BRAF was upregulated in heart samples from patients with heart failure compared with normal controls. We assessed the effects of activated BRAF in the heart using mice with tamoxifen-activated Cre for cardiomyocyte-specific knock-in of the activating V600E mutation into the endogenous gene. We used echocardiography to measure cardiac dimensions/function. Cardiomyocyte BRAFV600E induced cardiac hypertrophy within 10 d, resulting in increased ejection fraction and fractional shortening over 6 weeks. This was associated with increased cardiomyocyte size without significant fibrosis, consistent with compensated hypertrophy. The experimental Type 1 RAF inhibitor, SB590885, and/or encorafenib (a RAF inhibitor used clinically) increased ERK1/2 phosphorylation in cardiomyocytes, and promoted hypertrophy, consistent with a “RAF paradox” effect. Both promoted cardiac hypertrophy in mouse hearts in vivo, with increased cardiomyocyte size and no overt fibrosis. In conclusion, BRAF potentially plays an important role in human failing hearts, activation of BRAF is sufficient to induce hypertrophy, and Type 1 RAF inhibitors promote hypertrophy via the “RAF paradox”. Cardiac hypertrophy resulting from these interventions was not associated with pathological features, suggesting that Type 1 RAF inhibitors may be useful to boost cardiomyocyte function

Titin truncating mutations: a rare cause of dilated cardiomyopathy in the young

Truncating mutations in the TTN gene are the most common genetic cause of dilated cardiomyopathy in adults but their role in young patients is unknown. We studied 82 young dilated cardiomyopathy subjects and found that the prevalence of truncating TTN mutations in adolescents was similar to adults, but surprisingly few truncating TTN mutations were identified in affected children, including one confirmed de novo variant. In several cases, truncating TTN mutations in children with dilated cardiomyopathy had evidence of additional clinical or genetic risk factors. These findings have implications for genetic testing and suggest that single truncating TTN mutations are insufficient alone to cause pediatric-onset dilated cardiomyopathy

Titin truncating mutations: A rare cause of dilated cardiomyopathy in the young

10.1016/j.ppedcard.2016.01.003Progress in Pediatric Cardiology4041-45PPCA

Defining the genetic architecture of hypertrophic cardiomyopathy: re-evaluating the role of non-sarcomeric genes

Aim: Hypertrophic cardiomyopathy (HCM)exhibits genetic heterogeneity that is dominated by variation in eight sarcomericgenes.Genetic variation in a large number of non-sarcomeric genes has also been implicated in HCM but not formally assessed. Here we used very large case and control cohorts to determine the extent to which variation in non-sarcomeric genes contributes to HCM. Methods and results: We sequenced known and putative HCM genes ina new large prospective HCM cohort (n=804) and analysed data alongside the largest published series of clinically genotyped HCM patients (n=6179), previously published HCM cohorts and reference population samples from the Exome Aggregation Consortium (ExAC, n=60,706) to assess variation in 31 genes implicated in HCM. We foundno significant excess of rare (minor allele frequency < 1:10,000 in ExAC)protein-alteringvariants over controls for most genes tested and conclude that novel variantsin these genes are rarely interpretable, even for genes with previous evidence of co-segregation (e.g. ACTN2). To provide an aid for variant interpretation, weintegratedHCM gene sequencedata with aggregatedpedigreeand functional data and suggest ameans of assessing genepathogenicity in HCMusing this evidence. Conclusions: We show that genetic variation in the majority of non-sarcomeric genes implicated in HCM is not associated with the condition, reinforce the fact that the sarcomeric gene variation is the primary cause of HCM known to date and underscore that the aetiology of HCM is unknown in the majority ofpatients