Reduction of Filamin C Results in Altered Proteostasis, Cardiomyopathy, and Arrhythmias.

BACKGROUND: Many cardiomyopathy-associated FLNC pathogenic variants are heterozygous truncations, and FLNC pathogenic variants are associated with arrhythmias. Arrhythmia triggers in filaminopathy are incompletely understood. METHODS AND RESULTS: We describe an individual with biallelic FLNC pathogenic variants, p.Arg650X and c.970-4A>G, with peripartum cardiomyopathy and ventricular arrhythmias. We also describe clinical findings in probands with FLNC variants including Val2715fs87X, Glu2458Serfs71X, Phe106Leu, and c.970-4A>G with hypertrophic and dilated cardiomyopathy, atrial fibrillation, and ventricular tachycardia. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated. The FLNC truncation, Arg650X/c.970-4A>G, showed a marked reduction in filamin C protein consistent with biallelic loss of function mutations. To assess loss of filamin C, gene editing of a healthy control iPSC line was used to generate a homozygous FLNC disruption in the actin binding domain. Because filamin C has been linked to protein quality control, we assessed the necessity of filamin C in iPSC-CMs for response to the proteasome inhibitor bortezomib. After exposure to low-dose bortezomib, FLNC-null iPSC-CMs showed an increase in the chaperone proteins BAG3, HSP70 (heat shock protein 70), and HSPB8 (small heat shock protein B8) and in the autophagy marker LC3I/II. FLNC null iPSC-CMs had prolonged electric field potential, which was further prolonged in the presence of low-dose bortezomib. FLNC null engineered heart tissues had impaired function after low-dose bortezomib. CONCLUSIONS: FLNC pathogenic variants associate with a predisposition to arrhythmias, which can be modeled in iPSC-CMs. Reduction of filamin C prolonged field potential, a surrogate for action potential, and with bortezomib-induced proteasome inhibition, reduced filamin C led to greater arrhythmia potential and impaired function.

Susceptibility to innate immune activation in genetically-mediated myocarditis.

BACKGROUND: Myocarditis is clinically characterized by chest pain, arrhythmias, and heart failure, and treatment for myocarditis is often supportive. Mutations in DSP, a gene encoding the desmosomal protein desmoplakin, have been increasingly implicated in myocarditis with biomarkers and pathological features indistinguishable from other forms of myocarditis. DSP-associated myocarditis can progress to dilated cardiomyopathy with heightened arrhythmia risk. METHODS: To model the cardiomyocyte aspects of DSP-associated myocarditis and assess the role of innate immunity, we generated engineered heart tissues (EHTs) from human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from patients and gene-edited healthy control hiPSC lines. Homozygous and heterozygous DSP disrupted EHTs were generated to contain 90% hiPSC-CMs and 10% healthy control human cardiac fibroblasts. We measured innate immune activation and function at baseline and in response to Toll-like receptor (TLR) stimulation in EHTs. RESULTS: At baseline, DSP-/- EHTs displayed a transcriptomic signature of immune activation which was mirrored by EHT cytokine release. Importantly, DSP-/- EHTs were hypersensitive to TLR stimulation demonstrating greater contractile function impairment compared to isogenic controls. Compared to homozygous DSP-/- EHTs, heterozygous DSP patient-derived EHTs had less functionally impairment but also displayed heightened sensitivity to TLR stimulation. When subjected to strain, heterozygous DSP EHTs developed greater functional deficit indicating reduced contractile reserve compared to healthy control. Colchicine or NFΚB inhibitors improved baseline force production and strain-induced force deficits in DSP EHTs. Genomic correction of DSP p.R1951X using adenine base editing reduced inflammatory biomarker release from EHTs. CONCLUSIONS: Genetic reduction of DSP renders cardiomyocytes susceptible to innate immune activation and strain-dependent contractile deficits. EHTs replicate electrical and contractile phenotypes seen in human myocarditis implicating cytokine release as a key part of the myogenic susceptibility to inflammation. This heightened innate immune activation and sensitivity is a target for clinical intervention.

Genetic Variation in Enhancers Modifies Cardiomyopathy Gene Expression and Progression.

BACKGROUND: Inherited cardiomyopathy associates with a range of phenotypes, mediated by genetic and nongenetic factors. Noninherited cardiomyopathy also displays varying progression and outcomes. Expression of cardiomyopathy genes is under the regulatory control of promoters and enhancers, and human genetic variation in promoters and enhancers may contribute to this variability. METHODS: We superimposed epigenomic profiling from hearts and cardiomyocytes, including promoter-capture chromatin conformation information, to identify enhancers for 2 cardiomyopathy genes, MYH7 and LMNA. Enhancer function was validated in human cardiomyocytes derived from induced pluripotent stem cells. We also conducted a genome-wide search to ascertain genomic variation in enhancers positioned to alter cardiac expression and correlated one of these variants to cardiomyopathy progression using biobank data. RESULTS: Multiple enhancers were identified and validated for LMNA and MYH7, including a key enhancer that regulates the switch from MYH6 expression to MYH7 expression. Deletion of this enhancer resulted in a dose-dependent increase in MYH6 and faster contractile rate in engineered heart tissues. We searched for genomic variation in enhancer sequences across the genome, with a focus on nucleotide changes that create or interrupt transcription factor binding sites. The sequence variant, rs875908, disrupts a T-Box Transcription Factor 5 binding motif and maps to an enhancer region 2 kilobases from the transcriptional start site of MYH7. Gene editing to remove the enhancer that harbors this variant markedly reduced MYH7 expression in human cardiomyocytes. Using biobank-derived data, rs875908 associated with longitudinal echocardiographic features of cardiomyopathy. CONCLUSIONS: Enhancers regulate cardiomyopathy gene expression, and genomic variation within these enhancer regions associates with cardiomyopathic progression over time. This integrated approach identified noncoding modifiers of cardiomyopathy and is applicable to other cardiac genes.

Gene Editing and Gene-Based Therapeutics for Cardiomyopathies.

With an increasing understanding of genetic defects leading to cardiomyopathy, focus is shifting to correcting these underlying genetic defects. One approach involves treating mutant RNA through antisense oligonucleotides; the first drug has received regulatory approval to treat specific mutations associated with Duchenne muscular dystrophy. Gene editing is being evaluated in the preclinical setting. For inherited cardiomyopathies, genetic correction strategies require tight specificity for the mutant allele. Gene-editing methods are being tested to create deletions that may be useful to restore protein expression by through the bypass of mutations that restore protein production. Site-specific gene editing, which is required to correct many point mutations, is a less efficient process than inducing deletions.

Genetic modifiers of muscular dystrophy act on sarcolemmal resealing and recovery from injury.

Genetic disruption of the dystrophin complex produces muscular dystrophy characterized by a fragile muscle plasma membrane leading to excessive muscle degeneration. Two genetic modifiers of Duchenne Muscular Dystrophy implicate the transforming growth factor ß (TGFß) pathway, osteopontin encoded by the SPP1 gene and latent TGFß binding protein 4 (LTBP4). We now evaluated the functional effect of these modifiers in the context of muscle injury and repair to elucidate their mechanisms of action. We found that excess osteopontin exacerbated sarcolemmal injury, and correspondingly, that loss of osteopontin reduced injury extent both in isolated myofibers and in muscle in vivo. We found that ablation of osteopontin was associated with reduced expression of TGFß and TGFß-associated pathways. We identified that increased TGFß resulted in reduced expression of Anxa1 and Anxa6, genes encoding key components of the muscle sarcolemma resealing process. Genetic manipulation of Ltbp4 in dystrophic muscle also directly modulated sarcolemmal resealing, and Ltbp4 alleles acted in concert with Anxa6, a distinct modifier of muscular dystrophy. These data provide a model in which a feed forward loop of TGFß and osteopontin directly impacts the capacity of muscle to recover from injury, and identifies an intersection of genetic modifiers on muscular dystrophy.

BRD4 facilitates DNA damage response and represses CBX5/Heterochromatin protein 1 (HP1).

Ovarian cancer (OC) is a heterogeneous disease characterized by defective DNA repair. Very few targets are universally expressed in the high grade serous (HGS) subtype. We previously identified that CHK1 was overexpressed in most of HGSOC. Here, we sought to understand the DNA damage response (DDR) to CHK1 inhibition and increase the anti-tumor activity of this pathway. We found BRD4 suppression either by siRNA or BRD4 inhibitor JQ1 enhanced the cytotoxicity of CHK1 inhibition. Interestingly, BRD4 was amplified and/or upregulated in a subset of HGSOC with statistical correlation to overall survival. BRD4 inhibition increased CBX5 (HP1α) level. CHK1 inhibitor induced DDR marker, γ-H2AX, but BRD4 suppression did not. Furthermore, nuclear localization of CBX5 and γ-H2AX was mutually exclusive in BRD4-and CHK1-inhibited cells, suggesting BRD4 facilitates DDR by repressing CBX5. Our results provide a strong rationale for clinical investigation of CHK1 and BRD4 co-inhibition, especially for HGSOC patients with BRD4 overexpression.