CRISPR-Mediated Activation of Endogenous Gene Expression in the Postnatal Heart.

RATIONALE: Genome editing by CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 is evolving rapidly. Recently, second-generation CRISPR/Cas9 activation systems based on nuclease inactive dead (d)Cas9 fused to transcriptional transactivation domains were developed for directing specific guide (g)RNAs to regulatory regions of any gene of interest, to enhance transcription. The application of dCas9 to activate cardiomyocyte transcription in targeted genomic loci in vivo has not been demonstrated so far. OBJECTIVE: We aimed to develop a mouse model for cardiomyocyte-specific, CRISPR-mediated transcriptional modulation, and to demonstrate its versatility by targeting Mef2d and Klf15 loci (2 well-characterized genes implicated in cardiac hypertrophy and homeostasis) for enhanced transcription. METHODS AND RESULTS: A mouse model expressing dCas9 with the VPR transcriptional transactivation domains under the control of the Myh (myosin heavy chain) 6 promoter was generated. These mice innocuously expressed dCas9 exclusively in cardiomyocytes. For initial proof-of-concept, we selected Mef2d, which when overexpressed, led to hypertrophy and heart failure, and Klf15, which is lowly expressed in the neonatal heart. The most effective gRNAs were first identified in fibroblast (C3H/10T1/2) and myoblast (C2C12) cell lines. Using an improved triple gRNA expression system (TRISPR [triple gRNA expression construct]), up to 3 different gRNAs were transduced simultaneously to identify optimal conditions for transcriptional activation. For in vivo delivery of the validated gRNA combinations, we employed systemic administration via adeno-associated virus serotype 9. On gRNA delivery targeting Mef2d expression, we recapitulated the anticipated cardiac hypertrophy phenotype. Using gRNA targeting Klf15, we could enhance its transcription significantly, although Klf15 is physiologically silenced at that time point. We further confirmed specific and robust dCas9VPR on-target effects. CONCLUSIONS: The developed mouse model permits enhancement of gene expression by using endogenous regulatory genomic elements. Proof-of-concept in 2 independent genomic loci suggests versatile applications in controlling transcription in cardiomyocytes of the postnatal heart.

A context-specific cardiac ß-catenin and GATA4 interaction influences TCF7L2 occupancy and remodels chromatin driving disease progression in the adult heart.

Chromatin remodelling precedes transcriptional and structural changes in heart failure. A body of work suggests roles for the developmental Wnt signalling pathway in cardiac remodelling. Hitherto, there is no evidence supporting a direct role of Wnt nuclear components in regulating chromatin landscapes in this process. We show that transcriptionally active, nuclear, phosphorylated(p)Ser675-ß-catenin and TCF7L2 are upregulated in diseased murine and human cardiac ventricles. We report that inducible cardiomyocytes (CM)-specific pSer675-ß-catenin accumulation mimics the disease situation by triggering TCF7L2 expression. This enhances active chromatin, characterized by increased H3K27ac and TCF7L2 occupancies to cardiac developmental and remodelling genes in vivo. Accordingly, transcriptomic analysis of ß-catenin stabilized hearts shows a strong recapitulation of cardiac developmental processes like cell cycling and cytoskeletal remodelling. Mechanistically, TCF7L2 co-occupies distal genomic regions with cardiac transcription factors NKX2-5 and GATA4 in stabilized-ß-catenin hearts. Validation assays revealed a previously unrecognized function of GATA4 as a cardiac repressor of the TCF7L2/ß-catenin complex in vivo, thereby defining a transcriptional switch controlling disease progression. Conversely, preventing ß-catenin activation post-pressure-overload results in a downregulation of these novel TCF7L2-targets and rescues cardiac function. Thus, we present a novel role for TCF7L2/ß-catenin in CMs-specific chromatin modulation, which could be exploited for manipulating the ubiquitous Wnt pathway.

Human induced pluripotent stem cells for live cell cycle monitoring and endogenous gene activation.

The fluorescence ubiquitination cell cycle inhibitor (FUCCI) has been introduced to monitor cell cycle activity in living cells, including human induced pluripotent stem cells (hiPSC) and derived cell types. We have recently developed hiPSC with stable expression of dCas9VPR for endogenous gene activation and a Citrine-tagged ACTN2 cell line to monitor sarcomere development and function in muscle cells. Here, we present dual and triple transgenic hiPSC lines developed by genomic integration of FUCCI with and without dCas9VPR into the ROSA26 and AAVS1 loci, respectively, in the previously introduced ACTN2-Citrine line. Functionality of the transgenes was demonstrated in the novel hiPSC line, which we introduce as Myo-CCER and CraCCER.

Preclinical evaluation of CRISPR-based therapies for Noonan syndrome caused by deep-intronic LZTR1 variants.

Gene variants in LZTR1 are implicated to cause Noonan syndrome associated with a severe and early-onset hypertrophic cardiomyopathy. Mechanistically, LZTR1 deficiency results in accumulation of RAS GTPases and, as a consequence, in RAS-MAPK signaling hyperactivity, thereby causing the Noonan syndrome-associated phenotype. Despite its epidemiological relevance, pharmacological as well as invasive therapies remain limited. Here, personalized CRISPR-Cas9 gene therapies might offer a novel alternative for a curative treatment in this patient cohort. In this study, by utilizing a patient-specific screening platform based on iPSC-derived cardiomyocytes from two Noonan syndrome patients, we evaluated different clinically translatable therapeutic approaches using small Cas9 orthologs targeting a deep-intronic LZTR1 variant to cure the disease-associated molecular pathology. Despite high editing efficiencies in cardiomyocyte cultures transduced with lentivirus or all-in-one adeno-associated viruses, we observed crucial differences in editing outcomes in proliferative iPSCs vs. non-proliferative cardiomyocytes. While editing in iPSCs rescued the phenotype, the same editing approaches did not robustly restore LZTR1 function in cardiomyocytes, indicating critical differences in the activity of DNA double-strand break repair mechanisms between proliferative and non-proliferative cell types and highlighting the importance of cell type-specific screens for testing CRISPR-Cas9 gene therapies.

Physiologists as medical scientists: An early warning from the German academic system.

"Medical scientists" are postgraduate investigators who are engaged in biomedical research, and either hold a biomedical PhD or are qualified in medicine but do not participate in patient care. Medical scientists constitute ~40% of staff at medical faculties and >90% at nonuniversity medical research institutions in Germany. However, medical scientists in Germany face limited long-term career prospects and a lack of dedicated training and support programmes. They also face time limits on their career progression arising from national academic employment legislation, and imminent reforms by the German government are likely to make this worse. Nevertheless, recent developments in the educational landscape including the introduction of increasingly focused MSc, pre-PhD, and doctoral programmes to train medically aware basic scientists, as well as improved general recognition of the roles and relevance of medical scientists in health research, are encouraging. Physiologists have taken essential steps to improve the recognition of medical scientists in Germany by introducing a "specialist physiologist" qualification; this initiative could be applied to support medical scientists in other fields and countries. In this review, we describe the particular challenges facing medical scientists in Germany and make recommendations that may apply to other academic systems.

Combining a tetracycline (Tet)-inducible gRNA system and CRISPRa for titratable and timely controlled enhancement of endogenous SHISA3 activation in human induced pluripotent stem cells (hiPSC).

Towards increasing the possibility for temporal control of gene expression using CRISPR activation (a) systems, we generated homozygous human induced pluripotent stem cell (hiPSC) lines carrying a doxycycline (dox)-inducible guide(g)-RNA construct targeting the SHISA3 transciptional start site, as proof-of-principle, or a non targeting gRNA as a control. The dox-inducible gRNA cassette was inserted into the human ROSA26 locus in a line with dCas9VPR integrated at the AAVS1 locus (CRISPRa/Tet-iSHISA3). Pluripotency, genomic integrity and differentiation potential into all three germ layers were maintained. Dox-dependent gene induction was validated in hiPSCs as well as derived fibroblasts. These lines provide an attractive tool for cellular reprogramming in hiPSC-derived cells in a timely controlled manner.

Single-cell transcriptomics reveal extracellular vesicles secretion with a cardiomyocyte proteostasis signature during pathological remodeling.

Aberrant Wnt activation has been reported in failing cardiomyocytes. Here we present single cell transcriptome profiling of hearts with inducible cardiomyocyte-specific Wnt activation (ß-catΔex3) as well as with compensatory and failing hypertrophic remodeling. We show that functional enrichment analysis points to an involvement of extracellular vesicles (EVs) related processes in hearts of ß-catΔex3 mice. A proteomic analysis of in vivo cardiac derived EVs from ß-catΔex3 hearts has identified differentially enriched proteins involving 20 S proteasome constitutes, protein quality control (PQC), chaperones and associated cardiac proteins including α-Crystallin B (CRYAB) and sarcomeric components. The hypertrophic model confirms that cardiomyocytes reacted with an acute early transcriptional upregulation of exosome biogenesis processes and chaperones transcripts including CRYAB, which is ameliorated in advanced remodeling. Finally, human induced pluripotent stem cells (iPSC)-derived cardiomyocytes subjected to pharmacological Wnt activation recapitulated the increased expression of exosomal markers, CRYAB accumulation and increased PQC signaling. These findings reveal that secretion of EVs with a proteostasis signature contributes to early patho-physiological adaptation of cardiomyocytes, which may serve as a read-out of disease progression and can be used for monitoring cellular remodeling in vivo with a possible diagnostic and prognostic role in the future.

Enhancing Cardiomyocyte Transcription Using In Vivo CRISPR/Cas9 Systems.

Endogenous gene activation by programmable transcription factors offers gene-dose-dependent phenotyping of target cells embedded in their in vivo natural tissue environment. Modified CRISPR/Cas9 systems were developed to be used as guide (g) RNA programmable transcriptional activation platforms (CRISPRa) in vitro and in vivo allowing targeted or multiplexed gene activation studies. We specifically developed these tools to be applied in cardiomyocytes providing dCas9VPR expressing mice under the control of the Myosin heavy chain 6 (Myh6) promoter. Here, we describe a protocol for the efficient design and validation of newly identified gRNA for enhancing transcriptional activity of a selected gene of interest. Additionally, we are providing insights into a downstream application in a dCas9VPR expressing mouse model specifically for cardiomyocyte biology.

Establishment of two homozygous CRISPR interference (CRISPRi) knock-in human induced pluripotent stem cell (hiPSC) lines for titratable endogenous gene repression.

Using nuclease-deficient dead (d)Cas9 without enzymatic activity fused to transcriptional inhibitors (CRISPRi) allows for transcriptional interference and results in a powerful tool for the elucidation of developmental, homeostatic and disease mechanisms. We inserted dCas9KRAB (CRISPRi) cassette into the AAVS1 locus of hiPSC lines, which resulted in homozygous knock-in with an otherwise unaltered genome. Expression of dCas9KRAB protein, pluripotency and the ability to differentiate into all three embryonic germ layers were validated. Furthermore, functional cardiomyocyte generation was tested. The hiPSC-CRISPRi cell lines offer a valuable tool for studying endogenous transcriptional repression with single and multiplexed possibilities in all human cell types.

Tailoring Cardiac Synthetic Transcriptional Modulation Towards Precision Medicine.

Molecular and genetic differences between individual cells within tissues underlie cellular heterogeneities defining organ physiology and function in homeostasis as well as in disease states. Transcriptional control of endogenous gene expression has been intensively studied for decades. Thanks to a fast-developing field of single cell genomics, we are facing an unprecedented leap in information available pertaining organ biology offering a comprehensive overview. The single-cell technologies that arose aided in resolving the precise cellular composition of many organ systems in the past years. Importantly, when applied to diseased tissues, the novel approaches have been immensely improving our understanding of the underlying pathophysiology of common human diseases. With this information, precise prediction of regulatory elements controlling gene expression upon perturbations in a given cell type or a specific context will be realistic. Simultaneously, the technological advances in CRISPR-mediated regulation of gene transcription as well as their application in the context of epigenome modulation, have opened up novel avenues for targeted therapy and personalized medicine. Here, we discuss the fast-paced advancements during the recent years and the applications thereof in the context of cardiac biology and common cardiac disease. The combination of single cell technologies and the deep knowledge of fundamental biology of the diseased heart together with the CRISPR-mediated modulation of gene regulatory networks will be instrumental in tailoring the right strategies for personalized and precision medicine in the near future. In this review, we provide a brief overview of how single cell transcriptomics has advanced our knowledge and paved the way for emerging CRISPR/Cas9-technologies in clinical applications in cardiac biomedicine.

Establishment of a second generation homozygous CRISPRa human induced pluripotent stem cell (hiPSC) line for enhanced levels of endogenous gene activation.

CRISPR/Cas9 technology based on nuclease inactive dCas9 and fused to the heterotrimeric VPR transcriptional activator is a powerful tool to enhance endogenous transcription by targeting defined genomic loci. We generated homozygous human induced pluripotent stem cell (hiPSC) lines carrying dCas9 fused to VPR along with a WPRE element at the AAVS1 locus (CRISPRa2). We demonstrated pluripotency, genomic integrity and differentiation potential into all three germ layers. CRISPRa2 cells showed increased transgene expression and higher transcriptional induction in hiPSC-derived cardiomyocytes compared to a previously described CRISPRa line. Both lines allow studying endogenous transcriptional modulation with lower and higher transcript abundance.

Reversible reprogramming of cardiomyocytes to a fetal state drives heart regeneration in mice.

Cardiomyocyte (CM) replacement is very slow in adult mammalian hearts, preventing regeneration of damaged myocardium. By contrast, fetal hearts display considerable regenerative potential owing to the presence of less mature CMs that still have the ability to proliferate. In this study, we demonstrate that heart-specific expression of Oct4, Sox2, Klf4, and c-Myc (OSKM) induces adult CMs to dedifferentiate, conferring regenerative capacity to adult hearts. Transient, CM-specific expression of OSKM extends the regenerative window for postnatal mouse hearts and induces a gene expression program in adult CMs that resembles that of fetal CMs. Extended expression of OSKM in CMs leads to cellular reprogramming and heart tumor formation. Short-term OSKM expression before and during myocardial infarction ameliorates myocardial damage and improves cardiac function, demonstrating that temporally controlled dedifferentiation and reprogramming enable cell cycle reentry of mammalian CMs and facilitate heart regeneration.

Generation of homozygous CRISPRa human induced pluripotent stem cell (hiPSC) lines for sustained endogenous gene activation.

CRISPR/Cas9 technology is a powerful tool, owing to its robust on-target activity and high fidelity. Mutated Cas9 without nuclease activity (dCas9) fused to transcriptional modulators, can function as transcriptional inhibitors or activators (CRISPRa). We generated homozygous human induced pluripotent stem cell (hiPSC) lines with an inserted CRISPRa cassette into the AAVS1 locus whilst maintaining pluripotency and genomic integrity, the ability to differentiate into all three germ layers, generate functional cardiomyocytes, and validated Cas9-mediated induction of endogenous gene expression. Our generated hiPSC-CRISPRa offers a valuable tool for studying endogenous transcriptional modulation with single and multiplexed possibilities in all human cell types.

KLF15-Wnt-Dependent Cardiac Reprogramming Up-Regulates SHISA3 in the Mammalian Heart.

BACKGROUND: The combination of cardiomyocyte (CM) and vascular cell (VC) fetal reprogramming upon stress culminates in end-stage heart failure (HF) by mechanisms that are not fully understood. Previous studies suggest KLF15 as a key regulator of CM hypertrophy. OBJECTIVES: This study aimed to characterize the impact of KLF15-dependent cardiac transcriptional networks leading to HF progression, amenable to therapeutic intervention in the adult heart. METHODS: Transcriptomic bioinformatics, phenotyping of Klf15 knockout mice, Wnt-signaling-modulated hearts, and pressure overload and myocardial ischemia models were applied. Human KLF15 knockout embryonic stem cells and engineered human myocardium, and human samples were used to validate the relevance of the identified mechanisms. RESULTS: The authors identified a sequential, postnatal transcriptional repression mediated by KLF15 of pathways implicated in pathological tissue remodeling, including distinct Wnt-pathways that control CM fetal reprogramming and VC remodeling. The authors further uncovered a vascular program induced by a cellular crosstalk initiated by CM, characterized by a reduction of KLF15 and a concomitant activation of Wnt-dependent transcriptional signaling. Within this program, a so-far uncharacterized cardiac player, SHISA3, primarily expressed in VCs in fetal hearts and pathological remodeling was identified. Importantly, the KLF15 and Wnt codependent SHISA3 regulation was demonstrated to be conserved in mouse and human models. CONCLUSIONS: The authors unraveled a network interplay defined by KLF15-Wnt dynamics controlling CM and VC homeostasis in the postnatal heart and demonstrated its potential as a cardiac-specific therapeutic target in HF. Within this network, they identified SHISA3 as a novel, evolutionarily conserved VC marker involved in pathological remodeling in HF.