LMNA-mutated Rabbits: A Model of Premature Aging Syndrome with Muscular Dystrophy and Dilated Cardiomyopathy

LMNA-related cardiomyopathy: From molecular pathology to cardiac gene therapy

BACKGROUND

The genetic variants of LMNA cause an array of diseases that often affect the heart. LMNA-related cardiomyopathy exhibits high-penetrance and early-onset phenotypes that lead to late-stage heart failure or lethal arrhythmia. As a subtype of dilated cardiomyopathy and arrhythmogenic cardiomyopathy, LMNA-related cardiac dysfunction is resistant to existing cardiac therapeutic strategies, leaving a major unmet clinical need in cardiomyopathy management.

AIM OF REVIEW

Here we comprehensively summarize current knowledge about the genetic basis, disease models and pathological mechanisms of LMNA-related cardiomyopathy. Recent translational studies were highlighted to indicate new therapeutic modalities such as gene supplementation, gene silencing and genome editing therapy, which offer potential opportunities to overcome the difficulties in the development of specific drugs for this disease.

KEY SCIENTIFIC CONCEPTS OF REVIEW

LMNA-related cardiomyopathy involves many diverse disease mechanisms that preclude small-molecule drugs that target only a small fraction of the mechanisms. Agreeing to this notion, the first-in-human clinical trial for this disease recently reported futility. By contrast, gene therapy offers the new hope to directly intervene LMNA variants and demonstrates a tremendous potential for breakthrough therapy for this disease. Concepts in this review are also applicable to studies of other genetic diseases that lack effective therapeutics.

Skeletal phenotypes and molecular mechanisms in aging mice

Aging is an inevitable physiological process, often accompanied by age-related bone loss and subsequent bone-related diseases that pose serious health risks. Research on skeletal diseases caused by aging in humans is challenging due to lengthy study durations, difficulties in sampling, regional variability, and substantial investment. Consequently, mice are preferred for such studies due to their similar motor system structure and function to humans, ease of handling and care, low cost, and short generation time. In this review, we present a comprehensive overview of the characteristics, limitations, applicability, bone phenotypes, and treatment methods in naturally aging mice and prematurely aging mouse models (including SAMP6, POLG mutant, LMNA, SIRT6, ZMPSTE24, TFAM, ERCC1, WERNER, and KL/KL-deficient mice). We also summarize the molecular mechanisms of these aging mouse models, including cellular DNA damage response, senescence-related secretory phenotype, telomere shortening, oxidative stress, bone marrow mesenchymal stem cell (BMSC) abnormalities, and mitochondrial dysfunction. Overall, this review aims to enhance our understanding of the pathogenesis of aging-related bone diseases.

Genome-edited rabbits: Unleashing the potential of a promising experimental animal model across diverse diseases

Animal models are extensively used in all aspects of biomedical research, with substantial contributions to our understanding of diseases, the development of pharmaceuticals, and the exploration of gene functions. The field of genome modification in rabbits has progressed slowly. However, recent advancements, particularly in CRISPR/Cas9-related technologies, have catalyzed the successful development of various genome-edited rabbit models to mimic diverse diseases, including cardiovascular disorders, immunodeficiencies, aging-related ailments, neurological diseases, and ophthalmic pathologies. These models hold great promise in advancing biomedical research due to their closer physiological and biochemical resemblance to humans compared to mice. This review aims to summarize the novel gene-editing approaches currently available for rabbits and present the applications and prospects of such models in biomedicine, underscoring their impact and future potential in translational medicine.

Lp-PLA2 (Lipoprotein-Associated Phospholipase A2) Deficiency Lowers Cholesterol Levels and Protects Against Atherosclerosis in Rabbits

BACKGROUND

Elevated plasma Lp-PLA₂ (lipoprotein-associated phospholipase A₂) activity is closely associated with an increased risk of cardiovascular events. However, whether and how Lp-PLA₂ is directly involved in the pathogenesis of atherosclerosis is still unclear. To examine the hypothesis that Lp-PLA₂ could be a potential preventative target of atherosclerosis, we generated Lp-PLA₂ knockout rabbits and investigated the pathophysiological functions of Lp-PLA₂.

METHODS

Lp-PLA₂ knockout rabbits were generated using CRISPR/Cas9 system to assess the role of Lp-PLA₂ in plasma lipids regulation and identify its underlying molecular mechanisms. Homozygous knockout rabbits along with wild-type rabbits were fed a cholesterol-rich diet for up to 14 weeks and their atherosclerotic lesions were compared. Moreover, the effects of Lp-PLA₂ deficiency on the key cellular behaviors in atherosclerosis were assessed in vitro.

RESULTS

When rabbits were fed a standard diet, Lp-PLA₂ deficiency led to a significant reduction in plasma lipids. The decreased protein levels of SREBP2 (sterol regulatory element-binding protein 2) and HMGCR (3-hydroxy-3-methylglutaryl coenzyme A reductase) in livers of homozygous knockout rabbits indicated that the cholesterol biosynthetic pathway was impaired with Lp-PLA₂ deficiency. In vitro experiments further demonstrated that intracellular Lp-PLA₂ efficiently enhanced SREBP2-related cholesterol biosynthesis signaling independently of INSIGs (insulin-induced genes). When fed a cholesterol-rich diet, homozygous knockout rabbits exhibited consistently lower level of hypercholesterolemia, and their aortic atherosclerosis lesions were significantly reduced by 60.2% compared with those of wild-type rabbits. The lesions of homozygous knockout rabbits were characterized by reduced macrophages and the expression of inflammatory cytokines. Macrophages of homozygous knockout rabbits were insensitive to M1 polarization and showed reduced DiI-labeled lipoprotein uptake capacity compared with wild-type macrophages. Lp-PLA₂ deficiency also inhibited the adhesion between monocytes and endothelial cells.

CONCLUSIONS

These results demonstrate that Lp-PLA₂ plays a causal role in regulating blood lipid homeostasis and Lp-PLA₂ deficiency protects against dietary cholesterol-induced atherosclerosis in rabbits. Lp-PLA₂ could be a potential target for the prevention of atherosclerosis.

Effect of Occurrence of Lamin A/C (LMNA) Genetic Variants in a Cohort of 101 Consecutive Apparent “Lone AF” Patients: Results and Insights

Objective

Mutations in the Lamin A/C(LMNA) gene are commonly associated with cardiac manifestations, such as dilated cardiomyopathy (DCM) and conduction system disease. However, the overall spectrum and penetrance of rare LMNA variants are unknown. The present study described the presence of LMNAvariants in patients with “lone atrial fibrillation (AF)” as their sole clinical presentation.

Methods

One-hundred and one consecutive patients with “lone AF” criteria were initially screened by genetic testing. Genetic variants were classified according to the American College of Genetic and Genomic criteria. All subjects were evaluated through clinical and familial history, ECG, 24-h Holter monitoring, echocardiogram, cardiac magnetic resonance, treatment response, and the present relatives of LMNA carriers. In addition, whole-exome data from 49,960 UK Biobank (UKB) participants were analyzed to describe the overall penetrance of rare LMNA missense and loss of function (LOF) variants.

Results

Three missense variants in LMNA were identified in probands with AF as their first and unique clinical manifestation. Other five first-degree relatives, after the screening, also presented LMNA gene variants. Among 49,960 analyzed UKB participants, 331 carried rare LMNA missense or LOF variant. Participants who carried a rare LMNA variant were significantly associated with higher odds of arrhythmic events and of an abnormal ECG in the per-protocol ECG exam (p = 0.03 and p = 0.05, respectively).

Conclusion

Although a rare occurrence, our findings emphasize the possibility of an initial presentation of apparently “lone AF” in LMNA gene variant carriers.

Genetically modified rabbit models for cardiovascular medicine

Genetically modified (GM) rabbits are outstanding animal models for studying human genetic and acquired diseases. As such, GM rabbits that express human genes have been extensively used as models of cardiovascular disease. Rabbits are genetically modified via prokaryotic microinjection. Through this process, genes are randomly integrated into the rabbit genome. Moreover, gene targeting in embryonic stem (ES) cells is a powerful tool for understanding gene function. However, rabbits lack stable ES cell lines. Therefore, ES-dependent gene targeting is not possible in rabbits. Nevertheless, the RNA interference technique is rapidly becoming a useful experimental tool that enables researchers to knock down specific gene expression, which leads to the genetic modification of rabbits. Recently, with the emergence of new genetic technology, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated protein 9 (CRISPR/Cas9), major breakthroughs have been made in rabbit gene targeting. Using these novel genetic techniques, researchers have successfully modified knockout (KO) rabbit models. In this paper, we aimed to review the recent advances in GM technology in rabbits and highlight their application as models for cardiovascular medicine.

Research-Relevant Conditions and Pathology of Laboratory Mice, Rats, Gerbils, Guinea Pigs, Hamsters, Naked Mole Rats, and Rabbits

Animals are valuable resources in biomedical research in investigations of biological processes, disease pathogenesis, therapeutic interventions, safety, toxicity, and carcinogenicity. Interpretation of data from animals requires knowledge not only of the processes or diseases (pathophysiology) under study but also recognition of spontaneous conditions and background lesions (pathology) that can influence or confound the study results. Species, strain/stock, sex, age, anatomy, physiology, spontaneous diseases (noninfectious and infectious), and neoplasia impact experimental results and interpretation as well as animal welfare. This review and the references selected aim to provide a pathology resource for researchers, pathologists, and veterinary personnel who strive to achieve research rigor and validity and must understand the spectrum of "normal" and expected conditions to accurately identify research-relevant experimental phenotypes as well as unusual illness, pathology, or other conditions that can compromise studies involving laboratory mice, rats, gerbils, guinea pigs, hamsters, naked mole rats, and rabbits.

Generation of a laminopathies-specific iPSC line EHTJUi005-A-3 with homozygous knockout of the LMNA gene by CRISPR/Cas9 technology

LAMIN A/C, encoded by the LMNA gene, supports the normal structure of the cell nucleus and regulates the connection between the nucleus and the cytoskeleton as a component of the nucleus envelope. The loss of expression and function of the LMNA gene would lead to the occurrence of congenital muscular dystrophy and Emery-Dreifuss muscular dystrophy which are collectively named as laminopathies. Here, we report a human induced pluripotent stem cell (iPSC) line (EHTJUi005-A-3) generated from a wild iPSC (EHTJUi005-A) with homozygous knockout of the gene LMNA through CRISPR/Cas9. This iPSC line provides a useful research model for studying laminopathies disease.

Rescuing lethal phenotypes induced by disruption of genes in mice: a review of novel strategies

Approximately 35 % of the mouse genes are indispensable for life, thus, global knock-out (KO) of those genes may result in embryonic or early postnatal lethality due to developmental abnormalities. Several KO mouse lines are valuable human disease models, but viable homozygous mutant mice are frequently required to mirror most symptoms of a human disease. The site-specific gene editing systems, the transcription activator-like effector nucleases (TALENs), Zinc-finger nucleases (ZFNs) and the clustered regularly interspaced short palindrome repeat-associated Cas9 nuclease (CRISPR/Cas9) made the generation of KO mice more efficient than before, but the homozygous lethality is still an undesired side-effect in case of many genes. The literature search was conducted using PubMed and Web of Science databases until June 30th, 2020. The following terms were combined to find relevant studies: "lethality", "mice", "knock-out", "deficient", "embryonic", "perinatal", "rescue". Additional manual search was also performed to find the related human diseases in the Online Mendelian Inheritance in Man (OMIM) database and to check the citations of the selected studies for rescuing methods. In this review, the possible solutions for rescuing human disease-relevant homozygous KO mice lethal phenotypes were summarized.

Genome engineering technologies in rabbits

The rabbit has been recognized as a valuable model in various biomedical and biological research fields because of its intermediate size and phylogenetic proximity to primates. However, the technology for precise genome manipulations in rabbit has been stalled for decades, severely limiting its applications in biomedical research. Novel genome editing technologies, especially CRISPR/Cas9, have remarkably enhanced precise genome manipulation in rabbits, and shown their superiority and promise for generating rabbit models of human genetic diseases. In this review, we summarize the brief history of transgenic rabbit technology and the development of novel genome editing technologies in rabbits.

Transgenic Rabbit Models: Now and the Future

Transgenic rabbits have contributed to the progress of biomedical science as human disease models because of their unique features, such as the lipid metabolism system similar to humans and medium body size that facilitates handling and experimental manipulation. In fact, many useful transgenic rabbits have been generated and used in research fields such as lipid metabolism and atherosclerosis, cardiac failure, immunology, and oncogenesis. However, there have been long-term problems, namely that the transgenic efficiency when using pronuclear microinjection is low compared with transgenic mice and production of knockout rabbits is impossible owing to the lack of embryonic stem cells for gene targeting in rabbits. Despite these limitations, the emergence of novel genome editing technology has changed the production of genetically modified animals including the rabbit. We are finally able to produce both transgenic and knockout rabbit models to analyze gain- and loss-of-functions of specific genes. It is expected that the use of genetically modified rabbits will extend to various research fields. In this review, we describe the unique features of rabbits as laboratory animals, the current status of their development and use, and future perspectives of transgenic rabbit models for human diseases.

CRISPR-Cas systems: Overview, innovations and applications in human disease research and gene therapy

Genome editing is the modification of genomic DNA at a specific target site in a wide variety of cell types and organisms, including insertion, deletion and replacement of DNA, resulting in inactivation of target genes, acquisition of novel genetic traits and correction of pathogenic gene mutations. Due to the advantages of simple design, low cost, high efficiency, good repeatability and short-cycle, CRISPR-Cas systems have become the most widely used genome editing technology in molecular biology laboratories all around the world. In this review, an overview of the CRISPR-Cas systems will be introduced, including the innovations, the applications in human disease research and gene therapy, as well as the challenges and opportunities that will be faced in the practical application of CRISPR-Cas systems.

The Genetic Mutation of ANO5 in Rabbits Recapitulates Human Cardiomyopathy

The limb girdle muscular dystrophy type 2L (LGMD2L) is caused by mutations of the ANO5 gene in humans which encodes a 913 amino-acid integral membrane protein. Although cardiomyopathy has been reported in patients with an ANO5 mutation, the ANO5 mutant mice did not recapitulate this phenotype in previous studies. This study demonstrated that the ANO5−/− rabbits recapitulated the typical signs of cardiomyopathy with decreased ejection fraction (EF) and fraction shortening (FS) with increased interstitial fibrosis. This ANO5−/− rabbit model would promote basic research to comprehend the pathogenesis and mechanism of ANO5-related cardiomyopathy.

Identification of Target Genes and Transcription Factors in Mice with LMNA-Related Dilated Cardiomyopathy by Integrated Bioinformatic Analyses

BACKGROUND Dilated cardiomyopathy (DCM), which is characterized by enlarged ventricular dimensions and systolic dysfunction, is the most common type of cardiomyopathy. Mutations in the LMNA gene are reported in approximately 10% of familial DCM cases. However, the mechanism of LMNA mutations in human DCM remains unclear. MATERIAL AND METHODS We used the GSE36502 and GSE123916 datasets to obtain gene expression profiles from LMNA-related DCM mice and to identify differentially expressed genes (DEGs). Crucial function and pathway enrichment analyses of DEGs were performed. Protein-protein interaction (PPI) network analysis was carried out to identify the top 10 hub genes, which were validated using reverse transcription-polymerase chain reaction (RT-PCR) to find target genes. Weighted gene co-expression network analysis (WGCNA) was used to explore the module relevant to external traits of LMNA-related DCM mice. Transcription factors (TFs) for the selected genes were analyzed using NetworkAnalyst. RESULTS A total of 156 common DEGs (co-DEGs) were identified, including 80 up-regulated and 76 down-regulated genes. The enriched biological functions and pathways were oxidative stress, regulation of apoptosis, regulation of fibrosis, and MAPK pathways. Five target genes (Timp1, Hmox1, Spp1, Atf3, and Adipoq) were verified after RT-PCR. Most co-DEGs were discovered to be related to the development of external traits. Three TFs (ELF1, ETS1, and NRF1) showed close interactions with the hub genes. CONCLUSIONS Our study used integrated bioinformatic analyses and revealed some important genes in mice with LMNA-related DCM, which could provide novel insights into the mechanism underlying human LMNA-related DCM.

Emery-Dreifuss muscular dystrophy: focal point nuclear envelope

PURPOSE OF REVIEW

Emery-Dreifuss muscular dystrophy (EDMD) is caused by mutations in EMD encoding emerin and LMNA encoding A-type lamins, proteins of the nuclear envelope. In the past decade, there has been an extraordinary burst of research on the nuclear envelope. Discoveries resulting from this basic research have implications for better understanding the pathogenesis and developing treatments for EDMD.

RECENT FINDINGS

Recent clinical research has confirmed that EDMD is one of several overlapping skeletal muscle phenotypes that can result from mutations in EMD and LMNA with dilated cardiomyopathy as a common feature. Basic research on the nuclear envelope has provided new insights into how A-type lamins and emerin function in force transmission throughout the cell, which may be particularly important in striated muscle. Much of the recent research has focused on the heart and LMNA mutations. Prevalence and outcome studies have confirmed the relative severity of cardiac disease. Robust mouse models of EDMD caused by LMNA mutations has allowed for further insight into pathogenic mechanisms and potentially beneficial therapeutic approaches.

SUMMARY

Recent clinical and basic research on EDMD is gradually being translated to clinical practice and possibly novel therapies.