International Journal of Molecular Medical Science, 2024, Vol.14 http://medscipublisher.com/index.php/ijmms © 2024 MedSci Publisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved.
International Journal of Molecular Medical Science, 2024, Vol.14 http://medscipublisher.com/index.php/ijmms © 2024 MedSci Publisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. MedSci Publisher, operated by Sophia Publishing Group (SPG), is an international Open Access publishing platform that publishes scientific journals in the field of life science. Sophia Publishing Group (SPG), founded in British Columbia of Canada, is a multilingual publisher. Publisher MedSci Publisher Edited by Editorial Team of International Journal of Molecular Medical Science Email: edit@ijmms.medscipublisher.com Website: http://medscipublisher.com/index.php/ijmms Address: 11388 Stevenston Hwy, PO Box 96016, Richmond, V7A 5J5, British Columbia Canada International Journal of Molecular Medical Science (ISSN 1927-6656) is an open access, peer reviewed journal published online by MedSci Publisher. The journal publishes scientific articles like original research articles, case reports, review articles, editorials, short communications and correspondence of the high quality pertinent to all aspects of human biology, pathophysiology and molecular medical science, including genomics, transcriptomics, proteomics, metabolomics of disease therapy, clinical pharmacology, clinical biochemistry, vaccines, immunology, microbiology, epidemiology, aging, cancer biology, infectious diseases, neurological diseases and myopathies, stem cells and regenerative medicine, vascular and cardiovascular biology, as well as the important implications for human health and clinical practice research. All the articles published in International Journal of Molecular Medical Science are Open Access, and are distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. MedSci Publisher uses CrossCheck service to identify academic plagiarism through the world’s leading plagiarism prevention tool, iParadigms, and to protect the original authors’ copyrights.
International Journal of Molecular Medical Science (online), 2024, Vol. 14, No. 3 ISSN 1927-6656 http://medscipublisher.com/index.php/ijmms © 2024 MedSci Publisher, registered at the publishing platform that is operated by Sophia Publishing Group, founded in British Columbia of Canada. All Rights Reserved. Latest Content Harnessing AI for Revolutionary Advances in Medicine Design JimMason International Journal of Molecular Medical Science, 2024, Vol. 15, No. 3, 153-154 CRISPR/Cas9 Technology in Xenotransplantation: Current Applications and Future Challenges Huixian Li, Jingqiang Wang International Journal of Molecular Medical Science, 2024, Vol. 15, No. 3, 155-166 New Hope in Transplantation Medicine: The Role of Mesenchymal Stem Cells in the Prevention and Treatment of GVHD CaixinLi International Journal of Molecular Medical Science, 2024, Vol. 15, No. 3, 167-176 Genetic Determinants of Long-Term Graft Survival in Pig-to-Human Xenotransplantation Xiaofang Lin International Journal of Molecular Medical Science, 2024, Vol. 15, No. 3, 177-192 Prospects of Gene Editing Technologies in Sickle Cell Anemia Qiyan Lou, Xiaoying Xu International Journal of Molecular Medical Science, 2024, Vol. 15, No. 3, 193-202
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 153-154 http://medscipublisher.com/index.php/ijmms 153 Perspective Open Access Harnessing AI for Revolutionary Advances in Medicine Design JimMason MedSci Publisher of Sophia Publishing Plateform, Richmond, BC, V7A 4Z5, Canada Corresponding email: jim.mason@sophiapublisher.com International Journal of Molecular Medical Science, 2024, Vol.14, No.3 doi: 10.5376/ijmms.2024.14.0018 Received: 10 May, 2024 Accepted: 16 May, 2024 Published: 30 May, 2024 Copyright © 2024 Mason, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Mason J., 2024, Harnessing AI for revolutionary advances in medicine design, International Journal of Molecular Medical Science, 14(3): 153-154 (doi: 10.5376/ijmms.2024.14.0018) Abstract The integration of artificial intelligence (AI) in structural biology has revolutionized medicine design, notably through AlphaFold 3's accurate prediction of biomolecular interactions. With AI predicting over 600 million protein structures, the vast database enhances the identification of novel drug targets and optimization of therapeutic molecules. However, AI's limitations in capturing protein dynamics highlight the continued need for experimental validation. The synergy between AI's predictive power and empirical methods like cryo-EM and NMR spectroscopy fosters comprehensive drug design, accelerating the development of personalized medicine. This perspective underscores the necessity of balancing AI and experimental approaches to unlock unprecedented therapeutic innovations. Keywords AI; AlphaFold; Drug discovery; Experimental validation; Personalized medicine The integration of artificial intelligence (AI) into the realm of structural biology has opened new frontiers for the design and development of medical therapeutics. The recent publication by Abramson, Adler, Dunger, et al., in Nature on May 8, 2024, underscores the potential of AlphaFold 3 to predict biomolecular interactions with unprecedented accuracy (Abramson et al., 2024). This breakthrough is not merely a technical milestone; it represents a transformative shift in how we approach drug discovery and development. AlphaFold's ability to predict over 600 million protein structures has dramatically expanded our structural database, providing an extensive foundation for medicinal chemists and pharmacologists. As Westlake University's Professor Shi Yigong notes, "AI's rapid advancements have fundamentally altered our understanding of protein structures, offering a database that is several orders of magnitude larger than what we had before. This scale of change inevitably influences our comprehension of life sciences, drug discovery, and disease treatment" (Credit: Tai Media AGI, Video ID: sphMGEP2FvbOKcq). The vast array of predicted structures facilitates the identification of novel drug targets and the optimization of therapeutic molecules, accelerating the transition from conceptual design to clinical application. Despite these advancements, the process of translating AI predictions into viable medical products remains complex. Dr. Yan Ning, the current President of Shenzhen Medical Academy of Research and Translation, provides a more nuanced perspective. While recognizing AI's impressive capabilities, she emphasizes the importance of experimental validation in drug design. "AI can predict a static structure, but the true beauty and complexity of proteins lie in their dynamic states. To truly understand a protein’s function, we must observe it in various conformations, something AI currently struggles with" (Credit: Tai Media AGI, Video ID: sphMGEP2FvbOKcq ). This sentiment highlights a crucial aspect of drug design: the need to understand the dynamic nature of target proteins. Proteins do not exist in a single, static conformation; they adopt multiple shapes that are crucial for their biological functions. Drugs designed to interact with these proteins must therefore be effective across these various states. AI's predictions, while highly accurate, often provide a snapshot rather than a full dynamic picture. Thus, combining AI's predictive power with experimental techniques such as cryo-electron microscopy (cryo-EM) and nuclear magnetic resonance (NMR) spectroscopy is essential to capture these dynamic processes.
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 153-154 http://medscipublisher.com/index.php/ijmms 154 The integration of AI into medicinal chemistry has already shown promising results. For example, the rapid identification of binding sites and the optimization of ligand interactions have significantly shortened the lead optimization phase. Additionally, AI-driven models can predict off-target effects and potential toxicity earlier in the drug development process, reducing the risk of late-stage failures. This predictive capability is particularly valuable in the context of personalized medicine, where drugs can be tailored to the genetic and molecular profiles of individual patients. Moreover, the collaboration between computational scientists and experimental biologists is fostering innovative approaches to drug design. The synergy between AI's data-driven insights and the empirical rigor of laboratory experiments enables a more comprehensive understanding of drug-target interactions. This holistic approach not only enhances the efficacy and safety of new therapeutics but also expedites their development. As we move forward, the focus should be on creating a seamless integration between AI predictions and experimental validation. Investment in interdisciplinary research and training is crucial to equip the next generation of scientists with the skills needed to harness both computational and experimental tools. Regulatory frameworks must also evolve to accommodate the rapid advancements in AI-driven drug development, ensuring that new therapies are both safe and effective. In conclusion, the advent of AI technologies like AlphaFold represents a paradigm shift in medicine design and development. By leveraging AI's predictive power and combining it with rigorous experimental validation, we can accelerate the discovery of new therapeutics and bring life-saving treatments to patients faster than ever before. The future of medicine lies in this collaborative, interdisciplinary approach, where the strengths of AI and experimental science converge to drive innovation and improve human health. References Abramson J., Adle, J., Dunger J., Evans R., Green T., Pritzel A., Ronneberger O., Willmore L., Ballard A.J., Bambrick J., Bodenstein S.W., Evans D.A., Hung C.C., Neill M.O., Reiman D., Tunyasuvunakool K., Wu Z., Žemgulytė A., Arvaniti E., Beattie C., Bertolli O., Bridgland A., Cherepanov A., Congreve M., Cowen-Rivers A.I., Cowie A., Figurnov M., Fuchs F.B., Gladman H., Jain R., Khan Y.A., Low C.M.R., Perlin K., Potapenko A., Savy P., Singh S., Stecula A., Thillaisundaram A., Tong C., Yakneen S., Zhong E.D., Zielinski M., Žídek A., Bapst V., Kohli P., Jaderberg M., Hassabis D., and Jumper J.M., 2024, Accurate structure prediction of biomolecular interactions with AlphaFold 3, Nature, 1-3. https://doi.org/10.1038/s41586-024-07487-w PMid:38718835
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 155 Research Perspective Open Access CRISPR/Cas9 Technology in Xenotransplantation: Current Applications and Future Challenges Huixian Li, Jingqiang Wang Institute of Life Science, Jiyang College of Zhejiang A&F University, Zhuji, 311800, Zhejiang, China Corresponding author : jingqiang.wang@gmail.com International Journal of Molecular Medical Science, 2024, Vol.14, No.1 doi: 10.5376/ijmms.2024.14.0019 Received: 11 May, 2024 Accepted: 13 Jun., 2024 Published: 24 Jun., 2024 Copyright © 2024 Li and Wang, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Li H.X., and Wang J.Q., 2024, CRISPR/Cas9 technology in xenotransplantation: current applications and future challenges, International Journal of Molecular Medical Science, 14(3): 155-166 (doi: 10.5376/ijmms.2024.14.0019) Abstract Xenotransplantation has emerged as a promising solution to the severe shortage of human donor organs. The CRISPR/Cas9 technology, with its unprecedented precision and efficiency in gene editing, has revolutionized this field, making it possible to genetically modify donor animals to enhance compatibility and reduce immunogenicity. This study explores the current applications of CRISPR/Cas9 in xenotransplantation, highlighting significant advancements such as the knockout of genes producing xenogeneic antigens and the elimination of porcine endogenous retroviruses (PERVs) to prevent zoonotic disease transmission. Despite these achievements, challenges such as off-target effects, genetic mosaicism, and long-term organ survival rates still exist. The research also discusses emerging technologies and the integration of CRISPR/Cas9 with other biotechnological approaches, which have the potential to address these challenges and further advance the field. This study hopes to promote a broader understanding and acceptance of xenotransplantation as a life-saving medical intervention driven by innovations in CRISPR/Cas9 technology. Keywords CRISPR/Cas9; Xenotransplantation; Genetic modification; Immunological rejection; Gene editing 1 Introduction Xenotransplantation, the process of transplanting organs, tissues, or cells from one species to another, has emerged as a promising solution to the critical shortage of human donor organs. This biotechnological approach seeks to alleviate the gap between the demand and supply of transplantable organs, which remains a significant challenge in modern medicine. By utilizing organs from genetically modified animals, particularly pigs, xenotransplantation holds the potential to save countless lives (Ryczek et al., 2021). Pigs are considered the most suitable donors due to their physiological similarities to humans and the feasibility of breeding them in large numbers. However, significant immunological and virological barriers have historically impeded the clinical application of pig-to-human xenotransplantation. The phylogenetic distance between pigs and humans leads to acute immune rejection, and the presence of porcine endogenous retroviruses (PERVs) poses a risk of cross-species viral transmission (Salomon, 2016; Niu et al., 2017; Ryczek et al., 2021). The advent of CRISPR/Cas9 technology has revolutionized genetic engineering by providing a precise, efficient, and scalable method for genome editing. Derived from a microbial adaptive immune system, CRISPR/Cas9 allows for targeted modifications of DNA sequences, enabling the insertion, deletion, or replacement of genes with unprecedented accuracy. This technology has been instrumental in advancing various fields, including biotechnology, medicine, and basic biological research (Hsu et al., 2014; Kararoudi et al., 2018). This breakthrough has opened new avenues in biomedical research, including the potential for developing genetically modified animals that are better suited for xenotransplantation. The ability to engineer donor animals with reduced immunogenicity and enhanced compatibility with human physiology represents a significant leap forward in the quest to make xenotransplantation a viable clinical option (Cowan et al., 2019). This study aims to provide a comprehensive overview of the current applications of CRISPR/Cas9 technology in the field of xenotransplantation. By examining the latest advancements and identifying ongoing challenges, it highlights the transformative potential of genetic engineering in overcoming obstacles associated with
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 156 xenotransplantation. Additionally, the study explores future development directions and proposes strategies to address the remaining scientific, ethical, and regulatory issues. Through this analysis, the study seeks to promote a broader understanding and acceptance of xenotransplantation as a life-saving medical intervention driven by innovations in CRISPR/Cas9 technology. 2 Current State of Xenotransplantation 2.1 Definition and historical background Xenotransplantation refers to the transplantation of living cells, tissues, or organs from one species to another. Historically, the concept of xenotransplantation dates back to the early 20th century, with initial attempts involving the transplantation of animal organs into humans. However, these early efforts were largely unsuccessful due to severe immunological rejection and other complications. It wasn't until the advent of immunosuppressive drugs and advances in genetic engineering that xenotransplantation began to show promise as a potential solution to the shortage of human donor organs (Ryczek et al., 2021). The advent of genetic engineering, particularly the development of CRISPR/Cas9 technology, has significantly advanced the field by enabling precise genetic modifications to donor animals, primarily pigs (Figure 1), to reduce immunogenicity and improve compatibility with human recipients (Ryczek et al., 2021; Kararoudi et al., 2018; Zhang et al., 2021). Figure 1 Two methods for creating genetically edited pigs for xenotransplantation using the CRISPR/Cas system (Adapted from Ryczek et al., 2021) Image caption: Panel A involves microinjecting CRISPR/Cas-modified constructs or RNP complexes into pig zygotes. Panel B uses somatic cell nuclear transfer (SCNT) technology. The diagram details the workflows of these two methods, including how to provide CRISPR/Cas9 components to fertilized eggs at the early pronuclear stage to minimize genetic mosaicism. Research indicates that the presence of genetic mosaics is one of the major limiting factors in the application of CRISPR/Cas9 in large animal models. To reduce mosaicism caused by the CRISPR/Cas9 system in large animals, it is recommended to inject CRISPR/Cas9 components during the early pronuclear stage and use the appropriate form of Cas9-sgRNA (ribonucleoprotein complex). These two methods demonstrate the potential and challenges of generating gene-edited pigs for xenotransplantation research (Adapted from Ryczek et al., 2021) 2.2 Current challenges in xenotransplantation Despite significant advancements, xenotransplantation faces several critical challenges: 1) Immunological Rejection: One of the primary obstacles is the acute and chronic rejection of xenografts by the human immune system. Hyperacute rejection occurs within minutes to hours and is triggered by pre-existing antibodies against xenoantigens, such as those produced by genes like GGTA1, CMAH, and B4GALNT2. Efforts to knock out these genes using CRISPR/Cas9 have shown promise in reducing immunogenicity, but issues like genetic mosaicism still need to be addressed (Tanihara et al., 2021; Cowan et al., 2019). Subsequent acute and chronic rejections are driven by cellular immune responses, which involve T-cells and other immune mechanisms (Kararoudi et al., 2018).
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 157 2) Zoonotic Diseases: The risk of transmitting zoonotic diseases, particularly porcine endogenous retroviruses (PERVs), from donor pigs to human recipients is a significant concern. These viruses, embedded in the pig genome, have the potential to infect human cells and could lead to unforeseen diseases (Cowan et al., 2019). Recent studies have demonstrated the potential of CRISPR/Cas9 to inactivate PERVs in pigs, thereby reducing the risk of zoonotic infections and making xenotransplantation safer (Ross et al., 2018). 3) Genetic Modifications and Mosaicism: While CRISPR/Cas9 allows for precise genetic modifications, the generation of genetically uniform animals remains a challenge. Mosaicism, where not all cells in the organism carry the desired genetic modifications, can limit the effectiveness of xenotransplantation. Techniques such as electroporation have been developed to improve the efficiency of gene editing, but further optimization is required (Tanihara et al., 2021). 4) Ethical and Regulatory Issues: The use of animals for organ harvesting raises ethical concerns regarding animal welfare and the moral implications of genetic modifications. Additionally, stringent regulatory frameworks are necessary to ensure the safety and efficacy of xenotransplantation procedures (Naert and Vleminckx, 2018). 2.3 Overview of donor species and recipient compatibility Pigs are the primary donor species for xenotransplantation due to their physiological similarities to humans and the feasibility of genetic modifications. They are physiologically and anatomically similar to humans, including comparable organ size and function. Moreover, pigs have a relatively short gestation period and large litters, making them a practical choice for genetic modification and breeding programs (Tanihara et al., 2021).The compatibility between donor pigs and human recipients is enhanced through the targeted editing of specific genes to reduce immunogenicity and improve graft survival. Key genes involved in xenoantigen biosynthesis, such as GGTA1, CMAH, and B4GALNT2, are commonly targeted to create genetically modified pigs with reduced antigenicity (Kararoudi et al., Tanihara et al., 2021; Cowan et al., 2019). Additionally, the use of CRISPR/Cas9 has enabled the development of pigs with multiple genetic modifications, including the knockout of glycosyltransferases and the inactivation of PERVs, which are crucial for improving the safety and efficacy of xenotransplantation (Ryczek et al., Ross et al., 2018; Cowan et al., 2019). These advancements have paved the way for more successful preclinical trials and hold promise for future clinical applications. While xenotransplantation has made significant strides with the help of CRISPR/Cas9 technology, ongoing research is essential to overcome the remaining challenges and ensure the safety and efficacy of this promising therapeutic approach. 3 CRISPR/Cas9 Technology 3.1 Principles and mechanisms of CRISPR/Cas9 The CRISPR/Cas9 system, derived from the adaptive immune system of bacteria, has revolutionized genome editing. The technology relies on two key components: the Cas9 nuclease and a guide RNA (gRNA). The gRNA directs Cas9 to a specific DNA sequence through complementary base pairing, where Cas9 introduces a double-strand break. This break can then be repaired by the cell's endogenous repair mechanisms, primarily non-homologous end joining (NHEJ) or homology-directed repair (HDR) (Figure 2) (Wang et al., 2017; Liang et al., 2015). The precision of CRISPR/Cas9 is largely determined by the design of the gRNA, which can be optimized using various bioinformatics tools (Wang et al., 2017). 3.2 Advantages of CRISPR/Cas9 Over traditional gene editing techniques CRISPR/Cas9 offers several advantages over traditional gene-editing methods, such as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). These include: 1) Simplicity and Efficiency: CRISPR/Cas9 is easier to design and implement compared to ZFNs and TALENs, which require complex protein engineering. The design of guide RNA sequences for CRISPR/Cas9 is straightforward and can be rapidly synthesized (Blitz and Nakayama, 2021).
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 158 Figure 2 Mechanisms of CRISPR-Cas9 mediated genome engineering (Adapted from Jiang and Doudna, 2017) Image caption: The diagram illustrates how synthesized single-guide RNA (sgRNA) or the CRISPR RNA (crRNA)-tracrRNA complex directs the Cas9 endonuclease to target and cleave arbitrary DNA sequences in the genome, resulting in double-strand breaks (DSBs). Subsequently, host-mediated DNA repair mechanisms repair these breaks. In the absence of a repair template, the error-prone non-homologous end joining (NHEJ) pathway can lead to random insertions or deletions, thereby disrupting gene function. In the presence of a homologous repair template, the high-fidelity homologous recombination repair (HDR) pathway is activated to accurately repair or modify the target gene. The diagram also shows how altering the nuclease active sites of Cas9 (e.g., the D10A mutation) can achieve DNA targeting without cleavage, as well as other applications such as gene regulation, epigenetic modification, and live-cell imaging. These mechanisms demonstrate the wide potential applications of the CRISPR-Cas9 system in gene editing and regulation (Adapted from Jiang and Doudna, 2017) 2) Multiplexing Capability: CRISPR/Cas9 can target multiple genes simultaneously by using multiple guide RNAs. This multiplexing capability is crucial for comprehensive genetic modifications required in xenotransplantation to knock out multiple immunogenic genes (Tanihara et al., 2021). 3)Precision: The precision of CRISPR/Cas9 in targeting specific DNA sequences reduces the risk of off-target effects, which are common in other gene-editing technologies. This precision is enhanced by the ability to design highly specific guide RNAs (Matson et al., 2019). 4) Cost-Effectiveness: The lower cost and ease of designing CRISPR/Cas9 components make it more accessible for research and clinical applications compared to other gene-editing methods (Ryczek et al., 2021). 3.3 Applications of CRISPR/Cas9 in medical research CRISPR/Cas9 has been extensively utilized in medical research for disease modeling and therapeutic interventions. For instance, it has been employed to edit patient-derived xenografts (PDXs) to study genetic dependencies and mechanisms of drug resistance in cancer (Hulton et al., 2020). This approach allows for rapid in vivo functional genomics without the need for in vitro culture, significantly enhancing the utility of PDXs as models of human cancer.
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 159 The study by Hulton et al. (2020) explored a method for rapid in vivo functional genomics research by directly editing the genomes of patient-derived xenografts (PDXs) using CRISPR-Cas9. They developed a technique that employs a tightly regulated, inducible Cas9 vector, which allows for the selection of transduced cells without the need for in vitro culture. This approach enabled the analysis of genetic dependencies in PDXs through targeted gene disruption and the investigation of acquired drug resistance mechanisms via homology-directed repair (HDR). The results demonstrated that this versatile system could be broadly applied to other xenograft models, significantly enhancing the utility of PDXs as genetically programmable models for human cancer. This method greatly improves the value of PDX models in cancer biology research and the development of new therapeutic strategies. Figure 3 Design of the pSpCTRE vector and its validation in vitro (Adapted from Hulton et al., 2020) Image caption: a shows the structure of the pSpCTRE vector, which includes a TRE3GS promoter for doxycycline (dox)-induced expression of Cas9 and a CD4T cell surface marker to facilitate rapid selection of transduced cells; b demonstrates the results of CD4T selection and enrichment of A549 cells using flow cytometry; c and d validate the efficient expression of Cas9 and the effective editing of the GFP gene in the presence of dox, as well as the absence of Cas9 expression and GFP editing without dox, through a GFP editing experiment; e displays the dose-dependent effect of different dox concentrations on Cas9 expression and GFP editing efficiency; f shows that A549GFP-SpCTRE cells passaged multiple times do not exhibit leaky expression of Cas9 in the absence of dox, while effective GFP editing occurs upon dox addition (Adapted from Hulton et al., 2020) Hulton et al. (2020) discovered that by integrating the tightly regulated TRE3GS promoter and the CD4T cell surface marker, the pSpCTRE vector can rapidly select and enrich transduced cells in A549 cells (Figure 3a and 3b). Under tetracycline (dox) induction, the pSpCTRE vector efficiently expresses Cas9 and successfully edits the GFP gene, whereas in the absence of dox, Cas9 is not expressed and GFP remains unedited (Figure 3c and 3d). This indicates that the pSpCTRE system has low background activity without the inducer. The dose-dependent response of Cas9 expression and GFP editing to dox concentration demonstrates the vector system’s sensitive response to dox (Figure 3e). Even after multiple passages, A549GFP-SpCTRE cells remain stable under no-dox conditions and can perform effective editing immediately upon dox addition (Figure 3f). These results suggest that the pSpCTRE vector exhibits high controllability and efficacy in vitro, laying the foundation for further in vivo functional genomics research. Furthermore, CRISPR/Cas9 has been used to investigate gene editing in human tripronuclear zygotes, revealing insights into DNA repair mechanisms and the challenges of off-target effects and mosaicism in early embryos
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 160 (Liang et al., 2015). These studies underscore the potential and the current limitations of CRISPR/Cas9 technology in clinical applications. CRISPR/Cas9 technology has transformed the landscape of genome editing with its simplicity, efficiency, and versatility. Its applications in medical research are vast, ranging from cancer studies to early human embryo editing, although challenges such as off-target effects and repair mechanism fidelity remain to be addressed. 4 Applications of CRISPR/Cas9 in Xenotransplantation 4.1 Genetic modification of donor animals to reduce immunogenicity CRISPR/Cas9 technology has been instrumental in reducing the immunogenicity of donor animals, particularly pigs, which are commonly used in xenotransplantation. By targeting genes responsible for xenoantigen biosynthesis, such as GGTA1, CMAH, and B4GALNT2, researchers have successfully created genetically modified pigs with reduced levels of these antigens. This genetic modification helps to mitigate hyperacute rejection, a major barrier in xenotransplantation (Tanihara et al., 2021). The ability to perform precise gene editing with CRISPR/Cas9 has significantly improved the feasibility of using animal organs for human transplantation by reducing the immune response triggered by these foreign tissues (Ryczek et al., 2021). 4.2 Prevention of zoonotic disease transmission through genome editing One of the critical concerns in xenotransplantation is the potential transmission of zoonotic diseases from donor animals to human recipients. CRISPR/Cas9 technology offers a solution by enabling the precise editing of animal genomes to eliminate endogenous retroviruses and other pathogens that could pose a risk. This genome editing approach not only enhances the safety of xenotransplantation but also ensures that the transplanted organs are free from infectious agents that could compromise the health of the recipient (Ryczek et al., 2021; Wei et al., 2020). By targeting and inactivating specific viral sequences within the donor genome, CRISPR/Cas9 helps to create safer and more reliable sources of transplantable organs. 4.3 Enhancing organ compatibility and function Beyond reducing immunogenicity and preventing disease transmission, CRISPR/Cas9 technology is also being used to enhance the compatibility and function of transplanted organs. By editing genes that influence organ size, function, and metabolic compatibility, researchers can create donor organs that are better suited for human physiology. For example, modifications to genes involved in metabolic pathways can help ensure that the transplanted organs function more efficiently within the human body (Ryczek et al., 2021; Tanihara et al., 2021). This level of customization and optimization is crucial for improving the success rates and long-term outcomes of xenotransplantation procedures. 4.4 Case studies and successful experiments using CRISPR/Cas9 in xenotransplantation Several successful experiments have demonstrated the potential of CRISPR/Cas9 in advancing xenotransplantation. One notable study involved the one-step generation of multiple gene-edited pigs by introducing CRISPR/Cas9 into zygotes via electroporation. This approach targeted the GGTA1, CMAH, and B4GALNT2 genes simultaneously, resulting in pigs with significantly reduced xenoantigen levels (Tanihara et al., 2021). Tanihara et al. (2021) conducted a study that utilized electroporation technology to introduce the CRISPR/Cas9 system into fertilized eggs, as a method to rapidly generate multi-gene-edited pigs with reduced xenoantigen biosynthesis. The study targeted the GGTA1, CMAH, and B4GALNT2 genes to create pigs devoid of xenoantigens. Initially, the study optimized the gRNA combinations for GGTA1 and CMAH, and then introduced these gRNAs along with Cas9 into in vitro fertilized oocytes via electroporation. The electroporated embryos were subsequently transplanted into recipient sows. The results demonstrated that this method successfully generated pigs with dual gene edits for GGTA1/CMAH and triple gene edits for GGTA1/CMAH/B4GALNT2. Immunohistochemical analysis indicated a significant reduction in the expression levels of xenoantigens in these multi-gene-edited pigs, although some pigs exhibited gene mosaicism. The study suggests that, despite the issue of mosaicism that still needs to be addressed, the electroporation technique holds great potential for rapidly
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 161 generating multi-gene-modified pigs, which can significantly enhance the efficiency and success rate of pig-to-human xenotransplantation research. Figure 4 Immunohistochemical assessment of wild-type and GGTA1/CMAH double-edited piglets (Adopted from Tanihara et al., 2021) Image caption: Ear biopsies derived from wild-type (WT) and GGTA1/CMAH double-edited piglets (#2 and #3) were immunohistochemically stained for Galα(1,3)Gal (green) (a) and Neu5Gc (green) (b). These tissues were counterstained with 4’, 6-diamidino-2-phenylindole (DAPI) (blue); The scale bar in each panel represents 50 μm (Adopted from Tanihara et al., 2021) Figure 4 presents the analysis results of ear biopsy samples from wild-type and GGTA1/CMAH double-gene-edited pigs using the immunohistochemistry method. The samples were stained with Alexa 488-labeled heterologous lectin B4 and anti-Neu5Gc antibodies to detect the expression of Galα(1,3)Gal and Neu5Gc epitopes. The results show that in the GGTA1/CMAH double-gene-edited pigs, the absence of Galα(1,3)Gal is clearly visible in Figure 3a, while the expression of Neu5Gc epitopes is similar to that of the wild-type, as shown in Figure 3b. This indicates that despite gene editing, some xenoantigens are still expressed, demonstrating the variability in the effects of gene editing and the presence of mosaicism. Another study highlighted the use of CRISPR/Cas9 to edit patient-derived xenografts, showcasing the technology's ability to perform rapid in vivo functional genomics and analyze genetic dependencies and drug resistance mechanisms (Hulton et al., 2020). These case studies underscore the transformative impact of CRISPR/Cas9 on the field of xenotransplantation, paving the way for more effective and safer transplantation practices. By leveraging the precision and efficiency of CRISPR/Cas9, researchers are making significant strides in overcoming the challenges associated with xenotransplantation, ultimately bringing us closer to the goal of using animal organs to address the shortage of human donor organs. 5 Ethical and Regulatory Considerations 5.1 Ethical concerns surrounding genetic modification and xenotransplantation The application of CRISPR/Cas9 technology in xenotransplantation raises significant ethical concerns. One primary issue is the moral implications of genetic modification in animals, particularly pigs, which are commonly used as organ donors. The genetic alteration of these animals to reduce immunogenicity and improve compatibility with human recipients involves complex ethical considerations regarding animal welfare and the extent to which humans should interfere with natural genetic processes (Ryczek et al., 2021; Kararoudi et al., 2018). Additionally, the potential for unintended off-target effects and genetic mosaicism in edited animals poses further ethical dilemmas, as these could lead to unforeseen health issues in the animals or compromise the safety of the transplanted organs (Tanihara et al., 2021; Zhang, 2020). The broader ethical debate also encompasses the potential long-term impacts on biodiversity and the natural ecosystem, as well as the moral status of genetically modified organisms (Zhang et al., 2020). 5.2 Regulatory frameworks governing the use of CRISPR/Cas9 in xenotransplantation The regulatory landscape for the use of CRISPR/Cas9 in xenotransplantation is still evolving. Different countries have established varying frameworks to address the safety, efficacy, and ethical implications of this technology.
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 162 For instance, regulatory bodies such as the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have begun to develop guidelines for the clinical application of gene-editing technologies, including CRISPR/Cas9 (Zhang et al., 2020). These guidelines typically require rigorous preclinical testing to assess the potential risks and benefits, as well as long-term monitoring of recipients to detect any adverse effects. Additionally, there are specific regulations concerning the welfare of genetically modified animals, which mandate humane treatment and ethical considerations in their use for research and clinical purposes (Ryczek et al., 2021; Kararoudi et al., 2018). The global nature of xenotransplantation research necessitates international cooperation and harmonization of regulatory standards to ensure the safe and ethical application of CRISPR/Cas9 technology (Zhang et al., 2020). 5.3 Public perception and acceptance Public perception and acceptance of CRISPR/Cas9 technology in xenotransplantation are crucial for its successful implementation. Public concerns often revolve around the ethical implications of genetic modification, the potential risks associated with the technology, and the transparency of the research and regulatory processes (Memi et al., 2018; Zhang et al., 2020). Effective communication and public engagement are essential to address these concerns and build trust in the technology. This includes providing clear and accessible information about the benefits and risks of CRISPR/Cas9-mediated xenotransplantation, as well as involving the public in discussions about the ethical and social implications (Memi et al., 2018). Additionally, public acceptance may be influenced by cultural and religious beliefs, which can vary significantly across different communities and regions. Therefore, it is important for researchers and policymakers to consider these diverse perspectives and engage in meaningful dialogue with stakeholders to foster a supportive environment for the advancement of xenotransplantation research (Zhang et al., 2020). While CRISPR/Cas9 technology holds great promise for advancing xenotransplantation, it also presents significant ethical and regulatory challenges. Addressing these concerns through robust regulatory frameworks, ethical considerations, and public engagement is essential for the responsible development and application of this transformative technology. 6 Technical Challenges and Limitations 6.1 Off-target effects and genetic stability One of the primary technical challenges associated with the use of CRISPR/Cas9 technology in xenotransplantation is the occurrence of off-target effects. These unintended modifications can lead to genetic instability and potentially harmful consequences. Off-target effects occur when the CRISPR/Cas9 system introduces mutations at sites other than the intended target, which can result in unexpected and adverse alterations to the genome (Guo et al., 2023; Chen et al., 2019; Kimberland et al., 2018). Various strategies have been developed to minimize these off-target effects, including the use of high-fidelity Cas9 variants and improved guide RNA designs (Guo et al., 2023; Kimberland et al., 2018). However, despite these advancements, the risk of off-target mutations remains a significant concern that must be addressed to ensure the safety and efficacy of CRISPR/Cas9-mediated xenotransplantation (Ricci et al., 2019). 6.2 Long-term viability and functionality of edited organs Another critical challenge is ensuring the long-term viability and functionality of organs edited using CRISPR/Cas9 technology. While initial studies have demonstrated the potential of CRISPR/Cas9 to generate genetically modified pigs with reduced xenoantigen expression, the long-term effects of these genetic modifications on organ function and overall health remain unclear (Tanihara et al., 2021; Ryczek et al., 2021). Genetic mosaicism, where not all cells in the organism carry the intended genetic modifications, is a particular issue that can compromise the consistency and reliability of the edited organs (Tanihara et al., 2021). Additionally, the potential for immune responses against the edited tissues and the stability of the genetic modifications over time are areas that require further investigation to ensure the success of xenotransplantation (Ryczek et al., 2021).
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 163 6.3 Challenges in scaling up from laboratory to clinical applications Scaling up the use of CRISPR/Cas9 from laboratory research to clinical applications involves several logistical and technical challenges. Efficient delivery of CRISPR/Cas9 components to target cells or tissues is one major obstacle. Viral vectors have been widely used but pose risks such as immune responses and insertional mutagenesis. Non-viral delivery systems, including lipid nanoparticles and electroporation, are being explored to enhance delivery efficiency and safety (Li et al., 2018). Another challenge is the need for robust and reproducible protocols for generating and validating genetically modified donor animals. Achieving high efficiency and consistency in gene editing across multiple animals is critical for producing reliable and functional xenotransplantation organs (Zhang et al., 2021). Additionally, the regulatory landscape for CRISPR/Cas9-mediated xenotransplantation must be navigated carefully, ensuring compliance with safety and ethical standards while addressing public concerns and gaining acceptance for clinical use (Memi et al., 2018). 7 Future Directions and Opportunities 7.1 Emerging technologies and advancements in CRISPR/Cas9 The CRISPR/Cas9 technology has significantly advanced genetic engineering, enabling precise modifications in the genome of various organisms, including pigs, which are considered potential donors for xenotransplantation. Recent developments in CRISPR/Cas9 have focused on enhancing the specificity and efficiency of gene editing. Innovations such as base editing and prime editing have emerged, allowing for more precise genetic modifications without causing double-strand breaks (Ryczek et al., 2021). These advancements hold promise for reducing off-target effects and improving the safety of genetically modified pigs for xenotransplantation. 7.2 Potential breakthroughs in immunosuppression and tolerance induction One of the major challenges in xenotransplantation is the immunological barrier between species. CRISPR/Cas9 technology has been instrumental in addressing this issue by enabling the knockout of genes responsible for hyperacute rejection, such as the alpha-1,3-galactosyltransferase (GGTA1) gene in pigs (Ryczek et al., 2021). Future research may focus on further modifying the pig genome to induce tolerance and reduce the need for immunosuppressive drugs. For instance, the integration of CRISPR/Cas9 with other gene-editing tools could lead to the development of pigs with human-compatible immune markers, potentially minimizing immune rejection (Ryczek et al., 2021). 7.3 Integration of CRISPR/Cas9 with other biotechnological approaches The combination of CRISPR/Cas9 with other biotechnological approaches, such as stem cell therapy and regenerative medicine, offers exciting opportunities for xenotransplantation. For example, CRISPR/Cas9 can be used to create genetically modified pigs whose organs are more compatible with human physiology. These organs can then be further enhanced using stem cell therapy to improve their functionality and longevity (Ryczek et al., 2021). Additionally, regenerative medicine techniques could be employed to repair and regenerate damaged tissues in xenotransplanted organs, thereby extending their viability and reducing the risk of complications. 7.4 Long-term vision for xenotransplantation and its role in addressing organ shortages The long-term vision for xenotransplantation involves creating a sustainable and reliable source of organs to address the global shortage of human organs for transplantation. CRISPR/Cas9 technology plays a crucial role in this vision by enabling the production of genetically modified pigs that can serve as organ donors with reduced risk of rejection and other complications (Ryczek et al., 2021). As the technology continues to evolve, it is anticipated that xenotransplantation will become a viable and routine option for patients in need of organ transplants, ultimately saving countless lives and alleviating the burden on the organ donation system. In conclusion, the future of CRISPR/Cas9 technology in xenotransplantation is promising, with ongoing advancements and integration with other biotechnological approaches paving the way for significant breakthroughs. Continued research and development in this field will be essential to overcome existing challenges and realize the full potential of xenotransplantation in addressing organ shortages.
International Journal of Molecular Medical Science, 2024, Vol.14, No.3, 155-166 http://medscipublisher.com/index.php/ijmms 164 8 Concluding Remarks CRISPR/Cas9 technology has significantly advanced the field of xenotransplantation, particularly in the context of pig-to-human organ transplants. The ability to precisely edit the genome has allowed researchers to create genetically modified pigs that are less likely to trigger acute immunological reactions in human recipients. This has been achieved by targeting specific genes responsible for immune rejection and other incompatibilities. The technology has also been instrumental in studying the functions and mechanisms of various biological processes, thereby providing a deeper understanding of the genetic factors involved in xenotransplantation. Despite the remarkable progress, several challenges remain. One of the primary issues is the efficiency and specificity of CRISPR/Cas9-mediated gene editing. Off-target effects and incomplete gene edits can lead to unintended consequences, which are particularly concerning in a clinical setting. Additionally, the delivery methods for CRISPR/Cas9 components need to be optimized to ensure that the gene edits are made precisely and efficiently in the target tissues. Another significant challenge is the ethical and regulatory landscape surrounding the use of genetically modified organisms for medical purposes, which requires careful consideration and ongoing dialogue. The advancements in CRISPR/Cas9 technology hold immense promise for the future of xenotransplantation and organ transplantation. By overcoming the immunological barriers and improving the compatibility of pig organs for human use, CRISPR/Cas9 could potentially address the severe shortage of human organs available for transplantation. Furthermore, the technology's ability to make precise genetic modifications opens up new avenues for personalized medicine, where organs could be tailored to the genetic profile of individual patients, thereby reducing the risk of rejection and improving long-term outcomes. As research continues to address the existing challenges, the integration of CRISPR/Cas9 in clinical practice could revolutionize the field of organ transplantation, offering new hope to patients worldwide. Conflict of Interest Disclosure The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest. References Blitz I., and Nakayama T., 2021, CRISPR-Cas9 Mutagenesis in xenopus tropicalis for phenotypic analyses in the F0 generation and beyond, Cold Spring Harbor protocols, 8: 106971. https://doi.org/10.1101/pdb.prot106971 PMid:34244352 Chen M., Mao A., Xu M., Weng Q., Mao J., and Ji J., 2019, CRISPR-Cas9 for cancer therapy: Opportunities and challenges, Cancer letters, 447: 48-55. https://doi.org/10.1016/j.canlet.2019.01.017 PMid:30684591 Cowan P., Hawthorne W., and Nottle M., 2019, Xenogeneic transplantation and tolerance in the era of CRISPR-Cas9. Current Opinion in Organ Transplantation, 24: 5-11. https://doi.org/10.1097/MOT.0000000000000589 PMid:30480643 Guo C., Ma X., Gao F., and Guo Y., 2023, Off-target effects in CRISPR/Cas9 gene editing, Frontiers in Bioengineering and Biotechnology, 11: 1143157. https://doi.org/10.3389/fbioe.2023.1143157 PMid:36970624 PMCid:PMC10034092 Hsu P., Lander E., and Zhang F., 2014, Development and applications of CRISPR-Cas9 for genome engineering, Cell, 157: 1262-1278. https://doi.org/10.1016/j.cell.2014.05.010 PMid:24906146 PMCid:PMC4343198 Hulton C., Costa E., Shah N., Quintanal-Villalonga A., Heller G., Stanchina E., Rudin C., and Poirier J., 2020, Direct genome editing of patient-derived xenografts using CRISPR-Cas9 enables rapid in vivo functional genomics, Nature cancer, 1: 359-369. https://doi.org/10.1038/s43018-020-0040-8 PMid:33345196 PMCid:PMC7745982 Jiang F., and Doudna J.A., 2017, CRISPR–Cas9 structures and mechanisms, Annual review of biophysics, 46: 505-529. https://doi.org/10.1146/annurev-biophys-062215-010822 PMid:28375731
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