Genome-Wide PERV Inactivation in Pigs Using CRISPR/Cas9

Expert consensus on clinical trials of human xenotransplantation in China

The history of xenotransplantation started in the 19th century. After a few decades of investigation, significant breakthroughs and preclinical milestones have been achieved worldwide. With the recent transplantation of genetically modified porcine kidneys and heart into humans, these ground-breaking achievements have attracted great attention worldwide, in the hope that xenotransplantation might alleviate or even solve the problem of organ shortage. On January 20, 2022, the China Organ Transplantation Development Foundation convened a symposium on "The History, Current Situation and Future of Human Xenotransplantation Clinical Trials," where ways to promote the ethical and sustainable development of xenotransplantation in China were discussed among the participating experts. A formal consensus was reached as the product of the symposium, outlining the expert opinions on scientific, regulatory, and ethical issues of clinical trials of xenotransplantation in China.

Porcine Endogenous Retroviruses: Quantification of the Viral Copy Number for the Four Miniature Pig Breeds in China

Domestic pigs has served not only as one of the most important economy livestock but also as ideal organ-source animals owing to similarity in anatomy, physiology, and organ size to humans. Howerer, the barrier of the cross-species transmission risk of porcine endogenous retrovirus (PERVs) blocked the pig-to-human xenotransplantation. PERVs are integrated into pigs’ genomes and cannot be eliminated by designated or specified pathogen-free breeding. PERVs are an important biosafety issue in xenotransplantation because they can be released from normal pig cells and infect human cells in vitro under certain conditions. Screening and analyzing the presence of PERVs in pig genome will provide essential parameters for pig breed sources. In China, four miniature pig breeds, such as Guizhou miniature pig (GZ), Bama miniature pig (BM), Wuzhishan miniature pig (WZS), and Juema miniature pig (JM), were the main experimental miniature pig breeds, which were widely used. In this study, PCR was performed to amplify env-A, env-B, and env-C for all individuals from the four breeds. The results revealed that PERV env-A and env-B were detected in all individuals and the lowest ratios of PERV env-C was 17.6% (3/17) in the GZ breed. Then, PERV pol and GAPDH were detected using the droplet digital PCR (ddPCR) method. As the reference of GAPDH copy number, the copy numbers of PERVs were at the median of 12, 16, 14, and 16 in the four miniature pig breeds (GZ, BM, WZS, and JM), respectively. Furthermore, the copy number of the PERV pol gene in many organs from the GZ breed was analyzed using ddPCR. The copy numbers of PERV pol gene were at the median of 7 copies, 8 copies, 8 copies, 11 copies, 5 copies, 6 copies, and 7 copies in heart, liver, spleen, lung, kidney, muscle, and skin, and the maximum number was 11 copies in the lung. The minimum number was 5 copies in the kidney as the reference of GAPDH. These data suggest that GZ breed has the lower PERVs copy number in the genome, and may be an ideal organ-source miniature pig breed for the study of the pig-to-human xenotransplantation.

Virological and Parasitological Characterization of Mini-LEWE Minipigs Using Improved Screening Methods and an Overview of Data on Various Minipig Breeds

Minipigs play an important role in biomedical research and have also been used as donor animals in xenotransplantation. To serve as a donor in xenotransplantation, the animals must be free of potential zoonotic viruses, bacteria and parasites. Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs and cannot be eliminated as most of the other pig viruses can. PERV-A and PERV-B infect human cells in cell culture and are integrated in all pigs, whereas PERV-C infects only pig cells and it is found in many, but not all pigs. Minipigs are known for a high prevalence of recombinant PERV-A/C viruses able to infect human cells (Denner and Schuurman, Viruses, 2021;13:1869). Here, Mini-LEWE minipigs are screened for the first time for pig viruses including PERV. Peripheral blood mononuclear cells (PBMCs) from 10 animals were screened using PCR-based methods (PCR, RT-PCR, and real-time PCR). In comparison with our previous screening assays, numerous improvements were introduced, e.g., the usage of gene blocks as a PCR standard and foreign RNA to control reverse transcription in RT-PCR. Using these improved detection methods, Mini-LEWE pigs were found to be negative for porcine cytomegalovirus (PCMV), porcine lymphotropic herpesviruses (PLHV-1, -2 and -3), porcine circoviruses (PCV1, 2, 3 and 4), porcine parvovirus (PPV) and hepatitis E virus (HEV). All animals carried PERV-A, PERV-B and PERV-C in their genome. PERV-A/C was not found. In contrast to all other minipig breeds (Göttingen minipigs, Aachen minipigs, Yucatan micropig, Massachusetts General Hospital miniature pigs), Mini-LEWE minipigs have less viruses and no PERV-A/C. Parasitological screening showed that none of the Mini-LEWE minipigs harbored ecto- and gastrointestinal parasites, but at least one animal tested positive for anti-Toxoplasma gondii antibodies.

Application of CRISPR-Cas9 Editing for Virus Engineering and the Development of Recombinant Viral Vaccines

CRISPR-Cas technology, discovered originally as a bacterial defense system, has been extensively repurposed as a powerful tool for genome editing for multiple applications in biology. In the field of virology, CRISPR-Cas9 technology has been widely applied on genetic recombination and engineering of genomes of various viruses to ask some fundamental questions about virus-host interactions. Its high efficiency, specificity, versatility, and low cost have also provided great inspiration and hope in the field of vaccinology to solve a series of bottleneck problems in the development of recombinant viral vaccines. This review highlights the applications of CRISPR editing in the technological advances compared to the traditional approaches used for the construction of recombinant viral vaccines and vectors, the main factors affecting their application, and the challenges that need to be overcome for further streamlining their effective usage in the prevention and control of diseases. Factors affecting efficiency, target specificity, and fidelity of CRISPR-Cas editing in the context of viral genome editing and development of recombinant vaccines are also discussed.