Sequential disruption of ALV host receptor genes reveals no sharing of receptors between ALV subgroups A, B, and J

Salvage pathway of vitamin B12 absorption in chickens with mutant tumor virus a receptor

The tumor virus A receptor (TVA), a member of the low-density lipoprotein receptor (LDLR) family, serves as an entry receptor for Avian Leukosis Virus (ALV) subgroups A and K, as well as a receptor for vitamin B12 bound to transcobalamin. Naturally occurring genetic variants in the TVA gene determine susceptibility or resistance to ALV-A and -K, but the effects of these mutated TVA on vitamin B12 uptake have not been investigated systemically. We found four TVA variants comprising the wild type (TVAWT), a single nucleotide polymorphism variant (TVASNP), and two partial deletions in the splicing branch point region (TVAR). This study investigates the relationship between the various genotypes of TVA alleles and uptake of vitamin B12 in chickens. A protein interaction model suggested that mutant TVAs (i.e., TVASNP, TVAR) may have reduced ability to take up vitamin B12 due to a disrupted LDL-A domain, a pivotal region involved in vitamin B12 uptake; however, we found no significant difference in absorption of vitamin B12 in TVAWT and TVASNP chickens, or levels of its metabolite in serum. Notably, TVAR chickens had significantly higher levels of vitamin B12 than TVAWT chickens, a finding contrary to the predicted lower uptake. Expression of vitamin B12 carrier related genes (i.e., AMN, GIF, and TCN2) in chickens showed a stepwise increase: TVAWT > TVASNP > TVAR. These results suggest a mechanism by which mutant TVA chickens with a disrupted TVA protein acquire natural resistance to ALV-A -K, with no impairment of vitamin B12 metabolism.

Strategies for the Generation of Gene Modified Avian Models: Advancement in Avian Germline Transmission, Genome Editing, and Applications

Avian models are valuable for studies of development and reproduction and have important implications for food production. Rapid advances in genome-editing technologies have enabled the establishment of avian species as unique agricultural, industrial, disease-resistant, and pharmaceutical models. The direct introduction of genome-editing tools, such as the clustered regularly interspaced short palindromic repeats (CRISPR) system, into early embryos has been achieved in various animal taxa. However, in birds, the introduction of the CRISPR system into primordial germ cells (PGCs), a germline-competent stem cell, is considered a much more reliable approach for the development of genome-edited models. After genome editing, PGCs are transplanted into the embryo to establish germline chimera, which are crossed to produce genome-edited birds. In addition, various methods, including delivery by liposomal and viral vectors, have been employed for gene editing in vivo. Genome-edited birds have wide applications in bio-pharmaceutical production and as models for disease resistance and biological research. In conclusion, the application of the CRISPR system to avian PGCs is an efficient approach for the production of genome-edited birds and transgenic avian models.

Targeted Ablation of Exon 2 of the Avian Leukosis Virus-A (ALV-A) Receptor Gene in a Chicken Fibroblast Cell Line by CRISPR Abrogates ALV-A Infection

The U.S. Department of Agriculture Avian Disease and Oncology Laboratory currently relies on live birds of specific genetic backgrounds for producing chicken-embryo fibroblasts that are used for the diagnosis and subtyping of field isolates associated with avian leukosis virus (ALV) outbreaks. As an alternative to maintaining live animals for this purpose, we are currently developing cell lines capable of achieving the same result by ablation of the entry receptors utilized by ALV strains. We used CRISPR-Cas9 on the cell fibroblast-derived cell line DF-1 to disrupt the tva gene, which encodes the receptor required for binding and entry of ALV-A into cells. We ultimately identified seven DF-1 clones that had biallelic and homozygous indels at the Cas9 target site, exon 2 of tva. When tested in vitro for their ability to host ALV-A, the five clones that had frameshift mutations that disrupted the Tva protein were unable to support ALV-A replication. This result clearly demonstrates that modified cell lines can be used as part of a battery of tests to determine ALV subtype for isolate characterization, thus eliminating the need for live birds.

Advances on genetic and genomic studies of ALV resistance

Avian leukosis (AL) is a general term for a variety of neoplastic diseases in avian caused by avian leukosis virus (ALV). No vaccine or drug is currently available for the disease. Therefore, the disease can result in severe economic losses in poultry flocks. Increasing the resistance of poultry to ALV may be one effective strategy. In this review, we provide an overview of the roles of genes associated with ALV infection in the poultry genome, including endogenous retroviruses, virus receptors, interferon-stimulated genes, and other immune-related genes. Furthermore, some methods and techniques that can improve ALV resistance in poultry are discussed. The objectives are willing to provide some valuable references for disease resistance breeding in poultry.

Transcriptome-Wide Dynamics of m6A Methylation in Tumor Livers Induced by ALV-J Infection in Chickens

Avian Leukosis Virus Subgroup J (ALV-J) is a tumorigenic virus with high morbidity and rapid transmission. N6-methyladenosine (m⁶A) is a common epigenetic modification that may be closely related to the pathogenicity of ALV-J. Currently, there are no reports on whether m⁶A modification is related to ALV-J induced tumor formation. In this study, we used methylated RNA immunoprecipitation sequencing (MeRIP-seq) and RNA sequencing (RNA-seq) to examine the differences in m⁶A methylation and gene expression in normal livers and ALV-J-induced tumor livers systematically, with functional enrichment and co-expression analysis. The results identified 6,541 m⁶A methylated peaks, mainly enriched in CDS, and more than 83% of the transcripts contained 1-2 m⁶A peaks. For RNA-seq, 1,896 and 1,757 differentially expressed mRNAs and lncRNAs were identified, respectively. Gene enrichment analysis indicated that they may be involved in biological processes and pathways such as immunology-related and apoptosis. Moreover, we identified 17 lncRNAs, commonly existing in differently expressed methylome and transcriptome. Through co-expression analysis, 126 differentially expressed lncRNAs, and 18 potentially m⁶A-related methyltransferases were finally identified and connected, suggesting that m⁶A modifications might affect gene expression of lncRNAs and play a role in ALV-J induced tumor formation. This study provides the first comprehensive description of the m⁶A expression profile in tumor livers induced by ALV-J infection in chickens, which provides a basis for studying the role of m⁶A modification in ALV-J induced tumorigenesis. This study provides clues for studying the epigenetic etiology and pathogenesis of ALV-J.

Knock-Out of Retrovirus Receptor Gene Tva in the Chicken Confers Resistance to Avian Leukosis Virus Subgroups A and K and Affects Cobalamin (Vitamin B12)-Dependent Level of Methylmalonic Acid

The chicken Tva cell surface protein, a member of the low-density lipoprotein receptor family, has been identified as an entry receptor for avian leukosis virus of classic subgroup A and newly emerging subgroup K. Because both viruses represent an important concern for the poultry industry, we introduced a frame-shifting deletion into the chicken tva locus with the aim of knocking-out Tva expression and creating a virus-resistant chicken line. The tva knock-out was prepared by CRISPR/Cas9 gene editing in chicken primordial germ cells and orthotopic transplantation of edited cells into the testes of sterilized recipient roosters. The resulting tva −/− chickens tested fully resistant to avian leukosis virus subgroups A and K, both in in vitro and in vivo assays, in contrast to their susceptible tva +/+ and tva +/− siblings. We also found a specific disorder of the cobalamin/vitamin B₁₂ metabolism in the tva knock-out chickens, which is in accordance with the recently recognized physiological function of Tva as a receptor for cobalamin in complex with transcobalamin transporter. Last but not least, we bring a new example of the de novo resistance created by CRISPR/Cas9 editing of pathogen dependence genes in farm animals and, furthermore, a new example of gene editing in chicken.

Recent Advances in the Application of CRISPR/Cas9 Gene Editing System in Poultry Species

CRISPR/Cas9 system genome editing is revolutionizing genetics research in a wide spectrum of animal models in the genetic era. Among these animals, is the poultry species. CRISPR technology is the newest and most advanced gene-editing tool that allows researchers to modify and alter gene functions for transcriptional regulation, gene targeting, epigenetic modification, gene therapy, and drug delivery in the animal genome. The applicability of the CRISPR/Cas9 system in gene editing and modification of genomes in the avian species is still emerging. Up to date, substantial progress in using CRISPR/Cas9 technology has been made in only two poultry species (chicken and quail), with chicken taking the lead. There have been major recent advances in the modification of the avian genome through their germ cell lineages. In the poultry industry, breeders and producers can utilize CRISPR-mediated approaches to enhance the many required genetic variations towards the poultry population that are absent in a given poultry flock. Thus, CRISPR allows the benefit of accessing genetic characteristics that cannot otherwise be used for poultry production. Therefore CRISPR/Cas9 becomes a very powerful and robust tool for editing genes that allow for the introduction or regulation of genetic information in poultry genomes. However, the CRISPR/Cas9 technology has several limitations that need to be addressed to enhance its use in the poultry industry. This review evaluates and provides a summary of recent advances in applying CRISPR/Cas9 gene editing technology in poultry research and explores its potential use in advancing poultry breeding and production with a major focus on chicken and quail. This could aid future advancements in the use of CRISPR technology to improve poultry production.

Precise Genome Editing in Poultry and Its Application to Industries

Poultry such as chickens are valuable model animals not only in the food industry, but also in developmental biology and biomedicine. Recently, precise genome-editing technologies mediated by the CRISPR/Cas9 system have developed rapidly, enabling the production of genome-edited poultry models with novel traits that are applicable to basic sciences, agriculture, and biomedical industry. In particular, these techniques have been combined with cultured primordial germ cells (PGCs) and viral vector systems to generate a valuable genome-edited avian model for a variety of purposes. Here, we summarize recent progress in CRISPR/Cas9-based genome-editing technology and its applications to avian species. In addition, we describe further applications of genome-edited poultry in various industries.

Highly elevated base excision repair pathway in primordial germ cells causes low base editing activity in chickens

Base editing technology enables the generation of precisely genome-modified animal models. In this study, we applied base editing to chicken, an important livestock animal in the fields of agriculture, nutrition, and research through primordial germ cell (PGC)-mediated germline transmission. Using this approach, we successfully produced two genome-modified chicken lines harboring mutations in the genes encoding ovotransferrin (TF) and myostatin (MSTN); however, only 55.5% and 35.7% of genome-modified chickens had the desired base substitutions in TF and MSTN, respectively. To explain the low base-editing activity, we performed molecular analysis to compare DNA repair pathways between PGCs and the chicken fibroblast cell line DF-1. The results revealed that base excision repair (BER)-related genes were significantly elevated in PGCs relative to DF-1 cells. Subsequent functional studies confirmed that the editing activity could be regulated by modulating the expression of uracil N-glycosylase (UNG), an upstream gene of the BER pathway. Collectively, our findings indicate that the distinct DNA repair property of chicken PGCs causes low editing activity during genome modification, however, modulation of BER functions could promote the production of genome-modified organisms with the desired genotypes.