CRISPR-Cas9 effectors facilitate generation of single-sex litters and sex-specific phenotypes

Systematic identification of Y-chromosome gene functions in mouse spermatogenesis

The mammalian Y chromosome is essential for male fertility, but which Y genes regulate spermatogenesis is unresolved. We addressed this by generating 13 Y-deletant mouse models. In Eif2s3y, Uty, and Zfy2 deletants, spermatogenesis was impaired. We found that Uty regulates spermatogonial proliferation, revealed a role for Zfy2 in promoting meiotic sex chromosome pairing, and uncovered unexpected effects of Y genes on the somatic testis transcriptome. In the remaining single Y-gene deletants, spermatogenesis appeared unperturbed, but testis transcription was still altered. Multigene deletions, including a human-infertility AZFa model, exhibited phenotypes absent in single Y deletants. Thus, Y genes may regulate spermatogenesis even if they show no phenotypes when deleted individually. This study advances our knowledge of Y evolution and infertility and provides a resource to dissect Y-gene functions in other tissues.

Identification of Site in the UTY Gene as Safe Harbor Locus on the Y Chromosome of Pig

Genomic Safe Harbors (GSH) are loci used for the insertion of exogenous genetic elements, enabling exogenous gene expressing predictably without alterations of the host genome. These sites are becoming increasingly important as the gene editing technologies advance rapidly. Currently, only a few GSHs have been identified in the pig genome. In this study, a novel strategy was demonstrated for the efficient insertion of exogenous genetic material into the third exon of the UTY gene on the Y chromosome using CRISPR/Cas9-mediated homology arm-mediated end joining. The safety of the locus was verified according to the proper expression of the inserted EGFP gene without altering the expression of UTY. This approach enables the integration and expression of the exogenous gene at this locus, indicating that the UTY locus serves as a genomic safe harbor site for gene editing in the pig genome. Located on the Y chromosome, this site can be utilized for sex-biased pig breeding and developing biomedical models.

Gender Control of Mouse Embryos by Activation of TLR7/8 on X Sperm via Ligands dsRNA-40 and dsRNA-DR

Gender control technologies are promising for enhancing the production efficiency of the farm animal industry, and preventing sex-linked hereditary diseases in humans. It has been shown that the X sperm of mammalian animals specifically expresses X-chromosome-derived toll-like receptor 7/8 (TLR7/8), and the activation of TLR7/8 on the X sperm by their agonist, R848, can separate X and Y sperm via the specific inhibition of X sperm motility. The use of R848-preselected sperm for fertilization resulted in sex-ratio-skewed embryos or offspring. In this study, we aimed to investigate whether two other TLR7/8 ligands, double-stranded RNA-40 (dsRNA-40) and double-stranded RNA-DR (dsRNA-DR), are also effective in the separation of mouse X and Y sperm and the subsequent generation of gender-ratio-skewed in vitro fertilization (IVF) embryos. Our results indicated that cholesterol modification significantly enhances the transfection of dsRNA-40 and dsRNA-DR into sperm cells. dsRNA-40 and dsRNA-DR incubation with mouse sperm could separate X and Y sperm by the specific suppression of X sperm motility by decreasing its ATP level and mitochondrial activity. The use of a dsRNA-40- or dsRNA-DR-preselected upper layer of sperm, which predominantly contains high-motility Y sperm, for IVF caused a male-biased sex ratio shift in resulting embryos (with 65.90-74.93% of embryos being male). This study develops a simple new method for the efficient separation of mammalian X and Y sperm, enabling the selective production of male or female progenies.

Unmasking the Adverse Impacts of Sex Bias on Science and Research Animal Welfare

Sex bias in biomedical and natural science research has been prevalent for decades. In many cases, the female estrous cycle was thought to be too complex an issue to model for, and it was thought to be simpler to only use males in studies. At times, particularly when studying efficacy and safety of new therapeutics, this sex bias has resulted in over- and under-medication with associated deleterious side effects in women. Many sex differences have been recognized that are unrelated to hormonal variation occurring during the estrous cycle. Sex bias also creates animal welfare challenges related to animal over-production and wastage, insufficient consideration of welfare (and scientific) impact related to differential housing of male vs female animals within research facilities, and a lack of understanding regarding differential requirements for pain recognition and alleviation in male versus female animals. Although many funding and government agencies require both sexes to be studied in biomedical research, many disparities remain in practice. This requires further enforcement of expectations by the Institutional Animal Care and Use Committee when reviewing protocols, research groups when writing grants, planning studies, and conducting research, and scientific journals and reviewers to ensure that sex bias policies are enforced.

The History and Prospects of Rabbit Sperm Sexing

Sperm sex selection is a longstanding challenge in the field of animal reproduction. The cuniculture industry, in particular producers of males or females for breeding purposes, would greatly benefit from the pre-selection of the offspring's sex. This review article overviews the current and future developments in rabbit sperm sexing technologies, as well as the implications of implementing these methodologies in cuniculture. The first attempts of sperm sexing were performed in rabbits; however, a both efficient and cost-effective methodology was not yet developed for this species. Those included sperm sexing according to differences in sperm density, surface electric charge, pH susceptibility, antisera reaction, and flow cytometry. Separation by flow cytometry has proven to be efficient in rabbits, yielding fractions with approximately 81% and 86% purity for X- and Y-sperm, respectively. However, it is not cost-effective for cuniculture and decreases sperm quality. The advantages, limitations, and practical considerations of each method are presented, highlighting their applicability and efficiency. Furthermore, herein we explore the potential of immunological-based techniques that overcome some of the limitations of earlier methods, as well as recent advancements in sperm sexing technologies in other animal models, which could be applied to rabbits. Finally, the challenges associated with the development and widespread implementation of rabbit sperm sexing technologies are addressed. By understanding the advantages and limitations of existing and emerging methods, researchers can direct their efforts towards the most promising directions, ultimately contributing to a more efficient, profitable, and sustainable cuniculture.

Knockout of Rlim Results in a Sex Ratio Shift toward Males but Superovulation Cannot Compensate for the Reduced Litter Size

Technologies that can preselect offspring gender hold great promise for improving farm animal productivity and preventing human sex-related hereditary diseases. The maternal Rlim allele is required for imprinted X-chromosome inactivation, which is essential for the normal development of female mouse embryos. In this study, we inactivated the maternal Rlim allele in embryos by crossing a male transgenic mouse line carrying an X-linked CMV-Cre transgene with a female line carrying a loxP-flanked Rlim gene. Knockout of the maternal Rlim gene in embryos resulted in a male-biased sex ratio skew in the offspring. However, it also reduced litter size, and this effect was not compensated for by superovulation in the mother mice. In addition, we showed that siRNA-mediated knockdown of Rlim in mouse embryos leads to the birth of male-only progenies. This study provides a new promising method for male-biased sex selection, which may help to improve the productivity in livestock and prevent sex-associated hereditary diseases in humans.

The fate of surplus laboratory animals: Minimizing the production of surplus animals has the greatest potential to reduce the number of laboratory animals: Minimizing the production of surplus animals has greatest potential to reduce the number of laboratory animals

To meet regulatory requirements and the political pressure to minimize the number of animals used in research, it is critical to reduce the production of surplus animals.

Rapid, visual and highly sensitive sexing of bovine embryos by recombinase polymerase amplification with CFI staining

Early bovine embryo sexing both increases the number of offspring of the desired sex, and reduces the subsequent costs of processing unwanted offspring of the opposite sex. The need for cattle of different sexes varies from industry to industry, and a range of tools have been set up to meet this need, but most are energy- and time-consuming, hence it is important to establish a fast and convenient method for bovine embryo determination. Herein, we established a recombinase polymerase amplification (RPA) method combined with CFI dye (RPA-CFI) for sexing of bovine embryos. The assay is highly sensitive, specific, rapid and simple; it can be carried out in only 5 min at 37 °C in a metal bath, and results are visualised using a fluorescent colorimeter. Highly specific male-female common and male-specific primers were designed based on the 1399 bp repeating unit of bovine 1.715 satellite DNA and the male-specific S4 repeating sequence, respectively. The limit of detection (LOD) of RPA-CFI with male-female common primers was 1 pg/μL, and the LOD with male-specific primers was 2 pg/μL. RPA-CFI could determine the sex of bovine embryos from only two cells. This is the first report using RPA-CFI for sex determination of bovine embryos. The assay could be applied to other economically important animals to improve efficiency in livestock industries. Additionally, the assay could relieve pressure on food demand due to human population growth, and contribute to economic development of global stockbreeding.

Modern Approaches to Mouse Genome Editing Using the CRISPR-Cas Toolbox and Their Applications in Functional Genomics and Translational Research

Mice have been used in biological research for over a century, and their immense contribution to scientific breakthroughs can be seen across all research disciplines, with some of the main beneficiaries being the fields of medicine and life sciences. Genetically engineered mouse models (GEMMs), along with other model organisms, are fundamentally important research tools frequently utilised to enhance our understanding of pathophysiology and biological mechanisms behind disease. In the 1980s, it became possible to precisely edit the mouse genome to create gene knockout and knock-in mice, although with low efficacy. Recent advances utilising CRISPR-Cas technologies have considerably improved our ability to do this with ease and precision, while also allowing the generation of desired genetic variants from single nucleotide substitutions to large insertions/deletions. It is now quick and relatively easy to genetically edit somatic cells which were previously more recalcitrant to traditional approaches. Further refinements have created a 'CRISPR toolkit' that has expanded the use of CRISPR-Cas beyond gene knock-ins and knockouts. In this chapter, we review some of the latest applications of CRISPR-Cas technologies in GEMMs, including nuclease-dead Cas9 systems for activation or repression of gene expression, base editing and prime editing. We also discuss improvements in Cas9 specificity, targeting efficacy and delivery methods in mice. Throughout, we provide examples wherein CRISPR-Cas technologies have been applied to target clinically relevant genes in preclinical GEMMs, both to generate humanised models and for experimental gene therapy research.

Sex bias and omission exists in Batten disease research: Systematic review of the use of animal disease models

Batten disease, also known as the neuronal ceroid lipofuscinoses (NCL), is a group of inherited neurodegenerative disorders mainly affecting children. NCL are characterised by seizures, loss of vision, and progressive motor and cognitive decline, and are the most common form of childhood dementia. At least one type of Batten disease and three types of mouse disease models show sex differences in their severity and progression. Scientific research has a recognised prevalent omission of female animals when using model organisms for basic and preclinical research. Sex bias and omission in research using animal models of Batten disease may affect understanding and treatment development. We conducted a systematic review of research publications since the first identification of NCL genes in 1995, identifying those using animal models. We found that <10 % of these papers considered sex as a biological variable. There was consistent omission of female model organisms in studies. This varied over the period but is improving; one third of papers considered sex as a biological variable in the last decade, and there is a noticeable increase in the last 5 years. The wide-ranging reasons for this published sex bias are discussed, including misunderstanding regarding oestrogen, impact on sample size, and the underrepresentation of female scientists. Their implications for Batten disease and future research are considered. Recommendations going forward support requirements by funders for consideration of sex in all stages of experimental design and implementation, and a role for publishers, families and others with a particular interest in Batten disease.

Governance of Heritable Human Gene Editing World-Wide and Beyond

To date, the controversy surrounding the unknown risks and consequences of heritable genome editing has grown, with such work raising biosafety and ethical concerns for future generations. However, the current guideline of global governance is limited. In the context of the new framework for the governance of human genome editing developed by the World Health Organization (WHO) committee, this paper presents further analysis by highlighting predicaments of governance on germline engineering that merit the most attention: (1) building a scientific culture informed by a broader set of values and considerations in the internal scientific community at large, such as codes of ethics, and education, in addition to awareness-raising measures; and (2) reflecting on and institutionalizing policies in grassroots practice according to local conditions in external governance, such as the experimentalist governance, which is a multi-layered model of governance that establishes an open-ended framework from the top and offers stakeholders the freedom of discussion. The key to achieving these goals is more democratic deliberation between the public and the inclusive engagement of the global scientific community, which has been extensively used in the Biological and Toxin Weapons Convention (BTWC). On a global scale, we believe that practicing heritable human genome editing in accordance with the WHO and BTWC appears to be a good choice.

Sex differences in the intergenerational inheritance of metabolic traits

Strong evidence suggests that early-life exposures to suboptimal environmental factors, including those in utero, influence our long-term metabolic health. This has been termed developmental programming. Mounting evidence suggests that the growth and metabolism of male and female fetuses differ. Therefore, sexual dimorphism in response to pre-conception or early-life exposures could contribute to known sex differences in susceptibility to poor metabolic health in adulthood. However, until recently, many studies, especially those in animal models, focused on a single sex, or, often in the case of studies performed during intrauterine development, did not report the sex of the animal at all. In this review, we (a) summarize the evidence that male and females respond differently to a suboptimal pre-conceptional or in utero environment, (b) explore the potential biological mechanisms that underlie these differences and (c) review the consequences of these differences for long-term metabolic health, including that of subsequent generations.