Production of Sry knockout mouse using TALEN via oocyte injection

Validation of male-specific markers confirmed an XY/XX-type sex determination system in Chinese dark sleeper (Odontobutis sinensis)

Next generation sequencing (NGS) method has been successfully used to identify sex-specific markers in many fish species. The Chinese dark sleeper (Odontobutis sinensis) is a commercially important aquaculture species in China. Here, by conducting whole genome sequencing of three females, three males and one female mixed pool, we initially identified 658 male-specific sequences and 242 female-specific sequences of Chinese dark sleeper. Based on polymerase chain reaction (PCR) confirmation, three markers showed 100 % male-specific amplification in 16 female and 16 male individuals from the Wuhan population, and this result was further validated using an additional 16 females and 16 males from the Guangdong population, demonstrating the reliability of these markers across multiple populations. These findings suggest a male heterogametic sex determination system in Chinese dark sleeper. Moreover, male-specific genes including high mobility group box 3 (hmgxb3) and insulin-like growth factor 2b (igf2b) were identified through NR annotation, which might be candidate sex-determining genes in Chinese dark sleeper. Generally, these validated male-specific markers first demonstrated the existence of an XY-type sex determination system in Chinese dark sleeper and would make a significant contribution to the development of all-male breeding and understanding of sex determination mechanisms.

CRISPR-Mediated SRY Gene Mutation Increases the Expression of Female Lineage-Specific Gene in Pre-Implantation Buffalo Embryo

In mammals, sex determination is governed by the SRY gene on the Y chromosome, redirecting gonadal development from forming ovaries to testes. Mutations or alterations in the SRY gene can significantly affect phenotypic changes and lineage-specific markers. This study aims to elucidate the role of the SRY gene in buffalo embryos using CRISPR-Cas9 technology. We designed a crRNA targeting the HMG domain of the SRY gene using the CRISPOR algorithm. Nucleofection of sgRNA-Cas9 RNPs into buffalo fibroblasts confirmed efficient cleavage at the targeted site. Using this validated guide, we investigated the role of the SRY gene in sexual determination by electroporating CRISPR-Cas9-RNPs into single-stage zygotes of buffalo. Genetic changes in the SRY gene were confirmed through sequencing, revealing mosaic blastocysts with multiple alleles and non-mosaic mutants. Mutations in SRY gene increased the expression of female lineage-specific gene Wnt4 whereas decreased the expression of male specific gene SOX9 in blastocysts, suggesting reprogramming towards female sex determination pathways. Our findings provide insights into buffalo sex differentiation mechanisms and potential applications in reproductive strategies for breeding programmes.

The potential regulatory role of the non-coding RNAs in regulating the exogenous estrogen-induced feminization in Takifugu rubripes gonad

Estrogen plays a pivotal role in the early stage of sex differentiation in teleost. However, the underlying mechanisms of estrogen-induced feminization process are still needed for further clarification. Here, the comparative analysis of whole-transcriptome RNA sequencing was conducted between 17beta-Estradiol induced feminized XY (E-XY) gonads and control gonads (C) in Takifugu rubripes. A total of 57 miRNAs, 65 lncRNAs, and 4 circRNAs were found to be expressed at lower levels in control-XY (C-XY) than that in control-XX (C-XX), and were up-regulated in XY during E2-induced feminization process. The expression levels of 24 miRNAs, and 55 lncRNAs were higher in C-XY than that in C-XX, and were down-regulated in E2-treated XY. Furthermore, a correlation analysis was performed between miRNA-seq and mRNA-seq data. In C-XX/C-XY, 114 differential expression (DE) miRNAs were predicted to target to 904 differential expression genes (DEGs), while in C-XY/E-XY, 226 DEmiRNAs were predicted to target to 2,048 DEGs. In C-XX/C-XY, and C-XY/E-XY, KEGG pathway enrichment analysis showed that those targeted genes were mainly enriched in MAPK signaling, calcium signaling, steroid hormone biosynthesis and ovarian steroidogenesis pathway. Additionally, the competitive endogenous RNA (ceRNA) regulatory network was constructed by 24 miRNAs, 21 lncRNAs, 4 circRNAs and 5 key sex-related genes. These findings suggested that the expression of critical genes in sex differentiation were altered in E2-treated XY T. rubripes may via the lncRNA-miRNA-mRNA regulation network to facilitate the differentiation and maintenance of ovaries. Our results provide a new insight into the comprehensive understanding of the effects of estrogen signaling pathways on sex differentiation in teleost gonads.

Return of the forgotten hero: the role of Y chromosome-encoded Zfy in male reproduction

The Y-linked zinc finger gene ZFY is conserved across eutherians and is known to be a critical fertility factor in some species. The initial studies of the mouse homologues, Zfy1 and Zfy2, were performed using mice with spontaneous Y chromosome mutations and Zfy transgenes. These studies revealed that Zfy is involved in multiple processes during spermatogenesis, including removal of germ cells with unpaired chromosomes and control of meiotic sex chromosome inactivation during meiosis I, facilitating the progress of meiosis II, promoting spermiogenesis, and improving assisted reproduction outcomes. Zfy was also identified as a key gene in Y chromosome evolution, protecting this chromosome from extinction by serving as the executioner responsible for meiosis surveillance. Studies with targeted Zfy knock-outs revealed that mice lacking both homologues have severe spermatogenic defects and are infertile. Based on protein structure and in vitro assays, Zfy is expected to drive spermatogenesis as a transcriptional regulator. The combined evidence documents that the presence of at least one Zfy homologue is required for male fertility and that Zfy2 plays a more prominent role. This knowledge reinforces the importance of these factors for mouse spermatogenesis and informs our understanding of the human ZFY variants, which are homologous to the mouse Zfy1 and Zfy2.

Loss of circSRY reduces γH2AX level in germ cells and impairs mouse spermatogenesis

Sry on the Y chromosome is the master switch of sex determination in mammals. It has been well established that Sry encodes a transcription factor that is transiently expressed in somatic cells of the male gonad, leading to the formation of testes. In the testis of adult mice, Sry is expressed as a circular RNA (circRNA) transcript. However, the physiological function of Sry circRNA (circSRY) remains unknown since its discovery in 1993. Here we show that circSRY is mainly expressed in the spermatocytes, but not in mature sperm or somatic cells of the testis. Loss of circSRY led to germ cell apoptosis and the reduction of sperm count in the epididymis. The level of γH2AX was decreased, and failure of XY body formation was noted in circSRY KO germ cells. Further study demonstrated that circSRY directly bound to miR-138-5p in spermatocytes, and in vitro assay suggested that circSRY regulates H2AX mRNA through sponging miR-138-5p. Our study demonstrates that, besides determining sex, Sry also plays an important role in spermatogenesis as a circRNA.

First sex modification case in equine cloning

Somatic cell nuclear transfer (SCNT) is an asexual reproductive technique where cloned offspring contain the same genetic material as the original donor. Although this technique preserves the sex of the original animal, the birth of sex-reversed offspring has been reported in some species. Here, we report for the first time the birth of a female foal generated by SCNT of a male nuclear donor. After a single SCNT procedure, 16 blastocysts were obtained and transferred to eight recipient mares, resulting in the birth of two clones: one male and one female. Both animals had identical genetic profiles, as observed in the analysis of 15-horse microsatellite marker panel, which confirmed they are indeed clones of the same animal. Cytogenetic analysis and fluorescent in situ hybridization using X and Y specific probes revealed a 63,X chromosome set in the female offspring, suggesting a spontaneous Y chromosome loss. The identity of the lost chromosome in the female was further confirmed through PCR by observing the presence of X-linked markers and absence of Y-linked markers. Moreover, cytogenetic and molecular profiles were analyzed in blood and skin samples to detect a possible mosaicism in the female, but results showed identical chromosomal constitutions. Although the cause of the spontaneous chromosome loss remains unknown, the possibility of equine sex reversal by SCNT holds great potential for the preservation of endangered species, development of novel breeding techniques, and sportive purposes.

Two acquired mouse Y chromosome-linked genes, Prssly and Teyorf1, are dispensable for male fertility‡

Prssly (Protease, serine-like, Chr Y) and Teyorf1 (Testis expressed, chromosome Y open reading frame 1) are two acquired single-copy genes located on the distal tip of the non-pairing short arm of the mouse Y chromosome adjacent to telomeric sequence. Both genes lack X chromosome-linked homologues and are expressed in testicular germ cells. We first performed analysis of Prssly and Teyorf1 genomic sequences and demonstrated that previously reported Prssly sequence is erroneous and the true Prssly sequence is longer and encodes a larger protein than previously estimated. We also confirmed that both genes encode pseudogenes that are not expressed in testes. Next, using CRISPR/Cas9 genome targeting, we generated Prssly and Teyorf1 knockout (KO) mice and characterized their phenotype. To create Prssly KO mice, we targeted the conserved exon 5 encoding a trypsin domain typical for serine proteases. The targeting was successful and resulted in a frame shift mutation that introduced a premature stop codon, with the Prssly KO males retaining only residual transcript expression in testes. The Teyorf1 targeting removed the entire open reading frame of the gene, which resulted in no transcript expression in KO males. Both Prssly KO and Teyorf1 KO males were fertile and had normal testis size and normal sperm number, motility, and morphology. Our findings show that Prssly and Teyorf1 transcripts with potential to encode proteins are dispensable for male fertility.

Effects of endocrine disrupting chemicals on gonad development: Mechanistic insights from fish and mammals

Over the past century, evidence has emerged that endocrine disrupting chemicals (EDCs) have an impact on reproductive health. An increased frequency of reproductive disorders has been observed worldwide in both wildlife and humans that is correlated with accidental exposures to EDCs and their increased production. Epidemiological and experimental studies have highlighted the consequences of early exposures and the existence of key windows of sensitivity during development. Such early in life exposures can have an immediate impact on gonadal and reproductive tract development, as well as on long-term reproductive health in both males and females. Traditionally, EDCs were thought to exert their effects by modifying the endocrine pathways controlling reproduction. Advances in knowledge of the mechanisms regulating sex determination, differentiation and gonadal development in fish and rodents have led to a better understanding of the molecular mechanisms underlying the effects of early exposure to EDCs on reproduction. In this manuscript, we review the key developmental stages sensitive to EDCs and the state of knowledge on the mechanisms by which model EDCs affect these processes, based on the roadmap of gonad development specific to fish and mammals.

Advancements in mammalian X and Y sperm differences and sex control technology

Mammal sex determination depends on whether the X sperm or Y sperm binds to the oocyte during fertilization. If the X sperm joins in oocyte, the offspring will be female, if the Y sperm fertilizes, the offspring will be male. Livestock sex control technology has tremendous value for livestock breeding as it can increase the proportion of female offspring and improve the efficiency of livestock production. This review discusses the detailed differences between mammalian X and Y sperm with respect to their morphology, size, and motility in the reproductive tract and in in vitro conditions, as well as ’omics analysis results. Moreover, research progress in mammalian sex control technology has been summarized.

Human SRY Expression at the Sex-determining Period is Insufficient to Drive Testis Development in Mice

The sex-determining region of the Y chromosome, Sry/SRY, is an initiation factor for testis development in both humans and mice. Although the functional compatibility between murine SRY and human SRY was previously examined in transgenic mice, their equivalency remains inconclusive. Because molecular interaction and timeline of mammalian sex determination were mostly described in murine experiments, we generated a mouse model in which Sry was substituted with human SRY to verify the compatibility. The mouse model had the human SRY open reading frame at the locus of murine Sry exon 1-Sry(SRY) mice-and was generated using the CRISPR/Cas9 system. The reproductive system of the mice was analyzed. The expression of human SRY in the fetal gonadal ridge of Sry(SRY) mice was detected. The external and internal genitalia of adult Sry(SRY) mice were similar to those of wild-type females, without any significant difference in anogenital distance. Sry(SRY) mice obtained gonads, which were morphologically considered as ovaries. Histological analysis revealed that the cortical regions of gonads from adult Sry(SRY) mice contained few follicles. We successfully replaced genes on the Y chromosome with targeted genome editing using the CRISPR/Cas9 system. Because the Sry(SRY) XY mice did not develop testis, we concluded that human SRY was insufficient to drive testis development in mouse embryos. The difference in response elements and lack of glutamine-rich domains may have invalidated human SRY function in mice. Signal transduction between Sry/SRY expression and Sox9/SOX9 activation is possibly organized in a species-specific manner.

Y chromosome functions in mammalian spermatogenesis

The mammalian Y chromosome is critical for male sex determination and spermatogenesis. However, linking each Y gene to specific aspects of male reproduction has been challenging. As the Y chromosome is notoriously hard to sequence and target, functional studies have mostly relied on transgene-rescue approaches using mouse models with large multi-gene deletions. These experimental limitations have oriented the field toward the search for a minimum set of Y genes necessary for male reproduction. Here, considering Y-chromosome evolutionary history and decades of discoveries, we review the current state of research on its function in spermatogenesis and reassess the view that many Y genes are disposable for male reproduction.

Alternate Roles of Sox Transcription Factors beyond Transcription Initiation

Sox proteins are known as crucial transcription factors for many developmental processes and for a wide range of common diseases. They were believed to specifically bind and bend DNA with other transcription factors and elicit transcriptional activation or repression activities in the early stage of transcription. However, their functions are not limited to transcription initiation. It has been showed that Sox proteins are involved in the regulation of alternative splicing regulatory networks and translational control. In this review, we discuss the current knowledge on how Sox transcription factors such as Sox2, Sry, Sox6, and Sox9 allow the coordination of co-transcriptional splicing and also the mechanism of SOX4-mediated translational control in the context of RNA polymerase III.

Efficient TALEN-mediated gene knockin at the bovine Y chromosome and generation of a sex-reversal bovine

Functional elucidation of bovine Y-chromosome genes requires available genome editing technologies. Meanwhile, it has yet to be proven whether the bovine Sry gene is the main or single factor involved in the development of the male phenotype in bovine. Here, we efficiently knocked out four Y-linked genes (Sry, ZFY, DDX3Y, and EIF2S3Y) in bovine fetal fibroblasts (BFFs) with transcription activator-like effector nucleases (TALENs) individually. Furthermore, we used TALEN-mediated gene knockin at the Sry gene and generated a sex-reversal bovine by somatic cell nuclear transfer (SCNT). The resulting bovine had only one ovary and was sterile. We demonstrate, for the first time, that the Sry gene is an important sex-determining gene in bovine. Our method lays a solid foundation for detecting the biology of the bovine Y chromosome, as it may provide an alternative biological model system for the study of mammalian sex determination, and new methods for the practical application in agricultural, especially for sex predetermination.

Knockout of the HMG domain of the porcine SRY gene causes sex reversal in gene-edited pigs

The present work characterizes the porcine sex-determining region on the Y chromosome (SRY) gene and demonstrates its pivotal role in sex determination. We provide evidence that genetically male pigs with a knockout of the SRY gene undergo sex reversal of the external and internal genitalia. This discovery of SRY as the main switch for sex determination in pigs may provide an alternative for surgical castration in pig production, preventing boar taint. As the pig shares many genetic, physiological, and anatomical similarities with humans, it also provides a suitable large animal model for human sex reversal syndromes, allowing for the development of new interventions for human sex disorders.

The sex-determining region on the Y chromosome (SRY) is thought to be the central genetic element of male sex development in mammals. Pathogenic modifications within the SRY gene are associated with a male-to-female sex reversal syndrome in humans and other mammalian species, including rabbits and mice. However, the underlying mechanisms are largely unknown. To understand the biological function of the SRY gene, a site-directed mutational analysis is required to investigate associated phenotypic changes at the molecular, cellular, and morphological level. Here, we successfully generated a knockout of the porcine SRY gene by microinjection of two CRISPR-Cas ribonucleoproteins, targeting the centrally located “high mobility group” (HMG), followed by a frameshift mutation of the downstream SRY sequence. This resulted in the development of genetically male (XY) pigs with complete external and internal female genitalia, which, however, were significantly smaller than in 9-mo-old age-matched control females. Quantitative digital PCR analysis revealed a duplication of the SRY locus in Landrace pigs similar to the known palindromic duplication in Duroc breeds. Our study demonstrates the central role of the HMG domain in the SRY gene in male porcine sex determination. This proof-of-principle study could assist in solving the problem of sex preference in agriculture to improve animal welfare. Moreover, it establishes a large animal model that is more comparable to humans with regard to genetics, physiology, and anatomy, which is pivotal for longitudinal studies to unravel mammalian sex determination and relevant for the development of new interventions for human sex development disorders.

Mini-Review Regarding the Applicability of Genome Editing Techniques Developed for Studying Infertility

Infertility is a highly debated topic today. It has been long hypothesized that infertility has an idiopathic cause, but recent studies demonstrated the existence of a genetic substrate. Fortunately, the methods of editing the human genome proven to be revolutionary. Following research conducted, we identified a total of 21 relevant studies; 14 were performed on mice, 5 on zebrafish and 2 on rats. We concluded that over forty-four genes in total are dispensable for fertility in both sexes without affecting host homeostasis. However, there are genes whose loss-of-function induces moderate to severe phenotypic changes in both sexes. There were situations in which the authors reported infertility, exhibited by the experimental model, or other pathologies such as cryptorchidism, cataracts, or reduced motor activity. Overall, zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 are techniques that offer a wide range of possibilities for studying infertility, even to create mutant variants. It can be concluded that ZFNs, TALENs, and CRISPR/Cas9 are crucial tools in biomedical research.

Heterozygous deletion of Sox9 in mouse mimics the gonadal sex reversal phenotype associated with campomelic dysplasia in humans

Heterozygous mutations in the human SOX9 gene cause the skeletal malformation syndrome campomelic dysplasia which in 75% of 46, XY individuals is associated with male-to-female sex reversal. Although studies in homozygous Sox9 knockout mouse models confirmed that SOX9 is critical for testis development, mice heterozygous for the Sox9-null allele were reported to develop normal testes. This led to the belief that the SOX9 dosage requirement for testis differentiation is different between humans, which often require both alleles, and mice, in which one allele is sufficient. However, in prior studies, gonadal phenotypes in heterozygous Sox9 XY mice were assessed only by either gross morphology, histological staining or analyzed on a mixed genetic background. In this study, we conditionally inactivated Sox9 in somatic cells of developing gonads using the Nr5a1-Cre mouse line on a pure C57BL/6 genetic background. Section and whole-mount immunofluorescence for testicular and ovarian markers showed that XY Sox9 heterozygous gonads developed as ovotestes. Quantitative droplet digital PCR confirmed a 50% reduction of Sox9 mRNA as well as partial sex reversal shown by an upregulation of ovarian genes. Our data show that haploinsufficiency of Sox9 can perturb testis development in mice, suggesting that mice may provide a more accurate model of human disorders/differences of sex development than previously thought.

Sex Manipulation Technologies Progress in Livestock: A Review

Sex manipulation technologies allow predetermination of the sex of animal offspring by altering the normal reproductive process. In livestock production, the difference in type and gender can translate into significant economic benefits, including alleviation of severe food shortages. In livestock, however, the commercial application of sex manipulation technologies is currently available for cattle only. In this review, we described the brief history of sex manipulation, and the research progresses of common methods used in sex manipulation thus far. Information presented in this review can inform future studies on expanding the scope and use of sex manipulation technologies in livestock.

Sox8 and Sox9 act redundantly for ovarian-to-testicular fate reprogramming in the absence of R-spondin1 in mouse sex reversals

In mammals, testicular differentiation is initiated by transcription factors SRY and SOX9 in XY gonads, and ovarian differentiation involves R-spondin1 (RSPO1) mediated activation of WNT/β-catenin signaling in XX gonads. Accordingly, the absence of RSPO1/Rspo1 in XX humans and mice leads to testicular differentiation and female-to-male sex reversal in a manner that does not requireSry or Sox9 in mice. Here we show that an alternate testis-differentiating factor exists and that this factor is Sox8. Specifically, genetic ablation of Sox8 and Sox9 prevents ovarian-to-testicular reprogramming observed in XX Rspo1 loss-of-function mice. Consequently, Rspo1 Sox8 Sox9 triple mutant gonads developed as atrophied ovaries. Thus, SOX8 alone can compensate for the loss of SOX9 for Sertoli cell differentiation during female-to-male sex reversal.

In humans, mice and other mammals, genetic sex is determined by the combination of sex chromosomes that each individual inherits. Individuals with two X chromosomes (XX) are said to be chromosomally female, while individuals with one X and one Y chromosome (XY) are chromosomally males.

One of the major differences between XX and XY individuals is that they have different types of gonads (the organs that make egg cells or sperm). In mice, for example, before males are born, a gene called Sox9 triggers a cascade of events that result in the gonads developing into testes. In females, on the other hand, another gene called Rspo1 stimulates the gonads to develop into ovaries.

Loss of Sox9 in XY embryos, or Rspo1 in XX embryos, leads to mice developing physical characteristics that do not match their genetic sex, a phenomenon known as sex reversal. For example, in XX female mice lacking Rspo1, cells in the gonads reprogram into testis cells known as Sertoli cells just before birth and form male structures known as testis cords. The gonads of female mice missing both Sox9 and Rspo1 (referred to as “double mutants”) also develop Sertoli cells and testis cords, suggesting another gene may compensate for the loss of Sox9.

Previous studies suggest that a gene known as Sox8, which is closely related to Sox9, may be able to drive sex reversal in female mice. However, it was not clear whether Sox8 is able to stimulate testis to form in female mice in the absence of Sox9.

To address this question, Richardson et al. studied mutant female mice lacking Rspo1, Sox8 and Sox9, known as “triple mutants”. Just before birth, the gonads in the triple mutant mice showed some characteristics of sex reversal but lacked the Sertoli cells found in the double mutant mice. After the mice were born, the gonads of the triple mutant mice developed as rudimentary ovaries without testis cords, unlike the more testis-like gonads found in the double mutant mice.

The findings of Richardson et al. show that Sox8 is able to trigger sex reversal in female mice in the absence of Rspo1 and Sox9. Differences in sexual development in humans affect the appearance of individuals and often cause infertility. Identifying Sox8 and other similar genes in mice may one day help to diagnose people with such conditions and lead to the development of new therapies.

Analysis of the Mouse Y Chromosome by Single-Molecule Sequencing With Y Chromosome Enrichment

Since human and mouse Y chromosomes contain repeated sequences, it is difficult to determine the precise sequences and analyze the function of individual Y chromosome genes. Therefore, the causes of many diseases and abnormalities related to Y chromosome genes, such as male infertility, remain unclear. In this study, to elucidate the mouse Y chromosome, we enriched the mouse Y chromosome using a fluorescence-activated cell sorter (FACS) equipped with commonly used UV and blue 488 nm lasers and read the nucleotides using the Oxford Nanopore MinION long-read sequencer. This sequencing strategy allows us to cover the whole known region as well as the potential undetermined region of the Y chromosome. FACS-based chromosome enrichment and long-read sequencing are suitable for analysis of the Y chromosome sequences and may lead to further understanding of the physiological role of Y chromosome genes.

The History of Transgenesis

A transgenic mouse carries within its genome an artificial DNA construct (transgene) that is deliberately introduced by an experimentalist. These animals are widely used to understand gene function and protein function. When addressing the history of transgenic mouse technology, it is apparent that a number of basic science research areas laid the groundwork for success. These include reproductive science, genetics and molecular biology, and micromanipulation and microscopy equipment. From reproductive physiology came applications on how to optimize mouse breeding, how to superovulate mice to produce zygotes for DNA microinjection or preimplantation embryos for combination with embryonic stem (ES) cells, and how to return zygotes and embryos to a pseudopregnant surrogate dam for gestation and birth. From developmental biology, it was learned how to micromanipulate embryos for morula aggregation and blastocyst microinjection and how to establish germline competent ES cells. From genetics came the foundational principles governing the inheritance of genes, the interactions of gene products, and an understanding of the phenotypic consequences of genetic mutations. From molecular biology came a panoply of tools and reagents that are used to clone DNA transgenes, to detect the presence of transgenes, to assess gene expression by measuring transcription, and to detect proteins in cells and tissues. Technical advances in light microscopes, micromanipulators, micropipette pullers, and ancillary equipment made it possible for experimentalists to insert thin glass needles into zygotes or embryos under controlled conditions to inject DNA solutions or ES cells. To fully discuss the breadth of contributions of these numerous scientific disciplines to a comprehensive history of transgenic science is beyond the scope of this work. Examples will be used to illustrate scientific developments central to the foundation of transgenic technology and that are in use today.

Efficient production and transmission of CRISPR/Cas9-mediated mutant alleles at the IG-DMR via generation of mosaic mice using a modified 2CC method

Generation of mutant imprinting control region (ICR) mice using genome editing is an important approach for elucidating ICR functions. IG-DMR is an ICR in the Dlk1-Dio3 imprinted domain that contains functional regions—in both parental alleles—that are essential for embryonic development. One drawback of this approach is that embryonic lethality can occur from aberrant expression of the imprinted genes if IG-DMR gets mutated in either the paternal or maternal allele. To overcome this problem, we generated mosaic mice that contained cells with modified IG-DMR alleles and wild-type cells using the 2CC method that allowed for microinjection of the CRISPR/Cas9 constructs into a blastomere of 2-cell embryos. This method improved the birth rate of the founder pups relative to that obtained using the standard protocol. We also successfully produced mosaic mice in which the tandem repeat array sequence in the IG-DMR had been replaced by homology directed repair. Additionally, paternal transmission of the replaced allele caused aberrant expression of the imprinted genes due to hypomethylation of the IG-DMR, indicating that the replaced allele recapitulated our deletion model. Our results indicate that this method is useful for the generation of mutant mice in which a genomic locus essential for normal development has been genetically edited.

Pre-determination of sex in pigs by application of CRISPR/Cas system for genome editing

In livestock industries, one sex is usually preferred because of the impact on the production (e.g. milk from cows, eggs from laying hens). Furthermore, in pig production, the male-specific boar taint is a big hurdle for consumer acceptance. Consequently, a shift in the ratio towards the desired sex would be a great benefit. The most widely applied method for pre-determination of the sex is fluorescence-activated sperm sorting, which relies on the different DNA content of the X- and Y-chromosomal sperm. However, the successful practical adaption of this method depends on its ease of use. At present, sperm sexing via fluorescence-activated cell sorting (FACS) has only reached commercial application in cattle. Nevertheless, sperm sexing technology still needs to be improved with respect to efficiency and reliability, to obtain high numbers of sexed sperm and less invasive sperm treatment to avoid damage. New genome editing technologies such as Zinc finger nucleases (ZFN), Transcription-activator like endonucleases (TALENs) and the CRISPR/Cas system have emerged and offer great potential to affect determination of the sex at the genome level. The sex-determining region on the Y chromosome (SRY) serves as a main genetic switch of male gender development. It was previously shown that a knockout of the SRY gene in mice and rabbits displayed suppressed testis development in the fetal gonadal ridges resulting in a female phenotype. These new technologies hold great opportunities to pre-determine sex in pigs. However, further investigations are needed to exploit their full potential for practical application.

XY oocytes of sex-reversed females with a Sry mutation deviate from the normal developmental process beyond the mitotic stage†

The fertility of sex-reversed XY female mice is severely impaired by a massive loss of oocytes and failure of meiotic progression. This phenomenon remains an outstanding mystery. We sought to determine the molecular etiology of XY oocyte dysfunction by generating sex-reversed females that bear genetic ablation of Sry, a vital sex determination gene, on an inbred C57BL/6 background. These mutant mice, termed XYsry- mutants, showed severe attrition of germ cells during fetal development, resulting in the depletion of ovarian germ cells prior to sexual maturation. Comprehensive transcriptome analyses of primordial germ cells (PGCs) and postnatal oocytes demonstrated that XYsry- females had deviated significantly from normal developmental processes during the stages of mitotic proliferation. The impaired proliferation of XYsry- PGCs was associated with aberrant β-catenin signaling and the excessive expression of transposable elements. Upon entry to the meiotic stage, XYsry- oocytes demonstrated extensive defects, including the impairment of crossover formation, the failure of primordial follicle maintenance, and no capacity for embryo development. Together, these results suggest potential molecular causes for germ cell disruption in sex-reversed female mice, thereby providing insights into disorders of sex differentiation in humans, such as "Swyer syndrome," in which patients with an XY karyotype present as typical females and are infertile.

Mapping of a responsible region for sex reversal upstream of Sox9 by production of mice with serial deletion in a genomic locus

Sox9 plays critical roles in testis formation. By mapping four familial cases of disorders of sexual development, a 32.5 kb sequence located far upstream of SOX9 was previously identified as being a commonly deleted region and named the XY sex reversal region (XYSR). To narrow down a responsible sequence in XYSR, we generated mutant mice with a series of deletions in XYSR by application of the CRISPR/Cas9 system, using a mixture of sgRNAs targeting several kilobase (kb) intervals in the region. When the whole XYSR corresponding sequence in mice was deleted in XY karyotype individuals, the mutation resulted in female offspring, suggesting that an expression mechanism of SOX9/Sox9 through XYSR is conserved in human and mouse. Male-to-female sex reversal was found in mice with a 4.8 kb deletion. We identified a sequence conserved among humans, mice, and opossum, the deletion of which (783 bp) in mice resulted in male-to-female sex reversal. The sequence includes a recently reported critical gonad enhancer for Sox9. Although it cannot be concluded that the human sequence is responsible for XYSR, it is likely. This method is applicable for fine mapping of responsible sequences for disease-causing deletions especially with regard to rare diseases.

Peptidyl arginine deiminase 2 (Padi2) is expressed in Sertoli cells in a specific manner and regulated by SOX9 during testicular development

Peptidyl arginine deiminases (PADIs) are enzymes that change the charge of proteins through citrullination. We recently found Padi2 was expressed exclusively in fetal Sertoli cells. In this study, we analyzed the transcriptional regulation of Padi2 and the role of PADI2 in testicular development. We showed SOX9 positively regulated Padi2 transcription and FOXL2 antagonized it in TM3 cells, a model of Sertoli cells. The responsive region to SOX9 and FOXL2 was identified within the Padi2 sequence by reporter assay. In fetal testes from Sox9 knockout (AMH-Cre:Sox9^flox/flox) mice, Padi2 expression was greatly reduced, indicating SOX9 regulates Padi2 in vivo. In vitro analysis using siRNA suggested PADI2 modified transcriptional regulation by SOX9. However, Padi2^−/− XY mice were fertile and showed no apparent reproductive anomalies. Although, PADI2 is known as an epigenetic transcriptional regulator through H3 citrullination, no significant difference in H3 citrullination between wildtype and Padi2^−/− XY gonads was observed. These results suggest Padi2 is a novel gene involved in testis development that is specifically expressed in Sertoli cells through the regulation by SOX9 and FOXL2 and PADI2 supports regulation of target genes by SOX9. Analysis of the Padi2^−/− XY phenotype suggested a redundant factor compensated for PADI2 function in testicular development.

CRISPR/Cas9-mediated knock-in of the murine Y chromosomal Sry gene

Mammalian zygote-mediated genome editing via the clustered regularly interspaced short palindromic repeats/CRISPR-associated endonuclease 9 (CRISPR/Cas9) system is widely used to generate genome-modified animals. This system allows for the production of loss-of-function mutations in various Y chromosome genes, including Sry, in mice. Here, we report the establishment of a CRISPR-Cas9-mediated knock-in line of Flag-tag sequences into the Sry locus at the C-terminal coding end of the Y chromosome (Y^Sry-flag). In the F1 and successive generations, all male pups carrying the Y^Sry-flag chromosome had normal testis differentiation and proper spermatogenesis at maturity, enabling complete fertility and the production of viable offspring. To our knowledge, this study is the first to produce a stable Sry knock-in line at the C-terminal region, highlighting a novel approach for examining the significance of amino acid changes at the naive Sry locus in mammals.

Mutation of the Sp1 binding site in the 5' flanking region of SRY causes sex reversal in rabbits

Sex-determining region Y is a crucial gene that initiates male sex determination in mammals. Mutations of the Sp1-binding site in the 5' flanking region of SRY are associated with clinical male-to-female sex reversal syndrome, although such occurrences are rare and, until now, have not been reported in animal models. In this study, we mutated Sp1-binding sites in the 5' flanking region of the rabbit SRY gene using the CRISPR/Cas9 system. As expected, the SRY-Sp1 knockout rabbits had female external and internal genitalia and exhibited normal female copulatory behaviors, but they were infertile, and the adults displayed reduced follicles. Interestingly, we successfully obtained offspring from sex-reversed SRY-Sp1 knockout rabbits using embryo transfer. In summary, our study demonstrates that Sp1 is a major regulator in SRY gene transcription, and mutations of the Sp1 binding sites (Sp1-B and Sp1-C) in the 5' flanking region of SRY induce sex reversal in rabbits, which can be used as targets for clinical research of male-to-female sex reversal syndrome. Additionally, we provide the first evidence that sex reversal syndrome patients have the potential to become pregnant with the use of embryo transfer.

Brain Sexual Differentiation and Requirement of SRY: Why or Why Not?

Brain sexual differentiation is orchestrated by precise coordination of sex steroid hormones. In some species, programming of select male brain regions is dependent upon aromatization of testosterone to estrogen. In mammals, these hormones surge during the organizational and activational periods that occur during perinatal development and adulthood, respectively. In various fish and reptiles, incubation temperature during a critical embryonic period results in male or female sexual differentiation, but this can be overridden in males by early exposure to estrogenic chemicals. Testes development in mammals requires a Y chromosome and testis determining gene SRY (in humans)/Sry (all other therian mammals), although there are notable exceptions. Two species of spiny rats: Amami spiny rat (Tokudaia osimensis) and Tokunoshima spiny rat (Tokudaia tokunoshimensis) and two species of mole voles (Ellobius lutescens and Ellobius tancrei), lack a Y chromosome/Sry and possess an XO chromosome system in both sexes. Such rodent species, prototherians (monotremes, who also lack Sry), and fish and reptile species that demonstrate temperature sex determination (TSD) seemingly call into question the requirement of Sry for brain sexual differentiation. This review will consider brain regions expressing SRY/Sry in humans and rodents, respectively, and potential roles of SRY/Sry in the brain will be discussed. The evidence from various taxa disputing the requirement of Sry for brain sexual differentiation in mammals (therians and prototherians) and certain fish and reptilian species will be examined. A comparative approach to address this question may elucidate other genes, pathways, and epigenetic modifications stimulating brain sexual differentiation in vertebrate species, including humans.

One-step generation of complete gene knockout mice and monkeys by CRISPR/Cas9-mediated gene editing with multiple sgRNAs

The CRISPR/Cas9 system is an efficient gene-editing method, but the majority of gene-edited animals showed mosaicism, with editing occurring only in a portion of cells. Here we show that single gene or multiple genes can be completely knocked out in mouse and monkey embryos by zygotic injection of Cas9 mRNA and multiple adjacent single-guide RNAs (spaced 10-200 bp apart) that target only a single key exon of each gene. Phenotypic analysis of F0 mice following targeted deletion of eight genes on the Y chromosome individually demonstrated the robustness of this approach in generating knockout mice. Importantly, this approach delivers complete gene knockout at high efficiencies (100% on Arntl and 91% on Prrt2) in monkey embryos. Finally, we could generate a complete Prrt2 knockout monkey in a single step, demonstrating the usefulness of this approach in rapidly establishing gene-edited monkey models.

Creation of mutant mice with megabase-sized deletions containing custom-designed breakpoints by means of the CRISPR/Cas9 system

The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) system is a useful tool for creation of mutant mice with mutations mirroring those in human patients. Various methods have been developed for this purpose, including deletions, inversions, and translocations. So far, mutant mice with deletions of up to 1.2 megabases (Mb) have been generated by microinjection of the CRISPR/Cas9 system into fertilized eggs; however, a method for generation of mutant mice with a deletion of more than several Mb size is necessary because such deletions have often been identified as possible causes of human diseases. With an aim to enable the generation of disease models carrying large deletions with a breakpoint in custom-designed sequences, we developed a method for induction of an Mb-sized deletion by microinjection of a pair of sgRNAs, Cas9, and a donor plasmid into fertilized eggs. Using this method, we efficiently and rapidly generated mutant mice carrying deletions up to 5 Mb.

Complementary Critical Functions of Zfy1 and Zfy2 in Mouse Spermatogenesis and Reproduction

The mammalian Y chromosome plays a critical role in spermatogenesis. However, the exact functions of each gene in the Y chromosome have not been completely elucidated, partly owing to difficulties in gene targeting analysis of the Y chromosome. Zfy was first proposed to be a sex determination factor, but its function in spermatogenesis has been recently elucidated. Nevertheless, Zfy gene targeting analysis has not been performed thus far. Here, we adopted the highly efficient CRISPR/Cas9 system to generate individual Zfy1 or Zfy2 knockout (KO) mice and Zfy1 and Zfy2 double knockout (Zfy1/2-DKO) mice. While individual Zfy1 or Zfy2-KO mice did not show any significant phenotypic alterations in fertility, Zfy1/2-DKO mice were infertile and displayed abnormal sperm morphology, fertilization failure, and early embryonic development failure. Mass spectrometric screening, followed by confirmation with western blot analysis, showed that PLCZ1, PLCD4, PRSS21, and HTT protein expression were significantly deceased in spermatozoa of Zfy1/2-DKO mice compared with those of wild-type mice. These results are consistent with the phenotypic changes seen in the double-mutant mice. Collectively, our strategy and findings revealed that Zfy1 and Zfy2 have redundant functions in spermatogenesis, facilitating a better understanding of fertilization failure and early embryonic development failure.

CRISPR/Cas9-mediated simultaneous knockout of Dmrt1 and Dmrt3 does not recapitulate the 46,XY gonadal dysgenesis observed in 9p24.3 deletion patients

DM domain transcription factors play important roles in sexual development in a wide variety of species from invertebrate to humans. Among seven mammalian family members of DM domain transcription factors, DMRT1 has been studied in mouse and human for its conserved role in male gonadal identity. Chromosomal deletion of 9p24.3, the region in which DMRT1 is located, is associated with 46,XY gonadal dysgenesis. Dmrt1 knockout (KO) mice also showed male-to-female gonadal reprogramming. However, the phenotype of Dmrt1 KO mouse appears only after birth while 46,XY gonadal dysgenesis occurs during the developmental phase, and the cause behind this difference remained unknown. We hypothesized that in human the function of other DMRT genes clustered with DMRT1, namely DMRT3, might also be impaired by the chromosomal deletion, which leads to the gonadal dysgenesis phenotype. Thus, simultaneous loss of multiple DM domain genes in mice could have a more severe impact on gonadal development. To address this issue, we generated double KO mice for Dmrt1 and Dmrt3 via the CRISPR/Cas9 system. Comparing adult and neonatal testes of single and double KO mice, we found that loss of Dmrt1 or Dmrt3, or both, does not have apparent effect on male gonadal formation during embryonic development. Our study demonstrated that the discrepancy between human with 9p24.3 deletion and Dmrt1 KO mouse could not be explained by the simultaneous loss of Dmrt3 gene. CRISPR/Cas9 is a versatile and straightforward approach to elucidate the questions that were otherwise difficult to address with conventional methods.

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We have generated Dmrt1 KO, Dmrt3 KO and Dmrt1/3 DKO mice via the CRISPR/Cas9 system.

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.CRISPR/Cas9-mediated gene KO could generate 3 KO lines via single microinjection.

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Double KO of Dmrt1 and Dmrt3 could not recapitulate human 9p24.3 deletion symptom.

Efficient generation of B2m-null pigs via injection of zygote with TALENs

Donor major histocompatibility complex class I (MHC I) molecules are the main targets of the host immune response after organ allotransplantation. Whether and how MHC I-deficiency of pig donor tissues affects rejection after xenotransplantation has not been assessed. Beta2-microglobulin (B2M) is indispensable for the assembly of MHC I receptors and therefore provides an effective target to disrupt cell surface MHC I expression. Here, we report the one-step generation of mutant pigs with targeted disruptions in B2m by injection of porcine zygotes with B2m exon 2-specific TALENs. After germline transmission of mutant B2m alleles, we obtained F1 pigs with biallelic B2m frameshift mutations. F1 pigs lacked detectable B2M expression in tissues derived from the three germ layers, and their lymphocytes were devoid of MHC I surface receptors. Skin grafts from B2M deficient pigs exhibited remarkably prolonged survival on xenogeneic wounds compared to tissues of non-mutant littermates. Mutant founder pigs with bi-allelic disruption in B2m and B2M deficient F1 offspring did not display visible abnormalities, suggesting that pigs are tolerant to B2M deficiency. In summary, we show the efficient generation of pigs with germline mutations in B2m, and demonstrate a beneficial effect of donor MHC I-deficiency on xenotransplantation.

Roden C, Lu J, Nicoli S. In vivo mutagenesis of miRNA gene families using a scalable multiplexed CRISPR/Cas9 nuclease system

A large number of microRNAs (miRNAs) are grouped into families derived from the same phylogenetic ancestors. miRNAs within a family often share the same physiological functions despite differences in their primary sequences, secondary structures, or chromosomal locations. Consequently, the generation of animal models to analyze the activity of miRNA families is extremely challenging. Using zebrafish as a model system, we successfully provide experimental evidence that a large number of miRNAs can be simultaneously mutated to abrogate the activity of an entire miRNA family. We show that injection of the Cas9 nuclease and two, four, ten, and up to twenty-four multiplexed single guide RNAs (sgRNAs) can induce mutations in 90% of the miRNA genomic sequences analyzed. We performed a survey of these 45 mutations in 10 miRNA genes, analyzing the impact of our mutagenesis strategy on the processing of each miRNA both computationally and in vivo. Our results offer an effective approach to mutate and study the activity of miRNA families and pave the way for further analysis on the function of complex miRNA families in higher multicellular organisms.

Genome Editing in Mice Using TALE Nucleases

Gene engineering for generating targeted mouse mutants is a key technology for biomedical research. Using TALENs as sequence-specific nucleases to induce targeted double-strand breaks, the mouse genome can be directly modified in zygotes in a single step without the need for embryonic stem cells. By embryo microinjection of TALEN mRNAs and targeting vectors, knockout and knock-in alleles can be generated fast and efficiently. In this chapter we provide protocols for the application of TALENs in mouse zygotes.

Roles of the Y chromosome genes in human cancers

Male and female differ genetically by their respective sex chromosome composition, that is, XY as male and XX as female. Although both X and Y chromosomes evolved from the same ancestor pair of autosomes, the Y chromosome harbors male-specific genes, which play pivotal roles in male sex determination, germ cell differentiation, and masculinization of various tissues. Deletions or translocation of the sex-determining gene, SRY, from the Y chromosome causes disorders of sex development (previously termed as an intersex condition) with dysgenic gonads. Failure of gonadal development results not only in infertility, but also in increased risks of germ cell tumor (GCT), such as gonadoblastoma and various types of testicular GCT. Recent studies demonstrate that either loss of Y chromosome or ectopic expression of Y chromosome genes is closely associated with various male-biased diseases, including selected somatic cancers. These observations suggest that the Y-linked genes are involved in male health and diseases in more frequently than expected. Although only a small number of protein-coding genes are present in the male-specific region of Y chromosome, the impacts of Y chromosome genes on human diseases are still largely unknown, due to lack of in vivo models and differences between the Y chromosomes of human and rodents. In this review, we highlight the involvement of selected Y chromosome genes in cancer development in men.

Biology of the Sertoli Cell in the Fetal, Pubertal, and Adult Mammalian Testis

A healthy man typically produces between 50 × 10(6) and 200 × 10(6) spermatozoa per day by spermatogenesis; in the absence of Sertoli cells in the male gonad, this individual would be infertile. In the adult testis, Sertoli cells are sustentacular cells that support germ cell development by secreting proteins and other important biomolecules that are essential for germ cell survival and maturation, establishing the blood-testis barrier, and facilitating spermatozoa detachment at spermiation. In the fetal testis, on the other hand, pre-Sertoli cells form the testis cords, the future seminiferous tubules. However, the role of pre-Sertoli cells in this process is much less clear than the function of Sertoli cells in the adult testis. Within this framework, we provide an overview of the biology of the fetal, pubertal, and adult Sertoli cell, highlighting relevant cell biology studies that have expanded our understanding of mammalian spermatogenesis.

GeneKnockout by Targeted Mutagenesis in a Hemimetabolous Insect, the Two-Spotted Cricket Gryllus bimaculatus, using TALENs

Hemimetabolous, or incompletely metamorphosing, insects are phylogenetically basal. These insects include many deleterious species. The cricket, Gryllus bimaculatus, is an emerging model for hemimetabolous insects, based on the success of RNA interference (RNAi)-based gene-functional analyses and transgenic technology. Taking advantage of genome-editing technologies in this species would greatly promote functional genomics studies. Genome editing using transcription activator-like effector nucleases (TALENs) has proven to be an effective method for site-specific genome manipulation in various species. TALENs are artificial nucleases that are capable of inducing DNA double-strand breaks into specified target sequences. Here, we describe a protocol for TALEN-based gene knockout in G. bimaculatus, including a mutant selection scheme via mutation detection assays, for generating homozygous knockout organisms.

Direct production of XY(DMY-) sex reversal female medaka (Oryzias latipes) by embryo microinjection of TALENs

Medaka is an ideal model for sex determination and sex reversal, such as XY phenotypically female patients in humans. Here, we assembled improved TALENs targeting the DMY gene and generated XY^DMY− mutants to investigate gonadal dysgenesis in medaka. DMY-TALENs resulted in indel mutations at the targeted loci (46.8%). DMY-nanos3UTR-TALENs induced mutations were passed through the germline to F1 generation with efficiencies of up to 91.7%. XY^DMY− mutants developed into females, laid eggs, and stably passed the Y^DMY− chromosome to next generation. RNA-seq generated 157 million raw reads from WT male (WT_M_TE), WT female (WT_F_OV) and XY^DMY− female medaka (TA_F_OV) gonad libraries. Differential expression analysis identified 144 up- and 293 down-regulated genes in TA_F_OV compared with WT_F_OV, 387 up- and 338 down-regulated genes in TA_F_OV compared with WT_M_TE. According to genes annotation and functional prediction, such as Wnt1 and PRCK, it revealed that incomplete ovarian function and reduced fertility of XY^DMY− mutant is closely related to the wnt signaling pathway. Our results provided the transcriptional profiles of XY^DMY− mutants, revealed the mechanism between sex reversal and DMY in medaka, and suggested that XY^DMY− medaka was a novel mutant that is useful for investigating gonadal dysgenesis in phenotypic female patients with the 46, XY karyotype.

Generation of mutant mice via the CRISPR/Cas9 system using FokI-dCas9

Genome editing, which introduces mutations in genes of interest using artificial DNA nucleases such as the ZFN, TALEN, and CRISPR/Cas9 systems in living cells, is a useful tool for generating mutant animals. Although CRISPR/Cas9 provides advantages over the two other systems, such as an easier vector construction and high efficiency of genome editing, it raises concerns of off-target effects when single guide RNA (gRNA) is used. Recently, FokI-dCas9 (fCas9), a fusion protein comprised of the inactivated mutant form of Cas9 and the DNA nuclease domain of FokI, has been developed. It enables genome editing with reduced risks of off-target effects in mammalian cultured cell lines, as fCas9 requires gRNAs to bind opposite strands with an appropriate distance between them. Here, we demonstrated that fCas9 efficiently generates living mutant mice through microinjection of its mRNA and gRNAs into zygotes. A comparison of the relative efficiencies of genome editing using fCas9 and other modified Cas9s showed that these mutagenesis efficiencies are similar when the targets of two gRNAs are separated by an appropriate distance, suggesting that in addition to the ease of vector construction, fCas9 exhibit high efficiency in producing mutant mice and in reducing risks of off-target effects.

Severe myopathy in mice lacking the MEF2/SRF-dependent gene leiomodin-3

Maintenance of skeletal muscle structure and function requires a precise stoichiometry of sarcomeric proteins for proper assembly of the contractile apparatus. Absence of components of the sarcomeric thin filaments causes nemaline myopathy, a lethal congenital muscle disorder associated with aberrant myofiber structure and contractility. Previously, we reported that deficiency of the kelch-like family member 40 (KLHL40) in mice results in nemaline myopathy and destabilization of leiomodin-3 (LMOD3). LMOD3 belongs to a family of tropomodulin-related proteins that promote actin nucleation. Here, we show that deficiency of LMOD3 in mice causes nemaline myopathy. In skeletal muscle, transcription of Lmod3 was controlled by the transcription factors SRF and MEF2. Myocardin-related transcription factors (MRTFs), which function as SRF coactivators, serve as sensors of actin polymerization and are sequestered in the cytoplasm by actin monomers. Conversely, conditions that favor actin polymerization de-repress MRTFs and activate SRF-dependent genes. We demonstrated that the actin nucleator LMOD3, together with its stabilizing partner KLHL40, enhances MRTF-SRF activity. In turn, SRF cooperated with MEF2 to sustain the expression of LMOD3 and other components of the contractile apparatus, thereby establishing a regulatory circuit to maintain skeletal muscle function. These findings provide insight into the molecular basis of the sarcomere assembly and muscle dysfunction associated with nemaline myopathy.

TALEN-Mediated Gene Disruption on Y Chromosome Reveals Critical Role of EIF2S3Y in Mouse Spermatogenesis

The Y chromosome plays a critical role in spermatogenesis. Formerly, it had been difficult to generate knockout mice with specific Y chromosome mutations using conventional gene-targeting strategies. Recently, a transcription activator-like effector nuclease (TALEN) was successfully used for editing a mouse Y chromosome-linked gene. Here, we report the generation of a mouse model with a mutation in EIF2S3Y, a Y chromosome-linked gene, and analysis of its phenotype. The mouse carrying a targeted mutation of EIF2S3Y was infertile and had hypoplastic testes. Histological and electron microscopic analyses showed that differentiation of spermatogonia was arrested at the stage of spermatogonial stem cells (undifferentiated spermatogonia) and that the progression of spermatogenesis was interrupted, resulting in azoospermia. Using TALEN, we verified that EIF2S3Y performs a key function in differentiation of spermatogonial stem cells.

Generation of fertile and fecund F0 XY female mice from XY ES cells

Known examples of male to female sex reversal in mice are caused by either strain incompatibilities or mutations in genes required for male sex determination. The resultant XY females are often sterile or exhibit very poor fertility. We describe here embryonic stem (ES) cell growth conditions that promote the production of healthy, anatomically normal fertile and fecund female F0 generation mice completely derived from gene-targeted XY male ES cells. The sex reversal is a transient trait that is not transmitted to the F1 progeny. Growth media with low osmolality and reduced sodium bicarbonate, maintained throughout the gene targeting process, enhance the yield of XY females. As a practical application of the induced sex reversal, we demonstrate the generation of homozygous mutant mice ready for phenotypic studies by the breeding of F0 XY females with their isogenic XY male clonal siblings, thereby eliminating one generation of breeding and the associated costs.

TALEN-mediated somatic mutagenesis in murine models of cancer

Cancer genome sequencing has identified numerous somatic mutations whose biologic relevance is uncertain. In this study, we used genome-editing tools to create and analyze targeted somatic mutations in murine models of liver cancer. Transcription activator-like effector nucleases (TALEN) were designed against β-catenin (Ctnnb1) and adenomatous polyposis coli (Apc), two commonly mutated genes in hepatocellular carcinoma (HCC), to generate isogenic HCC cell lines. Both mutant cell lines exhibited evidence of Wnt pathway dysregulation. We asked whether these TALENs could create targeted somatic mutations after hydrodynamic transfection into mouse liver. TALENs targeting β-catenin promoted endogenous HCC carrying the intended gain-of-function mutations. However, TALENs targeting Apc were not as efficient in inducing in vivo homozygous loss-of-function mutations. We hypothesized that hepatocyte polyploidy might be protective against TALEN-induced loss of heterozygosity, and indeed Apc gene editing was less efficient in tetraploid than in diploid hepatocytes. To increase efficiency, we administered adenoviral Apc TALENs and found that we could achieve a higher mutagenesis rate in vivo. Our results demonstrate that genome-editing tools can enable the in vivo study of cancer genes and faithfully recapitulate the mosaic nature of mutagenesis in mouse cancer models. Cancer Res; 74(18); 5311-21. ©2014 AACR.

Highly efficient targeted mutagenesis in one-cell mouse embryos mediated by the TALEN and CRISPR/Cas systems

Since the establishment of embryonic stem (ES) cell lines, the combined use of gene targeting with homologous recombination has aided in elucidating the functions of various genes. However, the ES cell technique is inefficient and time-consuming. Recently, two new gene-targeting technologies have been developed: the transcription activator-like effector nuclease (TALEN) system, and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) system. In addition to aiding researchers in solving conventional problems, these technologies can be used to induce site-specific mutations in various species for which ES cells have not been established. Here, by targeting the Fgf10 gene through RNA microinjection in one-cell mouse embryos with the TALEN and CRISPR/Cas systems, we produced the known limb-defect phenotypes of Fgf10-deficient embryos at the F0 generation. Compared to the TALEN system, the CRISPR/Cas system induced the limb-defect phenotypes with a strikingly higher efficiency. Our results demonstrate that although both gene-targeting technologies are useful, the CRISPR/Cas system more effectively elicits single-step biallelic mutations in mice.

Switching on sex: transcriptional regulation of the testis-determining gene Sry

Mammalian sex determination hinges on the development of ovaries or testes, with testis fate being triggered by the expression of the transcription factor sex-determining region Y (Sry). Reduced or delayed Sry expression impairs testis development, highlighting the importance of its accurate spatiotemporal regulation and implying a potential role for SRY dysregulation in human intersex disorders. Several epigenetic modifiers, transcription factors and kinases are implicated in regulating Sry transcription, but it remains unclear whether or how this farrago of factors acts co-ordinately. Here we review our current understanding of Sry regulation and provide a model that assembles all known regulators into three modules, each converging on a single transcription factor that binds to the Sry promoter. We also discuss potential future avenues for discovering the cis-elements and trans-factors required for Sry regulation.

Rapid generation of mouse models with defined point mutations by the CRISPR/Cas9 system

Introducing a point mutation is a fundamental method used to demonstrate the roles of particular nucleotides or amino acids in the genetic elements or proteins, and is widely used in in vitro experiments based on cultured cells and exogenously provided DNA. However, the in vivo application of this approach by modifying genomic loci is uncommon, partly due to its technical and temporal demands. This leaves many in vitro findings un-validated under in vivo conditions. We herein applied the CRISPR/Cas9 system to generate mice with point mutations in their genomes, which led to single amino acid substitutions in proteins of interest. By microinjecting gRNA, hCas9 mRNA and single-stranded donor oligonucleotides (ssODN) into mouse zygotes, we introduced defined genomic modifications in their genome with a low cost and in a short time. Both single gRNA/WT hCas9 and double nicking set-ups were effective. We also found that the distance between the modification site and gRNA target site was a significant parameter affecting the efficiency of the substitution. We believe that this is a powerful technique that can be used to examine the relevance of in vitro findings, as well as the mutations found in patients with genetic disorders, in an in vivo system.

A site-specific, single-copy transgenesis strategy to identify 5' regulatory sequences of the mouse testis-determining gene Sry

The Y-chromosomal gene SRY acts as the primary trigger for male sex determination in mammalian embryos. Correct regulation of SRY is critical: aberrant timing or level of Sry expression is known to disrupt testis development in mice and we hypothesize that mutations that affect regulation of human SRY may account for some of the many cases of XY gonadal dysgenesis that currently remain unexplained. However, the cis-sequences involved in regulation of Sry have not been identified, precluding a test of this hypothesis. Here, we used a transgenic mouse approach aimed at identifying mouse Sry 5' flanking regulatory sequences within 8 kb of the Sry transcription start site (TSS). To avoid problems associated with conventional pronuclear injection of transgenes, we used a published strategy designed to yield single-copy transgene integration at a defined, transcriptionally open, autosomal locus, Col1a1. None of the Sry transgenes tested was expressed at levels compatible with activation of Sox9 or XX sex reversal. Our findings indicate either that the Col1a1 locus does not provide an appropriate context for the correct expression of Sry transgenes, or that the cis-sequences required for Sry expression in the developing gonads lie beyond 8 kb 5' of the TSS.