Method for generating genome-edited chickens

A method for generating a genome-edited chicken is provided, relating to the technical field of genetic engineering. Ovarian injection in situ is conducted on a hen, where a gene editing reagent is injected into ovarian medulla of the hen that is close to laying eggs, such that the exogenously-injected gene editing reagent can enter developing ovarian follicles through blood circulation. In resulting Go individuals, a chimera chicken with both somatic cells and germ cells edited is successfully and efficiently obtained, with an editing efficiency of the Go individuals reaching up to 36.36%. Compared with a traditional primordial germ cell (PGC)-mediated method, the ovarian injection in situ is time-saving and labor-saving, convenient and rapid, low-cost, and highly safe.

REFERENCE TO SEQUENCE LISTING

A computer readable XML file entitled “GWPCTP20240705146-sequence listing”, which was created on Dec. 6, 2024, with a file size of about 61,196 bytes, contains the sequence listing for this application, has been filed with this application, and is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the technical field of genetic engineering, and in particular to a method for generating gene-edited chickens.

BACKGROUND

As an oviparous animal, chicken embryonic development is divided into two stages: in vivo development and in vitro development. When a hen reaches sexual maturity, the primary follicles in the ovary develop rapidly and gradually increase in size. According to the diameter of the ovarian follicles, the follicles can be divided into prehierarchal follicles such as small white follicles, large white follicles, and small yellow follicles, as well as preovulatory follicles from the first largest follicle with diameter of 40 mm named F1 to the sixth largest follicle with diameter of 10 mm named F6. After ovulation, the F1 follicles are received by the infundibulum of the oviduct and the ovum finishes fertilization at the infundibulum. Then, the yolk carrying the zygote migrates in the oviduct, stays in the magnum for about 4 h to be covered by egg white, stays in the isthmus for about 1 h to complete the deposition of inner and outer eggshell membranes, and then the zygote begins the first mitosis in the isthmus. Next, the egg enters the shell gland and stays here for about 18 h to 19 h to finish eggshell mineralization and pigment deposition. Lastly, the egg is laid from the vagina. At this time, the chicken embryo reaches to blastocyst stage, and including approximately 55,000 cells, specifically a germinal disc with a diameter of about 3.5 mm visible on the surface of the yolk. If fertilized eggs are stored at a low temperature of 16° C., the embryonic development diapauses; if the fertilized eggs are placed back in incubation conditions at 37.8° C., the embryo can restart its development. After a 21-day incubation process, the chicks can hatch.

Since chickens have the above-mentioned unique reproductive physiological characteristics and early zygotes are not easy to obtain, methods applicable to mammals such as micromanipulation of zygotes or somatic cell nuclear transplantation to produce genome-modified individuals cannot be implemented in the chickens. Currently, genome-edited chickens are mainly produced by primordial germ cell (PGC)-mediated method. This method has high requirements on the experimenters and strict demand on laboratory environment. The culture of PGC cells is difficult, the transfection is inefficient, and the transplantation to chicken embryos is also a big challenge, which needs skilled techniques and rich experiences. In addition, due to the competition of endogenous PGCs, the G0 germline transmission efficiency is variable and low (generally less than 10%), and it is spend at least 18 months to obtain homozygous mutants in G2 offspring, which is time-consuming and labor-intensive. At present, the PGC-mediated method can generate sterile surrogate chicken strains. The endogenous PGCs of sterile surrogate chicken embryos with knock-in inducible lethal gene on the DAZL gene locus can be ablated by adding drugs. Then, the injected exogenous donor PGCs can transplant to the gonads of chicken embryos host. Hence, the application of the sterile surrogate chicken can not only improve the G0 germline transmission efficiency, but also shorten to G1 offspring to obtain homozygous mutants. However, the surrogate hosts are transgenic chickens, which poses a hidden danger to biosafety.

SUMMARY

In order to solve the above problems, the present disclosure provides a method for generating a gene-edited chicken. In the present disclosure, ovarian injection in situ of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) ribonucleoprotein (RNP) is conducted on a hen (G−1) through a new approach that is independent of PGC-mediated methods, such that a chimeric chicken with edited somatic cells and gonads can be efficiently obtained in a G0 generation. This is a new method for generating gene-edited chickens that is simple, convenient, safe, and efficient.

The present disclosure provides a method for generating gene-edited chickens, including the following steps: injecting a gene editing reagent into ovarian medulla of a hen to obtain a G−1 hen; wherein the hen that will lay the first egg after injection of 10 to 15 days are chosen to be operated, and the gene editing reagent includes CRISPR/Cas9 RNP; crossing the G−1 hen with wild-type roosters to obtain fertilized eggs, and hatching the fertilized eggs artificially to obtain a G0 population; and detecting gene editing results in the G0 population to obtain the gene-edited chickens.

Preferably, the reagent for gene editing using CRISPR/Cas9 RNP includes Cas9 proteins, guide RNAs (gRNA), and buffers; the Cas9 proteins and the gRNAs are at a molar ratio of 1:(1-2); and the Cas9 proteins are injected at a concentration of 3.5 μg/μL.

Preferably, the buffers include 20 mM of 4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid (HEPES) and 500 mM of NaCl.

Preferably, the Cas9 proteins include Cas9 protein obtained by using prokaryotic expression system.

Preferably, an editing efficiency of the gRNA is detected using a chicken tool cell line before the gene editing reagent is injected, and then a gRNA with an editing efficiency greater than or equal to 45% is selected.

Preferably, the chicken tool cell line includes DF-1 cells.

Preferably, the hen used to inject with the gene editing reagent are fasted on the day of injection.

Preferably, the method further includes: cockerels and hens among the gene-edited chickens are mated to each other to obtain homozygous mutants.

Preferably, a process of detecting the gene editing result includes steps 1) and/or 2):

Preferably, a process of detecting in 2) includes PCR amplification and/or sequencing.

Beneficial effects: ovarian injection in situ is conducted on a hen, where the gene editing reagents are injected into ovarian medulla of the hen that is close to laying eggs, such that the injected exogenous gene editing reagent can enter developing ovarian follicles through blood circulation. In G0 offspring, chimera chickens with both somatic cells and germ cells edited are successfully and efficiently obtained, with the G0 editing efficiency reaching up to 36.36%. Compared with the traditional primordial germ cell (PGC)-mediated method, the ovarian injection in situ is time-saving and labor-saving, convenient and rapid, low-cost, and highly safe. The specific advantages are reflected in the following aspects: (1) Since the G0 gene-edited chickens obtained by ovarian injection in situ are somatic cell chimeras, mutant individuals can be identified within one week after hatching by extracting genomic DNA from blood, thus shortening the time of identification of mutant individuals. (2) Since the G0 gene-edited chickens obtained by ovarian injection in situ are somatic chimeras, they can show the mutant phenotype of functional genes immediately in the G0 individuals. This makes it possible to determine in advance whether the mutation affects its normal function of the target gene, and is of great significance for the research of novel genes and accelerates the research progress of functional genes. (3) Mutants can be found in both G0 roosters and hens obtained by ovarian injection in situ. After the mutants mature, they are mated with each other, and homozygous mutant individuals can be obtained in the G1 offspring, thus shortening the generation cycle to 8 months. For the individuals, TA cloning results of genomic DNA have showed that the individual mosaic rate is 8.33% to 52.94%, and a target gene site is also detected to be edited in the rooster' semen (FIG. 2). The above results show that there is a high mosaic ratio of G0 individuals, and it is easier to obtain heterozygotes or homozygotes in the G1 offspring. (4) Since the injected gene editing reagent is Cas9 RNP, the Cas9 protein can be degraded after taking effect. In this way, gene editing events can occur without introducing exogenous genes and contaminating the chicken's own genome, thus ensuring biosafety. (5) Furthermore, the method bypasses the process of culturing and transfecting PGCs. After verifying the editing efficiency of the gRNA of target gene at the DF-1 cell line, only the Cas9 protein needs to be expressed and purified and the gRNA needs to be transcribed in vitro before surgical injection. Therefore, the method has a relatively mature process and a simple surgical procedure and is easy to learn. Only a universal animal anesthesia machine, surgical instruments, and 1 mL of a disposable sterile syringe are required to complete the ovarian injection in situ. (6) In addition, the costs of the method are mainly universal animal anesthesia machine, gRNA in vitro transcription kit, identification of the mutants and their feeding making the costs much lower than those of the PGC-mediated method.

In summary, the ovarian injection in situ provided by the present disclosure is expected to replace the traditional PGC-mediated method for generating the gene-edited chicken. The method of the present disclosure can be applied to non-professional laboratories for the study of functional genes, thereby accelerating the research process of the gene-edited chicken.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a method for generating gene-edited chickens, including the following steps: injecting a gene editing reagent into ovarian medulla of a hen to obtain a G−1 hen; where the hen that will lay the first egg after injection of 10 to 15 days are chosen to be operated; the genome editing reagent includes a reagent for gene editing using CRISPR/Cas9 RNP; crossing the G. 1 hen with wild-type roosters to obtain fertilized eggs, and hatching the fertilized eggs artificially to obtain a G0 population; and detecting gene editing results in the G0 population to obtain the gene-edited chickens.

In the present disclosure, a gene editing reagent is injected into ovarian medulla of a hen 10 days to 15 days before laying the first egg to obtain a G−1 hen. The gene editing reagent includes a reagent for gene editing using CRISPR/Cas9 RNP; the reagent for gene editing using CRISPR/Cas9 RNP preferably includes a Cas9 protein and a gRNA; the Cas9 protein is preferably injected at a concentration of 3.5 μg/μL; the Cas9 and the gRNA are at a molar ratio of preferably 1:(1-2), more preferably 1:1. When injecting Cas9 RNP, a buffer (Storage Buffer) preferably includes the following components at the final concentrations: 20 mM of HEPES and 500 mM of NaCl. On the day of injection, the Cas9 protein and the gRNA are preferably prepared immediately after thawing, and the reagent is prepared in a centrifuge tube, incubated at 37° C. for 5 min, and then transferred into a disposable sterile syringe to ensure that the Cas9 RNP is injected into the ovary of the hen within 1 h. If there are two or more gRNAs, each of the gRNAs is preferably incubated with an equal volume of the Cas9 protein to form RNPs and then mixed and transferred into a disposable sterile syringe for ovarian injection.

In the present disclosure, a candidate gRNA is preferably designed according to the target gene. The activity of the candidate gRNA is preferably identified on a chicken tool cell line, and a gRNA with an editing efficiency of not less than 45% is preferably selected to ensure that the target gRNA used for ovarian injection in vivo can achieve gene editing. This step is to screen for gRNAs with higher activity at the cellular level. The chicken tool cell line preferably includes a DF-1 cell line.

In the present disclosure, a large amount of the gRNA is preferably prepared by in vitro transcription. Two 10 μL in vitro transcription reaction systems can meet the gRNA injection requirements for one chicken. Preferably, the Cas9 protein is prepared by prokaryotic expression and the Cas9 protein is purified by Ni-NTA affinity chromatography, which is not only low-cost but also allows for designing different forms of Cas9 fusion proteins as needed.

In the present disclosure, after obtaining the Cas9 protein and the gRNA, in vitro cleavage is preferably conducted to verify the activity of the Cas9 protein and the gRNA. There are certain requirements for the age and ovarian development status of the hens to be injected. The hens that are close to laying eggs (hens 10 days to 15 days before laying eggs), about 15 to 17 weeks old, are selected depending on the breed of the hens selected. The advantages of selecting hens that are close to laying eggs are: 1) the ovarian follicles of hens close to laying have not yet developed into pre-ovulatory ovarian follicles. When the ovarian injection is conducted, there may be no pre-ovulatory ovarian follicles hindering operative view formed by the surgical incision, making it easy to find the location of the ovaries and determine the position when injecting exogenous materials, which is conducive to the injection operation. 2) When the ovarian follicles of hens close to laying egg, the ovary develops into small yellow follicles and large yellow follicles, the oocyte enter into the rapid yolk deposition phase. The injected exogenous protein is more easily taken up by the ovarian follicles and the G0 editing efficiency can be improved at this time.

In the present disclosure, the injection (ovarian injection in situ) is preferably conducted in the morning and hens to be operated on are preferably fasted on the same day of the operation. This is to prevent the full intestine from hindering the surgical view formed by the incision and to prevent accidents during the operation. The specific method is preferably step 7 in Example 1. The injection has low operating threshold and can be conducted by any technician in the field without the need for professional personnel to operate.

In the present disclosure, after a G−1 hen is obtained and the G−1 hen lays the first egg, the G. 1 hen is inseminated artificially to obtain fertilized eggs. The fertilized eggs are artificially hatched to obtain the G0 population. The injection on the hens does not affect the fertility rate and hatchability rate of the eggs.

In the present disclosure, after obtaining the G0 population, gene editing events in the G0 population are detected to obtain the gene-edited chicken. A process of detecting the gene editing events preferably includes: after hatching, detecting the offspring of the operated G−1 hens at both phenotypic and genetic levels. Phenotypically, the phenotype of the individuals in the G0 population is preferably observed based on mutation effect produced by the modified functional gene. At the genetic level, blood is preferably collected from 1-day-old chicks by the carotid artery blood sampling to extract genomic DNA, and the gRNA site of the target gene is amplified by PCR. The gene-edited individual is identified using LabChip® GXII Touch microfluidic capillary electrophoresis system, and the HT DNA High Sensitivity Labchip® and supporting reagents are preferably used. The Indel types of the edited individuals are detected using TA cloning and other methods, and the edited individual is confirmed to be kept for breeding. Thus, a chimeric chicken is obtained in the G0 individuals, in which both somatic cells and germ cells are edited.

In the present disclosure, the method further preferably includes: mating G0 roosters with G0 hens each other among the gene-edited chicken to obtain homozygous gene-edited chickens.

In order to further illustrate the present disclosure, the method for generating gene-edited chickens provided by the present disclosure will be described in detail below in conjunction with accompanying drawings and examples, but they should not be construed as limiting the protection scope of the present disclosure.

In nature, the loss of a single copy of the IHH gene causes the Xingyi bantam chicken to exhibit a creeping phenotype, characterized by pronounced chondrodystrophy characteristic with short shanks and small wings. This mutation can be identified in the embryonic stage. Homozygous deletion of the IHH gene can cause chicken embryo death during early embryonic development, indicating that mutation of the IHH gene can affect embryonic development and exhibit an easily observed mutant phenotype. The IHH gene as a target gene facilitates to screen edited individuals from a phenotypical perspective.

A construction process of the pX330-dual gRNA knockout plasmid included: (1) when a pX330-dual gRNA knockout vector was first constructed, sequences that could transcribe the dual gRNAs were obtained by gene synthesis, specifically:

The sequences shown in SEQ ID NO: 4 and SEQ ID NO: 5 were cloned into a pUC57 vector separately, and the vector was used as a template for PCR amplification of dual gRNA.

The editing efficiency of different gRNA combinations was detected by 1.5% agarose gel electrophoresis, and the results are shown in FIG. 1C. The fragment deletion efficiency of the gRNA-R1 and gRNA-R3 combination was 48.4%, while the fragment deletion efficiency of the gRNA-R1 and gRNA-R2 combination was 23.8%. Therefore, the gRNA-R1 and gRNA-R3 combination with a higher editing efficiency was selected for ovarian injection in vivo.

5′-TAATACGACTCACTATAGGG-3′ (SEQ ID NO: 14) in the upstream primer was the T7 promoter sequence, AAAAAA in the downstream primer was the polyA tail sequence as a transcription termination sequence, and 5′-gcaccgactcggtgccac-3′ (SEQ ID NO: 15) was a partial sequence of the gRNA scaffold.

The PCR product was purified to serve as DNA templates for gRNA in vitro transcription. According to the instructions of a MEGAshortscript™ T7 transcription kit (THERMO FISHER SCIENTIFIC, catalog number AM1354), the gRNA was purified using phenol-chloroform method and the obtained gRNA was dissolved in nuclease-free water. The concentration and OD value of gRNA were measured by NanoDrop™ 2000, where the concentration of gRNA is (1-5) μg/μL, and could satisfy the injection requirements.

The pET28a-Cas9-2NLS prokaryotic expression vector was transformed using the BL21(DE3) Escherichia coli strain and then plated onto LB agar plates containing 50 g/ml kanamycin and incubate at 37° C. overnight. Single clones were picked and transferred to 20 mL of 2×YT medium containing 50 g/ml kanamycin, and then incubate at 37° C. with shaking at 200 rpm overnight. The culture was transferred into 1 L of 2×YT medium containing 50 g/ml kanamycin and was continue to be shaken at 200 rpm and 37° C. until OD600 value reaches about 0.6. Then the culture was allowed to return to room temperature for 30 min, and 0.5 M IPTG solution was added to make a final concentration of 0.5 mM. The culture was then incubated at 16° C. and 120 rpm for 20 h. The obtained culture was centrifuged at 8,000 g for 10 min at 4° C. to harvest bacterial cell pellet, and the bacterial cell pellet were added with a lysis buffer (20 mM Tris-HCl, 500 mM NaCl, 10% glycerol, 0.1% Trixon λ-100, 1 mg/mL lysozyme, and 1 mM PMSF) to lyse the cells by sonication, and then centrifuged at 12,000 rpm and 4° C. for 30 min to collect a supernatant, which contained soluble Cas9-2NLS proteins.

The supernatant was combined with Ni Sepharose™ 6 Fast Flow filler (CYTIVA) at 4° C. for 1 h and eluted with 10, 20, 50, 100, and 250 mM imidazole buffers separately. The 250 mM imidazole buffer (20 mM Tris-HCl, 500 mM NaCl, and 250 mM imidazole) was collected and a 50-kDa MWCO concentration column was used to exchange the buffer of the purified protein with storage buffer. When the volume of the solution in the concentration column was less than 500 μL, the protein storage buffer (20 mM HEPES, 500 mM NaCl) was added to exchange the imidazole buffer, and centrifuged at 4,000 g and 4° C. until the volume of the solution was less than 1 mL. The solution was transferred into a 1.5 mL centrifuge tube and the concentration of Cas9-2NLS protein was determined with the Bradford assay. The protein was aliquot according to the amount used for each chicken, and then quickly frozen in liquid nitrogen and stored at −80° C. Generally, 1 clone was inoculated into 1 L of 2×YT medium, and the Cas9 protein produced by the cultured bacterial cells could satisfy the injection dosage of 2 to 3 hens.

Sample collection and DNA extraction: leg tissue or gonad tissue of chicken embryos was obtained on the 12.5 embryonic days to detect gene editing events. Blood was collected from 1-day-old chicks by carotid artery method after hatching. Genomic DNA was extracted using the cell/tissue/blood genomic DNA extraction kit (TIANGEN Biotech (Beijing) Co., Ltd.). The leg tissue or blood was added to 200 μL of GA solution, and then ground using a homogenizer after adding grinding beads. 220 μL of a mixed solution of proteinase K and GB solution was added; and after sufficient oscillation, the resulting mixture was bathed in 70° C. water and incubated for 2 h to 3 h, and the subsequent genomic DNA extraction operation was conducted according to the instructions.

PCR amplification: detection of gene editing events was conducted on individuals with obvious mutation phenotypes. The extracted genomic DNA was used as a template and PrimeSTAR® HS DNA Polymerase with GC Buffer (TAKARA) was used to amplify the IHH-gR3 target site using specific primers. The specific primer sequences were: F: 5′-gcccttcgctatttattg-3′ (SEQ ID NO: 21); R: 5′-cgtgagctccttgaagcg-3′ (SEQ ID NO: 22). A LabChip™ GXll Touch microfluidic capillary electrophoresis system was used to identify whether mutations occurred.

TA cloning: PrimeSTAR® HS DNA Polymerase with GC Buffer (TAKARA) was used to amplify the genomic DNA of the individuals with mutations, where primer pairs included F: 5′-gcccttcgctatttattgccgc-3′ (SEQ ID NO: 23); R: 5′-ggataaactcgctgctctgccca-3′ (SEQ ID NO: 24). A PCR product was purified, and a corresponding system was prepared using pEASY®-Blunt Cloning Kit (Beijing TransGen Biotech Co., Ltd.), and incubated at 37° C. to allow cloning. The colones were picked for Sanger sequencing and the sequencing results were aligned with the reference sequence. The partial results are shown in Table 1.

Detection results of TA cloning

Individual No.
No. of TA clones
No. of mutations
Mutation ratio

Table 1 shows that the individual mosaicism rate was at 8.33% to 52.94%.

G0 editing efficiency of ovarian injection in situ of Cas9 RNP

targeting chicken IHH gene

Hen 
of
unfertilized
fertilized
Fertility 
G0
editing

No.
eggs
eggs
eggs
rate
mutants
efficiency

Individual detection of chicks: some of the fertilized eggs were incubated until they hatched, and some of the chickens were raised until they reached sexual maturity. Currently, 4 chimeras were obtained, including 3 hens and 1 rooster. After detection, the blood of the hens and the semen of the roosters were edited (FIG. 2) and left for mating with each other to obtain next generation.

Comparative Example 1

In situ injection of the pX330 gene-editing plasmid into the ovaries of hens could also obtain chimeras in the G0 individuals in which both somatic cells and germ cells were edited, but the editing efficiency in the G0 individuals was low, at 1.71%. Since pX330 contained the Cas9 gene sequence, G0 individuals could carry the Cas9 gene. The specific plan was as follows:

PCR amplification of IHH target gene: for Cas9 positive individuals, PrimeSTAR® HS DNA Polymerase with GC Buffer was used to amplify the sequence including the two target sites with specific primers. Agarose gel electrophoresis was used to detect whether the pX330 plasmid injected into the ovaries in vivo could produce fragment deletions. The primer sequences were as follows: for the treatment of hen ovary injection of pX330-gRNA-(R2+R1) plasmid, the sequences of the detection primer pair are shown in SEQ ID NO: 9 and SEQ ID NO: 10; for the treatment of hen ovary injection of pX330-gRNA-(R3+R1) plasmid, the sequences of the detection primer pair are shown in SEQ ID NO: 21 and SEQ ID NO: 22.

T7E1 assay detection: for Cas9-positive individuals, the editing events of a single gRNA site were detected using a T7E1 assay. The fragments containing the target sites gRNA-R1, gRNA-R2, and gRNA-R3 were PCR amplified using genomic DNA as a template and the corresponding specific primers. The amplified products were digested with T7E1 enzyme, and the results of 1.5% agarose gel electrophoresis was performed color invert using ImageJ software to analyze the mutant ratio. Primer sequences are shown in Table 3.

Primer sequences for T7E1 assay

Primer name
Primer sequence (5′-3′)
SEQ ID NO:
Product length (bp)

TA cloning: TA cloning in Example 1 was conducted on the mutant individuals confirmed by T7E1.

G0 editing efficiency of ovarian injection in situ of pX330 plasmid targeting chicken IHH gene

dead
individuals

Dosage
No. of
Injection
No.
of

embryos
carrying
No. of G0

As shown in FIG. 4, 14 G0 chicks or embryos were detected to carry the Cas9 gene, of which 11 were from the 100 μg/hen treatment and 3 were from the 200 μg/hen treatment, indicating that the efficiency of G0 offspring carrying the Cas9 gene was higher when the injection dose was 100 μg/hen.

In the treatments with plasmid injection doses of 100 μg/hen and 200 μg/hen, gene editing events was detected in 8 (2.62%) and 2 (0.72%) individuals, respectively, indicating that the injection dose of 100 μg/hen had a higher efficiency in the G0 offspring. In the treatment of pX330-gRNA-(R3+R1) plasmid injection, 5 individuals were gene edited, among which 2 unhatched embryos (A16-3-4, A25-3-5) had obvious fragment deletions, 1 embryo (6100) had a mutation at the gRNA-R1 target site, and 2 chicks (50981, 50980) had a mutation at the gRNA-R3 target site. In the treatment of pX330-gRNA-(R2+R1) plasmid injection, gene editing occurred in 5 individuals, of which 2 chicks (60935, 50976) had fragment deletions, and 4 chicks (60935, 60957, 50954, 50955) had mutations at the gRNA-R1 target site (FIG. 4 and Table 4). The above results showed that increasing the plasmid dosage could not significantly improve the Cas9 positivity rate in the G0 offspring, and the choice of gRNA combination could affect the editing efficiency of fragment deletion, and not both gRNAs could work at the same time.

In order to detect whether the Cas9 gene also existed in other tissues, 9 tissues including heart, liver, spleen, lung, kidney, breast muscle, leg muscle, comb, and gonad were collected from 3 chicks (50980, 50981, and 50955), and genomic DNA was extracted and the Cas9 gene was amplified. The results are shown in FIG. 5A, where pX330 plasmid and H2O were used as DNA templates for positive and negative controls, respectively. The Cas9 gene was detected in the heart, spleen, lung, comb, and gonad of chicks 50980 and 50981 (FIG. 5A), indicating that the 2 chicks were chimeras; while the Cas9 gene was not detected in the tissues of chick 50955. Since the gRNA-R3 target site was edited in the blood of individuals 50980 and 50981, in order to evaluate whether the editing could be inherited to the next generation (G1), the gonads of these 2 chickens were used to detect whether the gRNA-R3 and gRNA-R1 target sites were mutated. The results of T7E1 assay showed that only the gRNA-R3 target site was mutated in these 2 individuals, which was consistent with the detection results in the blood (FIG. 5B). The Sanger sequencing results also showed that these 2 individuals had a 3-bp deletion in the signal peptide sequence upstream of the gRNA-R3 target site, resulting in loss of the 10th amino acid leucine (FIG. 5C), which might affect the secretion of IHH protein. These results suggested that it was feasible to create gene-edited chickens by ovarian injection in situ, but the editing efficiency required to be further improved.

Compared with Example 1, except for the different Cas9 proteins used for ovarian injection, the other steps were the same. A construction process of the pET28a-4NLS-Cas9-2NLS prokaryotic expression vector included:

The protein purification of 4NLS-Cas9-2NLS was the same as that of Cas9-2NLS in Example 1.

The effects of ovarian injection in situ of 4NLS-Cas9-2NLS RNP are shown in Table 5.

G0 editing efficiency of injection of 4NLS-Cas9-2NLS RNP

targeting chicken IHH gene

Hen
of
unfertilized 
fertilized 
Fertility
G0
editing

No.
eggs
eggs
eggs
rate
mutants
efficiency

Tables 1 and 5 show that Cas9 protein was not limited to proteins using 2 nuclear localization signals, and similar effects could be achieved using 4NLS-Cas9-2NLS.

A method similar to Example 1 was adopted, except that Cas9 RNP was prepared as follows: 41.27 mg of chloroquine diphosphate (SIGMA ALDRICH) was dissolved in 1 mL of Cas9 protein buffer (20 mM HEPES, 500 mM NaCl) to a concentration of 80 mM. When preparing Cas9-2NLS RNP, the corresponding volume was calculated such that the final concentration of chloroquine diphosphate was 2 mM, and the protein was prepared immediately before use and placed at room temperature away from light. The editing results are shown in Table 6.

G0 editing efficiency of injection of Cas9-2NLS RNP (2 mM

Hen 
of
unfertilized
fertilized
Fertility
G0
editing

No.
eggs
eggs
eggs
rate
mutants
efficiency

As shown in Table 6, when an endosomal escape agent such as chloroquine diphosphate was added to the buffer during the preparation of Cas9 RNP, the G0 editing efficiency was not affected.

A method similar to Example 1 was adopted, except that Cas9 protein used for ovarian injection was a recombinant fusion protein of Cas9 and fluorescent protein expressed by the pET28a-Cas9-mNG prokaryotic expression vector. A construction process of the pET28a-Cas9-mNG prokaryotic expression vector included:

The protein purification of Cas9-mNG was the same as that of Cas9-2NLS in Example 1.

The effects of ovarian injection in situ of Cas9-mNG RNP are shown in Table 7.

G0 editing efficiency of ovarian injection in situ of Cas9-mNG

RNP targeting chicken IHH gene

Hen
of
unfertilized 
fertilized 
Fertility
of G0
editing

No.
eggs
eggs
eggs
rate
mutants
efficiency

As shown in Table 7, the Cas9 protein was fused with a fluorescent protein, such as mNeongreen, to form the Cas9-mNG protein that could also achieve similar gene editing effects.

Although the present application has been described in detail through the above examples, the examples are merely some rather than all of the examples of the present application. All other examples obtained by a person based on these examples without creative efforts shall fall within the protection scope of the present application.