P-805 Sperm and embryo genome editing by CRISPR-Cas9 in a mouse model
Authors: De Jesus A  B , Pizzo E , Xie P , Rosenwaks Z , Palermo G 
Published: July 2024

Abstract

Study question
We wonder whether it is possible to edit a specific gene in individual mammalian spermatozoa as well as embryo by using CRISPR-Cas9?

Summary answer
We were able to successfully edit the coat pigment gene on individual spermatozoa as well as embryos by injecting CRISPR-Cas9 during Piezo-ICSI.

What is known already
Earlier investigations into embryo genome editing with CRISPR-Cas9 were conducted during the S-phase or zygote stage, encountering obstacles such as mosaicism and loss of heterozygosity. To overcome these challenges, performing genome editing at the gamete level is considered more favorable. While it may be achievable in oocytes, sperm genome editing can be ambitious due to the tightly supercoiled DNA around the protamine cores. In initial attempts, simple permeabilization of the sperm membrane allowed CRISPR-Cas9 penetration into the cell; however, the attempts at genomic editing proved to be unsuccessful.

Study design, size, duration
In the past several months, oocytes were divided into two groups: half of them were used to perform embryo genome editing, whilst the other half was used to edit exclusively the sperm genome through oocyte-mediated sperm decondensation (OMSD), where a single spermatozoon was injected into an enucleated oocyte and used the ooplasmic machinery for DNA decondensation and editing. In both models, CRISPR-Cas9 was used to knock out Tyr gene to create an albino phenotype.

Participants/materials, setting, methods
B6D2F1 mice were used to retrieve oocytes and spermatozoa. A cohort of oocytes used for the OSMD approach were enucleated, whilst the others were maintained with an intact nucleus. All oocytes, enucleated or not, were injected with a single spermatozoon with Tyr-sgRNA and Cas9 protein. Embryos were cultured until the 8-cell stage and processed for T7E1 cleavage analysis. Genome editing efficiency was calculated following the manufacturer protocol.

Main results and the role of chance
Of the 40 oocytes used for the study, 20 intact oocytes were used to perform embryo genome editing in the standard fashion, while 20 were enucleated for OMSD experiments. After undergoing Piezo-ICSI with CRISPR-Cas9 solution, 90.0% (18/20) of intact oocytes fertilized. After 48 hours in culture, diploid embryos reached the 8-cell stage at 88.9% (16/18). The 584-bp region of the target site of extracted DNA was amplified and gene modification of the edited diploid embryo cohort was confirmed in 87.5% (14/16). Compared to the OMSD cohort, enucleated oocytes fertilized at 85.0% (17/20) while cleavage rate was lower at 64.7% (11/17) and sperm genome modification at 81.8% (9/11). Both diploid embryos and haploid androgenetic embryos evidence an editing efficiency of 36.8%. Therefore, we verified that genome editing on embryos and sperm achieved the same editing yield.

Limitations, reasons for caution
While the experimental observations are still limited, it is important to note that these results in the embryo model may be underestimated by the T7E1 cleavage assay that only recognize heterozygous modification. The technique needs further refinement to optimize targeting and editing efficiency.

Wider implications of the findings
Sperm genome decondensation through an OMSD approach allows CRISPR-Cas9 to edit the sperm genome in order to correct eventual pathogenic mutations present in the gamete. Moreover, these would be the only method to absolutely ensure uniform generation of resulting embryos.

Trial registration number
not applicable