O-216 CRISPR/Cas9 mediated knock-out (KO) reveals a divergent role for trophectoderm markers GATA2/3 in the mouse and human preimplantation embryo

Study question

What is the effect of CRISPR/Cas9-mediated KO of trophectoderm (TE) markers GATA2/3 on embryo development and lineage commitment in both mouse and human preimplantation embryos?

Summary answer

CRISPR/Cas9-mediated KO of GATA2/3 points to interspecies differences in TE regulation and potential cross-talk between trophectoderm and inner cell mass (ICM) in mouse preimplantation embryos.

What is known already

GATA3 and its isoform GATA2 are major TE markers regulating the first lineage segregation, operating downstream of the HIPPO-pathway. Although recent evidence suggests that the HIPPO-pathway is conserved across mouse and human, it is unknown whether GATA2/3 share similar interspecies function during preimplantation development. GATA3 RNA depletion experiments in mouse embryos revealed a compensatory upregulation of GATA2, that potentially masked the observed phenotype. Upon double KO (DKO), the phenotype appeared more severe, as embryos were unable to cavitate. However, the precise effect of the (D)KO on embryo development was not investigated thoroughly, and should be expanded towards human preimplantation embryos.

Study design, size, duration

Mouse embryonic stem cells (mESCs), mouse zygotes and donated human spare oocytes were targeted. CRISPR/Cas9 ribonucleoprotein complexes, either targeting Gata3/GATA3, Gata2 or both, were delivered via nucleofection, electroporation or co-injected with sperm, in mESCs, mouse zygotes or human oocytes, respectively. Appropriate non-targeted control groups were included. Morphological analysis, immunofluorescence and next-generation sequencing were applied to check for gene editing efficiency and the impact of KO on embryonic development.

Participants/materials, setting, methods

The targeted embryos and controls were cultured for 4.5 (mouse) or 6.5 days (human) in vitro. They were stained for different developmental markers, including TEAD4 and CDX2 (TE), OCT4 and SOX2 (early ICM), NANOG (epibast, EPI) and SOX17 (hypoblast, PrE). Immunostaining was used to determine cell number, TE/ICM fraction, marker localization and fluorescence intensity. Embryos were subjected to genetic analysis to determine on-target efficiency, while in silico predicted off-target sites were evaluated in targeted mESCs.

Main results and the role of chance

GATA3 KO mouse embryos exhibited morula arrest (94%; n = 16). All GATA3-edited mouse embryos exhibited a reduction of CDX2-positive cells. From the 12 full KO embryos, four showed a decreased number of NANOG-positive blastomeres. No effect was observed for TEAD4 and OCT4. Complete KO morulas were devoid of SOX2 expression.

GATA2 KO mouse embryos could still form blastocysts (19% morula arrest, n = 21 embryos), even when harboring 100% frameshift mutations. KO did not noticeably influence cell number nor the expression of GATA3 or NANOG.

GATA2/3 DKO mouse embryos could still form blastocysts (38% morula arrest, n = 21), showing a milder phenotype compared to GATA3 KO embryos. In the presumed DKO blastocysts, the whole ICM is NANOG-positive and increased in cell number. SOX2 expression was still retained in the ICM, but presumed polar TE was also SOX2-positive.

Three out of four GATA3 KO human embryos harboring 100% frameshift mutations, were surprisingly able to form blastocysts. In one embryo, no morphological TE could be formed, while the others showed one or two CDX2-positive cells in TE. No effect was observed for TEAD4 and OCT4. In addition, all KO embryos displayed an increase in ICM/TE fraction. Some outer cells in KO blastocysts were NANOG-positive.

Limitations, reasons for caution

CRISPR/Cas9 is limited by the occurrence of mosaicism (more than one genotype present in an embryo) and potential off-target editing, which we will assess at in silico predicted off-target sites via NGS in mESCs. The observations of the study will be consolidated by increasing the sample size, especially in human.

Wider implications of the findings

Gene editing studies enable us to unravel the molecular interactions that are required for human preimplantation development. Obtaining novel insights into the molecular networks of the GATA transcription factor family could significantly improve our understanding of several pregnancy-related complications related to trophectoderm specification, such as early miscarriage or preeclampsia.

Trial registration number

NA

P-457 Spindle transfer rescues poor embryo development of in vitro matured ovarian tissue oocytes from transgender men

Study question

Could collection temperature and spindle transfer (ST) potentially improve development of embryos derived from in vitro matured (IVM) ovarian tissue oocytes (OTO) of transgender men?

Summary answer

Spindle transfer, but not collection temperature, significantly improved embryo development of OTO-IVM oocytes from transgender men.

What is known already

For transgender men, the fertility preservation strategy of ovarian stimulation may interfere with the desired masculine characteristics and enhance gender dysphoria. Alternatively, ovarian tissue oocytes collected ex vivo could serve as potential gametes, not requiring ovarian stimulation. Oocytes can be collected during gender affirming surgery, matured, and vitrified. Ovarian tissue oocyte in vitro maturation (OTO-IVM) has successfully been used for cancer patients, as live births have been reported. OTO-IVM in transgender men demonstrated sufficient maturation rates and survival following vitrification. Nevertheless, a decreased fertilization potential of these oocytes and severely compromised embryonic development have been observed.

Study design, size, duration

Patients between 18-24 years were recruited for this study from November 2020 to September 2021. Ovaries from 14 transgender men were collected in either cold (4oC, OTO-Cold) or warm (37oC, OTO-Warm) collection medium, to verify the best collection method. Following ovarian manipulation, cumulus oocyte complexes (COCs) were harvested from spent medium and underwent maturation for 48hrs. ST was performed to overcome inferior fertilization and embryonic development.

Participants/materials, setting, methods

Injected IVM oocytes underwent calcium imaging or were monitored for embryonic developmental potential. In vitro matured GV (germinal vesicle), MI (metaphase I) and in vivo matured oocytes with clusters of smooth endoplasmic reticulum (SERa) served as controls and cytoplasmic recipients for ST. OTO-IVM or control oocytes were used as spindle donors (ST-OTO or Control-ST respectively). Genetic analysis was performed to detect chromosomal abnormalities in embryos from all groups.

Main results and the role of chance

In total, we collected 252 OTO-Cold and 230 OTO-Warm oocytes, showing similar maturation rates (53%). For calcium imaging, 39 control, 33 OTO-cold and 31 OTO-warm oocytes were analysed, determining the product of amplitude per frequency, in arbitrary units (AU). The average value for control oocytes was 2.30AU, significantly higher than OTO-Cold (1.47AU, p=0.046) and OTO-Warm oocytes (1.03AU, p=0.036). Calcium release was similar between OTO-Cold and OTO-Warm oocytes. Following ICSI, 19/47 OTO-Cold and 24/48 OTO-Warm oocytes normally fertilized, significantly lower than the control group (42/52) (p < 0.001 and p = 0.001 respectively). Blastocyst formation was significantly higher in control oocytes (13/42,31%) when compared to OTO-Cold (1/19, p=0.027) and OTO-Warm (2/24, p=0.035). No statistically significant difference in fertilization rate and embryo development was detected between OTO-Cold and OTO-Warm oocytes. ST was performed to overcome poor embryo development in the OTO-Cold group. Following ST, 12/19 ST-OTO-Cold and 24/38 Control-ST oocytes were normally fertilized. Blastocyst development was similar between the two groups (4/12 and 7/24 respectively), but significantly higher than blastocyst development in OTO-Cold ICSI oocytes (p = 0.038 and p = 0.045). Genetic analysis revealed that 4/10 OTO-Cold, 4/11 OTO-Warm, 4/11 ICSI control, and 4/7 Control-ST embryos were chromosomally abnormal while 6/8 OTO-ST were abnormal, and 2/8 showed a suggestive low-grade mosaicism.

Limitations, reasons for caution

A major limitation of our study is the lack of ovaries from cis women. Control oocytes used in this study originate from infertility patients that underwent ovarian stimulation. High abnormality rate in ST-OTO embryos might be concerning for the safety of ST, but the number of embryos analysed is limited.

Wider implications of the findings

Our data indicate that OTO-IVM oocytes from transgender men display poor cytoplasmic quality, demonstrated by embryonic arrest and calcium imaging. ST was able to overcome poor embryo development, and it could be of interest to use freshly donated oocytes as cytoplasmic recipients for this.

Trial registration number

Not applicable