P-702 Secretion of beta-human chorionic gonadotropin (b-hCG) by post-implantation embryos differs according to different chromosomal status.
Authors: Palomar A , De la Rosa Carracedo P , Quiñonero A , Florensa M , Mercader A , Domínguez F , Sanchez-Ribas I 
Published: June 2023

Abstract

Study question
To evaluate secretion of b-hCG hormone by human post-implantation embryos cultured in-vitro up to day 10 according to their chromosomal status.

Summary answer
Secreted b-hCG levels differs among embryos with different chromosomal load, being severely diminished in monosomic embryos when compared either to euploid or trisomic embryos.

What is known already
Exploring early post-implantation embryo development is subject of growing interest because errors in correct embryo development at this period might cause early pregnancy losses or development of pathologies affecting postnatal health. Down's Syndrome (DS) is one of the most common examples of chromosomal supernumerary (trisomy 21; T21) aneuploidy that leads to live birth but associated to pathology in postnatal life while monosomy of chromosome 21 (M21) is more likely to cause embryo developmental arrest. Furthermore, conclusions extracted from our previous studies put forward the idea of different gene and metabolite expression according to different chromosomal load (Sanchez-Ribas et al., 2019).

Study design, size, duration
This is an exploratory comparative study which includes a total of 33 supernumerary science-donated human blastocysts on day 6 from in-vitro fertilization cycles followed by preimplantation genetic test for aneuploidies. Euploid (n = 10), pure T21 (n = 13) and pure M21 (n = 10) were included in the study. All day 6 blastocyst included were good-quality embryos and were initiating hatching at the time of vitrification. Embryos degenerated during culture were not used for b-hCG quantifications.

Participants/materials, setting, methods
Euploid, T21 and M21 day blastocysts were thawed and cultured in-vitro until day 10. Embryos were cultured in culture medium specifically designed to support post-implantation embryo development. Each day of culture, half volume of culture media was replaced by fresh media. Levels of b-hCG in conditioned embryo media were quantified by ELISA immunoassay at day 7, 9, and 10. Results are expressed as mean ± standard deviation. Embryo morphology was tracked by bright-field images.

Main results and the role of chance
All euploid and T21 embryos thawed managed to hatch completely and expand the blastocoel cavity, while 40% of M21 embryos failed to hatch being arrested inside the zona pellucida on day 7. On day 7, mean concentration of b-hCG secreted did not differ among euploid, T21 and M21 embryos (37,29±6,27; 36,05±6,61 and 27,16±9,95 ng/mL respectively). Interestingly, euploid and T21 embryos reported a progressive time-dependent increase in b-hCG levels along post-implantation in-vitro culture while secreted levels of b-hCG by M21 embryos showed a decreasing trend, reporting diminished b-hCG secretion at the end of the culture (day 10) compared to day 7 (14,86±2,57 vs 27,16±9,95 ng/mL; p &lt; 0.05). No significant differences in b-hCG secretions were observed between euploid and T21 embryos neither on day 9 d.p.f. (61,05±8,47 and 56,08±8,10 ng/mL respectively) nor on day 10 (74,88±6,36 and 71,75±7,01 ng/mL respectively). However, b-hCG secreted by M21 embryos on day 9. and day 10 (27,11±11,45 and 14,86±2,57 ng/mL respectively) were significantly reduced (p &lt; 0.01) compared with euploid or T21 embryos. Besides, while all euploid and T21 embryos developed until day 10, 16,67% of expanded M21 embryos degenerated before day 9.

Limitations, reasons for caution
This research focused only on embryo development, no maternal tissues were considered. Further proteins should be investigated to reveal other pathways implicated in healthy embryo development and not only trophoblast proliferation.

Wider implications of the findings
Monosomic embryos, associated to implantation failures, secrete lower b-hCG when compared to either euploid or trisomic embryos, which show similar behavior. Besides, b-hCG in culture media could become a non-invasive marker to track embryo progression along post-implantation stages and detect embryos with increased likelihood of developmental arrest or early miscarriage.

Trial registration number
Not Applicable