P-518 Differentially gene expression identified by RNA seq in aneuploid human blastocysts is more influenced by the phenomenon of aneuploidy than the involved chromosome
Authors: Bazrgar M , Khajehoseini F , Eftekhari-Yazdi P , Gourabi H , Pazhoomand R , Hosseinishenatal S , Mohammadi R 
Published: July 2024

Abstract

Study question
What insights do RNA-Seq profiles provide about gene expression in aneuploid blastocysts with small and large chromosomes compared to euploid blastocysts?

Summary answer
The difference in gene expression is affected by aneuploidy and is more related to the presence of aneuploidy than the size of the aneuploid chromosomes.

What is known already
Embryonic aneuploidy is one of the main common causes of implantation failure and miscarriage. RNA-Seq investigations of the embryo in the early stages of preimplantation indicate that with the gradual replacement of the embryonic profile and the reduction of the oocyte residues, the gene expression profile in the blastocyst is more dependent on the embryo’s genome. A recent study by comparing the gene expression profile of euploid blastocysts and specific aneuploidies such as trisomy 21, trisomy 18, XXY and XO syndromes using RNA-Seq, showed how the type of aneuploidy might affect gene expression.

Study design, size, duration
This study was conducted on surplus embryos of the patients with history of recurrent implantation failure or recurrent miscarriage following PGT by Day-5 trophectoderm biopsy. Inclusion criteria involved maternal age ≤37, normal parental karyotypes, and ovulation by long stimulation protocol. As advanced maternal age and ovarian stimulation method can affect the aneuploidy rate and expression profile, embryos belonging to mothers &gt; 37 years and included only patients with the long stimulation protocol were excluded.

Participants/materials, setting, methods
The normal embryos and those with single aneuploidy were selected for comparison of gene expression profiles using RNA sequencing. The single aneuploid embryos were classified in two groups: with defined aneuploidy(DA) of small chromosomes (including chromosomes 16, 20, 21, 22), and with undefined aneuploidy(UA) as single aneuploids for other chromosomes. To neutralize the effect of genomic content on gene expression profile in DA group, embryos were pooled reciprocally (trisomy and monosomy for the same chromosome).

Main results and the role of chance
Principal Component Analysis (PCA) validated embryos classification considering their differentially expressed genes (DEG) clustering into three distinct groups. To minimize the role of chance, only genes with significant adjusted P-values in both STAR and Hisat data analyses were included in consequent interpretations. In addition to comparison between and among the DA, UA and normal groups, the entire set of aneuploid groups was compared with the normal group. The DEG in both aneuploid groups compared to the normal group represented eminent upregulation rather than downregulation. Gene Ontology analyses indicated an upregulation of transcription, ubiquitination, autophagy, mitochondrial, chromosome segregation and DNA repair pathways in both DA and UA compared to normal group. Overexpression of the DNA repair pathway and other validated genes of interest in aneuploid embryos was in agreement with qPCR findings. Translation and protein synthesis pathways also showed overexpression as part of the functional response to aneuploidy. Upregulation of apoptotic process pathway was evident in the UA group. While comparison of gene expression between DA and UA showed overexpression of 143 genes, but cellular lipid synthesis was the only pathway with a significant difference. The distribution of DEG on chromosomes in the DA group was not associated with selected aneuploidies.

Limitations, reasons for caution
The number of biological replicates in each category was limited in order to provide more confident data by minimizing the time interval between providing the samples and RNA sequencing. Larger studies might change some affected pathways due to differences in identified DEG.

Wider implications of the findings
This study shed light on the black box of human preimplantation aneuploidy. Considering that the transcriptome is affected by the genomic status and some debates on genomic based current PGT approaches, this study is a step for selection of healthy and viable embryos by revealing the molecular mechanisms.

Trial registration number
not applicable