194 HOUSEKEEPING GENE EXPRESSION OF BOVINE FERTILIZED AND CLONED EMBRYOS

Abstract

Real-time RT-PCR can accurately quantify mRNA levels in pre-implantation embryos; however, comparisons among different embryonic stages and among embryos produced by different means often rely on a control gene, which is commonly assumed to remain constant across samples. The objective of this study was to compare housekeeping gene expression levels, relative to total mRNA, across different stages of bovine pre-implantation development in embryos generated by IVF and somatic cell nuclear transfer (SCNT). Embryos were produced according to standard protocols (Ross et al. 2006 Biotechniques 41, 741–750). Total RNA was collected from 3 pools of 10 oocyte/embryos at metaphase II (MII), PN, 2-cell, 4-cell, 8-cell, morula, and blastocyst stages and reverse-transcribed using oligo-dT primers. The cDNA was then amplified using PCR (Kocabas et al. 2006 PNAS 103, 14 027–14 032). All amplified cDNA samples were diluted to 1 ng μL–1, as determined by NanoDrop spectrophotometer (Thermo Fisher Scientific, Wilmington, DE, USA) and corroborated using a Qubit fluorometer (Invitrogen, Carlsbad, CA, USA). For real-time PCR, 2 μL of cDNA was analyzed in duplicates. Absolute quantification was performed as previously described (Iager et al. 2008 Cloning Stem Cells doi:10.1089), using SYBR-green chemistry and standard curves specific for each gene. The number of RNA copies per nanogram of amplified cDNA was compared among samples using ANOVA. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), cyclophilin A, β-actin, ribosomal protein L-15 (RPL-15), and β-glucuronidase (GUS) expression levels were similar in MII oocyte, 1-, 2-, 4-, and 8-cell embryos, while a significant increase at morula and blastocyst stages was observed (P < 0.05). A similar pattern of expression was observed for 18S ribosomal RNA, but with a significant decrease from morula to blastocyst stages (P < 0.05). Histone H2A expression was significantly higher at 1-cell stage, similar from 2-cell to morula stages and lowest at the blastocyst stage. We then compared expression between IVF and SCNT embryos at 2-, 4-, and 8-cell and blastocyst stages. GAPDH, RPL-15, GUS, and β-actin were significantly different among groups in at least 3 of the analyzed stages, which in all cases included blastocysts. 18s-rRNA was different among IVF and SCNT embryos only at the 8-cell stage, while no differences were observed at any stages for histone H2A and cyclophilin A. At the blastocyst stage, the lowest overall variability among IVF and SCNT embryos was observed for 18s-rRNA. In conclusion, none of the evaluated housekeeping genes showed consistent mRNA expression levels across developmental stages of IVF embryos. In addition, SCNT embryos, compared to IVF, had different levels of gene expression for commonly used housekeeping genes, which, if neglected, might result in data misinterpretation. In our conditions, histone H2A had similar expression levels between IVF and SCNT embryos across different stages and showed less variability than cyclophilin A. Finally, for comparisons at the blastocyst stage, 18s-rRNA had the least variability among IVF and SCNT embryos.