GSE107477
Hepatic transcriptome analysis for differential gene expression between steers of divergent feed efficiency phenotypes in three Canadian beef breeds
The objective of this study was to identify differentially expressed genes in the liver of steers with divergent Residual Feed Intake (RFI). Methods:In total 50 purebred Angus, 48 purebred Charolais and 158 Kinsella Composite breed steers were tested for individual feed intake using the GrowSafe system for an average period of 70 to 73 days. During the feedlot test animals were fed at ad libitum with a finishing diet composed of 75% barley grain, 20% barley silage and 5% rumensin pellet. Body weight of each animal was measured at an interval of 28days and ADG for each animal was obtained from a linear regression of serial body weight (BW) measurements (Kgs) on time (days). MWT was calculated as midpoint BW0.75, where midpoint BW was computed as the sum of initial BW of the animal and the product of its ADG multiplied by half the number test days. DMI of each animal was calculated as the average daily feed intake of the animals for the time during the feedlot test (days). The expected DMI for each animal was predicted using the regression intercept and regression coefficients of ADG and MWT on actual DMI, and RFI was computed as the difference between the standardized daily DMI and the expected DMI. At the end of the test, animals were slaughtered and liver tissue was collected immediately after slaughter separately bagged in plastic bags, labeled and flash frozen in liquid nitrogen. The frozen samples were transferred to the laboratory on ice and stored at -80C until RNA extraction. From the frozen samples, 20 samples from each breed were selected for total RNA extraction including six samples  with extreme high and six samples  extreme low-RFI phenotypes for each breed. Complementary DNA (cDNA) libraries were constructed for each of the 60 animals and consequently single end sequenced using Illumina Hiseq 2500 sequencer. The raw reads were aligned and mapped to the bovine reference genome UMD 3.1 using Tophat2 aligner with default alignment parameters. Thereafter, reads aligned uniquely to each annotated transcript of gene in the bovine genome were counted using HTSeq-count package with default parameters. Read counts from HTSeq-count, sample information (sample id, sire, feed efficiency group and sequencing mode) and gene annotation table from ENSEMBL Biomart were used for differential expression analysis within each breed using the edgeR package in R for the six extreme low-RFI and six extreme high-RFI samples. Results: At a false discovery rate of 0.05 and fold change > 2 , we identified 72, 41 and 175 differentially expressed genes for Angus, Charolais and KC RFI divergent steers respectively. Overall design: RNA-seq of Angus, Charolais and Kinsella Composite male liver with either low or high Residual Feed Intake (RFI).