Blood transcriptome analysis and identification of genes associated with supernumerary teats in Chinese Holstein cows

In dairy cows, supernumerary teats (SNT) are not desired as they are considered a repository for bacteria; thus, SNT are a risk factor for mastitis. Supernumerary teats are a heritable oligo- or polygenic trait. The incidence of SNT in offspring must be reduced by genomic selection. However, in modern dairy farming, farmers often ignore the effects of SNT on cows. The study aimed to elucidate the effects of SNT on dairy cows from the blood transcriptome level and identify genes associated with SNT in Chinese Holstein cows. We selected 6 SNT cows (Yes) and 6 non-SNT cows (No). In the 6 SNT cows, 3 cows had 1 SNT (One) and 3 cows had 2 SNT (Two). They were divided into 3 comparison groups (One vs. No; Two vs. No; and Yes vs. No). RNA was extracted from blood white membrane cells of 12 cows, and RNA sequencing was performed. Differential gene expression analysis based on the negative binomial distribution was used to detect differentially expressed genes in the One versus No and Two versus No comparison groups. Genes that were significantly upregulated or downregulated both in the One versus No and Two versus No groups (shared genes, SG) were obtained for further analysis. We also performed gene set enrichment analysis for all genes expressed in the Yes versus No group, correlation analysis between SG and the hematological parameters, protein-protein interaction network analysis of SG to select hub genes, and alternative splicing analysis for Yes versus No group to explore the functions of differentially spliced genes. We detected 289 SG. Gene set enrichment analysis, gene ontology, and the Kyoto Encyclopedia of Genes and Genomes enrichment analysis results showed that SNT affect immunity, inflammation, and lactation-related pathways in dairy cows. Correlation analysis showed that LOC104968484, SLC25A6, GADD45G, BAX, APAF1, ATM, XIAP, MDM4, BDP1, CEP350, MED13, TAOK1, SMG1, and RIF1 are associated with white blood cell count and absolute value of lymphocytes in SNT cows only, so they might be genes associated with SNT in Chinese Holstein cows. We found 2 genes (BAX and MDM4) were also differentially spliced genes. However, the causal relationship between these genes and the SNT phenotype needs to be further studied. This study is the first to reveal the adverse effects of SNT on dairy cows at a transcriptional level, and the genes we found can be used as a reference for further searching for candidate genes for the SNT phenotype.