24 Structural variants affecting mRNAs isoforms splice sites associated with marbling in Nellore cattle

Abstract

Abstract The mRNA identification and associated splice variants affecting meat marbling in Nellore cattle could contribute to a better understanding of the genetic architecture of this complex trait. Longissimus thoracis samples from the 20 most extreme bulls (out of 80 bulls set) for marbling [high (n=10) and low (n=10) groups] trait were used to perform transcriptomic analysis using RNA-Sequencing. The CLC Genomics Workbench software was used to align the fragments of each sample to the bovine reference genome (ARS-UCD1.2) and to perform differential expressed transcript analysis (DE). An average of 37,000 transcripts and 14 mRNA DE (P ≤ 0.001; FC > 2) were detected in the muscle transcriptome. Among them, three transcripts were up-regulated (COL4A2-202, GHRH-201 and ENSBTAT00000070459.1) and 11 down-regulated (HBB-201, HBA-201, RPL14-202, CATHL1-201, CATHL4-201, MAPKAPK2-204, PCBD1-201, RPL30-203, NEURL1-202, PGLYRP1-201 and ENSBTAT00000038384.2) in high marbling group in relation to low group. Structural variants, including Single Nucleotide Variant (SNV), insertion or deletion were also found in the mRNA DE (4 in high and 6 in low marbling groups), causing possible splice site disruption. For high marbling group, in the COL4A2-202 transcript, three SNVs [one guanine (G) located on chromosome 12 at 85,104,867 bp, two cytosine (C), at the positions: 85,119,139 and 85,145,937 bp] were found. From these, one is a non-synonyms mutation [SNV (G) 85,145,937bp], causing an amino acid change (ENSBTAP00000005916.5: p.Glu1088Asp) in Collagen alpha-2(IV) chain protein (COL4A2). Interestingly, animals with low marbling presented also deletions (85,100,930 pb) in this same gene, causing an amino acid change (ENSBTAP00000005916.5:p.Pro113fs)in the COL4A2 protein. Functional enrichment analysis, using the DE mRNAs list, identified significant metabolic pathways (FDR < 0.05), such as O2/CO2 exchange in erythrocytes, tyrosine biosynthesis and phenylalanine degradation. The results suggest potential key regulatory genes associated with marbling, an economically important trait for the beef industry and for the consumer. Funding FAPESP (#2009/16118-5, #11/21241-0, #2017/02470-3, #2017/10630–2, #2018/ 20026–8, #2018/11154-2) and CAPES (#001).