Detection of functional polymorphisms influencing the promoter activity of the SAA2 gene and their association with milk production traits in Chinese Holstein cows

Genetic variants in the serum amyloid A2 (SAA2) gene as a potential marker for milk production traits in Chinese Holstein cows

Background

This study was conducted to detect potential polymorphisms of the serum amyloid A2 (SAA2) gene and explore their relationships with milk production traits in Chinese Holstein cows.

Objectives: This study used

sequencing technology conducted in 532 Chinese Holstein cows.

Methods

Three single nucleotide polymorphisms (SNPs) were identified within intron 1, named g.14061A>G, g.14072G>C and g.14819C>T. Eight estimated haplotypes were identified, of which three major haplotypes had a frequency of Hap3 (‐ACC‐), Hap5 (‐GCC‐) and Hap2 (‐AGT‐), with 17.9%, 12.30% and 8.10%, respectively.

Results

The association analysis of single markers (g.14061A>G and g.14819C>T) and combined genotypes (Hap1/5) revealed prominent effects on milk production traits in Chinese Holstein cows (p < 0.05).

Conclusions

Our results suggest that the SAA2 gene is associated with economic traits in Chinese Holstein cows and may be used as candidate gene for marker‐assisted selection and management in breeding programs.

Complete CSN1S2 Characterization, Novel Allele Identification and Association With Milk Fatty Acid Composition in River Buffalo

The αs2-casein is one of the phosphoproteins secreted in all ruminants' milk, and it is the most hydrophilic of all caseins. However, this important gene (CSN1S2) has not been characterized in detail in buffaloes with only two alleles detected (reported as alleles A and B), and no association studies with milk traits have been carried out unlike what has been achieved for other species of ruminants. In this study, we sequenced the whole gene of two Mediterranean river buffalo homozygotes for the presence/absence of the nucleotide C (g.7539G>C) realized at the donor splice site of exon 7 and, therefore, responsible for the skipping of the same exon at mRNA level (allele B). A high genetic variability was found all over the two sequenced CSN1S2 alleles. In particular, 74 polymorphic sites were found in introns, six in the promoter, and three SNPs in the coding region (g.11072C>T, g.12803A>T, and g.14067A>G) with two of them responsible for amino acid replacements. Considering this genetic diversity, those found in the database and the SNP at the donor splice site of exon 7, it is possible to deduce at least eight different alleles (CSN1S2 A, B, B1, B2, C, D, E, and F) responsible for seven different possible translations of the buffalo αs2-casein. Haplotype data analysis suggests an evolutionary pathway of buffalo CSN1S2 gene consistent with our proposal that the published allele CSN1S2 A is the ancestral αs2-CN form, and the B2 probably arises from interallelic recombination (single crossing) between the alleles D and B (or B1). The allele CSN1S2 C is of new identification, while CSN1S2 B, B1, and B2 are deleted alleles because all are characterized by the mutation g.7539G>C. Two SNPs (g.7539G>C and g.14067A>G) were genotyped in 747 Italian buffaloes, and major alleles had a relative frequency of 0.83 and 0.51, respectively. An association study between these SNPs and milk traits including fatty acid composition was carried out. The SNP g.14067A>G showed a significant association (P < 0.05) on the content of palmitic acid in buffalo milk, thus suggesting its use in marker-assisted selection programs aiming for the improvement of buffalo milk fatty acid composition.

Determination of genetic associations between indels in 11 candidate genes and milk composition traits in Chinese Holstein population

Background

We have previously identified 11 promising candidate genes for milk composition traits by resequencing the whole genomes of 8 Holstein bulls with extremely high and low estimated breeding values for milk protein and fat percentages (high and low groups), including FCGR2B, CENPE, RETSAT, ACSBG2, NFKB2, TBC1D1, NLK, MAP3K1, SLC30A2, ANGPT1 and UGDH those contained 25 indels between high and low groups. In this study, the purpose was to further examine whether these candidates have significant genetic effects on milk protein and fat traits.

Results

With PCR product sequencing, 13 indels identified by whole genome resequencing were successfully genotyped. With association analysis in 769 Chinese Holstein cows, we found that the indel in FCGR2B was significantly associated with milk yield, protein yield and protein percentage (P = 0.0041 to 0.0297); five indels in CENPE and one indel in MAP3K1 were markedly relevant to milk yield, fat yield and protein yield (P < 0.0001 to 0.0073); polymorphism in RETSAT was evidently associated with milk yield, fat yield, protein yield and protein percentage (P = 0.0001 to 0.0237); variant in ACSBG2 affected fat yield and protein percentage (P = 0.0088 and 0.0052); one indel in TBC1D1 was with respect to fat percentage and protein percentage (P = 0.0224 and 0.0209). Significant associations were shown between indels in NLK and protein yield and protein percentage (P = 0.0012 to 0.0257); variant in UGDH was related to the milk yield (P = 0.0312). The two exonic indels in FCGR2B and CENPE were predicted to change the mRNA and protein secondary structures, and resulted in the corresponding protein dysfunction.

Conclusion

Our findings presented here provide the first evidence for the associations of eight functional genes with milk yield and composition traits in dairy cattle.

Electronic supplementary material

The online version of this article (10.1186/s12863-019-0751-y) contains supplementary material, which is available to authorized users.

Genetic association of DDIT3, RPL23A, SESN2 and NR4A1 genes with milk yield and composition in dairy cattle

Previously, we identified by RNA sequencing that DDIT3, RPL23A, SESN2 and NR4A1 genes were significantly differentially expressed between the mammary glands of lactating Holstein cows with extremely high and low milk protein and fat percentages; thus, these four genes are considered as promising candidates potentially affecting milk yield and composition traits in dairy cattle. In the present study, we further verified whether these genes have genetic effects on milk traits in a Chinese Holstein population. By re-sequencing part of the non-coding and the entire coding regions of the DDIT3, RPL23A, SESN2 and NR4A1 genes, a total of 35 SNPs and three insertions/deletions were identified, of which three were found in DDIT3, 12 in RPL23A, 16 in SESN2 and seven in NR4A1. Moreover, two of the insertions/deletions-g.125714860_125714872del and g.125714806delinsCCCC in SESN2-were novel and have not been reported previously. Subsequent single SNP analyses revealed multiple significant association with all 35 SNPs and three indels regressed against the dairy production traits (P-value = <0.0001-0.0493). In addition, with a linkage disequilibrium analysis, we found one, one, three, and one haplotype blocks in the DDIT3, RPL23A, SESN2 and NR4A1 genes respectively. Haplotype-based association analyses revealed that some haplotypes were also significantly associated with milk production traits (P-value = <0.0001-0.0461). We also found that 12 SNPs and two indels (two in DDIT3, two in RPL23A, nine in SESN2 and one in NR4A1) altered the specific transcription factor binding sites in the promoter, thereby regulating promoter activity, suggesting that they might be promising potential functional variants for milk traits. In summary, our findings first determined the genetic associations of DDIT3, RPL23A, SESN2 and NR4A1 with milk yield and composition traits in dairy cattle and also suggested potentially causal variants, which require in-depth validation.

Genetic Analyses Confirm SNPs in HSPA8 and ERBB2 are Associated with Milk Protein Concentration in Chinese Holstein Cattle

Heat shock 70 kDa protein 8 (HSPA8) and erb-b2 receptor tyrosine kinase 2 (ERBB2) were the promising candidates for milk protein concentration in dairy cattle revealed through previous RNA sequencing (RNA-Seq) study. The objective of this post-RNA-Seq study was to confirm genetic effects of HSPA8 and ERBB2 on milk protein concentration in a large Chinese Holstein population and to evaluate the genetic effects of both genes on other milk production traits. There were 2 single-nucleotide polymorphisms (SNPs) identified for HSPA8 and 11 SNPs for ERBB2 by sequencing 17 unrelated Chinese Holstein sires. The SNP-rs136632043 in HSPA8 had significant associations with all five milk production traits (p = 0.0086 to p < 0.0001), whereas SNP-rs132976221 was remarkably associated with three yield traits (p < 0.0001). Nine (ss1996900615, rs109017161, rs109122971, ss1996900614, rs110133654, rs109941438, rs110552983, rs133031530, and rs109763505) of 11 SNPs in ERBB2 were significantly associated with milk protein percentage (p = 0.0177 to p < 0.0001). A 12 Kb haplotype block was formed in ERBB2 and haplotype associations revealed similar effects on milk protein traits. Our findings confirmed the significant genetic effects of HSPA8 and ERBB2 on milk protein concentration and other milk production traits and SNP phenotypic variances above 1% may serve as genetic markers in dairy cattle breeding programs.

Genetic association of the ACACB gene with milk yield and composition traits in dairy cattle

Previously, we re-sequenced the whole genomes of eight Holstein bulls with high or low milk protein and fat percentage, and we detected two indels in the ACACB (acetyl-CoA carboxylase beta) gene that were polymorphic between the two groups. Thus, we considered ACACB as a promising candidate gene potentially affecting milk composition traits. Herein, we verified the genetic effects of ACACB on five milk traits in a Chinese Holstein population. We identified six SNPs in the 5'-promoter region, five in the 5'- untranslated region (UTR), 11 in exons, four in the 3'-UTR and three in the 3'-flanking region by re-sequencing the entire coding and regulatory regions of ACACB. One of these SNPs (ss1987461005) is reported here for the first time, and three of the SNPs (rs109482081, rs110819816 and rs109281947) were predicted to result in amino acid replacements. Genotype-phenotype association analyses showed that all the identified SNPs, except for ss1987461005, rs208919019 and rs134447911, were significantly associated with milk yield, fat yield, fat percentage, protein yield or protein percentage (P < 0.0001 to 0.0484). Linkage disequilibrium analyses were conducted among the identified SNPs to confirm the genetic associations. Two SNPs-rs135874354 (g.66218726T>C) and rs210928430 (g.66218117G>A)-were predicted to alter transcription factor binding sites in the 5'-promoter region of ACACB. A luciferase activity assay showed that the promoter activity of haplotype TG was significantly higher than that of CG (P = 0.0002) and that the promoter activity of haplotype TA was remarkably higher than that of CA (P = 7.4285E-09), showing that the T allele of rs135874354 increased promoter activity. Thus, rs135874354 was considered to be a potentially functional mutation. Our findings have, for the first time, profiled the genetic effect of ACACB on milk production traits in dairy cattle and revealed a potentially causal mutation that requires further the in-depth validation.