Gene expression profiles in liver of pigs with extreme high and low levels of androstenone

Hepatic Gene Expression and Metabolite Profiles of Androstenone and Skatole Relative to Plasma Estrone Sulfate Levels in Boars

Testicular steroids can alter the activity and expression of enzymes within the liver and may influence the metabolism of skatole and androstenone, which are responsible for boar taint. Plasma levels of estrone sulfate (E1S) are indicative of the steroidogenic capacity of the boar and are variable between animals of similar live weights at slaughter. This study aimed to characterize the relationship between steroidogenic capacity and the metabolism of boar taint compounds by relating plasma E1S levels at slaughter weight to the expression levels of genes regulating the metabolism of androstenone and skatole, along with their respective metabolite profiles. RT-qPCR was used to evaluate gene expression in the liver. Hepatocytes were also isolated and treated with androstenone or skatole, with metabolite levels in the incubation media quantified by high-performance liquid chromatography. Plasma E1S levels ranged from 2.2-108.5 ng/mL and were positively correlated with overall skatole metabolism (p = 0.038), the production of metabolites 3-methyloxindole (p = 0.026) and 3-hydroxy-3-methyloxindole (p = 0.036), and expression levels of key genes involved in skatole metabolism, specifically CYP2C33 (p = 0.0042), CYP2C49 (p = 0.022), and CYB5R1 (p = 0.017). There was no association between androstenone metabolism and plasma E1S concentrations; however, there was evidence of possible co-regulation amongst genes involved in the metabolism of androstenone, skatole, and estrogens. These findings indicate that steroidogenic capacity is related to the rate of skatole, but not androstenone metabolism, in slaughter-weight boars.

Expression genome-wide association study identifies key regulatory variants enriched with metabolic and immune functions in four porcine tissues

BACKGROUND

Integration of high throughput DNA genotyping and RNA-sequencing data enables the discovery of genomic regions that regulate gene expression, known as expression quantitative trait loci (eQTL). In pigs, efforts to date have been mainly focused on purebred lines for traits with commercial relevance as such growth and meat quality. However, little is known on genetic variants and mechanisms associated with the robustness of an animal, thus its overall health status. Here, the liver, lung, spleen, and muscle transcriptomes of 100 three-way crossbred female finishers were studied, with the aim of identifying novel eQTL regulatory regions and transcription factors (TFs) associated with regulation of porcine metabolism and health-related traits.

RESULTS

An expression genome-wide association study with 535,896 genotypes and the expression of 12,680 genes in liver, 13,310 genes in lung, 12,650 genes in spleen, and 12,595 genes in muscle resulted in 4,293, 10,630, 4,533, and 6,871 eQTL regions for each of these tissues, respectively. Although only a small fraction of the eQTLs were annotated as cis-eQTLs, these presented a higher number of polymorphisms per region and significantly stronger associations with their target gene compared to trans-eQTLs. Between 20 and 115 eQTL hotspots were identified across the four tissues. Interestingly, these were all enriched for immune-related biological processes. In spleen, two TFs were identified: ERF and ZNF45, with key roles in regulation of gene expression.

CONCLUSIONS

This study provides a comprehensive analysis with more than 26,000 eQTL regions identified that are now publicly available. The genomic regions and their variants were mostly associated with tissue-specific regulatory roles. However, some shared regions provide new insights into the complex regulation of genes and their interactions that are involved with important traits related to metabolism and immunity.

Differential Regulation of Male-Hormones-Related Enhancers Revealed by Chromatin Accessibility and Transcriptional Profiles in Pig Liver

Surgical castration can effectively avoid boar taint and improve pork quality by removing the synthesis of androstenone in the testis, thereby reducing its deposition in adipose tissue. The expression of genes involved in testis-derived hormone metabolism was altered following surgical castration, but the upstream regulatory factors and underlying mechanism remain unclear. In this study, we systematically profiled chromatin accessibility and transcriptional dynamics in liver tissue of castrated and intact full-sibling Yorkshire pigs. First, we identified 897 differentially expressed genes and 6864 differential accessible regions (DARs) using RNA- and ATAC-seq. By integrating the RNA- and ATAC-seq results, 227 genes were identified, and a significant positive correlation was revealed between differential gene expression and the ATAC-seq signal. We constructed a transcription factor regulatory network after motif analysis of DARs and identified a candidate transcription factor (TF) SP1 that targeted the HSD3B1 gene, which was responsible for the metabolism of androstenone. Subsequently, we annotated DARs by incorporating H3K27ac ChIP-seq data, marking 2234 typical enhancers and 245 super enhancers involved in the regulation of all testis-derived hormones. Among these, four typical enhancers associated with HSD3B1 were identified. Furthermore, an in-depth investigation was conducted on the androstenone-related enhancers, and an androstenone-related mutation was identified in a newfound candidatetypical enhancer (andEN) with dual-luciferase assays. These findings provide further insights into how enhancers function as links between phenotypic and non-coding area variations. The discovery of upstream TF and enhancers of HSD3B1 contributes to understanding the regulatory networks of androstenone metabolism and provides an important foundation for improving pork quality.

Revelation of genetic diversity and genomic footprints of adaptation in Indian pig breeds

In the present study, the genetic diversity measures among four Indian domestic breeds of pig namely Agonda Goan, Ghurrah, Ghungroo, and Nicobari, of different agro-climatic regions of country were explored and compared with European commercial breeds, European wild boar and Chinese domestic breeds. The double digest restriction site-associated DNA sequencing (ddRADseq) data of Indian pigs (102) and Landrace (10 animals) were generated and whole genome sequencing data of exotic pigs (60 animals) from public data repository were used in the study. The principal component analysis (PCA), admixture analysis and phylogenetic analysis revealed that Indian breeds were closer in ancestry to Chinese breeds than European breeds. European breeds exhibited highest genetic diversity measures among all the considered breeds. Among Indian breeds, Agonda Goan and Ghurrah were found to be more genetically diverse than Nicobari and Ghungroo. The selection signature regions in Indian pigs were explored using iHS and XP-EHH, and during iHS analysis, it was observed that genes related to growth, reproduction, health, meat quality, sensory perception and behavior were found to be under selection pressure in Indian pig breeds. Strong selection signatures were recorded in 24.25-25.25 Mb region of SSC18, 123.25-124 Mb region of SSC15 and 118.75-119.5 Mb region of SSC2 in most of the Indian breeds upon pairwise comparison with European commercial breeds using XP-EHH. These regions were harboring some important genes such as EPHA4 for thermotolerance, TAS2R16, FEZF1, CADPS2 and PTPRZ1 for adaptability to scavenging system of rearing, TRIM36 and PGGT1B for disease resistance and CCDC112, PIAS1, FEM1B and ITGA11 for reproduction.

Genomic Prediction and Genome-Wide Association Study for Boar Taint Compounds

With a perspective future ban on surgical castration in Europe, selecting pigs with reduced ability to accumulate boar taint (BT) compounds (androstenone, indole, skatole) in their tissues seems a promising strategy. BT compound concentrations were quantified in the adipose tissue of 1075 boars genotyped at 29,844 SNPs. Traditional and SNP-based breeding values were estimated using pedigree-based BLUP (PBLUP) and genomic BLUP (GBLUP), respectively. Heritabilities for BT compounds were moderate (0.30-0.52). The accuracies of GBLUP and PBLUP were significantly different for androstenone (0.58 and 0.36, respectively), but comparable for indole and skatole (~0.43 and ~0.47, respectively). Several SNP windows, each explaining a small percentage of the variance of BT compound concentrations, were identified in a genome-wide association study (GWAS). A total of 18 candidate genes previously associated with BT (MX1), reproduction traits (TCF21, NME5, PTGFR, KCNQ1, UMODL1), and fat metabolism (CTSD, SYT8, TNNI2, CD81, EGR1, GIPC2, MIGA1, NEGR1, CCSER1, MTMR2, LPL, ERFE) were identified in the post-GWAS analysis. The large number of genes related to fat metabolism might be explained by the relationship between sexual steroid levels and fat deposition and be partially ascribed to the pig line investigated, which is selected for ham quality and not for lean growth.

Hep3Gel: A Shape-Shifting Extracellular Matrix-Based, Three-Dimensional Liver Model Adaptable to Different Culture Systems

Drug-induced hepatotoxicity is a leading cause of clinical trial withdrawal. Therefore, in vitro modeling the hepatic behavior and functionalities is not only crucial to better understand physiological and pathological processes but also to support drug development with reliable high-throughput platforms. Different physiological and pathological models are currently under development and are commonly implemented both within platforms for standard 2D cultures and within tailor-made chambers. This paper introduces Hep3Gel: a hybrid alginate-extracellular matrix (ECM) hydrogel to produce 3D in vitro models of the liver, aiming to reproduce the hepatic chemomechanical niche, with the possibility of adapting its shape to different manufacturing techniques. The ECM, extracted and powdered from porcine livers by a specifically set-up procedure, preserved its crucial biological macromolecules and was embedded within alginate hydrogels prior to crosslinking. The viscoelastic behavior of Hep3Gel was tuned, reproducing the properties of a physiological organ, according to the available knowledge about hepatic biomechanics. By finely tuning the crosslinking kinetics of Hep3Gel, its dualistic nature can be exploited either by self-spreading or adapting its shape to different culture supports or retaining the imposed fiber shape during an extrusion-based 3D-bioprinting process, thus being a shape-shifter hydrogel. The self-spreading ability of Hep3Gel was characterized by combining empirical and numerical procedures, while its use as a bioink was experimentally characterized through rheological a priori printability evaluations and 3D printing tests. The effect of the addition of the ECM was evident after 4 days, doubling the survival rate of cells embedded within control hydrogels. This study represents a proof of concept of the applicability of Hep3Gel as a tool to develop 3D in vitro models of the liver.

Nuclear Receptor Pathways Mediating the Development of Boar Taint

The nuclear receptors PXR, CAR, and FXR are activated by various ligands and function as transcription factors to control the expression of genes that regulate the synthesis and metabolism of androstenone and skatole. These compounds are produced in entire male pigs and accumulate in the fat to cause the development of a meat quality issue known as boar taint. The extent of this accumulation is influenced by the synthesis and hepatic clearance of androstenone and skatole. For this reason, PXR, CAR, and FXR-mediated signaling pathways have garnered interest as potential targets for specialized treatments designed to reduce the development of boar taint. Recent research has also identified several metabolites produced by gut microbes that act as ligands for these nuclear receptors (e.g., tryptophan metabolites, short-chain fatty acids, bile acids); however, the connection between the gut microbiome and boar taint development is not clear. In this review, we describe the nuclear receptor signaling pathways that regulate the synthesis and metabolism of boar taint compounds and outline the genes involved. We also discuss several microbial-derived metabolites and dietary additives that are known or suspected nuclear receptor ligands and suggest how these compounds could be used to develop novel treatments for boar taint.

Recent genetic advances on boar taint reduction as an alternative to castration: a review

Boar taint is an unpleasant odor in male pig meat, mainly caused by androstenone, skatole, and indole, which are deposited in the fat tissue. Piglet castration is the most common practice to prevent boar taint. However, castration is likely to be banished in a few years due to animal welfare concerns. Alternatives to castration, such as genetic selection, have been assessed. Androstenone and skatole have moderate to high heritability, which makes it feasible to select against these compounds. This review presents the latest results obtained on genetic selection against boar taint, on correlation with other traits, on differences in breeds, and on candidate genes related to boar taint. QTLs for androstenone and skatole have been reported mainly on chromosomes 6, 7, and 14. These chromosomes were reported to contain genes responsible for synthesis and degradation of androstenone and skatole. A myriad of work has been done to find markers or genes that can be used to select animals with lower boar taint. The selection against boar taint could decrease performance of some reproduction traits. However, a favorable response on production traits has been observed by selecting against boar taint. Selection results have shown that it is possible to reduce boar taint in few generations. In addition, modifications in diet and environment conditions could be associated with genetic selection to reduce boar taint. Nevertheless, costs to measure and select against boar taint should be rewarded with incentives from the market; otherwise, it would be difficult to implement genetic selection.

SULFATION PATHWAYS: Expression of SULT2A1, SULT2B1 and HSD3B1 in the porcine testis and epididymis

In the porcine testis, in addition to estrogen sulfates, the formation of numerous sulfonated neutral hydroxysteroids has been observed. However, their functions and the underlying synthetic pathways are still widely unclear. To obtain further information on their formation in postpubertal boars, the expression of sulfotransferases considered relevant for neutral hydroxysteroids (SULT2A1, SULT2B1) was investigated in the testis and defined segments of the epididymis applying real-time RT-qPCR, Western blot and immunohistochemistry (IHC). Sulfotransferase activities were assessed in tissue homogenates or cytosolic preparations applying dehydroepiandrosterone and pregnenolone as substrates. A high SULT2A1 expression was confirmed in the testis and localized in Leydig cells by IHC. In the epididymis, SULT2A1 expression was virtually confined to the body. SULT2B1 expression was absent or low in the testis but increased significantly along the epididymis. Immunohistochemical observations indicate that both enzymes are secreted into the ductal lumen via an apocrine mechanism. The results from the characterization of expression patterns and activity measurements suggest that SULT2A1 is the prevailing enzyme for the sulfonation of hydroxysteroids in the testis, whereas SULT2B1 may catalyze the formation of sterol sulfates in the epididymis. In order to obtain information on the overall steroidogenic capacity of the porcine epididymis, the expression of important steroidogenic enzymes (CYP11A1, CYP17A1, CYP19, HSD3B1, HSD17B3, SRD5A2) was monitored in the defined epididymal segments applying real-time RT-qPCR. Surprisingly, in addition to a high expression of SRD5A2 in the epididymal head, a substantial expression of HSD3B1 was detected, which increased along the organ.

Transcriptome signature of liver tissue with divergent mutton odour and flavour using RNA deep sequencing

Mutton consumption is less popular in many Asian countries including Indonesia, whose consumers often complain about the unpleasant flavour and odour of the meat. The main causes of mutton odour are the two compounds of branched chain fatty acid (BCFA): methylnonanoic (MNA), phenol, 3-methyl (MP), 4-methylnonanoic (MNA) and 4-ethyloctanoic (EOA) present in all the adipose tissue; and the 3-methylindole (MI) or skatole and indole, which are originated from pastoral diets. It is crucial to understand the genetic mechanism of mutton odour and flavour (MOF) to select sheep for lower BCFA and indole thus reduce the unpleasant flavour of meat. The aim of the present study was to investigate transcriptome profiling in liver tissue with divergent MOF using RNA deep sequencing. Liver tissues from higher (n = 3) and lower (n = 3) MOF sheep were analysed using Illumina HiSeq 2500. The total number of reads produced for each liver sample ranged from 21.37 to 25.37 million. Approximately 103 genes were differentially expressed (DEGs) with significance level of p-adjusted value <0.05. Among them, 60 genes were up-regulated, and 43 were down-regulated (p < 0.01, FC > 1.5) in higher MOF group. Differentially regulated genes in high MOF liver samples were enriched in biological processes such as cellular response to chemical stimulus and endogenous stimulus; cellular components such as such as basement membrane and extracellular matrix; and molecular functions such as haeme binding and oxidoreductase activity. Among the DEGs, metabolic phase I related genes belonging to the cytochrome P450 CYP2A6 were dominantly expressed. Additionally, phase II conjugation genes including UDP glucuronosyltransferases UGT2B18, sulfotransferase SULT1C1, and glutathione S-transferase GSTM1 were identified. The dominant candidate genes for SOF could be cytochrome P450, sodium-channel protein, transmembrane protein, glutathione transferase, UDP glucuronosyltransferases and sulfotransferase. Pathway analysis identified steroid hormone biosynthesis and chemical carcinogenesis by cytochrome P450 pathways which may play important roles in MOF-related molecules metabolism. This work highlighted potential genes and gene-networks that may affect meat off flavour and odour in sheep.

17-beta-hydroxysteroid dehydrogenase 13 inhibits the progression and recurrence of hepatocellular carcinoma

BACKGROUND

Our previous study showed that 17-beta-hydroxysteroid dehydrogenase 13 (HSD17B13) is down-regulated in hepatocellular carcinoma (HCC). But its function in HCC remains unknown. This study aimed to reveal the function of HSD17B13 and its clinical significance in HCC.

METHODS

mRNA levels of HSD17B13 were analyzed in cohort 1 (30 normal, 30 HBV cirrhosis, 60 HBV-related HCC and 60 peritumoral tissue) by real-time PCR. HSD17B13 protein was evaluated in cohort 2 (15 normal, 33 HBV-cirrhosis, 12 dysplastic nodules, 34 HBV-related HCC, and 9 metastatic HCC) using immunohistochemistry. The association between HSD17B13 and the survival of HCC patients was analyzed in cohort 3 (n = 88). The inhibitory mechanism of HSD17B13 on HCC was explored .

RESULTS

The mRNA of HSD17B13 and its protein expression were significantly down-regulated in HCC compared to non-tumor specimens (P < 0.001). The sensitivity, specificity and area under curve (AUC) values of HSD17B13 expression levels for HCC detection were 81.7%, 83.7% and 0.856, respectively (P < 0.001). Lower HSD17B13 in peritumoral tissue was an independent risk factor of worse recurrence free survival of HCC patients (HR: 0.41; 95% CI: 0.20-0.83; P = 0.014). The study in Huh 7 and SK-HEP-1 cells showed that HSD17B13 induced an accumulation of cells in G1 phase and reduction of cells in S and G2 phases via up-regulating the expression of P21, P27 and MMP2.

CONCLUSIONS

Lower HSD17B13 in peritumoral tissues was associated with worse recurrence free survival and overall survival of HCC patients. HSD17B13 delayed G1/S progression of HCC cells. HSD17B13 may be a therapeutic target for the treatment of HCC.

Systems genomics study reveals expression quantitative trait loci, regulator genes and pathways associated with boar taint in pigs

Boar taint is an offensive odour and/or taste from a proportion of non-castrated male pigs caused by skatole and androstenone accumulation during sexual maturity. Castration is widely used to avoid boar taint but is currently under debate because of animal welfare concerns. This study aimed to identify expression quantitative trait loci (eQTLs) with potential effects on boar taint compounds to improve breeding possibilities for reduced boar taint. Danish Landrace male boars with low, medium and high genetic merit for skatole and human nose score (HNS) were slaughtered at ~100 kg. Gene expression profiles were obtained by RNA-Seq, and genotype data were obtained by an Illumina 60K Porcine SNP chip. Following quality control and filtering, 10,545 and 12,731 genes from liver and testis were included in the eQTL analysis, together with 20,827 SNP variants. A total of 205 and 109 single-tissue eQTLs associated with 102 and 58 unique genes were identified in liver and testis, respectively. By employing a multivariate Bayesian hierarchical model, 26 eQTLs were identified as significant multi-tissue eQTLs. The highest densities of eQTLs were found on pig chromosomes SSC12, SSC1, SSC13, SSC9 and SSC14. Functional characterisation of eQTLs revealed functions within regulation of androgen and the intracellular steroid hormone receptor signalling pathway and of xenobiotic metabolism by cytochrome P450 system and cellular response to oestradiol. A QTL enrichment test revealed 89 QTL traits curated by the Animal Genome PigQTL database to be significantly overlapped by the genomic coordinates of cis-acting eQTLs. Finally, a subset of 35 cis-acting eQTLs overlapped with known boar taint QTL traits. These eQTLs could be useful in the development of a DNA test for boar taint but careful monitoring of other overlapping QTL traits should be performed to avoid any negative consequences of selection.

Exploiting whole genome sequence data to fine map and characterize candidate genes within a quantitative trait loci region affecting androstenone on porcine chromosome 5

Male piglets are routinely castrated to eliminate boar taint. However, this treatment is undesirable, and alternative approaches, including genetic strategies to reduce boar taint, are demanded. Androstenone is one of the causative agents of boar taint, and a QTL region affecting this pheromone has previously been reported on SSC5: 22.6–24.8 Mb in Duroc. The QTL region is one of the few reported for androstenone that does not simultaneously affect levels of other sex steroids. The main objective of this study was to fine map this QTL. Whole genome sequence data from 23 Norwegian Duroc boars were analyzed to detect new polymorphisms within the QTL region. A subset of 161 SNPs was genotyped in 834 Duroc sires and analyzed for association with androstenone in adipose tissue and testosterone, estrone sulphate and 17β‐estradiol in blood plasma. Our results revealed 100 SNPs significantly associated with androstenone levels in fat (P < 0.001) with 94 of the SNPs being in strong linkage disequilibrium in the region 23.03–24.27 Mb. This haplotype block contains at least four positional candidate genes (HSD17B6,SDR9C7,RDH16 and STAT6) involved in androstenone biosynthesis. No significant associations were found between any of the SNPs and levels of testosterone and estrogens, confirming previous findings. The amount of phenotypic variance explained by single SNPs within the haplotype block was as high as 5.4%. As the SNPs in this region significantly affect levels of androstenone without affecting levels of other sex steroids, they are especially interesting as genetic markers for selection against boar taint.

Differential expression and co-expression gene networks reveal candidate biomarkers of boar taint in non-castrated pigs

Boar taint (BT) is an offensive odour or taste observed in pork from a proportion of non-castrated male pigs. Surgical castration is effective in avoiding BT, but animal welfare issues have created an incentive for alternatives such as genomic selection. In order to find candidate biomarkers, gene expression profiles were analysed from tissues of non-castrated pigs grouped by their genetic merit of BT. Differential expression analysis revealed substantial changes with log-transformed fold changes of liver and testis from -3.39 to 2.96 and -7.51 to 3.53, respectively. Co-expression network analysis revealed one module with a correlation of -0.27 in liver and three modules with correlations of 0.31, -0.44 and -0.49 in testis. Differential expression and co-expression analysis revealed candidate biomarkers with varying biological functions: phase I (COQ3, COX6C, CYP2J2, CYP2B6, ACOX2) and phase II metabolism (GSTO1, GSR, FMO3) of skatole and androstenone in liver to steroidgenesis (HSD17B7, HSD17B8, CYP27A1), regulation of steroidgenesis (STARD10, CYB5R3) and GnRH signalling (MAPK3, MAP2K2, MAP3K2) in testis. Overrepresented pathways included "Ribosome", "Protein export" and "Oxidative phosphorylation" in liver and "Steroid hormone biosynthesis" and "Gap junction" in testis. Future work should evaluate the biomarkers in large populations to ensure their usefulness in genomic selection programs.

Expression Levels of GABA-A Receptor Subunit Alpha 3, Gabra3 and Lipoprotein Lipase, Lpl Are Associated with the Susceptibility to Acetaminophen-Induced Hepatotoxicity

Drug-induced liver injury (DILI) is the serious and fatal drug-associated adverse effect, but its incidence is very low and individual variation in severity is substantial. Acetaminophen (APAP)-induced liver injury accounts for >50% of reported DILI cases but little is known for the cause of individual variations in the severity. Intrinsic genetic variation is considered a key element but the identity of the genes was not well-established. Here, pre-biopsy method and microarray technique was applied to uncover the key genes for APAP-induced liver injury in mice, and a cause and effect experiment employing quantitative real-time PCR was conducted to confirm the correlation between the uncovered genes and APAP-induced hepatotoxicity. We identified the innately and differentially expressed genes of mice susceptible to APAP-induced hepatotoxicity in the pre-biopsied liver tissue before APAP treatment through microarray analysis of the global gene expression profiles (Affymetrix GeneChip® Mouse Gene 1.0 ST for 28,853 genes). Expression of 16 genes including Gdap10, Lpl, Gabra3 and Ccrn4l were significantly different (t-test: FDR <10%) more than 1.5 fold in the susceptible animals than resistant. To confirm the association with the susceptibility to APAP-induced hepatotoxicity, another set of animals were measured for the expression level of selected 4 genes (higher two and lower two genes) in the liver pre-biopsy and their sensitivity to APAP-induced hepatotoxicity was evaluated by post hoc. Notably, the expressions of Gabra3 and Lpl were significantly correlated with the severity of liver injury (p<0.05) demonstrating that these genes may be linked to the susceptibility to APAP-induced hepatotoxicity.

Preliminary study of FMO1, FMO5, CYP21, ESR1, PLIN2 and SULT2A1 as candidate gene for compounds related to boar taint

An association study between polymorphisms of six genes and boar taint related compounds androstenone, skatole and indole was performed in a boar population (n=370). Significant association (P<0.05) was detected for SNP of FMO5 (g.494A>G) with all boar taint compounds, SNP of CYP21 (g.3911T>C) with skatole and indole, and SNP of ESR1 (g.672C>T) with androstenone and indole. mRNA expression of CYP21 and ESR1 was higher in CAB (castrated boar) compared to non-castrated boars; whereas, the expression of FMO5 and ESR1 was higher in LBT (low boar taint) compared to HBT (high boar taint) in liver tissue. FMO5, CYP21 and ESR1 proteins were less detectable in HBT compared with LBT and CAB in liver tissues. These findings suggest that FMO5, CYP21 and ESR1 gene variants might have effects on the boar taint compounds.

Identification of gene co-expression clusters in liver tissues from multiple porcine populations with high and low backfat androstenone phenotype

Background

Boar taint is principally caused by accumulation of androstenone and skatole in adipose tissues. Studies have shown high heritability estimates for androstenone whereas skatole production is mainly dependent on nutritional factors. Androstenone is a lipophilic steroid mainly metabolized in liver. Majority of the studies on hepatic androstenone metabolism focus only on a single breed and very few studies account for population similarities/differences in gene expression patterns. In this work, we concentrated on population similarities in gene expression to identify the common genes involved in hepatic androstenone metabolism of multiple pig populations. Based on androstenone measurements, publicly available gene expression datasets from three porcine populations were compiled into either low or high androstenone dataset. Gene expression correlation coefficients from these datasets were converted to rank ratios and joint probabilities of these rank ratios were used to generate dataset specific co-expression clusters. Finally, these networks were clustered using a graph clustering technique.

Results

Cluster analysis identified a number of statistically significant co-expression clusters in the dataset. Further enrichment analysis of these clusters showed that one of the clusters from low androstenone dataset was highly enriched for xenobiotic, drug, cholesterol and lipid metabolism and cytochrome P450 associated metabolism of drugs and xenobiotics. Literature references revealed that a number of genes in this cluster were involved in phase I and phase II metabolism. Physical and functional similarity assessment showed that the members of this cluster were dispersed across multiple clusters in high androstenone dataset, possibly indicating a weak co-expression of these genes in high androstenone dataset.

Conclusions

Based on these results we hypothesize that majority of the genes in this cluster forms a signature co-expression cluster in low androstenone dataset in our experiment and that majority of the members of this cluster might be responsible for hepatic androstenone metabolism across all the three populations used in our study. We propose these results as a background work towards understanding breed similarities in hepatic androstenone metabolism. Additional large scale experiments using data from multiple porcine breeds are necessary to validate these findings.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-014-0158-8) contains supplementary material, which is available to authorized users.

On the relationship between an Asian haplotype on chromosome 6 that reduces androstenone levels in boars and the differential expression of SULT2A1 in the testis

Background

Androstenone is one of the major compounds responsible for boar taint, a pronounced urine-like odor produced when cooking boar meat. Several studies have identified quantitative trait loci (QTL) for androstenone level on Sus scrofa chromosome (SSC) 6. For one of the candidate genes in the region SULT2A1, a difference in expression levels in the testis has been shown at the protein and RNA level.

Results

Haplotypes were predicted for the QTL region and their effects were estimated showing that haplotype 1 was consistently related with a lower level, and haplotype 2 with a higher level of androstenone. A recombinant haplotype allowed us to narrow down the QTL region from 3.75 Mbp to 1.94 Mbp. An RNA-seq analysis of the liver and testis revealed six genes that were differentially expressed between homozygotes of haplotypes 1 and 2. Genomic sequences of these differentially expressed genes were checked for variations within potential regulatory regions. We identified one variant located within a CpG island that could affect expression of SULT2A1 gene. An allele-specific expression analysis in the testis did not show differential expression between the alleles of SULT2A1 located on the different haplotypes in heterozygous animals. However a synonymous mutation C166T (SSC6: 49,117,861 bp in Sscrofa 10.2; C/T) was identified within the exon 2 of SULT2A1 for which the haplotype 2 only had the C allele which was higher expressed than the T allele, indicating haplotype-independent allelic-imbalanced expression between the two alleles. A phylogenetic analysis for the 1.94 Mbp region revealed that haplotype 1, associated with low androstenone level, originated from Asia.

Conclusions

Differential expression could be observed for six genes by RNA-seq analysis. No difference in the ratio of C:T expression of SULT2A1 for the haplotypes was found by the allele-specific expression analysis, however, a difference in expression between the C over T allele was found for a variation within SULT2A1, showing that the difference in androstenone levels between the haplotypes is not caused by the SNP in exon 2.

Identification of the novel candidate genes and variants in boar liver tissues with divergent skatole levels using RNA deep sequencing

Boar taint is the unpleasant odour of meat derived from non-castrated male pigs, caused by the accumulation of androstenone and skatole in fat. Skatole is a tryptophan metabolite produced by intestinal bacteria in gut and catabolised in liver. Since boar taint affects consumer's preference, the aim of this study was to perform transcriptome profiling in liver of boars with divergent skatole levels in backfat by using RNA-Seq. The total number of reads produced for each liver sample ranged from 11.8 to 39.0 million. Approximately 448 genes were differentially regulated (p-adjusted <0.05). Among them, 383 genes were up-regulated in higher skatole group and 65 were down-regulated (p<0.01, FC>1.5). Differentially regulated genes in the high skatole liver samples were enriched in metabolic processes such as small molecule biochemistry, protein synthesis, lipid and amino acid metabolism. Pathway analysis identified the remodeling of epithelial adherens junction and TCA cycle as the most dominant pathways which may play important roles in skatole metabolism. Differential gene expression analysis identified candidate genes in ATP synthesis, cytochrome P450, keratin, phosphoglucomutase, isocitrate dehydrogenase and solute carrier family. Additionally, polymorphism and association analysis revealed that mutations in ATP5B, KRT8, PGM1, SLC22A7 and IDH1 genes could be potential markers for skatole levels in boars. Furthermore, expression analysis of exon usage of three genes (ATP5B, KRT8 and PGM1) revealed significant differential expression of exons of these genes in different skatole levels. These polymorphisms and exon expression differences may have impacts on the gene activity ultimately leading to skatole variation and could be used as genetic marker for boar taint related traits. However, further validation is required to confirm the effect of these genetic markers in other pig populations in order to be used in genomic selection against boar taint in pig breeding programs.

RNA deep sequencing reveals novel candidate genes and polymorphisms in boar testis and liver tissues with divergent androstenone levels

Boar taint is an unpleasant smell and taste of pork meat derived from some entire male pigs. The main causes of boar taint are the two compounds androstenone (5α-androst-16-en-3-one) and skatole (3-methylindole). It is crucial to understand the genetic mechanism of boar taint to select pigs for lower androstenone levels and thus reduce boar taint. The aim of the present study was to investigate transcriptome differences in boar testis and liver tissues with divergent androstenone levels using RNA deep sequencing (RNA-Seq). The total number of reads produced for each testis and liver sample ranged from 13,221,550 to 33,206,723 and 12,755,487 to 46,050,468, respectively. In testis samples 46 genes were differentially regulated whereas 25 genes showed differential expression in the liver. The fold change values ranged from −4.68 to 2.90 in testis samples and −2.86 to 3.89 in liver samples. Differentially regulated genes in high androstenone testis and liver samples were enriched in metabolic processes such as lipid metabolism, small molecule biochemistry and molecular transport. This study provides evidence for transcriptome profile and gene polymorphisms of boars with divergent androstenone level using RNA-Seq technology. Digital gene expression analysis identified candidate genes in flavin monooxygenease family, cytochrome P450 family and hydroxysteroid dehydrogenase family. Moreover, polymorphism and association analysis revealed mutation in IRG6, MX1, IFIT2, CYP7A1, FMO5 and KRT18 genes could be potential candidate markers for androstenone levels in boars. Further studies are required for proving the role of candidate genes to be used in genomic selection against boar taint in pig breeding programs.

Effects of nuclear receptor transactivation on boar taint metabolism and gene expression in porcine hepatocytes

The accumulation of the testicular steroid androstenone (AND) and tryptophan degradation product skatole (3MI) in fat results in boar taint, an off odor and flavor in boar meat. Increasing boar taint metabolism in the liver may help limit the deposition of AND and 3MI in fat, thereby improving meat quality. The effects of transactivation of the nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR), and farnesoid X receptor (FXR) on the expression levels of several transcripts of interest and the metabolism of AND and 3MI in primary porcine hepatocytes were tested. Primary cells were isolated from mature boars, and transcript expression levels were assayed using real-time PCR. The transcripts of interest included porcine orthologs of common phase I and phase II metabolic enzymes and transcripts previously shown to be differentially expressed in boars with high boar taint levels. Transactivation of CAR, PXR, or FXR resulted in altered expression of several transcripts, including increased expression of cytochrome P450 (CYP) 2B22 by CAR, of CYP2A19, CYP2B22, CYP2C33, and CYP2C49 by PXR, of CYP2C33 and CYP2E1 by FXR, and of CYP19A2 by all three receptors. Only transactivation of PXR had a significant effect on AND metabolism, resulting in 7.5±1.5% of the initial level of AND remaining compared to 21.4±3.1% remaining with control dimethyl sulfoxide (DMSO) treatment. FXR had the greatest effect on 3MI metabolism, increasing the expression of CYP2E1 by 1.29-fold and increasing the production of the key metabolite 6-hydroxy-3-methylindole (6-OH-3MI), while decreasing 5-hydroxy-3-methylindole (5-OH-3MI) production. 3-Hydroxy-3-methyloxindole (HMOI) production was increased by CAR transactivation, while indol-3-carbinol (I3C) production was increased by PXR and FXR transactivation, and by treatment with 5β-dihydrotestosterone (5β-DHT). From this, it can be concluded that selective transactivation of PXR and FXR may be a viable means of decreasing boar taint by increasing the hepatic metabolism of AND and 3MI.

Effects of nuclear receptor transactivation on steroid hormone synthesis and gene expression in porcine Leydig cells

Male pigs are routinely castrated at a young age to prevent the formation of androstenone, a 16-androstene testicular steroid that is a major component of boar taint. The practice of castration has been increasingly viewed as unfavorable, due to both economic considerations and animal welfare concerns. Other means of controlling boar taint, including reducing the synthesis of androstenone in the testes, would eliminate the need for castration. In this study, we determined the effects of transactivation of three nuclear receptors, the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and farnesoid X receptor (FXR), on gene expression and steroid hormone metabolism in primary porcine Leydig cells. Primary cells were isolated from mature boars, and transcript expression levels were assayed using real-time PCR. The transcripts of interest included porcine orthologs of common phase I and phase II metabolic enzymes, enzymes involved in steroidogenesis, and transcripts previously shown to be differentially expressed in boars with high androstenone and boar taint levels. Transactivation of CAR, PXR, or FXR increased the expression of several genes involved in steroidogenesis, including cytochrome B5A (CYB5A) and cytochrome B5 reductase 1 (CYB5R1), as well as hydroxysteroid (17-beta) dehydrogenase 4 (HSD17B4) and retinol dehydrogenase 12 (RDH12). Treatment with (6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO), a CAR agonist, or rifampicin (RIF), a PXR agonist, resulted in significantly (p<0.05) decreased sex steroid production and significantly (p<0.05) increased production of 16-androstene steroids. Treatment with the FXR agonist chenodeoxycholic acid (CDCA) resulted in significantly (p<0.05) decreased sex steroid production. These results indicate that transactivation of these nuclear receptors may lead to increased levels of 16-androstene steroids, likely by altering the activity of CYP17A1 through CYB5A and CYB5R1 to the andien-β synthase reaction and away from the 17α-hydroxylase and C17, 20 lyase reactions.

Genome-wide association scan and phased haplotype construction for quantitative trait loci affecting boar taint in three pig breeds

Background

Boar taint is the undesirable smell and taste of pork meat derived from some entire male pigs. The main causes of boar taint are the two compounds androstenone and skatole (3-methyl-indole). The steroid androstenone is a sex pheromone produced in the testis of the boars. Skatole is produced from tryptophan by bacteria in the intestine of the pigs. In many countries pigs are castrated as piglets to avoid boar taint, however, this is undesirable for animal welfare reasons. Genetic variations affecting the level of boar taint have previously been demonstrated in many breeds. In the study presented in this paper, markers and haplotypes, which can be applied to DNA-based selection schemes in order to reduce or eliminate the boar taint problem, are identified.

Results

Approximately 30,000 SNPs segregating in 923 boars from three Danish breeds; Duroc, Landrace, and Yorkshire, were used to conduct genome wide association studies of boar taint compounds. At 46 suggestive quantitative trait loci (QTL), 25 haplotypes and three single markers with effects were identified. Furthermore, 40% of the haplotypes mapped to previously identified regions. Haplotypes were also analysed for effects of slaughter weight and meat content. The most promising haplotype was identified on Sus scrofa chromosome 1. The gain in fixed effect of having this haplotype on level of androstenone in Landrace was identified to be high (1.279 μg/g). In addition, this haplotype explained 16.8% of the phenotypic variation within the trait. The haplotype was identified around the gene CYB5A which is known to have an indirect impact on the amount of androstenone. In addition to CYB5A, the genes SRD5A2, LOC100518755, and CYP21A2 are candidate genes for other haplotypes affecting androstenone, whereas, candidate genes for the indolic compounds were identified to be SULT1A1 and CYP2E1.

Conclusions

Despite the small sample size, a total of 25 haplotypes and three single markers were identified including genomic regions not previously reported. The haplotypes that were analysed showed large effects on trait level. However, little overlap of QTL between breeds was observed.

Large scale genome-wide association and LDLA mapping study identifies QTLs for boar taint and related sex steroids

Background

Boar taint is observed in a high proportion of uncastrated male pigs and is characterized by an unpleasant odor/flavor in cooked meat, primarily caused by elevated levels of androstenone and skatole. Androstenone is a steroid produced in the testis in parallel with biosynthesis of other sex steroids like testosterone and estrogens. This represents a challenge when performing selection against androstenone in breeding programs, without simultaneously decreasing levels of other steroids. The aim of this study was to use high-density genome wide association (GWA) in combination with linkage disequilibrium-linkage analysis (LDLA) to identify quantitative trait loci (QTL) associated with boar taint compounds and related sex steroids in commercial Landrace (n = 1,251) and Duroc (n = 918) breeds.

Results

Altogether, 14 genome wide significant (GWS) QTL regions for androstenone in subcutaneous fat were obtained from the LDLA study in Landrace and 14 GWS QTL regions in Duroc. LDLA analysis revealed that 7 of these QTL regions, located on SSC 1, 2, 3, 7 and 15, were obtained in both breeds. All 14 GWS androstenone QTLs in Landrace are also affecting the estrogens at chromosome wise significance (CWS) or GWS levels, while in Duroc, 3 of the 14 QTLs affect androstenone without affecting any of the estrogens. For skatole, 10 and 4 QTLs were GWS in the LDLA analysis for Landrace and Duroc respectively, with 4 of these detected in both breeds. The GWS QTLs for skatole obtained by LDLA are located at SSC 1, 5, 6, 7, 10, 11, 13 and 14.

Conclusion

This is the first report applying the Porcine 60 K SNP array for simultaneous analysis of boar taint compounds and related sex hormones, using both GWA and LDLA approaches. Several QTLs are involved in regulation of androstenone and skatole, and most of the QTLs for androstenone are also affecting the levels of estrogens. Seven QTLs for androstenone were detected in one breed and confirmed in the other, i.e. in an independent sample, although the majority of QTLs are breed specific. Most QTLs for skatole do not negatively affect other sex hormones and should be easier to implement into the breeding scheme.

Expression levels of 25 genes in liver and testis located in a QTL region for androstenone on SSC7q1.2

A quantitative trait locus (QTL) for boar fat androstenone levels has been identified near the SSC7 centromere in a Large White × Meishan cross. Backcrosses were produced to isolate the Chinese haplotype in a European genetic background. The expression of 25 genes from the QTL region was studied in the testes and livers of 5-month-old backcross boars, with the aim of identifying the causal gene. Using Fluidigm, a new high-throughput technology, the expression of 25 genes was measured in a single real-time PCR experiment. This study found six significantly down-regulated genes (C6ORF106, C6ORF81, CLPS, SLC26A8, SRPK1 and MAPK14) in the testes of MS-LW backcross boars. However, according to current knowledge, none of the genes appear to be related to androstenone metabolism. In the livers, none of the genes were significantly up- or down-regulated, including TEAD3, which was previously designated as a possible candidate to explain this QTL.

Differences in testosterone, androstenone, and skatole levels in plasma and fat between pubertal purebred Duroc and Landrace boars in response to human chorionic gonadotrophin stimulation

The concentrations of the boar taint compounds androstenone and skatole in plasma and fat, together with those of testosterone in plasma, were investigated in pubertal purebred Duroc and Landrace boars following stimulation with human chorionic gonadotrophin (hCG). Higher initial levels of androstenone and testosterone were found in Duroc than Landrace boars. Duroc boars, which were approximately ten days older than the Landrace boars, also showed a more advanced stage of spermatogenesis than Landrace boars. While Landrace boars had the highest skatole levels. Following stimulation with hCG the relative increases in testosterone, androstenone, and skatole concentrations were highest in Landrace boars. The level of androstenone in fat three days after hCG stimulation exceeded 1 microg/g fat in all stimulated boars. The decreases in plasma levels of androstenone and testosterone on Days 2 and 3 after hCG stimulation were more pronounced in Landrace than Duroc boars. However, unlike the plasma androstenone and testosterone levels, the plasma concentrations of skatole did not decrease on Days 2 and 3 following stimulation, but remained elevated on Day 3. These results indicate that the lower levels of testicular steroids in Landrace boars compared with Duroc boars was not due to a lower production capacity, but more likely to a faster disappearance of steroids in Landrace boars. In the present study, age, live weight, and testicular development did not significantly contribute to the variation in fat androstenone. The present data and previous reports on candidate genes related to androstenone biosynthesis and metabolism suggests that future selection against factors associated with boar taint remains a possible solution for the problem of boar taint in the swine industry.

A genome-wide association study on androstenone levels in pigs reveals a cluster of candidate genes on chromosome 6

BACKGROUND

In many countries, male piglets are castrated shortly after birth because a proportion of un-castrated male pigs produce meat with an unpleasant flavour and odour. Main compounds of boar taint are androstenone and skatole. The aim of this high-density genome-wide association study was to identify single nucleotide polymorphisms (SNPs) associated with androstenone levels in a commercial sire line of pigs. The identification of major genetic effects causing boar taint would accelerate the reduction of boar taint through breeding to finally eliminate the need for castration.

RESULTS

The Illumina Porcine 60K+SNP Beadchip was genotyped on 987 pigs divergent for androstenone concentration from a commercial Duroc-based sire line. The association analysis with 47,897 SNPs revealed that androstenone levels in fat tissue were significantly affected by 37 SNPs on pig chromosomes SSC1 and SSC6. Among them, the 5 most significant SNPs explained together 13.7% of the genetic variance in androstenone. On SSC6, a larger region of 10 Mb was shown to be associated with androstenone covering several candidate genes potentially involved in the synthesis and metabolism of androgens. Besides known candidate genes, such as cytochrome P450 A19 (CYP2A19), sulfotransferases SULT2A1, and SULT2B1, also new members of the cytochrome P450 CYP2 gene subfamilies and of the hydroxysteroid-dehydrogenases (HSD17B14) were found. In addition, the gene encoding the ss-chain of the luteinizing hormone (LHB) which induces steroid synthesis in the Leydig cells of the testis at onset of puberty maps to this area on SSC6. Interestingly, the gene encoding the alpha-chain of LH is also located in one of the highly significant areas on SSC1.

CONCLUSIONS

This study reveals several areas of the genome at high resolution responsible for variation of androstenone levels in intact boars. Major genetic factors on SSC1 and SSC6 showing moderate to large effects on androstenone concentration were identified in this commercial breeding line of pigs. Known and new candidate genes cluster especially on SSC6. For one of the most significant SNP variants, the difference in the proportion of animals surpassing the threshold of consumer acceptance between the two homozygous genotypes was as much as 15.6%.

Association between SNPs within candidate genes and compounds related to boar taint and reproduction

Background

Boar taint is an unpleasant odour and flavour of the meat from some uncastrated male pigs primarily caused by elevated levels of androstenone and skatole in adipose tissue. Androstenone is produced in the same biochemical pathway as testosterone and estrogens, which represents a particular challenge when selecting against high levels of androstenone in the breeding programme, without simultaneously decreasing levels of other steroids. Detection of single nucleotide polymorphisms (SNPs) associated with compounds affecting boar taint is important both for gaining a better understanding of the complex regulation of the trait and for the purpose of identifying markers that can be used to improve the gain of breeding. The beneficial SNPs to be used in breeding would have the combinational effects of reducing levels of boar taint without affecting fertility of the animals. The aim of this study was to detect SNPs in boar taint candidate genes and to perform association studies for both single SNPs and haplotypes with levels of boar taint compounds and phenotypes related to reproduction.

Results

An association study involving 275 SNPs in 121 genes and compounds related to boar taint and reproduction were carried out in Duroc and Norwegian Landrace boars. Phenotypes investigated were levels of androstenone, skatole and indole in adipose tissue, levels of androstenone, testosterone, estrone sulphate and 17β-estradiol in plasma, and length of bulbo urethralis gland. The SNPs were genotyped in more than 2800 individuals and several SNPs were found to be significantly (LRT > 5.4) associated with the different phenotypes. Genes with significant SNPs in either of the traits investigated include cytochrome P450 members CYP2E1, CYP21, CYP2D6 and CYP2C49, steroid 5α-reductase SRD5A2, nuclear receptor NGFIB, catenin CTNND1, BRCA1 associated protein BAP1 and hyaluronoglucosaminidase HYAL2. Haplotype analysis provided additional evidence for an effect of CYP2E1 on levels of skatole and indole, and for BAP1, HYAL2 and SRD5A2 on levels of androstenone.

Conclusion

The findings in this study indicate that polymorphisms in CYP2E1, CYP21, CYP2D6, CYP2C49, NGFIB and CTNND1 might be used to reduce levels of boar taint without affecting levels of testosterone, estrone sulphate, 17β-estradiol or length of bulbo urethralis gland.