Study On Phenotypic Variation And Genetic Basis Of Purine Content In Pork

In past decades,because of the change in dietary structure and the growth of intakes of purines per capita,the morbidities of hyperuricaemia and gout have increased about tenfold in China,thus purine contents in food are gaining increasing attention of consumers and food scientists.The purine content of meat products is high,and pork accounts for about 65% of total meat consumption in China,but the knowledge about phenotypic variation and genetic basis for purine contents in pork remains very limited.In this study,we first assessed the contents of purine bases(including guanine,adenine,hypoxanthine,and xanthine)and multiple meat quality traits on 455 longissimus dorsi(LD)samples from six different pig breeds including 6 Duroc,18 DLY,18Bamaxiang(BMX),18 Erhualian(EHL),18 Laiwu(LW),and 377 F6 pigs of heterogeneous stock(HS).Meanwhile,three HS castrated boars were randomly selected for measurement of purine contents of 12 tissues or organs.We then conducted the whole genome resequencing of 377 F6 HS pigs and the RNA-seq of longissimus muscle from 40 HS F6 individuals with extreme low and high pork purine contents.Using these data,we investigated the impacts of breeds,tissues and sexes on purine contents and the relationships between purine contents and other meat quality traits,and the genetic loci(QTL)for purine base contents and total purine content(TPC)through a genome-wide association study(GWAS).The main results are as follows:(1)Dynamic change of purine contents in postmortem muscles of pigs: Adenine was rapidly converted into hypoxanthine in meat until 24 h postmortem,while the guanine and the total purine content(TPC)maintain constant levels.(2)Significant differences in TPC among breeds: The LW pigs had lower average TPC than that of other breeds(P<0.01),except Duroc.The average TPC was also lower in the EHL pigs than in the BMX pigs(P<0.05).(3)Significant differences in TPC between genders: The average TPC of castrated boars(137.71mg/100g)was significantly lower than that of sows(139.90mg/100g).(4)Significant differences in TPC among tissues: Lung,liver and kidney,containing beyond 200mg/100 g TPC,belong to high-purine tissues;Muscles,e.g.cardiac muscle,skeletal muscle and smooth muscle,are moderate abundant purine tissues with about130mg/100 g of TPC;Backfat,skin and hoof connective tissues,have only about30mg/100 g of TPC and thereby belong to low-purine tissues.(5)Significant correlations(P<0.05)were observed between TPC and all analyzed meat quality traits including p H,meat color,and intramuscular fat content(IMF),and their correlation coefficients ranged from 0.2 to 0.4.This result suggested that lower purine pork tend to exhibit higher ultimate p H,IMF,and water-holding capacity,as well as redder color,more tenderness and better taste.Therefore,decreasing purine contents in pork will be not in conflict with traditional breeding direction of some meat quality traits,and maybe benefit to improve other quality traits in some degree.(6)GWAS identified 40 SNPs(P<5.00×10-8)significantly associated with TPC,which represent about 11 QTLs.Among these SNPs,the most significant SNP rs32337327(P=2.75×10-9)was located in the introgenic region of LDB and QDPR,whereas the top SNP on SSC3,SSC11 and SSC14 fall in the introns of TGFBRAP1,MPHOSPH8 and C12orf43,respectively.Moreover,263 significant SNPs for guanine level were found to distribute on 26 QTL regions.Among them,the most significant SNP rs343436977(P=9.72×10-13)localizes in the intron of ABCC4 and additional 9 GWAS tag SNPs also reside within genes.No genome-wide significantly associated SNPs have been found for adenine and hypoxanthine content,but two suggestive QTLs(P<5.00×10-6)may exist on SSC7 and SSC8,where their top SNPs(rs324018311 and rs702472296)were found within the PAQR8 and SLC10A7 genes,respectively.The GWAS singals for the sum of adenine and hypoxanthin were similar to that of TPC.(7)Analysis of transcriptomic data of high-and low-purine pigs revealed 25 differential expression transcripts.However,only 8 transcripts were assigned to the corresponding Ensembl-annotated genes,including RYK,ORAOV1,MTERF1,MOSPD1,SLCO1A2,B3GNTL1,ZDHHC7 and FAM92 A.Of note,these differentially expressed genes(DEGs)were not presented in the identified QTLs,suggesting that the purine contents in pork may be regulated by complex genetic factors.(8)Collecting 70 annotated genes nearest to the GWAS tag SNPs and 8 DEGs,gene ontology(GO)analysis identified 11 functional terms.Notably,8 genes were enriched in calcium ion binding.KEGG pathway analysis indicated that ABCC4,ADCY2,and GRIA1 were involved in the c AMP signaling pathway.In addition,out of the 78 encoding genes,ten have ATP binding site and fourty-one belong to membrane protein.The results suggest that calcium binding porteins and c AMP signaling pathway may regulate the purine metabolism and transportation in muscle cells,thereby affecting the purine content in LD.This is the first study to reveal lots of genetic variations and candidate genes associated with the purine contents in pork,and to provide new insight into the role of calcium and c AMP signaling pathways in purine metabolism.These results lay a solid foundation for further revealling the genetic mechanism of purine content in muscle,but also provided theoretical and technical support for the cultivation of low purine pig breeds.