畜牧兽医学报  2020, Vol. 51 Issue (6): 1187-1195. DOI: 10.11843/j.issn.0366-6964.2020.003    PDF    
鸡血糖性状的全基因组关联分析
刘晓静1,2, 刘璐1,2, 王杰1,2, 崔焕先1,2, 赵桂苹1,2, 文杰1,2     
1. 中国农业科学院北京畜牧兽医研究所, 北京 100193;
2. 动物营养学国家重点实验室, 北京 100193
摘要:旨在挖掘影响鸡血糖性状的有效SNP位点及功能基因,为优质肉鸡分子育种工作提供有效的理论支撑。本试验选取407只京星黄母鸡于98日龄屠宰,酚仿法提取血液DNA,进行深度为10×的全基因组重测序;葡萄糖氧化酶法测定血清中血糖水平,基于全基因组重测序和血糖表型数据进行全基因组关联分析(GWAS)。结果,GWAS共筛选到6个血糖相关的SNPs位点(关联阈值P < 1.43×10-6)。基因注释发现,rs734134177在UBE3D基因第8内含子上,其编码蛋白为泛素蛋白连接酶。该位点携带野生型(AA)个体的血糖水平极显著高于突变型(GG)个体(P < 0.01);rs794554022位于ACAD9基因下游D 93.5 kb处。ACAD9蛋白为酰基辅酶A脱氢酶家族的成员之一,是细胞线粒体中脂肪酰基辅酶A进行β-氧化过程中的限速酶。rs794554022位点携带野生型(AA)个体的血糖水平极显著低于携带突变型(CC)个体的(P < 0.01)。以上位点可能是调控血糖水平的相关候选SNPs位点,这两个位点所在基因可能参与了肉鸡血糖代谢的调控过程,这些结果将为调控肉鸡血糖代谢进而改善肉品质的育种工作提供候选的分子标记,为肉鸡血糖代谢的调控提供了新的思路。
关键词血糖        全基因组重测序    SNP    全基因组关联分析(GWAS)    分子标记    
Genome-wide Association Study of Chicken Blood Glucose Traits Using Whole Genome Resequencing
LIU Xiaojing1,2, LIU Lu1,2, WANG Jie1,2, CUI Huanxian1,2, ZHAO Guiping1,2, WEN Jie1,2     
1. Institute of Animal Science, Chinese Academy of Agricultural Sciences, Beijing 100193, China;
2. State Key Laboratory of Animal Nutrition, Beijing 100193, China
Abstract: This experiment aimed to explore the effective SNPs loci and functional genes related to blood glucose traits in chickens, and provide effective theoretical support for molecular breeding of high quality broilers. In this experiment, 407 Jing Xing Yellow hens were slaughtered at 98 days of age, blood DNA was extracted by phenol-chloroform method, and whole genome resequencing was performed at a depth of 10×; Glucose oxidase method was used to determine blood glucose levels in serum. The genome-wide association study (GWAS) were performed based on whole genome resequencing and glycemic phenotype data. GWAS screened a total of 6 blood glucose-related SNPs loci (associated threshold P < 1.43×10-6). The gene annotation revealed that rs734134177 was on the 8th intron of UBE3D gene, and its encoded protein was ubiquitin protein ligase. The blood glucose level of wild-type (AA) individuals at this locus was extremely significantly higher than that of mutant (GG) individuals (P < 0.01); rs794554022 was extremely located at D 93.5 kb of ACAD9 gene downstream. ACAD9 protein was a member of the acyl-CoA dehydrogenase family and was the rate-limiting enzyme in the process of β-oxidation of fatty acyl-CoA in mitochondria. The blood glucose level of the wild-type(AA) individuals at the rs794554022 locus was extremely significantly lower than that of the mutant(CC) individuals (P < 0.01). The above loci may be related candidate SNPs for regulating blood glucose level. The genes containing the two loci may be involved in the regulation of blood glucose metabolism in broilers. These results will provide candidate molecular markers for regulating the blood glucose metabolism of broilers and improving the meat quality, which will provide a new idea for the regulation of blood glucose metabolism in broilers.
Key words: blood glucose    chicken    whole genome resequencing    SNP    genome-wide association study (GWAS)    molecular marker    

血糖作为动物机体能量的直接和重要来源,在正常生理状态下,血糖水平不会随环境的改变而发生大的变化,但在机体发生病变或者环境恶劣的情况下,血糖水平会发生变化,血糖值的波动预示着机体的健康情况[1-3]。数据统计发现,屠宰前猪的血糖值与肉色、风味等肉质性状的评分呈极显著正相关。组织中的糖类对肉色、嫩度、pH等多种指标有影响,葡萄糖的代谢途径之一即为合成肝糖原与肌糖原,肌糖原分解为乳酸影响pH,进而影响肉品质[4-5]。血糖和脂肪的代谢相互影响,一定条件下可以相互转化[6-7]。脂肪酸是脂肪的主要组成物质之一,血糖浓度升高情况下会有一部分葡萄糖转化为脂肪。血糖参与糖脂代谢过程,影响畜禽的肉品质。

随着测序技术的快速发展和成本的降低,全基因组重测序技术正在被广泛应用和报道[8-9]。全基因组重测序可以比对个体测序的序列和已知该物种的参考序列,由此找到大量的SNP、SV、Indel和CNV,然后利用生物信息学分析在全基因组水平上不同个体或群体基因组间的变异,同时完成变异注释[10]。全基因组关联分析是以整个基因组的SNP作为分子标记,在全基因组水平上进行相关性分析,从中找到影响某一性状的变异基因或SNP关键位点[11-12]。Yoo等[13]利用长白×韩国本地猪种构建的F2群体在1和8号染色体定位到了影响血糖浓度的QTL;研究报道,白色杜洛克×二花脸资源家系在14号染色体68 cM处定位到了一个5%基因组显著影响血糖的QTL[14]。由于鸡有特殊的生理结构且其新陈代谢不同于人和其他哺乳动物,目前,对猪、牛等动物血糖性状的研究较多,对鸡的研究较少。本研究旨在挖掘影响鸡血糖的有效SNP位点和基因,为肉鸡的选育和提高其生产性能提供理论依据。

1 材料与方法 1.1 试验动物

本研究选择407只京星黄鸡母鸡为试验素材,所有的鸡均来源于中国农业科学院北京畜牧兽医研究所昌平鸡场,饲养环境和管理相同,所有肉鸡自由采食和饮水,进行常规免疫。营养标准按照NRC国际标准。饲养至98日龄,翅静脉采血,EDTA抗凝,-20 ℃条件下保存,用于提取DNA。另外采集1 mL非抗凝血分离血清,-80 ℃保存用于血液中葡萄糖测定。

1.2 血清中葡萄糖含量的测定

血液样品在室温下静置5 h,3 000 r·min-1 4 ℃离心10 min,分离血清,用葡萄糖氧化酶法[15]测定血清中葡萄糖含量。

1.3 血液DNA提取

使用标准的酚仿法提取血液DNA,用Nanodrop1000核酸蛋白分析仪(Thermo Scientific,USA)测定DNA浓度,测定标准为A260 nm/A280 nm比值为1.8~2.0。质量合格的样品送到北京博奥晶典公司进行深度为10×的全基因组重测序。

1.4 全基因组重测序

407个个体进行全基因组重测序后,使用fastp(0.19.5)进行质控过滤。参数设置为:合格的phred质量值为30,质量不合格碱基小于等于30%,每个read至少有75个碱基,剔除碱基质量低于30%的碱基。利用BWA[16]软件,采用mem模式,将过滤后的Clean Reads比对到参考基因组,这一步去掉未比对上、低质量或重复的Reads。以上步骤完成后,需要重新校正碱基质量(base quality score recalibration, BQSR)。变异检测极度依赖测序碱基质量值。BQSR主要通过机器学习的方法构建测序碱基的错误率模型,然后对这些碱基的质量值进行相应的调整。第一步利用picard-tools和Samtools得到排序好的bam文件,将bam文件用于haplotypeCaller检测,获得单个样本的gvcf文件,将单个的gvcf文件合成一个gvcf文件,进行群体分析。

1.5 全基因组关联分析

获得的测序原始数据首先用Beagle5.0[17]软件填充SNP基因座,用Plink v1.9[18]软件进行质量控制,标准为去除最小等位基因频率小于5%(MAF < 0.05)的位点和缺失率大于5%的位点(geno>0.05)。

最后使用GEMMA[19]软件进行分析,计算模型为:

$ y = {W_\alpha } + {X_\beta } + Zu + \varepsilon $

其中,y是定量表型值,W是协方差矩阵,α是协变量,X是固定效应矩阵,β是有效SNP位点的数量,Z是负荷矩阵,u是随机效应向量,ε是误差向量。

在本试验的分析中,将第一、第二主成分作为协变量,利用不连锁的位点建立亲缘关系矩阵(plink, ld-prune, r2>0.2),逐个对单个位点进行求解,并通过极大似然比检验法(likelihood ratio test, LRT)检验位点的显著性。

1.6 基因注释

用Ensemble(oct2018.archive.ensembl.org/Gallus_gallus/Info/Index)或NCBI(http://www.ncbi.nlm.nih.gov/)在鸡参考基因组数据库中搜寻关联性显著的SNP所在区域的已知基因信息,根据基因注释情况分析可能的位置候选基因。

2 结果 2.1 表型和基因分型数据质控

407个个体血糖表型最大值和最小值分别为13.2和4.5 mmol·L-1,平均值为8.82 mmol·L-1,变异系数为16%。经质控后MAF < 0.05的位点有3 803 046个,位点缺失率大于5%的位点有96 575个,质控前个体407个,位点13 249 224个,质控后个体407个,位点9 349 603个。染色体质控结果见表 1

表 1 染色体质控结果 Table 1 Quality control results for each chromosome
2.2 全基因组关联分析结果

P值矫正首先使用PLINKv1.9软件进行blocks分析,设置D为0.8,连锁最大区域长度为500 kb,共获得699 341个独立LD区块。然后利用Bonferroni校正多重检验确定显著性阈值,染色体显著水平阈值为7.15×10-8(0.05/699 341),潜在显著水平阈值为1.43×10-6(1/699 341)。

曼哈顿图分析结果发现,达到潜在基因组水平显著差异的SNPs位点共6个(图 1表 2)。其中,1号染色体上的rs316791138和rs312803988位点附近是CHAMP1基因,其主要具有核酸结合的作用;3号染色体rs16326282位点加减100 kb范围内并未发现基因,而rs734134177位点位于UBE3D基因上,所编码的E3泛素蛋白连接酶可从特定的E2泛素结合酶中吸收泛素,并将其转移至底物,通常促进其被蛋白酶体降解;6号染色体上的10381728位点在一个lncRNA内;12号染色体的rs794554022位点处上游93.5 kb为基因ACAD9,该基因编码酰基脱氢酶家族成员,对棕榈酰辅酶A(C16:0)和硬脂酰辅酶A(C18:0)具有脱氢酶活性,催化棕榈酰辅酶A的活性是硬脂酰辅酶A的3倍。然而,它在体内长链脂肪酸氧化中不起主要作用[20-21]

蓝色线为达基因组水平5%显著水平阈值线,红色线为达基因组潜在关联阈值线 The blue line is the 5% significant level threshold line at the genomic level, and the red line is the potential associated threshold line for the genome 图 1 血糖的全基因组关联分析曼哈顿图 Fig. 1 Manhattan plot of genome-wide association study for blood glucose trait
表 2 血糖含量Bonferroni校正的潜在水平显著的SNPs位点信息 Table 2 The significant SNPs loci information at Bonferroni corrected potential level for blood glucose
2.3 位点加性效应分析

基于全基因组重测序数据的SNP位点分型结果,分别计算rs316791138、rs312803988、rs734134177、rs16326282、rs794554022以及6号染色体10381728等6个SNPs的基因型频率和表型均值(表 3),并进行等位基因的加性效应分析。结果表明,除去6号染色体位点,其他5个位点的优势等位基因都能够降低血糖含量。将显著SNP基因型添加到单变量模型中,以在逐步条件分析中阐明独立信号,发现rs734134177和rs794554022野生型和突变型个体表型间达到显著差异(图 2图 3)。

表 3 显著SNPs位点的等位基因和基因型频率以及加性效应 Table 3 Alleles and genotypes frequencies, additive effect of the significant SNPs loci
AA为野生型,AG为杂合型,GG为突变型。*表示差异显著(P < 0.05), **、***、****均表示差异极显著(P < 0.01),下同 AA is the wild type, AG is the heterozygous type, GG is the mutant type. *show the significant difference(P < 0.05), **, ***, **** show the extremely significant difference(P < 0.01), the same as below 图 2 rs734134177不同基因型个体的表型差异 Fig. 2 Phenotypic differences of individuals with different genotypes at rs734134177 locus
AA为野生型,AC为杂合型,CC为突变型。ns表示差异不显著(P>0.05) AA is the wild type, AC is the heterozygous type, CC is the mutant type. ns means no significant difference(P>0.05) 图 3 rs794554022不同基因型个体的表型差异 Fig. 3 Phenotypic differences of individuals with different genotypes at rs794554022 locus
3 讨论

肉鸡养殖中有一种常见病为肉鸡低血糖-尖峰死亡综合征[22],主要特征之一为低血糖,血糖供应不足,细胞有氧呼吸过程受到影响,新陈代谢下降,导致机体异常,鸡群大量死亡。血糖作为预示机体健康的重要指标之一,在肉鸡养殖中发挥着重要的作用。研究报道,猪的血糖值与肉色、大理石纹、风味评分存在极显著相关,血糖值较低时,猪肉的上述品质会呈现不同程度下降[23]。血糖作用于糖代谢过程合成肌糖原[24],肌糖原分解代谢生成乳酸,直接影响pH;血糖自身在无氧酵解过程中会生成乳酸,影响pH,pH是衡量肉质的指标之一[25]。总之,血糖在肉鸡的生长发育和肉品质形成等方面均有着重要的调控作用[26-27],在肉鸡生产中具有重要的参考价值。

目前,GWAS已经在鸡的肉品质和抗病育种开发分子标记等方面得到了广泛应用,并取得了很好的效果。Liu等[28]使用SNP芯片研究了北京油鸡屠宰和肉品质性状,发现了24个与鸡屠宰性状关联的SNPs位点;Sun等[29]通过芯片数据对北京油鸡和科宝肉鸡的F2群体共10个肉质性状进行了GWAS分析,最终得到33个关联位点和14个候选基因。本试验基于全基因组重测序,对407个京星黄母鸡个体进行了10×测序,与血糖表型进行的全基因组关联分析,共得到了6个存在潜在关联水平的SNPs位点。相较于大多数芯片的全基因组关联分析研究,重测序的数据位点覆盖全面,得到的结果更精确。对全基因组显著关联水平SNP分析发现,rs734134177在UBE3D基因的第8内含子上, UBE3D作为一个泛素蛋白连接酶,参与途径包括I类MHC介导的抗原加工和呈递以及先天免疫系统。在2015年的全基因组关联分析研究中报道了它对视网膜色素等的影响[30],牙周炎的全基因组关联分析中也挖掘到UBE3D基因[31]。迄今为止,没有关于UBE3D影响血糖含量的报道,但是rs734134177位点A突变为G后,血糖含量显著降低,提示该位点可能参与调控鸡的血糖含量。

rs794554022附近的ACAD9基因编码酰基辅酶A脱氢酶家族的成员,该蛋白质家族的成员定位于线粒体,并且催化脂肪酰基辅酶A的β-氧化过程的限速步骤[21],编码的蛋白质对棕榈酰辅酶A和长链不饱和底物具有特异活性[32-33]。酰基辅酶A为脂肪酸与辅酶A的硫醇脂化合物,也就是脂肪酸合成和分解的活性代谢中间产物,水解会生成脂肪酸和辅酶A。有研究表明,处在低血糖状态下,机体打破血糖平衡,促进脂肪酸的氧化过程[34-36]。脂肪酸是脂肪的主要组成物质之一,血糖浓度升高情况下会有一部分葡萄糖转化为脂肪。脂肪酰基辅酶A具有合成脂肪酸的作用,进而会影响到血糖含量的变化[37-38]。在酶法检测中发现,棕榈酰辅酶A会抑制完整微粒体中葡萄糖-6-磷酸酶的活性,进而得出大鼠肝微粒中脂酰辅酶A对葡萄糖-6-磷酸酶有抑制作用[39]。葡萄糖-6-磷酸酶是一种水解磷酸化合物的磷酸酶,在肝组织中通过水解葡萄糖-6-磷酸释放葡萄糖入血,饥饿时肝糖原能够补充血糖,维持机体血糖平衡。作为糖异生过程中的关键酶,棕榈酰辅酶A在血糖形成过程中发挥着重要作用。rs794554022位点突变基因型相比于其他位点具有较高的升血糖作用,但是突变个体较少,可能是群体样本较少,后续还需要做大样本、更深入的研究。ACAD9可以作为影响鸡血清中血糖含量的候选基因,为京星黄鸡的标记辅助选择提供参考,相关机理尚需进一步验证。

研究报道,与rs316791138和rs312803988位点相关的CHAMP1基因编码的蛋白质在线粒体-微管附着和染色体分离调控中起作用,主要影响神经发育[40]。ENSGALG00000035579未见关于其功能研究的报道。目前,对于这几个基因的相关研究极少,在鸡中更是鲜见报道,该研究可以为京星黄鸡选育血糖性状提供相关理论支持,对进一步改善肉品质等性状提供技术支持。前期报道有利用60K芯片对不同品种的鸡进行血糖性状的全基因组关联分析[41],本研究利用全基因组重测序数据对鸡血糖性状进行全基因组关联分析,揭示了几个影响血糖代谢过程的基因,共得到了6个主要相关的SNPs位点,在位点附近找到了候选基因ACAD9、UBE3D等。考虑到全基因组重测序的全面性,结合全基因组关联分析对显著SNP的可靠性及其对表型变异的解释,本研究结果将对以后肉鸡的肉品质育种提供新的见解且筛选出有效的分子标记。

4 结论

综上所述,通过全基因组重测序数据对鸡血糖性状的GWAS研究共筛选到6个显著位点,经过一系列分析后发现,rs734134177和rs794554022位点可能是调控血糖水平的相关候选SNPs位点,与两个位点相关的基因可能参与了肉鸡血糖代谢的调控过程,本研究结果将为调控肉鸡血糖代谢进而改善肉品质的育种工作提供候选分子标记,为肉鸡血糖代谢的调控提供新的理论支持。

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