上海海洋大学学报  2020, Vol. 29 Issue (6): 811-819    PDF    
基因编辑酪氨酸酶(TYR)基因不同功能区对鱼类体色的影响
许细丹1,2,3, 陈红林1,2,3, MANDAL Biplab Kumar1,2,3, 司周旋1,2,3, 王军1,2,3, 王成辉1,2,3     
1. 上海海洋大学 农业农村部淡水水产种质资源重点实验室, 上海 201306;
2. 上海海洋大学 水产科学国家级实验教学示范中心, 上海 201306;
3. 上海海洋大学 上海水产养殖工程技术研究中心, 上海 201306
摘要:以AB系斑马鱼(Danio rerio)为研究对象,应用CRISPR/Cas9基因编辑技术,对纯合亲鱼受精卵进行显微注射,通过编辑损坏Tyr基因的CDS区第二外显子和3'-UTR非poly-A加尾信号区,使基因发生突变,检测基因不同功能区经编辑后对鱼类体色的影响。结果显示:Tyr基因在野生型斑马鱼胚胎的各个发育时期均能正常表达,眼睛出现黑色素时的表达量最高;编辑损坏Tyr基因的CDS区后,突变型斑马鱼的胚胎和仔鱼均未出现黑色素细胞,表现为体表白化,而成鱼会再度出现黑色素细胞,表现为体表黑色条纹断裂。编辑损坏Tyr基因的3'-UTR非poly-A加尾信号区后,突变型胚胎、仔鱼和成鱼均未出现黑色素减褪,黑色素合成未受影响。研究表明,编辑损坏基因的CDS区会对生物表型产生显著影响,而编辑损坏3'-UTR非poly-A加尾信号区对生物表型的影响有限。
关键词Tyr    基因编辑    CDS    3'-UTR    黑色素    CRISPR/Cas9    

酪氨酸酶(Tyrosinase, Tyr)基因是黑色素合成通路中的关键基因, 通过参与L-多巴(Levodopa, L-DOPA)的烃化和氧化形成多巴醌(Dopaquinone, DOPA-quinone)来调控黑色素的形成, 其表达量和活性水平决定了黑色素生成的速度和种类[1-4]。已有研究表明, Tyr基因突变会导致人的眼皮白化病(Oculocutaneous albinism, OCA2)[1]。此外, Tyr基因的突变与动物的白化现象密切相关, 如丹麦奶牛会出现毛发、皮肤、眼睛白化现象[5], 兔出现毛发和眼睛虹膜的完全白化[6]Tyr基因突变导致青鳉(Oryzias latipes)的黑色素细胞异常, 表现出橘色皮肤[7]; 虹鳟(Oncorhynchus mykiss)Tyr基因突变体表现出嵌合白化和完全白化现象[8]; 黄颡鱼(Tochysurus fulvidraco)Tyr基因突变体出现黑色素缺乏, 表现为金黄色[9]。由此可见, Tyr基因突变会对动物的体色带来重要影响。

动物的基因由编码区(CDS)、5'非翻译区(5'-untranslated region, 5'-UTR)、3'非翻译区(3'-untranslated region, 3'-UTR)、调控区等结构组成。其中, 3'非翻译区作为非编码DNA调控区域, 可参与调控基因mRNA的衰变和翻译, 具有重要的转录调节功能[10-13]。目前, 基因的突变研究大多关注基因的CDS区突变对该基因功能的影响, 而较少关注3'-UTR突变对基因功能的影响。已有研究[14]表明, 3'-UTR是秀丽隐杆线虫生殖系基因表达的主要调节因子。人类气道上皮细胞趋化因子配体(Chemokineligand, CXCL)基因的3'-UTR突变可导致该基因表达水平下降[15]。敲除Ras相关的C3肉毒素底物1(Ras-related C3 botulinum toxin substrate 1, Rac1)基因的3'-UTR会损害小鼠初级皮层神经元树突的生长发育[16]。此外, 相关体色基因研究[17]发现, 酪氨酸酶相关蛋白1(Tyrosinase-related protein 1, Tyrp1)基因的3'-UTR碱基插入或缺失会导致欧洲家兔的毛色呈褐色。应用基因敲除技术损坏莲山兔Tyr基因的3'-UTR区域, 也得到了褐色毛色的突变体[18]。这些研究结果表明, 基因的3'-UTR突变会对动物带来重要影响。

CRISPR/Cas9基因编辑技术作为一种高效的基因编辑技术, 已广泛应用于动物[19-20]、植物[21-22]、微生物[23]的基因功能编辑, 并可实现大片段外源基因敲入[24]和定点突变[25]。在鱼类, CRISPR/Cas9靶向编辑 Slc45a2 基因得到白化斑马鱼[26], 靶向编辑Tyr基因得到白化大型泥鳅[27]。本实验室应用CRISPR/Cas9技术编辑瓯江彩鲤Asip基因, 发现突变体表现出体表黑色素细胞缩小, 黑色斑块弥散的现象[28]。以斑马鱼为对象, 利用CRISPR/Cas9技术分别对Tyr基因的CDS和3'-UTR非poly-A加尾信号区域进行编辑, 旨在探索Tyr基因不同功能区对体色的影响, 以便为瓯江彩鲤以及其他鱼类的体色遗传机制研究积累资料。

1 材料与方法 1.1 实验材料

实验材料为AB系斑马鱼, 饲养于28 ℃恒温循环水系统(上海海圣循环水养殖系统)。

Tyr基因表达时序分析材料:取3~4月龄斑马鱼亲鱼繁育授精, 在受精后第6、12、24、30、36、48和60小时取胚胎, 以及1、2、3、4、5、10、15 dph(孵化出膜后的天数)取稚幼鱼, 使用体式显微镜观察和拍照黑色素细胞出现时间、数量及大小等情况后, 置于液氮中速冻并保存于-80 ℃冰箱备用。

Tyr基因编辑材料:剪取3~4月龄斑马鱼亲鱼鳍条, 使用酚氯仿抽提法提取基因组DNA并检测纯度。在Ensembl网站(https://asia.ensembl.org/index.html)下载斑马鱼的Tyr基因CDS区、3'-UTR区序列, 使用SnapGene软件分别设计引物对它们进行PCR扩增(表 1), 扩增产物送公司测序(生工生物工程股份有限公司)。分别筛选出Tyr基因不同功能区纯合且无突变的候选亲鱼作为基因编辑实验亲鱼。 Tyr基因CDS和3'-UTR区的PCR扩增、定量表达检测引物以及基因编辑靶点序列 PCR amplification, qRT-PCR primers and gene editing target sequence of CDS and 3'-UTR regions in Tyr gene

Tyr下游基因(Tyrp1, Dct)表达分析材料:根据Tyr基因CDS成鱼突变体皮肤黑色素细胞密度以及分布情况, 取突变体体侧无黑色素区域、少量黑色素区域、大量黑色素区域等不同皮肤区域, 置于液氮中速冻并保存于-80 ℃冰箱备用。

1.2 Tyr基因和下游基因(Tyrp1, Dct)表达分析

使用Trizol (TaKaRa, 日本)法提取上述样本的总RNA, 经2%凝胶电泳和Nanodrop 2000检测其纯度和浓度。使用PrimeSriptTM RT reagent Kit (TaKaRa, 日本)试剂盒反转录得到cDNA, 并保存于-40 ℃冰箱。通过在线网址(https://www.genscript.com/tools/real-time-pcr-taqman-primer-design-tool)设计Tyr基因CDS区和下游基因(Tyrp1, Dct)CDS区的定量表达检测引物(表 1), 选用β-actin基因作为内参基因。使用2-ΔΔCT[29]法分析Tyr基因和下游基因的定量表达结果。

1.3 sgRNA的靶点设计与体外转录

斑马鱼Tyr基因CDS区第二外显子和3'-UTR非poly-A加尾信号区的编辑靶点通过在线网址(http://zifit.partners.org/ZiFiT/CSquare9Nuclease.aspx)设计。合成含有靶点的正向Target序列与反向Scaffold, 并进行sgRNA体外转录, PCR扩增程序:94 ℃预变性3 min; 94 ℃变性30 s, 65 ℃复性30 s, 72 ℃延伸1 min, 共34个循环; 72 ℃延伸5 min。取5 μL PCR产物经2%琼脂糖凝胶电泳, 确认明亮且清晰单一的条带, 大小为100 bp左右。根据AxyPrep PCR Cleanup Kit(AXYGEN, 美国)试剂盒回收PCR产物, 应用Promega P1300体外转录试剂盒(Promega, 美国)进行体外转录合成sgRNA。最后使用乙醇沉淀法纯化回收, DEPC水溶解, 经琼脂糖凝胶电泳和NanoDrop2000检测纯度和含量, 保存于-80 ℃冰箱[30]

1.4 显微注射与编辑效率检测

将gRNAs(150 ng/μL)和Cas9蛋白(127 ng/μL, NEB, 美国)混合后, 显微注射到斑马鱼受精卵的1期细胞中。注射2 d后随机采集20粒眼睛色素缺陷的CDS编辑胚胎, 以及20粒3'-UTR编辑胚胎, 提取DNA, 进行Tyr基因CDS和3'-UTR靶点区域PCR扩增, 产物测序以分析编辑效率。显微镜下观察野生型和突变型胚胎(48 h)、仔鱼(出膜后3 d)、成鱼(性成熟)的整体和局部特征, 并进行基因型和表型分析比较。

2 结果 2.1 Tyr基因在斑马鱼胚胎中的表达

斑马鱼胚胎发育形态与定量表达结果(图 1)显示, Tyr基因在14个检测时期都有表达, 其中以受精后36 h表达量最高, 此时胚胎眼部出现色素沉着, 形成可见黑色素细胞。受精后48 h表达量下降, 此时黑色素细胞延伸至胚胎全身, 形成早期背侧和腹部条纹。至出膜第3天后, 随着胚胎背侧和腹部条纹逐渐明确与密集, 稚鱼Tyr基因表达量逐渐稳定。至出膜后15天, 形成了细小的早期两侧条纹, 胸鳍、臀鳍和尾鳍均分布圆形细小的黑色素细胞。

dph.孵化出膜后的天数 dph.days post hatch 图 1 Tyr基因在斑马鱼胚胎和稚鱼发育时期的相对表达量变化 Fig. 1 Relative expression profiles of Tyr gene at different embryonic and larva stages in zebrafish
2.2 Tyr基因CDS区的编辑

通过检测显微注射组的胚胎, 发现20个胚胎中有18个胚胎出现不同程度的突变。将突变体饲养至成体时, 选取体表黑色素条带褪断明显的突变个体的鳍条DNA进行PCR检测, 单克隆测序结果显示, 存在15个突变型序列和5个野生型序列, 编辑效率达75%, 其中73.3%为核苷酸移码突变, 导致Tyr基因编码氨基酸的改变(图 2)。

W-E.野生型胚胎; W-L.野生型仔鱼; W-A.野生型成鱼; M-E.突变型胚胎; M-L.突变型仔鱼; M-A.突变型成鱼; 红色表示靶点序列; 蓝色表示替换序列; 绿色表示突变氨基酸序列; “-”缺失; “+”插入 W-E. wild-type embryo; W-L. wild-type larvae fish; W-A. wild-type adult fish; M-E. mutant embryo; M-L. mutant larvae fish; M-A. mutant adult fish; Targets are marked in red; Substitution sequences are presented in blue; Mutant amino acid sequences are located in green; deletions"-"; insertions"+" 图 2 Tyr基因CDS区经编辑后的突变体表型及其DNA序列 Fig. 2 Phenotypes and DNA sequence of mutant fish by targeting CDS region of Tyr gene in zebrafish

与野生型相比, 突变型胚胎和仔鱼均未出现黑色素细胞, 整体表现为白化状态。性成熟成鱼体表出现由色素细胞组成的体表条纹图案, 皮肤暗条纹沿背腹轴呈断裂现象, 断裂处缺失黑色素细胞, 非断裂处黑色素细胞排列整齐, 未出现黑色素细胞的缩小。眼睛和鳍条黑色素细胞排列紧密, 树突结构明显(图 2)。

下游基因(Tyrp1, Dct)定量表达结果(图 3)显示, 下游基因在Tyr基因CDS成鱼突变体体表侧面无黑色素区域、少量黑色素区域以及大量黑色素区域等不同皮肤区域均表达, 其中下游基因在无黑色素区域表达量较其他区域小。

M-A-1.突变型成鱼无黑色素区域; M-A-2.突变型成鱼少量黑色素区域; M-A-3.突变型成鱼大量黑色素区域 M-A-1.region with no melanophore in mutant adult fish; M-A-2.region with less melanophore in mutant adult fish; M-A-3.region with large amount of melanophore in mutant adult fish 图 3 下游基因在Tyr基因CDS成鱼突变体不同皮肤区域的相对表达量变化 Fig. 3 Relative expression profiles of the downstream gene at different regions of skins in adult zebrafish of editing CDS region of Tyr gene
2.3 Tyr基因3'-UTR区的编辑

显微注射组胚胎PCR测序检测结果显示, 20个胚胎中只有6个受到了不同程度的编辑影响。选取性成熟突变个体, 经单克隆测序检测, 发现20个克隆中突变型序列占90%, 即编辑效率为90%, 包括1/3的核苷酸插入突变和2/3的核苷酸删除突变, 插入片段长达18 bp, 删除片段长达78 bp(图 4)。

W-E.野生型胚胎; W-L.野生型仔鱼; W-A.野生型成鱼; M-E.突变型胚胎; M-L.突变型仔鱼; M-A.突变型成鱼; 红色.正向靶点序列; 绿色.反向靶点序列; 紫色.插入序列; (AA).正向和反向靶点共有序列; “-”缺失; “+”插入 W-E. wild-type embryo; W-L. wild-type larvae fish; W-A. wild-type adult fish; M-E. mutant embryo; M-L. mutant larvae fish; M-A. mutant adult fish; Forward targets are marked in red; Reverse targets are marked in green; Insertion sequences are presented in purple; Common sequences between forward and reverse target are shown with AA; deletions"-"; insertions "+" 图 4 Tyr基因3'-UTR区经编辑后的突变体表型及其DNA序列 Fig. 4 Phenotypes and DNA sequence of mutant fish by targeting 3'-UTR region of Tyr gene in zebrafish

观察发现突变型胚胎期、仔鱼期的卵黄囊和胚体上均分布黑色素细胞, 并形成正常的早期背侧和腹部条纹, 视网膜黑色素沉着形成可见黑色素。性成熟突变型成鱼体表图案明显, 黑色素细胞色素颗粒聚集, 暗条纹黑色素细胞排列紧密, 背侧、腹侧和两侧色素条纹加深, 未出现黑色素细胞减少以及树突结构的缩小(图 4)。

3 讨论

在脊椎动物中, 酪氨酸酶基因家族中Tyr基因是黑色素合成信号通路中的关键限速酶, 黑色素生成的速度和种类取决于Tyr基因的表达量和活性水平, 因此Tyr基因在组织中广泛表达。Tyr基因在黄颡鱼胚胎中的表达结果显示在胚胎发育阶段存在差异表达, 呈现由低逐渐升至最高后降低的趋势, 在皮肤和鳍条开始形成大量黑色素时表达量最高[9]。在橘色双冠丽鱼中, Tyr基因在胚胎发育各个时期均表达, 黑色素细胞开始形成时期表达量急速增至最高[30]。本研究发现, 斑马鱼Tyr基因在各胚胎发育阶段表达, 眼睛黑色素细胞开始形成阶段表达量最高, 这一结果与上述结果符合。有研究[31-32]表明斑马鱼胚胎黑色素细胞大部分形成后的12 d中只会有微量的黑色素细胞数量上的调节。与本实验胚胎发育后阶段表达无明显差异结果相符合。

生物体中基因的表达主要通过翻译蛋白表示, 而翻译起始在很大程度上依赖于起始密码子以及相对于CDS的上下游区域[33-34]。基因的CDS通过编码氨基酸来表达基因, 基因的UTR调控区主要通过与相关结合蛋白或miRNA作用介导基因转录的调控[35-36]。应用CRISPR/Cas9技术编辑连山黑兔Tyr基因CDS区的结果显示, 删除CDS区105 kb的大长度片段可导致连山黑兔的完全白化[37]。应用CRISPR/Cas9技术编辑连山黑兔Tyr基因的3'-UTR调控区发现, 删除其两个有效poly-A加尾信号, 突变体虹膜和毛色中的黑色素减少, 产生褐色连山兔[18]。在欧洲兔中, Tyrp1基因第二外显子的突变引起终止密码子的提前, 突变体毛色呈褐色。同样Tyrp1基因的3'-UTR核苷酸插入或缺失会导致欧洲兔毛呈褐色[17]。本研究通过CRISPR/Cas9技术编辑Tyr基因CDS区和3'-UTR非poly-A加尾信号区, 发现Tyr基因的CDS区突变型胚胎、仔鱼和性成熟成鱼的黑色素发育存在缺陷, 表明Tyr基因CDS区对斑马鱼各个发育时期黑色素的形成都发挥着重要作用。然而Tyr基因的3'-UTR突变型胚胎、仔鱼和性成熟成鱼黑色素细胞发育正常, 与野生型相比, 体表未出现明显差异。在哺乳动物, 加尾信号(AAUAAA)六碱基的缺失或突变会阻碍多腺苷酸化3'端的产生, 3'-UTR序列保守程度高于其他真核生物, 且某特定保守序列(UAAC/GUUAU)仅在3'-UTR有活性[38-40]。此外, 有研究[41-45]表明3'-UTR包含多种顺式调控元件, 通过RNA结合蛋白和miRNA识别并结合对基因进行调控, 且许多元件已被实验证实为活性元件。结合本实验, 实验结果引起了3'-UTR碱基的插入与缺失, 但可能未对poly-A加尾信号区造成影响, 推测斑马鱼Tyr基因3'-UTR非poly-A加尾信号区可能不存在活性保守区域, 并未参与Tyr基因的调控。此外, 也有可能本实验靶点作用区域并未影响3'-UTR活性元件以及与其他调控因子的作用。在生物体的黑色素合成过程中, α-促黑素细胞激素(α-Melanocyte-stimulating hormone, α-MSH)可通过与黑色素细胞膜上的黑色素皮质激素受体1(Mela-nocortin-1 receptor, Mc1r)结合促使Tyr上调合成多巴; 同样α-MSH还可通过激活小眼畸形相关转录因子(Micropht halmia-associtated transcription factor, Mitf)提高酪氨酸酶基因家族活性, 合成黑色素[46]。同样, 研究表明Mc1r[47], Mitf[48], Tyrp1[9], Dct[49], Slc24a5[26]等单基因的突变会导致生物存在不同程度的黑色素发育缺陷。本研究Tyr基因的CDS区突变型仔鱼和性成熟成鱼的黑色素发育模式不同, 表现为完全白化的仔鱼和体表黑色素分布不均的性成熟成鱼, 在Tyr基因突变导致其功能缺失时, 其下游基因( Tyrp1,Dct)在成鱼突变体不同皮肤区域表达存在差异, 推测突变体断裂条纹的形成可能受晚期黑色素合成相关通路中其他基因的影响, 形成旁路补偿现象。

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Fish coloration change from disrupting different functional regions in tyrosinase (TYR) gene by gene editing technology
XU Xidan1,2,3, CHEN Honglin1,2,3, MANDAL Biplab Kumar1,2,3, SI Zhouxuan1,2,3, WANG Jun1,2,3, WANG Chenghui1,2,3     
1. Key Laboratory of Freshwater Aquatic Genetic Resources, Ministry of Agriculture and Rural Affairs, Shanghai Ocean University, Shanghai 201306, China;
2. National Demonstration Center for Experimental Fisheries Science Education, Shanghai Ocean University, Shanghai 201306, China;
3. Shanghai Engineering Research Center of Aquaculture, Shanghai Ocean University, Shanghai 201306, China
Abstract: In order to detect the effect of editing different functional regions of genes on fish coloration, fertilized eggs of AB strain of homozygous zebrafish (Danio rerio) were microinjected by CRISPR/Cas9 technology, Tyr gene were disrupted in the exon 2 and non-poly (A) addition signal of 3'-UTR, and mutants with disruption of different functional regions were obtained. The results showed that:the relative expression level of Tyr gene was detected in the examined stages of fertilized egg to 15 dph larvae in wild-type zebrafish, and the relative expression level increased to the highest level at melanin formation of eyes; the melanocytes were not observed in mutant embryo and larvae with disrupting the CDSregion of Tyr gene, displaying complete albino, then, the melanocytes were observed in adult fishes with broken melanocyte stripes. The mutants with disrupting non-poly (A) addition signal of 3'-UTR region of Tyr gene showed the same melanocyte stripes with wild-type zebrafish, indicating normal melanin synthesis in them. It was indicated that significantly phenotypic change would occur by disrupting CDS region of gene, while no obvious phenotypic change was detected by disrupting non-poly (A) addition signal of 3'-UTR region of gene.
Key words: Tyr     gene editing     CDS     3'-UTR     melanin     CRISPR/Cas9