摘要
采用重叠PCR(overlap PCR)方法构建了pdr5与snq2基因敲除组件,研究了pdr5、snq2基因突变对酵母细胞传感器评估遗传毒性的影响。考察了野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变酵母细胞传感器暴露于系列浓度甲磺酸甲酯(MMS)、甲磺酸乙酯(EMS)、顺铂、4-硝基喹啉-N-氧化物(4NOQ)、5-氟尿嘧啶(5-FU)、羟基脲、水杨酸和葡萄糖溶液24 h后的细胞生长抑制情况与16 h后的荧光诱导情况。研究结果表明,overlap PCR方法能够高效率构建基因突变酵母细胞传感器;snq2单基因突变与pdr5、snq2双基因突变细胞传感器检测遗传毒性的准确度为100%,高于野生型与pdr5单基因突变细胞传感器(87.5%);pdr5、snq2双基因突变酵母细胞传感器表现出最高的遗传毒性检测灵敏度,为构建高准确度与灵敏度的酵母细胞传感器提供了思路与方法,为酵母细胞膜转运蛋白基因pdr5与snq2的进一步功能研究奠定了基础。
遗传毒性化合物由于其潜在的致突变性与致癌作用,引起了制药行业和监管机构的高度关
芽殖酵母酿酒酵母(Saccharomyces cerevisiae)是一种代表性的单细胞真核生物,在遗传毒性测试应用方面具有多种优势。酵母细胞生长迅速、易于培养、遗传背景简单,使其成为优良的生物传感工
通过采用细胞壁合成与药物转运突变体改善酵母细胞渗透性,已经实现酵母细胞传感器的检测灵敏度与特异性的提高,敲除细胞壁合成基因erg6、cwp1和cwp2与膜转运蛋白基因pdr5、snq2和yor1是提高酵母细胞传感器检测遗传毒性灵敏度的有效方
酵母重组质粒pRNR2-yEGFP由扬州大学医学院预防医学系李湘鸣教授赠送;大肠埃希菌DH5α感受态细胞(上海生工生物工程有限公司);酿酒酵母BY4741、pESC-Leu质粒、pESC-His质粒(上海柯雷生物科技有限公司)。
SD/-Ura、SD/-Leu、SD/-His、SD/-His-Leu、SD/ -Ura-Leu、SD/-Ura-His、SD/-Ura-His-Leu培养基(上海艾礼生物科技有限公司);PrimeSTAR Max DNA 聚合酶(日本TaKaRa生物股份有限公司);PCR产物纯化试剂盒、无毒核酸染料、TE缓冲液、TAE缓冲液(上海生工生物工程有限公司);质粒DNA小提试剂盒、胶回收/DNA纯化试剂盒(南京诺唯赞生物科技有限公司);DNA上样缓冲液(南京翼飞雪生物科技有限公司);甲磺酸甲酯(methyl methanesulfonate,MMS,98%,批号:EJ120007)、甲磺酸乙酯(ethyl methanesulfonate,EMS,98%,批号:DK190025)(上海阿拉丁生化科技股份有限公司);顺铂(65%)、4-硝基喹啉-N-氧化物(4-nitroquinoline-N-oxide,4NOQ,98%)、5-氟尿嘧啶(5-fluorouracil,5-FU,99%)、羟基脲(99%)、水杨酸(99.5%)、葡萄糖(99%)(上海麦克林生化科技有限公司);其他试剂均为市售分析纯。
按常规玻璃珠法从酿酒酵母BY4741中提取基因组DNA后,采用overlap PCR构建基因敲除组件,原理如

Figure 1 Schematic diagram of using overlap PCR to construct fragment of deletion (gene knockout)
*The bold are the fragments of homology arm on the primers
采用overlap PCR方法构建pdr5Δ∷LEU2基因敲除组件。分别设计引物q-1/q-6与q-4/q-5,采用overlap PCR连接pdr5-1与LEU2片段以及pdr5-2与LEU2片段,分别得到纯化产物pdr5-1-LEU2与pdr5-2-LEU2。设计引物q-7/q-8,采用overlap PCR连接pdr5-1-LEU2与pdr5-2-LEU2片段,得到纯化产物pdr5Δ∷LEU2。
采用类似方法构建snq2Δ∷HIS3基因敲除组件。首先设计引物q-9/q-10、q-11/q-12与q-13/q-14,分别扩增得到snq2上下游同源臂snq2-1、snq2-2以及HIS3片段;设计引物q-9/q-14与q-12/q-13采用overlap PCR扩增分别得到snq2-1-HIS3与snq2-2-HIS3片段;设计引物q-15/q-16,采用overlap PCR连接snq2-1-HIS3与snq2-2-HIS3片段,纯化后得到snq2Δ∷HIS3。
采用醋酸锂转化法将pdr5Δ∷LEU2与snq2Δ∷HIS3基因敲除组件分别转入野生型酵母细胞BY4741,经SD/-Leu与SD/-His平板筛选得到pdr5单基因突变与snq2单基因突变酵母细胞;再将snq2Δ∷HIS3基因敲除组件转入snq2单基因突变酵母细胞,经SD/-His-Leu平板筛选得到pdr5、snq2双基因突变酵母细胞。采用玻璃珠法提取pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变酵母细胞的基因组DNA,分别设计引物q-7/q-8、q-15/q-16与q-7/q-8和q-15/q-16扩增突变片段后进行测序验证。
采用醋酸锂转化法将pRNR2-yEGFP转化于感受态酵母细胞BY4741,经SD/-Ura、SD/-Ura-Leu、SD/-Ura-His、SD/-Ura-His-Leu平板筛选得到pRNR2调控的野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变酵母细胞传感器。提取酵母细胞传感器质粒DNA,设计引物q-17/q-18扩增质粒片段进行验证。
使用2%DMSO水溶液作为溶剂配制遗传毒性化合物与阴性对照储备液,并逐级稀释至以下质量浓度:MMS(10,30,60,80,100,180,250,300,400 μg/mL);EMS(50,100,200,400,800,1 000,1 300,1 500,2 000 μg/mL);顺铂(10,20,30,40,60,90,120,150,200 μg/mL);4NOQ(0.01,0.02,0.05,0.1,0.2,0.5,1,2,5 μg/mL);5-FU(0.01,0.05,0.1,0.2,0.4,0.6,0.8,1,2 μg/mL);羟基脲(100,200,400,500,800,1 200,1 500,2 000,2 500 μg/mL);水杨酸(0.1,0.5,1,2,5,10,15,30,50 μg/mL);葡萄糖(5,10,20,40,60,80,100,150,200 μg/mL)。过夜培养4种酵母细胞单克隆,用新鲜的酵母培养液稀释至A600为0.1。以底部透明的黑色微孔板为反应载体,每孔中加入稀释后酵母培养液190 μL与测试化合物溶液或溶剂对照10 μL,每种质量浓度设置3个平行对照。微孔板置于30 ℃,220 r/min环境下培养24 h后,使用酶标仪测量A600。最终测定时每孔中DMSO的浓度低于0.1%,对酵母细胞生长没有明显影响。
过夜培养4种酵母细胞单克隆,用新鲜的酵母培养液稀释至A600为0.1。以底部透明的黑色微孔板为反应载体,每孔中加入稀释后酵母培养液190 μL与MMS溶液10 μL,质量浓度为10,30,60,80,100,150,180,200,250 μg/mL,每种浓度设置3个平行对照。微孔板置于30 ℃,220 r/min环境下培养,使用酶标仪每间隔4 小时测量A600与EGFP强度(激发光波长485 nm,发射光波长535 nm),持续时间20 h。
根据实验“2.4”与“2.5”项下确定测试化合物溶液的最大非细胞毒性浓度与细胞传感器的最佳暴露时间,选用适宜浓度测试化合物溶液孵育4种酵母细胞传感器适当时间:MMS(10,30,60,100,150,200,250 μg/mL);EMS(50,100,300,600,1 000,1 300,1 500 μg/mL);顺铂(10,20,30,60,90,120,150 μg/mL);4NOQ(0.01,0.05,0.1,0.2,0.5,0.7,1 μg/mL);5-FU(0.01,0.05,0.1,0.15,0.2,0.3,0.4 μg/mL);羟基脲(100,200,400,800,1 000,1 200,1 500 μg/mL);水杨酸(1,2,5,10,15,20,30 μg/mL);葡萄糖(10,20,40,80,100,150,200 μg/mL)。每种质量浓度设置3个平行对照,其余操作同“2.5”项。
在“2.4”项下中,采用Microsoft Excel 2013软件计算测试化合物各浓度下4种酵母细胞传感器A600均值,取3次独立实验的平均值和标准差。定义Ax/A0为暴露组A600与溶剂对照组A600的比值,设定Ax/A0小于90%为遗传毒性数据的排斥阈值。
在“2.5”与“2.6”项中,通过扣除空白培养基校正EGFP与A600,采用Microsoft Excel 2013软件计算4种酵母细胞传感器的EGFP与A600的均值(n = 3)。定义单位细胞的平均相对荧光强度AF(average fluorescence)= EGFP/A600,取3次独立实验的平均值及标准差。定义荧光诱导倍数FI(fold induction)为暴露组与溶剂对照组的单位细胞的平均相对荧光强度之比,则FI=AFtreated/AFuntreated。设定遗传毒性阈值为1.5倍,即当AF增加50%,且荧光强度随剂量的增加表现出增长趋势时,判断化合物为遗传毒性阳性,反之为阴性。采用Graphpad Prism 7.00软件进行制图分析。
采用overlap PCR方法构建pdr5Δ∷LEU2与snq2Δ∷HIS3基因敲除组件,PCR扩增产物电泳结果见

Figure 2 Gene knockout of pdr5Δ∷LEU2
M: DNA markers; Lane 1-2: pdr5-1-LEU2 (2 789 bp); Lane 3-4: pdr5-2-LEU2 (2 817 bp); Lane 5-9: pdr5Δ∷LEU2 gene knockout (3 304 bp)

Figure 3 Gene knockout of snq2Δ∷HIS3
M: DNA markers; Lane 1-2: snq2-1-HIS3(1899 bp); Lane 3-4: snq2-2-HIS3 (1 948 bp); Lane 5-7: snq2Δ∷HIS3 gene knockout ( 2540 bp)
pdr5Δ∷LEU2与snq2Δ∷HIS3基因敲除组件转入酵母细胞后,经平板筛选得到的单克隆DNA的PCR扩增产物电泳结果见

Figure 4 Mutant gene fragment of pdr5
M: DNA markers; Lane 1-4: Single-gene mutation of pdr5 (3 293 bp); Lane 5-8: Double-gene mutation of pdr5 and snq2 (3 293 bp)

Figure 5 Mutant gene fragment of snq2
M: DNA markers; Lane 1-4: Single-gene mutation of snq2 (2 543 bp); Lane 5-8: Double-gene mutation of pdr5 and snq2 (2 543 bp)
pRNR2-yEGFP重组载体转入野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变4种酵母细胞,质粒DNA的PCR扩增产物电泳结果见

Figure 6 Fragment of recombinant plasmid pRNR2-yEGFP
M: DNA markers; Lane 1: Wild-type yeast cell sensor (1 493 bp); Lane 2: Yeast cell sensor of single-gene mutation of pdr5 (1 493 bp); Lane 3: Yeast cell sensor of single-gene mutation of snq2 (1 493 bp); Lane 4: Yeast cell sensor of double-gene mutation of pdr5 and snq2 (1 493 bp)
应用野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变酵母细胞传感器对8种测试化合物进行24 h生长抑制测试,4种细胞传感器的Ax/A0结果见

Figure 7 Cytotoxicity in yeast cells for tested compounds in 24 h ()
A: MMS (methyl methanesulfonate); B: EMS (ethyl methanesulfonate); C: Cisplatin; D: 4NOQ (4-nitroquinoline-N-oxide); E: 5-FU (5-fluorouracil); F: Hydroxyurea; G: Salicylic acid; H: Glucose
经“2.4”项确定MMS最大非细胞毒性浓度后,采用MMS进行最佳暴露时间测试,野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变4种细胞传感器结果见

Figure 8 Yeast cell sensors exposed to different concentrations of MMS solution for 0, 4, 8, 12, 16, 20 h ()
当MMS质量浓度最低为50 μg/mL时,16 h暴露组细胞传感器FI值超过1.5。pRNR2调控的野生型细胞传感器暴露于MMS溶液下16 h的倒置荧光显微镜图见

Figure 9 Wild-type cell sensor regulated by pRNR2 exposed to MMS solution for 16 h
A: Bright-field image at 0 μg/mL; B: Bright-field image at 50 μg/mL; C: Inverted fluorescence microscope image at 0 μg/mL; D: Inverted fluorescence microscope image at 50 μg/mL
野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变4种酵母细胞传感器暴露于系列浓度测试化合物16 h后,荧光蛋白表达的剂效关系见

Figure 10 Yeast cell sensors exposured to tested compounds for 16 h ()
A: MMS (methyl methanesulfonate); B: EMS (ethyl methanesulfonate); C: Cisplatin; D: 4NOQ (4-nitroquinoline-N-oxide); E: 5-FU (5-fluorouracil); F: Hydroxyurea; G: Salicylic acid; H: Glucose
+: Positive; -: Negative; /: None relevant data; a: Wild-type; b: Single-gene mutation of pdr5; c: Single-gene mutation of snq2; d: Double-gene mutation of pdr5 and snq2
对于测试的6种遗传毒性阳性模型化合物,pdr5、snq2双基因突变细胞传感器表现出明显高于其他3种细胞传感器的FI。在检测灵敏度方面,双基因突变细胞传感器同样表现出明显优势。在测试6种遗传毒性数据阳性化合物时,相较于其他3种细胞传感器,双基因突变细胞传感器达到遗传毒性阈值所需浓度降低了数倍。snq2单基因突变细胞传感器相较于pdr5单基因突变细胞传感器,在检测4NOQ中表现出更高的准确度;而在检测5-FU方面,pdr5单基因突变细胞传感器表现出更高的荧光响应。野生型细胞传感器在检测灵敏度与荧光响应强度方面并无突出表现。结果表明尽管snq2与pdr5高度同源,具有共同的反应底物,但对于特异性底物表现出不同的响应强度,snq2单基因突变细胞传感器的检测准确度优于pdr5单基因突变细胞传感器,双基因突变细胞传感器在测试中表现出最佳的准确度和灵敏度。
基于DNA损伤诱导型启动子的酵母细胞传感器进行遗传毒性测定具有成本低、样品用量少、检测迅速、准确度高的优点,已被广泛用于环境样品与药物化合物的遗传毒性评估。通过敲除膜转运蛋白基因或细胞壁合成基因增加酵母细胞渗透性已成为改善细胞传感器检测灵敏度的有效方法。本研究采用overlap PCR方法分别构建pdr5与snq2基因敲除组件pdr5Δ∷LEU2与snq2Δ∷HIS3,设计的上下游同源臂长度为600~700 bp。将基因敲除组件与pRNR2-yEGFP重组质粒通过醋酸锂转化法转入酵母细胞得到野生型、pdr5单基因突变、snq2单基因突变与pdr5、snq2双基因突变酵母细胞传感器,PCR扩增产物电泳结果与基因测序结果验证了基因突变细胞传感器构建的成功。采用24 h酵母生长抑制试验测得各化合物最大非细胞毒性浓度,测试的6种遗传毒性数据阳性化合物MMS、EMS、顺铂、4NOQ、5-FU、羟基脲与阴性化合物水杨酸都在一定浓度下表现出细胞毒性,遗传毒性数据阴性化合物葡萄糖在测试浓度范围内未表现出细胞毒性。4种酵母细胞传感器暴露于系列浓度化合物16 h后,结果表明snq2单基因突变与pdr5、snq2双基因突变细胞传感器检测准确度为100%,高于野生型与pdr5单基因突变细胞传感器(87.5%)。同一浓度下,双基因突变细胞传感器的荧光响应显著高于其他3种细胞传感器,pdr5单基因突变细胞传感器在5-FU检测中表现出显著高于野生型和snq2单基因突变细胞传感器的荧光响应。结果表明尽管snq2与pdr5高度同源,蛋白功能具有代偿性,本研究发现snq2单基因突变细胞传感器的检测准确度优于pdr5单基因突变细胞传感器,双基因突变细胞传感器检测准确度与灵敏度优于单基因突变和野生型细胞传感器。
综上,本研究采用overlap PCR方法构建组件对酵母细胞进行基因敲除,相较于一步PCR产物转化法基因敲除效率更高,相较于酶切连接构建法具有成本更低、操作更简便的优势。snq2单基因突变细胞传感器的遗传毒性检测准确度优于pdr5单基因突变细胞传感器,pdr5、snq2双基因突变酵母细胞传感器具有最佳的准确度和灵敏度。因此,本实验为构建高准确度与灵敏度的酵母细胞传感器提供了思路与方法,为酵母细胞膜转运蛋白基因pdr5与snq2的进一步功能研究奠定了基础。
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