两个携带线粒体tRNA~(Met)/tRNA~(Gln)A4401G和tRNA~(Cys)G5821A突变的中国汉族原发性高血压家系的临床及分子遗传学特征

线粒体tRNA基因可能是原发性高血压发病相关的突变热点区域。文章报道了两个具有母系遗传特征的中国汉族原发性高血压家系的临床和分子遗传学特征。家系先证者和其他成员的临床数据表明,家系中母系成员高血压发病的严重程度存在差异,发病年龄也从36~79岁不等。对两个家系先证者使用24对有部分重叠的引物进行线粒体DNA全序列扩增分析,结果发现这两个先证者均携带同质性的tRNAMet/tRNAGlnA4401G和tRNACysG5821A突变,多态性变异位点都属于东亚单体型C。A4401G突变可能通过影响tRNAMet和tRNAGln前体的加工引起线粒体tRNA代谢水平的改变,而tRNACysG5821A突变位于tRNACys氨基酸受体臂,该突变使tRNACys氨基酸受体臂上原有的G6-C67配对消失,可能影响tRNACys空间结构和功能的稳定性,致使线粒体功能障碍。因此,tRNAMet/tRNAGlnA4401G和tRNACysG5821A突变可能是这两个原发性高血压家系的分子致病基础。     

线粒体tRNAGlu A14693G可能是与Leber遗传性视神经病变相关的基因突变

收集了3个具有典型临床特征的中国汉族Leber遗传性视神经病变(Leber's hereditary optic neuropathy, LHON)家系。通过对先证者和家系其他成员进行眼科临床(如视力损害程度和发病年龄)检查, 发现这些家系成员中视力损害的外显率很低, 经mtDNA测序分析, 在tRNA^Glu 上发现了A14693G同质性突变位点, 多态性位点分别属于东亚单体型Y1b、Y1和Y1, 没有发现其他高度保守和有功能意义的突变位点。A14693G突变位于线粒体tRNAGlu高度保守区(通用位点为54位), 可能导致tRNA空间结构和稳定性发生改变, 继而影响tRNA的代谢, 导致线粒体蛋白合成功能受损和ATP障碍, 最终导致视力损害。所以, tRNAGlu A14693G突变可能是与视神经病变相关的致病性线粒体突变位点。     

中国汉族原发性高血压患者线粒体tRNAMet基因突变

为探讨线粒体tRNAMet基因突变与中国汉族人原发性高血压发生发展之间的关系,文章收集990名原发性高血压患者一般资料(性别、年龄、发病年龄、体重指数、家族遗传史等)、血常规、血生化及心脏彩色超声检查结果;采集入选者的静脉血,分离并提取白细胞中的DNA,PCR扩增线粒体tRNAMet,纯化后测序。以发生tRNAMet突变的原发性高血压患者为指示病例,匹配8例未发生突变的原发性高血压患者为对照病例,统计分析指示病例和对照病例在血常规、血生化及心脏彩色超声检查等方面的异同。结果显示在990名原发性高血压患者中,有8名患者发生线粒体tRNAMet突变,突变分布在6个位点,分别为A4401G、C4410A、U4418C、A4435G、U4454C和C4456U。与对照病例相比,指示病例的平均发病时间显著提前,这种早发现象与体重指数无关。指示病例高密度脂蛋白胆固醇的平均值明显高于对照病例;血红蛋白平均值明显低于对照病例;舒张末期左室内径、每搏输出量、和心脏指数平均值等明显高于对照病例,射血分数平均值低于对照病例。在指示病例中,有5名患者有高血压母系遗传史,1名患者具有父系遗传史,2名患者否认有家族遗传史。以上研究结果提示线粒体tRNAMet突变可能会导致其结构和功能的变化,进而通过干扰血脂代谢、血细胞的稳态以及心脏的结构与功能,参与了原发性高血压的发生和发展。     

线粒体T12338C突变可能是与Leber遗传性视神经病变相关的突变位点

Leber遗传性视神经病变变(Leber’s hereditary optic neuropathy,LHON)是一种与线粒体DNA(Mito-chondrial DNA,mtDNA)突变相关的母系遗传性眼科疾病。文章报道了两例具有典型LHON临床、分子遗传特征的中国汉族家系。首先通过对家系先证者和其他成员进行眼科相关检查,发现两个家系成员中视力都仅有先证者一人损害严重,即外显率很低。经常规的方法对母系成员进行mtDNA测序及相关软件分析,结果发现携带ND4 G11696A和ND5 T12338C同质性突变位点,多态性变异位点均属于东亚单体型F2。线粒体DNA ND4 G11696A是一个已知的与LHON相关的突变位点,而T12338C位于线粒体氧化磷酸化复合体I亚基ND5的第2个碱基,该突变使起始密码子由蛋氨酸转变成苏氨酸,并且紧连tRNALeu(CUN)的3′末端。这可能影响tRNA Leu(CUN)空间结构和稳定性发生改变,以及起始密码子改变导致线粒体ND5蛋白合成功能受损和ATP障碍,最终导致需求能量高的视神经受损和视力损害。因此,线粒体ND4 G11696A和ND5 T12338C突变可能协同作用Leber遗传性视神经病变的发生,是与LHON相关的mtDNA突变位点,但外显率很低说明突变本身不足以造成LHON的表型表达,提示其他修饰因子(核修饰基因、环境等)可能对这两个家系发病起协同作用。     

线粒体相关的细胞信号分子与心血管疾病

线粒体上与心血管疾病相关的位点突变(mtDNA突变)是心血管疾病发病的分子机制之一。线粒体的代谢与线粒体活性氧簇(mROS)、钙离子、一氧化氮、激素等信号分子相关。这些信号分子的变化,如mROS增加、ATP减少等,影响线粒体及细胞功能,进而引发相关生物应激反应,最终导致心血管相关疾病。现讨论线粒体相关的信号分子参与转录、细胞凋亡与自噬、血管紧张和炎症等活动的过程,并详细介绍线粒体相关的信号分子与心血管疾病(着重介绍高血压和冠心病)之间的关系。     

线粒体DNA异常与肿瘤

能量代谢重编程是肿瘤细胞一个重要的标志特征,而由线粒体DNA(mitochondrial DNA,mtDNA)结构及功能异常引起的线粒体功能障碍是其机制之一。人类mtDNA为位于线粒体基质中由16569bp组成的双链闭合环状分子,编码与氧化磷酸化电子传递链相关的13种多肽以及与线粒体蛋白合成相关的22种tRNA和2种rRNA。近年来,人们发现多种肿瘤组织及细胞中存在mtDNA序列的多类型突变或拷贝数的变异,且mtDNA的这些异常与肿瘤的发生发展、早期诊断及放化疗监测等密切相关。异常的mtDNA因削弱线粒体产能、增加细胞内活性氧(reactive oxygen species,ROS)水平、打破Ca2+稳态,从而赋予肿瘤细胞代谢重编程、凋亡抵抗等侵袭性进程。针对mtDNA异常在肿瘤发生发展中的作用及机制研究,将为肿瘤的早期诊断及靶向治疗提供新的策略。     

与耳聋相关的线粒体tRNA突变

线粒体tRNA基因突变是导致感音神经性耳聋的原因之一．有些tRNA突变可直接造成耳聋的发生,称之为原发突变．如tRNA^Leu(UUR) A3243G等突变与综合征型耳聋相关,而tRNA^Ser(UCN) T7511C等突变则与非综合征型耳聋相关．此外,继发突变如tRNA^Thr G15927A等突变则对原发突变起协同作用,影响耳聋的表型表达．这些突变可引起tRNA二级结构改变,从而影响线粒体蛋白质合成,降低细胞内ATP的产生,由此引起的线粒体功能障碍可导致耳聋的发生．主要讨论与耳聋相关的线粒体tRNA突变及其致聋机理．     

人线粒体tRNA修饰碱基与遗传性脑肌病

人的多种遗传疾病与线粒体tRNA基因突变有关,这些突变导致疾病发生的分子机理是当前研究的热点.通过研究线粒体tRNA分子上的碱基修饰情况,人们发现了一类特殊的带有牛磺酸衍生物基团的修饰,这类修饰主要位于线粒体tRNALys和线粒体tRNALeu(UUR)反密码子第一位摆动(wobble)位点的碱基上.最近的研究表明,位于这两种线粒体tRNA基因上的多种突变与遗传性脑肌病相关,包括A8344G,A3243G,T3271C等等,它们可以导致tRNA上相应摆动位点的碱基修饰缺失.无论是在体外培养的带有相应突变的细胞内,还是在来源于脑肌病病人的组织中,科学家都发现了相同的线粒体tRNA碱基修饰缺陷.通过分子手术证实,此类碱基修饰对于维持这两种tRNA的反密码子与mRNA上相应密码子的相互识别至关重要,缺失了这种修饰的tRNA将无法识别一些对应的密码子.通过进一步的实验,人们还鉴定出负责催化此类碱基修饰的酶.这些研究不但揭示了线粒体遗传性脑肌病相关突变的致病机理,也将为研究基因治疗提供可能的新手段.     

7例携带线粒体tRNAAlaC5601T突变的Leber遗传性视神经病变家系的相关研究

文章收集了7例携带线粒体tRNAAl。C5601T突变的中国Leber遗传性视神经病变(Leber’s hereditary opticneuropathy,LHON)的家系,通过眼科检查和遗传学分析,发现7个家系的外显率很低,分别为9.5%、14.3%、4.5%、8.3%、10.0%、22.2%和25.0%。用24对有部分重叠的引物对7个先证者线粒体DNA(Mitochondrial DNA,mtDNA)全序列进行扩增,并进行相关的分子生物学分析,结果发现这些家系均未携带G11778A、G3460A和T14484C这3个常见的原发突变位点,而在tRNAAla上发现了C5601T同质性突变,多态性位点分析分别属于东亚线粒体单体型G2、G2a1、G2a1、G2、G2b、G2a1、G2。C5601T突变位于线粒体tRNAAla的高度保守区(通用位点为59位),可能引起tRNA空间结构和稳定性发生改变,继而影响tRNA的代谢,导致线粒体蛋白和ATP合成障碍,最终导致视力损害。因此,tRNAAlaC5601T突变可能是与LHON相关的线粒体突变位点。同时低外显率提示其他因素(包括核修饰基因、环境因素)可能影响这7个中国C5601T突变家系的表型表达。     

人类线粒体tRNA生物合成与线粒体疾病

线粒体是普遍存在于真核细胞中的一类细胞器.每个线粒体含有多个拷贝的闭合环状双链DNA. 人类线粒体DNA (mitochondrial DNA, mtDNA)共编码22种线粒体tRNA(mitochondrial tRNA,mt tRNA), 2种rRNA 及13种多肽.mt tRNA独特的结构特点决定了它们与具有典型三叶草结构的细胞质 tRNA不同.编码mt tRNA的基因突变频率较高,这可能是引起线粒体功能障碍的主要原因之一. 同时 ,这与很多病理现象相关.目前发现,大量与mt tRNA生物代谢和功能相关的核因子包括加工内切酶、 tRNA修饰酶和氨酰-tRNA合成酶.这些核因子的异常导致了疾病相关的tRNA致病突变.由此可见mt tRNA功能对于线粒体活性的重要性.     

Chemical and physical properties of mammalian mitochondrial aminoacyl-transfer RNAs. II. Analysis of 7-methylguanosine in mitochondrial and cytosolic aminoacyl-transfer RNAs.

The 7-methylguanosine (m7G) content of two individual mitochondrial tRNAs, labelled in the aminoacyl moiety was assayed by the specific cleavage of the tRNA at this nucleotide followed by electrophoretic analysis to identify the 3'-terminal fragment of the tRNA. Neither Syriam hamster mitochondrial tRNALeu nor tRNAMet were found to contain m7G. In contrast, cytosolic tRNAMetS were cleaved indicating the presence of m7G, apparently 27--28 and 29 nucleotides from their 3' terminus. Cystolic tRNALeu was not cleaved. These results are discussed in relationship to the reported low content of methylated nucleosides in mitochondrial 4 S RNA.     

Mitochondrial tRNA import in Toxoplasma gondii

Apicomplexan parasites have the smallest known mitochondrial genome. It consists of a repeated element of approximately 6-7 kb in length and encodes three mitochondrial proteins, a number of rRNA fragments, but no tRNAs. It has therefore been postulated that in apicomplexans all tRNAs required for mitochondrial translation are imported from the cytosol. To provide direct evidence for this process we have established a cell fractionation procedure allowing the isolation of defined organellar RNA fractions from the apicomplexan Toxoplasma gondii. Analysis of T. gondii total and organellar RNA by Northern hybridization showed that except for the cytosol-specific initiator tRNAMet all nucleus-encoded tRNAs tested were present in the cytosol and in the mitochondrion but not in the plastid. Thus, these results provide the first experimental evidence for mitochondrial tRNA import in apicomplexans. The only other taxon that imports the whole set of mitochondrial tRNAs are the trypanosomatids. Interestingly, the initiator tRNAMet is the only cytosol-specific tRNA in trypanosomatids, indicating that the import specificity is identical in both groups. In agreement with this, the T. gondii initiator tRNAMet remained in the cytosol when expressed in Trypanosoma brucei. However, in contrast to trypanosomatids, no thio-modifications were detected in the tRNAGln of T. gondii indicating that, unlike what is suggested in Leishmania, they are not involved in regulating import.     

Characterization of a yeast mitochondrial locus necessary for tRNA biosynthesis. Deletion mapping and restriction mapping studies

Yeast mitochondrial DNA codes for a complete set of tRNAs. Although most components necessary for the biosynthesis of mitochondrial tRNA are coded by nuclear genes, there is one genetic locus on mitochondrial DNA necessary for the synthesis of mitochondrial tRNAs other than the mitochondrial tRNA genes themselves. Characterization of mutants by deletion mapping and restriction enzyme mapping studies has provided a precise location of this yeast mitochondrial tRNA synthesis locus. Deletion mutants retaining various segments of mitochondrial DNA were examined for their ability to synthesize tRNAs from the genes they retain. A subset of these strains was also tested for the ability to provide the tRNA synthesis function in complementation tests with deletion mutants unable to synthesize mature mitochondrial tRNAs. By correlating the tRNA synthetic ability with the presence or absence of certain wild-type restriction fragments, we have confined the locus to within 780 base pairs of DNA located between the tRNAMetf gene and tRNAPro gene, at 29 units on the wild-type map. Heretofore, no genetic function or gene product had been localized in this area of the yeast mitochondrial genome.     

Nucleotide sequence of non-initiator methionine tRNA from Bacillus subtilis

Non-initiator methionine tRNA (tRNAMet) was purified from Bacillus subtilis W 168 by a consecutive use of several column chromatographic systems. The nucleotide sequence was determined to be p-G-G-C-G-G-U-G-U-A-G-C-U-C-A-G-C-G-G-C-D-A-G-A-G-C-G-U-A-C-G-G-U-U-C-A-U-m6A-C-C-C -G-U-G-A-G-G(m7G)-U(D)-C-G-G-G-G-G-T-psi-C-G-A-U-C-C-C-C-U-C-C-G-C-C-G-C-U-A-C- C-A-OH. The nucleosides of G46 and U47 were partially modified to m7G and D, respectively. The nucleotide sequence shows a unique feature that the position adjacent to 3'-end of the anticodon C-A-U is occupied by m6A, not by t6A, although the tRNAMet belongs to a groups of tRNAs which recognize codons starting with A.     

Decreased CCA-addition in human mitochondrial tRNAs bearing a pathogenic A4317G or A10044G mutation

Pathogenic point mutations in mitochondrial tRNA genes are known to cause a variety of human mitochondrial diseases. Reports have associated an A4317G mutation in the mitochondrial tRNA(Ile) gene with fatal infantile cardiomyopathy and an A10044G mutation in the mitochondrial tRNA(Gly) gene with sudden infant death syndrome. Here we demonstrate that both mutations inhibit in vitro CCA-addition to the respective tRNA by the human mitochondrial CCA-adding enzyme. Structures of these two mutant tRNAs were examined by nuclease probing. In the case of the A4317G tRNA(Ile) mutant, structural rearrangement of the T-arm region, conferring an aberrantly stable T-arm structure and an increased T(m) value, was clearly observed. In the case of the A10044G tRNA(Gly) mutant, high nuclease sensitivity in both the T- and D-loops suggested a weakened interaction between the loops. These are the first reported instances of inefficient CCA-addition being one of the apparent molecular pathogeneses caused by pathogenic point mutations in human mitochondrial tRNA genes.     

Stable tRNA precursors in HeLa cells.

Two tRNA precursors were isolated from 32P-labeled or unlabeled HeLa cells by two dimensional polyacrylamide gel electrophoresis, and were sequenced. These were the precursors of tRNAMet and tRNALeu, and both contained four extra nucleotides including 5'-triphosphates at their 5'-end and nine extra nucleotides including oligo U at their 3'-end. These RNAs are the first naturally occurring tRNA precursors from higher eukaryotes whose sequences have been determined. In these molecules, several modified nucleosides such as m2G, t6A and ac4C in mature tRNAs were undermodified. Two additional hydrogen bonds were formed in the clover leaf structures of these tRNA precursors. These extra hydrogen bonds may be responsible for the stabilities of these tRNA precursors.