Respiratory chain supercomplexes set the threshold for respiration defects in human mtDNA mutant cybrids

Mitochondrial DNA (mtDNA) mutations cause heterogeneous disorders in humans. MtDNA exists in multiple copies per cell, and mutations need to accumulate beyond a critical threshold to cause disease, because coexisting wild-type mtDNA can complement the genetic defect. A better understanding of the molecular determinants of functional complementation among mtDNA molecules could help us shedding some light on the mechanisms modulating the phenotypic expression of mtDNA mutations in mitochondrial diseases. We studied mtDNA complementation in human cells by fusing two cell lines, one containing a homoplasmic mutation in a subunit of respiratory chain complex IV, COX I, and the other a distinct homoplasmic mutation in a subunit of complex III, cytochrome b. Upon cell fusion, respiration is recovered in hybrids cells, indicating that mitochondria fuse and exchange genetic and protein materials. Mitochondrial functional complementation occurs frequently, but with variable efficiency. We have investigated by native gel electrophoresis the molecular organization of the mitochondrial respiratory chain in complementing hybrid cells. We show that the recovery of mitochondrial respiration correlates with the presence of supramolecular structures (supercomplexes) containing complexes I, III and IV. We suggest that critical amounts of complexes III or IV are required in order for supercomplexes to form and provide mitochondrial functional complementation. From these findings, supercomplex assembly emerges as a necessary step for respiration, and its defect sets the threshold for respiratory impairment in mtDNA mutant cells.     

Tissue-specific mtDNA lesions and radical-associated mitochondrial dysfunction in human hearts exposed to doxorubicin

Doxorubicin causes a chronic cardiomyopathy. Although the exact pathogenesis is unknown, recent animal data suggest that somatically acquired alterations of mitochondrial DNA (mtDNA) and concomitant mitochondrial dysfunction play an important role in its onset. In this study, skeletal and myocardial muscles were examined from human autopsies. Compared to controls (n = 8), doxorubicin-exposed hearts (n = 6) showed low absolute enzyme activity of mtDNA-encoded nicotinamide adenine dinucleotide hydrogen dehydrogenase (NADH DH, 79% residual activity, p = 0.03) and cytochrome c oxidase (COX, 59% residual activity, p < 0.001), but not of succinate dehydrogenase (SDH), which is encoded exclusively by nuclear DNA. NADH DH/SDH and COX/SDH ratios were 37% (p < 0.001) and 27% (p < 0.001) of controls. Expression of the mtDNA-encoded subunit II of COX was reduced (82%, p = 0.04), compared to its unchanged nucleus-encoded subunit IV. MtDNA-content was diminished (56%, p = 0.02), but the 'common' mtDNA-deletion was increased (9.2-fold, p = 0.004). Doxorubicin-exposed hearts harboured numerous additional mtDNA rearrangements lacking direct repeats. They contained elevated levels of malondialdehyde (MDA) (p = 0.006, compared to controls), which correlated inversely with the COX/SDH ratio (r = -0.45, p = 0.02) and the mtDNA-content (r = -0.75, p = 0.002), and correlated positively with the levels of the 'common' deletion (r = 0.80, p < 0.001). Doxorubicin-exposed hearts also contained the highest levels of superoxide (p < 0.001, compared to controls), which correlated negatively with the mtDNA-encoded respiratory chain activities, such as the COX/SDH ratio (r = -0.57, p = 0.02) and the NADH/SDH ratio (r = -0.52, p = 0.04), as well as with the mtDNA content (r = -0.69, p = 0.003), and correlated positively with the frequency of the 'common' deletion (r = 0.76, p < 0.001) and the MDA levels (r = 0.86, p < 0.001). Doxorubicin-exposed hearts contained electron-dense deposits within mitochondria. Hearts exposed to other anthracyclines (n = 6) or skeletal muscle (all groups) had no mitochondrial dysfunction. Doxorubicin, unlike other anthracyclines, augments lipid peroxidation, induces mtDNA mutations and decreases mtDNA content in human hearts. These lesions have an impact on mitochondrial function and could be of importance in the pathogenesis of clinical cardiomyopathy.     

Metabolic consequences of a novel missense mutation of the mtDNA CO I gene

We have identified a novel heteroplasmic C6489A missense mutation in the mitochondrial DNA (mtDNA) CO I gene encoding the cytochrome c oxidase (COX) subunit I in a 17-year-old girl with epilepsia partialis continua. This point mutation leads to an exchange of the highly conserved Leu196 to Ileu196. Muscle biopsy showed in single fibers decreased COX activity and lowered binding of COX antibodies, indicating decreased stability of the mutated enzyme. The analysis of blood mtDNA revealed about 30% mutant mtDNA in the patients blood but about 90% mutant mtDNA in the blood of two non-affected family members. Quantitative analysis of the mutation gene dose effect on COX activity on single muscle fiber level revealed a very high threshold-a COX deficiency was observed only in fibers containing >95% mutant mtDNA. In apparent contrast to this high mutation gene dose threshold, in vivo investigations of mitochondrial function in saponin-permeabilized muscle fibers of the index patient containing approximately 90% mutated mtDNA showed decreased maximal rates of respiration and an increased sensitivity of fiber respiration to cyanide. This is due to a 2-fold increase of COX flux control on muscle fiber respiration and a 30% decrease of COX metabolic threshold, supporting the concept of tight COX control of oxidative phosphorylation in skeletal muscle.     

m.6267G>A: a recurrent mutation in the human mitochondrial DNA that reduces cytochrome c oxidase activity and is associated with tumors

Complete sequencing of the mitochondrial genome of 13 cell lines derived from a variety of human cancers revealed nine novel mitochondrial DNA (mtDNA) variations. One of them, m.6267G>A, is a recurrent mutation that introduces the Ala122Thr substitution in the mitochondrially encoded cytochrome c oxidase I (MT-CO1): p.MT-CO1: Ala122Thr (GenBank: NP_536845.1). Biochemical analysis of the original cell lines and the transmitochondrial cybrids generated by transferring mitochondrial DNAs to a common nuclear background, indicate that cytochrome c oxidase (COX) activity, respiration, and growth in galactose are impaired by the m.6267G>A mutation. This mutation, found twice in the cancer cell lines included in this study, has been also encountered in one out of 63 breast cancer samples, one out of 64 colon cancer samples, one out of 260 prostate cancer samples, and in one out of 15 pancreatic cancer cell lines. In all instances the m.6267G>A mutation was associated to different mtDNA haplogroups. These findings, contrast with the extremely low frequency of the m.6267G>A mutation in the normal population (1:2264) and its apparent absence in other pathologies, strongly suggesting that the m.6267G>A missense mutation is a recurrent mutation specifically associated with cancer.     

p53对缺血再灌注损伤后肾小管上皮细胞演变的影响

目的：观察缺血再灌注损伤（ischemia/reperfusioninjury,IRI）后,低龄和高龄p53（＋/＋）和p53（-/-）鼠肾小管上皮细胞的演变,探讨p53基因对IRI后肾小管上皮细胞演变的影响。方法：分别建立低龄（2月龄）和高龄（12月龄）p53（＋/＋）和p53（-/-）鼠左肾IRI模型。于IRI后0d、1d、3d、7d、1月、3月、6月,用HE染色观察肾小管组织学变化,免疫组织化学法检测肾小管上皮细胞增殖细胞核抗原（proliferating cell nuclear antigen,PCNA）的表达,组织化学染色观察肾小管上皮细胞衰老相关β-半乳糖苷酶（SA-β-gal）的活性,用脱氧核苷酸末端转移酶介导的缺口末端标记（terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick-end labeling,TUNEL）法检测肾小管上皮细胞凋亡。结果：IRI后0d,肾小管以坏死为主,高龄鼠比低龄鼠明显（P〈0.05）,p53（-/-）鼠比p53（＋/＋）鼠明显（P〈0.05）;细胞凋亡在7d时达到高峰,且高龄鼠比低龄鼠明显（P〈0.05）,p53（＋/＋）鼠比p53（-/-）鼠明显（P〈0.05）。p53（＋/＋）低龄鼠IRI后1月出现衰老细胞,3月和6月显著增多（P〈0.05）,对侧肾未见衰老细胞;而p53（-/-）低龄鼠IRI后6月才出现明显的衰老细胞;两种基因型高龄鼠在IRI后0d双肾均见衰老细胞,但p53（＋/＋）鼠显著多于p53（-/-）鼠（P〈0.05）,1d后,IRI肾的衰老细胞明显减少（P〈0.05）,对侧肾无变化。p53（＋/＋）高龄和低龄鼠细胞增殖无统计学意义（P〉0.05）,而p53（-/-）低龄和高龄鼠在IRI后3、7d细胞增殖进行性增加,且低龄鼠显著多于高龄鼠、p53（-/-）鼠多于p53（＋/＋）鼠（P〈0.05）。对高龄鼠IRI后1d点细胞衰老和凋亡进行相关分析显示,在p53（＋/＋）鼠二者显著负相关（r=-0.82,P〈0.05）,在p53（-/-）鼠,二者无显著相关性（r=0.26,P〉0.05）。结论：①IRI可促进正常肾小管上皮细胞衰老的进程;②已经进入衰老状态的肾小管上皮细胞在遭受IRI刺激后,更易走向死亡（坏死和/或凋亡）;③p53在IRI所致肾小管上皮细胞演变过程中发挥着重要的调控作用。     

An intragenic suppressor in the cytochrome c oxidase I gene of mouse mitochondrial DNA

We report here the identification of a cell line containing single and double missense mutations in cytochrome c oxidase (COX) subunit I gene of mouse mitochondrial DNA. When present in homoplasmy, the single mutant displays a normal complex IV assembly but a significantly reduced COX activity, while the double mutant almost completely compensates the functional defect of the first mutation. We discuss the potential structural consequences of those mutations based on the modeled structure of mouse complex IV. Based on genetic, biochemical and molecular analyses of cultured mouse cells we infer that: (i) deleterious mutations can arise and become predominant; (ii) cultured cells can maintain several mtDNA haplotypes at stable frequencies; (iii) the respiratory chain has little spare COX capacity; and (iv) the size of a cavity in the vicinity of Val421 in CO I of animal COX may affect the function of the enzyme.     

Immune recognition of cyclin B1 as a tumor antigen is a result of its overexpression in human tumors that is caused by non-functional p53

Cyclin B1, which plays a key role in the control of cell cycle progression from G(2) through M phase, was recently identified by us as a tumor antigen recognized by human T-cells. To understand what makes this normal molecule antigenic, we compared its expression in malignant versus normal cells. Immunohistology showed overexpression of cyclin B1 protein in tumors compared to surrounding normal tissue and localization in the cytoplasm rather than the nucleus. Cyclin B1 is overexpressed at protein and mRNA level in many tumor cell lines including breast, lung, colorectal carcinoma, lymphoma and leukemia. While overexpressed in tumor cells at all stages of the cell cycle, its expression still peaks at G(2)/M phase, as it does in normal cells. We compared cyclin B1 expression in two cell clones derived from the same colorectal tumor cell line, one wild type for p53 (HCT116p53(+/+)) and one with deleted p53 (HCT116p53(-/-)). HCT116p53(+/+) cells had undetectable (normal) level of cyclin B1 protein, while HCT116p53(-/-) cells showed overexpression. When reconstituted with p53, HCT116p53(-/-) cells reverted to normal cyclin B1 expression. We conclude that p53 plays an important role in cyclin B1 regulation and that tumors with mutated p53 will be good candidates for cyclin B1 based immunotherapy.     

A novel heteroplasmic tRNAleu(CUN) mtDNA point mutation in a sporadic patient with mitochondrial encephalomyopathy segregates rapidly in skeletal muscle and suggests an approach to therapy

A novel mtDNA point mutation was detected in the tRNAleu(CUN) gene (G to A at position 12315) in a sporadic patient with chronic progressive external ophthalmoplegia, ptosis, limb weakness, sensorineural hearing loss and a pigmentary retinopathy. The mutation disrupts base pairing in the T psi C stem at a site which has been conserved throughout evolution. Although the other mtDNA tRNAleu gene (UUR) is a hotspot for mutation, this is the first pathogenic mutation to be reported in the gene coding for tRNAleu(CUN). MtDNAs carrying the mutation constituted 94% of total mtDNAs in two separate muscle biopsies. Single fibre analysis showed that skeletal muscle fibres without detectable cytochrome c oxidase activity (COX-ve fibres) contained predominantly mutant mtDNAs (93-98%) while fibres with apparently normal COX activity had up to 90% mutant mtDNAs, demonstrating that the G12315A mutation is functionally recessive. Immunofluorescence studies with specific antibodies to mtDNA- or nuclear-encoded subunits of COX were consistent with a defect in mitochondrial protein translation. The mutation was not present in blood cells or cultured fibroblasts and surprisingly, it could not be detected in satellite cells cultured from the patient's muscle. This pattern, which may by typical of patients who have inherited new germline pathogenic mtDNA mutations, possibly reflects loss of the mutation by random genetic drift in mitotic tissues and proliferation of mitochondria containing the mutant mtDNA in post- mitotic cells. The absence of mtDNA carrying the mutation in satellite cells suggests that regeneration of skeletal muscle fibres from satellite cells could restore a wild-type mtDNA genotype and normal muscle function.      

In situ lineage tracking of human prostatic epithelial stem cell fate reveals a common clonal origin for basal and luminal cells

Stem cells accumulate mitochondrial DNA (mtDNA) mutations resulting in an observable respiratory chain defect in their progeny, allowing the mapping of stem cell fate. There is considerable uncertainty in prostate epithelial biology where both basal and luminal stem cells have been described, and in this study the clonal relationships within the human prostate epithelial cell layers were explored by tracing stem cell fate. Fresh-frozen and formalin-fixed histologically-benign prostate samples from 35 patients were studied using sequential cytochrome c oxidase (COX)/succinate dehydrogenase (SDH) enzyme histochemistry and COX subunit I immunofluorescence to identify areas of respiratory chain deficiency; mtDNA mutations were identified by whole mitochondrial genome sequencing of laser-captured areas. We demonstrated that cells with respiratory chain defects due to somatic mtDNA point mutations were present in prostate epithelia and clonally expand in acini. Lineage tracing revealed distinct patterning of stem cell fate with mtDNA mutations spreading throughout the whole acinus or, more commonly, present as mosaic acinar defects. This suggests that individual acini are typically generated from multiple stem cells, and the presence of whole COX-deficient acini suggests that a single stem cell can also generate an entire branching acinar subunit of the gland. Significantly, a common clonal origin for basal, luminal and neuroendocrine cells is demonstrated, helping to resolve a key area of debate in human prostate stem cell biology.     

Mitochondrial dysfunction and ultrastructural damage in the hippocampus of pilocarpine-induced epileptic rat

Mitochondrial dysfunction has been implicated as a contributing factor in epileptic seizures. Present studies were carried out to decipher seizure-dependent changes in mitochondrial function and ultrastructure in the chronic condition of temporal lobe epilepsy (TLE) induced by pilocarpine in rat hippocampus. Enzyme assay revealed significant depression of the activity of mitochondrial- and nuclear-encoded cytochrome oxidase (COX). Conversely, the activity of nuclear-encoded succinate dehydrogenase (SDH) remained unchanged. Discernible mitochondrial ultrastructural damage, varying from swelling to disruption of membrane, was observed in the hippocampus. Quantitative real-time PCR and Western blotting showed the expression of mitochondrial-encoded COX subunit III (COXIII) dropped significantly during the chronic seizure activity; the corresponding expression of COX subunit IV (COXIV) displayed no significant change. Most likely, our results suggest that dysfunction of mitochondrial COX respiratory enzyme and mitochondrial ultrastructural damage in the hippocampus are associated with prolonged seizure during experimental TLE and mitochondria are more vulnerable to epilepsy.     

Severe lactic acidosis caused by a novel frame-shift mutation in mitochondrial-encoded cytochrome c oxidase subunit II

We report the first frame-shift truncation mutation in a mitochondrial DNA (mtDNA)-encoded subunit II of cytochrome c oxidase (COXII). The mutation was identified by temporal temperature gradient gel electrophoresis (TTGE) followed by direct DNA sequencing in an infant who died at 12 days of age following a course of apnea, bradycardia, and severe lactic acidosis. The patient had a twin brother who died at two days of age of similar course. The mutation, 8042delAT, produced a truncated protein that was 72 amino acids shorter than the wild type protein. The mutant protein, missing one third of the amino acid residues at the C-terminal essential for hydrophilic interaction with cytochrome c, ligand binding to CuA and Mg, and the formation of proton and water channels, apparently has devastating effects on mitochondrial respiratory function.     

Correlation between DNA alterations and p53 and p16 protein expression in cancer cell lines

To investigate the interaction between DNA abnormalities, p53 and p16 gene mutations, and methylation and protein expression, 20 cancer cell lines were examined by Western blotting. A clear relation was found to exist between p53 accumulation and mutation status. Of 20 cell lines examined, 14 demonstrated p53 homozygous mutations in exons 3-10, including 12 missense mutations, one nonsense mutation, and one frameshift mutation. Overexpression of p53 was always linked to missense mutations in exons 6-8. Intermediate expression of p53 was noted in cells with missense mutations or polymorphism to proline at codon 72 in exons 4-5, whereas there was slight or no visible expression in wild type cells and in cells with nonsense and frameshift mutations. DNA aberration in the p16 promoter gene correlated significantly with protein expression of the p16 suppressor gene. Overexpression was noted in six cell lines, intermediate expression in two, and slight or no visible expression in 12. Methylation-caused disappearance of p16 protein was noted in 40% (8/20) of the cell lines. Of six cell lines overexpressing p16 protein, two could be amplified with primers for both unmethylated and methylated forms in a methylation-specific RCP analysis. One cell line with no visible expression could also be amplified with both primers. Overexpression or disappearance of p16 protein may readily occur when one of two alleles has been methylated.     

Introduction of heteroplasmic mitochondrial DNA (mtDNA) from a patient with NARP into two human rho degrees cell lines is associated either with selection and maintenance of NARP mutant mtDNA or failure to maintain mtDNA.

Mitochondria from a patient heteroplasmic at nucleo-tide position 8993 of mitochondrial DNA (mtDNA) were introduced into two human tumour cell lines lacking mtDNA. The donor mitochondria contained between 85 and 95% 8993G:C mtDNA. All detectable mtDNA in the mitochondrially transformed cells contained the pathological 8993G:C mutation 3 months after transformation. These results suggest that 8993G:C mtDNA had a selective advantage over 8993T:A mtDNA in both lung carcinoma and osteo-sarcoma cell backgrounds. In contrast, two other presumed pathological mtDNA variants were lost in favour of 'wild-type' mtDNA molecules in the same lung carcinoma cell background. Taken together, these findings suggest that the transmission bias of mtDNA variants is dependent upon a combination of nuclear background and mtDNA genotype. A second phenomenon observed was a marked decrease in the growth rate of many putative transformed cell lines after 6 weeks of culturing in selective medium, and in these cell lines mtDNA was not readily detectable by Southern blotting. Restriction endonuclease analysis and sequencing of amplified mtDNA demonstrated that the slow growing cells contained little or no mtDNA. It is concluded that these cells represented transient mitochondrial transformants.     

Severe lactic acidosis caused by a novel frame‐shift mutation in mitochondrial‐encoded cytochrome c oxidase subunit II

We report the first frame‐shift truncation mutation in a mitochondrial DNA (mtDNA)‐encoded subunit II of cytochrome c oxidase (COXII). The mutation was identified by temporal temperature gradient gel electrophoresis (TTGE) followed by direct DNA sequencing in an infant who died at 12 days of age following a course of apnea, bradycardia, and severe lactic acidosis. The patient had a twin brother who died at two days of age of similar course. The mutation, 8042delAT, produced a truncated protein that was 72 amino acids shorter than the wild type protein. The mutant protein, missing one third of the amino acid residues at the C‐terminal essential for hydrophilic interaction with cytochrome c, ligand binding to Cu_A and Mg, and the formation of proton and water channels, apparently has devastating effects on mitochondrial respiratory function. © 2001 Wiley‐Liss, Inc.