Maternally inherited mitochondrial DNA disease in consanguineous families

Mitochondrial respiratory chain disease represents one of the most common inborn errors of metabolism and is genetically heterogeneous, with biochemical defects arising from mutations in the mitochondrial genome (mtDNA) or the nuclear genome. As such, inheritance of mitochondrial respiratory chain disease can either follow dominant or recessive autosomal (Mendelian) inheritance patterns, the strictly matrilineal inheritance observed with mtDNA point mutations or X-linked inheritance. Parental consanguinity in respiratory chain disease is often assumed to infer an autosomal recessive inheritance pattern, and the analysis of mtDNA may be overlooked in the pursuit of a presumed nuclear genetic defect. We report the histochemical, biochemical and molecular genetic investigations of two patients with suspected mitochondrial disease who, despite being born to consanguineous first-cousin parents, were found to harbour well-characterised pathogenic mtDNA mutations, both of which were maternally transmitted. Our findings highlight that any diagnostic algorithm for the investigation of mitochondrial respiratory chain disease must include a full and complete analysis of the entire coding sequence of the mitochondrial genome in a clinically relevant tissue. An autosomal basis for respiratory chain disease should not be assumed in consanguineous families and that 'maternally inherited consanguineous' mitochondrial disease may thus be going undiagnosed.     

Respiratory chain complex I deficiency caused by mitochondrial DNA mutations

Defects of the mitochondrial respiratory chain are associated with a diverse spectrum of clinical phenotypes, and may be caused by mutations in either the nuclear or the mitochondrial genome (mitochondrial DNA (mtDNA)). Isolated complex I deficiency is the most common enzyme defect in mitochondrial disorders, particularly in children in whom family history is often consistent with sporadic or autosomal recessive inheritance, implicating a nuclear genetic cause. In contrast, although a number of recurrent, pathogenic mtDNA mutations have been described, historically, these have been perceived as rare causes of paediatric complex I deficiency. We reviewed the clinical and genetic findings in a large cohort of 109 paediatric patients with isolated complex I deficiency from 101 families. Pathogenic mtDNA mutations were found in 29 of 101 probands (29%), 21 in MTND subunit genes and 8 in mtDNA tRNA genes. Nuclear gene defects were inferred in 38 of 101 (38%) probands based on cell hybrid studies, mtDNA sequencing or mutation analysis (nuclear gene mutations were identified in 22 probands). Leigh or Leigh-like disease was the most common clinical presentation in both mtDNA and nuclear genetic defects. The median age at onset was higher in mtDNA patients (12 months) than in patients with a nuclear gene defect (3 months). However, considerable overlap existed, with onset varying from 0 to >60 months in both groups. Our findings confirm that pathogenic mtDNA mutations are a significant cause of complex I deficiency in children. In the absence of parental consanguinity, we recommend whole mitochondrial genome sequencing as a key approach to elucidate the underlying molecular genetic abnormality.     

Nonrandom tissue distribution of mutant mtDNA

Heteroplasmic mitochondrial DNA (mtDNA) defects are an important cause of inherited human disease. On a cellular level, the percentage of mutant mtDNA is the principal factor behind the expression of the genetic defect. Marked variation in the level of mutant mtDNA among tissues is thought to be responsible for the diverse clinical phenotypes associated with the same pathogenic mtDNA mutation. This study was designed to determine whether the percentage level of a pathogenic mtDNA molecule is determined by a purely random process. The tissue distribution of the A3243G MELAS point mutation was analyzed in five individuals who were members of a family with maternally inherited diabetes and deafness. The level of mutant mtDNA was measured in four tissues in three individuals and three tissues in two individuals. The highest level of mutant mtDNA occurred in skeletal muscle, followed by hair follicles, and then buccal mucosa, with the lowest levels in blood (leucocyte/platelet fraction). The probability of observing any strict hierarchy in family is 4.82 × 10⁻⁵. These results indicate that the distribution of the A3243G mutation is not solely determined by random processes. Am. J. Med. Genet. 85:498–501, 1999. © 1999 Wiley-Liss, Inc.     

Validation of microarray-based resequencing of 93 worldwide mitochondrial genomes

The human mitochondrial genome consists of a multicopy, circular dsDNA molecule of 16,569 base pairs. It encodes for 13 proteins, two ribosomal genes, and 22 tRNAs that are essential in the generation of cellular ATP by oxidative phosphorylation in eukaryotic cells. Germline mutations in mitochondrial DNA (mtDNA) are an important cause of maternally inherited diseases, while somatic mtDNA mutations may play important roles in aging and cancer. mtDNA polymorphisms are also widely used in population and forensic genetics. Therefore, methods that allow the rapid, inexpensive and accurate sequencing of mtDNA are of great interest. One such method is the Affymetrix GeneChip Human Mitochondrial Resequencing Array 2.0 (MitoChip v.2.0) (Santa Clara, CA). A direct comparison of 93 worldwide mitochondrial genomes sequenced by both the MitoChip and dideoxy terminator sequencing revealed an average call rate of 99.48% and an accuracy of > or =99.98% for the MitoChip. The good performance was achieved by using in-house software for the automated analysis of additional probes on the array that cover the most common haplotypes in the hypervariable regions (HVR). Failure to call a base was associated mostly with the presence of either a run of > or =4 C bases or a sequence variant within 12 bases up- or downstream of that base. A major drawback of the MitoChip is its inability to detect insertions/deletions and its low sensitivity and specificity in the detection of heteroplasmy. However, the vast majority of haplogroup defining polymorphism in the mtDNA phylogeny could be called unambiguously and more rapidly than with conventional sequencing.     

Heteroplasmic Segregation Associated with Trisomy-9 in Cultured Human Cells

In cybrid cells carrying the mitochondrial A3243G MELAS mutation, which were also heteroplasmic for the G12300A suppressor mutation, we observed a transient episode of heteroplasmic instability, resulting in a wide diversification in G12300A heteroplasmy levels and a shift in the average heteroplasmy level from 11 to 29%. These cells were found to be trisomic for chromosome 9, whereas a minority of cells that retained disomy-9 showed no instability. Coculture experiments implied that trisomy-9 cells exhibited a significant growth advantage, but neither heteroplasmy levels, respiratory phenotype nor trisomy-9 itself had direct selective value under standard culture conditions. Mitochondrial nucleoid number was the same (50–100) in cells that had or had not experienced transient heteroplasmic instability, but 1–2 orders of magnitude less than the segregation number in such cells. These findings support the idea that mtDNA partition is under nuclear genetic control, and implicate a locus on chromosome 9 in this regulation.     

线粒体DNA相关疾病的分子遗传学研究

在儿童退行性疾病中 ,诸如发育延迟、感觉运动障碍、惊厥、糖尿病以及器官衰竭等 ,线粒体功能紊乱是一个相当常见的病因[1] 。近年来线粒体ＤＮＡ(ｍｔＤＮＡ)突变及其致病作用也是一些晚发性疾病如Ａｌｚｈｅｉｍｅｒ’ｓ病、肿瘤及Ｐａｒｋｉｎｓｏｎ’ｓ病的研究热点。据Ｆｉｎｌａｎｄ的一项研究表明ｍｔＤＮＡ相关疾病的发病率≥ 16 3 10万。自从 1963年发现ｍｔＤＮＡ和 1988年首次报道致病性ｍｔＤＮＡ突变以来 ,迄今已发现至少 97个点突变和很多的ｍｔＤＮＡ重排 (缺失、重复 ) ,ｍｔＤＮＡ突变业已成为线粒体疾病的重要病因之一[1,…     

MT‐ND5 Mutation Causing Exercise Intolerance Displays Intercellular Heteroplasmy and Rapid Shifts Between Generations

We studied the inheritance and cellular segregation of a maternally inherited, heteroplasmic MT‐ND5 mutation, m.13271T>C, previously shown to cause only exercise intolerance despite being present in multiple tissues. The mutation was present at low levels in early passage, bulk muscle culture, but on subcloning, only homoplasmic clones were found. Studies of transmission showed that the mutation expanded from very low levels in the patient's mother to higher levels in the patient, particularly skeletal muscle, but was not found in the placenta and umbilical cord blood of her child. Our study suggests that the m.13271T>C is either already strictly segregated (intercellular heteroplasmy), or moves rapidly to this state in cultured cells. Transmission studies suggest that intercellular heteroplasmy may also be present in the patient's germline. Although rapid shifts in heteroplasmic mitochondrial DNA mutations reflect a bottleneck in the female germline, complete segregation will accentuate the effects of this and further complicate genetic counseling.     

Mitochondrial transfer between oocytes: potential applications of mitochondrial donation and the issue of heteroplasmy

The developmental competence of mouse and human early embryosappears to be directly related to the metabolic capacity ofa finite complement of maternally inherited mitochondria thatappear to begin to replicate after implantation. Mitochondrialdysfunctions resulting from a variety of intrinsic and extrinsicinfluences, including genetic abnormalities, hypoxia and oxidativestress, can profoundly influence the level of ATP generationin oocytes and early embryos, which in turn may result in aberrantchromosomal segregation or developmental arrest. Deletions andmutations in oocyte mitochondrial DNA may subtend metabolicdeficiencies or replication disorders in some infertile womenand in women of increased reproductive age. Here, we describemethods for (i) the compartmentalization of mouse and humanoocyte mitochondria into unique cytoplasts enriched for theseorganelles, and (ii) their transfer by microinjection into intactrecipient oocytes. Metabolically active mitochondria in donorand recipient metaphase II stage oocytes were labelled withmitochondria-specific fluorescent probes, and the fate and locationof donated mitochondria in recipient oocytes were followed byconventional epifluorescence and scanning laser confocal fluorescencemicroscopy. The net ATP content of undisturbed and recipientoocytes from the same cohort(s) was measured quantitativelyat timed intervals after mitochondrial injection. The resultsdemonstrate the feasibility of isolating and transferring mitochondriabetween oocytes, an apparent increase in net ATP productionin the recipients, and the persistence of activity in the transferredmitochondria. The findings are discussed with respect to mitochondrialfunction and dysfunction in mammalian oocytes and embryos, andto the potential clinical applications of mitochondrial donationas they relate to the creation of heteroplasmic embryos.     

Genetic counselling protocols for hereditary non-polyposis colorectal cancer: a survey of UK regional genetics centres

Predictive testing for hereditary non-polyposis colorectal cancer (HNPCC) is typically offered within an extended genetic counselling protocol, originally developed in the context of Huntington's Disease. We conducted a questionnaire survey of 20 UK regional genetics centres to obtain evidence regarding current approaches to HNPCC pre-test counselling. Centres were asked to describe the structure and content of pre-test counselling and their views on shortening the protocol. Sixteen centres responded to the survey. Four centres were considering shortening the protocol or had already done so. The remaining centres followed an extended protocol of two sessions separated by a 1-month period for reflection, although two centres conceded that the protocol had been reduced in certain cases. Different centres used different terminology to describe the content of pre-test counselling. Although content areas relating to education or impact of test results were covered more frequently than those relating to reflection, there was a marked tendency to consider all three areas as essential and to use both educational and reflective counselling, even in those centres that favoured a shortened protocol. This apparent dilemma highlights both the practical difficulty of how to shorten HNPCC pre-test counselling protocols and the need for controlled trials of different approaches.     

和田地区维吾尔族人群线粒体DNA遗传多样性分析

目的:分析和田维吾尔族人群线粒体DNA遗传多样性。方法:运用基因芯片技术对411个(高变区Ⅰ、编码区、高变区Ⅱ)SNP位点进行了检测和分型。结果:所测411个SNP位点中83个位点发生了突变,其中A750G、A8860G、T10873C、A25326G、T16189C5个位点的突变率为100%。依据突变位点共划分出25种单倍型。其中,单倍型H2a2a、M、G2a、HV和A有较高的频率(11.36%、9.09%、6.82%、6.82%、6.82%)。52.2%的单倍型属于亚洲单倍型,47.8%的单倍型属于欧洲单倍型,不同地区维吾尔族群体线粒体DNA单倍型有较大差异。结论:和田维吾尔族mtDNA-SNP具有较丰富的遗传多样性,具有明显的亚欧混合现象。维吾尔族母系遗传库较丰富,适用于个体识别、亲子鉴定、疾病诊断。     

Preimplantation genetic diagnosis for mitochondrial DNA disorders: ethical guidance for clinical practice

Although morally acceptable in theory, preimplantation genetic diagnosis (PGD) for mitochondrial DNA (mtDNA) disorders raises several ethical questions in clinical practice. This paper discusses the major conditions for good clinical practice. Our starting point is that PGD for mtDNA mutations should as far as possible be embedded in a scientific research protocol. For every clinical application of PGD for mtDNA disorders, it is not only important to avoid a ‘high risk of serious harm'' to the future child, but also to consider to what extent it would be possible, desirable and proportional to try to reduce the health risks and minimize harm. The first issue we discuss is oocyte sampling, which may point out whether PGD is feasible for a specific couple. The second issue is whether one blastomere represents the genetic composition of the embryo as a whole – and how this could (or should) be investigated. The third issue regards the cutoff points below which embryos are considered to be eligible for transfer. We scrutinize how to determine these cutoff points and how to use these cutoff points in clinical practice – for example, when parents ask to take more or less risks. The fourth issue regards the number of cycles that can (or should) justifiably be carried out to find the best possible embryo. Fifth, we discuss whether follow-up studies should be conducted, particularly the genetic testing of children born after IVF/PGD. Finally, we offer the main information that is required to obtain a truly informed consent.     

Single‐fiber studies for assigning pathogenicity of eight mitochondrial DNA variants associated with mitochondrial diseases

Whole mitochondrial DNA (mtDNA) sequencing is now systematically used in clinical laboratories to screen patients with a phenotype suggestive of mitochondrial disease. Next Generation Sequencing (NGS) has significantly increased the number of identified pathogenic mtDNA variants. Simultaneously, the number of variants of unknown significance (VUS) has increased even more, thus challenging their interpretation. Correct classification of the variants' pathogenicity is essential for optimal patient management, including treatment and genetic counseling. Here, we used single muscle fiber studies to characterize eight heteroplasmic mtDNA variants, among which were three novel variants. By applying the pathogenicity scoring system, we classified four variants as “definitely pathogenic” (m.590A>G, m.9166T>C, m.12293G>A, and m.15958A>T). Two variants remain “possibly pathogenic” (m.4327T>C and m.5672T>C) but should these be reported in a different family, they would be reclassified as “definitely pathogenic.” We also illustrate the contribution of single‐fiber studies to the diagnostic approach in patients harboring pathogenic variants with low level heteroplasmy.     

中国布依族、苗族人群线粒体DNA Region V区的遗传多态性

目的 研究中国布依族、苗族人群线粒体DNA Region V区的遗传多态性。方法 采用PCR直接测序法对布依族（96份）及苗族人（90份）样本线粒体DNA Region V区进行序列分析。结果 在布依族及苗族样本中只发现标准型和短型两种多态，短型多态（即9bp缺失）频率在布依族、苗族人群中分别为31．1％和33．3％。结论 中国布依族、苗族人群线粒体DNA Region V区有相似的多态性，而与其他民族或人种有一定差异。     

A comprehensive review of HVS-I mitochondrial DNA variation of 19 Iranian populations

Iran is located along the Central Asian corridor, a natural artery that has served as a cross-continental route since the first anatomically modern human populations migrated out of Africa. We compiled and reanalyzed the HVS-I (hypervariable segment-I) of 3840 mitochondrial DNA (mtDNA) sequences from 19 Iranian populations and from 26 groups from adjacent countries to give a comprehensive review of the maternal genetic variation and investigate the impact of historical events and cultural factors on the maternal genetic structure of modern Iranians. We conclude that Iranians have a high level of genetic diversity. Thirty-six haplogroups were observed in Iran's populations, and most of them belong to widespread West-Eurasian haplogroups, such as H, HV, J, N, T, and U. In contrast, the predominant haplogroups observed in most of the adjacent countries studied here are H, M, D, R, U, and C haplogroups. Using principal component analysis, clustering, and genetic distance-based calculations, we estimated moderate genetic relationships between Iranian and other Eurasian groups. Further, analyses of molecular variance and comparing geographic and genetic structures indicate that mtDNA HVS-I sequence diversity does not exhibit any sharp geographic structure in the country. Barring a few from some culturally distinct and naturally separated minorities, most Iranian populations have a homogenous maternal genetic structure.     

A new pathologic mitochondrial DNA mutation in the cytochrome oxidase subunit I (MT-CO1)

A disorder of mitochondrial energy metabolism may be missed in children with a very mild phenotype. Here, we described a patient with a moderate mental retardation and a mild exercise intolerance. This child harboured a mtDNA transition (m.6955G>A) in the subunit I of the cytochrome oxidase (MT-CO1) that fulfils most of the requirements to be pathologic. Despite this subunit is the second longest polypeptide encoded in the mtDNA, only one other missense mutation associated with a myopathy has been described. This suggests that we are missing other phenotypes and that the mitochondrial pathology field is broader that previously thought.     

Clinical anophthalmos in a family

Monolateral or bilateral anophthalmos recurring in the absence of other associated defects in six members of a family is reported. The malformation appears to be inherited as a dominant (autosomal or X-linked) trait with incomplete penetrance. Implications for genetic counselling are briefly discussed.     

线粒体肌病患者线粒体DNA的突变分析

目的分析线粒体肌病患者线粒体DNA的突变情况，为疾病诊断提供依据。方法用常规HE、酶组化染色和电镜检查等病理形态学方法对3例线粒体肌病疑似患者进行诊断，并用聚合酶链反应-单链构象多态和DNA测序等方法对患者线粒体DNA中全部22个tRNA基因进行突变筛查。结果3例患者均被确诊为线粒体肌病，其中例1tRNA—VaI基因发生A1627G纯合突变，例2tRNA—Val基因发生A1627G／A杂合突变，例3tRNA—Trp基因发生T5554C突变、tRNA—Arg基因发生A10412C／A杂合突变。结论线粒体DNA中的tRNA基因突变是线粒体肌病的重要病因之一。     

Mutations in FASTKD2 are associated with mitochondrial disease with multi‐OXPHOS deficiency

Mutations in FASTKD2, a mitochondrial RNA binding protein, have been associated with mitochondrial encephalomyopathy with isolated complex IV deficiency. However, deficiencies related to other oxidative phosphorylation system (OXPHOS) complexes have not been reported. Here, we identified three novel FASTKD2 mutations, c.808_809insTTTCAGTTTTG, homoplasmic mutation c.868C>T, and heteroplasmic mutation c.1859delT/c.868C>T, in patients with mitochondrial encephalomyopathy. Cell‐based complementation assay revealed that these three FASTKD2 mutations were pathogenic. Mitochondrial functional analysis revealed that mutations in FASTKD2 impaired the mitochondrial function in patient‐derived lymphocytes due to the deficiency in multi‐OXPHOS complexes, whereas mitochondrial complex II remained unaffected. Consistent results were also found in human primary muscle cell and zebrafish with knockdown of FASTKD2. Furthermore, we discovered that FASTKD2 mutation is not inherently associated with epileptic seizures, optic atrophy, and loss of visual function. Alternatively, a patient with FASTKD2 mutation can show sinus tachycardia and hypertrophic cardiomyopathy, which was partially confirmed in zebrafish with knockdown of FASTKD2. In conclusion, both in vivo and in vitro studies suggest that loss of function mutation in FASTKD2 is responsible for multi‐OXPHOS complexes deficiency, and FASTKD2‐associated mitochondrial disease has a high degree of clinical heterogenicity.     

Clinical,biochemical, cellular and molecular characterization of mitochondrial DNA depletion syndrome due to novel mutations in the MPV17 gene

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are severe autosomal recessive disorders associated with decreased mtDNA copy number in clinically affected tissues. The hepatocerebral form (mtDNA depletion in liver and brain) has been associated with mutations in the POLG, PEO1 (Twinkle), DGUOK and MPV17 genes, the latter encoding a mitochondrial inner membrane protein of unknown function. The aims of this study were to clarify further the clinical, biochemical, cellular and molecular genetic features associated with MDS due to MPV17 gene mutations. We identified 12 pathogenic mutations in the MPV17 gene, of which 11 are novel, in 17 patients from 12 families. All patients manifested liver disease. Poor feeding, hypoglycaemia, raised serum lactate, hypotonia and faltering growth were common presenting features. mtDNA depletion in liver was demonstrated in all seven cases where liver tissue was available. Mosaic mtDNA depletion was found in primary fibroblasts by PicoGreen staining. These results confirm that MPV17 mutations are an important cause of hepatocerebral mtDNA depletion syndrome, and provide the first demonstration of mosaic mtDNA depletion in human MPV17 mutant fibroblast cultures. We found that a severe clinical phenotype was associated with profound tissue-specific mtDNA depletion in liver, and, in some cases, mosaic mtDNA depletion in fibroblasts.     

Mitochondrial genetic analysis in a Chinese family suffering from both mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes and diabetes

To investigate the mitochondrial mutations in patients suffering from both mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) and maternally inherited diabetes. MELAS was confirmed by muscle biopsy performed from the biceps muscle of the proband. Mitochondrial DNA (mtDNA) was isolated from peripheral blood mononuclear cells. The significant mtDNA loci of other 14 family members were further detected according to the sequencing results of the proband. Direct sequencing of PCR products was used to identify the mitochondrial mutations. The proband (III 1) and her brother (III 3) both harbored the tRNALeu (UUR) A3243G mutation, with heteroplasmic levels of 50% and 33% respectively. Moreover, another two mitochondrial variants, A8860G and A15326G, were also detected in the samples of all the family members. MELAS and diabetes can coexist in one patient, and the main cause for these diseases is the tRNALeu (UUR) A3243G mutation. However, other gene variants may contribute to its pathogenesis. This case also supports the concept that both syndromes can be regarded as two phenotypes of the same disease.