Coexistence of mitochondrial DNA and beta myosin heavy chain mutations in hypertrophic cardiomyopathy with late congestive heart failure

Analysis of Mitochondrial Transfer RNA Mutations in Breast Cancer

Damage of mitochondrial functions caused by mitochondrial DNA (mtDNA) pathogenic mutations had long been proposed to be involved in breast carcinogenesis. However, the detailed pathological mechanism remained deeply undetermined. In this case-control study, we screened the frequencies of mitochondrial tRNA (mt-tRNA) mutations in 80 breast cancer tissues and matched normal adjacent tissues. PCR and Sanger sequence revealed five possible pathogenic mutations: tRNA^Val G1606A, tRNA^Ile A4300G, tRNA^Ser(UCN) T7505C, tRNA^Glu A14693G and tRNA^Thr G15927A. We noticed that these mutations resided at extremely conserved positions of tRNAs and would affect tRNAs transcription or modifications. Furthermore, functional analysis suggested that patients with these mt-tRNA mutations exhibited much lower levels of mtDNA copy number and ATP, as compared with controls (p<0.05). Therefore, it can be speculated that these mutations may impair mitochondrial protein synthesis and oxidative phosphorylation (OXPHOS) complexes, which caused mitochondrial dysfunctions that were involved in the breast carcinogenesis. Taken together, our data indicated that mutations in mt-tRNA were the important contributors to breast cancer, and mutational analyses of mt-tRNA genes were critical for prevention of breast cancer.

Skeletal Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

Whole mitochondrial genome analysis in Chinese patients with keratoconus

Purpose

Keratoconus (KC) is a corneal disorder characterized by corneal ectasia, progressive corneal thinning, and conical protrusion. This study aimed to elucidate the mitochondrial gene profile in Chinese patients with KC, analyze the mitochondrial haplogroup and heteroplasmy, and further explore the association between mitochondrial genes and KC.

Methods

Mitochondrial sequencing was conducted on 100 patients with KC and 100 matched controls. Haplogroup analysis was conducted with logistic regression analysis. The heteroplasmy was analyzed with ANOVA (ANOVA) and Student t test. Sequence kernel association tests (SKATs) were performed to analyze the association between mitochondrial genes and KC. Mtoolbox, Mitoclass.1, and APOGEE were used to estimate the impact of the identified variants in protein-coding genes. PON-mt-tRNA was used to annotate the impact of the variants in tRNA. RNAstructure was used to predict the secondary structures of native and mutated tRNAs.

Results

We identified 689 variants in patients with KC and 725 variants in controls (with 308 variants shared by both). The mitochondrial haplogroups exhibited no statistically significant differences between the two groups. Based on the heteroplasmy analysis, the number of heteroplasmic variants in the complete mitochondrial genome, RNA coding regions, and noncoding regions were statistically significantly different in the KC cases and controls (p<0.05). The heteroplasmic levels of the m.16180_16182delAA, m.16182insC, and m.14569 G>C variants in the KC cases were statistically significantly higher than those in the controls (p<0.05). The SKAT analysis showed that the COX3 and TRNH genes were statistically significantly associated with KC (p<0.05). Among the nine variants of COX3 included in the SKAT analysis (m.9300G>A, m.9316T>C, m.9327A>G, m.9355A>G, m.9468A>G, m.9612G>A, m.9804G>A, m.9957G>A, and m.9966 G>A), m.9612G>A was predicted to be deleterious by Mtoolbox. The m.9316T>C, m.9327A>G, m.9355A>G, m.9612G>A, m.9804G>A, and m.9957G>A variants were predicted to be damaging by Mitoclass.1. The m.9355A>G and m.9804G>A variants were predicted to be pathogenic by APOGEE. All identified variants located in TRNH (m.12153C>T, m.12178C>T, and m.12192G>A) were predicted to be neutral by the PON-mt-tRNA website.

Conclusions

This study presents the mitochondrial gene profile of Chinese patients with KC and demonstrated that the COX3 and TRNH genes were associated with KC.

Mitochondrial DNA: Hotspot for Potential Gene Modifiers Regulating Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a prevalent and untreatable cardiovascular disease with a highly complex clinical and genetic causation. HCM patients bearing similar sarcomeric mutations display variable clinical outcomes, implying the involvement of gene modifiers that regulate disease progression. As individuals exhibiting mutations in mitochondrial DNA (mtDNA) present cardiac phenotypes, the mitochondrial genome is a promising candidate to harbor gene modifiers of HCM. Herein, we sequenced the mtDNA of isogenic pluripotent stem cell-cardiomyocyte models of HCM focusing on two sarcomeric mutations. This approach was extended to unrelated patient families totaling 52 cell lines. By correlating cellular and clinical phenotypes with mtDNA sequencing, potentially HCM-protective or -aggravator mtDNA variants were identified. These novel mutations were mostly located in the non-coding control region of the mtDNA and did not overlap with those of other mitochondrial diseases. Analysis of unrelated patients highlighted family-specific mtDNA variants, while others were common in particular population haplogroups. Further validation of mtDNA variants as gene modifiers is warranted but limited by the technically challenging methods of editing the mitochondrial genome. Future molecular characterization of these mtDNA variants in the context of HCM may identify novel treatments and facilitate genetic screening in cardiomyopathy patients towards more efficient treatment options.

Clinical and laboratory interpretation of mitochondrial mRNA variants

Interpretation of mitochondrial protein-encoding (mt-mRNA) variants has been challenging due to mitochondrial characteristics that have not been addressed by American College of Medical Genetics and Genomics guidelines. We developed criteria for the interpretation of mt-mRNA variants via literature review of reported variants, tested and refined these criteria by using our new cases, followed by interpreting 421 novel variants in our clinical database using these verified criteria. A total of 32 of 56 previously reported pathogenic (P) variants had convincing evidence for pathogenicity. These variants are either null variants, well-known disease-causing variants, or have robust functional data or strong phenotypic correlation with heteroplasmy levels. Based on our criteria, 65.7% (730/1,111) of variants of unknown significance (VUS) were reclassified as benign (B) or likely benign (LB), and one variant was scored as likely pathogenic (LP). Furthermore, using our criteria we classified 2, 12, and 23 as P, LP, and LB, respectively, among 421 novel variants. The remaining stayed as VUS (91.2%). Appropriate interpretation of mt-mRNA variants is the basis for clinical diagnosis and genetic counseling. Mutation type, heteroplasmy levels in different tissues of the probands and matrilineal relatives, in silico predictions, population data, as well as functional studies are key points for pathogenicity assessments.

RNA Sequence Analyses throughout the Course of Mouse Cardiac Laminopathy Identify Differentially Expressed Genes for Cell Cycle Control and Mitochondrial Function

Lamin A/C (LMNA) gene mutations are a known cause of familial dilated cardiomyopathy, but the precise mechanisms triggering disease progression remain unknown. We hypothesize that analysis of differentially expressed genes (DEGs) throughout the course of Lmna knockout (Lmna-/-)-induced cardiomyopathy may reveal novel Lmna-mediated alterations of signaling pathways leading to dilated cardiomyopathy. Although Lmna was the only DEG down-regulated at 1 week of age, we identified 730 and 1004 DEGs in Lmna-/- mice at 2 weeks and 1 month of age, respectively. At 2 weeks, Lmna-/- mice demonstrated both down- and up-regulation of the key genes involving cell cycle control, mitochondrial dysfunction, and oxidative phosphorylation, as well as down-regulated genes governing DNA damage repair and up-regulated genes involved in oxidative stress response, cell survival, and cardiac hypertrophy. At 1 month, the down-regulated genes included those involved in oxidative phosphorylation, mitochondrial dysfunction, nutrient metabolism, cardiac β-adrenergic signaling, action potential generation, and cell survival. We also found 96 overlapping DEGs at both ages involved in oxidative phosphorylation, mitochondrial function, and calcium signaling. Impaired oxidative phosphorylation was observed at early disease stage, even before the appearance of disease phenotypes, and worsened with disease progression, suggesting its importance in the pathogenesis and progression of LMNA cardiomyopathy. Reduction of oxidative stress might therefore prevent or delay the development from Lmna mutation to LMNA cardiomyopathy.

Mitochondrial DNA Variation Dictates Expressivity and Progression of Nuclear DNA Mutations Causing Cardiomyopathy

Nuclear-encoded mutations causing metabolic and degenerative diseases have highly variable expressivity. Patients sharing the homozygous mutation (c.523delC) in the adenine nucleotide translocator 1 gene (SLC25A4, ANT1) develop cardiomyopathy that varies from slowly progressive to fulminant. This variability correlates with the mitochondrial DNA (mtDNA) lineage. To confirm that mtDNA variants can modulate the expressivity of nuclear DNA (nDNA)-encoded diseases, we combined in mice the nDNA Slc25a4^-/- null mutation with a homoplasmic mtDNA ND6^P25L or COI^V421A variant. The ND6^P25L variant significantly increased the severity of cardiomyopathy while the COI^V421A variant was phenotypically neutral. The adverse Slc25a4^-/- and ND6^P25L combination was associated with impaired mitochondrial complex I activity, increased oxidative damage, decreased l-Opa1, altered mitochondrial morphology, sensitization of the mitochondrial permeability transition pore, augmented somatic mtDNA mutation levels, and shortened lifespan. The strikingly different phenotypic effects of these mild mtDNA variants demonstrate that mtDNA can be an important modulator of autosomal disease.

Mitochondrial Function, Biology, and Role in Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is a major leading cause of morbidity and mortality in the United States and elsewhere. Alterations in mitochondrial function are increasingly being recognized as a contributing factor in myocardial infarction and in patients presenting with cardiomyopathy. Recent understanding of the complex interaction of the mitochondria in regulating metabolism and cell death can provide novel insight and therapeutic targets. The purpose of this statement is to better define the potential role of mitochondria in the genesis of cardiovascular disease such as ischemia and heart failure. To accomplish this, we will define the key mitochondrial processes that play a role in cardiovascular disease that are potential targets for novel therapeutic interventions. This is an exciting time in mitochondrial research. The past decade has provided novel insight into the role of mitochondria function and their importance in complex diseases. This statement will define the key roles that mitochondria play in cardiovascular physiology and disease and provide insight into how mitochondrial defects can contribute to cardiovascular disease; it will also discuss potential biomarkers of mitochondrial disease and suggest potential novel therapeutic approaches.

Cardiovascular Disease, Mitochondria, and Traditional Chinese Medicine

Recent studies demonstrated that mitochondria play an important role in the cardiovascular system and mutations of mitochondrial DNA affect coronary artery disease, resulting in hypertension, atherosclerosis, and cardiomyopathy. Traditional Chinese medicine (TCM) has been used for thousands of years to treat cardiovascular disease, but it is not yet clear how TCM affects mitochondrial function. By reviewing the interactions between the cardiovascular system, mitochondrial DNA, and TCM, we show that cardiovascular disease is negatively affected by mutations in mitochondrial DNA and that TCM can be used to treat cardiovascular disease by regulating the structure and function of mitochondria via increases in mitochondrial electron transport and oxidative phosphorylation, modulation of mitochondrial-mediated apoptosis, and decreases in mitochondrial ROS. However further research is still required to identify the mechanism by which TCM affects CVD and modifies mitochondrial DNA.

Private mitochondrial DNA variants in danish patients with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetic cardiac disease primarily caused by mutations in genes coding for sarcomeric proteins. A molecular-genetic etiology can be established in ~60% of cases. Evolutionarily conserved mitochondrial DNA (mtDNA) haplogroups are susceptibility factors for HCM. Several polymorphic mtDNA variants are associated with a variety of late-onset degenerative diseases and affect mitochondrial function. We examined the role of private, non-haplogroup associated, mitochondrial variants in the etiology of HCM. In 87 Danish HCM patients, full mtDNA sequencing revealed 446 variants. After elimination of 312 (69.9%) non-coding and synonymous variants, a further 109 (24.4%) with a global prevalence > 0.1%, three (0.7%) haplogroup associated and 19 (2.0%) variants with a low predicted in silico likelihood of pathogenicity, three variants: MT-TC: m.5772G>A, MT-TF: m.644A>G, and MT-CYB: m.15024G>A, p.C93Y remained. A detailed analysis of these variants indicated that none of them are likely to cause HCM. In conclusion, private mtDNA mutations are frequent, but they are rarely, if ever, associated with HCM.

Genetics of inherited cardiomyopathies

Cardiomyopathies are the most common disorders resulting in heart failure, with dilated cardiomyopathy being responsible for the majority of cases. Other forms of cardiomyopathy, especially hypertrophic forms, are also important causes of heart failure. The mortality rate due to cardiomyopathy in the USA is over 10,000 deaths per year, and the costs associated with heart failure are approximately 200 million US dollars per year in the USA alone. Over the past few years, breakthroughs have occurred in understanding the basic mechanisms of these disorders, potentially enabling clinicians to devise improved diagnostic strategies and therapies. As at least 30 to 40% of cases are inherited, it is now imperative that the genetic basis for these disorders is clearly recognized by caregivers and scientists. However, it has also become clear that these diseases are genetically highly heterogeneous, with multiple genes identified for each of the major forms of cardiomyopathy, and most patients having private mutations. These data suggest that the genetic diagnosis of most patients with cardiomyopathy will be impractical with current technologies. However, there are a few exceptions, such as patients with X-linked cardiomyopathies, with or without the concomitant abnormalities of cyclic neutropenia and 3-methylglutaconic aciduria, or patients with cardiomyopathy associated with conduction disease: these appear to be associated with mutations in a small subset of genes, and can be investigated by certified diagnostic laboratories. This review will summarize current knowledge of the genetics of inherited cardiomyopathies and how findings from research laboratories may be translated into the diagnostic laboratory.

The novel mitochondrial 16S rRNA 2336T>C mutation is associated with hypertrophic cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM) is a primary disorder characterised by asymmetric thickening of septum and left ventricular wall, with a prevalence of 0.2% in the general population.

Objective

To describe a novel mitochondrial DNA mutation and its association with the pathogenesis of HCM.

Methods and results

All maternal members of a Chinese family with maternally transmitted HCM exhibited variable severity and age at onset, and were implanted permanent pacemakers due to complete atrioventricular block (AVB). Nuclear gene screening (MYH7, MYBPC3, TNNT2 and TNNI3) was performed, and no potential pathogenic mutation was identified. Mitochondrial DNA sequencing analysis identified a novel homoplasmic 16S rRNA 2336T>C mutation. This mutation was exclusively present in maternal members and absent in non-maternal members. Conservation index by comparison to 16 other vertebrates was 94.1%. This mutation disturbs the 2336U-A2438 base pair in the stem–loop structure of 16S rRNA domain III, which is involved in the assembly of mitochondrial ribosome. Oxygen consumption rate of the lymphoblastoid cells carrying 2336T>C mutation had decreased by 37% compared with controls. A reduction in mitochondrial ATP synthesis and an increase in reactive oxidative species production were also observed. Electron microscopic analysis indicated elongated mitochondria and abnormal mitochondrial cristae shape in mutant cells.

Conclusions

It is suggested that the 2336T>C mutation is one of pathogenic mutations of HCM. This is the first report of mitochondrial 16S rRNA 2336T>C mutation and an association with maternally inherited HCM combined with AVB. Our findings provide a new insight into the pathogenesis of HCM.

MT-CYB mutations in hypertrophic cardiomyopathy

Mitochondrial dysfunction is a characteristic of heart failure. Mutations in mitochondrial DNA, particularly in MT-CYB coding for cytochrome B in complex III (CIII), have been associated with isolated hypertrophic cardiomyopathy (HCM). We hypothesized that MT-CYB mutations might play an important causal or modifying role in HCM. The MT-CYB gene was sequenced from DNA isolated from blood from 91 Danish HCM probands. Nonsynonymous variants were analyzed by bioinformatics, molecular modeling and simulation. Two germline-inherited, putative disease-causing, nonsynonymous variants: m.15024G>A; p.C93Y and m.15482T>C; p.S246P were identified. Modeling showed that the p.C93Y mutation leads to disruption of the tertiary structure of Cytb by helix displacement, interfering with protein–heme interaction. The p.S246P mutation induces a diproline structure, which alters local secondary structure and induces a kink in the protein backbone, interfering with macromolecular interactions. These molecular effects are compatible with a leaky phenotype, that is, limited but progressive mitochondrial dysfunction. In conclusion, we find that rare, putative leaky mtDNA variants in MT-CYB can be identified in a cohort of HCM patients. We propose that further patients with HCM should be examined for mutations in MT-CYB in order to clarify the role of these variants.

Severity of cardiomyopathy associated with adenine nucleotide translocator-1 deficiency correlates with mtDNA haplogroup

Mutations of both nuclear and mitochondrial DNA (mtDNA)-encoded mitochondrial proteins can cause cardiomyopathy associated with mitochondrial dysfunction. Hence, the cardiac phenotype of nuclear DNA mitochondrial mutations might be modulated by mtDNA variation. We studied a 13-generation Mennonite pedigree with autosomal recessive myopathy and cardiomyopathy due to an SLC25A4 frameshift null mutation (c.523delC, p.Q175RfsX38), which codes for the heart-muscle isoform of the adenine nucleotide translocator-1. Ten homozygous null (adenine nucleotide translocator-1(-/-)) patients monitored over a median of 6 years had a phenotype of progressive myocardial thickening, hyperalaninemia, lactic acidosis, exercise intolerance, and persistent adrenergic activation. Electrocardiography and echocardiography with velocity vector imaging revealed abnormal contractile mechanics, myocardial repolarization abnormalities, and impaired left ventricular relaxation. End-stage heart disease was characterized by massive, symmetric, concentric cardiac hypertrophy; widespread cardiomyocyte degeneration; overabundant and structurally abnormal mitochondria; extensive subendocardial interstitial fibrosis; and marked hypertrophy of arteriolar smooth muscle. Substantial variability in the progression and severity of heart disease segregated with maternal lineage, and sequencing of mtDNA from five maternal lineages revealed two major European haplogroups, U and H. Patients with the haplogroup U mtDNAs had more rapid and severe cardiomyopathy than those with haplogroup H.

Vulnerable myocardial interstitium in patients with isolated left ventricular hypertrophy and sudden cardiac death: a postmortem histological evaluation

BACKGROUND

Concentric left ventricular hypertrophy (LVH) is independently associated with increased risk of sudden cardiac death (SCD). Some animal models of LVH display specific alterations of the myocardial interstitium that could increase myocardial vulnerability to ventricular arrhythmias, but these merit evaluation in humans with LVH and SCD.

METHODS AND RESULTS

Twelve consecutive patients with isolated LVH and SCD (LVH+SCD) in the absence of hypertrophic cardiomyopathy, coronary disease, or other cardiac structural abnormality were ascertained in the Oregon Sudden Unexpected Death Study. Detailed postmortem comparisons were conducted with 18 controls who had isolated LVH and unnatural deaths (Control Group A) and 6 controls who had structurally normal hearts and unnatural deaths (Control Group B). Postmortem left ventricular myocardial sections were obtained for measurement of collagen volume fraction, characterization of gap junctions, and quantification of collagen subtypes. Heart weight normalized to body weight was higher in LVH+SCD cases (6.9±1.2 g/kg) than in Control Group A (5.3±1.4 g/kg) and Control Group B (4.2±0.3 g/kg); P=0.001. Collagen volume fraction was also higher in LVH+SCD cases (3.1±0.4) than in Control Group A (2.3±0.4) and Control Group B (1.6±0.3); P=0.0002. The relative amount of collagen III was significantly higher in LVH+SCD cases (33.0±4.4%) than in Control Group A (20.9±4.3%) and Control Group B (13.4±3.5%); P=0.0001. There was an overall increase in the number of connexin 43-labeled gap junctions with increasing myocyte size. No subject was found to have high-risk hypertrophic cardiomyopathy mutations.

CONCLUSIONS

In addition to the expected increase in myocardial mass and overall collagen content, SCD with isolated LVH was associated with relative abundance of type III collagen, a novel finding that warrants further mechanistic evaluation. (J Am Heart Assoc. 2012;1:e001511 doi: 10.1161/JAHA.111.001511.).

Hypertrophic cardiomyopathy in childhood

Hypertrophic cardiomyopathy has important differences in children compared with adults, particularly with regard to the range of causes and the outcomes in infants. Survival is highly dependent on etiology, particularly in the youngest patients, and pursuit of the specific cause is therefore necessary. The clinical utility of defining the genotype in children with familial hypertrophic cardiomyopathy exceeds that at other ages and has a highly favorable cost/benefit ratio. Although most of the available information concerning treatment and prevention of sudden death is derived in adults, management of children requires consideration of the differences in age-specific risk/benefit ratios.

Mitochondrial cardiomyopathies: how to identify candidate pathogenic mutations by mitochondrial DNA sequencing, MITOMASTER and phylogeny

Pathogenic mitochondrial DNA (mtDNA) mutations leading to mitochondrial dysfunction can cause cardiomyopathy and heart failure. Owing to a high mutation rate, mtDNA defects may occur at any nucleotide in its 16 569 bp sequence. Complete mtDNA sequencing may detect pathogenic mutations, which can be difficult to interpret because of normal ethnic/geographic-associated haplogroup variation. Our goal is to show how to identify candidate mtDNA mutations by sorting out polymorphisms using readily available online tools. The purpose of this approach is to help investigators in prioritizing mtDNA variants for functional analysis to establish pathogenicity. We analyzed complete mtDNA sequences from 29 Italian patients with mitochondrial cardiomyopathy or suspected disease. Using MITOMASTER and PhyloTree, we characterized 593 substitution variants by haplogroup and allele frequencies to identify all novel, non-haplogroup-associated variants. MITOMASTER permitted determination of each variant's location, amino acid change and evolutionary conservation. We found that 98% of variants were common or rare, haplogroup-associated variants, and thus unlikely to be primary cause in 80% of cases. Six variants were novel, non-haplogroup variants and thus possible contributors to disease etiology. Two with the greatest pathogenic potential were heteroplasmic, nonsynonymous variants: m.15132T>C in MT-CYB for a patient with hypertrophic dilated cardiomyopathy and m.6570G>T in MT-CO1 for a patient with myopathy. In summary, we have used our automated information system, MITOMASTER, to make a preliminary distinction between normal mtDNA variation and pathogenic mutations in patient samples; this fast and easy approach allowed us to select the variants for traditional analysis to establish pathogenicity.

Mitochondrial DNA variant discovery and evaluation in human Cardiomyopathies through next-generation sequencing

Mutations in mitochondrial DNA (mtDNA) may cause maternally-inherited cardiomyopathy and heart failure. In homoplasmy all mtDNA copies contain the mutation. In heteroplasmy there is a mixture of normal and mutant copies of mtDNA. The clinical phenotype of an affected individual depends on the type of genetic defect and the ratios of mutant and normal mtDNA in affected tissues. We aimed at determining the sensitivity of next-generation sequencing compared to Sanger sequencing for mutation detection in patients with mitochondrial cardiomyopathy. We studied 18 patients with mitochondrial cardiomyopathy and two with suspected mitochondrial disease. We “shotgun” sequenced PCR-amplified mtDNA and multiplexed using a single run on Roche's 454 Genome Sequencer. By mapping to the reference sequence, we obtained 1,300× average coverage per case and identified high-confidence variants. By comparing these to >400 mtDNA substitution variants detected by Sanger, we found 98% concordance in variant detection. Simulation studies showed that >95% of the homoplasmic variants were detected at a minimum sequence coverage of 20× while heteroplasmic variants required >200× coverage. Several Sanger “misses” were detected by 454 sequencing. These included the novel heteroplasmic 7501T>C in tRNA serine 1 in a patient with sudden cardiac death. These results support a potential role of next-generation sequencing in the discovery of novel mtDNA variants with heteroplasmy below the level reliably detected with Sanger sequencing. We hope that this will assist in the identification of mtDNA mutations and key genetic determinants for cardiomyopathy and mitochondrial disease.

Prevalence of systolic impairment in an unselected regional population with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HC) may transition to a phase characterized by systolic impairment resembling dilated cardiomyopathy. This study retrospectively assessed the incidence of left ventricular (LV) systolic impairment at initial clinical evaluation in 248 consecutive patients with HC (mean age 53 +/- 16 years). HC with systolic impairment was diagnosed if the LV ejection fraction was <50%, calculated by echocardiography. Twenty patients (8%) had HC with LV systolic impairment at initial evaluation. Patients with systolic impairment had a greater incidence of family histories of sudden cardiac death (SCD) than patients with preserved systolic function (25% vs 5.3%, p = 0.006) and more severe functional limitations (New York Heart Association class >or=III, p <0.001). All-cause mortality and cardiovascular mortality did not differ between the 2 groups. The incidence of SCD was 1.7% in patients with normal LV ejection fractions, and no SCD was observed in patients with systolic impairment. The latter group had more frequent major cardiac events (SCD, ventricular fibrillation, aborted cardiac arrest, and first implantable cardioverter-defibrillator discharge; p = 0.03). During follow-up, 2 patients progressed to HC with systolic impairment (annual incidence 0.85%). In conclusion, systolic impairment is not exceptional in patients with HC at initial evaluation and is associated with functional deterioration and major cardiac events.

Two novel mutations in the beta-myosin heavy chain gene associated with dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is familial in approximately 20-35% of cases of idiopathic DCM. Several mutations in the different sarcomere protein genes have been reported to cause DCM.

AIMS

We wanted to investigate the role of sarcomere protein gene variants in Finnish DCM patients.

METHODS AND RESULTS

We screened all coding exons of five sarcomere protein genes (beta-myosin heavy chain, alpha-tropomyosin, troponin C, troponin I and troponin T) in a well-characterized population of 52 DCM patients in Eastern Finland by the PCR-SSCP and sequencing method. Two novel mutations, Arg1053Gln and Arg1500Trp, in the beta-myosin heavy chain gene in two index patients were detected. The proband with the Arg1053Gln mutation had a dilated left ventricle and impaired systolic function, but other family members carrying this mutation presented with septal hypertrophy. It thus seems that the Arg1053Gln mutation is primarily a HCM mutation, which can also lead to DCM. The other mutation, Arg1500Trp, was associated with a typical DCM phenotype. The Arg1500Trp mutation carrier had only one family member alive, but she did not carry the mutation and, therefore, cosegregation of the mutation and the disease in this family could not be reliably verified. No disease-causing mutations were found in the other sarcomere protein genes.

CONCLUSIONS

Two novel mutations in the beta-myosin heavy chain gene were detected in patients with DCM. Overall, mutations in the beta-myosin heavy chain gene seem to be relatively uncommon in Finnish DCM patients.

Two cases of severe neonatal hypertrophic cardiomyopathy caused by compound heterozygous mutations in the MYBPC3 gene

BACKGROUND

Idiopathic (primary) hypertrophic cardiomyopathy (HCM) is mainly caused by mutations in genes encoding sarcomeric proteins. One of the most commonly mutated HCM genes is the myosin binding protein C (MYBPC3) gene. Mutations in this gene lead mainly to truncation of the protein which gives rise to a relatively mild phenotype. Pure HCM in neonates is rare and most of the time childhood HCM occurs in association with another underlying condition.

OBJECTIVE

To study the presence of mutations in the MYBPC3 gene in idiopathic childhood HCM.

METHODS

MYBPC3 coding region and splice junction variation were analysed by denaturing high performance liquid chromatography (DHPLC) and sequencing in DNA isolated from two neonates with severe unexplained HCM, who died within the first weeks of life.

RESULTS

Truncating mutations were found in both alleles of the MYBPC3 gene in both patients, suggesting there was no functional copy of the MYBPC3 protein. Patient 1 carried the maternally inherited c.2373_2374insG mutation and the paternally inherited splice-donor site mutation c.1624+1G-->A. Patient 2 carried the maternally inherited frameshift mutation c.3288delA (p.Glu1096fsX92) and the paternally inherited non-sense mutation c.2827C-->T (p.Arg943X).

CONCLUSIONS

The findings indicate the need for mutation analysis of genes encoding sarcomeric proteins in childhood HCM and the possibility of compound heterozygosity.

The pathology of hypertrophic cardiomyopathy

Sudden cardiac death (SCD) is devastating at any age, but even more so when the individual affected is young and asymptomatic, and the death is entirely unexpected. SCD is a catastrophic complication of hypertrophic cardiomyopathy (HCM) and may be the first manifestation of this disease. HCM is an inherited intrinsic disease of the myocardium characterized by left ventricular hypertrophy without chamber dilatation, in the absence of either a systemic or other cardiac disease, which may cause a similar magnitude of hypertrophy. HCM may be a clinically silent disease. Indeed, the pathologist may be the first to encounter a case of HCM at autopsy. HCM has wide-ranging implications for affected families, who will require cardiac screening and genetic counselling even if mutations are not known. Therefore, prompt and accurate diagnosis of HCM is vital. This review article will focus on the pathological diagnosis of HCM, recent advances in the genetics of this disease, and common pitfalls which may arise, leading to diagnostic uncertainty.

Identification of a new missense mutation in the mtDNA of hereditary hypertrophic, but not dilated cardiomyopathic hamsters

The cardiomyopathic hamster is characterized by a naturally occurring deletion in the delta-sarcoglycan gene generating either the hypertrophic or the dilatative phenotype of cardiomyopathy. This evidence suggests that other genetic or environmental factors might concur to the pathogenesis of cardiomyopathy. The aim of the present study was to investigate on the possibility that other genes are involved in the pathogenesis of hamster cardiomyopathy. For this purpose, a series of genes of cardiomyopathic and healthy hamsters were compared by the differential display technique. The hamster cytochrome c oxidase mitochondrial subunit III (COIII) gene has been sequenced and identified as the gene upregulated in brain and skeletal muscle. The gene sequencing and restriction analysis demonstrated that a missense mutation is present in the COIII gene of hamsters exhibiting hypertrophic cardiomyopathy while no mutations were present in dilatative cardiomyopathic hamsters. The mutation was heteroplasmic and the heteroplasmy level was increased with age in skeletal muscle and heart. The ultrastructural analysis of cardiac tissue showed severe damage in the mitochondrial structure of hypertrophic but not dilatative hamster hearts. These results suggest that the pathogenesis of the cardiac damage in hypertrophic cardiomyopathic hamster may be sustained by multiple mutations exerting a cumulative effect on both structure and function of cardiac muscle.

Hypertrophic cardiomyopathy: two homozygous cases with "typical" hypertrophic cardiomyopathy and three new mutations in cases with progression to dilated cardiomyopathy

About 10% of cases of hypertrophic cardiomyopathy (HCM) evolve into dilated cardiomyopathy (DCM) with unknown causes. We studied 11 unrelated patients (pts) with HCM who progressed to DCM (group A) and 11 who showed "typical" HCM (group B). Mutational analysis of the beta-myosin heavy chain (MYH7), myosin-binding protein C (MYBPC3), and cardiac troponin T (TNNT2) genes demonstrated eight mutations affecting MYH7 or MYBPC3 gene, five of which were new mutations. In group A-pts, the first new mutation occurred in the myosin head-rod junction and the second occurred in the light chain-binding site. The third new mutation leads to a MYBPC3 lacking titin and myosin binding sites. In group B, two pts with severe HCM carried two homozygous MYBPC3 mutations and one with moderate hypertrophy was a compound heterozygous for MYBPC3 gene. We identified five unreported mutations, potentially "malignant" defects as for the associated phenotypes, but no specific mutations of HCM/DCM.

Pathogenic expression of homoplasmic mtDNA mutations needs a complex nuclear-mitochondrial interaction

Here we define a category of human, maternally inherited disorders that are characterized by a homoplasmic mtDNA pathogenic mutation with variable penetrance and a stereotypical clinical expression, usually restricted to a single tissue. Examples of such disorders include Leber's hereditary optic neuropathy, mitochondrial non-syndromic sensorineural hearing loss, and a form of mitochondrial hypertrophic cardiomyopathy. The mtDNA mutation is necessary, but not sufficient to induce the pathology, and multiple lines of evidence suggest a two-locus genetic model involving a primary mitochondrial mutation and a nuclear modifier. The nuclear modifier does not induce any pathology per se, but it contributes to the pathogenic effect of the mitochondrial mutation. The nuclear modifier could be a common functional polymorphism in a tissue-specific protein, possibly with mitochondrial location.

Hypertrophic cardiomyopathy:a paradigm for myocardial energy depletion

Genetic analysis of hypertrophic cardiomyopathy (HCM), a mendelian form of cardiac hypertrophy, indicates that the primary defect is in sarcomeric function. However, the initial proposal that depressed myocardial contraction leads to a 'compensatory' hypertrophy has proven inconsistent with laboratory and clinical evidence. Drawing on observations of mutant contractile protein function, together with mouse models and clinical studies, we propose that sarcomeric HCM mutations lead to inefficient ATP utilization. The suggestion that energy depletion underlies HCM is supported by the HCM-like phenotype found with mutations in a variety of metabolic genes. A central role for compromised energetics would also help explain the unresolved clinical observations of delayed onset and asymmetrical hypertrophy in HCM, and would have implications for therapy in HCM and, potentially, in more-common forms of cardiac hypertrophy and failure.

Alterations in mitochondrial function in a mouse model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by varying degrees of ventricular hypertrophy and myofibrillar disarray. Mutations in cardiac contractile proteins cause HCM. However, there is an unexplained wide variability in the clinical phenotype, and it is likely that there are multiple contributing factors. Because mitochondrial dysfunction has been described in heart disease, we tested the hypothesis that mitochondrial dysfunction contributes to the varying HCM phenotypes. Mitochondrial function was assessed in two transgenic models of HCM: mice with a mutant myosin heavy chain gene (MyHC) or with a mutant cardiac troponin T (R92Q) gene. Despite mitochondrial ultrastructural abnormalities in both models, the rate of state 3 respiration was significantly decreased only in the mutant MyHC mice by approximately 23%. Notably, this decrease in state 3 respiration preceded hemodynamic dysfunction. The maximum activity of alpha-ketogutarate dehydrogenase as assayed in isolated disrupted mitochondria was decreased by 28% compared with isolated control mitochondria. In addition, complexes I and IV were decreased in mutant MyHC transgenic mice. Inhibition of beta-adrenergic receptor kinase, which is elevated in mutant MyHC mouse hearts, can prevent mitochondrial respiratory impairment in mutant MyHC mice. Thus our results suggest that mitochondria may contribute to the hemodynamic dysfunction seen in some forms of HCM and offer a plausible mechanism responsible for some of the heterogeneity of the disease phenotypes.

Clinical and epidemiological aspects of cardiomyopathies: a critical review of current knowledge

Five years after the second report of the WHO/ISFC Task Force on the definition and classification of cardiomyopathies (CM), a critical review of the matter appears well-timed. The need for a correct definition of myocardial diseases is emphasized by considering them the result of a 'direct' injury due to different known and unknown causes and not a consequence of ischemic disease or of pressure and/or volume overload. This is in order to eliminate terms like ischemic CM, valvular CM, and hypertensive CM, which are a source of confusion. The concept of myocardial injury is also reviewed. This should not only include the structural/organic macroscopic injury, but also the subcellular, ultrastructural, and molecular damage (mostly of genetic origin) of the contracting element proteins, of citosol, sarcolemma and cell membrane ion channels. As the myocardium is a complex structure, made of common fibers and of specific conduction tissue, injury may be clinically identified either by ventricular function impairment or by bioelectric function defects, i.e. tachyarrhythmias and/or bradyarrhythmias, which sometimes are the unique manifestation of the disease (arrhythmogenic CM, in the strict sense). On the basis of the morpho-functional alterations, CMs may be classified as dilated CM (which could be better identified as hypokinetic CM, referring to the functional aspect, because the morphologic aspect is not always present), hypertrophic CM, restrictive CM, and arrhythmogenic CM (including not only arrhythmogenic right ventricular CM, but also other forms, like the so-called arrhythmias of the 'apparently' healthy heart, due to 'occult' myocardial injury). Moreover, these forms may present in association, like mixed CM (dilated-arrhythmogenic, dilated-hypertrophic, etc.). From an etiologic point of view, it is advisable to maintain the distinction between specific CM, due to a known cause, and primary or idiopathic CM, including, together with sporadic forms of an unknown origin, familial forms of a genetic origin, depending on alterations of contractile or regulating functional proteins, when myocardial injury is the sole manifestation (idiopathic) of clinical picture. The most modern etiopathogenetic, pathophysiological, and clinical features of each form of CM are briefly described in order to suggest a complete definition of the disease and to state a clinical-epidemiological setting that encompasses the current knowledge.

Prevalence and severity of "benign" mutations in the beta-myosin heavy chain, cardiac troponin T, and alpha-tropomyosin genes in hypertrophic cardiomyopathy

BACKGROUND

Genotype-phenotype correlative studies have implicated 8 particular mutations that cause hypertrophic cardiomyopathy (HCM) as "benign defects," associated with near-normal survival: N232S, G256E, F513C, V606M, R719Q, and L908V of beta-myosin heavy chain (MYH7); S179F of troponin T (TNNT2); and D175N of alpha-tropomyosin (TPM1). Routine genetic screening of HCM patients for specific mutations is anticipated to provide important diagnostic and prognostic information. The frequency and associated phenotype of these mutations in a large, unselected cohort of HCM is unknown.

METHODS AND RESULTS

A total of 293 unrelated HCM patients were genotyped for the presence of a benign mutation. DNA was obtained after informed consent; specific MHY7, TNNT2, and TPM1 fragments were amplified by polymerase chain reaction; and the mutations were detected by denaturing high-performance liquid chromatography and automated DNA sequencing. Only 5 (1.7%) of the 293 patients possessed a benign mutation. Moreover, all 5 subjects with an ascribed benign mutation had already manifested clinically severe expression of HCM, with all 5 requiring surgical myectomy, 3 of the 5 having a family history of sudden cardiac death, and 1 adolescent requiring an orthotopic heart transplant.

CONCLUSIONS

These findings demonstrate the rarity of specific mutations in HCM and challenge the notion of mutation-specific clinical outcomes. Fewer than 2% of the subjects harbored a benign mutation, and those patients with a benign mutation experienced a very serious clinical course.

[The mitochondrial organelle and the heart]

The heart is highly dependent for its function on oxidative energy generated in mitochondria, primarily by fatty acid beta-oxidation, respiratory electron chain and oxidative phosphorylation. Defects in mitochondrial structure and function have been found in association with cardiovascular diseases such as dilated and hypertrophy cardiomyopathy, cardiac conduction defects and sudden death, ischemic and alcoholic cardiomyopathy, as well as myocarditis. While a subset of these mitochondrial abnormalities have a defined genetic basis (e.g. mitochondrial DNA changes leading to oxidative phosphorylation dysfunction,fatty acid beta-oxidation defects due to specific nuclear DNA mutations), other abnormalities appear to be due to a more sporadic or environmental cardiotoxic insult or have not yet been characterized.This review focuses on abnormalities in mitochondrial bioenergetic function and mitochondrial DNA defects associated with cardiovascular diseases, their significance in cardiac pathogenesis as well as on the available diagnostic and therapeutic options. A concise background concerning mitochondrial biogenesis and bioenergetic pathways during cardiac growth,development and aging will also be provided.

Prevalence and age-dependence of malignant mutations in the beta-myosin heavy chain and troponin T genes in hypertrophic cardiomyopathy: a comprehensive outpatient perspective

OBJECTIVES

The goal of this study was to determine the prevalence of "malignant" mutations in hypertrophic cardiomyopathy (HCM).

BACKGROUND

Previous genotype-phenotype studies have implicated four mutations (R403Q, R453C, G716R and R719W) as highly malignant defects in the beta-myosin heavy chain (MYH7). In the cardiac troponin T gene (TNNT2), a specific mutation (R92W) has been associated with high risk of sudden death. Routine clinical screening for these malignant mutations has been suggested to identify high-risk individuals.

METHODS

We screened 293 unrelated individuals with HCM seen at the Mayo Clinic in Rochester, Minnesota, between April 1997 and October 2000. Deoxyribonucleic acid (DNA) was obtained after informed consent; amplification of MYH7 exons 13 (R403Q), 14 (R453C) and 19 (G716R and R719W), and TNNT2 exon 9 (R92W) was performed by polymerase chain reaction. The mutations were detected using denaturing high-performance liquid chromatography and automated DNA sequencing.

RESULTS

The mean age at diagnosis was 42 years with 53 patients diagnosed before age 25. The mean maximal left ventricular wall thickness was 21 mm. Nearly one-third of cases were familial and one-fourth had a family history of sudden cardiac death. Only 3 of the 293 patients possessed one of the five "malignant" mutations, and all 3 patients were <25 years of age at presentation (p < 0.006).

CONCLUSIONS

This finding underscores the profound genetic heterogeneity in HCM. Only 1% of unrelated individuals seen at a tertiary referral center for HCM possessed one of the five "malignant" mutations that were examined. Routine clinical testing for these specific mutations is of low yield.

Mutations of the light meromyosin domain of the beta-myosin heavy chain rod in hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) is caused by mutations in 9 sarcomeric protein genes. The most commonly affected is beta-myosin heavy chain (MYH7), where missense mutations cluster in the head and neck regions and directly affect motor function. Comparable mutations have not been described in the light meromyosin (LMM) region of the myosin rod, nor would these be expected to directly affect motor function. We studied 82 probands with HCM in whom no mutations had been found in MYH7 exons encoding the head and neck regions of myosin nor in the other frequently implicated disease genes. Primers were designed to amplify exons 24 to 40 of MYH7. These amplimers were subjected to temperature modulated heteroduplex analysis by denaturing high-performance liquid chromatography. An Ala1379Thr missense mutation in exon 30 segregated with disease in three families and was not present in 200 normal chromosomes. The mutation occurred on two haplotypes, indicating that it was not a polymorphism linked with another disease-causing mutation. The position of this residue within the LMM region of myosin suggests that it may be important for thick filament assembly or for accessory protein binding. A further missense mutation in exon 37, Ser1776Gly, segregated with disease in a single family and was absent from 400 population-matched control chromosomes. Because the Ser1776 residue occupies a core position in the myosin rod at which the substitution of glycine is extremely energetically unfavorable, it is likely to disrupt the coiled-coil structure. We conclude that mutation of the LMM can cause HCM and that such mutations may act through novel mechanisms of disease pathogenesis involving myosin filament assembly or interaction with thick filament binding proteins.

Mitochondria and the heart

Since the identification of the first pathogenic mutations of mitochondrial DNA in 1988, a plethora of information about human mitochondrial diseases has been brought to light. Not surprisingly, many of these disorders affect the myocardium, because this tissue relies heavily upon oxidative metabolism. This review focuses on disorders of the respiratory chain, the only area of mammalian cellular metabolism under the control of two genomes, nuclear and mitochondrial. Consequently, defects of aerobic synthesis of adenosine triphosphate (ATP) can be due to mutations of either genome. We describe genetic mitochondrial cardiomyopathies and briefly review mouse models and the mitochondrial theory of presbycardia.

The emerging concept of mitochondrial cardiomyopathies

OBJECTIVE

Our purpose was to present an updated review on the spectrum of mitochondrial DNA-related syndromes relevant to cardiac disturbances.

BACKGROUND

The advent of molecular genetics has provided important insight into the mechanisms underlying a variety of inherited heart disorders, including cardiac arrhythmias and cardiomyopathies. These studies pointed to defects in ion channels, contractile proteins, structural proteins, and signaling molecules as key players in disease pathogenesis, and they have opened up new mechanism-based approaches to therapy.

RESULTS AND CONCLUSIONS

Mitochondrial DNA defects and faulty oxidative phosphorylation are infrequently considered as causes of cardiomyopathies. This is surprising given the heavy dependence of the heart on oxidative metabolism and the recent advances in understanding the molecular features of mitochondrial disorders. This remarkable progress and the implications it may have for more common forms of cardiovascular disease are reviewed.

Functional, structural, and genetic mitochondrial abnormalities in myocardial diseases

Myocardial tissue is highly dependent on energy supplied by normal mitochondrial function. Therefore defects of energy production or utilization affect the heart in both syndromic and isolated disorders. Knowledge of the peculiar structural, functional, and genetic characteristics of mitochondria provides the basis for identification and classification of mitochondrial defects as well as for establishment of a diagnostic workup useful for related cardiac disorders. This review is therefore dedicated to the characteristics of normal mitochondria and the pathologic alterations of these organelles in various cardiovascular diseases.

Biochemical and molecular basis for mitochondrial cardiomyopathy in neonates and children

Defects in myocardial bioenergetics have been reported in patients with cardiomyopathy but their molecular basis and role in pathophysiology remain unclear. We sought to establish a molecular basis for cardiac mitochondrial respiratory enzyme abnormalities frequently present (75%) in a group of 16 children (including 2 neonates) with end-stage cardiomyopathy. Decreased specific activity levels were found in complexes I, III, IV and V but not in II, the only complex that is entirely nuclear encoded. Sequence analysis of cardiac mtDNA revealed 4 patients harbouring heteroplasmic mtDNA mutations in cytb, tRNAArg, and ND5 at highly conserved positions. These mutations were present neither in controls nor in patients without enzymatic defect. In addition, 4 patients exhibited marked reduction in cardiac mtDNA levels. The basis for respiratory enzyme abnormalities can be explained in a subset of our patients as a result of either pathogenic mtDNA mutation or depletion. Patients harbouring both DNA and enzymatic defects fulfil rigorous criteria defining mitochondrial cardiomyopathy.

Prevalence and characteristics of dystrophin defects in adult male patients with dilated cardiomyopathy

OBJECTIVES

To assess the prevalence of dystrophin defects in dilated cardiomyopathy (DCM) in male patients and to formulate investigation strategies for their identification.

BACKGROUND

Dystrophin defects presenting with predominant or exclusive cardiac involvement may be clinically indistinguishable from "idiopathic" DCM. Diagnosis may be missed, unless specifically investigated.

METHODS

Clinical and biochemical evaluation, right ventricular endomyocardial biopsy (EMB), light and electron microscopic and immunohistochemical studies of biopsy samples, six multiplex and two single polymerase chain reactions for 38 exons and automated sequencing of exon 9 and muscle promoter-exon 1 were undertaken in 201 consecutive male patients presenting with DCM, with (n = 14) and without (n = 187) increased serum creatine phosphokinase (sCPK).

RESULTS

Dystrophin defects were identified in 13 of the 201 patients (6.5%, age 16-50). Family history was positive in four patients. Serum CPK levels were increased in 11 of 13 patients. Light microscopy examination of EMB was uninformative; ultrastructural study showed multiple membrane defects. Dystrophin immunostain was abnormal. Eight patients, all older than 20, had deletions affecting midrod domain, normal or mildly increased CPK and better outcome than the five remaining cases all younger than 20, with more than five-fold increase of sCPK. Two of these latter had proximal and rod-domain deletions. Sisters of two patients were diagnosed as noncarriers with microsatellite analysis.

CONCLUSIONS

Although the overall prevalence of dystrophin defects in our consecutive DCM male series is low (6.5%), immunohistochemical and molecular studies are essential to identify protein and gene defects; screening studies are justified to define prevalence, clinical profile and genotype-phenotype correlation.

Mitochondrial DNA mutations and mitochondrial abnormalities in dilated cardiomyopathy

Mitochondrial (mt)DNA defects, both deletions and tRNA point mutations, have been associated with cardiomyopathies. The aim of the study was to determine the prevalence of pathological mtDNA mutations and to assess associated defects of mitochondrial enzyme activity in dilated cardiomyopathy (DCM) patients with ultrastructural abnormalities of cardiac mitochondria. In a large cohort of 601 DCM patients we performed conventional light and electron microscopy on endomyocardial biopsy samples. Cases with giant organelles, angulated, tubular, and concentric cristae, and crystalloid or osmiophilic inclusion bodies were selected for mtDNA analysis. Mutation screening techniques, automated DNA sequencing, restriction enzyme digestion, and densitometric assays were performed to identify mtDNA mutations, assess heteroplasmy, and quantify the amount of mutant in myocardial and blood DNA. Of 601 patients (16 to 63 years; mean, 43.5 +/- 12.7 years), 85 had ultrastructural evidence of giant organelles, with abnormal cristae and inclusion bodies; 19 of 85 (22.35%) had heteroplasmic mtDNA mutations (9 tRNA, 5 rRNA, and 4 missense, one in two patients) that were not found in 111 normal controls and in 32 DCM patients without the above ultrastructural mitochondrial abnormalities. In all cases, the amount of mutant was higher in heart than in blood. In hearts of patients that later underwent transplantation, cytochrome c oxidase (Cox) activity was significantly lower in cases with mutations than in those without or controls (P = 0.0008). NADH dehydrogenase activity was only slightly reduced in cases with mutations (P = 0.0388), whereas succinic dehydrogenase activity did not significantly differ between DCM patients with mtDNA mutations and those without or controls. The present study represents the first attempt to detect a morphological, easily identifiable marker to guide mtDNA mutation screening. Pathological mtDNA mutations are associated with ultrastructurally abnormal mitochondria, and reduced Cox activity in a small subgroup of non-otherwise-defined, idiopathic DCMs, in which mtDNA defects may constitute the basis for, or contribute to, the development of congestive heart failure.