Mitochondrial DNA defects in cardiomyopathy

Abnormalities in mitochondrial DNA (mtDNA) including specific deletions and point mutations have been found in an increasing number of cases of both dilated and hypertrophic cardiomyopathy. The role that these mutations may play in contributing to the cardiomyopathic phenotype is discussed in this survey of the recent literature.

Abnormal cardiac and skeletal muscle mitochondrial function in pacing-induced cardiac failure

BACKGROUND

Previous studies have shown that marked changes in myocardial mitochondrial structure and function occur in human cardiac failure. To further understand the cellular events and to clarify their role in the pathology of cardiac failure, we have examined mitochondrial enzymatic function and peptide content, and mitochondrial DNA (mtDNA) integrity in a canine model of pacing-induced cardiac failure.

METHODS

Myocardium and skeletal muscle tissues were evaluated for levels of respiratory complex I-V and citrate synthase activities, large-scale mtDNA deletions as well as peptide content of specific mitochondrial enzyme subunits. Levels of circulating and cardiac tumor necrosis factor-alpha (TNF-alpha), and of total aldehyde content in left ventricle were also assessed.

RESULTS

Specific activity levels of complex III and V were significantly lower in both myocardial and skeletal muscle tissues of paced animals compared to controls. In contrast, activity levels of complex I, II, IV and citrate synthase were unchanged, as was the peptide content of specific mitochondrial enzyme subunits. Large-scale mtDNA deletions were found to be more likely present in myocardial tissue of paced as compared to control animals, albeit at a relatively low proportion of mtDNA molecules (<0.01% of wild-type). In addition, the reduction in complex III and V activities was correlated with elevated plasma and cardiac TNF-alpha levels. Significant increases in left ventricle aldehyde levels were also found.

CONCLUSIONS

Our data show reductions in specific mitochondrial respiratory enzyme activities in pacing-induced heart failure which is not likely due to overall decreases in mitochondrial number, or necrosis. Our findings suggest a role for mitochondrial dysfunction in the pathogenesis of cardiac failure and may indicate a commonality in the signaling for pacing-induced mitochondrial dysfunction in myocardial and skeletal muscle. Increased levels of TNF-alpha and oxidative stress appear to play a contributory role.

The complete sequence of mtDNA genes in idiopathic dilated cardiomyopathy shows novel missense and tRNA mutations

BACKGROUND

Previous studies have shown that mitochondrial DNA (mtDNA) mutations are often present in patients with myocardial dysfunction. We sought to assess the prevalence and significance of heart mtDNA sequence changes in patients with idiopathic dilated cardiomyopathy (DCM).

METHODS AND RESULTS

DNA sequence of all the transfer ribonucleic acid (tRNA), ribosomal RNA (rRNA), and structural genes in cardiac mtDNA of 28 patients with DCM was determined and compared with a control group that had no evidence of heart disease. An increased number of point mutations were found in DCM cardiac mtDNA when compared with controls. Both novel and previously reported mutations were found in mitochondrial tRNA and structural genes. One of these mutations was heteroplasmic and resulted in changing a highly conserved nucleotide in tRNAArg. Novel, heteroplasmic mtDNA mutations (n = 4) specifying changes in moderate to highly conserved amino acid residues were found in COII, COIII, ND5, and cytb. These novel mtDNA mutations were found only in patients with severe reduction in mitochondrial enzyme activities.

CONCLUSIONS

Our results indicate that a high incidence of mtDNA nucleotide sequence changes in both tRNA and structural genes are present in DCM. Five heteroplasmic mutations were detected that both changed evolutionarily conserved residues (which may impair the function of proteins or tRNAs) and were associated with specific enzymatic defects. These mutations could play an important role in the pathogenesis of cardiomyopathy.

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.

Kearns-Sayre syndrome with a novel mitochondrial DNA deletion

We describe a 17-year-old boy with a clinical neurologic picture consistent with Kearns-Sayre syndrome. His manifestations included progressive external ophthalmoplegia, bilateral ptosis, retinitis pigmentosa, and muscle weakness. He was found to harbor an abundant novel deletion in skeletal muscle mitochondrial DNA. Biochemical analysis of the patient's biopsied skeletal muscle showed that the specific activities of all four respiratory complexes with mitochondrial DNA-encoded subunits were markedly reduced in contrast to normal activity levels of entirely nuclear DNA-encoded enzyme activities (eg, complex II and citrate synthase). Ultrastructural analysis also indicated the presence of strikingly abnormal mitochondria with both unusual cristae and frequent paracrystalline inclusions. The great amount of the deleted mitochondrial DNA in this patient's muscle, as well as the concomitant reduction in specific respiratory complex activity, suggests that the mitochondrial DNA deletion plays a role in the pathogenesis of this neurologic disease.

Heart mitochondrial DNA and enzyme changes during early human development

Previous studies in our laboratory demonstrated significant changes in bovine heart mitochondrial bioenergetics during fetal growth and development. To further understand mitochondrial biogenesis in early human development, the activity and subunit content levels of specific mitochondrial enzymes in fetal and neonatal heart were determined. Comparing early gestation (EG, 45-65 day) later gestation (LG, 85-110 day) and neonate (birth-1 month), specific activity of citrate synthase (CS), a Krebs cycle enzyme showed a 2 fold increase from EG to LG and a 2 fold increase from LG to neonate. Specific activities of complex IV and complex V increased similarly 1.8-2 fold from EG to LG. However during the later fetal period from LG to neonate, complex IV activity increased only 1.3 fold and complex V showed no significant increase. Peptide content of COX-II subunit increased 2 fold from EG to LG and by 3.5 fold from LG to neonate. Levels of COX-IV and ATP synthase alpha subunits were undetectable in EG hearts, clearly detectable in LG heart and 3 fold increased from LG to neonate. Unexpectedly, mitochondrial transcription factor A (mt-TFA) levels were not significantly different during these developmental stages. Mitochondrial DNA (mtDNA) levels increased 1.8 fold from EG to LG, and 3.8 fold increase from EG to neonate and correlated with CS activity levels. In conclusion, these data indicate coordinated regulation of some nuclear-encoded (COX-IV and CS activity) and mitochondrial components (COX-II and mtDNA), and strongly suggest that mitochondrial content increases particularly during the early fetal cardiac development and reveal a distinct pattern of regulation for mt-TFA.

Mitochondrial biogenesis defects and neuromuscular disorders

A variety of mitochondrial DNA (mtDNA) defects, ranging from point mutations and large-scale deletions to severe reduction in the overall quantity of mtDNA (mtDNA depletion), may be associated with neuromuscular disorders. The nuclear genome, which encodes most of the proteins involved in mitochondrial biogenesis (regulation of maintenance, replication, and transcription of mtDNA), appears to be implicated in many of the mtDNA defects. In this review, we describe some of the mtDNA defects discovered by our laboratory and others in patients with neurologic disorders and analyze their potential relationship with the pathways and mechanisms involved in mitochondrial biogenesis.

Skeletal muscle mitochondrial defects in nonspecific neurologic disorders

A group of 25 children (5 months to 20 years of age) presenting with intractable seizures, developmental delay, and severe hypotonia, who did not fall into the known categories of mitochondrial encephalomyopathies, underwent muscle biopsy for evaluation of mitochondrial function and were compared with age-matched control subjects. Biopsied skeletal muscle was analyzed for six mitochondrial enzyme-specific activities, mitochondrial DNA point mutations and deletions, and mitochondrial DNA levels. The data reveal a high incidence of specific mitochondrial enzyme activity defects. Reduced activity levels were evident in complex I (11 patients), III (24 patients), IV (nine patients), and V (10 patients). Two patients also exhibited pronounced reduction in mitochondrial DNA levels (80% reduction compared with control subjects). Two patients manifested increased levels of 5-kb and 7.4-kb mitochondrial DNA deletions. Pathogenic mutations previously described in association with mitochondrial encephalomyopathies were not evident. The data suggest that mitochondrial dysfunction, including extensive defects in specific enzyme activities, may be frequently present in children with seizures, developmental delay, and hypotonia that do not fall within the known mitochondrial encephalomyopathies. These mitochondrial deficiencies can be primarily ascertained by biochemical analysis and are rarely accompanied by mitochondrial ultrastructural changes. The molecular basis of these defects, their role in these disorders, and potential treatment warrant further study.

Is age a contributory factor of mitochondrial bioenergetic decline and DNA defects in idiopathic dilated cardiomyopathy?

While mitochondrial abnormalities are increasingly recognized in cardiac diseases including hypertrophic cardiomyopathy, their presence in idiopathic dilated cardiomyopathy and the role that age plays in their incidence and severity have yet not been assessed. Levels of cardiac respiratory enzyme activities and mitochondrial DNA (mtDNA) were examined in 55 subjects with idiopathic dilated cardiomyopathy divided into 3 age groups. Respiratory enzyme activity levels were significantly lower in 37 patients (67%) compared to age-matched controls and increased activity levels were noted in 9 (16%). Decreased activities were found in complex I (n = 11), III (n = 16), IV (n = 12) and V (n = 13), but not in II, the only respiratory complex entirely nuclear-encoded. No age-specific differences were found in the overall frequency of enzymatic abnormalities. However, older patients had significantly increased multiple enzyme activity defects as well as increases in abundance and frequency of the 7.4 kb deletion. In addition, 3 patients were noted with marked reduction in mtDNA levels. None of the pathogenic mtDNA mutations previously associated with hypertrophic cardiomyopathy were found, nor was there any relationship that could be established between levels of specific mtDNA deletions and enzyme activities. In summary, specific mitochondrial abnormalities are heterogenous and frequent in both adults and children with idiopathic dilated cardiomyopathy. Older patients are more likely to have mtDNA deletions and multiple enzyme activity defects. The molecular basis for these abnormalities remains undefined.

Mitochondrial dysfunction in skeletal muscle of children with cardiomyopathy

OBJECTIVES

This study sought to examine skeletal muscle of children with cardiomyopathy (CM) for changes in mitochondrial enzyme activities and in mitochondrial DNA (mtDNA).

BACKGROUND

Heart mitochondrial enzymatic activity defects have been often found in dilated and hypertrophic CM. The defects primarily involve the activities of the electron transport system and oxidative phosphorylation pathway including respiratory complexes I, III, IV, and V.

METHODS

Skeletal muscle biopsies of 8 children with CM were examined for specific mitochondrial enzyme activities, mtDNA copy number and the presence of pathogenic mutations and deletions in mtDNA.

RESULTS

A marked deficiency in specific mitochondrial enzyme activities was found in 6 of 8 patients in skeletal muscle as well as in 2 of 3 hearts of those in whom cardiac tissue was available. Specific activity defects were found in complex I (2 cases), complex III (5 cases), complex IV (3 cases), and complex V (4 cases). Complex II and citrate synthase activities were unaffected. None of the previously reported pathogenic mutations associated with CM were detected, nor was there any evidence of mtDNA depletion. The incidence of defective respiratory complex activities in skeletal muscle was similar to the incidence of defective complex activities previously reported in cardiac tissue.

CONCLUSIONS

Mitochondrial analysis of skeletal muscle is warranted in the overall clinical evaluation of children with CM, and particularly before consideration for cardiac transplantation.

Cloning and molecular analysis of the human citrate synthase gene

The nucleotide sequence encoding the citrate synthase (CS) gene was determined from the sequencing of the CS cDNA isolated from a human heart cDNA library. The primary sequence of CS deduced from its nucleotide sequence reveals a highly conserved, albeit slightly larger, protein of 466 amino acids, with 95% homology to its pig homologue. The data also indicate that the human genomic CS gene contains no introns, and confirms the location of the human CS gene on chromosome 12.

Hypertrophic cardiomyopathy with mitochondrial DNA depletion and respiratory enzyme defects

We report the case of a child with severe hypertrophic cardiomyopathy, with decreased activity levels of cardiac mitochondrial respiratory complex I and III, and with a pronounced reduction in cardiac mitochondrial DNA copy number level. Mitochondrial DNA depletion has not been previously reported in hypertrophic cardiomyopathy and it may play a role in its pathogenesis.

Human mitochondrial function during cardiac growth and development

Little information is presently available concerning mitochondrial respiratory and oxidative phosphorylation function in the normal human heart during growth and development. We investigated the levels of specific mitochondrial enzyme activities and content during cardiac growth and development from the early neonatal period (10-20 days) to adulthood (67 years). Biochemical analysis of enzyme specific activities and content and mitochondrial DNA (mtDNA) copy number was performed with left ventricular tissues derived from 30 control individuals. The levels of cytochrome c oxidase (COX) and complex V specific activity, mtDNA copy number and COX subunit II content remained unchanged in contrast to increased citrate synthase (CS) activity and content. The developmental increase in CS activity paralleled increasing CS polypeptide content, but was neither related to overall increases in mitochondrial number nor coordinately regulated with mitochondrial respiratory enzyme activities. Our findings of unchanged levels of cardiac mitochondrial respiratory enzyme activity during the progression from early childhood to older adult contrasts with the age-specific regulation found with CS, a Krebs cycle mitochondrial enzyme.

Cardiac mitochondrial dysfunction and DNA depletion in children with hypertrophic cardiomyopathy

Abnormalities in specific mitochondrial respiratory enzymes and DNA (mtDNA) have been reported in cardiomyopathy. In this study, we report 4 cases of severe hypertrophic cardiomyopathy (HCM) in which specific cardiac mitochondrial enzyme activity defects were found, including complex I (n = 2), complex III (n = 2), complex IV (n = 2) and complex V (n = 1). Other abnormalities were also noted including a marked depletion of mtDNA (n = 1) and decreased content of subunit 2 of cytochrome c oxidase (n = 1). None of the mtDNA point mutations and common deletions previously found in association with cardiomyopathy were detected in these patients. These data indicate that specific respiratory enzyme activity defects are frequently present in HCM. Also, our finding of a marked depletion of mtDNA in 1 patient suggests that cardiac mtDNA depletion, previously unreported in HCM, needs further examination in order to establish whether it plays a primary role in its pathogenesis.

Mitochondrial cardiomyopathy: molecular and biochemical analysis

Abnormalities in cardiac mitochondrial respiratory enzymes and mitochondrial DNA have been found in an increasing number of pediatric cases of both dilated and hypertrophic cardiomyopathy, giving rise to the entity known as mitochondrial cardiomyopathy. Histochemical, biochemical, and molecular findings are described in this review of mitochondrial cardiomyopathy, which should provide assistance in its diagnostic identification.

Mitochondrial gene expression in rat heart and liver during growth and development

The levels of mitochondrial enzyme activities involved in respiration and oxidative phosphorylation and of specific mitochondrial gene transcripts were examined in rat heart and liver tissues during early growth, development, and aging. Increases were shown in cardiac respiratory complex activities I, III, IV, and V and ATPase6 and CoxII transcript levels during the transition from neonate to young adult. This increased mitochondrial gene expression is not associated with a proportionate increase in mitochondrial number. In contrast, no significant changes in liver mitochondrial activities or transcripts were detected during this transition. Marked reductions in the activities of complexes I, III, IV, and V and in ATPase6 and COXII transcripts were demonstrated in older adult as compared with young adult cardiac tissue with no concomitant reduction in cardiac citrate synthase activity and content, and mtDNA copy number. No decline was noted in liver mitochondrial enzyme activity levels and transcripts of old adult rats. These findings suggest that cardiac mitochondrial gene expression is developmentally regulated at a pretranslational level. The pattern of increasing mitochondrial gene expression in the young adult and decreasing gene expression in the aging heart stands in clear contrast to liver mitochondrial gene expression or nuclear-encoded genes such as citrate synthase.

Specific mitochondrial DNA deletions in canine myocardial ischemia

The effect of myocardial ischemia on mitochondrial DNA (mtDNA) structure and the presence of specific mtDNA deletions was determined using the model of Ameroid constriction of canine coronary arteries. The incidence of specific deletions was high in both the endocardial and epicardial tissues perfused by the occluded vessel as compared to myocardial tissues perfused by the unconstricted vessels. Our results show that specific mtDNA deletions similar to the 5 kb and 7.4 kb human mtDNA deletions occur following canine myocardial ischemia. However the presence of these deletions did not correlate with specific mitochondrial enzyme defects.

Cardiac mitochondrial dysfunction in Leigh syndrome

An infant with Leigh syndrome and associated cardiomyopathy is described. Abnormal activities of mitochondrial respiratory complexes III and V and a change in mtDNA at nt 8993 were detected in heart and skeletal muscle but not in liver.

A point mutation in the cytb gene of cardiac mtDNA associated with complex III deficiency in ischemic cardiomyopathy

We report a high incidence of reduced respiratory Complex III activity in heart muscle concomitant with the presence of a specific mutation in cytochrome b (cytb) in patients with ischemic cardiomyopathy. This C-->A mutation at nt 15452 converts the 236th residue of cytb from a leucine to isoleucine, is heteroplasmic and was observed in only 2 of 43 controls. Complex III activity is reduced (> 50%) in 5 of 6 patients with the C-->A15452 mutation suggesting that the cytb mutation is responsible for decreased Complex III activity and may play a role in the pathophysiology of ischemic cardiomyopathy.

Mitochondrial dysfunction after fetal alcohol exposure

Specific mitochondrial enzyme activities and mRNA levels were measured in the heart, brain, and liver tissues of a group of 1-day-old neonatal rats whose mothers were alcohol-fed during pregnancy and compared with a control group. The results show a significant decrease in mitochondrial ATP synthase activity in both the brain and liver, as well as a decrease in complex III activity in the liver of rats exposed to alcohol. Other mitochondrial enzymes activities (e.g., citrate synthase, cytochrome c oxidase, and complex I), as well as specific mitochondrial transcript levels, were not significantly affected. Heart mitochondrial enzyme activities were not significantly affected. These data reveal that a tissue-specific response occurs after fetal exposure to alcohol and may explain some of the cellular events occurring in fetal alcohol syndrome resulting in abnormal growth and neurological development.

Mitochondrial dysfunction in spontaneous inbred turkey cardiomyopathy

Mitochondrial enzyme activities were examined in cardiac tissues of turkeys with spontaneous inbred cardiomyopathy. Marked declines in specific enzyme activities were noted for respiratory complexes III and V ranging from 65-90% of the control values. No significant differences in complexes I, IV and citrate synthase nor in mitochondrial DNA copy number were detected. These results suggest that specific mitochondrial enzyme defects occur in cardiac tissues during spontaneous inbred turkey cardiomyopathy.

Specific mitochondrial DNA deletions in idiopathic dilated cardiomyopathy

OBJECTIVE

Structural changes in human mitochondrial DNA (mtDNA) have been implicated in a number of clinical conditions with dysfunctions in oxidative phosphorylation called OX-PHOS diseases, some of which have cardiac involvement. The objective of this study was to assess the frequency and extent of specific mitochondrial DNA deletions in idiopathic dilated cardiomyopathy.

METHODS

DNA extracted from tissue derived from the left ventricle of 41 patients with idiopathic dilated cardiomyopathy and 17 controls was amplified by polymerase chain reaction using specific primers to assess the incidence and proportion of 5-kb and 7.4-kb deletions in mitochondrial DNA.

RESULTS

In reactions using primers to detect the 5-kb deletion, an amplified product of 593 bp was found in low abundance relative to undeleted mitochondrial DNA but with high frequency in a number of controls and patients. A second deletion of 7.4 kb in size was also frequently present in controls and patients. In contrast to previous reports, these deletions were found to be present in both controls and in cardiomyopathic patients, 18 years and younger, including several infants. The 7.4-kb deletion was prominently increased in both frequency and in its proportion relative to undeleted mitochondrial DNA in patients 40 years and older with idiopathic dilated cardiomyopathy.

CONCLUSIONS

At variance with current literature our study reports a significant presence of both 5 and 7.4-kb deletions in the young and a higher frequency and quantity of the 7.4-kb deletion in the older cardiomyopathic patients in comparison with controls. The increased accumulation of the 7.4-kb deletion as both a function of aging and cardiomyopathy is suggestive that this specific mitochondrial DNA deletion arises more likely as an effect of heart dysfunction rather than as a primary cause of cardiomyopathy.

Heart mitochondria response to alcohol is different than brain and liver

Specific mitochondrial enzyme activities, mitochondrial DNA copy number, and mRNA levels were measured in heart, brain, and liver tissues of a group of alcohol-fed rats and compared with a control group. The results show a significant increase in mitochondrial enzyme activities (citrate synthase, complex IV, complex III, complex I, and complex V), as well as an increase in mitochondrial DNA in the cardiac tissue of the alcohol-fed animals. These data are indicative of an increase in mitochondrial number in the cardiac tissue that may occur as the result of an adaptive response to the alcoholic insult. However, in the liver and brain of the alcohol-treated rat, specific mitochondrial activities were decreased, in particular, complex III and ATP synthase, whereas levels of other mitochondrial enzymes (e.g., citrate synthase, specific mitochondrial transcripts, and mitochondrial DNA levels) do not seem to be affected. These data suggest that a tissue-specific response to alcohol exists that may have a common molecular mechanism in brain and liver, but is different in the heart.

Impaired mitochondrial function in idiopathic dilated cardiomyopathy: biochemical and molecular analysis

Mitochondrial defects at the biochemical and molecular levels are increasingly recognized in diseases involving the heart. The objective of this study was to assess the frequency and extent of mitochondrial defects in idiopathic dilated cardiomyopathy. Left ventricular tissues of 27 patients with idiopathic dilated cardiomyopathy undergoing orthotopic cardiac transplantation because of severe cardiac failure were examined to assess the specific activity levels of mitochondrial respiratory enzymes and changes in mtDNA structure and copy number. Abnormal specific activities of several mitochondrial enzymes were found in 55% of the cardiomyopathic tissues examined (15 patients), with six patients displaying single enzyme defects, including five in complex III and one in complex I. Multiple mitochondrial enzyme defects were found in nine patients, with the most frequent combination of defects seen in complex III and complex IV (5 cases). These enzymatic changes were shown not to be accompanied by changes in mtDNA copy number. In seven cases, however, including three young adults, there was a marked decrease in the levels of polymerase chain reaction products derived from specific mtDNA regions, which may be an indication of specific mtDNA damage. Specific mitochondrial abnormalities are frequently found in idiopathic dilated cardiomyopathy, with a variety of mitochondrial loci affected. These findings are not age dependent.

Localized mitochondrial dysfunction in canine myocardial ischemia

Effects of myocardial ischemia on mitochondrial enzymes and mitochondrial DNA (mtDNA) were examined using the model of Ameroid constriction of canine cardiac vessels. Endocardium supplied by constricted coronary arteries was found to have significantly lower citrate synthase and complex IV activities compared to values obtained from either epicardium supplied by constricted vessels or endocardium supplied by unconstricted coronary arteries. Neither significant differences in mtDNA copy number nor changes in respiratory complexes I, III and V were detected. These results suggest that highly localized, specific mitochondrial enzyme changes result from chronic myocardial ischemia.

Mitochondrial cardiomyopathy

An adolescent with mitochondrial cardiomyopathy is described. Skeletal and cardiac biopsies revealed abnormal mitochondria, with biochemical analysis showing cytochrome c oxidase deficiency.

Mitochondrial gene expression during bovine cardiac growth and development

The expression of both mitochondrial and nuclear genes encoding enzymes involved in electron transport and oxidative phosphorylation was examined in bovine cardiac tissue during early growth, development and aging. The steady state level of mRNAs for mitochondrial genes including ATPase 6. COXII and cyt b increased 2.5-4-fold relative to early fetal levels in late fetal and young adult tissues and showed a marked decline (30-50%) in older adult tissues. Similar results were found with the nuclear genes, COXVB and ATP-beta synthase showing coordinate regulation of the two genomes. An increase in mtDNA copy number correlated with the increase in transcript level. Enzyme activity levels for NADH dehydrogenase and cytochrome c oxidase showed a similar trend, albeit of lesser magnitude. These activity levels contrasted with the activity level of an entirely nuclear-encoded mitochondrial enzyme, citrate synthase, which increased not only throughout development but in the older adult tissue. This study indicates that there is a pattern of increasing mitochondrial and nuclear gene expression for OXPHOS enzymes in developing cardiac tissue and decreasing OXPHOS gene expression in the aging heart.

Presence of a mitochondrial DNA deletion in fetal and adult bovine cardiac tissue

A deletion of about 5.3 kilobases has been detected in the mitochondrial DNA of bovine cardiac tissue. This deletion appears to be somatic in origin given its sporadic presence in the various heart compartments examined. Cardiac tissue derived from developmental stages including fetal, early and older adult animals harbored this mutation with increased levels (100-1000 fold) found in older adults. The deleted region of the mitochondrial genome maps to relatively the same area (deleting ATPase6, COXIII, ND2, ND4 and a portion of ND5) as the common 5 kb deletion reported in humans, but its presence in fetal tissue, as well as its decreased age dependence distinguish it relative to the reported human deletion.

Cardiomyopathy and abnormal mitochondrial function

The diagnosis of cardiomyopathy is mainly based on clinical and morphological criteria. While metabolic, viral, chemical, genetic, and immunological factors are often proposed as causes of cardiomyopathy, little is known about the role of the respiratory chain and other biochemical mitochondrial defects in this group of diseases. Research on the biochemistry and molecular biology of cardiomyopathies offers an opportunity for a better understanding of the pathogenesis and for finding specific therapy.

Mitochondrial DNA of the blowfly Phormia regina: restriction analysis and gene localization

A study of an invertebrate mitochondrial genome, that of the blowfly Phormia regina, has been initiated to compare its structural and functional relatedness to other metazoan mitochondrial genomes. A restriction map of mitochondrial DNA (mtDNA) isolated from sucrose gradient-purified mitochondria has been established using a combination of single and double restriction endonuclease digestions and hybridizations with isolated mtDNA fragments, revealing a genome size of 17.5 kilobases (kb). A number of mitochondrial genes including those encoding the 12 S and 16 S ribosomal RNA, the cytochrome c oxidase I subunit (COI) and an unidentified open reading frame (URF2) have been located on the Phormia mtDNA by Southern blot analysis using as probes both isolated mtDNA fragments and oligonucleotides derived from the sequences of previously characterized genes from rat and Drosophila yakuba mtDNAs. These data indicate that for those regions examined, the mitochondrial genome organization of blowfly mtDNA is the same as that of Drosophila yakuba, the order being COI-URF2-12 S-16 S. These data also report the presence of an A + T-rich region, located as a 2.5-kb region between the URF2 and the 12 S rRNA genes, and its amplification by the polymerase chain reaction is described.

Isolation and characterization of maltose non utilizing (mnu) mutants mapping outside the MAL1 locus in Saccharomyces cerevisiae

The MAL1 locus of Saccharomyces cerevisiae comprises three genes necessary for maltose utilization. They include regulatory, maltose transport and maltase genes designated MAL1R, MAL1T and MAL1S respectively. Using a MAL1 strain transformed with an episomal, multicopy plasmid carrying the MAL2 locus, five recessive and one dominant mutant unable to grow on maltose, but still retaining a functional MAL1 locus were isolated. All the mutants could use glycerol, ethanol, raffinose and sucrose as a sole carbon source; expression of the maltase and maltose permease genes was severely and coordinately reduced. Only the dominant mutant failed to accumulate the MAL1R mRNA.

Genetic mapping and biochemical analysis of mutants in the maltose regulatory gene of the MAL1 locus of Saccharomyces cerevisiae

The MAL1 locus of Saccharomyces cerevisiae comprises three genes necessary for maltose utilization: a regulatory (MALR), a maltose transport (MALT) and a maltase gene (MALS). A fine structure genetic map of the MAL1R gene was constructed and the order of mutations was confirmed by plasmid-mediated chromosomal recombination. The mutations cluster non-randomly within the 5' half of the gene, where the putative DNA binding domain of the encoded protein is located. Only mutations mal1R-22 and MAL1R-72 map in the 3' terminal half of the gene; these mutations cause a different pattern of transcriptional regulation of plasmid-borne MAL6T genes. Experiments supporting a direct involvement of the MALR-encoded protein in carbon catabolite repression of MAL gene expression are reported.

Induction of the lutropin/choriogonadotropin receptor in rat ovary during luteinization

Molecular analysis of the induction of the lutropin/choriogonadotropin receptor during the process of luteinization of the rat ovary was performed. The appearance of receptor binding activity and an immunological analysis of the receptor using Triton solubilized membrane proteins show little receptor present in luteal tissue through day 3 subsequent to hCG treatment, with some in day 4, and a marked increase by day 5. A similar pattern was found in the analysis of RNA hybridizing to several probes derived from the published cDNA sequence suggesting that receptor induction occurs primarily at the level of transcription.

Regulation of MAL gene expression in yeast: gene dosage effects

Both the MAL1 and MAL6 loci in Saccharomyces strains have been shown by functional and structural studies to comprise a cluster of at least three genes necessary for maltose utilization. They include regulatory, maltose transport and maltase genes designated MALR, MALT and MALS, respectively. Subclones of each gene derived from the MAL6 locus were inserted into the multicopy shuttle plasmid YEp13, introduced into MAL1 and mal1 strains and the effects of altered gene dosage of each gene, or a combination of them, on MAL gene expression investigated. MAL1 strains transformed with a plasmid carrying the MAL6S gene showed coordinate four to five fold increases in both maltase enzyme activity and its mRNA, whereas no increase in maltose transport activity or of MALT mRNA was observed when MAL6T was present on multicopy plasmids. The presence of the MAL6R gene on a multicopy plasmid led to greatly increased transcription of both inducible and constitutive mRNAs with homology to the regulatory gene; it also gave rise to two fold increases in both induced maltase mRNA levels and enzyme activity, but only in the presence of maltose. However, it had no apparent effect on the accumulation of MALT mRNA. Finally, the induction kinetics of plasmid-borne and chromosomal MALS and MALT gene expression were examined under conditions of altered gene dosage of the MAL6 regulatory and structural genes. The results of these experiments indicate that MALR encodes a trans-acting positive activator that requires maltose for induction of MALS and MALT transcription even when the regulatory gene is present on a multicopy plasmid. Maltose transport can be a rate-limiting factor in MAL gene expression, at least in the early stages of induction. The regulation of the MALS and MALT genes, whose activities are coordinately induced in MAL1 strains by maltose, may in fact exhibit some important differences.

Organization of the MAL loci of Saccharomyces. Physical identification and functional characterization of three genes at the MAL6 locus

We have physically and functionally identified three genes at the MAL6 locus of Saccharomyces carlsbergensis. Using multicopy yeast plasmid vectors, we have subcloned various segments of the entire MAL6 locus. The functional characterization of the MAL6 subcloned regions was determined by (1) analyzing biochemically the levels of MAL-encoded proteins (maltase [alpha-D-glucosidase, E.C. 3.2.1.20] and maltose transport protein) in cells transformed with various MAL6 subclones, and (2) testing the ability of the subclones to complement the maltose fermentation defects of well characterized Mal- mutants in the highly homologous MAL1 locus. The physical homology between MAL6 and MAL1 is in part demonstrated by the gene disruption of MAL1 using subcloned MAL6 DNA sequences. The results demonstrate that the MAL6 locus is a complex of at least three genes: MAL6R, MAL6T and MAL6S. These genes specify, respectively, a regulatory function, a maltose transport activity (presumably the maltose permease) and the structural gene for maltase. The functional organization of the MAL6 locus is thus identical to that which we had previously determined by mutational analysis for the MAL1 locus.

Mutational analysis of the MAL1 locus of Saccharomyces: identification and functional characterization of three genes

Fermentation of maltose by Saccharomyces strains depends on the presence of any one of five unlinked MAL loci (MAL1, MAL2, MAL3, MAL4 or MAL6). Earlier mutational analyses of MAL2 and MAL6 containing strains have identified a single complementation group at each of these two loci. However complementation analysis between naturally occurring Mal- Saccharomyces strains isolated from the wild demonstrated the presence of two complementation groups (designated MALp and MALg) at the MAL1, MAL3 and MAL6 loci. The available evidence suggests that the MALp gene is functionally equivalent to the complementation group identified by mutational analysis at the MAL6 locus and that this gene encodes a protein involved in the regulation of the coordinate induction of both maltase and maltose permease synthesis. In this paper we report the isolation, in a well characterized MAL1 strain, of 47 mutants unable to ferment maltose. All the mutants, with one exception, map at the MAL1 locus. These mal1 mutants, except for one, are recessive to MAL1 and fall into two major complementation groups. Evidence is presented that these two classes of mutants identify both a gene involved in the regulation of maltose identify both a gene involved in the regulation of maltose fermentation (MAL1R) and a gene involved in maltose transport (MAL1T). We also report here the isolation of a temperature sensitive maltose nonfermenting mutant mapping at the MAL1 locus identifying a third gene (MAL1S) at this locus. The maltase synthesized by this mutant, when assayed in cell-free extracts, is significantly more thermolabile than the wild type enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and physical characterization of yeast maltase structural genes

Each of at least five unlinked MAL loci (MAL1 through MAL4 and MAL6) on the yeast genome controls the ability to synthesize an inducible alpha-D-glucosidase (maltase). A subcloned fragment of the coding sequence of the MAL6 maltase structural gene was used as a hybridization probe to investigate the physical structure of the family of MAL structural genes in the genomes of different Saccharomyces strains. MAL+ strains, each carrying a genetically defined MAL locus, were crossed with a MAL- strain and the segregation behavior of the functional locus and of sequences complementary to the maltase structural gene at that locus analyzed. The maltase structural gene sequences of each MaL locus were detected by Southern blot hybridization using BamH1 digests of genomic DNA of the meiotic products. This restriction enzyme was previously shown to cleave outside the confines of the MAL 6 locus. The results of such experiments indicate that each MAL locus encompasses at least one maltase structural gene sequence homologous to that of MAL6, that yeast strains that lack functional MAL loci may or may not contain the corresponding maltase structural gene sequence, that the MAL1 maltase structural gene sequence or one of its alleles can be detected in all laboratory yeast strains examined and that each MAL locus can be identified as a characteristic BamH1 fragment of genomic DNA which includes a maltase structural gene. Yeast strains vary in the number of maltase structural gene sequences that they carry. By using the approach described in this report, the ones corresponding to the different functional MAL loci and residing within a BamH1 generated restriction fragment can be identified.