β-adrenergic Receptor Polymorphisms and Responses during Titration of Metoprolol Controlled Release/extended Release in Heart Failure*

OBJECTIVE

beta-Blockers require careful initiation and titration when used in patients with heart failure. Some patients tolerate beta-blocker therapy initiation without difficulty, whereas in other patients this period presents clinical challenges. We tested the hypothesis that polymorphisms at codons 389 (Arg389Gly) and 49 (Ser49Gly) of the beta(1)-adrenergic receptor would be associated with differences in initial tolerability of beta-blocker therapy in patients with heart failure. We also tested whether polymorphisms in the beta(2)-adrenergic receptor, G-protein alpha s subunit (G(s)alpha), and cytochrome P450 (CYP) 2D6 genes or S-metoprolol plasma concentrations were associated with beta-blocker tolerability.

METHODS

Sixty-one beta-blocker-naive patients with systolic heart failure were prospectively enrolled. Patients began taking 12.5 to 25 mg metoprolol controlled release/extended release with titration every 2 weeks (as tolerated) to 200 mg/d or the maximum tolerated dose over a period of 8 to 10 weeks. Decompensation was the composite of death, heart failure hospitalization, increase in other heart failure medications, or need to discontinue metoprolol. End points were assessed during the titration period.

RESULTS

The overall rate of decompensation was not different between the codon 49 or 389 genotypes. However, a significantly greater percentage of patients with the Gly389 variant required increases in heart failure medications as compared with Arg389 homozygotes (48% versus 14%, respectively; P = .006). Similarly, patients with the Ser49 homozygous genotype were significantly more likely to require increases in concomitant heart failure therapy as compared with Gly49 carriers (41% versus 11%, respectively; P = .03). Neither CYP2D6 genotypes nor metoprolol pharmacokinetics differed between patients with and those without a decompensation event. There was no association between the beta(2)-adrenergic receptor or G(s)alpha polymorphisms with decompensated heart failure.

CONCLUSIONS

Patients with the Gly389 variant and Ser49Ser genotype were significantly more likely to require increases in heart failure medications during beta-blocker titration and thus may require more frequent follow-up during titration.

PPARGC1A genotype (Gly482Ser) predicts exceptional endurance capacity in European men

Animal and human data indicate a role for the peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PPARGC1A) gene product in the development of maximal oxygen uptake (V(O2 max)), a determinant of endurance capacity, diabetes, and early death. We tested the hypothesis that the frequency of the minor Ser482 allele at the PPARGC1A locus is lower in World-class Spanish male endurance athletes (cases) [n = 104; mean (SD) age: 26.8 (3.8) yr] than in unfit United Kingdom (UK) Caucasian male controls [n = 100; mean (SD) age: 49.3 (8.1) yr]. In cases and controls, the Gly482Ser genotype met Hardy-Weinberg expectations (P > 0.05 in both groups tested separately). Cases had significantly higher V(O2 max) [73.4 (5.7) vs. 29.4 ml x kg(-1) x min(-1) (3.8); P < 0.0001] and were leaner [body mass index: 20.6 (1.5) vs. 27.6 kg/m2 (3.9); P < 0.0001] than controls. In unadjusted chi2 analyses, the frequency of the minor Ser482 allele was significantly lower in cases than in controls (29.1 vs. 40.0%; P = 0.01). To assess the possibility that genetic stratification could confound these observations, we also compared Gly482Ser genotype frequencies in Spanish (n = 164) and UK Caucasian men (n = 381) who were unselected for their level of fitness. In these analyses, Ser482 allele frequencies were very similar (36.9% in Spanish vs. 37.5% in UK Caucasians, P = 0.83), suggesting that confounding by genetic stratification is unlikely to explain the association between Gly482Ser genotype and endurance capacity. In summary, our data indicate a role for the Gly482Ser genotype in determining aerobic fitness. This finding has relevance from the perspective of physical performance, but it may also be informative for the targeted prevention of diseases associated with low fitness such as Type 2 diabetes.

Angiotensin-converting enzyme gene polymorphism, left ventricular remodeling, and exercise capacity in strength-trained athletes

The mechanisms that regulate the development of human physiological cardiac hypertrophy remain poorly understood. The renin-angiotensin system, which is modulated by genetic polymorphism, plays an important role in the regulation of vascular tone and myocardial hypertrophy. Although a few studies have analyzed the association of angiotensin-converting enzyme (ACE) polymorphism and left ventricular (LV) hypertrophy in isotonic exercise-trained subjects who developed eccentric cardiac hypertrophy, there has been no research done in power athletes who developed concentric cardiac hypertrophy. We have hypothesized that ACE genotypic modulation characteristics may affect LV mass in power athletes. This study included 29 elite Caucasian wrestlers (mean age, 22.6 years) and 51 age-matched sedentary subjects. According to the absence or presence of the insertion segment in the polymerase chain reaction (PCR) product, the subjects were classified as homozygous deletion-deletion (DD), insertion-insertion (II), or heterozygous insertion-deletion (ID). The association of LV hypertrophy with ACE gene insertion/deletion (I/D) polymorphism was analyzed. Left ventricular mass and index were determined by echocardiography. Angiotensin-converting enzyme genotyping was performed on peripheral leukocytes using the polymerase chain reaction technique. The study and control group subjects were similar in height and weight. Left ventricular hypertrophy in the athletes was more apparent than in the controls. Angiotensin-converting enzyme genotype II frequency was 17.2% (5) in the athletes, 17.6% (9) in the controls; ID frequency was 51.7% (15) in the athletes, 56.8% (29) in the controls; and the DD frequency was 31% (9) in the athletes and 25.4% (13) in the controls. Left ventricular mass and mass index were found to be higher in genotype DD (126.2 +/- 2.9g/m2) than genotype II (85.5 +/- 4.0g/m2) or genotype ID (110.1 +/- 2.3g/m2) in the athletes (P < 0.001). Furthermore, maximal oxygen consumption in genotype DD was found to be higher than in II and ID. An association was found between ACE gene I/D polymorphism and LV hypertrophy in strength-trained athletes.

Progression despite replacement of a myopathic form of coenzyme Q10 defect

The authors report 7 years of follow-up evaluation of a patient with coenzyme Q10 (CoQ10) deficiency. Initial symptoms of exercise intolerance and hyperlactatemia improved markedly with substitutive treatment. However, CoQ(10) supplementation did not prevent the onset of a cerebellar syndrome. A switch to idebenone treatment resulted in clinical and metabolic worsening, which disappeared with subsequent CoQ10 treatment. CoQ10 defects may cause progressive neurologic disease despite supplementation.

Adenoviral-mediated transfer of vascular endothelial growth factor 121 cDNA enhances myocardial perfusion and exercise performance in the nonischemic state

BACKGROUND

Angiogenic gene therapy has been demonstrated to enhance perfusion to ischemic tissues, but it is unknown whether the administration of angiogenic growth factors will increase blood flow to nonischemic tissues. This study investigates whether enhanced myocardial perfusion can be mediated by adenovirus-mediated transfer of vascular endothelial growth factor 121 cDNA to nonischemic myocardium.

METHODS

New Zealand White rabbits received adenovirus (5 x 10(10) particle units) encoding for vascular endothelial growth factor 121 (n = 14) or a control vector without a transgene (n = 13) or saline solution (n = 9) via direct myocardial injection. Fluorescent microsphere perfusion studies and histologic analyses were performed 4 weeks later. In a parallel study, exercise treadmill testing was performed to assess the functional effects of this therapy in Sprague-Dawley rats.

RESULTS

Microsphere assessment of myocardial perfusion in rabbits 4 weeks after adenovirus-encoding vascular endothelial growth factor administration was greater than that for rats injected with control vector without a transgene or saline solution (3.2 +/- 0.5 vs 2.7 +/- 0.7 and 2.4 +/- 0.4, respectively; P <.03). The endothelial cell count per high power field was increased in animals injected with adenovirus-encoding vascular endothelial growth factor versus animals injected with control vector without a transgene or saline solution (147 +/- 27 vs 123 +/- 14 and 125 +/- 16 cells, respectively), although this did not reach statistical significance (P =.12). Rats treated with adenovirus-encoding vascular endothelial growth factor also demonstrated prolonged exercise tolerance compared with rats injected with control vector without a transgene or saline solution (exhaustion time: 26 +/- 5 minutes vs 19 +/- 2 minutes and 20 +/- 3 minutes, respectively; P =.006).

CONCLUSIONS

Adenovirus encoding-mediated transfer of vascular endothelial growth factor 121 induces an enhancement in regional perfusion in nonischemic myocardium that corresponds to changes in exercise tolerance. Adenovirus-encoding vascular endothelial growth factor therapy may be useful for inducing angiogenesis in the nonischemic state, such as for prophylactic therapy of early coronary artery disease.

Muscular dystrophy with truncated dystrophin in a family of Japanese Spitz dogs

Muscular dystrophy was diagnosed in seven male Japanese Spitz dogs with clinical signs of slowly progressive exercise intolerance, generalized weakness, myalgia, difficulty chewing and dysphagia. Serum creatine kinase (CK) concentrations were markedly elevated. Histopathology showed degeneration and regeneration of muscle, consistent with a dystrophic phenotype. Immunohistochemical staining for dystrophin and related proteins showed no staining with a monoclonal antibody against the rod domain of dystrophin but near-normal staining with an antibody against the C terminus. Immunoblot analysis in two affected dogs showed a truncated dystrophin protein of approximately 70-80 kDa. The severity of disease showed that this fragment was not large enough to protect from the dystrophic process.

Quantitative trait loci for maximal exercise capacity phenotypes and their responses to training in the HERITAGE Family Study

The purpose of this study was to identify regions of the human genome linked to maximal oxygen uptake (V̇o2 max) and maximal power output (MPO), and their response to a standardized 20-wk endurance-training program in sedentary black and white subjects. A total of 509 polymorphic markers covering the 22 autosomes were used in the genome-wide linkage scan. Baseline phenotypes were adjusted for age, sex, and body mass, whereas the training responses were adjusted for age, sex, and the baseline values. Regression-based single- and multipoint linkage analyses were used. In the sedentary state, a total of 351 and 102 sibling pairs were available for whites and blacks, respectively, and 329 and 90 sibling pairs, respectively, for the training response phenotypes. Baseline V̇o2 maxshowed promising linkage ( P < 0.0023) with 11p15.1 (whites), and suggestive evidence of linkage (0.01 > P > 0.0023) was found on 1p31, 7q32, and 7q36 (blacks). Baseline MPO exhibited promising linkage on 10q23 and suggestive evidence of linkage on 13q33 and 18q11-q12 (whites). V̇o2 maxtraining response yielded promising linkages with markers on 1p31 (blacks) and suggestive on 4q27, 7q34, and 13q12 (whites) and on 16q22 and 20q13.1 (blacks). Training-induced changes in MPO showed promising linkages on 5q23 (whites) and suggestive on 1q21, 4p15.1, and 4p13 (whites) and on 1q22 and 13q11 (blacks). In conclusion, the strongest evidence of linkage was found on chromosomal regions 11p15 and 10q23 for V̇o2 maxand MPO in the sedentary state and on chromosomes 1p31 and 5q23 for their responsiveness to training. These chromosomal regions harbor several candidate genes that deserve further investigation.

Late onset of stroke-like episode associated with a 3256C-->T point mutation of mitochondrial DNA

Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) are usually associated with the common 3243A-->G mutation of mtDNA. Onset of stroke-like episodes usually occurs before age 30. We report a patient with late onset MELAS harboring a rare 3256C-->T mutation in the tRNA(Leu(UUR)) gene of mtDNA. The patient presented with a stroke-like episode at age 36. MRI showed a stroke-like lesion in the right parietooccipital brain region. Proton MR spectroscopy showed elevated lactate concentrations in the lesion (8.4 mmol/l), and in the mid-occipital region (2.3-3.2 mmol/l) that appeared normal on MRI. Further tests revealed evidence of a severe oxidative defect of muscle metabolism as well.

FKBP12.6 deficiency and defective calcium release channel (ryanodine receptor) function linked to exercise-induced sudden cardiac death

Arrhythmias, a common cause of sudden cardiac death, can occur in structurally normal hearts, although the mechanism is not known. In cardiac muscle, the ryanodine receptor (RyR2) on the sarcoplasmic reticulum releases the calcium required for muscle contraction. The FK506 binding protein (FKBP12.6) stabilizes RyR2, preventing aberrant activation of the channel during the resting phase of the cardiac cycle. We show that during exercise, RyR2 phosphorylation by cAMP-dependent protein kinase A (PKA) partially dissociates FKBP12.6 from the channel, increasing intracellular Ca(2+) release and cardiac contractility. FKBP12.6(-/-) mice consistently exhibited exercise-induced cardiac ventricular arrhythmias that cause sudden cardiac death. Mutations in RyR2 linked to exercise-induced arrhythmias (in patients with catecholaminergic polymorphic ventricular tachycardia [CPVT]) reduced the affinity of FKBP12.6 for RyR2 and increased single-channel activity under conditions that simulate exercise. These data suggest that "leaky" RyR2 channels can trigger fatal cardiac arrhythmias, providing a possible explanation for CPVT.

Breakdown of adenine nucleotide pool in fatiguing skeletal muscle in McArdle's disease: a noninvasive 31P-MRS and EMG study

Energy metabolism and electrical muscle activity were studied in the calf muscles of 19 patients with proven McArdle's disease and in 25 healthy subjects. Phosphorus magnetic resonance spectroscopy and surface electromyography (S-EMG) were performed during two isometric muscle contractions of 3 min at 30% maximum voluntary contraction, one performed during normal perfusion and the other during applied ischemia. After about 1 min of ischemic muscle contraction in diseased muscle a significant acceleration in phosphocreatine breakdown was observed, along with a significant decrease in adenosine triphosphate. During both contractions the absence of glycolysis was shown by a significant alkalinization. Furthermore, in patients we observed a greater increase in the S-EMG amplitude than in control subjects. We conclude that early on during moderate exercise, a small number of muscle fibers reach metabolic depletion, indicated by a reduction in the adenine nucleotide pool. An increasing number of motor units, which are still in a high-energy state, are continuously recruited to compensate for muscle fatigue. This functional compartmentation may contribute to the pathophysiology of exercise intolerance in McArdle's disease.

Mitochondrial myopathy and complex III deficiency in a patient with a new stop-codon mutation (G339X) in the cytochrome b gene

A 19-year-old woman complained of life-long exercise intolerance and had chronic lactic acidosis. Neurological examination was normal, but muscle biopsy showed cytochrome c oxidase-positive fibers and marked complex III deficiency. Sequence analysis showed a novel stop-codon mutation (G15761A) in the mitochondrial DNA (mtDNA)-encoded cytochrome b gene, resulting in loss of the last 41 amino acids of the protein. By PCR/restriction fragment-length polymorphism (RFLP) analysis, the G15761A mutation was very abundant (73%) in the patient's muscle, barely detectable (less than 1%) in her urine, and absent in her blood; it was also absent in muscle, urine and blood from the patient's mother. This mutation fulfills all accepted criteria for pathogenicity.

Dynamic changes of deleted mitochondrial DNA in human leucocytes after endurance exercise

We examined the time course of the appearance and disappearance of deleted mitochondrial DNA (mtDNA) in human leucocytes after endurance exercise. Five untrained healthy females [aged 20.2 (0.4) years] exercised for 30 min at 50-60 W at 60 rpm on a cycle ergometer for 2 or 3 consecutive days. Blood samples were collected immediately before exercise, and on three occasions after the end of the last exercise bout. Levels of mtDNA were analyzed using the nested polymerase chain reaction method. Prior to exercise, a common mtDNA deletion was identified in all subjects. This common deletion was again identified in all subjects 1 day after completion of the exercise regime. However, mtDNA with this common deletion was found to have disappeared 2 days after exercise in three subjects, and after 4 days in the other two subjects. The deletion then reappeared 5 or 6 days after the final exercise bout in three of the five subjects. These findings demonstrate that a common deletion in leucocytes disappears over a period of several days after endurance exercise and reappears a number of days thereafter. Therefore, the status of the appearance and disappearance of common mtDNA deletion in leucocytes is highly dynamic.

Exercise intolerance, muscle pain and lactic acidaemia associated with a 7497G>A mutation in the tRNASer(UCN) gene

A 13-year-old girl with non-familial exercise intolerance, muscle pain and lactic acidaemia underwent a muscle biopsy for suspected mitochondrial disease. Muscle morphology showed 25% ragged-red fibres and 80% COX-negative staining. Enzymatic activities of mitochondrially co-encoded respiratory chain enzymes (complexes I, III, and IV) were decreased in muscle but normal in cultured skin fibroblasts. mtDNA analysis revealed the presence of the 7497G>A mutation in the tRNASer(UCN) gene, homoplasmic in skeletal muscle and 90% in leukocytes. Analysis of the mother's mtDNA showed 10% heteroplasmy in blood. It may be concluded that the 7497G>A mutation is associated with a muscle-only disease presentation for which high levels of mutated mtDNA are required. Exercise intolerance and muscle pain in otherwise normal children warrants further mitochondrial evaluation.

Polymorphisms of the beta1-adrenergic receptor predict exercise capacity in heart failure

BACKGROUND

Exercise performance in patients with congestive heart failure is partially dependent on cardiac beta1-adrenergic receptor (beta1AR) function. There are 2 common polymorphisms of the beta1AR gene that alter the encoded amino acids at positions 49 (Ser or Gly) and 389 (Gly or Arg) and alter receptor function in vitro. Their relevance to modification of cardiac function in heart failure is not known.

METHODS

Exercise testing was performed in 263 patients with idiopathic or ischemic cardiomyopathy (left ventricular ejection fraction approximately 25%). Potential associations were sought between beta1AR genotypes and the primary outcome variables of peak oxygen consumption (VO2), heart rate response, and exercise time.

RESULTS

The major determinants of exercise capacity were the polymorphisms at position 389, where patients homozygous for Gly389 had significantly lower peak VO2 compared with those with Arg389 (14.5 +/- 0.6 vs 17.7 +/- 0.4 mL/kg/min, P =.006), despite similar clinical characteristics including left ventricular ejection fraction. Consistent with a gene dose-response, heterozygosity was associated with an intermediate response (16.9 +/- 0.6 mL/kg/min, P <.05). When position 49 genotypes were included, a graded relationship between the 5 2-locus haplotypes and VO2 was found. Two haplotypes displayed the most divergent peak VO2: homozygous Gly389/Ser49, and homozygous Arg389/Gly49 carriers (14.4 +/- 0.5 vs 18.2 +/- 0.8 mL/kg/min, P =.001). Genotype did not predict the heart rate response. The above results were independent of beta-blocker or other medication use, left ventricular ejection fraction, beta2AR genotype, or other demographic and clinical characteristics.

CONCLUSION

beta1AR polymorphisms are a significant determinant of exercise capacity in patients with congestive heart failure. Early identification, by genetic testing for these polymorphisms, of heart failure patients at risk for development of depressed exercise capacity may be useful for initiation of specific therapy tailored to genotype.

Angiotensin-converting enzyme genotype modulates pulmonary function and exercise capacity in treated patients with congestive stable heart failure

BACKGROUND

The gene encoding ACE exhibits an insertion/deletion polymorphism resulting in 3 genotypes (DD, ID, and II), which affects serum and tissue ACE activity as well as other vasoactive substances. Pulmonary function is frequently abnormal in patients with congestive heart failure (CHF), the mechanism of which has not been completely characterized. ACE inhibition has been shown to improve diffusion across the alveolar-capillary membrane and to improve exercise capacity and gas exchange in CHF. The aim of the current study was to determine if ACE genotype is associated with altered pulmonary function and exercise intolerance in patients with treated CHF.

METHODS AND RESULTS

Fifty-seven patients (stratified according to ACE genotype as 17 DD, 28 ID, 12 II) with ischemic and dilated cardiomyopathy, left ventricular ejection fraction (LVEF) <35%, and <10 pack-years of smoking history were studied. All patients were receiving standard therapy for left ventricular systolic dysfunction. Pulmonary function, LVEF, serum ACE, plasma angiotensin II, atrial natriuretic peptide, and brain natriuretic peptide were measured at baseline. Peak VO2 and gas exchange measurements were assessed with graded exercise. Resting LVEF was similar among the genotype groups (25% to 28%), and no differences were observed in diastolic function or pulmonary artery pressures (P>0.05). Mean peak VO2 and forced vital capacity (% Pred) were significantly reduced (P<0.05), whereas mean serum ACE activity and plasma angiotensin II concentration were highest in DD homozygotes. Subjects homozygous for the D-allele also demonstrated higher mean ventilatory equivalents for carbon dioxide (VE/VCO2) during exercise (P<0.05).

CONCLUSIONS

ACE DD genotype is associated with decreased exercise tolerance in CHF, possibly mediated by altered pulmonary function. Pharmacological strategies effecting more complete inhibition of serum and tissue ACE and/or potentiation of bradykinin may improve exercise capacity in patients with CHF and ACE DD genotype.

Caveolin-2-deficient mice show evidence of severe pulmonary dysfunction without disruption of caveolae

Caveolin-2 is a member of the caveolin gene family with no known function. Although caveolin-2 is coexpressed and heterooligomerizes with caveolin-1 in many cell types (most notably adipocytes and endothelial cells), caveolin-2 has traditionally been considered the dispensable structural partner of the widely studied caveolin-1. We now directly address the functional significance of caveolin-2 by genetically targeting the caveolin-2 locus (Cav-2) in mice. In the absence of caveolin-2 protein expression, caveolae still form and caveolin-1 maintains its localization in plasma membrane caveolae, although in certain tissues caveolin-1 is partially destabilized and shows modestly diminished protein levels. Despite an intact caveolar membrane system, the Cav-2-null lung parenchyma shows hypercellularity, with thickened alveolar septa and an increase in the number of endothelial cells. As a result of these pathological changes, these Cav-2-null mice are markedly exercise intolerant. Interestingly, these Cav-2-null phenotypes are identical to the ones we and others have recently reported for Cav-1-null mice. As caveolin-2 expression is also severely reduced in Cav-1-null mice, we conclude that caveolin-2 deficiency is the clear culprit in this lung disorder. Our analysis of several different phenotypes observed in caveolin-1-deficient mice (i.e., abnormal vascular responses and altered lipid homeostasis) reveals that Cav-2-null mice do not show any of these other phenotypes, indicating a selective role for caveolin-2 in lung function. Taken together, our data show for the first time a specific role for caveolin-2 in mammalian physiology independent of caveolin-1.

A new stop codon mutation (Y52X) in the myophosphorylase gene in a Greek patient with McArdle's disease

We identified a novel stop codon mutation in the myophosphorylase gene in a Greek patient with typical symptoms of McArdle's disease. This is the first genetic study of myophosphorylase deficiency in a Greek family, showing that the proband was a compound heterozygous for the common "caucasian" mutation (R49X) and a new nonsense mutation (Y52X), both within exon 1. The new point mutation, a C-to-G transversion at codon 52, converts an encoded tyrosine to a stop codon. Our study confirms that the R49X is also present in the Greek population. The Y52X may represent a private mutation or a common mutation among Greeks. Our data further expand the already remarkable genetic heterogeneity of McArdle's disease. The prevalence of the Y52X mutation in Greek patients with McArdle's disease remains to be determined.

Mutations in mitochondrial DNA as a cause of exercise intolerance

Exercise intolerance is a common presentation of metabolic myopathies, especially of congenital errors of glycogen and lipid metabolism. Recently, however, exercise intolerance has been associated with specific defects in protein-coding genes of mitochondrial DNA (mtDNA), including mutations in genes for complex I, complex III, and complex IV. Contrary to the general rules of mitochondrial genetics, all patients were sporadic cases and all mutations were restricted to skeletal muscle, suggesting that they were somatic mutations not affecting the germ line.

Exercise intolerance resulting from a muscle-restricted mutation in the mitochondrial tRNA(Leu (CUN)) gene

BACKGROUND

Some patients presenting with isolated lifelong exercise intolerance and ragged-red fibres, harbour skeletal-muscle restricted mutations in their mitochondrial DNA.

AIM

To identify the molecular defect in a patient presenting with lifelong exercise intolerance, ragged-red fibres and deficiencies of complexes III and IV in skeletal muscle.

METHODS

The muscle biopsy was studied for activities of the respiratory chain, histochemical stains, and sequencing the tRNA genes of mitochondrial DNA.

RESULTS

The patient had a heteroplasmic mutation in the tRNA(Leu (CUN)) gene of mitochondrial DNA (G12334A). Clinical and morphological data as well as restriction fragment length polymorphism (RFLP) and single-fibre polymerase chain reaction (PCR) analyses strongly indicate that this molecular defect is the primary cause of the myopathy.

CONCLUSION

Mutations in any mitochondrial gene should be considered in the differential diagnosis of patients with lifelong exercise intolerance, even when the neurological examination is normal.

Cardiac performance in inbred rat genetic models of low and high running capacity

1. Previous work demonstrating that DA inbred rats are superior to COP inbred rats in aerobic treadmill running capacity has indicated their utility as genetic models to explore this trait. We tested the general hypothesis that intermediate phenotypes of cardiac function and calcium metabolism are responsible for the difference in capacity between these strains. 2. Logical cardiac trait differences were estimated at a tissue (isolated papillary muscle), cellular (isolated left ventricular cells), and biochemical level of organization. 3. DA hearts were found to give significantly higher values than COP hearts for: (1) maximal developed tension (38.3 % greater), and rates of tension change in contraction (61 %) or relaxation (59 %) of isolated papillary muscle, (2) fractional shortening (50 %), amplitude of the Ca(2+) transient (78.6 %), and caffeine-induced release of Ca(2+) from the sarcoplasmic reticulum (SR; 260 %) in isolated ventricular myocytes, and (3) Na(+),K(+)-ATPase activity of isolated myocytes (17.3 %). 4. Our results suggest that these trait differences may prove useful for further studies into the genes responsible for natural variations in both ventricular function and aerobic endurance capacity. Understanding the genetic basis of aerobic capacity will help define the continuum between health and disease.

A novel mutation in the mitochondrial tRNA(TYr) gene associated with exercise intolerance

The authors report a novel A5874G mutation in the mitochondrial tRNA tyrosine (tRNA(TYr)) gene associated with exercise intolerance, limb weakness, and complex III deficiency. The mutation was absent in blood from the patient and all maternal family members, indicating that it may be a spontaneous somatic mutation in muscle. This is the first point mutation in the tRNA(TYr) gene associated with human disease and is further evidence that exercise intolerance associated with complex III deficiency is genetically heterogeneous.

Segregation analysis of apolipoproteins A-1 and B-100 measured before and after an exercise training program: the HERITAGE Family Study

Complex segregation analyses of apolipoproteins (apo) A-1 and B-100 were performed in a sample of 520 individuals from 99 white families who participated in the HERITAGE Family Study. In these sedentary families, plasma apo A-1 and B-100 concentrations were measured before and after a 20-week endurance exercise training program. Baseline apo A-1 and B-100 were adjusted for the effects of age (age-adjusted baseline apo A-1 and B-100) and for the effects of age and BMI (age-BMI-adjusted baseline apo A-1 and B-100). The change in response to training was computed as a simple Delta (posttraining minus baseline) and was adjusted for age and the baseline (age-baseline-adjusted apo A-1 and B-100 responses to training). In the present study, a major gene could not be inferred for baseline apo A-1. Rather, we found a major effect along with a multifactorial effect accounting for 8% to 9% and 51% to 56% of the variance, respectively. In addition, no clear evidence supported a major-gene effect for its response to training, whereas the transmission of a major effect from parents to offspring was ambiguous, ie, genetic in nature or familial environmental in origin. The major effect accounted for 15% of the variance, with an additional 21% and 58% of the variance being accounted for by a multifactorial effect in parents and offspring, respectively. It is interesting to have obtained evidence of a putative recessive major locus for baseline apo B-100, which accounted for 50% to 56% of the variance, with an additional 25% to 29% of the variance due to a multifactorial effect. In contrast, no major effect for its response to training was identified, although a multifactorial effect was found that accounted for 27% of the variance. The novel findings arising from the present study are summarized as follows. Baseline apo A-1 and its response to training were influenced by a major effect and a multifactorial effect. Baseline apo B-100 was influenced by a putative major recessive gene with a multifactorial component, but its response to training was influenced solely by a multifactorial component in these sedentary families.

Exercise intolerance due to mutations in the cytochrome b gene of mitochondrial DNA

BACKGROUND

The mitochondrial myopathies typically affect many organ systems and are associated with mutations in mitochondrial DNA (mtDNA) that are maternally inherited. However, there is also a sporadic form of mitochondrial myopathy in which exercise intolerance is the predominant symptom. We studied the biochemical and molecular characteristics of this sporadic myopathy.

METHODS

We sequenced the mtDNA cytochrome b gene in blood and muscle specimens from five patients with severe exercise intolerance, lactic acidosis in the resting state (in four patients), and biochemical evidence of complex III deficiency. We compared the clinical and molecular features of these patients with those previously described in four other patients with mutations in the cytochrome b gene.

RESULTS

We found a total of three different nonsense mutations (G15084A, G15168A, and G15723A), one missense mutation (G14846A), and a 24-bp deletion (from nucleotide 15498 to 15521) in the cytochrome b gene in the five patients. Each of these mutations impairs the enzymatic function of the cytochrome b protein. In these patients and those previously described, the clinical manifestations included progressive exercise intolerance, proximal limb weakness, and in some cases, attacks of myoglobinuria. There was no maternal inheritance and there were no mutations in tissues other than muscle. The absence of these findings suggests that the disorder is due to somatic mutations in myogenic stem cells after germ-layer differentiation. All the point mutations involved the substitution of adenine for guanine, but all were in different locations.

CONCLUSIONS

The sporadic form of mitochondrial myopathy is associated with somatic mutations in the cytochrome b gene of mtDNA. This myopathy is one cause of the common and often elusive syndrome of exercise intolerance.

Insulin resistance, exercise capacity and body composition in subjects with two hypertensive parents

OBJECTIVE

To study insulin resistance in subjects with strong genetic predisposition to essential hypertension, compared with non-disposed subjects.

SUBJECTS

Thirty normotensive subjects aged 18-35 years whose parents both had essential hypertension, and 30 age- and sex matched subjects whose parents were both normotensive, were studied. Subjects or parents with diabetes and morbid obesity were excluded.

METHODS

The study comprised (1) a frequent sampling oral glucose tolerance test; (2) an isoglycemic hyperinsulinemic clamp study; (3) an analysis of body composition by dual-energy X-ray absorptiometry; (4) an exercise test with gas exchange analysis; and (5) investigation of composition of usual diet by diet registration for 5 days.

RESULTS

The 24-h diastolic blood pressure was higher in subjects predisposed to hypertension compared with the controls: 78.1 versus 74.0 mmHg (confidence interval for the difference between the means; -0.5; -7.9), but the insulin sensitivity index was similar: 312 versus 362 I(2) min(-1) pmol(-1) kg(-1) (28; -129). The two groups were similar in terms of body composition, exercise capacity and composition of usual diet. Resting and 24-h diastolic blood pressures were correlated to abdominal fat mass but not to insulin sensitivity.

CONCLUSION

Subjects with a strong genetic predisposition to essential hypertension had increased diastolic blood pressure compared with subjects with normotensive parents, but they were not insulin resistant. This may be due to the subjects being highly selected as to confounding factors. The increased blood pressure in the hypertension prone subjects could not be attributed to differences in body composition, exercise capacity or dietary habits.