miR-1183 Is a Key Marker of Remodeling upon Stretch and Tachycardia in Human Myocardium

Many cardiac insults causing atrial remodeling are linked to either stretch or tachycardia, but a comparative characterization of their effects on early remodeling events in human myocardium is lacking. Here, we applied isometric stretch or sustained tachycardia at 2.5 Hz in human atrial trabeculae for 6 h followed by microarray gene expression profiling. Among largely independent expression patterns, we found a small common fraction with the microRNA miR-1183 as the highest up-regulated transcript (up to 4-fold). Both, acute stretch and tachycardia induced down-regulation of the predicted miR-1183 target genes ADAM20 and PLA2G7. Furthermore, miR-1183 was also significantly up-regulated in chronically remodeled atrial samples from patients with persistent atrial fibrillation (3-fold up-regulation versus sinus rhythm samples), and in ventricular myocardium from dilative cardiomyopathy hearts (2-fold up-regulation) as compared to non-failing controls. In sum, although stretch and tachycardia show distinct transcriptomic signatures in human atrial myocardium, both cardiac insults consistently regulate the expression of miR-1183 and its downstream targets in acute and chronic remodeling. Thus, elevated expression of miR-1183 might serve as a tissue biomarker for atrial remodeling and might be of potential functional significance in cardiac disease.

Coffee Consumption and Incident Tachyarrhythmias: Reported Behavior, Mendelian Randomization, and Their Interactions

Importance

The notion that caffeine increases the risk of cardiac arrhythmias is common. However, evidence that the consumption of caffeinated products increases the risk of arrhythmias remains poorly substantiated.

Objective

To assess the association between consumption of common caffeinated products and the risk of arrhythmias.

Design, Setting, and Participants

This prospective cohort study analyzed longitudinal data from the UK Biobank between January 1, 2006, and December 31, 2018. After exclusion criteria were applied, 386 258 individuals were available for analyses.

Exposures

Daily coffee intake and genetic polymorphisms that affect caffeine metabolism.

Main Outcomes and Measures

Any cardiac arrhythmia, including atrial fibrillation or flutter, supraventricular tachycardia, ventricular tachycardia, premature atrial complexes, and premature ventricular complexes.

Results

A total of 386 258 individuals (mean [SD] age, 56 [8] years; 52.3% female) were assessed. During a mean (SD) follow-up of 4.5 (3.1) years, 16 979 participants developed an incident arrhythmia. After adjustment for demographic characteristics, comorbid conditions, and lifestyle habits, each additional cup of habitual coffee consumed was associated with a 3% lower risk of incident arrhythmia (hazard ratio [HR], 0.97; 95% CI, 0.96-0.98; P < .001). In analyses of each arrhythmia alone, statistically significant associations exhibiting a similar magnitude were observed for atrial fibrillation and/or flutter (HR, 0.97; 95% CI, 0.96-0.98; P < .001) and supraventricular tachycardia (HR, 0.96; 95% CI, 0.94-0.99; P = .002). Two distinct interaction analyses, one using a caffeine metabolism-related polygenic score of 7 genetic polymorphisms and another restricted to CYP1A2 rs762551 alone, did not reveal any evidence of effect modification. A mendelian randomization study that used these same genetic variants revealed no significant association between underlying propensities to differing caffeine metabolism and the risk of incident arrhythmia.

Conclusions and Relevance

In this prospective cohort study, greater amounts of habitual coffee consumption were associated with a lower risk of arrhythmia, with no evidence that genetically mediated caffeine metabolism affected that association. Mendelian randomization failed to provide evidence that caffeine consumption was associated with arrhythmias.

Therapeutic Effects of Short Cyclic and Combined Epitope Peptides in a Long-Term Model of Graves' Disease and Orbitopathy

BACKGROUND

Cyclic peptides derived from some cylindrical loops of the leucine-rich repeat domain (LRD) of the thyrotropin receptor (TSHR) have been shown to treat disease manifestations in a mouse model of Graves' disease during a long-term protocol of four-weekly immunizations with adenovirus coding for the TSHR A-subunit (Ad-TSHR289).

METHODS

In a follow-up study, two additional cyclic peptides were tested, which were shortened in order to obtain additional information on the minimally involved epitopes and to enable easier production conditions. In addition, a linear peptide was tested, which mimics parts of three loops of the native TSHR LRD structure, and is potentially able to block the discontinuous epitopes of anti-TSHR antibodies.

RESULTS

The novel peptides markedly reduced thyroid size, serum thyroxine levels, retro-orbital fibrosis, and tachycardia in Ad-TSHR289-immunized mice. In immunologically naïve mice, administration of the peptides did not induce any immune response.

CONCLUSIONS

In summary, novel cyclic peptides mitigate many clinical findings in a mouse model of established Graves' disease and orbitopathy, and may therefore provide an additional therapeutic option compared to existing drugs or interventions.

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

KCNN2 polymorphisms and cardiac tachyarrhythmias

Potassium calcium-activated channel subfamily N member 2 (KCNN2) encodes an integral membrane protein that forms small-conductance calcium-activated potassium (SK) channels. Recent studies in animal models show that SK channels are important in atrial and ventricular repolarization and arrhythmogenesis. However, the importance of SK channels in human arrhythmia remains unclear. The purpose of the present study was to test the association between genetic polymorphism of the SK2 channel and the occurrence of cardiac tachyarrhythmias in humans. We enrolled 327 Han Chinese, including 72 with clinically significant ventricular tachyarrhythmias (VTa) who had a history of aborted sudden cardiac death (SCD) or unexplained syncope, 98 with a history of atrial fibrillation (AF), and 144 normal controls. We genotyped 12 representative tag single nucleotide polymorphisms (SNPs) across a 141-kb genetic region containing the KCNN2 gene; these captured the full haplotype information. The rs13184658 and rs10076582 variants of KCNN2 were associated with VTa in both the additive and dominant models (odds ratio [OR] 2.89, 95% confidence interval [CI] = 1.505-5.545, P = 0.001; and OR 2.55, 95% CI = 1.428-4.566, P = 0.002, respectively). After adjustment for potential risk factors, the association remained significant. The population attributable risks of these 2 variants of VTa were 17.3% and 10.6%, respectively. One variant (rs13184658) showed weak but significant association with AF in a dominant model (OR 1.91, CI = 1.025-3.570], P = 0.042). There was a significant association between the KCNN2 variants and clinically significant VTa. These findings suggest an association between KCNN2 and VTa; it also appears that KCNN2 variants may be adjunctive markers for risk stratification in patients susceptible to SCD.

Calcium Imaging in Pluripotent Stem Cell-Derived Cardiac Myocytes

The possibility to generate cardiomyocytes (CMs) from disease-specific induced pluripotent stem cells (iPSCs) is a powerful tool for the investigation of various cardiac diseases in vitro. The pathological course of various cardiac conditions, causatively heterogeneous, often converges into disturbed cellular Ca(2+) cycling. The gigantic Ca(2+) channel of the intracellular Ca(2+) store of CMs, the ryanodine receptor type 2 (RyR2), controls Ca(2+) release and therefore plays a crucial role in Ca(2+) cycling of CMs. In the present protocol we describe ways to measure and analyze global as well as local cellular Ca(2+) release events in CMs derived from a patient carrying a CPVT-causing RyR2 mutation.

Identification of four novel mutations in the thyroid hormone receptor-β gene in 164 Spanish and 2 Greek patients with resistance to thyroid hormone

OBJECTIVE

The aim of the study was to describe the clinical, biochemical, and genetic features of a sample of Mediterranean patients with RTH (resistance to thyroid hormone) due to mutations in TRβ (thyroid hormone receptor beta) referred to our institution during the last 15 years.

DESIGN

166 blood samples were received for RTH genetic testing between January 1997 and December 2011. Genetic testing was performed by PCR amplification followed by sequencing of exons 7, 8, 9, and 10. Clinical and biochemical features were obtained from available information sent by referring hospitals.

RESULTS

Mutations were identified in 50 patients (29 probands and 21 relatives). 64% were women, and mean ± stdev age at diagnosis among probands was 33.2 ± 20.5 years. The following clinical features were recorded: goiter in 50%, hyperkinetic behavior in 32%, and tachycardia in 29%. Up to 19% of the probands had undergone some type of thyroidal ablative therapy before diagnosis. As for biochemical features, mean ± stdev TSH was 10.2 ± 21.4 mUI/L, and mean ± stdev fT4 was 35.5 ± 10.8 pmol/L. We found four new mutations: p.Phe451Leu, p.Pro452Arg, p.Glu457Gly, and p.Phe459Leu.

CONCLUSIONS

The clinical and biochemical characteristics of our samples of Mediterranean populations with RTH were similar to those described in the published literature. Interestingly, in our populations we have identified four novel mutations in the TRβ gene.

MicroRNA-1 downregulation increases connexin 43 displacement and induces ventricular tachyarrhythmias in rodent hypertrophic hearts

Downregulation of the muscle-specific microRNA-1 (miR-1) mediates the induction of pathologic cardiac hypertrophy. Dysfunction of the gap junction protein connexin 43 (Cx43), an established miR-1 target, during cardiac hypertrophy leads to ventricular tachyarrhythmias (VT). However, it is still unknown whether miR-1 and Cx43 are interconnected in the pro-arrhythmic context of hypertrophy. Thus, in this study we investigated whether a reduction in the extent of cardiac hypertrophy could limit the pathological electrical remodeling of Cx43 and the onset of VT by modulating miR-1 levels. Wistar male rats underwent mechanical constriction of the ascending aorta to induce pathologic left ventricular hypertrophy (LVH) and afterwards were randomly assigned to receive 10mg/kg valsartan, VAL (LVH+VAL) delivered in the drinking water or placebo (LVH) for 12 weeks. Sham surgery was performed for control groups. Programmed ventricular stimulation reproducibly induced VT in LVH compared to LVH+VAL group. When compared to sham controls, rats from LVH group showed a significant decrease of miR-1 and an increase of Cx43 expression and its ERK1/2-dependent phosphorylation, which displaces Cx43 from the gap junction. Interestingly, VAL administration to rats with aortic banding significantly reduced cardiac hypertrophy and prevented miR-1 down-regulation and Cx43 up-regulation and phosphorylation. Gain- and loss-of-function experiments in neonatal cardiomyocytes (NCMs) in vitro confirmed that Cx43 is a direct target of miR-1. Accordingly, in vitro angiotensin II stimulation reduced miR-1 levels and increased Cx43 expression and phosphorylation compared to un-stimulated NCMs. Finally, in vivo miR-1 cardiac overexpression by an adenoviral vector intra-myocardial injection reduced Cx43 expression and phosphorylation in mice with isoproterenol-induced LVH. In conclusion, miR-1 regulates Cx43 expression and activity in hypertrophic cardiomyocytes in vitro and in vivo. Treatment of pressure overload-induced myocyte hypertrophy reduces the risk of life-threatening VT by normalizing miR-1 expression levels with the consequent stabilization of Cx43 expression and activity within the gap junction.

Atrial tachyarrhythmia in Rgs5-null mice

AIMS

The aim of this study was to elucidate the effects of regulator of G-protein signaling 5 (Rgs5), a negative regulator of G protein-mediated signaling, on atrial repolarization and tachyarrhythmia (ATA) in mice.

METHODS AND RESULTS

In present study, the incidence of ATA were increased in Rgs5(-/-) Langendorff-perfused mouse hearts during program electrical stimulation (PES) (46.7%, 7 of 15) and burst pacing (26.7%, 4 of 15) compared with wild-type (WT) mice (PES: 7.1%,1 of 14; burst:7.1%,1 of 14) (P<0.05). And the duration of ATA also shown longer in Rgs5(-/-) heart than that in WT, 2 out of 15 hearts exhibited sustained ATA (>30 s) but none of them observed in WT mice. Atrial prolonged repolarization was observed in Rgs5(-/-) hearts including widened P wave in surface ECG recording, increased action potential duration (APD) and atrial effective refractory periods (AERP), all of them showed significant difference with WT mice (P<0.05). At the cellular level, whole-cell patch clamp recorded markedly decreased densities of repolarizing K(+) currents including I(Kur) (at +60 mV: 14.0±2.2 pF/pA) and I(to) (at +60 mV: 16.7±1.3 pA/pF) in Rgs5(-/-) atrial cardiomyocytes, compared to those of WT mice (at +60 mV I(to): 20.4±2.0 pA/pF; I(kur): 17.9±2.0 pF/pA) (P<0.05).

CONCLUSION

These results suggest that Rgs5 is an important regulator of arrhythmogenesis in the mouse atrium and that the enhanced susceptibility to atrial tachyarrhythmias in Rgs5(-/-) mice may contribute to abnormalities of atrial repolarization.

Fine-mapping and initial characterization of QT interval loci in African Americans

The QT interval (QT) is heritable and its prolongation is a risk factor for ventricular tachyarrhythmias and sudden death. Most genetic studies of QT have examined European ancestral populations; however, the increased genetic diversity in African Americans provides opportunities to narrow association signals and identify population-specific variants. We therefore evaluated 6,670 SNPs spanning eleven previously identified QT loci in 8,644 African American participants from two Population Architecture using Genomics and Epidemiology (PAGE) studies: the Atherosclerosis Risk in Communities study and Women's Health Initiative Clinical Trial. Of the fifteen known independent QT variants at the eleven previously identified loci, six were significantly associated with QT in African American populations (P≤1.20×10(-4)): ATP1B1, PLN1, KCNQ1, NDRG4, and two NOS1AP independent signals. We also identified three population-specific signals significantly associated with QT in African Americans (P≤1.37×10(-5)): one at NOS1AP and two at ATP1B1. Linkage disequilibrium (LD) patterns in African Americans assisted in narrowing the region likely to contain the functional variants for several loci. For example, African American LD patterns showed that 0 SNPs were in LD with NOS1AP signal rs12143842, compared with European LD patterns that indicated 87 SNPs, which spanned 114.2 Kb, were in LD with rs12143842. Finally, bioinformatic-based characterization of the nine African American signals pointed to functional candidates located exclusively within non-coding regions, including predicted binding sites for transcription factors such as TBX5, which has been implicated in cardiac structure and conductance. In this detailed evaluation of QT loci, we identified several African Americans SNPs that better define the association with QT and successfully narrowed intervals surrounding established loci. These results demonstrate that the same loci influence variation in QT across multiple populations, that novel signals exist in African Americans, and that the SNPs identified as strong candidates for functional evaluation implicate gene regulatory dysfunction in QT prolongation.

Atrial tachycardia/fibrillation in the connexin 43 G60S mutant (Oculodentodigital dysplasia) mouse

Atrial fibrillation (AF), the most common cardiac arrhythmia seen in general practice, can be promoted by conduction slowing. Cardiac impulse conduction depends on gap junction channels, which are composed of connexins (Cxs). While atrial Cx40 and Cx43 are equally expressed, AF studies have primarily focused on Cx40 reductions. The G60S Cx43 mutant (Cx43(G60S/+)) mouse model of Oculodentodigital dysplasia has a 60% reduction in Cx43 in the atria. Cx43(G60S/+) mice were compared with Cx40-deficient (Cx40(-/-)) mice to determine the role of Cxs in atrial tachycardia/fibrillation (AT/F). Intracardiac electrophysiological studies were done in 6-mo-old male C57BL/6 Cx43(G60S/+) mutant, littermate (Cx43(+/+)), Cx40(-/-), and C57BL/6 wild-type (WT) mice. AT/F induction used an extra stimulus during sinus rhythm, programmed electrical stimulation, or burst pacing (1-ms pulses, 50-Hz, 400-ms train) in the absence and presence of carbachol (CCh). Atrial effective refractory periods did not differ between strains. Cx43(G60S/+) mice were more susceptible to induction of sustained AT/F (duration >2 min, 9 of 12; maximum >35 min) compared with Cx43(+/+) mice (3 of 11; χ(2) = 5.24; P = 0.02). CCh enhanced sustained AT/F susceptibility in WT (from 1 of 12 without, to 7 of 10 with CCh; χ(2) = 8.98; P < 0.01) but not in Cx40(-/-) mice (1 of 13 without vs. 2 of 9 with CCh; χ(2) = 0.95; P = NS). The pattern of epicardial recordings during AT/F in Cx43(G60S/+) mice was left preceding right, with left atrial fractionated activation patterns consistent with clinical observations of AF. In conclusions, while Cx43(G60S/+) mice had severe AT/F, Cx40(-/-) mice were resistant to CCh-induced AT/F.

Ventricular arrhythmias and left ventricular dysfunction in familial cardiomyopathy

In familial cardiomyopathy (CM), different forms of myocardial abnormalities including asymmetric and symmetric hypertrophy and dilated left ventricles are presented, mostly showing varying hereditary penetrance. This study presents a family with CM including three major clinical manifestations: severe ventricular arrhythmias, repolarization abnormalities and left ventricular hypertrophy. This triad was strikingly consistent in the two generations examined. The familial pattern with an autosomal dominant inheritance did not show any linkage to the HLA region.

Beneficial effects of propylthiouracil plus L-thyroxine treatment in a patient with a mutation in MCT8

CONTEXT

Mutations of the monocarboxylate transporter 8 (MCT8) gene determine a distinct X-linked phenotype of severe psychomotor retardation and consistently elevated T(3) levels. Lack of MCT8 transport of T(3) in neurons could explain the neurological phenotype.

OBJECTIVE

Our objective was to determine whether the high T(3) levels could also contribute to some critical features observed in these patients.

RESULTS

A 16-yr-old boy with severe psychomotor retardation and hypotonia was hospitalized for malnutrition (body weight = 25 kg) and delayed puberty. He had tachycardia (104 beats/min), high SHBG level (261 nmol/liter), and elevated serum free T(3) (FT(3)) level (11.3 pmol/liter), without FT(4) and TSH abnormalities. A missense mutation of the MCT8 gene was present. Oral overfeeding was unsuccessful. The therapeutic effect of propylthiouracil (PTU) and then PTU plus levothyroxine (LT(4)) was tested. After PTU (200 mg/d), serum FT(4) was undetectable, FT(3) was reduced (3.1 pmol/liter) with high TSH levels (50.1 mU/liter). Serum SHBG levels were reduced (72 nmol/liter). While PTU prescription was continued, high LT(4) doses (100 microg/d) were needed to normalize serum TSH levels (3.18 mU/liter). At that time, serum FT(4) was normal (16.4 pmol/liter), and FT(3) was slightly high (6.6 pmol/liter). Tachycardia was abated (84 beats/min), weight gain was 3 kg in 1 yr, and SHBG was 102 nmol/liter.

CONCLUSIONS

1) When thyroid hormone production was reduced by PTU, high doses of LT(4) (3.7 microg/kg.d) were needed to normalize serum TSH, confirming that mutation of MCT8 is a cause of resistance to thyroid hormone. 2) High T(3) levels might exhibit some deleterious effects on adipose, hepatic, and cardiac levels. 3) PTU plus LT(4) could be an effective therapy to reduce general adverse features, unfortunately without benefit on the psychomotor retardation.

Mitochondrial dysfunction and redox signaling in atrial tachyarrhythmia

Accumulating evidence links calcium-overload and oxidative stress to atrial remodeling during atrial fibrillation (AF). Furthermore, atrial remodeling appears to increase atrial thrombogeneity, characterized by increased expression of adhesion molecules. The aim of this study was to assess mitochondrial dysfunction and oxidative stress-activated signal transduction (nuclear factor-kappaB [NF-kappa B], lectin-like oxidized low-density lipoprotein receptor [LOX-1], intercellular adhesion molecule-1 [ICAM-1], and hemeoxgenase-1 [HO-1]) in atrial tissue during AF. Ex vivo atrial tissue from patients with and without AF and, additionally, rapid pacing of human atrial tissue slices were used to study mitochondrial structure by electron microscopy and mitochondrial respiration. Furthermore, quantitative reverse transcription polymerase chain reaction (RT-PCR), immunoblot analyses, gel-shift assays, and enzyme-linked immunosorbent assay (ELISA) were applied to measure nuclear amounts of NF-kappa B target gene expression. Using ex vivo atrial tissue samples from patients with AF we demonstrated oxidative stress and impaired mitochondrial structure and respiration, which was accompanied by nuclear accumulation of NF-kappa B and elevated expression levels of the adhesion molecule ICAM-1 and the oxidative stress-induced markers HO-1 and LOX-1. All these changes were reproduced by rapid pacing for 24 hours of human atrial tissue slices. Furthermore, the blockade of calcium inward current with verapamil effectively prevented both the mitochondrial changes and the activation of NF-kappa B signaling and target gene expression. The latter appeared also diminished by the antioxidants apocynin and resveratrol (an inhibitor of NF-kappa B), or the angiotensin II receptor type 1 antagonist, olmesartan. This study demonstrates that calcium inward current via L-type calcium channels contributes to oxidative stress and increased expression of oxidative stress markers and adhesion molecules during cardiac tachyarrhythmia.

Cardiac Involvement in Juvenile Ceroid Lipofuscinosis of the Spielmeyer-Vogt-Sjögren Type: Prospective Noninvasive Findings in Two Siblings

The results of prospective noninvasive cardiologic investigations, including echocardiography and Holter monitoring are described in 2 siblings with juvenile ceroid lipofuscinosis of the Spielmeyer-Vogt-Sjögren type. In the elder patient, echocardiography revealed ventricular hypertrophy with slowed ventricular relaxation. Holter monitoring showed not only bradycardia but also slow and fast ectopic atrial rhythms, sinus arrests and complex ventricular ectopic activity including ventricular tachycardia. In the younger patient the findings were less severe. These functional disturbances due to cardiac involvement, never reported before in this disease, are discussed.

Dopamine-β-hydroxylase in postural tachycardia syndrome

Norepinephrine is frequently elevated in postural tachycardia syndrome (POTS), a syndrome of heterogeneous etiology characterized by a >30 beats/min increase in heart rate with standing. Norepinephrine is synthesized from dopamine by dopamine-β-hydroxylase (DBH). The results of a preliminary study suggested that the T allele frequency of the DBH −1021C→T polymorphism is elevated in POTS. This allele correlates with low DBH activity and might predict reduced serum DBH activity in patients with POTS. To test the hypothesis that low DBH activity and the underlying −1021C→T polymorphism are associated with increased susceptibility to POTS, we measured serum DBH activity in POTS and determined its relationship to the DBH genotype and plasma norepinephrine. Serum DBH was similar for 83 normal volunteers and 42 patients with POTS: median (range) = 22.5 (0.5–94.2) and 19.6 (0.1–68.8) nmol·min−1·ml−1, respectively ( P = 0.282). The genotype frequencies for 254 control and 157 POTS patients were not different between groups (∼63% CC genotype and ∼5% TT genotype, P = 0.319). The T allele associated with lower serum DBH in both groups [control serum DBH = 15.7 (SD 12.3) and 35.1 nmol·min−1·ml−1(SD 18.6) for T carriers and noncarriers, respectively; POTS serum DBH = 8.2 (SD 5.6) and 28.5 nmol·min−1·ml−1(SD 14.7) for T carriers and noncarriers, respectively]. High DBH in POTS was linked to elevated plasma levels of norepinephrine. Although DBH activity and genotype are unlikely to be primary determinants of susceptibility to POTS, differences in DBH activity in POTS may reflect differences in the level of sympathetic activation.

Familial orthostatic tachycardia

PURPOSE OF REVIEW

Postural tachycardia syndrome is an autonomic disorder primarily of younger women. The patient population is heterogeneous, making diagnosis and treatment a challenge. A mutation in the norepinephrine (noradrenaline) transporter gene prompted further genetic analysis.

RECENT FINDINGS

Eleven new mutations were found in the human norepinephrine transporter gene, although none were directly associated with postural tachycardia syndrome. The 5'-flanking -1012C --> T variant of the dopamine beta-hydroxylase gene was slightly increased and protection was associated with a reduced incidence of two mutations in the endothelial nitric oxide synthase gene, and one in endothelin-1. Mutations in other disease-related genes suggest a potential relationship with the pathogenesis of postural tachycardia syndrome. Benign joint hypermobility syndrome, for example, shares similar autonomic symptoms and is linked to a mutation in tenascin-X. Additional genetic findings are discussed as potential contributors to vascular health and neurodegeneration.

SUMMARY

Genetic testing can reveal molecular mechanisms of disease and provide an additional strategy for diagnosis and treatment of heterogeneous patient populations such as postural tachycardia syndrome. It is quite likely that the pathogenesis of this disorder will be attributed to numerous genetic mutations, both subtle and overt. Therefore, continued study of the relationships between genotype and phenotype are necessary to better understand this syndrome and others with associated dysautonomia.

Catecholaminergic polymorphic ventricular tachycardia: RYR2 mutations, bradycardia, and follow up of the patients

BACKGROUND

The aim of the study was to assess underlying genetic cause(s), clinical features, and response to therapy in catecholaminergic polymorphic ventricular tachycardia (CPVT) probands.

METHODS AND RESULTS

We identified 13 missense mutations in the cardiac ryanodine receptor (RYR2) in 12 probands with CPVT. Twelve were new, of which two are de novo mutations. A further 11 patients were silent gene carriers, suggesting that some mutations are associated with low penetrance. A marked resting sinus bradycardia off drugs was observed in all carriers. On beta blocker treatment, 98% of the RYR2 mutation carriers remained symptom free with a median follow up of 2 (range: 2-37) years.

CONCLUSION

CPVT patients with RYR2 mutation have bradycardia regardless of the site of the mutation, which could direct molecular diagnosis in (young) patients without structural heart disease presenting with syncopal events and a slow heart rate but with normal QTc at resting ECG. Treatment with beta blockers has been very effective in our CPVT patients during initial or short term follow up. Given the risk of sudden death and the efficacy of beta blocker therapy, the identification of large numbers of RYR2 mutations thus calls for genetic screening, early diagnosis, and subsequent preventive strategies.

β2-Adrenoceptor Genotype and Function Affect Hemodynamic Profile Heterogeneity in Postural Tachycardia Syndrome

Previous studies suggest that the β2-adrenoceptor functions abnormally in patients with postural tachycardia syndrome (POTS) and may contribute to their altered hemodynamic profile. To test the hypothesis that the β2-adrenoceptor response is decreased in POTS, we studied: (1) the arterial vasodilation response to the β agonist, isoproterenol, and (2) the distribution of common polymorphisms (codons 16 and 27) of the gene coding the receptor (β 2 -AR) in a large population with POTS. We measured plasma catecholamines and monitored hemodynamics and changes in forearm and leg blood flow to incremental doses of intraarterial isoproterenol in 9 patients with POTS compared with 8 healthy subjects. For polymorphism assessment we collected DNA from 57 patients with POTS and compared with 67 age-sex matched healthy subjects. Circulating catecholamines were significantly higher in POTS subjects compared with controls. Intrabrachial and intrafemoral isoproterenol infusion elicited a dose-dependent increase in blood flow. In healthy subjects, blood flow increased (mean±SEM) 400±70% in the forearm and 170±40% in the leg, but only 280±60% in forearms and 120±20% in legs of patients with POTS (ANOVA for both P<0.001). The genotype and allele distributions for codons 16 and 27 β 2 -AR variants were not different in the 2 groups. However, the blood pressure and plasma norepinephrine levels diverged in patients according to their genotype. Patients with Gly16Gly and patients with Glu27Glu had lower plasma catecholamines and higher supine and upright blood pressure, compared with other genotypes. Therefore, both decreased β2-adrenoceptor-related vasodilation and β 2 -AR polymorphisms may contribute to the hemodynamic diversity of patients with POTS.

Experimental therapy of genetic arrhythmias: disease-specific pharmacology

The integration between molecular biology and clinical practice requires the achievement of fundamental steps to link basic science to diagnosis and management of patients. In the last decade, the study of genetic bases of human diseases has achieved several milestones, and it is now possible to apply the knowledge that stems from the identification of the genetic substrate of diseases to clinical practice. The first step along the process of linking molecular biology to clinical medicine is the identification of the genetic bases of inherited diseases. After this important goal is achieved, it becomes possible to extend research to understand the functional impairments of mutant protein(s) and to link them to clinical manifestations (genotype-phenotype correlation). In genetically heterogeneous diseases, it may be possible to identify locus-specific risk stratification and management algorithms. Finally, the most ambitious step in the study of genetic disease is to discover a novel pharmacological therapy targeted at correcting the inborn defect (locus-specific therapy) or even to "cure" the DNA abnormality by replacing the defective gene with gene therapy. At present, this curative goal has been successful only for very few diseases. In the field of inherited arrhythmogenic diseases, several genes have been discovered, and genetics is now emerging as a source of information contributing not only to a better diagnosis but also to risk stratification and management of patients. The functional characterization of mutant proteins has opened new perspectives about the possibility of performing gene-specific or mutation-specific therapy. In this chapter, we will briefly summarize the genetic bases of inherited arrhythmogenic conditions and we will point out how the information derived from molecular genetics has influenced the "optimal use of traditional therapies" and has paved the way to the development of gene-specific therapy.

Arg16 homozygosity of the β2-adrenergic receptor improves the outcome after β2-agonist tocolysis for preterm labor

BACKGROUND

Beta(2)-adrenergic receptor (beta(2)AR) agonists are not consistently successful when administered as tocolytic therapy. The beta(2)AR displays genetic variability; an arginine-to-glycine substitution at codon 16 (Arg16Gly) has been shown to increase receptor desensitization in response to agonist exposure, whereas a substitution of glutamate for glutamine at codon 27 (Gln27Glu) decreases down-regulation. We have demonstrated that homozygosity for Arg16 protects against preterm delivery. Our goal was to determine whether beta(2)-agonists are more effective in women with the Arg16 genotype and preterm labor.

METHODS

Sixty white women with preterm labor between 24 and 34 weeks' gestation were treated for 48 hours with intravenous hexoprenaline. The effect of tocolysis and outcome of pregnancy were recorded. The beta(2)AR genotypes at codons 16 and 27 of ADRB2 were determined. A control group of 116 women delivered at term was also genotyped.

RESULTS

Preterm labor was not associated with beta(2)AR genotype at codon 16 (17% of patients with preterm labor were Arg16 homozygotes versus 19% of control subjects) or codon 27. Gestation was significantly prolonged in Arg16 homozygotes (median, 69 days; interquartile range, 63-79 days) compared with the other 2 genotypes (median, 58 days; interquartile range, 2-72 days) (P = .04). Tocolysis was 100% successful in delaying delivery for 48 hours in Arg16 homozygotes (n = 10), just failing to achieve statistical significance (P = .069). In contrast, only 37 of 50 women carrying 1 or 2 glycine alleles (74%) had delivery delayed by more than 48 hours with tocolysis. Neonatal outcomes were significantly better in babies born to mothers homozygous for arginine than in women with 1 or 2 Gly16 alleles.

CONCLUSIONS

This is the first study examining the pharmacogenetics of beta(2)AR agonist therapy for preterm labor. It appears that Arg16 homozygosity improves pregnancy outcome after beta(2)-agonist tocolysis. The relatively low frequency of Arg16 homozygotes in our population limited the power of this investigation. Future assessments of tocolytic therapy may need to assess beta(2)AR genotype.

Transcriptomic profiling of the canine tachycardia-induced heart failure model: global comparison to human and murine heart failure

Alterations of cardiac gene expression are central to ventricular dysfunction in human heart failure (HF). The canine tachycardia pacing-induced HF model is known to reproduce the main hemodynamic, echocardiographic and electrophysiological changes observed in human HF. In this study, we use this HF model to compare gene expression profiles in the left and right ventricles (LV, RV) of normal and end-stage failing canine hearts and compare the transcription profiles to those in human and murine models of HF. In end-stage HF, the LV exhibits down regulation of genes involved in energy production, cardiac contraction, and modulation of excitation-contraction coupling as compared with normal LV. The majority of transcriptomic changes between normal and end-stage canine HF were shared by the RV and LV. Genes down regulated only in the LV included those involved in aerobic energy production pathways, regulation of actin filament length, and enzyme-linked receptor protein signaling pathways. In normal canine hearts, genes encoding specific components of the contractile apparatus exhibit LV-RV asymmetric expression patterns; in failing hearts, cardiac fetal transcription factors MEF2 and MITF and the stress-responsive transcription factor ATF4 showed interventricular differences in expression. The comparison among the canine tachypacing, mouse transgenic, and human HF reveals that human disease involves down regulation of genes in a broad range of biological processes while experimentally induced HF is associated with down regulation of energy pathways, and that human ischemic HF and canine HF share a similar over representation of transcriptional pathways in the up regulated genes. This study provides insights into the molecular pathways leading to end-stage tachycardia-induced HF, and into global transcriptomic differences between the animal HF models and human HF.

Endothelial NO synthase polymorphisms and postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is a heterogeneous disorder characterized by an excessive rise in heart rate and symptoms consistent with cerebral hypoperfusion in the upright position. NO produced by endothelial NO synthase is a significant factor in the regulation of blood flow. Genetic polymorphisms in the promoter region (T-786C) and exon 7 (E298D) of the NO synthase isoform 3 gene affect enzyme activity and have been associated with a number of cardiovascular diseases. Because some findings in POTS suggest aberrant NO-mediated functions, we postulated that the variant genotypes of these polymorphisms may increase the risk of developing POTS and correlate with more severe symptoms. We genotyped 136 patients with POTS (mean age 32.2+/-9.9 years; 46 men and 90 women) from Nashville, Tenn, and Vienna, Austria, and compared them with 191 healthy volunteers (mean age 29.1+/-8.0 years; 127 men and 64 women). Participants also underwent orthostatic testing with blood pressure, heart rate, and plasma norepinephrine measurements while supine and upright. The frequencies of the -786CC and 298DD genotypes were significantly lower in patients with POTS than in control subjects (odds ratio [OR], 0.28; 95% confidence interval [CI], 0.14 to 0.57; P=0.001 for -786CC; and OR, 0.44; 95% CI, 0.21 to 0.91; P=0.033 for 298DD). According to 2-locus genotype analyses, patients with -786CC and 298EE or 298ED experienced the largest changes in heart rate and plasma norepinephrine with standing. These results indicate that NO may influence the development of POTS and the severity of POTS symptoms.

A screen of candidate genes and influence of β2-adrenergic receptor genotypes in postural tachycardia syndrome

OBJECTIVE

To screen candidate genes, encoding beta2-adrenergic receptor (beta2AR), alpha2C-adrenergic receptor (alpha(2C)AR), norepinephrine transporter (NET), and mitochondrial complex I (COI), for common single nucleotide polymorphisms (SNPs) in patients with postural tachycardia syndrome (POTS); alterations could potentially cause or aggravate orthostatic tachycardia and to relate beta2AR SNPs, known to effect venomotor tone, to heart rate (HR) and blood pressure measurements during 10-min head-up tilt.

METHODS

(a) DNA extraction from leukocytes of 29 patients with POTS; (b) Denaturing high performance liquid chromatography analysis to screen for the 12-bp deletion (Del322-325) in alpha(2C)AR and for the alanine to proline mutation at amino acid 457 (Ala457Pro) in NET; (c) Systematic direct sequence analysis to screen for SNPs in beta2AR, NET, and COI.

RESULTS

Three common polymorphisms were abundant in at least one allele in beta2AR resulting in a cysteine to arginine in the 5' promoter region (72% of patients), an arginine to glycine at amino acid-16 (Gly16; 86%), and a glutamine to glutamic acid at amino acid-27 (Glu27; 66%), a frequency that was no different to the normal Caucasian population. Orthostatic HR was significantly greater in patients with Glu27. Diastolic blood pressure (DBP) was significantly lower in a subset of patients with Gly16 whose HR were > or =120 beats/min with head-up tilt. All patients did not show the Ala457Pro mutation of NET; all sequence variants detected in alpha(2C)AR, NET, and COI were not considered causally related to POTS.

CONCLUSIONS

Of the candidate genes screened, none harbored a SNP considered to be causally related to POTS. There was significant association of HR and DBP with SNPs of the gene encoding beta2AR; Gly16 or Glu27 could aggravate orthostatic tachycardia by excessive venous pooling.

Norepinephrine transporter-deficient mice exhibit excessive tachycardia and elevated blood pressure with wakefulness and activity

BACKGROUND

Norepinephrine (NE) is a primary neurotransmitter of central autonomic regulation and sympathetic nerve conduction, and the norepinephrine transporter (NET) is crucial in limiting catecholaminergic signaling. NET is sensitive to antidepressants, cocaine, and amphetamine. NET blockade often is associated with cardiovascular side effects, and NET deficiency is linked to tachycardia in familial orthostatic intolerance.

METHODS AND RESULTS

We telemetrically monitored NET-deficient (NET(-/-)) mice to determine the cardiovascular effects of reduced NE reuptake. Mean arterial pressure was elevated in resting NET(-/-) mice compared with NET(+/+) controls (103+/-0.6 versus 99+/-0.4 mm Hg; P<0.01), and corresponding pressures increased to 122+/-0.3 and 116+/-0.3 mm Hg (P<0.0001) with activity. Heart rate was also greater in resting NET(-/-) mice (565+/-5 versus 551+/-3 bpm; P<0.05), and genotypic differences were highly significant during the active phase (640+/-5 versus 607+/-3 bpm; P<0.0001). Conversely, the respiratory rate of resting NET(-/-) mice was dramatically reduced, whereas increases after the day/night shift surpassed those of controls. Plasma catecholamines in NET(-/-) and NET(+/+) mice were as follows: NE, 69+/-8 and 32+/-7; dihydroxyphenylglycol, 2+0.4 and 17+/-3; epinephrine, 15+/-3 and 4+/-0.6; and dopamine, 13+/-4 and 4+/-1 pmol/mL. Catechols in urine, brain, and heart also were determined.

CONCLUSIONS

Resting mean arterial pressure and heart rate are maintained at nearly normal levels in NET-deficient mice, most likely as a result of increased central sympathoinhibition. However, sympathetic activation with wakefulness and activity apparently overwhelms central modulation, amplifying peripheral catecholaminergic signaling, particularly in the heart.

Severe form of thyroid hormone resistance in a patient with homozygous/hemizygous mutation of T3 receptor gene

Resistance to thyroid hormone syndrome (RTH) is a rare disorder, usually inherited as an autosomal dominant trait. Patients with RTH are usually euthyroid but can occasionally present with signs and symptoms of thyrotoxicosis or rarely with hypothyroidism. Affected individuals are usually heterozygous for mutations in the thyroid hormone receptor beta gene (TR-beta). We present a patient with RTH found to be homo-/hemizygous for a mutation in the TR-beta gene. The single nucleotide substitution I280S (1123T-->G) was present either on both alleles or in a hemizygous form with complete deletion of the second allele. The I280S mutation was recently reported in a heterozygous patient. The severe phenotype with seriously impaired intellectual development, hyperkinetic behaviour, tachycardia, hearing and visual impairment is probably due to the dominant negative effect of the I280S mutant protein and the absence of any functional TR-beta.

The novel missense mutation methionine 442 threonine in the thyroid hormone receptor beta causes thyroid hormone resistance: a case report

We report a family with thyroid hormone resistance caused by a novel mutation M442T in the thyroid hormone receptor beta (TRbeta) gene. The 59-year-old propositus and one of his two daughters had typical clinical signs of reduced responsiveness of tissues to thyroid hormones. Thus, elevated free T (3) and T (4) plasma concentrations in coexistance with a diffuse nodular goiter, nonsuppressed TSH, and atrial fibrillation with tachycardia were present in the propositus. His affected daughter also had increased levels of free T (3) and T (4) with slightly elevated TSH concentrations. Both affected members harboured a heterozygous M442T TRbeta mutation. The unaffected child had no mutation in the TRbeta gene and no clinical manifestations.

Association of angiotensin converting enzyme gene polymorphism with tachycardia cardiomyopathy

Despite incessant tachycardia, not all patients develop tachycardia-mediated cardiomyopathy. The cardiac renin-angiotensin system may be involved in cardiac remodelling and fibrosis. The level of angiotension-converting enzyme (ACE) in the serum is associated with a 287 bp insertion (I)/deletion (D) polymorphism in intron 16 of the ACE gene. The DD genotype is associated with increased serum ACE levels and a higher incidence of idiopathic dilated and ischemic cardiomyopathy. The objective of this study was to assess whether the ACE gene I/D polymorphism is responsible for development of tachycardia-mediated cardiomyopathy. We identified 20 consecutive patients with persistent tachycardia and cardiomyopathy who showed significant improvement in ejection fraction after rate control (group A, tachycardia cardiomyopathy group). We compared the I/D genotype frequency of group A with the gene frequency of a separate group of 20 patents who, despite rapid atrial arrhythmias had preserved left ventricular ejection fraction (group B, tachycardia without cardiomyopathy group). These two groups were then compared with 24 healthy normal volunteers (group C). After a mean follow-up of 30 months, group A patients showed improvement in ejection fraction from 20+/-7 to 43+/-9% (p<0.001). Group A had a significantly higher frequency of the DD genotype than groups B and C (p-value <0.035 and <0.009 respectively). The profile of group B patients was intermediate between normal and patients with tachycardia-mediated cardiomyopathy. I/D polymorphism of the ACE gene may account for cardiomyopathy secondary to tachycardia.

Compound heterozygosity for mutations (W156X and R225W) in SCN5A associated with severe cardiac conduction disturbances and degenerative changes in the conduction system

Cardiac conduction defects associate with mutations in SCN5A, the gene encoding the cardiac Na+ channel. In the present study, we characterized a family in which the proband was born in severe distress with irregular wide complex tachycardia. His older sister died at 1 year of age from severe conduction disease with similarly widened QRS-complexes. Mutational analysis of SCN5A in the proband demonstrated compound heterozygosity for a nonsense mutation (W156X), inherited from the father, and a missense mutation (R225W), inherited from the mother. Genotyping on DNA extracted from tissue from the deceased sibling revealed the same SCN5A genotype. Injection of cRNA encoding the W156X mutation in Xenopus oocytes did not produce any current. The R225W substitution neutralizes the third Arg residue within the voltage-sensing segment of domain I. Expression studies showed that this mutation leads to a severe reduction in I(Na) and is also associated with gating changes. Histological examination of the heart from the deceased sibling revealed changes consistent with a dilated type of cardiomyopathy and severe degenerative abnormalities of the specialized conduction system. The occurrence of compound heterozygosity for these two mutations implies that the proband carries solely severely dysfunctional cardiac Na+ channels. This explains his severe phenotype and that of his deceased sister who had been a carrier of the same genotype. The morphological changes within the heart of the deceased sibling may have occurred secondary to the Na+ channel abnormality and contributed to the severity of the disorder in this individual.

[Arterial hypertension and tachycardia: monitoring the patient's personal drug therapy]

The case report of a young patient with an increase in blood pressure and heart rate offers the opportunity to discuss the clinical guidelines to explore and treat high blood pressure. The value of the 24 h blood pressure monitoring and the need for precise information on all drugs taken are stressed.

Cardiac Characteristics of Transgenic Mice Overexpressing Refsum Disease Gene-Associated Protein within the Heart

Arrhythmia is a common cardiac symptom of Refsum disease. Recently, we identified a novel neuron-specific PAHX-associated protein (PAHX-AP1), which binds to the Refsum disease gene (PAHX). In this report, we developed heart-targeted transgenic (TG) mice under the control of alpha-myosin heavy chain promoter to determine whether cardiac overexpression of PAHX-AP1 provokes cardiac involvement symptoms. Northern and in situ hybridization analyses revealed PAHX-AP1 transcript was overexpressed in TG atrium, especially in the sinoatrial node. TG mice showed tachycardia, and tachyarrhythmia was observed in 20% of TG mice. Isolated TG atria showed higher frequency beating and were more sensitive to aconitine-induced tachyarrhythmia than the wild-type, and 40% of the TG atria showed irregular beating. Action potential duration in TG atrial fiber was shortened much more than the wild-type. Systemic administration of arrhythmogenic agents induced arrhythmia in TG mice, while no arrhythmia with the same dose in nonTG mice. Our results indicate that the chronic atrial tachycardia by overexpressed neuron-specific PAHX-AP1 transgene in atrium may be responsible for the increased susceptibility to arrhythmia.

Characterization of mice with a combined suppression of I(to) and I(K,slow)

Cardiac-specific expression of a truncated Kv1.1 polypeptide (Kv1DN) attenuates the slow inactivating outward K(+) current (I(K,slow)), increases action potential duration (APD) and Q-T intervals, and induces spontaneous ventricular arrhythmias. Expression of the pore mutant of Kv4.2 (Kv4DN) eliminates the fast component of the transient outward current (I(to)) and prolongs APDs and Q-T intervals markedly; however, no arrhythmias are seen in Kv4DN mice, suggesting that APD and Q-T prolongation are not per se proarrhythmic. To test this hypothesis, the Kv1DN and Kv4DN lines were crossbred to produce animals (Kv1/Kv4DN) expressing both transgenes in an identical genetic background. Whole cell voltage-clamp recordings from left ventricular apex cells confirmed that in Kv1/Kv4DN left ventricular apex cells, both components (fast and slow) of I(to) and the 4-aminopyridine-sensitive component of I(K,slow) are eliminated, resulting in marked APD prolongation compared with wild-type, Kv1DN, or Kv4DN cells. Telemetric electrocardiogram monitoring (n = 10 mice/group) revealed a significant prolongation of Q-Tc and P-R intervals in Kv1/Kv4DN animals compared with Kv1DN or Kv4DN animals. Spontaneous arrhythmias were observed mainly in Kv1DN mice. Thus the attenuation of fast I(to) in addition to I(K,slow) in Kv1/Kv4DN mice causes significant prolongation of APD and Q-T intervals and attenuation of spontaneous arrhythmias.

Familial orthostatic tachycardia due to norepinephrine transporter deficiency

Orthostatic intolerance (OI) or postural tachycardia syndrome (POTS) is a syndrome primarily affecting young females, and is characterized by lightheadedness, palpitations, fatigue, altered mentation, and syncope primarily occurring with upright posture and being relieved by lying down. There is typically tachycardia and raised plasma norepinephrine levels on upright posture, but little or no orthostatic hypotension. The pathophysiology of OI is believed to be very heterogeneous. Most studies of the syndrome have focused on abnormalities in norepinephrine release. Here the hypothesis that abnormal norepinephrine transporter (NET) function might contribute to the pathophysiology in some patients with OI was tested. In a proband with significant orthostatic symptoms and tachycardia, disproportionately elevated plasma norepinephrine with standing, impaired systemic, and local clearance of infused tritiated norepinephrine, impaired tyramine responsiveness, and a dissociation between stimulated plasma norepinephrine and DHPG elevation were found. Studies of NET gene structure in the proband revealed a coding mutation that converts a highly conserved transmembrane domain Ala residue to Pro. Analysis of the protein produced by the mutant cDNA in transfected cells demonstrated greater than 98% reduction in activity relative to normal. NE, DHPG/NE, and heart rate correlated with the mutant allele in this family.

CONCLUSION

These results represent the first identification of a specific genetic defect in OI and the first disease linked to a coding alteration in a Na+/Cl(-)-dependent neurotransmitter transporter. Identification of this mechanism may facilitate our understanding of genetic causes of OI and lead to the development of more effective therapeutic modalities.

Orthostatic intolerance and the postural tachycardia syndrome: genetic and environment pathophysiologies. Neurolab Autonomic Team

Orthostatic intolerance is a common problem for inbound space travelers. There is usually tachycardia on standing but blood pressure may be normal, low or, rarely, elevated. This condition is analogous to the orthostatic intolerance that occurs on Earth in individuals with orthostatic tachycardia, palpitations, mitral valve prolapse, and light-headedness. Our studies during the Neurolab mission indicated that sympathetic nerve traffic is raised in microgravity and that plasma norepinephrine is higher than baseline supine levels but lower than baseline upright levels. A subgroup of patients with familial orthostatic intolerance differ from inbound space travelers in that they have an alanine-to-to-proline mutation at amino acid position 457 in their norepinephrine transporter gene. This leads to poor clearance of norepinephrine from synapses, with consequent raised heart rate. Clinical features of these syndromes are presented.

Molecular genetics of arrhythmias--a new paradigm

The molecular genetic background of inherited cardiac arrhythmias has only recently been uncovered. This late development in comparison to other inherited cardiac disorders has partly been due to the high mortality and early disease onset of these arrhythmias resulting in mostly small nucleus families. Thus, traditional genetic linkage studies, which are based on the genetic information obtained from large multi-generation families, were made difficult. Inherited arrhythmogenic disorders can be divided into 'primary electrical disorders' (e.g., long-QT [LQT] syndrome) in which a detectable, organic heart disease is not evident, and into inherited diseases of the myocardial structure (e.g., hypertrophic cardiomyopathies) in which the arrhythmias occur combined with the structural alterations. To date, all inherited arrhythmogenic disorders in which the causative genes have been identified turned out to be channelopathies, since the genes encode channel subunits that regulate important ion currents that tune the cardiac action potential. The discovery of the genetic bases of the LQT syndrome became a new methodologic paradigm; because with the use of 'classical' genetic linkage strategies (named [positional] candidate strategies) not only the causative genes have been found, but moreover, functional components with a previously unknown but fundamental role for a normal repolarization process were discovered. Disease mutations turned out to be not only a family-specific event with a distinct phenotype and the potential of an additional diagnostic tool, but also, when expressed in heterologous expression systems, characterize the defective ion channel in a topological way and lead to a more specific understanding of ion channel function. Most, if not all, primary electrical cardiac disorders show a high genetic diversity. For the LQT syndromes, sixth disease loci and the responsible gene have been recently discovered (so-called locus or genetic heterogeneity). Within all disease genes, the mutations are spread over the entire gene (allelic heterogeneity); in addition, more than one disease mutation may be present. This complexity requires, at least, complete mutation analysis of all LQT genes before medical advice should be given. Meanwhile, genotype-phenotype correlations in large families are being used to evaluate intergene, interfamilial and intrafamilial differences in the clinical phenotype, reflecting gene specific, gene-site specific and individual consequences of a given mutation. A widespread phenotypic heterogeneity even within mutation carriers in the same family raises the importance of modifying factors and genes that are mostly unknown to date. The reduced penetrance and variable expressivity associated with the LQT mutations remain still to be explained. First insights into the complex actions of mutations are being extracted, from expression data; these preliminary results may lead to potential implications for a specific (gene-site directed) therapy. This paper discusses the current data on molecular genetics and genotype-phenotype correlations in LQT syndrome and related disorders and the potential implications for diagnosis and treatment.

Orthostatic intolerance and tachycardia associated with norepinephrine-transporter deficiency

BACKGROUND

Orthostatic intolerance is a syndrome characterized by lightheadedness, fatigue, altered mentation, and syncope and associated with postural tachycardia and plasma norepinephrine concentrations that are disproportionately high in relation to sympathetic outflow. We tested the hypothesis that impaired functioning of the norepinephrine transporter contributes to the pathophysiologic mechanism of orthostatic intolerance.

METHODS

In a patient with orthostatic intolerance and her relatives, we measured postural blood pressure, heart rate, plasma catecholamines, and systemic norepinephrine spillover and clearance, and we sequenced the norepinephrine-transporter gene and evaluated its function.

RESULTS

The patient had a high mean plasma norepinephrine concentration while standing, as compared with the mean (+/-SD) concentration in normal subjects (923 vs. 439+/-129 pg per milliliter [5.46 vs. 2.59+/-0.76 nmol per liter]), reduced systemic norepinephrine clearance (1.56 vs. 2.42+/-0.71 liters per minute), impairment in the increase in the plasma norepinephrine concentration after the administration of tyramine (12 vs. 56+/-63 pg per milliliter [0.07 vs. 0.33+/-0.37 pmol per liter]), and a disproportionate increase in the concentration of plasma norepinephrine relative to that of dihydroxyphenylglycol. Analysis of the norepinephrine-transporter gene revealed that the proband was heterozygous for a mutation in exon 9 (encoding a change from guanine to cytosine at position 237) that resulted in more than a 98 percent loss of function as compared with that of the wild-type gene. Impairment of synaptic norepinephrine clearance may result in a syndrome characterized by excessive sympathetic activation in response to physiologic stimuli. The mutant allele in the proband's family segregated with the postural heart rate and abnormal plasma catecholamine homeostasis.

CONCLUSIONS

Genetic or acquired deficits in norepinephrine inactivation may underlie hyperadrenergic states that lead to orthostatic intolerance.

Tachycardia: an autosomal, monogenic, biallelic, recessive trait

A brief commentary of the genetics of blood pressure is presented. The importance of the mechanisms of blood pressure regulation, among which heart rate is relevant, is emphasized. The analysis of a small population to test the quantitative model of the trait heart rate, considered as a metric character, is presented. The analysis of heart rate fitting to a qualitative model of inheritance is carried out. The results displayed might support the hypothesis that tachycardia could be an autosomal, monogenic, biallelic recessive trait.

Interaction of genetic predisposition and environmental factors in the pathogenesis of idiopathic orthostatic intolerance

BACKGROUND

The hemodynamic and autonomic abnormalities in idiopathic orthostatic intolerance (IOI) have been studied extensively. However, the mechanisms underlying these abnormalities are not understood. If genetic predisposition were important in the pathogenesis of IOI, monozygotic twins of patients with IOI should have similar hemodynamic and autonomic abnormalities.

METHODS

We studied two patients with IOI and their identical twins. Both siblings in the first twin pair had orthostatic symptoms, significant orthostatic tachycardia, increased plasma norepinephrine levels with standing, and a greater than normal decrease in systolic blood pressure with trimethaphan infusion.

RESULTS

Both siblings had a normal response of plasma renin activity to upright posture. In the second twin pair, only one sibling had symptoms of orthostatic intolerance, an orthostatic tachycardia, and raised plasma catecholamines with standing. The affected sibling had inappropriately low plasma renin activity with standing and was 8-fold more sensitive to the pressor effect of phenylephrine than the unaffected sibling.

CONCLUSIONS

We conclude that in some patients, IOI seems to be strongly influenced by genetic factors. In others, however, IOI may be mainly caused by nongenetic factors. These findings suggest that IOI is heterogenous, and that both genetic and environmental factors contribute individually or collectively to create the IOI phenotype.

Familial neonatal atrial tachycardia

A father and his two sons each presented with atrial tachycardia in the newborn period. The father went on to develop dilated cardiomyopathy. The first son (who also had transposition of the great arteries) died from the arrhythmia after surgery. The second son is currently successfully managed pharmacologically.

A newly recognized autosomal dominant limb girdle muscular dystrophy with cardiac involvement

Sixty-five members of three families with limb girdle muscular dystrophy (LGMD) underwent neurological, cardiological, and ancillary investigations. Thirty-five individuals were diagnosed as having slowly progressive autosomal dominant LGMD. Symmetrical weakness started in the proximal lower limb muscles, and gradually upper limb muscles also became affected. Early contractures of the spine were absent. Contractures of elbows and Achilles tendons were either minimal or late. Serum creatine kinase activity was normal to moderately elevated. Electromyogram and muscle biopsy were consistent with a mild muscular dystrophy. Cardiological abnormalities, found in more than one-half the patients, included dysrhythmias and atrioventricular (AV) conduction disturbances presenting as bradycardia, syncopal attacks necessitating pacemaker implantation, and sudden cardiac death. There was a significant relation between the severity of AV conduction disturbances and age. In nearly all patients, neuromuscular symptomatology preceded cardiological involvement. The early recognition of this previously not described, autosomal dominant LGMD with life-threatening cardiac involvement offers an opportunity for therapeutic intervention.

Familial bidirectional ventricular tachycardia

The occurrence of incessant ventricular arrhythmia with bouts of polymorphous and bidirectional ventricular tachycardia is described in two totally asymptomatic sisters, with structurally normal hearts and no QT prolongation. Familial survey revealed the occurrence of increased ventricular ectopic activity in the father, brother and another sister.

Familial ventricular tachycardia: a report of four families

Four cases of familial ventricular tachycardia are presented. In each family the proband was an adolescent girl. Twelve members in the four families were affected and all were female. The clinical and electrocardiographic features were relatively constant within each family but there were striking differences between families. These differences argue against a common electrophysiological or pathophysiological basis for the ventricular tachycardia.

Juvenile sudden death and effort ventricular tachycardias in a family with right ventricular cardiomyopathy

A family with occurrence of juvenile sudden death and effort polymorphous ventricular tachycardias is reported. Nineteen members aged 9 to 63 years were investigated. Four of them died suddenly in their youth. Postmortem investigation performed in 2 deceased subjects disclosed an apparently normal heart at macroscopy but fibro-fatty substitution of the right ventricular free wall was noted at histologic examination. The 14 living members underwent physical examination, resting electrocardiography, chest X-radiography, Holter monitoring, exercise stress testing, and M-mode and cross-sectional echocardiography. Four patients underwent hemodynamic and electrophysiologic studies. All 14 subjects had normal physical examination as well as normal electrocardiographic and cardiothoracic indices. Localized right ventricular structural and dynamic abnormalities were noted at cross-sectional echocardiographic and angiographic investigation of 9 of the patients. The right ventricular volumes in these subjects were normal or slightly increased. In 7 of them, polymorphous ventricular tachycardias were induced by exercise stress testing. The arrhythmias which were responsive to beta-blockade, do not seem to depend on reentry. Enhanced automaticity appeared to be the more likely mechanism of their production. These data demonstrate that right ventricular cardiomyopathy may occur in an occult form with life-threatening electrical instability.