Orthostatic Intolerance Is Not Necessarily Related to a Specific Mutation (Ala457Pro) in the Human Norepinephrine Transporter Gene

doi:10.1097/00000441-200302000-00002

The American Journal of the Medical Sciences

Volume 325, Issue 2, February 2003, Pages 63-65

https://doi.org/10.1097/00000441-200302000-00002 Get rights and content

ABSTRACT

Background

Orthostatic intolerance (OI) is a syndrome characterized by lightheadedness, palpitations, fatigue, blurred vision, dizziness, chest discomfort, cognitive impairment, and occasionally syncope. These symptoms usually occur after upright posture and are associated with tachycardia and high plasma concentrations of norepinephrine. It has been proposed that a mutation in exon 9 of the norepinephrine transporter gene (Ala457Pro), resulting in more than 98% loss of function compared with the wild type, might provide a pathogenetic mechanism to explain the clinical symptoms of patients with OI.

Methods

We studied 46 young men from military service who had sought medical advice because of dizziness while standing. Every patient underwent a tilt-table test, with monitoring of blood pressure, heart rate, and plasma catecholamines in supine position and during 30 minutes of standing. Fourteen patients showing the full-blown OI syndrome (30 bpm increase in heart rate and 600 pg/mL plasma norepinephrine levels while standing) underwent direct DNA sequencing of exon 9 of the norepinephrine-transporter gene.

Results and Conclusions

The specific mutation (Ala457Pro) was not detected in any of the 14 OI patients. Based on these findings, we doubt that this specific genetic transport defect is a frequent cause of the impaired uptake of norepinephrine in OI patients. Its routine determination will therefore not be helpful to establish the clinical diagnosis of OI.

Section snippets

Methods

From December 2000 to October 2001, we studied 46 soldiers from the Austrian Army who sought medical advice because of dizziness while standing. All patients had a medical check when entering the army several months before and had been found healthy. The patients did not exhibit symptoms of a systemic illness that might affect the autonomic nervous system and did not take medications. The Austrian Science Fund review board and the ethics committee of the university Vienna approved the study.

Results and Discussion

Basic characteristics of the 14 OI patients are presented in Table 1. Patients with OI exhibited a mean heart rate of 108 ± 7 beats/min and substantially higher plasma norepinephrine concentration in the upright position. These 14 patients with OI syndrome underwent direct DNA sequencing of exon 9 of the NET gene, with respect to the specific mutation (Ala457Pro). Figure 1 shows DNA sequence patterns of the identified mutant and of an investigated patient with OI.

The specific mutation could not

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Cited by (15)

Postural Orthostatic Tachycardia Syndrome: JACC Focus Seminar
2019, Journal of the American College of Cardiology
Citation Excerpt :
A single case of POTS with elevated standing NE levels was found to have a mutation in NET, encoded by the SLC6A2 gene, causing loss of function; this observation led to speculation that impairment of synaptic NE clearance and excessive sympathetic activation may underlie some forms of hyperadrenergic POTS. More focused studies attempting to tie genetic variations in the SLC6A2 gene to elevated NE levels, however, have been inconclusive (39,40). More common than genetic NET deficiency is NET blockade from pharmacological agents, such as NE reuptake inhibitors (e.g., bupropion), tricyclic antidepressants (e.g., nortriptyline), and other sympathomimetic drugs (e.g., methylphenidate) (15).
Postural orthostatic tachycardia syndrome (POTS), the most common form of orthostatic intolerance in young people, affects approximately 500,000 people in the United States alone, typically young women at the peak of their education and the beginning of their working lives. This is a heterogeneous disorder, the pathophysiology and mechanisms of which are not well understood. There are multiple contributing factors and numerous potential mimics. This review details the most current views on the potential causes, comorbid conditions, proposed subtypes, differential diagnoses, evaluations, and treatment of POTS from cardiological and neurological perspectives.
Epigenomic changes associated with impaired norepinephrine transporter function in postural tachycardia syndrome
2017, Neuroscience and Biobehavioral Reviews
Citation Excerpt :
Over expression of miR19a-3p in SK-N-BE (2) and HEK293 cells down regulated SLC6A2 transcript by creating microRNA binding site (Marques et al., 2016). The specific polymorphism A457P was not detected in 14 male POTS patients with increased heart rate and plasma NE levels (Ivancsits et al., 2003). Nikander et al. screened 29 POTS patients for A457P mutation using denatured high performance liquid chromatography (DHPLC) and failed to show association of A457P mutation with POTS.
The postural tachycardia syndrome (POTS) is characterised clinically by symptoms of light-headedness, palpitations, fatigue and exercise intolerance occurring with standing and relieved by lying down. Symptoms occur in association with an inappropriate rise in heart rate in the absence of a fall in blood pressure with the assumption of standing. The pathophysiology of POTS is complicated and poorly understood. Plasma norepinephrine (NE) is often elevated in patients with POTS, resulting in consideration of dysfunction of the norepinephrine transporter (NET) encoded by SLC6A2 gene. Whilst some studies have implicated a defect in the SLC6A2 gene, the cause of reduced SLC6A2 expression and function remains unclear. The search to explain the molecular mechanism of NET dysfunction has focused on genetic variation in the SLC6A2 gene and remains inconclusive. More recent studies show epigenetic mechanisms implicated in the regulation of SLC6A2 expression. In this article, we discuss the epigenetic mechanisms involved in SLC6A2 repression and highlight the potential therapeutic application of targeting these mechanisms in POTS.
Influence of an insertion variant in the 5′UTR of the endothelin-1 gene on orthostatic intolerance
2005, American Journal of the Medical Sciences
Orthostatic intolerance is a multifactorial disease in which the genetic contribution is probably the result of a number of genes acting in combination. Recent work has shown that orthostatic intolerance is influenced by endothelial nitric oxide synthase gene polymorphisms. Since endothelin-1 (ET-1) is one of the most important vasoconstrictor peptides, a frequent adenine insertion polymorphism within the 5ʹ-untranslated region (5ʹUTR), which is of functional importance for ET-1 expression, could influence orthostatic intolerance. The aim of this study was therefore to ascertain whether this frequent variant of the endothelin-1 gene influences the risk for orthostatic intolerance.
We studied 257 white patients (120 cases with orthostatic intolerance and 137 controls) for genotyping of the 5ʹUTR I variant. From this cohort, 111 patients and 99 control subjects underwent a tilttable test or an upright posture study, including monitoring of blood pressure, heart rate, and plasma catecholamines, in the supine position and during 30 minutes of standing. Genotyping was performed in all participants. χ² tests of independence were used to test for associations between orthostatic intolerance and genotype. In addition, an association of the insertion polymorphism with hemodynamic variables (heart rate, supine and upright blood pressure) was ascertained using one-way analysis of variance.
The 5ʹUTR I variant was significantly less common in patients with orthostatic intolerance (allele frequency 0.36 and 0.28, in controls and cases, respectively). Additionally, we found a significant decrease in the risk of orthostatic intolerance among people who were homozygous for the 5ʹUTR variant (I/I) compared with the wild-type variant (D/D) (odds ratio, 0.41; 95% confidence interval, 0.17 to 0.97; P = 0.04). No association between the 5ʹUTR variant and heart rate or blood pressure regardless of diagnosis was found.
Our current results suggest that the hereditary adenine insertion variant in the 5ʹ-UTR of the endothelin-1 gene is protective for orthostatic intolerance. The increased ET-1 protein expression that has been linked with the I variant might be associated with a more efficient hemodynamic response to standing. This is likely one of several common genetic loci that may represent modifiers of orthostatic intolerance phenotypes.
Regulated Internalization and Phosphorylation of the Native Norepinephrine Transporter in Response to Phorbol Esters: Evidence for localization in lipid rafts and lipid raft-mediated internalization
2004, Journal of Biological Chemistry
The effects of norepinephrine in the brain and periphery are terminated primarily by active reuptake of the catecholamine via cocaine- and amphetamine-sensitive norepinephrine transporters (NETs). Activation of protein kinase C (PKC) down-regulates NET by sequestering it from the plasma membrane, although the underlying mechanism is not yet known. Previously, we showed robust expression of endogenous NETs in rat placental trophoblasts (Jayanthi, L. D., Vargas, G., and DeFelice, L. J. (2002) Br. J. Pharmacol. 135, 1927-1934). Here we report a significant reduction in native NET function and surface expression in these cells following phorbol ester (β-PMA) treatment. The β-PMA-mediated down-regulation of NET occurs by a rapid sequestration of NETs from the plasma membrane and is calcium-independent. Reversible biotinylation experiments revealed a significant enhancement of NET endocytosis following β-PMA treatment. Chemical treatments and expression of dominant negative mutants of dynamin 1 and 2 failed to prevent the β-PMA effect, suggesting a clathrin-independent pathway. In contrast, treatment with the cholesterol-disrupting agent filipin, which blocks caveolae/lipid raft-mediated internalization, completely blocked the β-PMA-mediated NET sequestration. Discontinuous sucrose density gradient centrifugation revealed NET in the lipid raft fractions. Following β-PMA treatment, there was reduced NET levels in the lipid raft fractions suggesting that cholesterol-rich lipid rafts mediate PKC-triggered NET internalization. Metabolic labeling and immunoprecipitation studies revealed that NET phosphorylation is stimulated severalfold by PKC activation and protein phosphatase 1/2A inhibition. Together, these findings demonstrate for the first time that in trophoblasts (i) PKC activation regulates NET function and surface expression by an enhanced internalization process that is lipid raft-mediated and (ii) PKC and protein phosphatase(s) modulation regulates NET phosphorylation.
Recent advances in the understanding of the mechanisms underlying postural tachycardia syndrome in children: Practical implications for treatment
2017, Cardiology in the Young
The Important Role of Transporter Structures in Drug Disposition, Efficacy, and Toxicity
2023, Drug Metabolism and Disposition

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ArticlesOrthostatic Intolerance Is Not Necessarily Related to a Specific Mutation (Ala457Pro) in the Human Norepinephrine Transporter Gene

ABSTRACT

Background

Methods

Results and Conclusions

Section snippets

Methods

Results and Discussion

Am J Med Sci

Am J Med

Am J Med

Pathogenesis of hyperadrenergic orthostatic hypotension. Evidence of disordered venous innervation exclusively in the lower limbs

J Clin Invest

Hyperdynamic beta-adrenergic circulation state. Increased beta-receptor responsiveness

Arch Intern Med

Articles
Orthostatic Intolerance Is Not Necessarily Related to a Specific Mutation (Ala457Pro) in the Human Norepinephrine Transporter Gene