Alternative titles; symbols
HGNC Approved Gene Symbol: SLC6A2
SNOMEDCT: 870368003;
Cytogenetic location: 16q12.2 Genomic coordinates (GRCh38): 16:55,655,988-55,706,192 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 16q12.2 | ?Orthostatic intolerance | 604715 | Autosomal dominant | 3 |
The SLC6A2 gene encodes a norepinephrine (noradrenaline) transporter, which is responsible for reuptake of norepinephrine into presynaptic nerve terminals and is a regulator of norepinephrine homeostasis (Kim et al., 2006).
Pacholczyk et al. (1991) isolated a cDNA encoding a human noradrenaline transporter. The cDNA sequence predicted a protein of 617 amino acids, with 12-13 highly hydrophobic regions compatible with membrane-spanning domains. Expression of the cDNA clone in transfected HeLa cells indicated that noradrenaline transport activity is sodium-dependent and sensitive to selective noradrenaline transport inhibitors. Transporter RNA was localized to the brainstem and adrenal gland. The predicted protein sequence demonstrated significant amino acid identity with the Na(+)/gamma-aminobutyric acid transporter, thus identifying a new gene family for neurotransmitter transporter proteins.
Fritz et al. (1998) cloned the murine norepinephrine transporter gene, which they symbolized Slc6a5 (rather than Slc6a2).
The primary mechanism of inactivation of norepinephrine in the synapse is uptake into the neuron by the norepinephrine transporter. Approximately 80 to 90% of the norepinephrine released into many synapses is cleared by this mechanism, and the remaining 10 to 20% spills over into the circulation or extraneuronal tissue (Esler et al., 1990).
Pacholczyk et al. (1991) noted that uptake systems for the biogenic amines are the initial site of action for therapeutic antidepressants and drugs such as cocaine and the amphetamines. The reuptake of noradrenaline occurs via a specific Na(+)- and Cl(-)-dependent transport system which is the target for tricyclic antidepressants such as desipramine and imipramine.
Porzgen et al. (1995) isolated genomic phage clones of NET1. They reported that the gene is encoded by 14 exons spanning 45 kb. The intron/exon organization of the gene is homologous to other known neurotransmitter transporter genes, except for the presence of an additional exon encoding the C terminus of the protein.
Crystal Structure
Zhou et al. (2007) determined the crystal structure at 2.9 angstroms of the bacterial leucine transporter (LeuT), a homolog of SERT (182138), NET, and DAT (126455), in complex with leucine and the antidepressant desipramine. Desipramine binds at the inner end of the extracellular cavity of the transporter and is held in place by a hairpin loop and by a salt bridge. This binding site is separated from the leucine-binding site by the extracellular gate of the transporter. By directly locking the gate, desipramine prevents conformational changes and blocks substrate transport. Mutagenesis experiments on human SERT and DAT indicate that both the desipramine-binding site and its inhibition mechanism are probably conserved in the human neurotransmitter transporters.
By hybridization of a panel of somatic cell hybrids and by fluorescence in situ hybridization to metaphase chromosomes, Bruss et al. (1993) mapped the NET1 gene to chromosome 16q12.2. Gelernter et al. (1993) reported a TaqI RFLP at the NET1 locus. Gelernter et al. (1993) used PCR with a somatic cell hybrid panel to obtain a provisional assignment of the NET1 gene to chromosome 16. They typed the genetic polymorphism at the NET1 locus in 3 large multigenerational families and, by linkage analysis, confirmed the preliminary assignment and refined the localization to 16q, near the haptoglobin locus (HP; 140100). They then typed the NET1 RFLP on the CEPH families; the additional linkage data localized NET1 to 16q13-q21, flanked by D16S71 centromerically and HP telomerically.
Fritz et al. (1998) mapped the corresponding mouse gene to mouse chromosome 8 in a region with homology to human 16q and noted that this site in the mouse is in the region of a quantitative trait locus for ethanol sensitivity.
Keller et al. (2004) found that NET-deficient (NET -/-) mice had increased norepinephrine (NE) and decreased intracellular metabolites dihydroxyphenylglycol (DHPG) in plasma and urine compared with controls. However, NE was significantly decreased in brain and heart of NET -/- mice. Mean arterial pressure (MAP) was elevated in NET -/- mice both at rest and with activity. Resting heart rate was greater in NET -/- than in wildtype mice, and genotypic differences were highly significant during the active phase compared with control mice. NET -/- mice had consistently lower resting respiratory rates than wildtype mice, with highly significant genotypic differences; active values, however, were nearly equivalent. Locomotor activity was evaluated as a possible factor in the elevated hemodynamic measurements of NET -/- mice. Movements at rest did not vary between genotypes, although NET-/- mice were significantly more active.
Shirey-Rice et al. (2013) generated NET-deficient mice by knocking in a mutation orthologous to the human A457P mutation (163970.0001) found in individuals with postural orthostatic tachycardia syndrome (POTS). The knockin mice appeared healthy, bred with expected genotype distributions, and exhibited no differences in body weight among genotypes for either male or female mice. Mice carrying one A457P allele (NET +/P) exhibited reduced NE transport levels compared with wildtype (NET +/+) mice, and such transport deficits extended to noradrenergic neurons in the central nervous system (CNS) as NE transport levels were reduced in brain and sympathetic nerves, resembling the phenotype in heterozygous human A457P carriers. The transport activity in mice carrying two A457P alleles (NET P/P) was nearly abolished due to nonfunctional transporter despite high protein expression levels. NET deficiency altered cardiovascular function in the knockin mice similar to that of human A457P carriers, as NET +/P and NET P/P mice exhibited elevation of plasma and urine NE levels, reduced 3,4-dihydroxyphenylglycol (DHPG), and reduced DHPG:NE ratios; NET +/P mice did not show changes in blood pressure or baroreceptor sensitivity; and NET +/P mice demonstrated behavioral changes consistent with CNS NET dysfunction.
Orthostatic intolerance (604715) is a syndrome characterized by lightheadedness, fatigue, altered mentation, and syncope, and is associated with postural tachycardia and plasma norepinephrine concentrations that are disproportionately high in relation to sympathetic outflow. Most attempts to explain the abnormalities associated with orthostatic intolerance had focused on an increased release of norepinephrine in response to the change from a supine to an upright position. An alternative explanation was that there is an abnormality in the clearance of norepinephrine from the synaptic cleft. Shannon et al. (2000) noted that drugs that inhibit the norepinephrine transporter (e.g., cocaine, amphetamines, and tricyclic antidepressants) cause features typical of orthostatic intolerance (tachycardia, orthostatic symptoms, and high plasma catecholamine concentrations). In a proband and her identical twin with orthostatic intolerance and clinical and laboratory signs of disordered uptake of norepinephrine, Shannon et al. (2000) identified a heterozygous mutation in the SLC6A2 gene (163970.0001) that resulted in more than 98% loss of function as compared with the wildtype gene. The mutant allele in the proband's family segregated with the postural heart rate and abnormal plasma catecholamine homeostasis. Allele-specific oligonucleotide hybridization studies showed that the mother and 4 of the proband's sibs, including her twin, were heterozygous for the mutant allele. While supine, their heart rates were slightly but not significantly greater than those of their family members with the wildtype genotype, but their heart rates while standing were significantly higher. Likewise, plasma norepinephrine concentrations in a supine position were somewhat higher, and plasma norepinephrine concentrations in the upright position were significantly higher in the heterozygous family members than in those with the normal genotype.
Long-term weight-restored patients with anorexia nervosa (606788) have lower norepinephrine levels than controls (Kaye et al., 1985; Pirke et al., 1992). Since this may reflect altered reuptake by the norepinephrine transporter, Urwin et al. (2002) hypothesized that the NET gene may be involved in the genetic component of anorexia nervosa. PCR amplification of an AAGG repeat island (AAGG1) in the NET gene promoter region revealed a novel 343-bp sequence with 5 additional AAGG repeat islands (AAGG2-AAGG6). The sequence from AAGG1 to AAGG6, inclusive, was designated the NET gene promoter polymorphic region. A 4-bp deletion or insertion in AAGG4 resulted in the net loss or gain, respectively, of a putative Elk1 transcription factor site. The transmission disequilibrium test with 87 Australian trios (patient plus parents) demonstrated significant preferential transmission of the 4-bp insertion from parent to child with restricting anorexia nervosa, suggesting that this allele, or a DNA variant in linkage disequilibrium with it, doubles the risk for developing RAN.
Kim et al. (2006) identified a -3081A/T polymorphism in the promoter of the SLC6A2 gene. In vitro studies showed that the -3081T allele significantly decreased promoter function compared to the A allele. Database and EMSA analysis showed that the T allele creates a palindromic E2-box motif that interacts with Slug (SNAI2; 602150) and Scratch (SCRT1; 605858), which are neural-expressed transcription repressors. Kim et al. (2006) presented preliminary evidence suggesting an association between the -3081T allele and attention-deficit hyperactivity disorder (ADHD; 143465) among 81 ADHD patients who were not of African American background.
In affected members of a family with orthostatic intolerance (604715), Shannon et al. (2000) identified a heterozygous G-to-C transversion in exon 9 (nucleotide 237 of that exon) of the SLC6A2 gene, resulting in an ala457-to-pro (A457P) substitution. The mutation was identified in a mother and 5 of her 9 children.
To explore the reason for the deficiency in NET function associated with heterozygosity for the A457P variant, Paczkowski et al. (2002) compared the pharmacology of the variant with that of wildtype NET in COS-7 cells transfected with the 2 forms of the transporter. Compared to wildtype NET, the A457P variant exhibited a 5-fold higher affinity for cocaine, but a 2-fold lower affinity for a NET inhibitor and an unchanged affinity for an antidepressant. Other findings indicated that loss of function of the A457P variant is only partly due to a reduction in plasma membrane expression of the transporter, and is mainly caused by the pronounced reduction in the apparent affinity of norepinephrine.
Bruss, M., Kunz, J., Lingen, B., Bonisch, H. Chromosomal mapping of the human gene for the tricyclic antidepressant-sensitive noradrenaline transporter. Hum. Genet. 91: 278-280, 1993. [PubMed: 8478011] [Full Text: https://doi.org/10.1007/BF00218272]
Esler, M., Jennings, G., Lambert, G., Meredith, I., Horne, M., Eisenhofer, G. Overflow of catecholamine neurotransmitters to the circulation: source, fate, and functions. Physiol. Rev. 70: 963-985, 1990. [PubMed: 1977182] [Full Text: https://doi.org/10.1152/physrev.1990.70.4.963]
Fritz, J. D., Jayanthi, L. D., Thoreson, M. A., Blakely, R. D. Cloning and chromosomal mapping of the murine norepinephrine transporter. J. Neurochem. 70: 2241-2251, 1998. [PubMed: 9603188] [Full Text: https://doi.org/10.1046/j.1471-4159.1998.70062241.x]
Gelernter, J., Kruger, S., Kidd, K. K., Amara, S. TaqI RFLP at norepinephrine transporter protein (NET) locus. Hum. Molec. Genet. 2: 820 only, 1993. [PubMed: 8102573] [Full Text: https://doi.org/10.1093/hmg/2.6.820-a]
Gelernter, J., Kruger, S., Pakstis, A. J., Pacholczyk, T., Sparkes, R. S., Kidd, K. K., Amara, S. Assignment of the norepinephrine transporter protein (NET1) locus to chromosome 16. Genomics 18: 690-692, 1993. [PubMed: 7905857] [Full Text: https://doi.org/10.1016/s0888-7543(05)80375-1]
Kaye, W. H., Jimerson, D. C., Lake, C. R., Ebert, M. H. Altered norepinephrine metabolism following long-term weight recovery in patients with anorexia nervosa. Psychiat. Res. 14: 333-342, 1985. [PubMed: 3860886] [Full Text: https://doi.org/10.1016/0165-1781(85)90101-5]
Keller, N. R., Diedrich, A., Appalsamy, M., Tuntrakool, S., Lonce, S., Finney, C., Caron, M. G., Robertson, D. Norepinephrine transporter-deficient mice exhibit excessive tachycardia and elevated blood pressure with wakefulness and activity. Circulation 110: 1191-1196, 2004. [PubMed: 15337696] [Full Text: https://doi.org/10.1161/01.CIR.0000141804.90845.E6]
Kim, C.-H., Hahn, M. K., Joung, Y., Anderson, S. L., Steele, A. H., Mazei-Robinson, M. S., Gizer, I., Teicher, M. H., Cohen, B. M., Robertson, D., Waldman, I. D., Blakely, R. D., Kim, K.-S. A polymorphism in the norepinephrine transporter gene alters promoter activity and is associated with attention-deficit hyperactivity disorder. Proc. Nat. Acad. Sci. 103: 19164-19169, 2006. [PubMed: 17146058] [Full Text: https://doi.org/10.1073/pnas.0510836103]
Pacholczyk, T., Blakely, R. D., Amara, S. G. Expression cloning of a cocaine- and antidepressant-sensitive human noradrenaline transporter. Nature 350: 350-354, 1991. [PubMed: 2008212] [Full Text: https://doi.org/10.1038/350350a0]
Paczkowski, F. A., Bonisch, H., Bryan-Lluka, L. J. Pharmacological properties of the naturally occurring ala457pro variant of the human norepinephrine transporter. Pharmacogenetics 12: 165-173, 2002. [PubMed: 11875370] [Full Text: https://doi.org/10.1097/00008571-200203000-00010]
Pirke, K. M., Kellner, M., Philipp, E., Laessle, R., Krieg, J. C., Fichter, M. M. Plasma norepinephrine after a standardized test meal in acute and remitted patients with anorexia nervosa and in healthy controls. Biol. Psychiat. 31: 1074-1077, 1992. [PubMed: 1511079] [Full Text: https://doi.org/10.1016/0006-3223(92)90102-6]
Porzgen, P., Bonisch, H., Bruss, M. Molecular cloning and organization of the coding region of the human norepinephrine transporter gene. Biochem. Biophys. Res. Commun. 215: 1145-1150, 1995. Note: Erratum: Biochem. Biophys. Res. Commun. 227: 642-643, 1996. [PubMed: 7488042] [Full Text: https://doi.org/10.1006/bbrc.1995.2582]
Shannon, J. R., Flattem, N. L., Jordan, J., Jacob, G., Black, B. K., Biaggioni, I., Blakely, R. D., Robertson, D. Orthostatic intolerance and tachycardia associated with norepinephrine-transporter deficiency. New Eng. J. Med. 342: 541-549, 2000. [PubMed: 10684912] [Full Text: https://doi.org/10.1056/NEJM200002243420803]
Shirey-Rice, J. K., Klar, R., Fentress, H. M., Redmon, S. N., Sabb, T. R., Krueger, J. J., Wallace, N. M., Appalsamy, M., Finney, C., Lonce, S., Diedrich, A., Hahn, M. K. Norepinephrine transporter variant A457P knock-in mice display key features of human postural orthostatic tachycardia syndrome. Dis. Model. Mech. 6: 1001-1011, 2013. [PubMed: 23580201] [Full Text: https://doi.org/10.1242/dmm.012203]
Urwin, R. E., Bennetts, B., Wilcken, B., Lampropoulos, B., Beumont, P., Clarke, S., Russell, J., Tanner, S., Nunn, K. P. Anorexia nervosa (restrictive subtype) is associated with a polymorphism in the novel norepinephrine transporter gene promoter polymorphic region. Molec. Psychiat. 7: 652-657, 2002. [PubMed: 12140790] [Full Text: https://doi.org/10.1038/sj.mp.4001080]
Zhou, Z., Zhen, J., Karpowich, N. K., Goetz, R. M., Law, C. J., Reith, M. E. A., Wang, D.-N. LeuT-desipramine structure reveals how antidepressants block neurotransmitter reuptake. Science 317: 1390-1393, 2007. [PubMed: 17690258] [Full Text: https://doi.org/10.1126/science.1147614]