Germline De Novo Mutations in ATP1A1 Cause Renal Hypomagnesemia, Refractory Seizures, and Intellectual Disability
2018; Elsevier BV; Volume: 103; Issue: 5 Linguagem: Inglês
10.1016/j.ajhg.2018.10.004
ISSN1537-6605
AutoresKarl P. Schlingmann, Sascha Bandulik, Cherry Mammen, Maja Tarailo‐Graovac, Rikke Holm, Matthias Baumann, Jens König, Jessica J. Y. Lee, Britt I. Drögemöller, Katrin Imminger, Bodo B. Beck, Janine Altmüller, Hölger Thiele, Siegfried Waldegger, William van’t Hoff, Robert Kleta, Richard Warth, Clara van Karnebeek, Bente Vilsen, Detlef Böckenhauer, Martin Konrad,
Tópico(s)Magnesium in Health and Disease
ResumoOver the last decades, a growing spectrum of monogenic disorders of human magnesium homeostasis has been clinically characterized, and genetic studies in affected individuals have identified important molecular components of cellular and epithelial magnesium transport. Here, we describe three infants who are from non-consanguineous families and who presented with a disease phenotype consisting of generalized seizures in infancy, severe hypomagnesemia, and renal magnesium wasting. Seizures persisted despite magnesium supplementation and were associated with significant intellectual disability. Whole-exome sequencing and conventional Sanger sequencing identified heterozygous de novo mutations in the catalytic Na+, K+-ATPase α1 subunit (ATP1A1). Functional characterization of mutant Na+, K+-ATPase α1 subunits in heterologous expression systems revealed not only a loss of Na+, K+-ATPase function but also abnormal cation permeabilities, which led to membrane depolarization and possibly aggravated the effect of the loss of physiological pump activity. These findings underline the indispensable role of the α1 isoform of the Na+, K+-ATPase for renal-tubular magnesium handling and cellular ion homeostasis, as well as maintenance of physiologic neuronal activity. Over the last decades, a growing spectrum of monogenic disorders of human magnesium homeostasis has been clinically characterized, and genetic studies in affected individuals have identified important molecular components of cellular and epithelial magnesium transport. Here, we describe three infants who are from non-consanguineous families and who presented with a disease phenotype consisting of generalized seizures in infancy, severe hypomagnesemia, and renal magnesium wasting. Seizures persisted despite magnesium supplementation and were associated with significant intellectual disability. Whole-exome sequencing and conventional Sanger sequencing identified heterozygous de novo mutations in the catalytic Na+, K+-ATPase α1 subunit (ATP1A1). Functional characterization of mutant Na+, K+-ATPase α1 subunits in heterologous expression systems revealed not only a loss of Na+, K+-ATPase function but also abnormal cation permeabilities, which led to membrane depolarization and possibly aggravated the effect of the loss of physiological pump activity. These findings underline the indispensable role of the α1 isoform of the Na+, K+-ATPase for renal-tubular magnesium handling and cellular ion homeostasis, as well as maintenance of physiologic neuronal activity. Magnesium is essential for numerous cellular processes, including energy metabolism, protein and nucleic acid synthesis, and the maintenance of the electrical potential of nervous tissues and cell membranes. Genetic investigations in children with inherited forms of hypomagnesemia could identify critical components of epithelial magnesium transport at the molecular level.1Viering D.H.H.M. de Baaij J.H.F. Walsh S.B. Kleta R. Bockenhauer D. Genetic causes of hypomagnesemia, a clinical overview.Pediatr. Nephrol. 2017; 32: 1123-1135Crossref PubMed Scopus (84) Google Scholar Affected children commonly present with seizures, muscle spasms, or tetany. In the majority of cases, magnesium supplementation leads to relief of clinical symptoms and allows for a normal motor and cognitive development despite persistence of subnormal serum magnesium levels. In contrast with this favorable clinical course, we noted a small group of nine children who, despite appropriate magnesium supplementation, experienced prolonged and repeated seizure activity associated with severe intellectual disability. Of this cohort, two individuals had previously been diagnosed with hypomagnesemia, seizures, and mental retardation [HOMGSMR, MIM: 616418] due to bi-allelic mutations in CNNM2 [MIM: 607803].2Arjona F.J. de Baaij J.H. Schlingmann K.P. Lameris A.L. van Wijk E. Flik G. Regele S. Korenke G.C. Neophytou B. Rust S. et al.CNNM2 mutations cause impaired brain development and seizures in patients with hypomagnesemia.PLoS Genet. 2014; 10: e1004267Crossref PubMed Scopus (90) Google Scholar Here, we identified heterozygous de novo mutations in ATP1A1 [MIM: 182310] (RefSeqGene: NG_047036, GenBank: NM_000701), encoding the α1 isoform of Na+, K+-ATPase, in three children from this cohort. Data on clinical symptoms and biochemical measures at the time of disease manifestation were collected retrospectively from medical charts. Affected children with ATP1A1 mutations were clinically reevaluated during follow-up, and biochemical data were obtained. The three infants initially presented between 6 days and 6 months of age with generalized convulsions (Table 1, for the full dataset please refer to Table S1 in the Supplemental Data). At the time of manifestation, severe hypomagnesemia (0.30–0.36 mmol/L) was noted. Calculation of urinary fractional excretion rates of magnesium indicated massive renal magnesium wasting. Although urinary calcium excretion was not uniformly elevated, initial renal ultrasound examinations indicated medullary hyperechogenicity compatible with incipient nephrocalcinosis in individuals B-II-1 and C-II-2. All children were treated with antiepileptic drugs and received intravenous magnesium followed by ongoing oral supplementation. However, seizure activity persisted with frequent generalized seizures and repeated status epilepticus despite amelioration of serum magnesium levels. After a status epilepticus with both tonic-clonic seizure activity and hypoxemia for more than one hour, the cerebral magnetic resonance imaging (MRI) of individual A-II-1 showed bilateral parietooccipital cortical and subcortical diffusion restriction compatible with hypoxic ischemic encephalopathy. This resulted in a marked developmental setback and impaired vision. Follow-up MRI showed cerebral volume loss. All three children uniformly had significant global developmental delay, displayed limited motor skills, and spoke only in single words. Two individuals (B-II-1 and C-II-2) showed clinical features compatible with an autism spectrum disorder. MRI examinations revealed cerebral volume loss in individual C-II-2 also. Blood-pressure measurements repeatedly demonstrated normotension in all children, and cardiac examinations performed in individuals A-II-1 and C-II-2 were unremarkable. Individuals B-II-1 and C-II-2 exhibited significant polyuria of 4–8 ml/kg/h, but renal concentrating ability remained at least partially intact (random urine osmolalities of >400 mosmol/L). Although laboratory analyses did not reveal renal salt wasting or a significant activation of the renin-aldosterone system and serum potassium levels were mostly within the reference range, all affected individuals exhibited repeated episodes with significant hypokalemia (S-K+ of 2.1 to 2.6 mmol/L). Calculation of the transtubular potassium gradient during hypokalemic episodes indicated renal potassium wasting. The most recent laboratory examinations in individuals A-II-1 and B-II-1 demonstrated persisting hypomagnesemia despite high doses of oral magnesium supplementation, and fractional urinary excretion rates of magnesium confirmed major renal magnesium wasting. Parents of all three children were clinically unaffected and showed normal serum magnesium levels.Table 1Overview of Clinical Characteristics and GenotypesIndividualA-II-1B-II-1C-II-2DemographicsOriginof European descentof European descentFirst Nations CanadianGenderfemalefemalemaleAge at manifestation6 months2 months6 daysFirst symptomgeneralized seizuresgeneralized seizuresgeneralized seizuresInitial Laboratory FindingsS-Mg (mmol/L) (0.75–1.1)0.360.350.30FE-Mg (%) (3-5%)26.033.8ndMost Recent FindingsAge at last follow-up4 years10 years6 yearsS-Mg (mmol/L) (0.75.-1.1)0.570.280.62FE-Mg (%) (3-5%)15.327.021.3seizure activityrepeated status epilepticusmonthly seizuresfrequent seizures,repeated status epilepticusneurological outcomeglobal developmental delay, hyperactive behaviorglobal developmental delay, suspected autism spectrum disorderglobal developmental delay, speech delay, diagnosis of severe autism, self-biting behaviorATP1A1 Mutationsnucleotide levelc.905T>Cc.907G>Cc.2576T>Gprotein levelp.Leu302Argp.Gly303Argp.Met859Arg Open table in a new tab Extraction of DNA from whole blood was performed according to standard protocols. All genetic studies were approved by the respective ethics committees of the involved centers. The parents provided written informed consent. The clinical phenotype initially suggested the diagnosis of hypomagnesemia with secondary hypocalcemia (HSH) [HOMG1, MIM: 602014]; however, mutations in TRPM6 [MIM: 607009] were excluded.3Schlingmann K.P. Weber S. Peters M. Niemann Nejsum L. Vitzthum H. Klingel K. Kratz M. Haddad E. Ristoff E. Dinour D. et al.Hypomagnesemia with secondary hypocalcemia is caused by mutations in TRPM6, a new member of the TRPM gene family.Nat. Genet. 2002; 31: 166-170Crossref PubMed Scopus (647) Google Scholar, 4Walder R.Y. Landau D. Meyer P. Shalev H. Tsolia M. Borochowitz Z. Boettger M.B. Beck G.E. Englehardt R.K. Carmi R. Sheffield V.C. Mutation of TRPM6 causes familial hypomagnesemia with secondary hypocalcemia.Nat. Genet. 2002; 31: 171-174Crossref PubMed Scopus (470) Google Scholar Under the assumption of an unknown disease phenotype, we performed whole-exome sequencing in individual A-II-1 as well as the family C trio in order to identify the underlying genetic defect. Details on target enrichment, sequencing, and data analysis are provided in the Supplemental Data. We focused on missense, nonsense, splice-site, and frameshift variants upon all modes of inheritance. After performing sequential filtering and keeping variants predicted as pathogenic, we could identify no common gene with homozygous or compound-heterozygous variants in the two affected individuals. In contrast, both were found to carry a single heterozygous mutation, c.905T>G (p.Leu302Arg) and c. 2576T>G (p.Met859Arg) in ATP1A1, respectively. In silico analyses predicted the variants to be pathogenic; they were predicted to affect highly conserved amino acid residues of the Na+, K+-ATPase α1 protein (Figure 1, Table S2). None of the identified mutations is listed in publicly available exome databases, i.e., ExAC and gnomAD browsers. Identified mutations were confirmed by Sanger sequencing (details of primers are available on request); the trio analysis of family C exomes as well as sequencing of the parents of individual A-II-1 demonstrated that both mutations occurred de novo. We could not identify any additional gene with heterozygous variants shared by both affected individuals (A-II-1 and C-II-2). Therefore, we do not have any genetic evidence that the phenotype observed here results from additive effects of variants in a gene other than ATP1A1. Subsequently, ATP1A1 screening by conventional Sanger sequencing revealed that a third variant, c.907G>C (p.Gly303Arg), existed in a heterozygous state in individual B-II-1 but was not found in either parent and also occurred de novo. In families A and B, paternity was confirmed by analysis of seven independent polymorphic microsatellite markers. Paternity in family C was confirmed by segregation analysis of rare variants identified in the trio exome. For biochemical studies, mutations were introduced into full-length cDNA encoding the ouabain-insensitive rat α1 isoform of Na+, K+-ATPase and expressed in COS-1 cells. Ouabain selection was used for obtaining stable viable cell lines.5Vilsen B. Glutamate 329 located in the fourth transmembrane segment of the alpha-subunit of the rat kidney Na+,K+-ATPase is not an essential residue for active transport of sodium and potassium ions.Biochemistry. 1993; 32: 13340-13349Crossref PubMed Scopus (67) Google Scholar However, although COS cells transfected with wild-type rat Atp1a1 grew normally, several attempts to keep COS cells growing after transfection with mutant rat Atp1a1 cDNA and under ouabain selection failed; this indicates that, in contrast to the wild-type enzyme, none of the three mutants (p.Leu302Arg, p.Gly303Arg, or p.Met859Arg) was able to carry out the Na+ and K+ transport required to support cell growth. Therefore, transient expression was performed in the presence of siRNA to knock down endogenous Na+, K+-ATPase.6Beuschlein F. Boulkroun S. Osswald A. Wieland T. Nielsen H.N. Lichtenauer U.D. Penton D. Schack V.R. Amar L. Fischer E. et al.Somatic mutations in ATP1A1 and ATP2B3 lead to aldosterone-producing adenomas and secondary hypertension.Nat. Genet. 2013; 45 (e1–e2): 440-444Crossref PubMed Scopus (414) Google Scholar Leaky plasma membranes were assayed functionally by previously described methods7Toustrup-Jensen M. Hauge M. Vilsen B. Mutational effects on conformational changes of the dephospho- and phospho-forms of the Na+,K+-ATPase.Biochemistry. 2001; 40: 5521-5532Crossref PubMed Scopus (28) Google Scholar (details are provided in the Supplemental Data). Na+, K+-ATPase pump function follows the Post-Albers reaction cycle (Figure 2A), starting with phosphorylation after binding of intracellular Na+, which facilitates "pumping" of Na+ to the outside, before binding of K+, which, after dephosphorylation of the enzyme, is "pumped" into the cell. Measurements of phosphorylation under maximal conditions where the Na+ sites were saturated and dephosphorylation was blocked with oligomycin demonstrated that all three mutants became phosphorylated, indicating that they were expressed, although at a significantly reduced level for p.Gly303Arg and p.Met859Arg, and that they were able to perform the Na+ limb of the Post-Albers cycle (Figure 2B). A significantly reduced Na+ affinity was, however, seen for the p.Leu302Arg mutant. The other two mutants showed reduced cooperativity of Na+ binding, as deduced from Hill coefficients (Figure 2C and Figure S1). The K+ affinity was also significantly reduced for p.Leu302Arg, whereas the other two mutants showed less effect on K+ affinity but reduced cooperativity of K+ binding (Figure 2D). Determination of the E1P/E2P distribution by taking advantage of the ADP sensitivity of the phosphoenzyme showed no reduction of the level of E2P (Figure S2), indicating that the defective K+ binding is a direct effect on K+ interaction with the E2P state rather than a defect in the critical conformational change from E1P to E2P of Na+, K+-ATPase. Next, electroporation was used for transfecting adrenocortical carcinoma NCI-H295R cells (Cell Line Service) as well as HEK293 cells with plasmids containing full-length cDNA sequences encoding wild-type or mutant ouabain-resistant rat Atp1a1. Transfected cells were identified with anti-CD8-coated dynabeads (Life Technologies). Whole-cell patch-clamp recordings were performed as described.8Tauber P. Aichinger B. Christ C. Stindl J. Rhayem Y. Beuschlein F. Warth R. Bandulik S. Cellular pathophysiology of an adrenal adenoma-associated mutant of the plasma membrane Ca(2+)-ATPase ATP2B3.Endocrinology. 2016; 157: 2489-2499Crossref PubMed Scopus (43) Google Scholar For the determination of intracellular Na+ and K+ contents, flame photometry was used.8Tauber P. Aichinger B. Christ C. Stindl J. Rhayem Y. Beuschlein F. Warth R. Bandulik S. Cellular pathophysiology of an adrenal adenoma-associated mutant of the plasma membrane Ca(2+)-ATPase ATP2B3.Endocrinology. 2016; 157: 2489-2499Crossref PubMed Scopus (43) Google Scholar Intracellular Na+ and K+ content was measured under control conditions and after treatment with 10 μM ouabain-inhibiting endogenous human Na+, K+-ATPase. Patch-clamp analyses of NCI-H295R cells expressing mutant ATP1A1 constructs p.Leu302Arg and p.Gly303Arg revealed an abnormal Na+ permeability compared to that of wild-type cells, as manifested by Na+-dependent inward currents at negative voltages (Figure 2E). Moreover, cells expressing mutant ATP1A1 p.Leu302Arg and p.Gly303Arg also showed a depolarized membrane potential in the presence of Na+, but upon removal of extracellular Na+, the membrane potential was hyperpolarized to the level of wild-type-expressing cells (Figure 2F). In these experiments, cells expressing the mutant p.Met859Arg were indistinguishable from wild-type-ATP1A1-expressing cells. Inhibition of endogenous Na+, K+-ATPase by ouabain produced pronounced changes in intracellular Na+ and K+ in HEK293 cells (Figure 2G). These changes were significantly attenuated after expression of wild-type rat Atp1a1, whereas all three mutants failed to compensate for the block of endogenous ATP1A1. Expression of mutant p.Gly303Arg led to even more pronounced disturbances of intracellular Na+ and K+ content compared to that in non-transfected cells, possibly reflecting abnormal ion fluxes (Figure 2G). Analyses of intracellular pH levels revealed an abnormal H+ permeability and significant changes of intracellular pH upon alteration of extracellular pH for mutant p.Leu302Arg (Figure S3). The Na+, K+-ATPase is an integral membrane protein that catalyzes the transport of three Na+ ions out of and two K+ ions into the cell at the expense of one molecule of ATP. It generates the electrochemical driving force that powers essential functions, such as neuronal firing, muscle contraction, and transepithelial ion transport. The Na+, K+-ATPase is a heterodimer consisting of α and β subunits and is complemented by auxiliary FXYD proteins.9Sweadner K.J. Arystarkhova E. Donnet C. Wetzel R.K. FXYD proteins as regulators of the Na,K-ATPase in the kidney.Ann. N Y Acad. Sci. 2003; 986: 382-387Crossref PubMed Scopus (51) Google Scholar The catalytic α subunit binds translocating Na+ and K+ as well as ATP, coupling ionic movements across the cell membrane to ATP hydrolysis.10Post R.L. Hegyvary C. Kume S. Activation by adenosine triphosphate in the phosphorylation kinetics of sodium and potassium ion transport adenosine triphosphatase.J. Biol. Chem. 1972; 247: 6530-6540Abstract Full Text PDF PubMed Google Scholar In mammals, there exist four α isoforms that are expressed in a developmental- and tissue-specific manner, suggesting specific functional roles (α1–α4, ATP1A1–ATP1A4).11Blanco G. Na,K-ATPase subunit heterogeneity as a mechanism for tissue-specific ion regulation.Semin. Nephrol. 2005; 25: 292-303Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar, 12Lingrel J.B. Na,K-ATPase: isoform structure, function, and expression.J. Bioenerg. Biomembr. 1992; 24: 263-270PubMed Google Scholar The ubiquitously expressed α1 subunit (ATP1A1) represents the major α isoform in the kidney and is present in virtually all cell types and structures of the central nervous system (CNS).11Blanco G. Na,K-ATPase subunit heterogeneity as a mechanism for tissue-specific ion regulation.Semin. Nephrol. 2005; 25: 292-303Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar Others studied the effects of a targeted disruption of α1 (ATP1A1) almost 20 years ago in mice; the focus then was on the cardiac phenotype.13James P.F. Grupp I.L. Grupp G. Woo A.L. Askew G.R. Croyle M.L. Walsh R.A. Lingrel J.B. Identification of a specific role for the Na,K-ATPase alpha 2 isoform as a regulator of calcium in the heart.Mol. Cell. 1999; 3: 555-563Abstract Full Text Full Text PDF PubMed Scopus (328) Google Scholar Bi-allelic knock-out of ATP1A1 led to early fetal lethality, suggesting that complete loss of ATP1A1 function is not compatible with life. In contrast, heterozygous mice were fertile and generally healthy; however, they exhibited a hypocontractile cardiac phenotype mimicking cardiac glycoside toxicity.13James P.F. Grupp I.L. Grupp G. Woo A.L. Askew G.R. Croyle M.L. Walsh R.A. Lingrel J.B. Identification of a specific role for the Na,K-ATPase alpha 2 isoform as a regulator of calcium in the heart.Mol. Cell. 1999; 3: 555-563Abstract Full Text Full Text PDF PubMed Scopus (328) Google Scholar The severe phenotype in the affected children differs from these heterozygous (Atp1a1+/−) mice. The cardiac examination of the affected children did not reveal any pathologic findings, possibly indicating inter-species differences or suggesting compensatory mechanisms in the human heart. Seizures and hypomagnesemia have not been reported in (Atp1a1+/−) mice; however, they represent pivotal findings in the affected children. Hypomagnesemia in the affected individuals presented here is caused by massive renal Mg2+ wasting. Within the nephron, the distal convoluted tubule (DCT) represents the segment mediating active transcellular Mg2+ transport. Here, the basolaterally expressed Na+, K+-ATPase establishes favorable electrochemical gradients for apical cation influx through Mg2+-permeable ion channels and also provides the exit mechanism for reabsorbed Na+ ions. The DCT exhibits the highest density and activity of the Na+, K+-ATPase; α1 (ATP1A1) represents the predominating α isoform.14Lücking K. Nielsen J.M. Pedersen P.A. Jørgensen P.L. Na-K-ATPase isoform (alpha 3, alpha 2, alpha 1) abundance in rat kidney estimated by competitive RT-PCR and ouabain binding.Am. J. Physiol. 1996; 271: F253-F260PubMed Google Scholar The critical role of Na+, K+-ATPase activity for Mg2+ reabsorption in the DCT has already been highlighted by the discovery of genetic defects in its auxiliary γ subunit (encoded by FXYD2 [MIM: 601814]) in persons with dominant hypomagnesemia [HOMG2, MIM: 154020],15Meij I.C. Koenderink J.B. van Bokhoven H. Assink K.F. Groenestege W.T. de Pont J.J. Bindels R.J. Monnens L.A. van den Heuvel L.P. Knoers N.V. Dominant isolated renal magnesium loss is caused by misrouting of the Na(+),K(+)-ATPase gamma-subunit.Nat. Genet. 2000; 26: 265-266Crossref PubMed Scopus (216) Google Scholar as well as by the hypomagnesemia observed in individuals with SeSAME (EAST) syndrome [SESAMES, MIM: 612780].16Bockenhauer D. Feather S. Stanescu H.C. Bandulik S. Zdebik A.A. Reichold M. Tobin J. Lieberer E. Sterner C. Landoure G. et al.Epilepsy, ataxia, sensorineural deafness, tubulopathy, and KCNJ10 mutations.N. Engl. J. Med. 2009; 360: 1960-1970Crossref PubMed Scopus (428) Google Scholar This autosomal-recessive disorder is caused by mutations in KCNJ10 [MIM: 602208], encoding the Kir4.1 potassium channel that is co-expressed basolaterally in the DCT and recycles K+ as a prerequisite for maintenance of Na+, K+-ATPase activity. Despite considerable variability, serum Mg2+ levels in the affected children remained persistently low during follow-up despite a high dose oral Mg2+ supplementation (Table 1). Individuals with genetic defects in TRPM6 or FXYD2 also present with seizures; however, in these individuals, Mg2+ supplementation usually leads to a rapid cessation of epileptic activity, and physical and mental development are generally undisturbed even though Mg2+ levels remain subnormal.17Schlingmann K.P. Sassen M.C. Weber S. Pechmann U. Kusch K. Pelken L. Lotan D. Syrrou M. Prebble J.J. Cole D.E. et al.Novel TRPM6 mutations in 21 families with primary hypomagnesemia and secondary hypocalcemia.J. Am. Soc. Nephrol. 2005; 16: 3061-3069Crossref PubMed Scopus (133) Google Scholar In contrast, the children with ATP1A1 defects exhibited persistent seizures and uniformly developed significant intellectual disability. Therefore, the neurologic phenotype has to be considered as a primary feature of ATP1A1 deficiency due to a genuine disturbance of Na+, K+-ATPase function in the CNS. Here, α1 is ubiquitously expressed and thought to maintain neuronal housekeeping functions, whereas the α3 isoform potentially acts as a reserve pump that is only required during phases of increased intracellular Na+ concentrations, i.e., after repeated action potentials.11Blanco G. Na,K-ATPase subunit heterogeneity as a mechanism for tissue-specific ion regulation.Semin. Nephrol. 2005; 25: 292-303Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar, 18Munzer J.S. Daly S.E. Jewell-Motz E.A. Lingrel J.B. Blostein R. Tissue- and isoform-specific kinetic behavior of the Na,K-ATPase.J. Biol. Chem. 1994; 269: 16668-16676Abstract Full Text PDF PubMed Google Scholar Expression of the α2 isoform is restricted to astrocytes and developing neurons. In the CNS, constant Na+, K+-ATPase activity is required for generating the resting membrane potential and for buffering and clearance of extracellular K+ transients during neuronal activity.19Larsen B.R. Stoica A. MacAulay N. Managing brain extracellular K(+) during neuronal activity: the physiological role of the Na(+)/K(+)-ATPase subunit isoforms.Front. Physiol. 2016; 7: 141Crossref PubMed Scopus (79) Google Scholar Decreases in Na+, K+-ATPase activity have been detected in animal models of epilepsy and in forms of human myoclonus epilepsy and mitochondrial disorders.20Marquezan B.P. Funck V.R. Oliveira C.V. Pereira L.M. Araújo S.M. Zarzecki M.S. Royes L.F. Furian A.F. Oliveira M.S. Pentylenetetrazol-induced seizures are associated with Na+,K+-ATPase activity decrease and alpha subunit phosphorylation state in the mice cerebral cortex.Epilepsy Res. 2013; 105: 396-400Crossref PubMed Scopus (10) Google Scholar, 21Clapcote S.J. Duffy S. Xie G. Kirshenbaum G. Bechard A.R. Rodacker Schack V. Petersen J. Sinai L. Saab B.J. Lerch J.P. et al.Mutation I810N in the alpha3 isoform of Na+,K+-ATPase causes impairments in the sodium pump and hyperexcitability in the CNS.Proc. Natl. Acad. Sci. 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Neurophysiol. 2002; 88: 2963-2978Crossref PubMed Scopus (112) Google Scholar Very recently, germline mutations in ATP1A1 have been reported in individuals with Charcot-Marie-Tooth type 2 disease (CMT2) [CMT2DD, MIM: 618036].25Lassuthova P. Rebelo A.P. Ravenscroft G. Lamont P.J. Davis M.R. Manganelli F. Feely S.M. Bacon C. Brožková D.S. Haberlova J. et al.Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2.Am. J. Hum. Genet. 2018; 102: 505-514Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar Combining extensive data from different exome sequencing projects, the authors identified heterozygous ATP1A1 missense mutations in seven CMT2-affected families. Segregation analyses were compatible with dominant inheritance. In accordance with the peripheral nervous system (PNS) phenotype, strong α1 expression was demonstrated in axolemma and myelin sheaths of sensory and motor neurons. The identified missense mutations affect conserved amino acid residues in different regions of the α1 protein. Interestingly, a mutational clustering is observed within the helical linker region (residues 592–608). Mutations in this region have been shown to reduce the rate of E1P to E2P conformational transition during the Post-Albers reaction cycle, but without completely blocking the conversion.26Clarke D.M. Loo T.W. MacLennan D.H. Functional consequences of alterations to amino acids located in the nucleotide binding domain of the Ca2(+)-ATPase of sarcoplasmic reticulum.J Biol Chem. 1990; 265: 22223-22227PubMed Google Scholar Ouabain survival assays demonstrated a significant decrease in cell viability, and functional studies in Xenopus oocytes showed a significant Na+-current reduction compatible with a deleterious effect on Na+, K+-ATPase ion-transport function.25Lassuthova P. Rebelo A.P. Ravenscroft G. Lamont P.J. Davis M.R. Manganelli F. Feely S.M. Bacon C. Brožková D.S. Haberlova J. et al.Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2.Am. J. Hum. Genet. 2018; 102: 505-514Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar In contrast to findings in these previously reported CMT2-affected families, the clinical evaluation of the affected children with de novo ATP1A1 mutations did not reveal any signs of peripheral neuropathy. This phenotypic discrepancy could possibly be attributed to the relatively young age of the children; the age of onset of clinical symptoms in the previously reported CMT2-affected families varied between 8 and 50 years of age.25Lassuthova P. Rebelo A.P. Ravenscroft G. Lamont P.J. Davis M.R. Manganelli F. Feely S.M. Bacon C. Brožková D.S. Haberlova J. et al.Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2.Am. J. Hum. Genet. 2018; 102: 505-514Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar Conversely, data on serum magnesium levels as well as a potential concomitant epileptic phenotype were not reported in the CMT2-affected families. Yet, the authors detail migraine headaches in one CMT2-affected family and explicitly note the possibility of combined CNS and PNS symptoms and an expanding spectrum of phenotypes associated with ATP1A1 mutations. Clearly, the predominant renal and CNS symptoms as well as the unfavorable clinical course of the children presented here with profound intellectual disability indicate a distinct clinical entity caused by ATP1A1 defects. Such a pleiotropic effect with variable phenotypes has already been described for mutations in the Na+, K+-ATPase homologs α2 and α3 (ATP1A2 [MIM: 182340] and ATP1A3 [MIM: 182350]). The disease spectrum caused by germline mutations in these two isoforms comprises f
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