ABCC9 mutations identified in human dilated cardiomyopathy disrupt catalytic KATP channel gating

doi:10.1038/ng1329

Nat Genet. Author manuscript; available in PMC 2007 October 1.

Published in final edited form as:

Nat Genet. 2004 April; 36(4): 382–387.

Published online 2004 March 21. doi: 10.1038/ng1329

PMCID: PMC1995438

NIHMSID: NIHMS23424

ABCC9 mutations identified in human dilated cardiomyopathy disrupt catalytic K_ATP channel gating

Martin Bienengraeber,^1,² Timothy M Olson,^1,³ Vitaliy A Selivanov,¹ Eva C Kathmann,^1,² Fearghas O’Cochlain,¹ Fan Gao,² Amy B Karger,^1,² Jeffrey D Ballew,¹ Denice M Hodgson,¹ Leonid V Zingman,^1,² Yuan-Ping Pang,² Alexey E Alekseev,^1,² and Andre Terzic^1,²

¹Division of Cardiovascular Diseases, Department of Medicine, Mayo Clinic College of Medicine, Mayo Foundation, Rochester, Minnesota 55905, USA.

²Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic College of Medicine, Mayo Foundation, Rochester, Minnesota 55905, USA.

³Division of Pediatric Cardiology, Department of Pediatric and Adolescent Medicine, Mayo Clinic College of Medicine, Mayo Foundation, Rochester, Minnesota 55905, USA.

Correspondence should be addressed to A.T. (Email: terzic.andre/at/mayo.edu).

The publisher's final edited version of this article is available at Nat Genet

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Abstract

Stress tolerance of the heart requires high-fidelity metabolic sensing by ATP-sensitive potassium (K_ATP) channels that adjust membrane potential-dependent functions to match cellular energetic demand. Scanning of genomic DNA from individuals with heart failure and rhythm disturbances due to idiopathic dilated cardiomyopathy identified two mutations in ABCC9, which encodes the regulatory SUR2A subunit of the cardiac K_ATP channel. These missense and frameshift mutations mapped to evolutionarily conserved domains adjacent to the catalytic ATPase pocket within SUR2A. Mutant SUR2A proteins showed aberrant redistribution of conformations in the intrinsic ATP hydrolytic cycle, translating into abnormal K_ATP channel phenotypes with compromised metabolic signal decoding. Defective catalysis-mediated pore regulation is thus a mechanism for channel dysfunction and susceptibility to dilated cardiomyopathy.

Cardiac K_ATP channels are heteromultimers composed of Kir6.2, an inwardly rectifying potassium channel pore, and the regulatory SUR2A subunit, an ATPase-harboring ATP-binding cassette protein¹^-⁴. The SUR2A subunit recognizes and processes intracellular energetic signals, through its nucleotide binding domains, endowing the K_ATP channel-enzyme complex with a metabolic decoding capacity⁵. Potassium movement through Kir6.2 does not require energy expenditure⁶, yet ATP hydrolysis at SUR2A is integral in the transduction of metabolic signals from cellular energetic pathways to the channel pore⁵^,⁷. In this way, K_ATP channels set membrane excitability in response to stress challenge and preserve cellular energy-dependent functions⁸^-¹². Thereby, the K_ATP channel complex has a vital role in securing cellular homeostasis under stress¹².

Disease-induced K_ATP channel dysregulation, recently recognized in a transgenic model of cardiomyopathy, has been associated with compromised stress tolerance¹³. In fact, in the stressed heart, knockout of K_ATP channel genes precipitates intracellular calcium overload predisposing to myocardial damage, arrhythmia and death preventable by calcium-channel blockade¹². Indeed, K_ATP channel-deficient hearts are susceptible to calcium-dependent maladaptive remodeling under chronic hemodynamic load, progressing to congestive heart failure (G.C. Kane, F.O., D.M.H., T. Miki, S. Seino and A.T., unpublished data). Although improper myocellular calcium handling contributes to the pathogenesis of dilated cardiomyopathy¹⁴^,¹⁵, a malignant disorder characterized by heart failure and increased susceptibility to metabolic stressors¹⁶^-¹⁸, little is known about K_ATP channels in human heart disease. Here, we report the first mutations in ABCC9, encoding SUR2A. Identified in individuals with idiopathic dilated cardiomyopathy, these defects in the regulatory K_ATP channel subunit disrupt catalysis-dependent gating and impair metabolic decoding, establishing a previously unrecognized mechanism of channel malfunction in human disease.

Scans for mutation in genomic DNA in a cohort of 323 individuals with idiopathic dilated cardiomyopathy identified two heterozygous mutations in exon 38 of ABCC9, which encodes the C-terminal domain of SUR2A specific to the cardiac splice variant of the regulatory K_ATP channel subunit (Fig. 1a,b). Both individuals with mutations in ABCC9 had severely dilated hearts with compromised contractile function and rhythm disturbances (Table 1). DNA sequencing of one mutated allele identified a 3-bp deletion and 4-bp insertion mutation (4570-4572delTTAinsAAAT), causing a frameshift at Leu1524 and introducing four anomalous terminal residues followed by a premature stop codon (Fs1524; Fig. 1a). The second mutated allele harbored a missense mutation (4537G→A) causing the amino acid substitution A1513T (Fig. 1a). The identified frameshift and missense mutations occurred in evolutionarily conserved domains of the C terminus of SUR2A (Fig. 1b), and neither mutation was present in 500 unrelated control individuals.

Figure 1

K_ATP channel mutations in dilated cardiomyopathy. (a) The regulatory SUR2A subunit (nucleotide-binding domains NBD1 and NBD2 with Walker A and B motifs and a linker L region) forms cardiac K_ATP channels by assembling with the pore-forming Kir6.2 subunit (more ...)

Table 1

Summary of clinical phenotypes

The C terminus of SUR proteins contributes to K_ATP channel trafficking¹⁹^,²⁰, and Fs1524 and A1513T SUR2A mutants, reconstituted with Kir6.2, had reduced expression in the plasma membrane (Fig. 2a). Yet, mutant K_ATP channel complexes formed functional channels with intact pore properties (Fig. 2b). Structural molecular dynamics simulation showed that the residues Ala1513 and Leu1524 flank the C-terminal β-strand in close proximity to the signature Walker A motif (Fig. 2c,d), required for coordination of nucleotides in the catalytic pocket of ATP-binding cassette proteins²¹^,²². Replacement of Ala1513 with a sterically larger and more hydrophilic threonine residue or truncation of the C terminus caused by the Fs1524 mutation would disrupt folding of the C-terminal β-strand and, thus, the tertiary organization of the adjacent second nucleotide binding domain (NBD2) pocket in SUR2A. ATP-induced K_ATP channel gating was aberrant in both channel mutants (Fig. 2e), suggesting that structural alterations induced by the mutations A1513T and Fs1524 of SUR2A distorted ATP-dependent pore regulation.

Figure 2

SUR2A mutant proteins, coexpressed with Kir6.2, alter K_ATP channel function. (a) Fs1524 and A1513T reduced K_ATP channel trafficking by ~70% and ~30%, probed immunologically by SUR surface expression in Xenopus laevis oocytes. (b) Single (more ...)

On further examination, purified wild-type and mutant NBD2 constructs had similar ATP binding but reduced ATP hydrolytic activities (Fig. 3a-c). The A1513T and Fs1524 mutations substantially diminished the maximal rate of the NBD2 ATPase reaction without altering the Michaelis-Menten constant of catalysis (Fig. 3d). A1513T reduced the product-dependent inhibition of the NBD2 ATPase more substantially than Fs1524 (Fig. 3e) but produced a less severe delay in the pre-steady state profile of product accumulation (Fig. 3f). Thus, the mutations A1513T and Fs1524 compromise ATP hydrolysis at SUR2A NBD2, generating distinct reaction kinetic defects.

Figure 3

SUR2A NBD2 mutants have normal ATP binding but altered ATPase properties. (a) Affinity-purified mutant or wild-type SUR2A NBD2, cloned in-frame with maltose binding protein (MBP), on SDS gels. An antibody against the last 12 amino acids of SUR2A recognized (more ...)

Aberrant catalytic properties in the A1513T and Fs1524 mutants translated into abnormal interconversion of discrete conformations in the NBD2 ATPase cycle (Fig. 4a). Each mutation doubled the rate constant of the SUR-ATP to SUR-ADP-P_i conversion (k₂), reducing the lifetime of SUR2A in the prehydrolytic state (Fig. 4a,b). Moreover, the rate constant of the SUR-ADP-P_i to SUR-ADP transition (k₃) was one hundred times lower in the Fs1524 mutant, markedly extending the lifetime of the SUR-ADP-P_i conformation and ‘jamming’ the ATPase cycle (Fig. 4a-c). Thus, in contrast to the catalytic reaction in the wild type, where the rate-limiting step is ADP dissociation (k₄), the Fs1524 ATPase is characterized by rate-limiting P_i dissociation (k₃; Fig. 4c). In contrast, the A1513T mutation delayed the ATPase cycle in the SUR-ADP conformation, by reducing the rate constant defining ADP dissociation (k₄) by a factor of 2, and reduced the ADP association rate constant (k₀₄) by a factor of 10 (Fig. 4a-c).

Figure 4

Altered kinetics of the ATPase cycle lead to disrupted metabolic decoding by K_ATP channels. (a) Rate constants defining the SUR2A ATPase reaction were derived from pre-steady state kinetics of P_i liberation in wild-type (WT) and mutant NBD2 constructs (more ...)

The ATPase cycle in both A1513T and Fs1524 mutants was abnormally delayed in a posthydrolytic conformation, SUR-ADP-P_i or SUR-ADP. Although they had distinct patterns of lifetime distribution, each mutation diminished the likelihood that SUR2A could adopt a prehydrolytic conformation and increased the probability of posthydrolytic conformations (Fig. 4d). Individual conformations of the SUR2A ATPase cycle have distinct impacts on K_ATP channel regulation, with the prehydrolytic SUR-ATP state promoting channel closure and the posthydrolytic SUR-ADP-P_i and SUR-ADP states favoring channel activation⁵. Consequently, alterations in hydrolysis-driven SUR2A conformational probability induced by A1513T and Fs1524 translated into abnormal ATP sensitivity of mutant channels (Fig. 2e).

Under metabolic stress, the increase in intracellular ADP favors posthydrolytic ADP-bound conformations associated with antagonism of ATP-induced K_ATP channel pore inhibition⁵^,¹⁰. ADP-dependent K_ATP channel regulation, and thus the channel’s stress responsiveness, are represented by the relative shift from prehydrolytic to posthydrolytic of the conformational probability (P) of the ATPase induced by changes in ADP (dP/d[ADP]). Compared to the wild type and at any given ATP level, A1513T and Fs1524 mutants were less responsive in ADP-induced redistribution of post- (Fig. 4e) and prehydrolytic (Fig. 4f) conformations. Accordingly, metabolic pathways, like the creatine kinase phosphotransfer system²³, effectively regulated ATPase activity of wild-type but not mutant SUR2A (Fig. 4g). In fact, aberrant catalysis in mutant SUR2A generated defective K_ATP channel phenotypes characterized by abnormal responses to both ATP (Fig. 2e) and ADP (Fig. 4h,i), mediators of the cellular energetic state. Thus, the mutations A1513T and Fs1524 altered intrinsic catalytic properties of the SUR2A ATPase, compromising proper translation of cellular energetic signals into K_ATP channel-mediated membrane electrical events.

K_ATP channel mutations, identified here in two individuals with dilated cardiomyopathy, underscore the essential role of the intrinsic enzymatic reaction in cardiac channel pore regulation. Aberrant kinetics of hydrolysis at the regulatory channel subunit, despite unaltered nucleotide binding, produced defective metabolic signal decoding. Thus, nucleotide-dependent regulation of the stress-responsive K_ATP channel is based not only on conventional competition between ATP and ADP at nucleotide binding domains²⁴ but also on the profile of conformational interconversion driven by the catalytic cycle in the regulatory channel subunit. Traditionally linked to defects in ligand interaction, subunit trafficking or pore conductance¹⁰^,²⁰^,²⁵, human cardiac K_ATP channel dysfunction provoked by alterations in the catalytic module of the channel complex establishes a new mechanism for channelopathy. In this way, defective ion channel function would confer susceptibility to dilated cardiomyopathy.

METHODS

Mutational analysis

We carried out mutation scans of K_ATP channel genes in individuals, predominantly of European descent, with idiopathic dilated cardiomyopathy who gave informed consent with approval from the Mayo Clinic Institutional Review Board. According to established diagnostic criteria, individuals had left ventricular dimensions >95th percentile indexed for body surface area and age and left ventricular ejection fractions <50% as determined by echocardiography. We designed primers for exon-specific PCR amplification using the OLIGO v6.51 Primer Analysis Software (National Biosciences) and the WAVEMAKER version 4.0.32 Software (Transgenomic). We identified sequence variants in PCR-amplified DNA fragments by denaturing high-performance liquid chromatography heteroduplex analysis (WAVE DNA Fragment Analysis System, Transgenomic). We amplified fragments that formed heteroduplexes by PCR and cycle-sequenced them using a Thermo Sequenase kit (Amersham Life Sciences). To confirm mutations, we isolated mutated alleles on 1X Mutation Detection Enhancement gels (FMC Bioproducts), reamplified them and sequenced them²⁶. Healthy blood donors served as controls.

Homology modeling and molecular dynamics simulation

We generated the three-dimensional model of SUR2A NBD2 using the homology modeling program MODELLER 6 using as a template the crystal structure of the ATPase domain in the human TAP1 protein with ADP in the binding pocket (PDB code 1JJ7) identified by the FASTA search²⁷. We refined the initial atomic NBD2 three-dimensional model by energy minimizations and a 2.0-ns (1.0-fs time step) molecular dynamics simulation using the AMBER 5.0/6.0 program with a second generation force field (parm96.dat) according to established protocols²⁸.

Cloning of K_ATP channel subunits and purification of SUR2A ATPase

We subcloned cDNAs encoding SUR2A and Kir6.2 cDNA into the expression vector pcDNA3.1. We carried out PCR-based site-directed mutagenesis using primers incorporating identified mutations (QuickChange, Stratagene). We sequenced the constructs to verify mutations and rule out additional changes in sequence. We purified soluble SUR2A NBD2 (Thr1220-Thr1546) by over-expressing a fusion protein construct containing the maltose binding protein (MBP) used for affinity chromatography on an amylose resin in 600 mM NaCl, 1 mM EDTA, 20 mM Tris, 1 mM dithiothreitol and 10% glycerol³^,⁵^,⁷

Enzymology and nucleotide binding

We measured ATPase activity of purified NBD2, in the presence of Mg²⁺, as P_i generation detected spectrophotometrically at 360 nm with the EnzChek Phosphate Assay Kit or by monitoring conversion of [γ−³²P]ATP using thin layer chromatography and quantified it with PhosphorImager and ImageQuant software (Molecular Dynamics)³^,⁵^,⁷. We captured the initial kinetics of ATP hydrolysis by stopped-flow spectroscopy with instantaneous mixing of purified protein and substrate on an SX.18MV spectrometer (Applied Photophysics). We determined binding of fluorescent ATP analog TNP-ATP to purified NBD2 constructs by fluorescence spectrophotometry (QuantaMaster, PTI) at excitation and emission wavelengths of 410 nm and 540 nm, respectively.

Surface channel subunit expression

We injected cRNA encoding wild-type or mutant SUR2A (6 ng), tagged with an extracellular hemagglutinin epitope, with cRNA encoding Kir6.2 (2 ng) into Xenopus laevis oocytes²⁹. After 72 h, we labeled oocytes at 4 °C with 0.4 μg ml⁻¹ monoclonal antibody to hemagglutinin and horseradish peroxidase-coupled secondary antibody. Chemiluminescence at the plasmalemma of individual oocytes, generated from channel proteins that trafficked to the surface, was captured in Power Signal ELISA solution (Pierce) and quantified using a plate reader (Lab Systems).

Electrophysiology

We cotransfected human embryonic kidney (HEK293) cells, cultured at 5% CO₂ in Dulbecco’s modified Eagle medium with 10% fetal calf serum and 2 mM glutamine, with 0.5 μg of reporter green fluorescent protein and 6 μg of total plasmid DNA (cDNAs encoding wild-type or mutant SUR2A and Kir6.2 at a 5:1 ratio) using 15 μl of Fugene (Roche). We applied the patch-clamp technique using patch electrodes filled with 140 mM KCl, 1 mM CaCl₂, 1 mM MgCl₂ and 5 mM HEPES buffer (pH 7.3) and superfused the cells with 140 mM KCl, 1 mM MgCl₂, 5 mM EGTA and 5 mM HEPES-KOH buffer (pH 7.3). We measured K_ATP channel activity at 31 ± 1 °C using a temperature controller (HCC-100A; Dagan)⁵.

Kinetics of SUR2A ATPase

In the ATPase catalytic cycle,

E is SUR2A and stars indicate distinct conformations during Mg²⁺-dependent ATP hydrolysis. Transitions are bidirectional, with the exception of the P_i liberation step (k₃), which is irreversible in SUR2A NBD2 constructs. Therefore, the steady-state rate equation of the ATPase reaction was derived by the King-Altman method:

(1)

and

(2)

where E_t is total enzyme amount, v is the reaction rate, slope₁ is the slope of double-reciprocal dependence of the reaction rate v on ATP (at [ADP] = 0), v_max is the maximal reaction rate, K_ATP = k₋₁/k₁, K_ADP-Pi = k₂/k₋₂ and K_ADP = k₄/k₋₄.

We determined the ADP dissociation constant, K_ADP, from spectrometric measurement of steady-state NBD2 ATPase activity at [ADP] = 0 with variable [ATP]. Equation 1 can thus be simplified:

(3)

where slope₁ and v_max are assessable for experimental determination. At variable [ADP], with constant [ATP] ([ATP]_c), equation 1 is a linear function of [ADP]:

(4)

where the slope of the reciprocal reaction rate slope₂ = slope₁/K_ADP[ATP]_c. Thus, K_ADP was defined:

(5)

We defined kinetic rate constants of the ATPase reaction using stopped-flow spectroscopy where virtually instantaneous initiation of the ATPase reaction was achieved by mixture of NBD2 constructs and MgATP (2 mM ATP, 3 mM MgCl₂). During the first catalytic cycle, the time required to reach the step of P_i liberation is shorter than in subsequent reaction cycles. Therefore, the rate of reaction product accumulation in the initial pre-steady state is higher than in the following steady-state reaction. Kinetic parameters of the ATPase cycle were defined by fitting the kinetics of product accumulation with a system of differential equations with equations 2 and 5:

(6)

where E_t is the total enzyme amount. We solved the system using the Runge-Kutta algorithm. We calculated probabilities for NBD2 ATPase conformations (E-ATP, E-ADP-P_i and E-ADP) as:

(7)

(8)

and

(9)

where D_E = (k₂k₃ + k₋₁k₃ + k₋₁k₋₂)k₄; D_E-ATP = (k₃ + k₋₂)k₁k₄[ATP]; D_E-ADP-Pi = k₁k₂k₄[ATP]; and D_E-ADP = k₁k₂k₃[ATP] + (k₂k₃ + k₋₁k₃ + k₋₁k₋₂)k₋₄[ADP].

We defined lifetimes of conformations in the ATPase cycle as reciprocal values of the sum of rates leading away from particular conformations:

(10)

ACKNOWLEDGMENTS

We thank J. Bryan, Y. Kurachi and S. Seino for K_ATP channel clones; B. Schwappach for constructs used in trafficking studies; and T.P. Burghardt and A.J. Caride for technical advice. This work was supported by the US National Institutes of Health, American Heart Association, Miami Heart Research Institute, Marriott Foundation, Siragusa Foundation, University of Minnesota Supercomputing Institute, Mayo-Dubai Healthcare City Research Project and Mayo Foundation. A.T. is an Established Investigator of the American Heart Association.

Footnotes

COMPETING INTERESTS STATEMENT

The authors declare that they have no competing financial interests.

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