A gain-of-function I_K-ATP mutation and its role in sudden cardiac death associated with J-wave syndromes

Early repolarization (ER), consisting of a J-point elevation, a notch or slur on the QRS (J wave), and tall/symmetric T waves, is predominantly found in healthy young male patients and has traditionally been regarded as totally benign.^1,2 The observation in 2000 that an ER pattern in the coronary-perfused wedge preparation can easily convert to one in which phase 2 reentry gives rise to polymorphic ventricular tachycardia/ventricular fibrillation (VT/VF) prompted the suggestion that ER may in some cases predispose to malignant arrhythmias in the clinic.^3,4 Sporadic case reports and experimental studies have long suggested a critical role for the J wave in the pathogenesis of idiopathic ventricular fibrillation (IVF).^5–12 A definitive association between ER and IVF has been presented in more recent reports.^13–16

The high prevalence of ER in patients with IVF suggests that ER may be a necessary condition for the development of IVF. Although not a sensitive marker for sudden cardiac death (SCD), due to its high prevalence in the general population, ER when observed in patients with syncope or a malignant family history of sudden cardiac death may be prognostic of risk. A transient J-wave augmentation secondary to a shift of the balance of currents active in the early phases of the action potential portends a high risk for VF in patients with ER.

Based on the available data pointing to an association of risk with spatial localization of the early repolarization pattern, a classification scheme was proposed in which type 1 is associated with an ER pattern predominantly in the lateral precordial leads; this form is very prevalent among healthy male athletes and is thought to be largely benign. Type 2, showing an ER pattern predominantly in the inferior or inferolateral leads, is associated with a moderate level of risk, and type 3, showing an ER pattern globally in the inferior, lateral, and right precordial leads, seems to be associated with the highest level of risk and is often associated with electrical storms.⁴ Brugada syndrome (BrS) represents a fourth variant, in which ER is limited to the right precordial leads.

Electrocardiographic (ECG) changes in both BrS and ERS are dynamic,^12,17,18 with the most prominent ECG changes appearing just before the onset of VT/VF.^{5–12,17–19} The amplitude of J waves, barely noticeable during sinus rhythm, may become progressively augmented with increased vagal tone and bradycardia and still further accentuated after successive extrasystoles and compensatory pauses giving rise to short-long-short sequences.

Because of the similarity in ECG characteristics, clinical outcomes, and risk factors and because they share a common arrhythmic platform related to amplification of I_to-mediated J waves, these congenital and acquired syndromes have been grouped under the heading of J-wave syndromes.⁴

Data relative to the genetic and molecular basis for J-wave syndromes are relatively limited. BrS has previously been associated with mutations in 7 different genes, including SCN5A, GPD1L, CACNAC1C, CACNb2b, SCN1B, KCNE3, and SCN3B, whereas ERS has been associated with mutations in KCNJ8, CACNA1C, and CACNB2.^4,20,21 Haissaguerre et al²¹ were the first to associate KCNJ8 with ERS in the case of a young female patient with more than 100 recurrences of VF unresponsive to beta-blockers, lidocaine/mexiletine, verapamil, and amiodarone. Recurrences of VF were associated with marked accentuation of the early re-polarization pattern at times mimicking acute myocardial ischemia, although coronary angiography during one of the episodes was normal. However, functional expression data for the S422L missense mutation in KCNJ8 were not presented to substantiate the association of the genotype with the phenotype.²¹

This important deficiency has been partially addressed by a study reported by Medeiros-Domingo et al²² in this issue of the Journal. This talented group of investigators genetically screened 87 probands with BrS and 14 with ERS and found 1 BrS and 1 ERS proband with an S422L-KCNJ8 (Kir6.1) mutation; the variation was absent in 600 control subjects. The investigators coexpressed the KCNJ8 mutation with adenosine triphosphate (ATP) regulatory subunit SUR2A in COS-1 cells and measured I_K-ATP using whole-cell patch-clamp techniques. The investigators report a significantly larger I_K-ATP for the mutant versus wild type in response to a high concentration of pinacidil (100 μM). Although no ATP was added to the pipette solution, presumably to induce I_K-ATP, no data are presented in the absence of pinacidil. Under baseline conditions, normal intracellular ATP levels maintain the I_K-ATP channels closed. The presumption is that the S422L-KCNJ8 mutant channels fail to close properly at normal intracellular ATP concentrations, thus resulting in a gain of function. The gold standard for demonstrating a change in sensitivity to ATP involves the study of inside-out patches with the internal membrane exposed to different ATP concentrations. These data unfortunately are not provided, and a full characterization of the functional consequence of this mutation awaits such studies. This notwithstanding, the findings presented further implicate KCNJ8 as a novel J-wave syndrome-susceptibility gene and a gain-of-function in the cardiac K_ATP Kir6.1 channel (in the presence of pinacidil) secondary to KCNJ8-S422L mutation as a novel pathogenic mechanism in BrS and ERS.

ATP-sensitive channels (K_ATP), originally discovered by Noma et al²³ in heart, have since been found in many tissues, including pancreatic, skeletal muscle, kidney, and brain cells.²⁴ K_ATP channels, which remain closed at normal [ATP]_i, open as [ATP]_i and the ATP/adenosine diphosphate ratio decline during hypoxia or ischemia, thus regulating calcium entry, contraction, and oxygen utilization.²⁵ K_ATP channels are thought to be hetero-octameric in structure, consisting of 4 pore-forming subunits (KCNJ8-encoded Kir6.1 or KCNJ11-encoded Kir6.2), and 4 regulatory subunit sulfonylurea receptors (SUR): ABCC8-encoded SUR1 or ABCC9-encoded SUR2A in the heart.^26,27

Coassociation of Kir6.2 and SUR2A was long thought to be the principal subunit combination comprising K_ATP channels in the heart and conferring cardioprotection under ischemic conditions. As discussed by Medeiros-Domingo et al, ²² a number of studies have raised some serious questions regarding the predominant role of Kir6.2. Recent studies suggest that within a single K_ATP channel, more than 1 Kir6.x or SURx subunit can coexist.^28–32

A number of studies point to relatively high expression of Kir6.1 in cardiomyocytes.^33–37 In the mouse, Morrissey et al³⁸ used immunolocalization assays to demonstrate that both Kir6.1 and Kir6.2 as well as SUR2 (but not SUR1) are expressed in a sarcomeric striated pattern, suggesting their presence in T-tubules in ventricular cardiomyocytes and that Kir6.1 is more strongly expressed in epicardial ventricular myocytes. This transmural distribution is consistent with the observation that K_ATP channels are more prominently activated by ischemia in epicardium vs. endocardium^39,40 and may be related to the finding by Furukawa et al⁴¹ that ATP-regulated K+ channels are activated by a smaller reduction in intracellular ATP in feline epicardial versus endocardial cells.

This heterogeneous transmural distribution of Kir6.1, if present in humans, likely contributes to the development of ST-segment elevation observed in patients with BrS- and ERS-carrying mutations in KCNJ8 or SUR2A. The presence of an additional repolarization force during the early phases of the epicardial action potential, due to a gain of function of I_K-ATP, can generate an early repolarization pattern in the ECG by causing depression of the epicardial action potential dome.⁴ In the presence of a prominent transient outward current (I_to), commonly seen in right ventricular epicardium,⁴² the addition of I_K-ATP can lead to an accentuation of the action potential notch, thus amplifying the normal J wave, resulting in an ST-segment elevation, characteristic of BrS. A further outward shift in the balance of currents active during the early phases of the epicardial action potential, due to augmented vagal influence (I_K-Ach activation), bradycardia (greater availability of I_to), or mild ischemia (more I_K-ATP), can then lead to heterogeneous loss of the action potential dome, thus creating a dispersion of repolarization within epicardium and between epicardium and endocardium. This marked electrical heterogeneity may then facilitate the development of phase 2 reentry and polymorphic VT.^4,43

The study by Medeiros-Domingo et al²² advances our understanding of this sudden cardiac death susceptibility gene and its role in the pathogenesis of J-wave syndromes. The KCNJ8-S422L mutation, now reported in 2 ERS and 1 BrS probands, provides further evidence for a role for I_K-ATP in inherited and possibly acquired life-threatening channelopathies. This mutation in KCNJ8 may prove to be a hotspot, and we look forward to additional studies of its action to alter the biophysical properties of I_K-ATP, as well as assessment of its pathogenicity.