Glibenclamide reverses cardiovascular abnormalities of Cantu syndrome driven by KATP channel overactivity

Multiple vascular anomalies and refractory pericardial effusion in a young patient with Cantu syndrome: a case report and review of the literature

BACKGROUND

Cantu syndrome is a rare and complex multisystem disorder characterized by hypertrichosis, facial dysmorphism, osteochondroplasia and cardiac abnormalities. With only 150 cases reported worldwide, Cantu syndrome is now gaining wider recognition due to molecular testing and a growing body of literature that further characterizes the syndrome and some of its most important features. Cardiovascular pathology previously described in the literature include cardiomegaly, pericardial effusion, vascular dilation and tortuosity, and other congenital heart defects. However, cardiovascular involvement is highly variable amongst individuals with Cantu syndrome. In some instances, it can be extensive and severe requiring surgical management and long term follow up.

CASE PRESENTATION

Herein we report a case of a fourteen-year-old female who presented with worsening pericardial effusion of unknown etiology, and echocardiographic findings of concentric left ventricular hypertrophy, a mildly dilated aortic root and ascending aorta. Her medical history was notable for hemoptysis and an episode of pulmonary hemorrhage secondary to multiple aortopulmonary collaterals that were subsequently embolized in early childhood. She was initially managed with Ibuprofen and Colchicine but continued to worsen, and ultimately required a pericardial window for the management of refractory pericardial effusion. Imaging studies obtained on subsequent visits revealed multiple dilated and tortuous blood vessels in the head, neck, chest, and pelvis. A cardiomyopathy molecular studies panel was sent, and a pathogenic variant was identified in the ABCC9 gene, confirming the molecular diagnosis of autosomal dominant Cantu syndrome.

CONCLUSIONS

Vascular anomalies and significant cardiac involvement are often present in Cantu syndrome, however there are currently no established screening recommendations or surveillance protocols in place. The triad of hypertrichosis, facial dysmorphism, and unexplained cardiovascular involvement in any patient should raise suspicion for Cantu syndrome and warrant further investigation. Initial cardiac evaluation and follow up should be indicated in any patient with a clinical and/or molecular diagnosis of Cantu syndrome. Furthermore, whole body imaging should be utilized to evaluate the extent of vascular involvement and dictate long term monitoring and care.

Rapid Characterization of the Functional and Pharmacological Consequences of Cantú Syndrome KATP Channel Mutations in Intact Cells

Gain-of-function of KATP channels, resulting from mutations in either KCNJ8 (encoding inward rectifier sub-family 6 [Kir6.1]) or ABCC9 (encoding sulphonylurea receptor [SUR2]), cause Cantú syndrome (CS), a channelopathy characterized by excess hair growth, coarse facial appearance, cardiomegaly, and lymphedema. Here, we established a pipeline for rapid analysis of CS mutation consequences in Landing pad HEK 293 cell lines stably expressing wild type (WT) and mutant human Kir6.1 and SUR2B. Thallium-influx and cell membrane potential, reported by fluorescent Tl-sensitive Fluozin-2 and voltage-sensitive bis-(1,3-dibutylbarbituric acid)trimethine oxonol (DiBAC4(3)) dyes, respectively, were used to assess channel activity. In the Tl-influx assay, CS-associated Kir6.1 mutations increased sensitivity to the ATP-sensitive potassium (KATP) channel activator, pinacidil, but there was strikingly little effect of pinacidil for any SUR2B mutations, reflecting unexpected differences in the molecular mechanisms of Kir6.1 versus SUR2B mutations. Compared with the Tl-influx assay, the DiBAC4(3) assay presents more significant signal changes in response to subtle KATP channel activity changes, and all CS mutants (both Kir6.1 and SUR2B), but not WT channels, caused marked hyperpolarization, demonstrating that all mutants were activated under ambient conditions in intact cells. Most SUR2 CS mutations were markedly inhibited by <100 nM glibenclamide, but sensitivity to inhibition by glibenclamide, repaglinide, and PNU37883A was markedly reduced for Kir6.1 CS mutations. Understanding functional consequences of mutations can help with disease diagnosis and treatment. The analysis pipeline we have developed has the potential to rapidly identify mutational consequences, aiding future CS diagnosis, drug discovery, and individualization of treatment. SIGNIFICANCE STATEMENT: We have developed new fluorescence-based assays of channel activities and drug sensitivities of Cantú syndrome (CS) mutations in human Kir6.1/SUR2B-dependent KATP channels, showing that Kir6.1 mutations increase sensitivity to potassium channel openers, while SUR2B mutations markedly reduce K channel opener (KCO) sensitivity. However, both Kir6.1 and SUR2B CS mutations are both more hyperpolarized than WT cells under basal conditions, confirming pathophysiologically relevant gain-of-function, validating DiBAC4(3) fluorescence to characterize hyperpolarization induced by KATP channel activity under basal, non KCO-activated conditions.

Electrophysiology of human iPSC-derived vascular smooth muscle cells and cell autonomous consequences of Cantu Syndrome mutations

Objective

Cantu Syndrome (CS), a multisystem disease with a complex cardiovascular phenotype, is caused by GoF variants in the Kir6.1/SUR2 subunits of ATP-sensitive potassium (K ATP ) channels, and is characterized by low systemic vascular resistance, as well as tortuous, dilated vessels, and decreased pulse-wave velocity. Thus, CS vascular dysfunction is multifactorial, with distinct hypomyotonic and hyperelastic components. To dissect whether such complexities arise cell-autonomously within vascular smooth muscle cells (VSMCs), or as secondary responses to the pathophysiological milieu, we assessed electrical properties and gene expression in human induced pluripotent stem cell-derived VSMCs (hiPSC-VSMCs), differentiated from control and CS patient-derived hiPSCs, and in native mouse control and CS VSMCs.

Approach and Results

Whole-cell voltage-clamp of isolated aortic and mesenteric VSMCs isolated from wild type (WT) and Kir6.1[V65M] (CS) mice revealed no difference in voltage-gated K + (K v ) or Ca 2+ currents. K v and Ca 2+ currents were also not different between validated hiPSC-VSMCs differentiated from control and CS patient-derived hiPSCs. Pinacidil-sensitive K ATP currents in control hiPSC-VSMCs were consistent with those in WT mouse VSMCs, and were considerably larger in CS hiPSC-VSMCs. Consistent with lack of any compensatory modulation of other currents, this resulted in membrane hyperpolarization, explaining the hypomyotonic basis of CS vasculopathy. Increased compliance and dilation in isolated CS mouse aortae, was associated with increased elastin mRNA expression. This was consistent with higher levels of elastin mRNA in CS hiPSC-VSMCs, suggesting that the hyperelastic component of CS vasculopathy is a cell-autonomous consequence of vascular K ATP GoF.

Conclusions

The results show that hiPSC-VSMCs reiterate expression of the same major ion currents as primary VSMCs, validating the use of these cells to study vascular disease. The results further indicate that both the hypomyotonic and hyperelastic components of CS vasculopathy are cell-autonomous phenomena driven by K ATP overactivity within VSMCs.

Skeletal muscle delimited myopathy and verapamil toxicity in SUR2 mutant mouse models of AIMS

ABCC9-related intellectual disability and myopathy syndrome (AIMS) arises from loss-of-function (LoF) mutations in the ABCC9 gene, which encodes the SUR2 subunit of ATP-sensitive potassium (KATP ) channels. KATP channels are found throughout the cardiovascular system and skeletal muscle and couple cellular metabolism to excitability. AIMS individuals show fatigability, muscle spasms, and cardiac dysfunction. We found reduced exercise performance in mouse models of AIMS harboring premature stop codons in ABCC9. Given the roles of KATP channels in all muscles, we sought to determine how myopathy arises using tissue-selective suppression of KATP and found that LoF in skeletal muscle, specifically, underlies myopathy. In isolated muscle, SUR2 LoF results in abnormal generation of unstimulated forces, potentially explaining painful spasms in AIMS. We sought to determine whether excessive Ca2+ influx through CaV 1.1 channels was responsible for myopathology but found that the Ca2+ channel blocker verapamil unexpectedly resulted in premature death of AIMS mice and that rendering CaV 1.1 channels nonpermeable by mutation failed to reverse pathology; results which caution against the use of calcium channel blockers in AIMS.

KATP channels are necessary for glucose-dependent increases in amyloid-β and Alzheimer's disease-related pathology

Elevated blood glucose levels, or hyperglycemia, can increase brain excitability and amyloid-β (Aβ) release, offering a mechanistic link between type 2 diabetes and Alzheimer's disease (AD). Since the cellular mechanisms governing this relationship are poorly understood, we explored whether ATP-sensitive potassium (KATP) channels, which couple changes in energy availability with cellular excitability, play a role in AD pathogenesis. First, we demonstrate that KATP channel subunits Kir6.2/KCNJ11 and SUR1/ABCC8 were expressed on excitatory and inhibitory neurons in the human brain, and cortical expression of KCNJ11 and ABCC8 changed with AD pathology in humans and mice. Next, we explored whether eliminating neuronal KATP channel activity uncoupled the relationship between metabolism, excitability, and Aβ pathology in a potentially novel mouse model of cerebral amyloidosis and neuronal KATP channel ablation (i.e., amyloid precursor protein [APP]/PS1 Kir6.2-/- mouse). Using both acute and chronic paradigms, we demonstrate that Kir6.2-KATP channels are metabolic sensors that regulate hyperglycemia-dependent increases in interstitial fluid levels of Aβ, amyloidogenic processing of APP, and amyloid plaque formation, which may be dependent on lactate release. These studies identify a potentially new role for Kir6.2-KATP channels in AD and suggest that pharmacological manipulation of Kir6.2-KATP channels holds therapeutic promise in reducing Aβ pathology in patients with diabetes or prediabetes.

Lymphatic contractile dysfunction in mouse models of Cantú Syndrome with KATP channel gain-of-function

Cantú Syndrome (CS) is an autosomal dominant disorder caused by gain-of-function (GoF) mutations in the Kir6.1 and SUR2 subunits of KATP channels. KATP overactivity results in a chronic reduction in arterial tone and hypotension, leading to other systemic cardiovascular complications. However, the underlying mechanism of lymphedema, developed by >50% of CS patients, is unknown. We investigated whether lymphatic contractile dysfunction occurs in mice expressing CS mutations in Kir6.1 (Kir6.1[V65M]) or SUR2 (SUR2[A478V], SUR2[R1154Q]). Pressure myograph tests of contractile function of popliteal lymphatic vessels over the physiological pressure range revealed significantly impaired contractile strength and reduced frequency of spontaneous contractions at all pressures in heterozygous Kir6.1[V65M] vessels, compared to control littermates. Contractile dysfunction of intact popliteal lymphatics in vivo was confirmed using near-infrared fluorescence microscopy. Homozygous SUR2[A478V] vessels exhibited profound contractile dysfunction ex vivo, but heterozygous SUR2[A478V] vessels showed essentially normal contractile function. However, further investigation of vessels from all three GoF mouse strains revealed significant disruption in contraction wave entrainment, decreased conduction speed and distance, multiple pacemaker sites, and reversing wave direction. Tests of 2-valve lymphatic vessels forced to pump against an adverse pressure gradient revealed that all CS-associated genotypes were essentially incapable of pumping under an imposed outflow load. Our results show that varying degrees of lymphatic contractile dysfunction occur in proportion to the degree of molecular GoF in Kir6.1 or SUR2. This is the first example of lymphatic contractile dysfunction caused by a smooth muscle ion channel mutation and potentially explains the susceptibility of CS patients to lymphedema.

Zoledronic Acid Blocks Overactive Kir6.1/SUR2-Dependent KATP Channels in Skeletal Muscle and Osteoblasts in a Murine Model of Cantú Syndrome

Cantú syndrome (CS) is caused by the gain of function mutations in the ABCC9 and KCNJ8 genes encoding, respectively, for the sulfonylureas receptor type 2 (SUR2) and the inwardly rectifier potassium channel 6.1 (Kir6.1) of the ATP-sensitive potassium (KATP) channels. CS is a multi-organ condition with a cardiovascular phenotype, neuromuscular symptoms, and skeletal malformations. Glibenclamide has been proposed for use in CS, but even in animals, the drug is incompletely effective against severe mutations, including the Kir6.1wt/V65M. Patch-clamp experiments showed that zoledronic acid (ZOL) fully reduced the whole-cell KATP currents in bone calvaria cells from wild type (WT/WT) and heterozygous Kir6.1wt/V65MCS mice, with IC50 for ZOL block < 1 nM in each case. ZOL fully reduced KATP current in excised patches in skeletal muscle fibers in WT/WT and CS mice, with IC50 of 100 nM in each case. Interestingly, KATP currents in the bone of heterozygous SUR2wt/A478V mice were less sensitive to ZOL inhibition, showing an IC50 of ~500 nM and a slope of ~0.3. In homozygous SUR2A478V/A478V cells, ZOL failed to fully inhibit the KATP currents, causing only ~35% inhibition at 100 μM, but was responsive to glibenclamide. ZOL reduced the KATP currents in Kir6.1wt/VMCS mice in both skeletal muscle and bone cells but was not effective in the SUR2[A478V] mice fibers. These data indicate a subunit specificity of ZOL action that is important for appropriate CS therapies.

Cantú syndrome: A new case and evolution of clinical conditions during first 2-year follow-up

Cantú syndrome, or hypertrichotic osteochondrodysplasia, is a rare autosomal dominant disease characterized by congenital hypertrichosis, characteristic dysmorphisms, skeletal abnormalities and cardiomegaly. We report on a 7-year-old girl with congenital generalized hypertrichosis, coarse facial appearance and cardiac involvement, with a de novo heterozygous mutation (c.3461G > A) in the ABCC9 gene. During the annual cardiac follow-up at the age of nine the echocardiogram showed mild left ventricular dilatation in consideration of which she started ramipril treatment. The progression of the clinical manifestations of Cantú syndrome highlights the relevance of an early diagnosis, including genetic analysis, and a multidisciplinary approach with long-term follow-up.

NLRP3 Inflammasome: a Novel Insight into Heart Failure

Among numerous cardiovascular diseases, heart failure is a final and fatal stage, and its morbidity, mortality, and rehospitalization rate remain high, which reduces the exercise tolerance of patients and brings great medical burden and economic pressure to the society. Inflammation takes on a major influence in the occurrence, development, and prognosis of heart failure (HF). The NLRP3 inflammasome is a key node in a chronic inflammatory response, which can accelerate the production of pro-inflammatory cytokines IL-1β and IL-18, leading to the inflammatory response. Therefore, whether it is possible to suppress the downstream factors of NLRP3 inflammasome and its signaling path is expected to provide a new intervention mediator for the therapy of heart failure. This article synopsizes the research progress of NLRP3 inflammasome in heart failure, to provide a reference for clinical treatment. CLINICAL RELEVANCE: This study explored the downstream factors of NLRP3 inflammasome and its signal pathway. Targeted drug therapy for NLRP3 inflammasome is expected to provide a new intervention target for the treatment of heart failure.

Lymphatic Clearance and Pump Function

Lymphatic vessels have an active role in draining excess interstitial fluid from organs and serving as conduits for immune cell trafficking to lymph nodes. In the central circulation, the force needed to propel blood forward is generated by the heart. In contrast, lymphatic vessels rely on intrinsic vessel contractions in combination with extrinsic forces for lymph propulsion. The intrinsic pumping features phasic contractions generated by lymphatic smooth muscle. Periodic, bicuspid valves composed of endothelial cells prevent backflow of lymph. This work provides a brief overview of lymph transport, including initial lymph formation along with cellular and molecular mechanisms controlling lymphatic vessel pumping.

Lymphedema as first clinical presentation of Cantu Syndrome: reversed phenotyping after identification of gain-of-function variant in ABCC9

Cantu Syndrome (CS), [OMIM #239850] is characterized by hypertrichosis, osteochondrodysplasia, and cardiomegaly. CS is caused by gain-of-function (GOF) variants in the KCNJ8 or ABCC9 genes that encode pore-forming Kir6.1 and regulatory SUR2 subunits of ATP-sensitive potassium (KATP) channels. Many subjects with CS also present with the complication of lymphedema. A previously uncharacterized, heterozygous ABCC9 variant, p.(Leu1055_Glu1058delinsPro), termed indel1055, was identified in an individual diagnosed with idiopathic lymphedema. The variant was introduced into the equivalent position of rat SUR2A, and inside-out patches were used to characterize the KATP channels formed by Kir6.2 and WT or mutant SUR2A subunits coexpressed in Cosm6 cells. The indel1055 variant causes gain-of-function of the channel, with an increase of the IC50 for ATP inhibition compared to WT. Retrospective consideration of this individual reveals clear features of Cantu Syndrome. An additional heterozygous ABCC9 variant, p.(Ile419Thr), was identified in a second individual diagnosed with lymphedema. In this case, there were no additional features consistent with CS, and the properties of p.(Ile416Thr) (the corresponding mutation in rat SUR2A)--containing channels were not different from WT. This proof-of-principle study shows that idiopathic lymphedema may actually be a first presentation of otherwise unrecognized Cantu Syndrome, but molecular phenotyping of identified variants is necessary to confirm relevance.

Personalized Therapeutics for KATP-Dependent Pathologies

Ubiquitously expressed throughout the body, ATP-sensitive potassium (K_ATP) channels couple cellular metabolism to electrical activity in multiple tissues; their unique assembly as four Kir6 pore-forming subunits and four sulfonylurea receptor (SUR) subunits has resulted in a large armory of selective channel opener and inhibitor drugs. The spectrum of monogenic pathologies that result from gain- or loss-of-function mutations in these channels, and the potential for therapeutic correction of these pathologies, is now clear. However, while available drugs can be effective treatments for specific pathologies, cross-reactivity with the other Kir6 or SUR subfamily members can result in drug-induced versions of each pathology and may limit therapeutic usefulness. This review discusses the background to K_ATP channel physiology, pathology, and pharmacology and considers the potential for more specific or effective therapeutic agents.

A Unique High-Output Cardiac Hypertrophy Phenotype Arising From Low Systemic Vascular Resistance in Cantu Syndrome

Background Cardiomegaly caused by left ventricular hypertrophy is a risk factor for development of congestive heart failure, classically associated with decreased systolic and/or diastolic ventricular function. Less attention has been given to the phenotype of left ventricular hypertrophy with enhanced ventricular function and increased cardiac output, which is potentially associated with high-output heart failure. Lack of recognition may pose diagnostic ambiguity and management complexities. Methods and Results We sought to systematically characterize high-output cardiac hypertrophy in subjects with Cantu syndrome (CS), caused by gain-of-function variants in ABCC9, which encodes cardiovascular KATP (ATP-sensitive potassium) channel subunits. We studied the cardiovascular phenotype longitudinally in 31 subjects with CS with confirmed ABCC9 variants (median [interquartile range] age 8 years [3-32 years], body mass index 19.9 [16.5-22.9], 16 male subjects). Subjects with CS presented with significant left ventricular hypertrophy (left ventricular mass index 86.7 [57.7-103.0] g/m2 in CS, n=30; 26.6 [24.1-32.8] g/m2 in controls, n=17; P<0.0001) and low blood pressure (systolic 94.5 [90-103] mm Hg in CS, n=17; 109 [98-115] mm Hg in controls, n=17; P=0.0301; diastolic 60 [56-66] mm Hg in CS, n=17; 69 [65-72] mm Hg in control, n=17; P=0.0063). Most (21/31) subjects with CS exhibited eccentric hypertrophy with normal left ventricular wall thickness. Congestive heart failure symptoms were evident in 4 of the 5 subjects with CS aged >40 years on long-term follow-up. Conclusions The data define the natural history of high-output cardiac hypertrophy resulting from decreased systemic vascular resistance in subjects with CS, a defining population for long-term consequences of high-output hypertrophy caused by low systemic vascular resistance, and the potential for progression to high-output heart failure.

KATP channels in lymphatic function

KATP channels function as negative regulators of active lymphatic pumping and lymph transport. This review summarizes and critiques the evidence for the expression of specific KATP channel subunits in lymphatic smooth muscle and endothelium, the roles that they play in normal lymphatic function, and their possible involvement in multiple diseases, including metabolic syndrome, lymphedema, and Cantú syndrome. For each of these topics, suggestions are made for directions for future research.

Reconstruction and analysis of potential biomarkers for hypertrophic cardiomyopathy based on a competing endogenous RNA network

Hypertrophic cardiomyopathy (HCM) is a common heritable cardiomyopath. Although considerable effort has been made to understand the pathogenesis of HCM, the mechanism of how long noncoding RNA (lncRNA)-associated competing endogenous RNA (ceRNA) network result in HCM remains unknown. In this study, we acquired a total of 520 different expression profiles of lncRNAs (DElncRNAs) and 371 messenger RNAs (mRNA, DEGs) by microarray and 33 microRNAs (DEmiRNAs) by sequencing in plasma of patients with HCM and healthy controls. Then lncRNA–miRNA pairs were predicted using miRcode and starBase and crossed with DEmiRNAs. MiRNA–mRNA pairs were retrieved from miRanda and TargetScan and crossed with DEGs. Combined with these pairs, the ceRNA network with eight lncRNAs, three miRNAs, and 22 mRNAs was constructed. lncRNA RP11-66N24.4 and LINC00310 were among the top 10% nodes. The hub nodes were analyzed to reconstruct a subnetwork. Furthermore, quantitative real-time polymerase chain reaction results showed that LINC00310 was significantly decreased in patients with HCM. For LINC00310, GO analysis revealed that biological processes were enriched in cardiovascular system development, sprouting angiogenesis, circulatory system development, and pathway analysis in the cGMP-PKG signaling pathway. These results indicate that the novel lncRNA-related ceRNA network in HCM and LINC00310 may play a role in the mechanism of HCM pathogenesis, which could provide insight into the pathogenesis of HCM.

Kir6.1 and SUR2B in Cantú syndrome

Kir6.1 and SUR2 are subunits of ATP-sensitive potassium (KATP) channels expressed in a wide range of tissues. Extensive study has implicated roles of these channel subunits in diverse physiological functions. Together they generate the predominant KATP conductance in vascular smooth muscle and are the target of vasodilatory drugs. Roles for Kir6.1/SUR2 dysfunction in disease have been suggested based on studies of animal models and human genetic discoveries. In recent years, it has become clear that gain-of-function (GoF) mutations in both genes result in Cantú syndrome (CS)-a complex, multisystem disorder. There is currently no targeted therapy for CS, but studies of mouse models of the disease reveal that pharmacological reversibility of cardiovascular and gastrointestinal pathologies can be achieved by administration of the KATP channel inhibitor, glibenclamide. Here we review the function, structure, and physiological and pathological roles of Kir6.1/SUR2B channels, with a focus on CS. Recent studies have led to much improved understanding of the underlying pathologies and the potential for treatment, but important questions remain: Can the study of genetically defined CS reveal new insights into Kir6.1/SUR2 function? Do these reveal new pathophysiological mechanisms that may be important in more common diseases? And is our pharmacological armory adequately stocked?

Functional Regulation of KATP Channels and Mutant Insight Into Clinical Therapeutic Strategies in Cardiovascular Diseases

ATP-sensitive potassium channels (K_ATP channels) play pivotal roles in excitable cells and link cellular metabolism with membrane excitability. The action potential converts electricity into dynamics by ion channel-mediated ion exchange to generate systole, involved in every heartbeat. Activation of the K_ATP channel repolarizes the membrane potential and decreases early afterdepolarization (EAD)-mediated arrhythmias. K_ATP channels in cardiomyocytes have less function under physiological conditions but they open during severe and prolonged anoxia due to a reduced ATP/ADP ratio, lessening cellular excitability and thus preventing action potential generation and cell contraction. Small active molecules activate and enhance the opening of the K_ATP channel, which induces the repolarization of the membrane and decreases the occurrence of malignant arrhythmia. Accumulated evidence indicates that mutation of K_ATP channels deteriorates the regulatory roles in mutation-related diseases. However, patients with mutations in K_ATP channels still have no efficient treatment. Hence, in this study, we describe the role of K_ATP channels and subunits in angiocardiopathy, summarize the mutations of the K_ATP channels and the functional regulation of small active molecules in K_ATP channels, elucidate the potential mechanisms of mutant K_ATP channels and provide insight into clinical therapeutic strategies.

Bisphosphonates Targeting Ion Channels and Musculoskeletal Effects

Bisphosphonates (BPs) are the most used bone-specific anti-resorptive agents, often chosen as first-line therapy in several bone diseases characterized by an imbalance between osteoblast-mediated bone production and osteoclast-mediated bone resorption. BPs target the farnesyl pyrophosphate synthase (FPPS) in osteoclasts, reducing bone resorption. Lately, there has been an increasing interest in BPs direct pro-survival/pro-mineralizing properties in osteoblasts and their pain-relieving effects. Even so, molecular targets involved in these effects appear now largely elusive. Ion channels are emerging players in bone homeostasis. Nevertheless, the effects of BPs on these proteins have been poorly described. Here we reviewed the actions of BPs on ion channels in musculoskeletal cells. In particular, the TRPV1 channel is essential for osteoblastogenesis. Since it is involved in bone pain sensation, TRPV1 is a possible alternative target of BPs. Ion channels are emerging targets and anti-target for bisphosphonates. Zoledronic acid can be the first selective musculoskeletal and vascular KATP channel blocker targeting with high affinity the inward rectifier channels Kir6.1-SUR2B and Kir6.2-SUR2A. The action of this drug against the overactive mutants of KCNJ9-ABCC9 genes observed in the Cantu’ Syndrome (CS) may improve the appropriate prescription in those CS patients affected by musculoskeletal disorders such as bone fracture and bone frailty.

Cantù syndrome: Report of a patient with a novel variant in KCNJ8 and revision of literature

Cantù syndrome (CS) is a rare multisystemic disorder, characterized by congenital hypertrichosis, macrocephaly, facial dysmorphisms, cardiomegaly, vascular, and skeletal anomalies. From the cognitive point of view, most of the patients show a mild speech delay and a few of them present intellectual disability and learning difficulties. To date, most CS-reported cases are caused by heterozygous ABCC9 gene mutations. Only three patients with CS and heterozygous KCNJ8 gene variants have been reported. The authors here present the fourth case of CS with a variant in KCNJ8 in a 6-month-old baby. Diagnosis was reached through Trio-Whole Exome analysis that revealed a de novo missense variant in KCNJ8.

ATP-sensitive potassium channels in zebrafish cardiac and vascular smooth muscle

ATP-sensitive potassium channels (K_ATP channels) are hetero-octameric nucleotide-gated ion channels that couple cellular metabolism to excitability in various tissues. In the heart, K_ATP channels are activated during ischaemia and potentially during adrenergic stimulation. In the vasculature, they are normally active at a low level, reducing vascular tone, but the ubiquitous nature of these channels leads to complex and poorly understood channelopathies as a result of gain- or loss-of-function mutations. Zebrafish (ZF) models of these channelopathies may provide insights to the link between molecular dysfunction and complex pathophysiology, but this requires understanding the tissue dependence of channel activity and subunit specificity. Thus far, direct analysis of ZF K_ATP expression and functional properties has only been performed in pancreatic β-cells. Using a comprehensive combination of genetically modified fish, electrophysiology and gene expression analysis, we demonstrate that ZF cardiac myocytes (CM) and vascular smooth muscle (VSM) express functional K_ATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. However, in contrast to mammalian cardiovascular K_ATP channels, ZF channels are insensitive to potassium channel opener drugs (pinacidil, minoxidil) in both chambers of the heart and in VSM. The results provide a first characterization of the molecular properties of fish K_ATP channels and validate the use of such genetically modified fish as models of human Cantú syndrome and ABCC9-related Intellectual Disability and Myopathy syndrome. KEY POINTS: Zebrafish cardiac myocytes (CM) and vascular smooth muscle (VSM) express functional K_ATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. In contrast to mammalian cardiovascular K_ATP channels, zebrafish channels are insensitive to potassium channel opener drugs (pinacidil, minoxidil) in both chambers of the heart and in VSM. We provide a first characterization of the molecular properties of fish K_ATP channels and validate the use of such genetically modified fish as models of human Cantú syndrome and ABCC9-related Intellectual Disability and Myopathy syndrome.

Vascular KATP channel structural dynamics reveal regulatory mechanism by Mg-nucleotides

Vascular K_ATP channels formed by the potassium channel Kir6.1 and its regulatory protein SUR2B maintain blood pressure in the physiological range. Overactivity of the channel due to genetic mutations in either Kir6.1 or SUR2B causes severe cardiovascular pathologies known as Cantú syndrome. The cryogenic electron microscopy structures of the vascular K_ATP channel reported here show multiple, dynamically related conformations of the regulatory subunit SUR2B. Molecular dynamics simulations reveal the negatively charged ED-domain in SUR2B, a stretch of 15 glutamate (E) and aspartate (D) residues not previously resolved, play a key MgADP-dependent role in mediating interactions at the interface between the SUR2B and Kir6.1 subunits. Our findings provide a mechanistic understanding of how channel activity is regulated by intracellular MgADP.

Vascular tone is dependent on smooth muscle K_ATP channels comprising pore-forming Kir6.1 and regulatory SUR2B subunits, in which mutations cause Cantú syndrome. Unique among K_ATP isoforms, they lack spontaneous activity and require Mg-nucleotides for activation. Structural mechanisms underlying these properties are unknown. Here, we determined cryogenic electron microscopy structures of vascular K_ATP channels bound to inhibitory ATP and glibenclamide, which differ informatively from similarly determined pancreatic K_ATP channel isoform (Kir6.2/SUR1). Unlike SUR1, SUR2B subunits adopt distinct rotational “propeller” and “quatrefoil” geometries surrounding their Kir6.1 core. The glutamate/aspartate-rich linker connecting the two halves of the SUR-ABC core is observed in a quatrefoil-like conformation. Molecular dynamics simulations reveal MgADP-dependent dynamic tripartite interactions between this linker, SUR2B, and Kir6.1. The structures captured implicate a progression of intermediate states between MgADP-free inactivated, and MgADP-bound activated conformations wherein the glutamate/aspartate-rich linker participates as mobile autoinhibitory domain, suggesting a conformational pathway toward K_ATP channel activation.

Consequences of SUR2[A478V] Mutation in Skeletal Muscle of Murine Model of Cantu Syndrome

(1) Background: Cantu syndrome (CS) arises from gain-of-function (GOF) mutations in the ABCC9 and KCNJ8 genes, which encode ATP-sensitive K⁺ (KATP) channel subunits SUR2 and Kir6.1, respectively. Most CS patients have mutations in SUR2, the major component of skeletal muscle KATP, but the consequences of SUR2 GOF in skeletal muscle are unknown. (2) Methods: We performed in vivo and ex vivo characterization of skeletal muscle in heterozygous SUR2[A478V] (SUR2^wt/AV) and homozygous SUR2[A478V] (SUR2^AV/AV) CS mice. (3) Results: In SUR2^wt/AV and SUR2^AV/AV mice, forelimb strength and diaphragm amplitude movement were reduced; muscle echodensity was enhanced. KATP channel currents recorded in Flexor digitorum brevis fibers showed reduced MgATP-sensitivity in SUR2^wt/AV, dramatically so in SUR2^AV/AV mice; IC₅₀ for MgATP inhibition of KATP currents were 1.9 ± 0.5 × 10⁻⁵ M in SUR2^wt/AV and 8.6 ± 0.4 × 10⁻⁶ M in WT mice and was not measurable in SUR2^AV/AV. A slight rightward shift of sensitivity to inhibition by glibenclamide was detected in SUR2^AV/AV mice. Histopathological and qPCR analysis revealed atrophy of soleus and tibialis anterior muscles and up-regulation of atrogin-1 and MuRF1 mRNA in CS mice. (4) Conclusions: SUR2[A478V] “knock-in” mutation in mice impairs KATP channel modulation by MgATP, markedly so in SUR2^AV/AV, with atrophy and non-inflammatory edema in different skeletal muscle phenotypes.

Young adult with Cantú syndrome: dealing with a rare genetic skin disorder

This case report of a young adult with Cantú syndrome (CS) illustrates a remarkable journey of learning how to cope with symptom management and emotional impact associated with a rare skin condition. We describe a 20-year-old woman with a CS-related mutation in ABCC9 resulting in clinical manifestations, including congenital hypertrichosis, facial dysmorphism and cardiomegaly. As of yet, no treatment is available for CS.Little is known about the impact of CS and similar (skin) conditions on the life of affected individuals, and about their needs and preferences in this regard. Hence, we describe the psychosocial implications our case had to deal with immediately after her diagnosis. In addition, we outline her significant progress in managing disease-associated features and emotional stress prompted by considerable personal development and an increase in confidence. This example shows that a normal lifestyle is achievable for (newly diagnosed) individuals despite suffering from CS or a related skin disorder.

The Pharmacology of ATP-Sensitive K+ Channels (KATP)

ATP-sensitive K⁺ channels (K_ATP) are inwardly-rectifying potassium channels, broadly expressed throughout the body. K_ATP is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels thus playing an important physiological role by coupling cellular metabolism with membrane excitability. The hetero-octameric channel complex is formed of 4 pore-forming inward rectifier Kir6.x subunits (Kir6.1 or Kir6.2) and 4 regulatory sulfonylurea receptor subunits (SUR1, SUR2A, or SUR2B). These subunits can associate in various tissue-specific combinations to form functional K_ATP channels with distinct electrophysiological and pharmacological properties. K_ATP channels play many important physiological roles and mutations in channel subunits can result in diseases such as disorders of insulin handling, cardiac arrhythmia, cardiomyopathy, and neurological abnormalities. The tissue-specific expression of K_ATP channel subunits coupled with their rich and diverse pharmacology makes K_ATP channels attractive therapeutic targets in the treatment of endocrine and cardiovascular diseases.

Behavioral and cognitive functioning in individuals with Cantú syndrome

Cantú syndrome (CS) is caused by pathogenic variants in ABCC9 and KCNJ8 encoding the regulatory and pore-forming subunits of ATP-sensitive potassium (K_ATP ) channels. CS is characterized by congenital hypertrichosis, distinctive facial features, peripheral edema, and cardiac and neurodevelopmental abnormalities. Behavioral and cognitive issues have been self-reported by some CS individuals, but results of formal standardized investigations have not been published. To assess the cognitive profile, social functioning, and psychiatric symptoms in a large group of CS subjects systematically in a cross-sectional manner, we invited 35 individuals (1-69 years) with confirmed ABCC9 variants and their relatives to complete various commonly applied standardized age-related questionnaires, including the Kaufman brief intelligence test 2, the social responsiveness scale-2, and the Achenbach system of empirically based assessment. The majority of CS individuals demonstrated average verbal and nonverbal intelligence compared to the general population. Fifteen percent of cases showed social functioning strongly associated with a clinical diagnosis of autism spectrum disorder. Both externalizing and internalizing problems were also present in this cohort. In particular, anxiety, anxiety or attention deficit hyperactivity disorder, and autism spectrum behaviors were predominantly observed in the younger subjects in the cohort (≥25%), but this percentage decreased markedly in adults.

Syndromic disorders caused by gain-of-function variants in KCNH1, KCNK4, and KCNN3-a subgroup of K+ channelopathies

Decreased or increased activity of potassium channels caused by loss-of-function and gain-of-function (GOF) variants in the corresponding genes, respectively, underlies a broad spectrum of human disorders affecting the central nervous system, heart, kidney, and other organs. While the association of epilepsy and intellectual disability (ID) with variants affecting function in genes encoding potassium channels is well known, GOF missense variants in K⁺ channel encoding genes in individuals with syndromic developmental disorders have only recently been recognized. These syndromic phenotypes include Zimmermann–Laband and Temple–Baraitser syndromes, caused by dominant variants in KCNH1, FHEIG syndrome due to dominant variants in KCNK4, and the clinical picture associated with dominant variants in KCNN3. Here we review the presentation of these individuals, including five newly reported with variants in KCNH1 and three additional individuals with KCNN3 variants, all variants likely affecting function. There is notable overlap in the phenotypic findings of these syndromes associated with dominant KCNN3, KCNH1, and KCNK4 variants, sharing developmental delay and/or ID, coarse facial features, gingival enlargement, distal digital hypoplasia, and hypertrichosis. We suggest to combine the phenotypes and define a new subgroup of potassium channelopathies caused by increased K⁺ conductance, referred to as syndromic neurodevelopmental K⁺ channelopathies due to dominant variants in KCNH1, KCNK4, or KCNN3.

Pathophysiological Consequences of KATP Channel Overactivity and Pharmacological Response to Glibenclamide in Skeletal Muscle of a Murine Model of Cantù Syndrome

Cantù syndrome (CS) arises from mutations in ABCC9 and KCNJ8 genes that lead to gain of function (GOF) of ATP-sensitive potassium (KATP) channels containing SUR2A and Kir6.1 subunits, respectively, of KATP channels. Pathological consequences of CS have been reported for cardiac and smooth muscle cells but consequences in skeletal muscle are unknown. Children with CS show muscle hypotonia and adult manifest fatigability. We analyzed muscle properties of Kir6.1[V65M] CS mice, by measurements of forelimb strength and ultrasonography of hind-limb muscles, as well as assessing KATP channel properties in native Flexor digitorum brevis (FDB) and Soleus (SOL) fibers by the patch-clamp technique in parallel with histopathological, immunohistochemical and Polymerase Chain Reaction (PCR) analysis. Forelimb strength was lower in Kir6.1^wt/VM mice than in WT mice. Also, a significant enhancement of echodensity was observed in hind-limb muscles of Kir6.1^wt/VM mice relative to WT, suggesting the presence of fibrous tissue. There was a higher KATP channel current amplitude in Kir6.1^wt/VM FDB fibers relative to WT and a reduced response to glibenclamide. The IC₅₀ of glibenclamide to block KATP channels in FDB fibers was 1.3 ± 0.2 × 10⁻⁷ M in WT and 1.2 ± 0.1 × 10⁻⁶ M in Kir6.1^wt/VM mice, respectively; and it was 1.2 ± 0.4 × 10⁻⁷ M in SOL WT fibers but not measurable in Kir6.1^wt/VM fibers. The sensitivity of the KATP channel to MgATP was not modified in Kir6.1^wt/VM fibers. Histopathological/immunohistochemical analysis of SOL revealed degeneration plus regressive-necrotic lesions with regeneration, and up-regulation of Atrogin-1, MuRF1, and BNIP3 mRNA/proteins in Kir6.1^wt/VM mice. Kir6.1^wt/VM mutation in skeletal muscle leads to changes of the KATP channel response to glibenclamide in FDB and SOL fibers, and it is associated with histopathological and gene expression changes in slow-twitch muscle, suggesting marked atrophy and autophagy.

Kir6.1- and SUR2-dependent KATP over-activity disrupts intestinal motility in murine models of Cantu Syndrome

Cantύ Syndrome (CS), caused by gain-of-function (GOF) mutations in pore-forming (Kir6.1, KCNJ8) and accessory (SUR2, ABCC9) ATP-sensitive potassium (KATP) channel subunit genes, is frequently accompanied by gastrointestinal (GI) dysmotility, and we describe one CS patient who required an implanted intestinal irrigation system for successful stooling. We used gene-modified mice to assess the underlying KATP channel subunits in gut smooth muscle, and to model the consequences of altered KATP channels in CS gut. We show that Kir6.1/SUR2 subunits underlie smooth muscle KATP channels throughout the small intestine and colon. Knock-in mice, carrying human KCNJ8 and ABCC9 CS mutations in the endogenous loci, exhibit reduced intrinsic contractility throughout the intestine, resulting in death when weaned onto solid food in the most severely affected animals. Death is avoided by weaning onto a liquid gel diet, implicating intestinal insufficiency and bowel impaction as the underlying cause, and GI transit is normalized by treatment with the KATP inhibitor glibenclamide. We thus define the molecular basis of intestinal KATP channel activity, the mechanism by which overactivity results in GI insufficiency, and a viable approach to therapy.

The surprising complexity of KATP channel biology and of genetic diseases

The ATP-sensitive K+ channel (KATP) is formed by the association of four inwardly rectifying K+ channel (Kir6.x) pore subunits with four sulphonylurea receptor (SUR) regulatory subunits. Kir6.x or SUR mutations result in KATP channelopathies, which reflect the physiological roles of these channels, including but not limited to insulin secretion, cardiac protection, and blood flow regulation. In this issue of the JCI, McClenaghan et al. explored one of the channelopathies, namely Cantu syndrome (CS), which is a result of one kind of KATP channel mutation. Using a knockin mouse model, the authors demonstrated that gain-of-function KATP mutations in vascular smooth muscle resulted in cardiac remodeling. Moreover, they were able to reverse the cardiovascular phenotypes by administering the KATP channel blocker glibenclamide. These results exemplify how genetic mutations can have an impact on developmental trajectories, and provide a therapeutic approach to mitigate cardiac hypertrophy in cases of CS.

Cantú syndrome versus Zimmermann-Laband syndrome: Report of nine individuals with ABCC9 variants

Cantú syndrome (CS) is a rare developmental disorder characterized by a coarse facial appearance, macrocephaly, hypertrichosis, skeletal and cardiovascular anomalies and caused by heterozygous gain-of-function variants in ABCC9 and KCNJ8, encoding subunits of heterooctameric ATP-sensitive potassium (K_ATP) channels. CS shows considerable clinical overlap with Zimmermann-Laband syndrome (ZLS), a rare condition with coarse facial features, hypertrichosis, gingival overgrowth, intellectual disability of variable degree, and hypoplasia or aplasia of terminal phalanges and/or nails. ZLS is caused by heterozygous gain-of-function variants in KCNH1 or KCNN3, and gain-of-function KCNK4 variants underlie the clinically similar FHEIG (facial dysmorphism, hypertrichosis, epilepsy, intellectual disability/developmental delay, and gingival overgrowth) syndrome; KCNH1, KCNN3 and KCNK4 encode potassium channels. Within our research project on ZLS, we performed targeted Sanger sequencing of ABCC9 in 15 individuals tested negative for a mutation in the ZLS-associated genes and found two individuals harboring a heterozygous pathogenic ABCC9 missense variant. Through a collaborative effort, we identified a total of nine individuals carrying a monoallelic ABCC9 variant: five sporadic patients and four members of two unrelated families. Among the six detected ABCC9 missense variants, four [p.(Pro252Leu), p.(Thr259Lys), p.(Ala1064Pro), and p.(Arg1197His)] were novel. Systematic assessment of the clinical features in the nine cases with an ABCC9 variant highlights the significant clinical overlap between ZLS and CS that includes early developmental delay, hypertrichosis, gingival overgrowth, joint laxity, and hypoplasia of terminal phalanges and nails. Gain of K⁺ channel activity possibly accounts for significant clinical similarities of CS, ZLS and FHEIG syndrome and defines a new subgroup of potassium channelopathies.

The Mechanism of High-Output Cardiac Hypertrophy Arising From Potassium Channel Gain-of-Function in Cantú Syndrome

Dramatic cardiomegaly arising from gain-of-function (GoF) mutations in the ATP-sensitive potassium (K_ATP) channels genes, ABCC9 and KCNJ8, is a characteristic feature of Cantú syndrome (CS). How potassium channel over-activity results in cardiac hypertrophy, as well as the long-term consequences of cardiovascular remodeling in CS, is unknown. Using genome-edited mouse models of CS, we therefore sought to dissect the pathophysiological mechanisms linking K_ATP channel GoF to cardiac remodeling. We demonstrate that chronic reduction of systemic vascular resistance in CS is accompanied by elevated renin-angiotensin signaling, which drives cardiac enlargement and blood volume expansion. Cardiac enlargement in CS results in elevation of basal cardiac output, which is preserved in aging. However, the cardiac remodeling includes altered gene expression patterns that are associated with pathological hypertrophy and are accompanied by decreased exercise tolerance, suggestive of reduced cardiac reserve. Our results identify a high-output cardiac hypertrophy phenotype in CS which is etiologically and mechanistically distinct from other myocardial hypertrophies, and which exhibits key features of high-output heart failure (HOHF). We propose that CS is a genetically-defined HOHF disorder and that decreased vascular smooth muscle excitability is a novel mechanism for HOHF pathogenesis.

"Electrifying dysmorphology": Potassium channelopathies causing dysmorphic syndromes

Potassium channels are a heterogeneous group of membrane-bound proteins, whose functions support a diverse range of biological processes. Genetic disorders arising from mutations in potassium channels are classically recognized by symptoms arising from acute channel dysfunction, such as periodic paralysis, ataxia, seizures, or cardiac conduction abnormalities, often in a patient with otherwise normal examination findings. In this chapter, we review a distinct subgroup of rare potassium channelopathies whose presentations are instead suggestive of a developmental disorder, with features including intellectual disability, craniofacial dysmorphism or other physical anomalies. Known conditions within this subgroup are: Andersen-Tawil syndrome, Birk-Barel syndrome, Cantú syndrome, Keppen-Lubinsky syndrome, Temple-Baraitser syndrome, Zimmerman-Laband syndrome and a very similar disorder called Bauer-Tartaglia or FHEIG syndrome. Ion channelopathies are unlikely to be routinely considered in the differential diagnosis of children presenting with developmental concerns, and so detailed description and photographs of the clinical phenotype are provided to aid recognition. For several of these disorders, functional characterization of the genetic mutations responsible has led to identification of candidate therapies, including drugs already commonly used for other indications, which adds further impetus to their prompt recognition. Together, these cases illustrate the potential for mechanistic insights gained from genetic diagnosis to drive translational work toward targeted, disease-modifying therapies for rare disorders.

Three-dimensional facial morphology in Cantú syndrome

Cantú syndrome (CS) was first described in 1982, and is caused by pathogenic variants in ABCC9 and KCNJ8 encoding regulatory and pore forming subunits of ATP-sensitive potassium (K_ATP ) channels, respectively. It is characterized by congenital hypertrichosis, osteochondrodysplasia, extensive cardiovascular abnormalities and distinctive facial anomalies including a broad nasal bridge, long philtrum, epicanthal folds, and prominent lips. Many genetic syndromes, such as CS, involve facial anomalies that serve as a significant clue in the initial identification of the respective disorder before clinical or molecular diagnosis are undertaken. However, an overwhelming number of CS patients receive misdiagnoses based on an evaluation of coarse facial features. By analyzing three-dimensional images of CS faces, we quantified facial dysmorphology in a cohort of both male and female CS patients with confirmed ABCC9 variants. Morphometric analysis of different regions of the face revealed gender-specific significant differences in face shape. Moreover, we show that 3D facial photographs can distinguish between CS and other genetic disorders with specific facial dysmorphologies that have been mistaken for CS-associated anomalies in the past, hence assisting in an earlier clinical and molecular diagnosis. This optimizes genetic counseling and reduces stress for patients and parents by avoiding unnecessary misdiagnosis.