Novel SCA19/22-associated KCND3 mutations disrupt human KV 4.3 protein biosynthesis and channel gating

A novel KCND3 variant in the N-terminus impairs the ionic current of Kv4.3 and is associated with SCA19/22

Spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of autosomal dominant movement disorders. Among the SCAs associated with impaired ion channel function, SCA19/22 is caused by pathogenic variants in KCND3, which encodes the voltage-gated potassium channel Kv4.3. SCA19/22 is clinically characterized by ataxia, dysarthria and oculomotor dysfunction in combination with other signs and symptoms, including mild cognitive impairment, peripheral neuropathy and pyramidal signs. The known KCND3 pathogenic variants are localized either in the transmembrane segments, the connecting loops, or the C-terminal region of Kv4.3. We have identified a novel pathogenic variant, c.455A>G (p.D152G), localized in the N-terminus of Kv4.3. It is located in the immediate neighbourhood of the T1 domain, which is responsible for multimerization with the β-subunit KChIP2b and thus for the formation of functional heterooctamers. Electrophysiological studies showed that p.D152G does not affect channel gating, but reduces the ionic current in Kv4.3, even though the variant is not located in the transmembrane domains. Impaired channel trafficking to the plasma membrane may contribute to this effect. In a patient with a clinical picture corresponding to SCA19/22, p.D152G is the first pathogenic variant in the N-terminus of Kv4.3 to be described to date with an effect on ion channel activity.

The Phenotypic Spectrum of Spinocerebellar Ataxia Type 19 in a Series of Latin American Patients

Spinocerebellar ataxia 19 (SCA19) represents a rare autosomal dominant genetic disorder resulting in progressive ataxia and cerebellar atrophy. SCA19 is caused by variants in the KCND3 gene, which encodes a voltage-gated potassium channel subunit essential for cerebellar Purkinje cell function. We describe six cases from Chile and Mexico, representing the largest report on SCA19 in Latin America. These cases encompass a range of clinical presentations, highlighting the phenotypic variability within SCA19 from an early-onset, severe disease to a late-onset, slowly progressive condition with normal lifespan. While some patients present with pure ataxia, others also show cognitive impairment, dystonia, and other neurological symptoms. The correlations between specific KCND3 variants and phenotypic outcomes are complex and warrant further investigation. As the genomic landscape of spinocerebellar ataxias evolves, comprehensive genetic testing is becoming pivotal in improving diagnostic accuracy. This study contributes to a better understanding of the clinical spectrum of SCA19, laying the groundwork for further genotype-phenotype correlations and functional studies to elucidate the underlying pathophysiology.

An E280K Missense Variant in KCND3/Kv4.3-Case Report and Functional Characterization

A five-year-old girl presented with headache attacks, clumsiness, and a history of transient gait disturbances. She and her father, mother, twin sister, and brother underwent neurological evaluation, neuroimaging, and exome sequencing covering 357 genes associated with movement disorders. Sequencing revealed the new variant KCND3 c.838G>A, p.E280K in the father and sisters, but not in the mother and brother. KCND3 encodes voltage-gated potassium channel D3 (Kv4.3) and mutations have been associated with spinocerebellar ataxia type 19/22 (SCA19/22) and cardiac arrhythmias. SCA19/22 is characterized by ataxia, Parkinsonism, peripheral neuropathy, and sometimes, intellectual disability. Neuroimaging, EEG, and ECG were unremarkable. Mild developmental delay with impaired fluid reasoning was observed in both sisters, but not in the brother. None of the family members demonstrated ataxia or parkinsonism. In Xenopus oocyte electrophysiology experiments, E280K was associated with a rightward shift in the Kv4.3 voltage-activation relationship of 11 mV for WT/E280K and +17 mV for E280K/E280K relative to WT/WT. Steady-state inactivation was similarly right-shifted. Maximal peak current amplitudes were similar for WT/WT, WT/E280K, and E280K/E280K. Our data indicate that Kv4.3 E280K affects channel activation and inactivation and is associated with developmental delay. However, E280K appears to be relatively benign considering it does not result in overt ataxia.

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

K+ channel Kv4.1 is expressed in the nociceptors/secondary nociceptive neurons and participates in pain regulation

BACKGROUND

Kv4 channels are key components controlling neuronal excitability at membrane potentials below action potential thresholds. It remains elusive whether Kv4.1 participates in pain regulation.

METHODS

We raised a Kv4.1 antibody to map Kv4.1⁺ neurons in the superficial dorsal horn of spinal cord and dorsal root ganglion (DRG) of rats. Behavioral, biochemical, and immunohistochemical methods were used to examine whether the activity of Kv4.1⁺ neurons or Kv4.1 expression level is altered after peripheral nerve injury.

RESULTS

In lamina I of spinal cord, Kv4.1 immunoreactivity (IR) was detected in neurokinin-1 receptor positive (NK1R)⁺ projection neurons (the secondary nociceptive neurons) and NK1R⁺ excitatory interneurons. Kv4.1, KChIP2 and DPP10 were co-expressed in these neurons. Peripheral nerve injury evoked by lumbar spinal nerve ligation (SNL) immediately induced phosphorylated extracellular regulated protein kinase (pERK, an indicator of enhanced neuronal activity) in lamina I Kv4.1⁺ neurons and lamina II Kv4.2/Kv4.3⁺ neurons of the spinal cord. Furthermore, Kv4.1 appeared in 59.9% of DRG neurons with variable sizes. Kv4.1 mRNA and protein levels in DRG neurons were gradually decreased after SNL. Following intrathecal injection of Kv4.1 antisense oligodeoxynucleotide (ASO) into naive rats, Kv4.1 protein level was reduced in the DRG, and mechanical but not thermal hypersensitivity was induced.

CONCLUSIONS

Kv4.1 appears in the secondary nociceptive neurons, and peripheral nerve injury increases the activity of these neurons. Kv4.1 expression in DRG neurons (including half of the nociceptors) is gradually reduced after peripheral nerve injury, and knockdown of Kv4.1 in DRG neurons induces pain. Thus, Kv4.1 participates in pain regulation.

SIGNIFICANCE

Based on the expression of Kv4.1 and Kv4.3 in the nociceptors, Kv4.1 in the secondary nociceptive neurons, Kv4.1 in spinal lamina I excitatory interneurons that regulate the activity of the secondary nociceptive neurons, as well as Kv4.2 and Kv4.3 in spinal lamina II excitatory interneurons that also regulate the activity of the secondary nociceptive neurons, developing Kv4 activators or genetic manipulation to increase Kv4 channel activity in these pain-related Kv4⁺ neurons will be useful in future pain therapeutics.

Rare KCND3 Loss-of-Function Mutation Associated With the SCA19/22

Spinocerebellar ataxia 19/22 (SCA19/22) is a rare neurodegenerative disorder caused by mutations of the KCND3 gene, which encodes the Kv4. 3 protein. Currently, only 22 KCND3 single-nucleotide mutation sites of SCA19/22 have been reported worldwide, and detailed pathogenesis remains unclear. In this study, Sanger sequencing was used to screen 115 probands of cerebellar ataxia families in 67 patients with sporadic cerebellar ataxia and 200 healthy people to identify KCND3 mutations. Mutant gene products showed pathogenicity damage, and the polarity was changed. Next, we established induced pluripotent stem cells (iPSCs) derived from SCA19/22 patients. Using a transcriptome sequencing technique, we found that protein processing in the endoplasmic reticulum was significantly enriched in SCA19/22-iPS-derived neurons and was closely related to endoplasmic reticulum stress (ERS) and apoptosis. In addition, Western blotting of the SCA19/22-iPS-derived neurons showed a reduction in Kv4.3; but, activation of transcription factor 4 (ATF4) and C/EBP homologous protein was increased. Therefore, the c.1130 C>T (p.T377M) mutation of the KCND3 gene may mediate misfold and aggregation of Kv4.3, which activates the ERS and further induces neuron apoptosis involved in SCA19/22.

Rare Gain-of-Function KCND3 Variant Associated with Cerebellar Ataxia, Parkinsonism, Cognitive Dysfunction, and Brain Iron Accumulation

Loss-of-function mutations in the K_V4.3 channel-encoding KCND3 gene are linked to neurodegenerative cerebellar ataxia. Patients suffering from neurodegeneration associated with iron deposition may also present with cerebellar ataxia. The mechanism underlying brain iron accumulation remains unclear. Here, we aim to ascertain the potential pathogenic role of KCND3 variant in iron accumulation-related cerebellar ataxia. We presented a patient with slowly progressive cerebellar ataxia, parkinsonism, cognitive impairment, and iron accumulation in the basal ganglia and the cerebellum. Whole exome sequencing analyses identified in the patient a heterozygous KCND3 c.1256G>A (p.R419H) variant predicted to be disease-causing by multiple bioinformatic analyses. In vitro biochemical and immunofluorescence examinations revealed that, compared to the human K_V4.3 wild-type channel, the p.R419H variant exhibited normal protein abundance and subcellular localization pattern. Electrophysiological investigation, however, demonstrated that the K_V4.3 p.R419H variant was associated with a dominant increase in potassium current amplitudes, as well as notable changes in voltage-dependent gating properties leading to enhanced potassium window current. These observations indicate that, in direct contrast with the loss-of-function KCND3 mutations previously reported in cerebellar ataxia patients, we identified a rare gain-of-function KCND3 variant that may expand the clinical and molecular spectra of neurodegenerative cerebellar disorders associated with brain iron accumulation.

Novel KCND3 Variant Underlying Nonprogressive Congenital Ataxia or SCA19/22 Disrupt KV4.3 Protein Expression and K+ Currents with Variable Effects on Channel Properties

KCND3 encodes the voltage-gated potassium channel K_V4.3 that is highly expressed in the cerebellum, where it regulates dendritic excitability and calcium influx. Loss-of-function K_V4.3 mutations have been associated with dominant spinocerebellar ataxia (SCA19/22). By targeted NGS sequencing, we identified two novel KCND3 missense variants of the K_V4.3 channel: p.S347W identified in a patient with adult-onset pure cerebellar syndrome and p.W359G detected in a child with congenital nonprogressive ataxia. Neuroimaging showed mild cerebellar atrophy in both patients. We performed a two-electrode voltage-clamp recording of K_V4.3 currents in Xenopus oocytes: both the p.G345V (previously reported in a SCA19/22 family) and p.S347W mutants exhibited reduced peak currents by 50%, while no K+ current was detectable for the p.W359G mutant. We assessed the effect of the mutations on channel gating by measuring steady-state voltage-dependent activation and inactivation properties: no significant alterations were detected in p.G345V and p.S347W disease-associated variants, compared to controls. K_V4.3 expression studies in HEK293T cells showed 53% (p.G345V), 45% (p.S347W) and 75% (p.W359G) reductions in mutant protein levels compared with the wildtype. The present study broadens the spectrum of the known phenotypes and identifies additional variants for KCND3-related disorders, outlining the importance of SCA gene screening in early-onset and congenital ataxia.

V374A KCND3 Pathogenic Variant Associated With Paroxysmal Ataxia Exacerbations

Objective

Ataxia channelopathies share common features such as slow motor progression and variable degrees of cognitive dysfunction. Mutations in potassium voltage-gated channel subfamily D member 3 (KCND3), encoding the K+ channel, Kv4.3, are associated with spinocerebellar ataxia (SCA) 19, allelic with SCA22. Mutations in potassium voltage-gated channel subfamily C member 3 (KCNC3), encoding another K+ channel, Kv3.3, cause SCA13. First, a comprehensive phenotype assessment was carried out in a family with autosomal dominant ataxia harboring 2 genetic variants in KCNC3 and KCND3. To evaluate the physiological impact of these variants on channel currents, in vitro studies were performed.

Methods

Clinical and psychometric evaluations, neuroimaging, and genotyping of a family (mother and son) affected by ataxia were carried out. Heterozygous and homozygous Kv3.3 A671V and Kv4.3 V374A variants were evaluated in Xenopus laevis oocytes using 2-electrode voltage-clamp. The influence of Kv4 conductance on neuronal activity was investigated computationally using a Purkinje neuron model.

Results

The main clinical findings were consistent with adult-onset ataxia with cognitive dysfunction and acetazolamide-responsive paroxysmal motor exacerbations in the index case. Despite cognitive deficits, fluorodeoxyglucose (FDG)-PET displayed hypometabolism mainly in the severely atrophic cerebellum. Genetic analyses revealed the new variant c.1121T>C (V374A) in KCND3 and c.2012T>C (A671V) in KCNC3. In vitro electrophysiology experiments on Xenopus oocytes demonstrated that the V374A mutant was nonfunctional when expressed on its own. Upon equal co-expression of wild-type (WT) and V374A channel subunits, Kv4.3 currents were significantly reduced in a dominant negative manner, without alterations of the gating properties of the channel. By contrast, Kv3.3 A671V, when expressed alone, exhibited moderately reduced currents compared with WT, with no effects on channel activation or inactivation. Immunohistochemistry demonstrated adequate cell membrane translocation of the Kv4.3 V374A variant, thus suggesting an impairment of channel function, rather than of expression. Computational modeling predicted an increased Purkinje neuron firing frequency upon reduced Kv4.3 conductance.

Conclusions

Our findings suggest that Kv4.3 V374A is likely pathogenic and associated with paroxysmal ataxia exacerbations, a new trait for SCA19/22. The present FDG PET findings contrast with a previous study demonstrating widespread brain hypometabolism in SCA19/22.

Key Modules and Hub Genes Identified by Coexpression Network Analysis for Revealing Novel Biomarkers for Spina Bifida

Spina bifida is a common neural tube defect (NTD) accounting for 5–10% of perinatal mortalities. As a polygenic disease, spina bifida is caused by a combination of genetic and environmental factors, for which the precise molecular pathogenesis is still not systemically understood. In the present study, we aimed to identify the related gene module that might play a vital role in the occurrence and development of spina bifida by using weighted gene co-expression network analysis (WGCNA). Transcription profiling according to an array of human amniocytes from patients with spina bifida and healthy controls was downloaded from the Gene Expression Omnibus database. First, outliers were identified and removed by principal component analysis (PCA) and sample clustering. Then, genes in the top 25% of variance in the GSE4182 dataset were then determined in order to explore candidate genes in potential hub modules using WGCNA. After data preprocessing, 5407 genes were obtained for further WGCNA. Highly correlated genes were divided into nineteen modules. Combined with a co-expression network and significant differentially expressed genes, 967 candidate genes were identified that may be involved in the pathological processes of spina bifida. Combined with our previous microRNA (miRNA) microarray results, we constructed an miRNA–mRNA network including four miRNAs and 39 mRNA among which three key genes were, respectively, linked to two miRNA-associated gene networks. Following the verification of qRT-PCR and KCND3 was upregulated in the spina bifida. KCND3 and its related miR-765 and miR-142-3p are worthy of further study. These findings may be conducive for early detection and intervention in spina bifida, as well as be of great significance to pregnant women and clinical staff.

Novel Cardiocerebral Channelopathy Associated with a KCND3 V392I Mutation

While a KCND3 V392I mutation uniquely displays a mixed electrophysiological phenotype of Kv4.3, only limited clinical information on the mutation carriers is available. We report two teenage siblings exhibiting both cardiac (early repolarization syndrome and paroxysmal atrial fibrillation) and cerebral phenotypes (epilepsy and intellectual disability), in whom we identified the KCND3 V392I mutation. We propose a link between the KCND3 mutation with a mixed electrophysiological phenotype and cardiocerebral phenotypes, which may be defined as a novel cardiocerebral channelopathy.

KCND3-Related Neurological Disorders: From Old to Emerging Clinical Phenotypes

KCND3 encodes the voltage-gated potassium ion channel subfamily D member 3, a six trans-membrane protein (Kv4.3), involved in the transient outward K⁺ current. KCND3 defect causes both cardiological and neurological syndromes. From a neurological perspective, Kv4.3 defect has been associated to SCA type 19/22, a complex neurological disorder encompassing a wide spectrum of clinical features beside ataxia. To better define the phenotypic spectrum and course of KCND3-related neurological disorder, we review the clinical presentation and evolution in 68 reported cases. We delineated two main clinical phenotypes according to the age of onset. Neurodevelopmental disorder with epilepsy and/or movement disorders with ataxia later in the disease course characterized the early onset forms, while a prominent ataxic syndrome with possible cognitive decline, movement disorders, and peripheral neuropathy were observed in the late onset forms. Furthermore, we described a 37-year-old patient with a de novo KCND3 variant [c.901T>C (p.Ser301Pro)], previously reported in dbSNP as rs79821338, and a clinical phenotype paradigmatic of the early onset forms with neurodevelopmental disorder, epilepsy, parkinsonism-dystonia, and ataxia in adulthood, further expanding the clinical spectrum of this condition.

Genetic screening for potassium channel mutations in Japanese autosomal dominant spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a genetically heterogeneous disease characterized by cerebellar ataxia. Many causative genes have been identified to date, the most common etiology being the abnormal expansion of repeat sequences, and the mutation of ion channel genes also play an important role in the development of SCA. Some of them encode calcium and potassium channels. However, due to limited reports about potassium genes in SCA, we screened 192 Japanese individuals with dominantly inherited SCA who had no abnormal repeat expansions of causative genes for potassium channel mutations (KCNC3 for SCA13 and KCND3 for SCA19/SCA22) by target sequencing. As a result, two variants were identified from two patients: c.1973G>A, p.R658Q and c.1018G>A, p.V340M for KCNC3, and no pathogenic variant was identified for KCND3. The newly identified p.V340M exists in the extracellular domain, and p.R658Q exists in the intracellular domain on the C-terminal side, although most of the reported KCNC3 mutations are present at the transmembrane site. Adult-onset and slowly progressive cerebellar ataxia are the main clinical features of SCA13 and SCA19 caused by potassium channel mutations, which was similar in our cases. SCA13 caused by KCNC3 mutations may present with deep sensory loss and cognitive impairment in addition to cerebellar ataxia. In this study, mild deep sensory loss was observed in one case. SCA caused by potassium channel gene mutations is extremely rare, and more cases should be accumulated in the future to elucidate its pathogenesis due to channel dysfunction.