The frissonnant mutant mouse, a model of dopamino-sensitive, inherited motor syndrome

Translating Genetic Discovery into a Mechanistic Understanding of Pediatric Movement Disorders: Lessons from Genetic Dystonias and Related Disorders

The era of next-generation sequencing has increased the pace of gene discovery in the field of pediatric movement disorders. Following the identification of novel disease-causing genes, several studies have aimed to link the molecular and clinical aspects of these disorders. This perspective presents the developing stories of several childhood-onset movement disorders, including paroxysmal kinesigenic dyskinesia, myoclonus-dystonia syndrome, and other monogenic dystonias. These stories illustrate how gene discovery helps focus the research efforts of scientists trying to understand the mechanisms of disease. The genetic diagnosis of these clinical syndromes also helps clarify the associated phenotypic spectra and aids the search for additional disease-causing genes. Collectively, the findings of previous studies have led to increased recognition of the role of the cerebellum in the physiology and pathophysiology of motor control-a common theme in many pediatric movement disorders. To fully exploit the genetic information garnered in the clinical and research arenas, it is crucial that corresponding multi-omics analyses and functional studies also be performed at scale. Hopefully, these integrated efforts will provide us with a more comprehensive understanding of the genetic and neurobiological bases of movement disorders in childhood.

Vulnerability of Human Cerebellar Neurons to Degeneration in Ataxia-Causing Channelopathies

Mutations in ion channel genes underlie a number of human neurological diseases. Historically, human mutations in ion channel genes, the so-called channelopathies, have been identified to cause episodic disorders. In the last decade, however, mutations in ion channel genes have been demonstrated to result in progressive neurodegenerative and neurodevelopmental disorders in humans, particularly with ion channels that are enriched in the cerebellum. This was unexpected given prior rodent ion channel knock-out models that almost never display neurodegeneration. Human ataxia-causing channelopathies that result in even haploinsufficiency can result in cerebellar atrophy and cerebellar Purkinje neuron loss. Rodent neurons with ion channel loss-of-function appear to, therefore, be significantly more resistant to neurodegeneration compared to human neurons. Fundamental differences in susceptibility of human and rodent cerebellar neurons in ataxia-causing channelopathies must therefore be present. In this review, we explore the properties of human neurons that may contribute to their vulnerability to cerebellar degeneration secondary to ion channel loss-of-function mutations. We present a model taking into account the known allometric scaling of neuronal ion channel density in humans and other mammals that may explain the preferential vulnerability of human cerebellar neurons to degeneration in ataxia-causing channelopathies. We also speculate on the vulnerability of cerebellar neurons to degeneration in mouse models of spinocerebellar ataxia (SCA) where ion channel transcript dysregulation has recently been implicated in disease pathogenesis.

KCNN2 Mutation in Pediatric Tremor Myoclonus Dystonia Syndrome with Electrophysiological Evaluation

Background:

Here we combine clinical, electrophysiological, and genetic findings to phenotype an unusual childhood movement disorder in a patient with a rare form of KCNN2 mutation.

Case Report:

A 10-year-old male presented with a clinical syndrome of tremor and myoclonus. Electrophysiology demonstrated muscle activity indicative of myoclonus dystonia, an observation that was not appreciated clinically. Genetic testing revealed an abnormality in the KCNN 2 gene, not present in the parents, known to cause dystonia, as the etiology.

Discussion:

The value of utilizing noninvasive, electrophysiological recording in pediatric movement disorders expands the precision of diagnosis, potentially informing treatment when correlated with clinical and genetic findings.

Variants in the SK2 channel gene (KCNN2) lead to dominant neurodevelopmental movement disorders

KCNN2 encodes the small conductance calcium-activated potassium channel 2 (SK2). Rodent models with spontaneous Kcnn2 mutations show abnormal gait and locomotor activity, tremor and memory deficits, but human disorders related to KCNN2 variants are largely unknown. Using exome sequencing, we identified a de novo KCNN2 frameshift deletion in a patient with learning disabilities, cerebellar ataxia and white matter abnormalities on brain MRI. This discovery prompted us to collect data from nine additional patients with de novo KCNN2 variants (one nonsense, one splice site, six missense variants and one in-frame deletion) and one family with a missense variant inherited from the affected mother. We investigated the functional impact of six selected variants on SK2 channel function using the patch-clamp technique. All variants tested but one, which was reclassified to uncertain significance, led to a loss-of-function of SK2 channels. Patients with KCNN2 variants had motor and language developmental delay, intellectual disability often associated with early-onset movement disorders comprising cerebellar ataxia and/or extrapyramidal symptoms. Altogether, our findings provide evidence that heterozygous variants, likely causing a haploinsufficiency of the KCNN2 gene, lead to novel autosomal dominant neurodevelopmental movement disorders mirroring phenotypes previously described in rodents.

KCNN2 mutation in autosomal-dominant tremulous myoclonus-dystonia

BACKGROUND AND PURPOSE

Despite recent advances in neurogenetics that have facilitated the identification of a number of dystonia genes, many familial dystonia syndromes remain without known cause. The aim of the study was to identify the cause of autosomal dominant tremulous myoclonus-dystonia in a UK kindred with affected individuals in three generations.

METHODS

Known genetic causes of myoclonus-dystonia were excluded. We combined clinical and electrophysiological phenotyping with whole-exome sequencing and Sanger sequencing to identify candidate causal variants in a family with tremulous myoclonus-dystonia.

RESULTS

The core phenotype consisted of childhood-onset dystonia predominantly affecting hands and neck, with a fast tremor with superimposed myoclonus and, in some individuals, subtle cerebellar signs. We identified a novel missense variant in potassium calcium-activated channel subfamily N member 2 (KCNN2) [NM_021614:c.1112G>A:p.(Gly371Glu)], which was the only variant that we were able to identify as segregating with the phenotype over three generations. This variant, which is absent from the most recent version of gnomAD, was predicted to be deleterious by SIFT and PolyPhen-2 and had an overall CADD score of 29.7.

CONCLUSIONS

KCNN2, a member of the KCNN family of potassium channel genes, is highly conserved across species and in humans is highly expressed in the brain, particularly the cerebellum. KCNN2 mutations have never been described as pathological in human disease, but are recognized abnormalities in two rodent models of fast, jerky tremor. Segregation, absence of the variant in the normal population and in-silico prediction of a deleterious effect together with animal models compatible with the clinical phenotype are all in line with KCNN2 mutations being a plausible cause underlying myoclonus-dystonia.

Tremor dominant Kyoto (Trdk) rats carry a missense mutation in the gene encoding the SK2 subunit of small-conductance Ca2+-activated K+ channel

Tremor dominant Kyoto (Trdk) is an autosomal dominant mutation that appeared in F344/NSlc rats mutagenized with N-ethyl-N-nitrosourea (ENU). In this study, we characterized and genetically analyzed F344-Trdk/+ heterozygous rats. The rats exhibited a tremor that was especially evident around weaning but persisted throughout life. The tremors of F344-Trdk/+ rats were attenuated by drugs effective against essential tremor (ET) but not drugs used to treat Parkinson's disease-related tremor, indicating that the pharmacological phenotype of F344-Trdk/+ rats was similar to human ET. Using positional candidate approach, we identified the Trdk mutation as a missense substitution (c. 866T>A, p. I289N) in Kcnn2, which encodes the SK2 subunit of the small-conductance Ca²⁺-activated K⁺ channel. In vitro electrophysiological studies revealed that the I289N mutation diminished SK2 channel activity. These findings demonstrate that F344-Trdk/+ rats represent a novel model of ET, and strongly suggest that Kcnn2 is the causative gene for the tremor phenotype in F344-Trdk/+ rats.

Selegiline Ameliorates Depression-Like Behavior in Mice Lacking the CD157/BST1 Gene, a Risk Factor for Parkinson's Disease

Parkinson’s disease (PD), a neurodegenerative disorder, is accompanied by various non-motor symptoms including depression and anxiety, which may precede the onset of motor symptoms. Selegiline is an irreversible monoamine oxidase-B (MAO-B) inhibitor, and is widely used in the treatment of PD and major depression. However, there are few reports about the effects of selegiline on non-motor symptoms in PD. The aim of this study was to explore the antidepressant and anxiolytic effects of selegiline, using CD157/BST1 knockout (CD157 KO) mouse, a PD-related genetic model displaying depression and anxiety, compared with other antiparkinsonian drugs and an antidepressant, and was to investigate the effects of selegiline on biochemical parameters in emotion-related brain regions. A single administration of selegiline (1–10 mg/kg) dose-dependently reduced immobility time in the forced swimming test (FST) in CD157 KO mice, but not C57BL/6N wild-type (WT) mice. At 10 mg/kg, but not 3 mg/kg, selegiline significantly increased climbing time in CD157 KO mice. A single administration of the antiparkinsonian drugs pramipexole (a dopamine (DA) D2/D3 receptor agonist) or rasagiline (another MAO-B inhibitor), and repeated injections of a noradrenergic and specific serotonergic antidepressant (NaSSA), mirtazapine, also decreased immobility time, but did not increase climbing time, in CD157 KO mice. The antidepressant-like effects of 10 mg/kg selegiline were comparable to those of 10 mg/kg rasagiline, and tended to be stronger than those of 1 mg/kg rasagiline. After the FST, CD157 KO mice showed decreases in striatal and hippocampal serotonin (5-HT) content, cortical norepinephrine (NE) content, and plasma corticosterone concentration. A single administration of selegiline at 10 mg/kg returned striatal 5-HT, cortical NE, and plasma corticosterone levels to those observed in WT mice. In the open field test (OFT), repeated administration of mirtazapine had anxiolytic effects, and selegiline nonsignificantly ameliorated anxiety-like behaviors in CD157 KO mice. In the social interaction and preference tests, repeated mirtazapine ameliorated the high anxiety and low sociability of CD157 KO mice, whereas selegiline did not. These results indicate that selegiline has antidepressant and mild anxiolytic effects in CD157 KO mice, and suggest that it is an effective antiparkinsonian drug for depressive and anxiety symptoms in PD patients with a CD157 single nucleotide polymorphism (SNP).

Selective positive modulator of calcium-activated potassium channels exerts beneficial effects in a mouse model of spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by a polyglutamine expansion within the Ataxin-2 (Atxn2) protein. Purkinje cells (PC) of the cerebellum fire irregularly and eventually die in SCA2. We show here that the type 2 small conductance calcium-activated potassium channel (SK2) play a key role in control of normal PC activity. Using cerebellar slices from transgenic SCA2 mice we demonstrate that SK channel modulators restore regular pacemaker activity of SCA2 PCs. Furthermore, we also show that oral delivery of a more selective positive modulator of SK2/3 channels (NS13001) alleviates behavioral and neuropathological phenotypes of aging SCA2 transgenic mice. We conclude that SK2 channels constitute a therapeutic target for SCA2 treatment and that the developed selective SK2/3 modulator NS13001 holds promise as a potential therapeutic agent for treatment of SCA2 and possibly other cerebellar ataxias.

Behavioral effects of a deletion in Kcnn2, the gene encoding the SK2 subunit of small-conductance Ca2+-activated K+ channels

Small-conductance Ca(2+)-activated potassium (SK) channels are heteromeric complexes of SK alpha-subunits and calmodulin that modulate membrane excitability, are responsible for part of the after-hyperpolarization (AHP) following action potentials, and thus control the firing patterns and excitability of most central neurons. An engineered knockout allele for the SK2 subunit has previously been reported. The hippocampal neurons of these mice lacked the medium latency component of the AHP, but the animals were not described as presenting any overt behavioral phenotype. In this report, we describe a deletion in the 5' region of the Kcnn2 gene encoding the SK2 subunit in the mouse neurological frissonnant (fri) mutant. The frissonnant mutant phenotype is characterized by constant rapid tremor and locomotor instability. It has been suggested, based merely on its phenotype, as a potential model for human Parkinson disease. We used a positional cloning strategy to identify the mutation underlying the frissonnant phenotype. We narrowed the genetic disease interval and identified a 3,441-bp deletion in the Kcnn2 gene, one of the three candidate genes present in the interval. Expression studies showed complete absence of normal Kcnn2 transcripts while some tissue-specific abnormal truncated variants were detected. Intracellular electrophysiological recordings of central vestibular neurons revealed permanent alterations of the AHP and firing behavior that might cause the tremor and associated locomotor deficits. Thus, the fri mutation suggests a new, potentially important physiological role, which had not been described, for the SK2 subunit of small-conductance Ca(2+)-activated potassium channels.