ANO3 Mutations in Chinese Dystonia: A Genetic Screening Study Using Next-Generation Sequencing

Broadening the clinical spectrum: molecular mechanisms and new phenotypes of ANO3-dystonia

Anoctamin 3 (ANO3) belongs to a family of transmembrane proteins that form phospholipid scramblases and ion channels. A large number of ANO3 variants were identified as the cause of craniocervical dystonia, but the underlying pathogenic mechanisms remain obscure. It was suggested that ANO3 variants may dysregulate intracellular Ca2+ signalling, as variants in other Ca2+ regulating proteins like hippocalcin were also identified as a cause of dystonia. In this study, we conducted a comprehensive evaluation of the clinical, radiological and molecular characteristics of four individuals from four families who carried heterozygous variants in ANO3. The median age at follow-up was 6.6 years (ranging from 3.8 to 8.7 years). Three individuals presented with hypotonia and motor developmental delay. Two patients exhibited generalized progressive dystonia, while one patient presented with paroxysmal dystonia. Additionally, another patient exhibited early dyskinetic encephalopathy. One patient underwent bipallidal deep brain stimulation (DBS) and showed a mild but noteworthy response, while another patient is currently being considered for DBS treatment. Neuroimaging analysis of brain MRI studies did not reveal any specific abnormalities. The molecular spectrum included two novel ANO3 variants (V561L and S116L) and two previously reported ANO3 variants (A599D and S651N). As anoctamins are suggested to affect intracellular Ca2+ signals, we compared Ca2+ signalling and activation of ion channels in cells expressing wild-type ANO3 and cells expressing anoctamin variants. Novel V561L and S116L variants were compared with previously reported A599D and S651N variants and with wild-type ANO3 expressed in fibroblasts isolated from patients or when overexpressed in HEK293 cells. We identified ANO3 as a Ca2+-activated phospholipid scramblase that also conducts ions. Impaired Ca2+ signalling and compromised activation of Ca2+-dependent K+ channels were detected in cells expressing ANO3 variants. In the brain striatal cells of affected patients, impaired activation of KCa3.1 channels due to compromised Ca2+ signals may lead to depolarized membrane voltage and neuronal hyperexcitability and may also lead to reduced cellular viability, as shown in the present study. In conclusion, our study reveals the association between ANO3 variants and paroxysmal dystonia, representing the first reported link between these variants and this specific dystonic phenotype. We demonstrate that ANO3 functions as a Ca2+-activated phospholipid scramblase and ion channel; cells expressing ANO3 variants exhibit impaired Ca2+ signalling and compromised activation of Ca2+-dependent K+ channels. These findings provide a mechanism for the observed clinical manifestations and highlight the importance of ANO3 for neuronal excitability and cellular viability.

The Clinical Spectrum of ANO3-Report of a New Family and Literature Review

BACKGROUND

Mutations in ANO3 are a rare cause of autosomal dominant isolated or combined dystonia, mainly presenting in adulthood.

CASES

We extensively characterize a new, large ANO3 family with six affected carriers. The proband is a young girl who had suffered from tremor and painful dystonic movements in her right arm since the age of 11 years. She later developed a diffuse dystonic tremor and mild extrapyramidal signs (ie, rigidity and hypodiadochokinesis) in her right arm. She also suffered from psychomotor delay and learning difficulties. Repeated structural and functional neuroimaging were unremarkable. A dystonic tremor was also present in her two sisters. Her paternal aunt, father, and a third older sister presented episodic postural tremor in the arms. The father and one sister also presented learning difficulties. The heterozygous p.G6V variant in ANO3 was identified in all affected subjects.

LITERATURE REVIEW

Stratification by age at onset divided ANO3 cases into two major groups, where younger patients displayed a more severe phenotype, probably due to variants near the scrambling domain.

CONCLUSIONS

We describe the phenotype of a new ANO3 family and highlight the need for functional studies to explore the impact of ANO3 variants on its phospholipid scrambling activity.

A novel ANO3 variant in two siblings with different phenotypes

INTRODUCTION

Dystonia type 24 is due to the mutation of the ANO3 gene. It generally consists of craniocervical dystonia associated with tremor; however, other neurological manifestations may also occur. Scientific literature has been expanding on its phenotype over the past few years.

CASE

Here we present two siblings affected by dystonia 24 associated to a novel missense mutation of the ANO3 gene. Description of their phenotype, with regard to motor and non-motor features, may improve the knowledge on DYT 24. Consistent with previous reports, our patients presented with cranio-cervical involvement, and they also exhibited different severity and phenotypes. However non-motor symptoms were present too.

CONCLUSION

Dystonia 24 spectrum is continuously expanding. This case suggests that the ANO3 missense mutation should be sought in all cases of dystonia and isolated tremor and that non-motor symptoms are an integral part of dystonic syndromes. It also shows that clinical and treatment features may vary from patient to patient, even if they may present the same mutation.

A novel variant in the transmembrane 4 domain of ANO3 identified in a two-year-old girl with developmental delay and tremor

ANO3 encodes Anoctamin-3, also known as TMEM16C, a calcium-activated chloride channel. Heterozygous variants of ANO3 can cause dystonia 24, an adult-onset focal dystonia. Some pediatric cases have been reported, but most patients were intellectually normal with some exceptions. Here, we report a two-year-old girl who showed mild to moderate developmental delay, tremor, and ataxic gait, but no obvious dystonia. Trio exome sequencing identified a heterozygous de novo missense variant NM_031418.4:c.1809T>G, p.(Asn603Lys) in the ANO3 gene. Three cases with ANO3 variants and intellectual disability have been reported, including the present case. These variants were predicted to face in the same direction on the same alpha-helix (the transmembrane 4 domain), suggesting an association between these variants and childhood-onset movement disorder with intellectual disability. In pediatric cases with developmental delay and movement disorders such as tremor and ataxia, specific variants in the transmembrane 4 domain of ANO3 may be a cause, even in the absence of dystonia.

A Novel Heterozygous ANO3 Mutation in a Child Presenting Tremor with Dystonia and Review of the Literature

Mutations in ANO3 have recently been identified as an autosomal dominant cause of dystonia (dystonia-24). Since then, the phenotypic spectrum has also been extended in children. Here, we reported a case of a 10-year-old Turkish girl child patient with a novel variant (NM_001313726: c.221dupA, p.Tyr74*), who exhibited tremor with mild dystonia. This report expands the phenotype caused by ANO3 variants and reveals an essential clinical aspect for patients and medical staff.

TMEM16C is involved in thermoregulation and protects rodent pups from febrile seizures

As the most common convulsive disorder in infancy and childhood, affecting 2 to 5% of American children in their first 5 y of life, febrile seizures (FSs) are associated with genetic risk factors, including the Tmem16c (Ano3) gene. Whereas central neuronal hyperexcitability has been implicated in FSs, whether FSs may result from compromised body temperature regulation is unknown. To approach this question, we developed rodent models of FSs associated with deficient thermoregulation, including conditional knockout mice with TMEM16C eliminated from a hypothalamic neuronal population important for maintaining body temperature but not from most of the cortical and hippocampal neurons and sensory neurons. Our findings raise the possibility that impaired homeostatic thermoregulation could contribute to the risk of FSs.

Febrile seizures (FSs) are the most common convulsion in infancy and childhood. Considering the limitations of current treatments, it is important to examine the mechanistic cause of FSs. Prompted by a genome-wide association study identifying TMEM16C (also known as ANO3) as a risk factor of FSs, we showed previously that loss of TMEM16C function causes hippocampal neuronal hyperexcitability [Feenstra et al., Nat. Genet. 46, 1274–1282 (2014)]. Our previous study further revealed a reduction in the number of warm-sensitive neurons that increase their action potential firing rate with rising temperature of the brain region harboring these hypothalamic neurons. Whereas central neuronal hyperexcitability has been implicated in FSs, it is unclear whether the maximal temperature reached during fever or the rate of body temperature rise affects FSs. Here we report that mutant rodent pups with TMEM16C eliminated from all or a subset of their central neurons serve as FS models with deficient thermoregulation. Tmem16c knockout (KO) rat pups at postnatal day 10 (P10) are more susceptible to hyperthermia-induced seizures. Moreover, they display a more rapid rise of body temperature upon heat exposure. In addition, conditional knockout (cKO) mouse pups (P11) with TMEM16C deletion from the brain display greater susceptibility of hyperthermia-induced seizures as well as deficiency in thermoregulation. We also found similar phenotypes in P11 cKO mouse pups with TMEM16C deletion from Ptgds-expressing cells, including temperature-sensitive neurons in the preoptic area (POA) of the anterior hypothalamus, the brain region that controls body temperature. These findings suggest that homeostatic thermoregulation plays an important role in FSs.

Arching deep brain stimulation in dystonia types

Although medical treatment including botulinum toxic injection is the first-line treatment for dystonia, response is insufficient in many patients. In these patients, deep brain stimulation (DBS) can provide significant clinical improvement. Mounting evidence indicates that DBS is an effective and safe treatment for dystonia, especially for idiopathic and inherited isolated generalized/segmental dystonia, including DYT-TOR1A. Other inherited dystonia and acquired dystonia also respond to DBS to varying degrees. For Meige syndrome (craniofacial dystonia), other focal dystonia, and some rare inherited dystonia, further evidences are still needed to evaluate the role of DBS. Because short disease duration at DBS surgery and absence of fixed musculoskeletal deformity are associated with better outcome, DBS should be considered as early as possible when indicated after careful evaluation including genetic work-up. This review will focus on the factors to be considered in DBS for patients with dystonia and the outcome of DBS in the different types of dystonia.

Pallidal Deep Brain Stimulation for Monogenic Dystonia: The Effect of Gene on Outcome

Globus pallidus internus deep brain stimulation (GPi DBS) is the most effective intervention for medically refractory segmental and generalized dystonia in both children and adults. Predictive factors for the degree of improvement after GPi DBS include shorter disease duration and dystonia subtype with idiopathic isolated dystonia usually responding better than acquired combined dystonias. Other factors contributing to variability in outcome may include body distribution, pattern of dystonia and DBS related factors such as lead placement and stimulation parameters. The responsiveness to DBS appears to vary between different monogenic forms of dystonia, with some improving more than others. The first observation in this regard was reports of superior DBS outcomes in DYT-TOR1A (DYT1) dystonia, although other studies have found no difference. Recently a subgroup with young onset DYT-TOR1A, more rapid progression and secondary worsening after effective GPi DBS, has been described. Myoclonus dystonia due to DYT-SCGE (DYT11) usually responds well to GPi DBS. Good outcomes following GPi DBS have also been documented in X-linked dystonia Parkinsonism (DYT3). In contrast, poorer, more variable DBS outcomes have been reported in DYT-THAP1 (DYT6) including a recent larger series. The outcome of GPi DBS in other monogenic isolated and combined dystonias including DYT-GNAL (DYT25), DYT-KMT2B (DYT28), DYT-ATP1A3 (DYT12), and DYT-ANO3 (DYT24) have been reported with varying results in smaller numbers of patients. In this article the available evidence for long term GPi DBS outcome between different genetic dystonias is reviewed to reappraise popular perceptions of expected outcomes and revisit whether genetic diagnosis may assist in predicting DBS outcome.

The expanding clinical and genetic spectrum of ANO3 dystonia

INTRODUCTION

Dystonia is a movement disorder with high clinical and genetic heterogeneity. Mutations in Anoctamin-3 (ANO3) gene have been reported to cause dystonia 24 (DYT24). This study aims to clarify the spectrum and frequency of ANO3 rare variants in Chinese populations with primary dystonia and understand the clinical and genetic features of DYT24.

METHODS

Sanger sequencing was used to screen all exons and exon-intron boundaries of ANO3 for rare variants in 115 primary dystonia patients. The clinical manifestations of patients with ANO3 variants in our study and previously reported literatures were further characterized.

RESULTS

Four distinct variants of ANO3 (c.1127A > G, c.1235 T > A, c.1531-3T > C, c.-11G > T) were identified in six unrelated individuals. Combined with our work and literature review, a total of 35 different rare variants distributed in ANO3 were identified in 62 dystonia patients. The predominant phenotype is cranio-cervical dystonia and more than half of patients develop head/limb tremor. Most of patients presented with isolated dystonia whereas few of them showed combined dystonia. The age of onset ranged from 1 to 69 years and peaked in late adulthood, while for generalized dystonia it peaked in a young age. Half of patients with generalized dystonia experienced deep brain stimulation (DBS). And all of them showed improvement of dystonia by DBS.

CONCLUSIONS

This study confirms a relatively high frequency of rare ANO3 variants in Chinese patients with dystonia and indicates that the late adulthood-onset, cranio-cervical dystonia seems to be an important feature of the ANO3 phenotype. Further functional studies are warranted to understand the role of ANO3 in dystonia.