Anoctamin 10‐Related Autosomal Recessive Cerebellar Ataxia: Comprehensive Clinical Phenotyping of an Irish Sibship

Abstract

The autosomal recessive cerebellar ataxias are a heterogeneous group of neurodegenerative disorders. Mutations in the anoctamin 10 gene (ANO10) recently have been identified as a cause of autosomal recessive spinocerebellar ataxia type 10. Comprehensive phenotypic data are provided on 3 siblings with homozygous ANO10 mutations, including detailed ocular and cognitive assessments and bladder involvement not previously described in the literature. Data also are provided on unblinded therapy with coenzyme Q10, previously reported as a possible therapy in ANO10‐related ataxia. A genetic diagnosis in this family was obtained through next‐generation sequencing techniques after over 10 years of expensive sequencing of individual genes using the traditional Sanger approach. Greater commercial availability of gene panels will improve the ability to obtain a genetic diagnosis in the uncommon “non‐Friedreich's” recessive ataxias. Clinical recognition of these recessive ataxic syndromes will also inevitably improve as the full phenotypic spectrum is identified.

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The autosomal recessive cerebellar ataxias (ARCAs) comprise a diverse group of clinically and genetically heterogeneous disorders. Friedreich's ataxia (FA) remains the most common ARCA, with a distinct clinical phenotype. Many sporadic and recessive cases are not identified as FA and often remain undiagnosed. Next‐generation sequencing (NGS) has heralded a significant shift in our ability to identify a genetic diagnosis in a proportion of this “Friedreich's‐negative” cohort. Mutations in the anoctamin 10 (ANO10) gene recently have been linked to spinocerebellar ataxia autosomal recessive type 10 (SCAR10),1 with classification as autosomal recessive cerebellar ataxia type 3 (ARCA3) subsequently proposed.2 The role of ANO10 in the pathogenesis of cerebellar degeneration remains uncertain. It is expressed largely within the cerebellum and the cortex in the adult brain.1 The phenotype is evolving, and only 26 patients with ANO10‐associated ataxia have been reported to date. Recently observed low levels of white cell ubiquinone (coenzyme‐Q10 [CoQ10]) and possible beneficial effects of CoQ10 supplementation were of interest given the paucity of therapeutic strategies in inherited ataxias.3 Three siblings with ANO10‐associated ataxia are reported here to provide more comprehensive phenotypic data and observational unblinded data on responses to a CoQ10 trial.

Case Series

Clinical Phenotype

A full summary of clinical characteristics are provided in Table 1. The 3 affected siblings originate from a sibship of 11 (Fig. S1), born to nonconsanguineous Irish parents. The proband (II‐2), a 61‐year‐old woman, had “always been last” in races at school, but no specific childhood concerns were raised. Her first subjective complaint was dysarthria at 43 years of age, followed by progressive gait ataxia and recurrent falls. Prominent urinary urge‐incontinence developed within 1 to 2 years of onset and has progressed, requiring intravesical botulinum toxin therapy. Progressive memory difficulties were first reported at 51 years of age. On examination, she has a moderate‐to‐marked cerebellar dysarthria, downbeat nystagmus (see Video 1) and prominent ocular vessels (Fig. 1). Tendon reflexes are brisk, and extensor plantar responses. She walks with a spastic ataxic gait, requiring a rollator for stability.

Table 1.

Phenotypic Characteristics of 3 Siblings with Autosomal Recessive Cerebellar Ataxia due to Homozygous Anoctamin 10 Mutations

	Sibling
	II‐2	II‐10	II‐11
Sex	Female	Male	Male
Age at onset, y	43	31	35
Age at examination, y	61	47	44
Ocular findings	Hypermetric saccades	Hypometric saccades	Hypermetric saccades
	Downbeat nystagmus in primary position	Downbeat nystagmus in primary position
	Horizontal and vertical GEN	Horizontal GEN	Horizontal GEN
	Tortuous scleral and conjunctival vessels	Tortuous scleral and conjunctival vessels
	Scleral and conjunctival telangiectasia	Scleral and conjunctival telangiectasia
Gait ataxia	+++	++	+
Dysarthria	+++	++	+
Appendicular dysmetria	++	+	+
Tendon reflexes UL	Increased	Normal	Normal
Tendon reflexes LL	Increased	Increased	Increased
Plantar responses	Extensor	Flexor	Unilateral extensor
LL spasticity	+	+	++
Autonomic dysfunction	Detrusor overactivity	Detrusor overactivity	–
Amyotrophy	–	–	Distal lower limbs
Seizures	–	Single	–
MoCA score	12/30	23/30	16/30
SARA score	18/40	9.5/40	9.5/40
ICARS score	45/100	22/100	24/100

GEN, gaze‐evoked nystagmus; +++, marked; ++, moderate; +, mild; UL, upper limb; LL, lower limb; MoCA, Montreal Cognitive Assessment; SARA, Scale for the Assessment and Rating of Ataxia; ICARS, International Cooperative Ataxia Rating Scale.

Figure 1.

(A) Magnetic resonance images of the brain from all 3 affected siblings (II‐2, II‐10, and II‐11). Axial T1 views of the cerebellum are shown in the top row, and sagittal T2 views thorough the corpus callosum are shown in the bottom row. Diffuse cerebellar atrophy is observed, with additional frontal atrophy most evident in the eldest and most severely affected sibling (II‐2). (B) Anterior segment photographs of the eyes of II‐2 (right eye [R]) and II‐10 (left eye [L]). Conjunctival and scleral telangiectasias are indicated by black arrows, and tortuosity is indicated by dashed arrows.

The 47‐year‐old brother of the proband (II‐10) presented at 32 years of age, having experienced an unwitnessed collapse, with urinary incontinence and post‐ictal confusion attributed to a seizure. He described poor balance with onset at 31 years of age. For some years before this, he had been nicknamed “Mumbles” because of perceived slurring of speech. He developed bladder urgency and incontinence at 40 years of age. Over the last 5 years, he has been complaining of naming and word‐finding difficulty. He shares similar ocular findings with II‐2 (Fig. 1) but otherwise has a spastic ataxic phenotype without clinical evidence of a neuropathy (Video 2).

The youngest sibling (II‐11), a man aged 44 years, had played football and participated in Thai boxing up until his early 30s. He reported memory difficulty throughout his life and had been ironically named “Brains” at school due to academic difficulties. His first subjective complaint was slurring of speech at the age of 35 years, followed by balance difficulty. Objectively, he has gaze‐evoked nystagmus and mild cerebellar dysarthria. He has moderate spasticity, brisk knee reflexes, and unilateral extensor plantar response. Clinical ataxia rating scales for all 3 siblings are provided in Table 1.

Neurophysiology

Nerve‐conduction studies from all 3 siblings showed no evidence of generalized large‐fiber peripheral neuropathy, and motor and sensory nerve conduction data from upper and lower limbs were within normal limits.

Neuroimaging

Magnetic resonance imaging of the brain revealed generalized cerebellar atrophy, which was most severe in the eldest and longest affected sibling (II‐2) and least severe in the youngest and clinically least affected sibling (II‐11). In addition, II‐2 has moderate‐to‐marked frontal cortical atrophy (Fig. 1).

Ophthalmology Assessment

Conjunctival and scleral vessels in II‐2 and II‐10 demonstrated tortuosity and telangiectasia, encroaching upon the cornea. Optical coherence tomography performed in II‐2 and II‐11 showed normal overall retinal nerve fiber layer and macular thickness.

Formal Neuropsychology

Cognition was assessed using a more extensive battery than in a previous study.4 In particular, an assessment of cognitive speed was incorporated, controlling for motor retardation. Raw outcomes for cognitive domains tested are provided in Table S1. Outcomes were transformed into z‐scores by relating them to the best matched normative samples. Given this kinship's educational history, a reliably matched population control sample was not available; therefore, 3 clinically unaffected siblings (II‐7, II‐8, and II‐9) of a similar age were recruited.

All affected siblings performed markedly less well than their unaffected siblings across a range of measures, with the most pronounced relative impairments in executive function, attention, concentration, learning, and memory tasks (although II‐10 performed strongly on verbal learning). Visuospatial perception was impaired (particularly in II‐2, perhaps linked to reported oscillopsia), with less impairment of visuospatial reasoning. The affected processing speed was relatively slowed with respect to unaffected siblings. Confrontational naming performance showed no clear disparity.

Genetic Testing

A commercially available 91‐gene panel (www.ouh.nhs.uk/geneticslab) revealed a homozygous ANO10 c.132dupA p.(Asp45fs) mutation in II‐2. Subsequent molecular genetic analysis confirmed this known pathogenic variant in II‐10 and II‐11. This is a frameshift mutation leading to premature termination of translation. The mother was heterozygous for c.132dupA p.(Asp45fs), and other family members were not available for testing.

CoQ10

CoQ10 levels were within the normal range (37–133 pmol/mg) for siblings II‐2, II‐10, and II‐11 at 92, 65, and 79 pmol/mg, respectively; however, muscle tissue samples were not available for CoQ10 analysis.

CoQ10 Supplementation

Supplementation with 300 mg/day CoQ10 for 3 months resulted in subjective improvement of energy levels and walking in II‐11, although scores on the Scale for the Assessment and Rating of Ataxia remained unchanged. Evaluation with the International Cooperative Ataxia Rating Scale in II‐2 showed an improvement in the kinetic score from 17/52 to 15/52 but progression in the static score from 19/34 to 20/34. International Cooperative Ataxia Rating Scale scores in II‐10 remained unchanged; and II‐11, who has been exercising daily recently, showed an improvement only in the gait and posture score from 8/34 to 6/34.

Discussion

The ANO10 product, a transmembrane protein 16K (TMEM16K), is a member of the human anoctamin family. Its physiological role is currently unknown,5 but it may play a role in influencing Ca²⁺ signaling in Purkinje cells, and abnormal protein synthesis could lead to cerebellar ataxia through this mechanism.1, 6 ANO10 has high frontal and occipital cortical and cerebellar expression,1 with levels higher in adults than in children, suggesting a potentially homeostatic rather than a developmental role.

Only 1 ANO10‐associated ataxia report has described cognitive functioning.4 Cognitive characteristics described here are broadly consistent with those in that previously reported family,4 with impaired executive functioning, attention, memory, and visuospatial perception, although bradyphrenia was indicated directly in our cohort. The role of the cerebellum in cognitive impairment in developmental and degenerative cerebellar disorders is increasingly recognized.7 The pattern of cognitive impairment and imaging findings in this sibship are in keeping with the reported prominent frontal cortical ANO10 expression. More accurate characterization of the profile was challenging in this group because of their particular characteristics, including limited educational attainment. Cognitive outcomes from affected kinships with backgrounds more consistent with the general population would be informative. The cortical phenotypic component is further supported by the presentation of 1 sibling who had a seizure, as described previously in other cases.3, 8 We recently identified a patient with compound heterozygous ANO10 mutations who, in addition to progressive ataxia, also demonstrated stimulus‐sensitive craniofacial myoclonic jerks (unpublished observation).

A notable oculomotor finding in ANO10‐associated ataxia is the presence of downbeat nystagmus, which was reported in 27% of patients.1, 2, 3, 4 In contrast to previously described cases,1, 4 in which only tortuosity of ocular vessels has been reported, II‐2 and II‐10 had both tortuosity and telangiectasia of sclera and conjunctiva (Fig. 1). Telangiectatic vessels are distinct from tortuous vessels, in that the vessel wall is irregular in shape (i.e., parts of the vessel are dilated), whereas the tortuous vessels have a regular vessel wall but run an irregular course, and no part of the vessel is dilated. These findings may be important discriminating factors to differentiate SCAR10 from other spastic ataxia phenotypes without neuropathy.

Bladder autonomic dysfunction, although in accordance with the spastic paraparesis, has not been previously reported in ANO10‐associated ataxia. The bladder involvement is important, because it potentially changes the localization focus from the frontal lobes. Spinal cord expression, although lower, may contribute to bladder dysfunction given the detrusor instability phenotype. Sensory or sensorimotor polyneuropathies are common in many ARCAs, but neuropathy has not been recognized to date as a component of the ANO10‐associated phenotype, leading to a call for its categorization as ARCA3, among other recessive ataxias without neuropathy. Our neurophysiological findings, in addition to those reported previously, confirm that sensory neuropathy is not a part of the phenotype.

The same ANO10 c.132dupA p.(Asp45fs) mutation has been previously reported in 2 individuals with ataxia and CoQ10 deficiency in muscle, plasma, and cerebrospinal fluid.3 In 1 of those individuals, short‐term, high‐dose CoQ10 was beneficial for fatigue, mobility, and cognition. In the other individual, lower dose CoQ10 led to mild improvement of ataxia, dysarthria, and oculomotor findings. In our patients, short‐term CoQ10 supplementation resulted in mild, predominantly subjective improvement in 1 of them. Early initiation of more prolonged supplementation with appropriate blinding will be required to address this question further.

A commercial panel approach has provided an exciting new avenue to achieve a genetic diagnosis where traditional methods have been unsuccessful.9 This kindred was investigated for 15 separate genes using Sanger sequencing before availability of the NGS ataxia panel, which ultimately was used to identify the diagnosis. ANO10 mutations may be a more common cause of unexplained sporadic and non‐Friedreich's ARCA than other rare recessive syndromes, e.g., ataxia‐telangiectasia and ataxia with oculomotor apraxia, possibly previously identified to a disproportionate degree due to characteristic phenotypic and biochemical markers. In this family, the cruel but accurate observation of “Mumbles” and “Brains” by their contemporaries identified the main features of the syndrome, upon which we have further expanded. Such clinical phenotyping remains critical to build a phenotypic spectrum and to interpret genetic variants of uncertain significance from the complex output of exome and genome NGS going forward.

Author Roles

1. Research project: A. Conception, B. Organization, C. Execution; 2. Statistical Analysis: A. Design, B. Execution, C. Review and Critique; 3. Manuscript Preparation: A. Writing of the first draft, B. Review and Critique.

P.B.M.: 1A, 1C, 3A, 3B

N.A.: 1C, 3B

M.D.A.: 1C, 3B

L.C.: 1C

A.E.: 1B

R.P.M.: 3B

S.M.M.: 3B

R.A.W.: 3B

Disclosures

Ethical Compliance Statement: We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines.

Funding Sources and Conflicts of interest: The authors declare that there are no conflicts of interest relevant to this work.

Financial Disclosures for the previous 12 months: Petya Bogdanova‐Mihaylova reports research funding support from Ataxia Ireland and The Meath Foundation; Sinéad M. Murphy reports research funding support from Ataxia Ireland. Richard A. Walsh reports unrestricted grant support from AbbVie, Royalties from Oxford University Press, and an educational grant from The Meath Foundation.

Supporting information

Videos accompanying this article are available in the supporting information here.

Figure S1. Pedigree of this Irish family with the c.132dupA p.(Asp45fs) mutation. Affected individuals are indicated in black. The proband is indicated by an arrow.

Table S1. Formal neuropsychological assessment of affected and unaffected siblings

Video 1. Video of II‐2, demonstrating cerebellar dysarthria, downbeat nystagmus, appendicular ataxia, and spastic ataxic gait.

Video 2. Video of II‐10, demonstrating cerebellar dysarthria, downbeat nystagmus in primary position, dysmetric saccades, and spastic ataxic gait.