Widening the spectrum of spinocerebellar ataxia autosomal recessive type 10 (SCAR10)

Abstract

Biallelic pathogenic variants in the ANO10 gene cause spinocerebellar ataxia recessive type 10. We report two patients, both compound heterozygous for ANO10 variants, including two novel variants. Both patients had onset of cerebellar ataxia in adulthood with slow progression and presented corticospinal tract signs, eye movement abnormalities and cognitive executive impairment. One of them had temporal lobe epilepsy and she also carried a heterozygous variant in CACNB4, a potential risk gene for epilepsy. Both patients had pronounced cerebellar atrophy on cerebral magnetic resonance imaging (MRI) and reduced metabolic activity in cerebellum as well as in the frontal lobes on 2-deoxy-2-(¹⁸F)fluoro-D-glucose positron emission tomography ((¹⁸F)FDG PET) scans. We provide comprehensive clinical, radiological and genetic data on two patients carrying likely pathogenic ANO10 gene variants. Furthermore, we provide evidence for a cerebellar as well as a frontal involvement on brain (¹⁸F)FDG PET scans which has not previously been reported.

Background

Autosomal recessive cerebellar ataxias (ARCAs) comprise a clinically and genetically heterogeneous group of progressive neurodegenerative disorders. Clinically, the patients are characterised by progressive gait and balance disturbances, with limb dyscoordination and often impairment of speech and eye movements.¹ In addition, ARCAs are frequently associated with other neurological and non-neurological symptoms.¹

ARCA type 3 and spinocerebellar ataxia recessive type 10 (SCAR10) define the same genetic subtype of an ARCA, which is associated with pathogenic variants in the ANO10 gene.² The role of ANO10 in the pathogenesis of cerebellar degeneration remains unclear³; however, it encodes a transmembrane protein, anoctamin 10, which is suggested to be a calcium activated chloride channel.^{2 4} ANO10 has the highest expression in the adult brain, especially in frontal cortex and occipital cortex, as well as in cerebellum.²

In this case report, we describe two patients carrying likely pathogenic ANO10 variants to widen the clinical, radiological and genetic spectrum, and we compare the patients with previously reported cases to further refine the SCAR10 phenotype.

Both patients as well as the parents of patient 1 were provided genetic counselling and a written informed consent for genetic testing as well as for publication was obtained.

Case presentation

Patient 1 is a Danish woman, in her 30s, born to non-consanguineous and healthy parents. The pregnancy and delivery were described as uneventful. The patient had febrile seizures at 6 months of age. After an otherwise normal early development, the patient presented with focal seizures at 4 years of age, with the onset shortly after a head trauma. The seizures displayed as stereotyped complex auditory hallucinations of hearing the same melody, accompanied by déja vu and impaired awareness. At the age of 26, the patient experienced her first focal to bilateral tonic-clonic seizure. Since then, she has been treated with lamotrigine, oxcarbazepine, levetiracetam and lacosamide but due to side effects, the patient had low compliance. Her temporal lobe epilepsy was, however, well controlled.

At the age of 30 years, the patient had onset of progressing gait disturbances, ataxia and slurred speech.

Neurological examination at age of 35 years, she had cerebellar dysarthria and saccadic pursuit of eye movements both in the horizontal and vertical level. Saccades were at normal speed but hypermetric. No nystagmus. There was trunk, limb and gait ataxia. No weakness was found but the tendon reflexes were equally increased. The patient scored 17/40 on Scale for the Assessment and Rating of Ataxia (SARA)⁵ corresponding to moderate to severe ataxia.

Neuropsychological examination revealed language impairment and problems with tests that required high tempo and attention.

Cerebral magnetic resonance imaging (MRI) at the age of 26 revealed marked cerebellar atrophy but no supratentorial lesions, in particular no temporal lobe abnormalities. At that time, the patient had no ataxia. An electroencephalogram (EEG) showed sharp-waves and low-frequency activity over the left temporal lobe.

Cerebral MRI at the age 35 years, revealed stationary cerebellar atrophy, as well as minor atrophy of the medulla oblongata and pons with a normal mesencephalon. Brain 2-deoxy-2-(¹⁸F)fluoro-D-glucose positron emission tomography ((¹⁸F)FDG PET) showed severely reduced metabolic activity of cerebellum and the metabolic activity was also found reduced in pons and in the frontal lobes (figure 1). Cerebrospinal fluid analyses revealed slightly elevated neurofilament light chain protein concentration and oligoclonal bands were present.

Figure 1.

MRI and 2-deoxy-2-(¹⁸F)fluoro-D-glucose positron emission tomography of the brain of patient 1. Lower row shows statistical surface projections with global normalisation, where blue indicates reduced metabolism. Yellow/red indicates relatively higher uptake, but is a result of global normalisation and not indicative of true hypermetabolism. Notice severe atrophy and hypometabolism in the cerebellum (white arrow) and also reduced metabolism in pons and frontal cortex (blue arrows) extending into the mesial frontal cortex.

Next-generation sequencing (NGS)-based testing of 200 genes related to movement disorders (see supplementary materials for details) identified two heterozygous variants of unknown significance (VUS) in the ANO10 gene, c.478_480del, p.(Arg160del) in exon 5 and c.1516G>C, p.(Gly506Arg) in exon 10. The c.478_480del variant is a deletion involving the evolutionary conserved amino acid, p.(Arg160), while the c.1516G>C variant is a missense change involving the evolutionary conserved amino acid, p.(Gly506). The two variants, have to our knowledge, not been reported earlier, and they are not present in the Genome Aggregation Database (gnomAD; https://gnomad.broadinstitute.org/). The variants have also not been identified in 2000 whole exomes investigated in Danish individuals.

The mother of the patient was heterozygous for the c.478_480del variant and the father of the patient was heterozygous for the c.1516G>C variant, thus the patient was compound heterozygous for the two ANO10 variants. Moreover the analysis detected a heterozygous variant in the CACNB4 gene (c.1184T>C p.(Leu395Pro) of unknown significance, which was also detected in her healthy mother.

Patient 2 is a Danish woman in her 40s, born to non-consanguineous parents. At the age of 31 years, she had onset of speech problems after pregnancy and delivery, with a progressive worsening. During the maternity leave, the patient’s mother recognised that the patient was getting clumsy, followed by problems with balance and coordination.

Neurological examination at age 32 years revealed dysarthria, saccadic pursuit of eye movements, slow saccades and increased tendon reflexes. SARA score was 7/40, equivalent to a mild ataxia.

Neuropsychological examination was performed at the age of 34 years, and revealed a reduction of cognitive processing speed and impairment of executive functions, but no memory or praxic impairment was found.

Cerebral MRI showed an isolated pronounced cerebellar atrophy. Brain (¹⁸F)FDG PET, at the age of 43 years, showed severely reduced cerebellar metabolic activity as well as supratentorial hypometabolism to a lesser degree, primarly of the frontal lobes (figure 2). The cerebrospinal fluid was normal.

Figure 2.

MRI and 2-deoxy-2-(¹⁸F)fluoro-D-glucose positron emission tomography of the brain of patient 2. Lower row shows statistical surface projections with global normalisation, where blue indicates reduced metabolism. Notice similarity with findings in patient 1 in figure 1.

During the past 13 years, the disease has progressed slowly, with a SARA score of now 12/40.

NGS-based testing of 81 genes associated with ataxia showed compound heterozygosity of the previously published pathogenetic variants c.96delA; p.(Glu33Asnfs*3)⁶ and c.132dupA; p.(Asp45Argfs*9)⁷ in transposition, in the ANO10 gene in exon 2. Both are frame-shift variants predicted to lead to premature termination of translation and nonsense mediated decay of ANO10 mRNA. The variant c.96delA is observed in 19 alleles out of 128 968 investigated alleles from non-Finnish Europeans (0.015%) and the variant c.132dupA is identified in 70 out of 120 656 investigated alleles from non-Finnish Europeans (0.058%) in gnomAD.

Outcome and follow-up

We report two patients, who are compound heterozygous for ANO10 variants and have a clinical phenotype compatible with SCAR10. Both patients had onset of cerebellar ataxia in adulthood with slow progression and presented corticospinal tract signs, eye movement abnormalities and cognitive impairment, primarily of executive functions. In addition, patient 1 has temporal lobe epilepsy.

Both patients have pronounced cerebellar atrophy on brain MRI scans and reduced metabolic activity in cerebellum as well as in the frontal lobes on (¹⁸F)FDG PET scans, which is in line with ANO10 expression in the brain.² To our knowledge, brain (¹⁸F)FDG PET scans in SCAR10 patients have not previously been reported.

Patient 1 carries two novel ANO10 variants, both affecting evolutionary conserved amino acids. We classify the two variants as VUS, but as the clinical phenotype of patient 1 is compatible with SCAR10 it is most likely that both variants are pathogenic. We classify both variants identified in patient 2 as pathogenic. The frameshift variant c.132dupA is the most frequently described ANO10 variant, causing protein truncation.⁷ Other variants known to cause dominant or recessive hereditary ataxia were not identified in neither of the patients.

Discussion

The clinical presentation of our patients resembles the previously reported cases of ANO10 disease^{2 7–17} where reported age of onset spans from 6 to 53 years.^{7 10} In addition to cerebellar ataxia, many SCAR10 patients present with eye movement abnormalities, corticospinal tract signs and in some patients also lower motor neuron involvement,^{2 7 17} bradykinesia,¹⁰ tremor^{2 10} and cognitive deficits7 10–14 16 illustrating the clinical variability.

Two out of three SCAR10 patients previously reported with epilepsy^{11 16} had no identified cause of epilepsy other than SCAR10. The third patient had a large temporo-parieto-occipital porencephalic cyst.¹⁶ Two SCAR10 patients reported by Nanetti et al¹⁰ had no history of seizures but had bilateral or asymmetric spikes over central regions on EEG and giant cortical somatosensory evoked potentials, indicating that cortical hyperexcitability may be a part of the SCAR10 phenotype. Besides having SCAR10, patient 1 carries a CACNB4 gene variant, c.1184T>C inherited from her healthy mother. One study reported an association between CACNB4 variants and increased risk of developing generalised epilepsy.¹⁸ Despite the patient’s mother, carrying the same CACNB4 gene variant, is healthy, one may hypothesise that the CACNB4 gene variant c.1184T>C may act as a potential risk allele for temporal lobe epilepsy in patient 1.

Cognitive evaluations of our patients showed deficits in executive function, attention, language performance and a reduction of processing speed, which are broadly consistent with cognitive profiles previously reported in SCAR10.7 10 12–14 16 Three studies^{10 12 13} have reported impaired visuospatial perception in SCAR10 patients, whereas our patients have normal visuospatial perception. The cognitive profile may be well explained by the cerebellar dysfunction,¹⁹ in combination with frontal deficits as illustrated also by the brain PET-FDG scans.

As a general finding, brain MRI scans of SCAR10 patients demonstrate cerebellar atrophy. Besides cerebellar atrophy, a few studies have reported supratentorial cortical atrophy in frontal regions,¹² frontalparietal regions,¹⁰ parieto-occipital regions¹¹ as well as brain stem atrophy^{8 9} on MRI. These patients were 53–70 years of age. No significant supratentorial atrophy was found on MRI scans in our patients, but patient 1 had atrophy of pons and medulla oblongata. Despite no supratentorial atrophy was found on MRI, both patients had reduced metabolic activity in the frontal lobes on (¹⁸F)FDG PET scans.

In conclusion, we report two SCAR10 patients, including one carrying two novel ANO10 gene variants. We provide evidence for a cerebellar as well as a frontal involvement on brain (¹⁸F)FDG PET scans and hypothesise that the CACNB4 gene variant c.1184T>C may act as a potential risk allele for epilepsy in SCAR10.

Learning points.

• The two patients, reported, are both compound heterozygous for ANO10 variants, including two novel variants. Biallelic pathogenic variants in the ANO10 gene cause spinocerebellar ataxia recessive type 10.

• Both patients had onset of cerebellar ataxia in adulthood with slow progression and presented corticospinal tract signs, eye movement abnormalities and cognitive executive impairment.

• Both patients had pronounced cerebellar atrophy on cerebral MRI and both patients had reduced metabolic activity in cerebellum as well as in the frontal lobes on 2-deoxy-2-(¹⁸F)fluoro-D-glucose positron emission tomography ((¹⁸F)FDG PET) scans. The neurological findings in the patients may be explained by the cerebellar impairment, in combination with frontal deficits, as illustrated by the brain PET-FDG scans.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s)