Introduction

The alpha-1A subunit of neuronal voltage-dependent P/Q-type Ca2+ channels is encoded by the CACNA1A gene, and mutations in CACNA1A result in channelopathies originally thought to cause distinct, well-known allelic disorders: Spinocerebellar ataxia type 6 (SCA6), familiar hemiplegic migraine (FHM), and episodic ataxia type 2 (EA2) [4]. Certain CACNA1A mutation types are more frequently associated with distinct phenotypes: missense mutations with FHM, mutations resulting in a premature stop with EA2, and an exonic CAG trinucleotide repeat expansion with SCA6 (http://www.hgmd.cf.ac.uk/ac/gene.php?gene=CACNA1A). Recent reports, however, revealed an extensive clinical overlap between these three phenotypes [1, 9] and a high intrafamilial phenotypic variability [5, 7]. In addition, new phenotypes including seizures and mental retardation have been reported [1].

Here, we describe the identification and functional characterization of a novel CACNA1A mutation in a patient with EA2. We highlight that mutations in genes being associated with paroxysmal disorders may be overlooked as the symptomatology may be misclassified as psychogenic especially when the family history is negative.

The genetic study was approved by the ethics committee of the University of Luebeck.

Case Report

A 27-year-old male patient was referred to our movement disorders outpatient clinic with a 10-year history of episodic double and blurred vision, dysarthria, clumsiness, nausea, and rotating vertigo being triggered by physical exercise and alcohol lasting for minutes to hours. Apart from intermittent occipital headache, the patient was completely asymptomatic and reported no other signs between the attacks. EEG, cMRI, and ECG were normal. The first attack occurred at the age of 13 years when the patient was swimming during school class requiring assistance to leave the pool. Subsequent attacks were thus considered panic attacks (ICD10 F41.0) as a consequence of the event of near-drowning. Neither behavioral therapy nor the administration of an SSRI for 2 to 3 months was successful. Treatment with acetazolamide, however, resulted in a clear improvement with regard to the frequency of the attacks.

Family history was negative for movement disorders and headaches. His mother was reported to suffer from multiple sclerosis since young adulthood and his brother had an inborn hydrocephalus.

On examination, the patient showed slightly hypometric horizontal saccades, mild action-induced perioral jerks, a slightly dysmetric finger-to-nose test with his left hand, and ataxic spiral drawing (SARA score 0.5/40, video). He displayed no nystagmus, and the remaining examination was otherwise normal. Given the frequency and length of the reported attacks and very mild interictal cerebellar signs, episodic ataxia type 2 (EA) was considered as clinical diagnosis and genetic testing was initiated demonstrating a novel, heterozygous, pathogenic variant in CACNA1A (c.3901delC; p.Leu1301Sefs*19) [6]. This variant has previously not been linked to any CACNA1A-related disorder, was absent in the server of the Genome Aggregation Database (gnomAD; http://gnomad.broadinstitute.org/variant/19-13386754-AG-A), and had a Combined Annotation Dependent Depletion score of 35 (CADD; http://cadd.gs.washington.edu/home). Unfortunately, further family members were not available for examination.

The video shows slight intermittent perioral jerks and very subtle upper limb dysmetria and an otherwise normal neurological examination. (MP4 28570 kb)

To investigate the functional consequences of the frameshift mutation, a fibroblast cell culture from skin biopsy of the patient was established. Comparing the dosage of wildtype and aberrant transcript levels by pyrosequencing analysis, only 30% of total CACNA1A mRNA was shown to carry the deletion. In addition, cells were treated with 100 μM cycloheximide (CHX) for 6–18 h to block nonsense-mediated mRNA decay; CACNA1A transcripts were sequenced and compared with those of untreated cells to prove a specific degradation of the aberrant CACNA1A transcript (Fig. 1).

Fig. 1
figure 1

Molecular analysis of CACNA1A expression. a Quantitative pyrosequencing analysis revealed only 30% of aberrant CACNA1A transcript in patient’s fibroblasts. b Sequencing analysis of CACNA1A cDNA products extracted from patient’s fibroblasts treated with 100 μM cycloheximide to block nonsense-mediated decay of aberrant CACNA1A transcripts (upper part) was compared to cDNA of the untreated cells (lower part). While the aberrant transcript (T) resulting in a premature translation stop is less prominently presented in native cells, it partially escaped nonsense-mediated mRNA decay in cycloheximide-treated cells (as indicated by arrows)

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Discussion

This patient with a novel pathogenic frameshift mutation in CACNA1A presenting with EA2-typical features highlights the importance of underlying genetic causes in the setting of triggered attacks of cerebellar dysfunction, even without a positive family history. The frequency and symptomatology of attacks, the age of onset lower the age of 20 years, and the response to acetazolamide were clearly in keeping with patients harboring other previously described CACNA1A mutations [2, 3]. Migraine and nystagmus are frequently but not obligatory found in the interictal state of EA2 [2] but were not present in our patient. Pathogenicity of this mutation was confirmed by functional investigations to prove reduced levels of mutated CACNA1A transcripts caused by specific degradation of the aberrant mRNA by nonsense-mediated mRNA decay.

Clinicians should discern that patients with paroxysmal movement disorders, e.g., EA2, are often misdiagnosed, as the distinction by clinical presentation alone is challenging. Interictal clinical signs, e.g., mild ataxia, should direct attention towards a paroxysmal movement disorder and should lead the way to a movement disorder specialist. In our patient, we observed perioral motor activity that should not be confused with myokymia as an interictal sign of EA1 [8]. Finally, the large age range of disease manifestation calls for close collaboration of pediatric neurologists, neurologists, and human geneticists.