Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): a family with five affected sibs from Turkey

Abstract

Background

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS), a relatively common cause of late-onset progressive ataxia, is a genetic disease characterised by biallelic pentanucleotide AAGGG repeat expansion in intron 2 of the replication factor complex subunit 1 gene. Herein, we describe the first molecularly confirmed CANVAS family with five affected siblings from Turkey.

Case presentation

The family comprised seven siblings born from healthy non-consanguineous parents. CANVAS phenotype was present in five of them; two were healthy and asymptomatic. Chronic cough was the first symptom reported in all five siblings, followed by the development of sensory symptoms, oscillopsia and imbalance. Clinical head impulse test (HIT) was positive in all cases and video HIT performed on three patients revealed very low vestibulo-ocular reflex gains bilaterally. Magnetic resonance imaging and nerve conduction studies revealed cerebellar atrophy and sensory neuronopathy, respectively. RP-PCR confirmed the homozygous presence of the AAGGG repeat expansion in all five cases.

Conclusion

Genetic screening for CANVAS should be considered in all patients with late-onset ataxia, sensory disturbances and vestibular involvement, especially in the presence of chronic cough.

Background

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) is a genetic neurological disorder that typically presents in midlife with progressive imbalance due to impairments in vestibular and somatosensory inputs and defective cerebellar integration [1]. In 2016, a set of clinical criteria were proposed for diagnosing CANVAS, which include abnormal vestibulo-ocular reflex (VOR) on video-oculography, video-nystagmography or rotational chair testing, cerebellar atrophy evident on magnetic resonance imaging (MRI), particularly involving the anterior and dorsal vermis and the lateral hemispheres (predominantly affecting Crus I), neurophysiologic evidence of neuronopathy (ganglionopathy) and exclusion of other genetic ataxias (specifically spinocerebellar ataxia type 3 and Friedreich ataxia) [1]. Dry spasmodic cough is a significant symptom of the syndrome, antedating the appearance of imbalance by several decades, as well as preserved tendon reflexes in the presence of neuronopathy [2, 3]. Histopathologic examinations demonstrate loss of Purkinje cells and the dorsal root ganglion cells of the vestibular, trigeminal and facial nerves [4]. The genetic background of CANVAS was disclosed by the identification of biallelic pentanucleotide AAGGG repeat expansion in intron 2 of the replication factor complex subunit 1 (RFC1) gene as the cause [5, 6]. Herein, we describe the first molecularly confirmed Turkish family with CANVAS, having five affected members.

Case presentation

Three siblings from a Turkish family were evaluated for symptoms of unsteady gait at the outpatient clinic of the Department of Neurology, Ege University Medical School. The family comprised seven siblings born from healthy non-consanguineous parents who had immigrated from the same village in Macedonia.

Two other siblings were also symptomatic; since they were non-ambulatory, they were examined at home. Although the remaining two siblings were healthy, we performed detailed neurologic examinations on all seven siblings. CANVAS phenotype was present in five of them; no other family members from other generations were reportedly affected (Fig. 1).

Fig. 1

Family pedigree

We performed a video head impulse test (vHIT) on five of the seven siblings (the three hospitalised siblings and the two unaffected ones). Additionally, nerve conduction studies (NCS) and cranial MRI were done on the three hospitalised siblings. Before conducting the RFC1 gene analysis, we performed the following examinations to rule out other causes of acquired and inherited late-onset ataxia and neuropathy, blood tests, including routine biochemistry, protein electrophoresis, serum and urine immunofixation electrophoresis, ANA, anti-dsDNA, anti-SSA, anti-SSB, paraneoplastic autoantibodies (anti-Hu, anti-Yo, anti-Ri, anti-MA2, anti-CV2, anti-amphiphysin and celiac autoantibodies (anti-endomysial and antigliadin), cerebrospinal fluid analysis, whole body computerised tomography and spinocerebellar ataxia gene panel.

The RFC1 gene screening using polymerase chain reaction (PCR) was performed on all seven siblings. The repeat expansion flanking sequence PCR bands were only present in the two unaffected siblings and not in the affected five. Consequently, all seven were subjected to RP-PCR which confirmed the presence of the homozygous AAGGG repeat expansion in the index case (III-3) and the other four affected sibs (III-1, 2, 5, 6) [5].

The index patient (III-3) was a 67-year-old male with a 10-year history of progressively unsteady gait which increased in the dark; he also reported oscillopsia during head movements. Additionally, he developed numbness and tingling in the distal parts of his extremities at 55 years of age and had a chronic cough for over thirty years that was resistant to antitussive medications. He also complained of erectile dysfunction for the last five years. There were no signs/symptoms of cardiac arrhythmia in the past medical history. On neurological examination, eye movements in all directions were within the normal range without any spontaneous or gaze-evoked nystagmus. Clinical HIT revealed catch-up saccades in the horizontal plane bilaterally. Ankle jerks were absent; other deep tendon reflexes were normal. We noted hypoesthesia with a stocking-and-glove distribution. He had dysarthria, but there was no dysmetria or dysdiadochokinesia on the cerebellar system examination; however, he had a wide-base gait with an inability to walk in tandem. He scored 13 on the scale for the assessment and rating of ataxia (SARA). NCS revealed the absence of sensory nerve action potentials, indicating sensory axonal neuropathy or ganglionopathy. Cranial MRI showed cerebellar atrophy mainly involving the vermis (Fig. 2).

Fig. 2

Sagittal view of T1-weighted brain magnetic resonance image (MRI) of the index patient showing vermian atrophy, mainly involving lobules VI and VII, and enlargement of the primary fissure (PF) and the pre-pyramidal fissure (PPF)

Table 1 summarises the demographic, clinical and imaging findings of all patients. The eldest affected sibling was female, the other four were males. Chronic cough was the first presenting symptom and occurred in all five siblings; it was followed by sensory symptoms, oscillopsia and imbalance. Patients 1 and 2 with a longer disease history had additional cerebellar features, including dysmetria, dysdiadochokinesia and dysarthria. Additionally, these patients had severe ataxia, necessitating assistance for ambulation (SARA scores: III-1 = 32, III-2 = 28). Patient III-5 also defined erectile dysfunction. No other features of movement disorders or cognitive impairment were present. Clinical HIT was positive in all; the vHIT performed on three patients III-3, 5 and 6 revealed very low VOR gains in all three canals bilaterally. The VOR gains recorded from unaffected siblings were in the normal range (Table 2). Sensory potentials could not be recorded from patient III-3; recordings from patients III-5 and 6 revealed findings consistent with sensory axonal neuropathy. Cranial MRI of patients III-3, 5 and 6 displayed cerebellar atrophy, mainly involving the vermis.

Table 1 Demographic features, symptomatology, age of symptom onset and neurological examination, investigation findings of the patients

Table 2 Vestibulo-ocular reflex gains recorded by video head impulse test

The study was approved by the ethics committee of Ege University Medical School (reference number: 99166796-050.04-1646946); all patients gave their written informed consent for publication.

Discussion

In this article, we describe the clinical characteristics of five genetically confirmed cases of CANVAS from the same family. A previous report from Turkey described five patients of CANVAS from different families [7]. Another study on the phenotypic spectrum and discriminative features of RFC1 disease identified biallelic AAGGG RFC1 repeat expansions in 18 patients from 15 Turkish families with unselected late-onset ataxia [8]. However, to the best of our knowledge, there is no account of this disease being diagnosed in five members of the same family in Turkey.

A study including 100 genetically confirmed patients carrying a biallelic AAGGG repeat expansion in RFC1 found that half of these cases were sporadic. The median age for the onset of neurological symptoms of the disease (excluding cough) was 52 years (range = 19–76 years). Early involvement of sensory neurons, characterised by distal sensory complaints, was followed by oscillopsia and gait ataxia indicating vestibular and cerebellar dysfunction. By the time ~ 80% of patients had vestibular failure and ~ 60% had cerebellar dysfunction, peripheral neuropathy was detected by NCS in all patients. Half of the patients (n = 50) required walking aids after 10 years of disease onset and 25 were wheelchair dependent after 15 years. Dry cough occurring up to three decades before the onset of unsteadiness was reported in over 60% of cases [9]. We also observed a similar disease pattern in our patients. The mean age of neurological symptom onset was 50 years (range = 48–55 years). Dry cough was the first symptom in all patients, starting generally by the third decade; however, it was only reported when asked. In a recent study that included 13 CANVAS patients, 92% of the patients developed dry cough at an average of 16 years before the onset of walking difficulties [10]. Although the exact underlying mechanism is not yet clear, the proposed explanation is the involvement of C-fibres in the upper respiratory tract, leading to hypersensitivity reactions in the second neurons of the solitary nucleus [2]. All our five patients developed sensory complaints as the initial neurological symptoms, followed by oscillopsia and imbalance within a few years. Sensory axonal neuropathy and bilateral vestibular involvement were noted in the three patients (III-3, 5, 6) investigated. The two elder siblings (III-1, 2) with severe ataxia leading to limited and assisted ambulation indoors also had dysmetria, dysdiadochokinesia and dysarthria. Two patients (III-3, 5) had erectile dysfunction. Dysautonomia going with sympathetic and parasympathetic involvement has been shown both by clinical [11, 12] and electrophysiological [13] testing in CANVAS. Notably, post-mortem autopsy studies of genetically confirmed cases have revealed ganglionopathy leading to axonal damage [13, 14] and autonomic dysfunction is characterized by a small fiber autonomic axonopathy [12].

In a Portuguese family of 15 members with four affected cases, CANVAS was diagnosed based on the identification of the biallelic AAGGG repeat expansion in all affected individuals [15]. In another Asia-Pacific cohort screened for CANVAS, a novel RFC1 repeat expansion motif, ACAGG, was identified in three affected individuals from two different families [16].

Dominik et al. [17], identified in addition to the common (AAGGG)n, three novel repeat motifs, associated with CANVAS in the homozygous and compound heterozygous states (AGGGC, AAGGC, AGAGG). While AAAAG, AAAGGG and AAGAG expansions appear to be benign, AAAGG repeat configuration may be pathogenic. All pathogenic motifs have arisen from a common haplotype. The evidence of novel repeat motifs suggests a dynamic nature of the pentanucleotide expansion in the RFC1 gene.

A small number of cases with typical CANVAS do not carry the above biallelic repeat expansions. The study by Ronco et al. [18], further expands the genotypic spectrum of the disease by identifying the first cases of CANVAS associated with truncating variants in trans position in RFC1 coding regions. The observation of patients carrying a compound heterozygous truncating variant in combination with (AAGGG)n repeat expansion in RFC1, has important implications on the disease pathogenesis, supporting a loss of function mechanism model (reduction of RFC1 expression was shown). Patients carrying a completely nonfunctional allele, show a relatively severe phenotype compared to most patients carrying a biallelic (AAGGG)n expansion.

In addition to the core CANVAS phenotype, chronic cough, oculomotor signs, dysautonomia and parkinsonism may be observed in CANVAS patients. Huin et al. [19], confirm RFC1 as a major cause of sensory neuronopathy and expand the clinical symptoms associated with CANVAS by the frequent presence of upper and lower motor neuron involvement.

It is noteworthy that a rare missense variant in the ELF2 gene was identified by Ahmad et al. [20], using exome sequencing in three patients with presumed autosomal dominant CANVAS from a non-consanguineous family from England.

Our patient cohort of CANVAS is the largest reported account of patients from Turkey with very typical clinical and laboratory features confirmed by the identification of biallelic AAGGG expansion in the RFC1 gene.

Conclusions

CANVAS is one of the common causes of late-onset progressive ataxia. Significant progress has been made regarding its diagnosis following the discovery of the role of the RFC1 pentanucleotide repeat expansion in its pathogenesis. Novel repeat motifs were identified in recent studies. In addition, by whole-genome or whole-exome sequencing, individuals with typical clinical features and carrying only a heterozygous AAGGG expansion were found to be associated with a second truncating variant in RFC1 gene. Accordingly, genetic testing for CANVAS is recommended in patients experiencing progressive imbalance, sensory and visual symptoms (such as oscillopsia with head movements and tingling and numbness in the distal extremities), and unexplained chronic dry cough regardless of the family history. In the light of the recent studies, assessment of novel configurations is warranted in CANVAS patients with negative or inconclusive genetic testing and full sequencing is recommended in cases affected by typical CANVAS and carrying monoallelic AAGGG expansions.