Dear Sirs,

In 2019, non-parametric linkage analyses and genome sequencing revealed that biallelic AAGGG expansions in the replication factor C subunit 1 (RFC1) gene are a frequent cause of late-onset ataxia [1]. Subsequent studies described the phenotypic spectrum of patients with pathological RFC1 expansion: they mainly presented in their fifth decade of life with a triad of cerebellar dysfunction (i.e., gait ataxia, dysarthria, ocular motor disorders), sensory neuropathy with concomitant sensory ataxia, and vestibular areflexia bilaterally, denoted by the acronym CANVAS [1]. Quite recently, a multicentre observational study has shown that RFC1 expansion comprises a multisystemic disease with a chronic dry cough, dysautonomia, and bradykinesia as additional clinical features of variable degree [2].

It is still an outstanding issue, whether biallelic AAGGG expansion in RFC1 are not associated with an even broader phenotypic spectrum of neurodegenerative diseases.

A 64-year-old male presented with a 1-year history of progressive and painless weakness of both hands. Neurological examination revealed generalized polytopic muscle fasciculations in 4/4 levels, muscle paresis for finger adduction/abduction (r: MRC 4–5, l: MRC 4), finger extension (r: MRC 4–5, l: MRC 4), thumb opposition (r: MRC 4–5, l: MRC 4), wrist extension/flexion (r: MRC 4–5, l: MRC 4) and hip flexion (r/l: MRC 4–5). A split hand sign was conspicuous on both sides. Muscle reflexes were brisk on the left upper limb with decreased ankle jerks bilaterally. There were neither relevant sensory/proprioceptive deficits nor clinical signs of ataxia/vestibulopathy. Clinical suspicion of a degenerative motor neuron disease was confirmed by electromyography, muscle ultrasound and transcranial magnetic stimulation (for details see Table 1). Sural and superficial peroneal nerve potentials and sensory evoked potentials of the tibial nerves (P40) were absent bilaterally.

Table 1 An overview of the diagnostic procedures and findings in our patient
Full size table

Finally, amyotrophic lateral sclerosis (ALS) was diagnosed according to the current diagnostic criteria (see Table 1). Additionally, regarding sensory nerve conduction studies and evoked potentials subclinical sensory neuropathy/neuronopathy was diagnosed.

Acquired conditions for sensory neuropathies/neuronopathies were excluded (see Table 1). The patient did not consent to a recommended additional CSF analysis.

Genetic analysis by CRISPR/Cas9 target enrichment and Oxford Nanopore long-read sequencing [3], revealed biallelic AAGGG repeat expansions (~ 400) of the RFC1 locus. Negative results of all genetic testing are listed in Table 1.

Due to the detected biallelic RFC1 repeat expansions we post-hoc performed vestibular testing by inner ear calorics and video-assisted head-impulse-test, which revealed isolated bilateral presbyvestibulopathy in the low-frequency range (see Table 1).

This case with a diagnosis of ALS, additional subclinical sensory neuro(no)pathy and bilateral presbyvestibulopathy in the low-frequency range in association with a biallelic RFC1 expansion raises the following noteworthy future question: is ALS/motor neuron disease within the phenotypic spectrum of biallelic RFC1 repeat expansions?

To our knowledge, this is the first case of an ALS patient with a concomitant subclinical sensory neuro(no)pathy and bilateral presbyvestibulopathy carrying a biallelic RFC1 repeat expansion. The number of genes associated with monogenic forms or increased risk of ALS is constantly growing including intermediate expansions of the SCA 1,2 genes and huntingtin-trinucleotide expansions [4, 5]. Acknowledging previous reports with abnormal findings in sensory nerve conduction studies in up to 20% of patients with ALS [6] and earlier morphological findings in sensory nerve biopsies suggesting loss of sensory root ganglion neurons [7], an involvement of pathological RFC1 expansions as additional monogenic form or at least genetic risk factor for ALS might be discussed. However, one must admit, that we cannot differentiate an association of pathological RFC1 expansions with a combined phenotype of ALS and sensory neuro(no)pathy in our patient from a bare coincidence of ALS with a beginning CANVAS phenotype due to RFC1 pathology. A recent study indeed found RFC1 expansions exclusively in so far “idiopathic” sensory neuropathies, but not in patients with “idiopathic” sensorimotor neuropathies [8]. Thus a clear link of RFC1 pathology with the motor system is missing so far. Since RFC1 mediated pathology affects the sensory ganglion cells and not the peripheral sensory nerves, the findings from Currò et al. are plausible [8]. A recent study revealed that pathogenic SPTLC1 mutations are not only associated with the phenotype of sensory and autonomic neuropathy (i.e. HSAN type 1), but also with juvenile onset ALS [9]. And, for the rare syndrome of “facial onset sensory motor neuronopathy” (FOSMN) typically beginning with sensory symptoms of the trigeminal nerves, underlying TDP-43 pathology in sensory ganglion cells as well as motor neurons was confirmed [10,11,12], thus classifying FOSMN currently as a rare form of motor neuron disease.

An important limitation of the presented case is, that we cannot assess the influence of rare genetic variants with small effect size or their combinatory effect in terms of polygenic risk modification.

In conclusion, the presented case with a concomitant sensory neuro(no)pathy and proven RFC1 expansion in addition to ALS should prompt a more systematical search for RFC1 expansion in larger patient cohorts with ALS and unexplained sensory involvement in order to disentangle a possible role of RFC1 pathology in ALS.