Mutation in RNF170 causes sensory ataxic neuropathy with vestibular areflexia: a CANVAS mimic

Overarching pathomechanisms in inherited peripheral neuropathies, spastic paraplegias, and cerebellar ataxias

International consortia collaborating on the genetics of rare diseases have significantly boosted our understanding of inherited neurological disorders. Historical clinical classification boundaries were drawn between disorders with seemingly different etiologies, such as inherited peripheral neuropathies (IPNs), spastic paraplegias, and cerebellar ataxias. These clinically defined borders are being challenged by the identification of mutations in genes displaying wide phenotypic spectra and by shared pathomechanistic themes, which are valuable indications for therapy development. We highlight common cellular alterations that underlie this genetic landscape, including alteration of cytoskeleton, axonal transport, mitochondrial function, and DNA repair response. Finally, we discuss venues for future research using the long axonopathies of the PNS as a model to explore other neurogenetic disorders.

Strategy for genetic analysis in hereditary neuropathy

Inherited neuropathies are a heterogeneous group of slowly progressive disorders affecting either motor, sensory, and/or autonomic nerves. Peripheral neuropathy may be the major component of a disease such as Charcot-Marie-Tooth disease or a feature of a more complex multisystemic disease involving the central nervous system and other organs. The goal of this review is to provide the clinical clues orientating the genetic diagnosis in a patient with inherited peripheral neuropathy. This review focuses on primary inherited neuropathies, amyloidosis, inherited metabolic diseases, while detailing clinical, neurophysiological and potential treatment of these diseases.

Novel stop-gain RNF170 variation detected in a Chinese family with adolescent-onset hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a heterogeneous group of inherited neurodegenerative disease characterized by progressive lower limb spasticity. Recent studies revealed that biallelic variants in RNF170 gene cause autosomal recessive complicated HSP with infancy onset. Here, we report an adolescent-onset HSP patient from a consanguineous Chinese family, with lower extremity stiffness, spastic gait, and unstable straight-line walking as the main manifestations. Whole-exome sequencing identifies a novel RNF170 mutation c.190C>T (p.R64*), which co-segregates with the disease in this pedigree. Functional analysis, including quantitative real-time PCR (RT-qPCR) and Western blot, indicates that both the mRNA and protein levels of mutant RNF170 are significantly reduced, which confirms the loss-of-function mechanism. Our study expands the spectrum of RNF170-associated HSP, while the RNF170 protein-involved degradation of the inositol 1,4,5-trisphosphate receptor in neurodegenerative motor neuron disorders deserves further investigation.

Sensory neuronopathies, diagnostic criteria and causes

PURPOSE OF REVIEW

To stress on the diagnostic strategy of sensory neuronopathies (SNN), including new genes and antibodies.

RECENT FINDING

SNN involve paraneoplastic, dysimmune, toxic, viral and genetic mechanisms. About one-third remains idiopathic. Recently, new antibodies and genes have reduced this proportion. Anti-FGFR3 and anti-AGO antibodies are not specific of SNN, although SNN is predominant and may occur with systemic autoimmune diseases. These antibodies are the only marker of an underlying dysimmune context in two-thirds (anti-FGFR3 antibodies) and one-third of the cases (anti-AGO antibodies), respectively. Patients with anti-AGO antibodies may improve with treatment, which is less clear with anti-FGFR3 antibodies. A biallelic expansion in the RFC1 gene is responsible for the cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) in which SNN is a predominant manifestation. Most of the patients have an adult onset and are sporadic. The RFC1 mutation may represent one-third of idiopathic sensory neuropathies. Finally, the criteria for the diagnosis of paraneoplastic SNN have recently been updated.

SUMMARY

The diagnostic of SNN relies on criteria distinguishing SNN from other neuropathies. The strategy in search of their cause now needs to include these recent findings.

Genetic Workup for Charcot–Marie–Tooth Neuropathy: A Retrospective Single-Site Experience Covering 15 Years

Charcot–Marie–Tooth (CMT) disease is the most commonly inherited neurological disorder. This study includes patients affected by CMT during regular follow-ups at the CMT clinic in Genova, a neuromuscular university center in the northwest of Italy, with the aim of describing the genetic distribution of CMT subtypes in our cohort and reporting a peculiar phenotype. Since 2004, 585 patients (447 index cases) have been evaluated at our center, 64.9% of whom have a demyelinating neuropathy and 35.1% of whom have an axonal neuropathy. A genetic diagnosis was achieved in 66% of all patients, with the following distribution: CMT1A (48%), HNPP (14%), CMT1X (13%), CMT2A (5%), and P0-related neuropathies (7%), accounting all together for 87% of all the molecularly defined neuropathies. Interestingly, we observe a peculiar phenotype with initial exclusive lower limb involvement as well as lower limb involvement that is maintained over time, which we have defined as a “strictly length-dependent” phenotype. Most patients with this clinical presentation shared variants in either HSPB1 or MPZ genes. The identification of distinctive phenotypes such as this one may help to address genetic diagnosis. In conclusion, we describe our diagnostic experiences as a multidisciplinary outpatient clinic, combining a gene-by-gene approach or targeted gene panels based on clinical presentation.

Motor neuron pathology in CANVAS due to RFC1 expansions

CANVAS caused by RFC1 biallelic expansions is a major cause of inherited sensory neuronopathy. Detection of RFC1 expansion is challenging and CANVAS can be associated with atypical features.

We clinically and genetically characterized 50 patients, selected based on the presence of sensory neuronopathy confirmed by EMG. We screened RFC1 expansion by PCR, repeat-primed PCR, and Southern blotting of long-range PCR products, a newly developed method. Neuropathological characterization was performed on the brain and spinal cord of one patient.

Most patients (88%) carried a biallelic (AAGGG)n expansion in RFC1. In addition to the core CANVAS phenotype (sensory neuronopathy, cerebellar syndrome, and vestibular impairment), we observed chronic cough (97%), oculomotor signs (85%), motor neuron involvement (55%), dysautonomia (50%), and parkinsonism (10%). Motor neuron involvement was found for 24 of 38 patients (63.1%). First motor neuron signs, such as brisk reflexes, extensor plantar responses, and/or spasticity, were present in 29% of patients, second motor neuron signs, such as fasciculations, wasting, weakness, or a neurogenic pattern on EMG in 18%, and both in 16%. Mixed motor and sensory neuronopathy was observed in 19% of patients. Among six non-RFC1 patients, one carried a heterozygous AAGGG expansion and a pathogenic variant in GRM1. Neuropathological examination of one RFC1 patient with an enriched phenotype, including parkinsonism, dysautonomia, and cognitive decline, showed posterior column and lumbar posterior root atrophy. Degeneration of the vestibulospinal and spinocerebellar tracts was mild. We observed marked astrocytic gliosis and axonal swelling of the synapse between first and second motor neurons in the anterior horn at the lumbar level. The cerebellum showed mild depletion of Purkinje cells, with empty baskets, torpedoes, and astrogliosis characterized by a disorganization of the Bergmann's radial glia. We found neuronal loss in the vagal nucleus. The pars compacta of the substantia nigra was depleted, with widespread Lewy bodies in the locus coeruleus, substantia nigra, hippocampus, entorhinal cortex, and amygdala.

We propose new guidelines for the screening of RFC1 expansion, considering different expansion motifs. Here, we developed a new method to more easily detect pathogenic RFC1 expansions. We report frequent motor neuron involvement and different neuronopathy subtypes. Parkinsonism was more prevalent in this cohort than in the general population, 10% versus the expected 1% (p < 0.001). We describe, for the first time, the spinal cord pathology in CANVAS, showing the alteration of posterior columns and roots, astrocytic gliosis and axonal swelling, suggesting motor neuron synaptic dysfunction.

RNF170 mutation causes autosomal dominant sensory ataxia with variable pyramidal involvement

Background

Although hereditary ataxias are a group of clinically and genetically heterogeneous disorders, specific clinical clues can sometimes incriminate certain genes. This can trigger genetic testing in sporadic patients or prompt dissecting certain genes more thoroughly when initial genetic testing is negative. Also for the assembly of gene panels and interpretation of the results, genotype−phenotype correlations remain important to establish.

Methods

We clinically evaluated a Belgian family with autosomal dominant inherited sensory ataxia and variable pyramidal involvement and performed targeted clinical exome sequencing. Secondly, we retrospectively screened sequencing data of an in‐house cohort of 404 patients with neuromuscular disorders for variants in the identified gene RNF170.

Results

All affected family members showed sensory ataxia on examination. Pyramidal involvement, and sometimes slow‐pursuit abnormalities and/or a sensory neuropathy, were more variable findings. We identified the heterozygous variant p.Arg199Cys in RNF170 in all three affected siblings of our family. We did not find additional pathogenic variants in RNF170 in our in‐house neuromuscular cohort.

Conclusions

We confirm the heterozygous variant p.Arg199Cys in RNF170 in a Belgian family with autosomal dominant sensory ataxia and variable pyramidal involvement. This constitutes a rare but clinically recognizable phenotype that warrants testing of RNF170. Unlike the distinctive bi‐allelic loss of function variants in RNF170 associated with hereditary spastic paraplegia (HSP), the p.Arg199Cys variant is the only one reported in sensory ataxia. It is important for neurologists to be aware of this characteristic phenotype and to include this gene in gene panels for ataxia and HSP.

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS): genetic and clinical aspects

Cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) typically presents in middle life with a combination of neuropathy, ataxia and vestibular disease, with patients reporting progressive imbalance, oscillopsia, sensory disturbance and a dry cough. Examination identifies a sensory neuropathy or neuronopathy and bilaterally impaired vestibulo-ocular reflex. The underlying genetic basis is of biallelic AAGGG expansions in the second intron of replication factor complex subunit 1 (RFC1). The frequency and phenotype spectrum of RFC1 disease is expanding, ranging from typical CANVAS to site-restricted variants affecting the sensory nerves, cerebellum and/or the vestibular system. Given the wide phenotype spectrum of RFC1, the differential diagnosis is broad. RFC1 disease due to biallelic AAGGG expansions is probably the most common cause of recessive ataxia. The key to suspecting the disease (and prompt genetic testing) is a thorough clinical examination assessing the three affected systems and noting the presence of chronic cough.

Challenges and Controversies in the Genetic Diagnosis of Hereditary Spastic Paraplegia

Purpose of Review

The hereditary spastic paraplegias (HSPs) are a group of disorders characterised by progressive lower limb weakness and spasticity. We address the challenges and controversies involved in the genetic diagnosis of HSP.

Recent Findings

There is a large and rapidly expanding list of genes implicated in HSP, making it difficult to keep gene testing panels updated. There is also a high degree of phenotypic overlap between HSP and other disorders, leading to problems in choosing the right panel to analyse. We discuss genetic testing strategies for overcoming these diagnostic hurdles, including the use of targeted sequencing gene panels, whole-exome sequencing and whole-genome sequencing. Personalised treatments for HSP are on the horizon, and a genetic diagnosis may hold the key to access these treatments.

Summary

Developing strategies to overcome the challenges and controversies in HSP may hold the key to a rapid and accurate genetic diagnosis.

Sensory neuronopathies: new genes, new antibodies and new concepts

Degeneration of dorsal root ganglia (DRG) and its central and peripheral projections provokes sensory neuronopathy (SN), a rare disorder with multiple genetic and acquired causes. Clinically, patients with SN usually present with proprioceptive ataxia, patchy and asymmetric sensory abnormalities, widespread areflexia and no weakness. Classic causes of SN include cancer, Sjögren's syndrome, vitamin deficiency, chemotherapy, mitochondrial disorders and Friedreich ataxia. More recently, new genetic and dysimmune disorders associated with SN have been described, including RFC1 gene-linked cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) and anti-FGFR3 antibodies. In this review, we detail the pathophysiology of DRG degeneration, and the genetic and acquired causes of SN, with a special focus on the recently described CANVAS and anti-FGFR3 antibodies. We also propose a user-friendly and easily implemented SN diagnostic strategy.