Role of the repeat expansion size in predicting age of onset and severity in RFC1 disease.

RFC1 disease, caused by biallelic repeat expansion in RFC1, is clinically heterogeneous in terms of age of onset, disease progression and phenotype. We investigated the role of the repeat size in influencing clinical variables in RFC1 disease. We also assessed the presence and role of meiotic and somatic instability of the repeat. In this study, we identified 553 patients carrying biallelic RFC1 expansions and measured the repeat expansion size in 392 cases. Pearson's coefficient was calculated to assess the correlation between the repeat size and age at disease onset. A Cox model with robust cluster standard errors was adopted to describe the effect of repeat size on age at disease onset, on age at onset of each individual symptoms, and on disease progression. A quasi-Poisson regression model was used to analyse the relationship between phenotype and repeat size. We performed multivariate linear regression to assess the association of the repeat size with the degree of cerebellar atrophy. Meiotic stability was assessed by Southern blotting on first-degree relatives of 27 probands. Finally, somatic instability was investigated by optical genome mapping on cerebellar and frontal cortex and unaffected peripheral tissue from four post-mortem cases. A larger repeat size of both smaller and larger allele was associated with an earlier age at neurological onset [smaller allele hazard ratio (HR) = 2.06, P < 0.001; larger allele HR = 1.53, P < 0.001] and with a higher hazard of developing disabling symptoms, such as dysarthria or dysphagia (smaller allele HR = 3.40, P < 0.001; larger allele HR = 1.71, P = 0.002) or loss of independent walking (smaller allele HR = 2.78, P < 0.001; larger allele HR = 1.60; P < 0.001) earlier in disease course. Patients with more complex phenotypes carried larger expansions [smaller allele: complex neuropathy rate ratio (RR) = 1.30, P = 0.003; cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) RR = 1.34, P < 0.001; larger allele: complex neuropathy RR = 1.33, P = 0.008; CANVAS RR = 1.31, P = 0.009]. Furthermore, larger repeat expansions in the smaller allele were associated with more pronounced cerebellar vermis atrophy (lobules I-V ß = -1.06, P < 0.001; lobules VI-VII ß = -0.34, P = 0.005). The repeat did not show significant instability during vertical transmission and across different tissues and brain regions. RFC1 repeat size, particularly of the smaller allele, is one of the determinants of variability in RFC1 disease and represents a key prognostic factor to predict disease onset, phenotype and severity. Assessing the repeat size is warranted as part of the diagnostic test for RFC1 expansion.

Late-onset Tay-Sachs disease presenting with a neuromuscular phenotype-a case series.

BACKGROUND AND PURPOSE: Tay-Sachs disease is a rare and often fatal, autosomal recessive, lysosomal storage disease. Deficiency in ß-hexosaminidase leads to accumulation of GM2 ganglioside resulting in neuronal swelling and degeneration. Typical onset is in infancy with developmental regression and early death. Late-onset Tay-Sachs disease (LOTS) is extremely rare, especially in the non-Ashkenazi Jewish population, and is characterized by a more indolent presentation typically encompassing features of cerebellar and anterior horn cell dysfunction in addition to extrapyramidal and neuropsychiatric symptoms. CASES: A case series of four unrelated patients of non-Ashkenazi Jewish origin with a predominantly, and in some cases pure, neuromuscular phenotype with evidence of a motor neuronopathy on electromyography is presented. Cerebellar atrophy, reported to be a ubiquitous feature in LOTS, was absent in all patients. CONCLUSION: This case series provides evidence to support a pure neuromuscular phenotype in LOTS, which should be considered in the differential diagnosis of anterior horn cell disorders.

Health-Related Quality of Life in Patients with Inherited Ataxia in Ireland.

Inherited cerebellar ataxias (CA) are heterogeneous progressive neurological conditions associated with significant functional limitations. This study aimed to assess the implications of inherited CA on patients' self-reported quality of life (QoL) and impairments in work and activities. 129 individuals with ataxia responded to a survey focused on QoL. Health-related QoL was measured using the RAND 36-Item Short Form Survey. An adaptation of the validated Work Productivity and Activity Impairment questionnaire was used to assess the effect of health on work productivity and ability to perform activities over the past week. Nine percent of respondents were currently employed. Individuals with inherited ataxia experienced significant activity impairment, and 75% required professional or informal care. Health-related quality of life (HRQoL) was significantly worse in all areas for the individuals with inherited ataxia compared with Irish population normative values. Participants with Friedreich's ataxia (n = 56) demonstrated worse physical functioning then those with undetermined ataxia (n = 55). Female gender, younger age at symptom onset, current employment, retirement due to age or ataxia, and living in a long-term care facility were associated with higher sub-scores in different domains of HRQoL, while disease duration correlated with worse physical functioning sub-scores. This study is the first cross-sectional study on HRQoL in patients with inherited ataxia in Ireland. It highlights high rates of unemployment, difficulty with daily activities and physical functioning limitations, which is worse than comparative international studies. Given the limited therapeutic options currently available, optimising HRQoL is an important aspect of managing ataxia.

DNMT1-associated sensory neuropathy and cerebellar ataxia: A novel variant and review of genotype-phenotype correlation.

AIM: Hereditary sensory neuropathy (HSN) 1E is a neurodegenerative disorder caused by pathogenic variants in DNA methyltransferase 1 (DNMT1). It is characterised by sensorineural deafness, sensory neuropathy and cognitive decline. Variants in DNMT1 are also associated with autosomal dominant cerebellar ataxia, deafness and narcolepsy. METHODS: A 42-year-old man presented with imbalance, lancinating pain, numerous paucisymptomatic injuries, progressive deafness since his mid-20s, mild cognitive decline and apathy. Examination revealed abnormalities of eye movements, distal sensory loss to all modalities, areflexia without weakness and lower limb ataxia. MRI brain and FDG-PET scan demonstrated biparietal and cerebellar atrophy/hypometabolism. Whole exome sequencing detected a heterozygous likely pathogenic missense variant in DNMT1, c.1289G > A, p.Cys430Tyr. Cochlear implant was performed at 44 years for the bilateral high frequency sensorineural hearing loss with improvement in hearing and day-to-day function. RESULTS AND CONCLUSION: We describe a novel variant in DNMT1 and confirm that an overlapping HSN1E-cerebellar phenotype can occur. Only one prior case of cochlear implant in HSN1E has been reported to date but this case adds to that literature, suggesting that cochlear implant can be successful in such patients. We further explore the clinical and radiological signature of the cognitive syndrome associated with this disorder.

The Cost of Living with Inherited Ataxia in Ireland.

Inherited ataxias are a heterogenous group of neurodegenerative disorders characterised by progressive impairment of balance and coordination, typically leading to permanent and progressive disability. Diagnosis and management of these disorders incurs a range of direct and indirect financial costs. The aim of this study was to collect individual ataxia-related healthcare resources in a large cohort of individuals with different subtypes of inherited ataxia and calculate the associated cost of illness in the Republic of Ireland. One hundred twenty-nine respondents completed a cross-sectional study on healthcare resource utilisation for progressive ataxia in Ireland. Costs were calculated using a prevalence-based approach and bottom-up methodology. The COI for inherited ataxia in 2016 was €59,993 per person per year. Results were similar between participants with Friedreich's ataxia (FRDA, n = 56), non-FRDA (n = 18) and those with undetermined ataxia (n = 55). Indirect costs, based on productivity losses by participants or caregivers, accounted for 52% of the cost of illness. Inherited ataxia is associated with significant health and social care costs. Further funding for inherited ataxia to ease the financial burden on patients, caregivers and healthcare system and improve standards of care compliance is warranted.

Inherited Cerebellar Ataxias: 5-Year Experience of the Irish National Ataxia Clinic.

Establishing a molecular diagnosis in patients with progressive ataxia is often challenging due to significant genetic and clinical heterogeneity and requires a methodical approach with expert clinical evaluation and investigations. We describe the 5-year experience of the National Ataxia Clinic (NAC), Ireland. All adults with ataxia attending the NAC between 2014 and 2019 were evaluated. All individuals underwent detailed clinical assessment and investigations including, where appropriate, genetic testing using next-generation sequencing. For all patients, acquired causes were ruled out. A total of 254 patients from 196 families were assessed; with growth of the clinic cohort by 82% from 133 to 242 over the 5-year period. The underlying genetic cause was identified in 128/196 probands (65.3%). The detection rate for repeat expansion disorder gene testing was 47.7% (82/172) and using NGS gene panel, a genetic diagnosis was obtained in 30/84 (35.7%). Whole exome sequencing identified the molecular diagnosis in 4/20 (20%), and whole genome sequencing provided genetic diagnosis in 1/5 (20%). The commonest diagnosis was Friedreich's ataxia (68/128, 53.1%). SPG7-associated ataxia was the second most common diagnosis (21/128, 16.4%), followed by ANO10-associated spastic ataxia, ataxia telangiectasia (AT), and other rarer phenotypes. Our results highlight that careful clinical phenotyping in a dedicated ataxia clinic is crucial for appropriate genetic testing in selected patients in a timely manner. Advanced genetic testing has significantly improved the diagnostic yield in patients with suspected genetic ataxia and should be considered in all individuals with negative repeat expansion testing.

Ataxia pancytopenia syndrome due to SAMD9L mutation presenting as demyelinating neuropathy.

Ataxia pancytopenia (ATXPC) syndrome due to gain-of-function pathogenic variants in the SAMD9L gene has been described in 38 patients to date. It is characterized by variable neurological and hematological phenotypes including ataxia, pyramidal signs, cytopenias, and hematological malignancies. Peripheral neuropathy with slowing of conduction velocities has been reported in only two affected individuals. We describe a female with childhood onset neuropathy diagnosed as Charcot-Marie-Tooth disease type 1 with onset of cerebellar ataxia in her 50s. Cerebellar, pyramidal, and neuropathic features were found on examination. Additionally, she also had conjunctival telangiectasia. Nerve conduction studies confirmed a demyelinating neuropathy. MRI brain showed cerebellar atrophy with diffuse white matter hyperintensities. OCT demonstrated global thinning of the retinal nerve fiber layer (RNFL). Full blood count has always been normal. A previously described pathogenic variant in SAMD9L [c.2956C>T p.(Arg986Cys)] was identified on whole exome sequencing. This case extends the previously described phenotype to include conjunctival telangiectasia and RNFL thinning and suggests that ATXPC syndrome should be considered in the differential for inherited demyelinating neuropathies.

Clinical spectrum of AIFM1-associated disease in an Irish family, from mild neuropathy to severe cerebellar ataxia with colour blindness.

Mutations in apoptosis-inducing factor mitochondrion-associated-1 (AIFM1) cause X-linked peripheral neuropathy (Cowchock syndrome, CMT4X); however, more recently a cerebellar presentation has been described. We describe a large Irish family with seven affected males. They presented with a variable age of onset, 18 months to 39 years of age. All developed variably present sensorineural deafness, peripheral neuropathy, cerebellar ataxia, and pyramidal involvement. In addition, three had colour vision deficiency. Scale for the assessment and rating of ataxia ranged 2 to 23/40, while Charcot-Marie-Tooth neuropathy score 2 varied between 7 and 13/36. All individuals had normal cognitive assessment. Neurophysiology demonstrated length-dependent large-fibre sensorimotor axonal neuropathy, with particular involvement of superficial radial sensory responses. Brain imaging, performed in four, revealed varying extent of cerebellar atrophy, and white matter changes in one. Optical coherence tomography was abnormal in one, who had unrelated eye pathology. Four obligate female carriers were assessed clinically, two of them neurophysiologically; all were unaffected. Whole genome sequencing demonstrated a previously reported hemizygous AIFM1 mutation. Analysis for mutations in other genes associated with colour deficiency was negative. AIFM1-associated phenotype in this family demonstrated significant variability. To our knowledge, this is the first report of AIFM1-associated colour blindness. Superficial radial nerve was particularly affected neurophysiologically, which could represent a phenotypic marker towards this specific genetic diagnosis.

Waardenburg syndrome: a rare cause of inherited neuropathy due to SOX10 mutation.

Waardenburg syndrome (WS) is a rare disorder comprising sensorineural deafness and pigmentation abnormalities. Four distinct subtypes are defined based on the presence or absence of additional symptoms. Mutations in six genes have been described in WS. SOX10 mutations are usually associated with a more severe phenotype of WS with peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, and Hirschsprung disease. Here we report a 32-year-old man with a novel heterozygous missense variant in SOX10 gene, who presented with congenital deafness, Hirschsprung disease, iris heterochromia, foot deformity, and intermediate conduction velocity length-dependent sensorimotor neuropathy. This case highlights that the presence of other non-neuropathic features in a patient with presumed hereditary neuropathy should alert the clinician to possible atypical rare causes.

Phenotypic and molecular insights into spinal muscular atrophy due to mutations in BICD2.

Spinal muscular atrophy is a disorder of lower motor neurons, most commonly caused by recessive mutations in SMN1 on chromosome 5q. Cases without SMN1 mutations are subclassified according to phenotype. Spinal muscular atrophy, lower extremity-predominant, is characterized by lower limb muscle weakness and wasting, associated with reduced numbers of lumbar motor neurons and is caused by mutations in DYNC1H1, which encodes a microtubule motor protein in the dynein-dynactin complex and one of its cargo adaptors, BICD2. We have now identified 32 patients with BICD2 mutations from nine different families, providing detailed insights into the clinical phenotype and natural history of BICD2 disease. BICD2 spinal muscular atrophy, lower extremity predominant most commonly presents with delayed motor milestones and ankle contractures. Additional features at presentation include arthrogryposis and congenital dislocation of the hips. In all affected individuals, weakness and wasting is lower-limb predominant, and typically involves both proximal and distal muscle groups. There is no evidence of sensory nerve involvement. Upper motor neuron signs are a prominent feature in a subset of individuals, including one family with exclusively adult-onset upper motor neuron features, consistent with a diagnosis of hereditary spastic paraplegia. In all cohort members, lower motor neuron features were static or only slowly progressive, and the majority remained ambulant throughout life. Muscle MRI in six individuals showed a common pattern of muscle involvement with fat deposition in most thigh muscles, but sparing of the adductors and semitendinosus. Muscle pathology findings were highly variable and included pseudomyopathic features, neuropathic features, and minimal change. The six causative mutations, including one not previously reported, result in amino acid changes within all three coiled-coil domains of the BICD2 protein, and include a possible 'hot spot' mutation, p.Ser107Leu present in four families. We used the recently solved crystal structure of a highly conserved region of the Drosophila orthologue of BICD2 to further-explore how the p.Glu774Gly substitution inhibits the binding of BICD2 to Rab6. Overall, the features of BICD2 spinal muscular atrophy, lower extremity predominant are consistent with a pathological process that preferentially affects lumbar lower motor neurons, with or without additional upper motor neuron involvement. Defining the phenotypic features in this, the largest BICD2 disease cohort reported to date, will facilitate focused genetic testing and filtering of next generation sequencing-derived variants in cases with similar features.

A pilot study of proximal strength training in Charcot-Marie-Tooth disease.

Gait analysis of people with Charcot-Marie-Tooth (CMT) disease revealed proximal adaptive gait strategies to compensate for foot drop. We previously demonstrated that hip flexor muscle fatigue can limit walking endurance. This pilot study used a single-blinded cross over design to investigate the effect of a 16-week home-based programme of resistance training on hip flexor muscle strength. Measures of walking endurance, gait speed, exertion, fatigue, and general activity were also recorded. The exercise protocol was based on American College of Sports Medicine recommendations. A mixed effects model was used for analysis. Twenty-six people finished the study, with average reported exercise participation of 93%. No negative effects of exercise were observed. Significant increase in hip flexor muscle strength was observed on the left, but not the right. No changes were observed in walking speed and endurance measures. This pilot study of home-based resistance training showed a modest improvement in hip strength but only on one side. The lack of a more significant improvement and no improvement in walking measures suggests that this training protocol may not be optimal for people with CMT and that patients may need to stratified differently for training studies in CMT.

The impact of headache disorders: a prospective analysis of headache referrals to outpatient and inpatient neurology and emergency services in an Irish University teaching hospital.

BACKGROUND: Headache represents a significant proportion of disability globally in general practice, neurology outpatient settings, and emergency departments. There is scant literature regarding the impact of headache on healthcare services in Ireland. AIMS: We aimed to investigate headache burden across the emergency department, inpatient stays, and neurology outpatient department referrals in an Irish University teaching hospital. METHODS: We prospectively collected data regarding emergency department presentations, inpatient neurology consultations, and neurology outpatient referrals for patients with headache between 13th January and 8th March 2020. Data were analyzed using descriptive statistics. RESULTS: There were 180 emergency department attendances, 50 inpatient consultations, and 76 outpatient referrals with headache. Neurological examinations were often incomplete; neuroimaging was commonly employed. Migraine was the most frequent headache diagnosis at discharge in the emergency department and among inpatients after neurology review. Diagnostic uncertainty was identified-33% of patients left the emergency department with no diagnosis, and "unknown/unspecified headache" was recorded on 49% of outpatient referrals and 30% of inpatient consult requests. Medication overuse headache coexisted with migraine in nine patients in the inpatient group. Prophylaxis had been trialed in 56% of patients with migraine referred to outpatients. CONCLUSIONS: Primary headache disorders have a large impact on hospital services. Diagnostic uncertainty is common; neuroimaging is relied upon. Appropriate care pathways, education, and resource allocation should be prioritized.

Unintended effects of orphan product designation for rare neurological diseases.

Since the introduction of the Orphan Drug Act in 1983, designed to promote development of treatments for rare diseases, at least 378 orphan drugs have been approved. Incentives include financial support, tax credits, and perhaps most importantly, extended market exclusivity. These incentives have encouraged industry interest and accelerated research on rare diseases, allowing patients with orphan diseases access to treatments. However, extended market exclusivity has been associated with unacceptably high drug costs, both for newly developed drugs and for drugs that were previously widely available. We suggest that a paradoxical effect of orphan product exclusivity can be reduced patient access to existing drugs. In addition, the costs of each new drug are arguably unsustainable for patients and for the American health care system. Of all the specialties, neurology has the third highest number of orphan product designations, and neurological diseases account for at least one-fifth of rare diseases. Citing the use of tetrabenazine for chorea in Huntington disease, adrenocorticotropic hormone for infantile spasms, and enzyme replacement therapy with alglucosidase alpha for Pompe disease, we highlight these paradoxical effects.

Asymmetric sensory ganglionopathy: a case series.

INTRODUCTION: Sensory ganglionopathies are uncommon but potentially very disabling. They have heterogeneous etiologies including autoimmune, paraneoplastic, toxic, and inflammatory although many remain idiopathic despite intensive investigation. Asymmetric sensory loss is relatively common at the onset, but with time, symptoms usually spread to involve all limbs symmetrically. METHODS: We report 6 patients with a persistent strikingly asymmetrical sensory ganglionopathy with acute or subacute onset and slow progression. RESULTS: Peripheral nerve biopsies in 5 patients showed axonal loss without significant inflammation; a dorsal root ganglion biopsy in 1 patient showed neuronal loss and inflammatory infiltrate. Four patients received immunomodulatory treatment, but overall the response to treatment was poor. CONCLUSIONS: Asymmetrical sensory ganglionopathies may have an inflammatory basis. Immunomodulatory therapy may be considered early in the disease course, although in this series there was a limited response to treatment.

Frequency of inter-specialty consensus decisions and adherence to advice following discussion at a weekly neurovascular multidisciplinary meeting.

BACKGROUND/AIMS: Data are limited on the frequency of 'consensus decisions' between sub-specialists attending a neurovascular multidisciplinary meeting (MDM) regarding management of patients with extracranial carotid/vertebral stenoses and post-MDM 'adherence' to such advice. METHODS: This prospective audit/quality improvement project collated prospectively-recorded data from a weekly Neurovascular/Stroke Centre MDM documenting the proportion of extracranial carotid/vertebral stenosis patients in whom 'consensus management decisions' were reached by neurologists, vascular surgeons, stroke physicians-geriatricians and neuroradiologists. Adherence to MDM advice was analysed in asymptomatic carotid stenosis (ACS), symptomatic carotid stenosis (SCS), 'indeterminate symptomatic status stenosis' (ISS) and vertebral artery stenosis (VAS) patients, including intervals between index event to MDM + / - intervention. RESULTS: One hundred fifteen patients were discussed: 108 with carotid stenosis and 7 with VAS. Consensus regarding management was noted in 96.5% (111/115): 100% with ACS and VAS, 96.2% with SCS and 92.9% with ISS. Adherence to MDM management advice was 96.4% (107/111): 100% in ACS, ISS and VAS patients; 92% (46/50) in SCS patients. The median interval from index symptoms to revascularisation in 50-99% SCS patients was 12.5 days (IQR: 9-18.3 days; N = 26), with a median interval from MDM to revascularisation of 5.5 days (IQR: 1-7 days). Thirty patients underwent revascularisation. Two out of twenty-nine patients (6.9%) with either SCS or ISS had a peri-procedural ipsilateral ischaemic stroke, with no further strokes/deaths during 3-months follow-up. CONCLUSIONS: The high frequency of inter-specialty consensus regarding management and adherence to proposed treatment supports a collaborative/multidisciplinary model of care in patients with extracranial arterial stenoses. Service development should aim to shorten times between MDM discussion-intervention and optimise prevention of stroke/death.

Contactin-1 links autoimmune neuropathy and membranous glomerulonephritis.

Membranous glomerulonephritis (MGN) is a common cause of nephrotic syndrome in adults, mediated by glomerular antibody deposition to an increasing number of newly recognised antigens. Previous case reports have suggested an association between patients with anti-contactin-1 (CNTN1)-mediated neuropathies and MGN. In an observational study we investigated the pathobiology and extent of this potential cause of MGN by examining the association of antibodies against CNTN1 with the clinical features of a cohort of 468 patients with suspected immune-mediated neuropathies, 295 with idiopathic MGN, and 256 controls. Neuronal and glomerular binding of patient IgG, serum CNTN1 antibody and protein levels, as well as immune-complex deposition were determined. We identified 15 patients with immune-mediated neuropathy and concurrent nephrotic syndrome (biopsy proven MGN in 12/12), and 4 patients with isolated MGN from an idiopathic MGN cohort, all seropositive for IgG4 CNTN1 antibodies. CNTN1-containing immune complexes were found in the renal glomeruli of patients with CNTN1 antibodies, but not in control kidneys. CNTN1 peptides were identified in glomeruli by mass spectroscopy. CNTN1 seropositive patients were largely resistant to first-line neuropathy treatments but achieved a good outcome with escalation therapies. Neurological and renal function improved in parallel with suppressed antibody titres. The reason for isolated MGN without clinical neuropathy is unclear. We show that CNTN1, found in peripheral nerves and kidney glomeruli, is a common target for autoantibody-mediated pathology and may account for between 1 and 2% of idiopathic MGN cases. Greater awareness of this cross-system syndrome should facilitate earlier diagnosis and more timely use of effective treatment.

Comprehensive analysis of the TRPV4 gene in a large series of inherited neuropathies and controls.

BACKGROUND: TRPV4 mutations have been identified in Charcot-Marie-Tooth type 2 (CMT2), scapuloperoneal spinal muscular atrophy and distal hereditary motor neuropathy (dHMN). OBJECTIVE: We aimed to screen the TRPV4 gene in 422 British patients with inherited neuropathy for potentially pathogenic mutations. METHODS: We sequenced TRPV4 coding regions and splice junctions in 271 patients with CMT2 and 151 patients with dHMN. Mutations were clinically and genetically characterised and screened in ≥345 matched controls. RESULTS: 13 missense and nonsense variants were identified, of which five were novel and absent from controls (G20R, E218K, N302Y, Y567X and T701I). N302Y and T701I mutations were present in typical CMT2 cases and are potentially pathogenic based on in silico analyses. G20R was detected in a patient with dHMN and her asymptomatic father and is possibly pathogenic with variable expressivity. The Y567X variant segregated with disease in a family with severe CMT2 but also with a MFN2 mutation reported to cause a mild CMT2 phenotype. Although Y567X caused nonsense mediated mRNA decay, the amount of TRPV4 protein on western blotting of patient lymphoblasts was no different to control. Y567X is therefore unlikely to be pathogenic. E218K is unlikely to be pathogenic based on segregation. CONCLUSIONS: In this comprehensive analysis of the TRPV4 gene, we identified mutations in <1% of patients with CMT2/dHMN. We found that TRPV4 likely harbours many missense and nonsense non-pathogenic variants that should be analysed in detail to prove pathogenicity before results are given to patients.

Charcot-Marie-Tooth disease: frequency of genetic subtypes and guidelines for genetic testing.

BACKGROUND: Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous group of diseases with approximately 45 different causative genes described. The aims of this study were to determine the frequency of different genes in a large cohort of patients with CMT and devise guidelines for genetic testing in practice. METHODS: The genes known to cause CMT were sequenced in 1607 patients with CMT (425 patients attending an inherited neuropathy clinic and 1182 patients whose DNA was sent to the authors for genetic testing) to determine the proportion of different subtypes in a UK population. RESULTS: A molecular diagnosis was achieved in 62.6% of patients with CMT attending the inherited neuropathy clinic; in 80.4% of patients with CMT1 (demyelinating CMT) and in 25.2% of those with CMT2 (axonal CMT). Mutations or rearrangements in PMP22, GJB1, MPZ and MFN2 accounted for over 90% of the molecular diagnoses while mutations in all other genes tested were rare. CONCLUSION: Four commonly available genes account for over 90% of all CMT molecular diagnoses; a diagnostic algorithm is proposed based on these results for use in clinical practice. Any patient with CMT without a mutation in these four genes or with an unusual phenotype should be considered for referral for an expert opinion to maximise the chance of reaching a molecular diagnosis.