Journal of Neurology 2016 Feb; 263(2): 361-9
NEFL N98S mutation: another cause of dominant intermediate Charcot-Marie-Tooth disease with heterogeneous early-onset phenotype
Berciano J1*, Peeters K2*, García A1, López-Alburquerque T3, Gallardo E1, Hernández-Fabián A3, Pelayo-Negro AL1, De Vriendt E2, Infante J1, Jordanova A2
1Services of Neurology, Clinical Neurophysiology and Radiology, University Hospital “Marqués de Valdecilla (IDIVAL)”, University of Cantabria (UC) and “Centro de Investigación Biomédica en Red de Enfermedades Neurodegenerativas (CIBERNED)”, Santander, Spain
2Molecular Neurogenomics Group, VIB Department of Molecular Genetics, University of Antwerp, Antwerpen, Belgium. Neurogenetics Laboratory, Institute Born-Bunge, University of Antwerp, Antwerpen, Belgium
3Services of Neurology and Pediatric Neurology, University Hospital of Salamanca, Salamanca, Spain
*Authorship: J Berciano and K Peeters contributed equally
The purpose of this study was to describe a pedigree with NEFL N98S mutation associated with a dominant intermediate Charcot-Marie-Tooth disease (DI-CMT) and heterogeneous early-onset phenotype. The pedigree comprised two patients, the proband and her son, aged 38 and 5 years. The proband, evaluated at age 31, showed delayed motor milestones that, as of the second decade, evolved into severe phenotype consisting of sensorimotor neuropathy, pes cavus, clawing hands, gait and kinetic cerebellar ataxia, nystagmus and dysarthria, she being wheelchair bound. By then, a working diagnosis of sporadic early onset cerebellar ataxia with peripheral neuropathy was established. Screening of mutations associated with SCA and autosomal recessive cerebellar ataxias was negative. Her son showed a mild phenotype characterized by delayed motor milestones, and lower-limb hypotonia and areflexia. Electrophysiology in both patients showed nerve conduction slowing in the intermediate range, both in proximal and distal nerve segments, but where compound muscle action potentials exhibited severe attenuation there was conduction slowing down to the demyelinating range. In the proband, cranial magnetic resonance imaging (MRI) showed cerebellar atrophy, electromyography disclosed active denervation in tibialis anterior, and MRI of lower-limb musculature demonstrated widespread and distally accentuated muscle fatty atrophy; furthermore, on water sensitive MRI sequences there was edema of calf muscles. We conclude that the NEFL N98S mutation is associated with a DI-CMT phenotype characterized by early-onset sensorimotor neuropathy delaying motor milestones, which may evolve into a severe and complex clinical picture including cerebellar ataxia.
Supplement: Abbreviated version of the paper for World Biomedical Frontiers
Charcot-Marie-Tooth disease (CMT) is the most frequent form of sensorimotor inherited neuropathy with a prevalence ratio of 28 cases/100,000 inhabitants1. CMT was initially classified according to the mode of transmission (autosomal dominant, autosomal recessive or X-linked) and electrophysiological or nerve biopsy features. Characteristically, motor conduction velocities (MCV) in median nerve are below 38 m/s for the demyelinating form (CMT1) and above 38 m/s for the axonal form2. For want of a better name, Davis et al introduced the term intermediate for designating dominant CMT (DI-CMT) families with clinico-electrophysiological and pathological data not fitting into either CMT1 or CMT23. In short, DI-CMT was originally characterized by: i/ absence of clinically observed nerve hypertrophy; ii/ median nerve MCV between 25 and 45 m/s (mean in their series, 34.6 m/s); iii/ preserved mean compound muscle action potential (CMAP) amplitude (mean, 4.6 mV); and iv/ nerve biopsy showing axonal changes, clusters of regenerating myelinated fibres, loss of the larger fibres noted from the unimodal diameter histogram, and onion bulbs with fewer lamellae than in CMT1.
The number of disease genes identified in CMT has expended rapidly over the last decades, particularly after the development of next-generation sequencing, such that more than 60 CMT-associated genes have now been discovered4. Leaving CMTX1 aside, DI-CMT encompasses six loci with five cloned genes (DNM2, YARS, MPZ, INF2, and GNB4)4, 5.
Neurofilament light-chain polypeptide gene (NEFL) mutations may cause either dominant axonal (CMT2E) or dominant demyelinating (CMT1F) phenotype, and rarely recessive axonal phenotype. NEFL mutations have occasionally been associated with intermediately slowed MCV, just one pedigree having been reported under the rubric of DI-CMT6. As a whole, NEFL mutations represent between 0.8 and 2% of all patients with CMT. NEFL-associated CMT is a highly variable disorder that comprises more than 18 disease-causing mutations, targeting the head or rod protein domains, with highly variable phenotypic expression, N98S (N97 in the old nomenclature) being reported in five pedigrees with severe early-onset disease phenotype7-10.
Figure 1. (A) Pedigree of the family. Patients of the first generation died at ages between 40 and 92 years. Case II-2 died, but no further details are available. Genotypes at position c.293 in NEFL are shown below the examined individuals (indicated with horizontal bars). (B) Electropherograms depicting the mutated (MUT) and reference (REF) sequences surrounding the NEFL c.392 position.
Here we describe the study of a heterogeneous DI-CMT pedigree, comprising the proband and her affected child, associated to NEFL N98S mutation.
- The study is based upon a single Spanish CMT family comprising two affected subjects over two generations (Figure 1).
- We carried out nerve conduction studies, and MR imaging examination of brain and lower-limb musculature.
- Molecular study comprised whole exome sequencing (WES) performed on the two affected individuals (III-2 and IV-1), and mutational analysis of NEFL.
Figure 2. Clinical and MRI pictures of the proband patient at age 31. (A) Close-up picture of hands showing marked hand amyotrophy with clawing deformity. (B) Note bilateral lower-leg amyotrophy with drop feet. (C) Axial T1 weighted MR image of middle third of thighs showing moderate and diffuse muscle fatty atrophy. (D) Axial T1 weighted MR image of mid calves showing marked and widespread fatty infiltration of all four muscle compartments. (E) Axial T2FS weighted MR image, obtained at same level of the previous one, illustrating muscle symmetric hypersignal, involving posterior tibialis, anterior tibialis and gastrocnemious medialis muscles, indicative of edema. (F) T1 weighted MR image through metatarsal bones showing massive fatty infiltration of foot musculature.
- The propositus, aged 38, presented with delayed motor milestones; she was firstly evaluated in 2008 at age 31. As of the second decade, there appeared distal weakness and sensory loss, pes cavus, clawing hands, cerebellar ataxia, nystagmus and dysarthria, she being wheelchair bound with severe CMT neuropathy score (CMTNS) (Figure 2). Her initial laboratory screening included molecular analysis of current causes of autosomal dominant or autosomal recessive ataxia, which was negative. The proband’s affected son, aged 5, showed delayed motor milestones, his examination demonstrating subtle signs that consisted of lower-limb areflexia and hypotonia with mild CMTNS.
- Nerve conduction studies, comprising both distal and proximal nerve segments, revealed the characteristic pattern of intermediate CMT. In nerves with severe distal compound muscle action potential (CMAP) attenuation, resulting in motor conduction velocities (MCV) in the demyelinating range, exploration of proximal nerve segments was particularly helpful for an accurate detection of intermediate nerve conduction slowing (cf. figure 3 in reference 6).
- Brain MR imaging of lower-limb musculature showed extensive and predominantly distal fatty atrophy of lower-limb musculature (Figure 2); on T2 fat suppressed (T2FS) weighted images, there was muscle oedema of calf muscles
- Brain MR imaging demonstrated marked cerebellar wasting (Figure 3).
- We investigated the six DI-CMT categories so far reported5 using WES of the two affected individuals (see figure 1). The analysis revealed a c.293A>G (p.N98S) missense mutation in NEFL, co-segregating with disease in the proband (III-2) and her affected son (IV-1).
Figure 3. Cranial sagittal T1-weighted MR image in the proband showing severe atrophy of the cerebellar vermis.
Conclusions and significance
- NEFL N98S mutation is another cause of DI-CMT to be added to the six genetic subtypes so far recognized.
- Detection of MCVs in the intermediate range was essential for accurate guidance of molecular diagnosis.
- The phenotype is characterized by early onset sensorimotor neuropathy delaying motor milestones, which may evolve into a severe and complex clinical picture including cerebellar ataxia.
- Differential diagnosis should include early onset cerebellar ataxias with peripheral neuropathy.
- For an accurate evaluation, electrophysiology should include examination of distal and proximal nerve trunks, particularly when distal CMAPs are severely reduced resulting in an apparent demyelinating nerve conduction pattern.
- MR imaging study of lower-limb musculature illustrates predominantly distal fatty atrophy, which suggests a process of length-dependent muscle denervation. The presence of muscle edema, a sign characteristic of subacute denervation, suggests that nerve inflammatory changes superimposed on the genetic condition might be pathogenic in the process of denervation (for further details, see reference 6).
- In good correlation with complex clinical findings, combining peripheral neuropathy and cerebellar semeiology, brain MR imaging may show cerebellar atrophy.
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Acknowledgements: We are grateful to Dr Victor Volpini for molecular screening of SCA and ARCA genes, Mrs Marta de la Fuente for Secretarial assistance, and Mrs. Coro Sánchez for laboratory help, and Miss Mar Ruiz for technical support. The study was supported by Research Institute of University Hospital “Marqués de Valdecilla” (IDIVAL) Grants BFR 05/10 and WLA 03/12. The Antwerp team was funded in part by the University of Antwerp (TOP BOF 29069); the Fund for Scientific Research-Flanders (FWO) and the Belgian Association against Neuromuscular Disorders (ABMM). KP is supported by a PhD fellowship from the Fund for Scientific Research-Flanders (FWO).
Professor Emeritus of Neurology
Department of Medicine and Psychiatry
University of Cantabria
39008 Santander, Spain
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