A family with axonal sensorimotor polyneuropathy with TUBB3 mutation
- Authors:
- Published online on: December 4, 2014 https://doi.org/10.3892/mmr.2014.3047
- Pages: 2729-2734
Abstract
Introduction
Heterozygous missense mutations in β-tubulin isotype III (TUBB3), which is exclusively expressed in neurons (1), result in the TUBB3 syndrome, which includes congenital fibrosis of the extraocular muscle type 3 (CFEOM3), intellectual impairments, facial paralysis and/or an axonal sensorimotor neuropathy (2,3). These findings suggested that TUBB3 has a role in mediating axon guidance and maturation (4,5). Brain magnetic resonance imaging (MRI) of patients with TUBB3 mutations has demonstrated evidence of dysgenesis of the corpus callosum, anterior commissure and internal capsule as well as generalized loss of white matter (4). However, to the best of our knowledge, there has been no report evaluating the features of lower limb MRIs of patients with TUBB3 mutations.
Patients in the same family who harbored the D417N mutation in TUBB3 exhibited a broad clinical spectrum, including CFEOM3 only; mixed features of CFEOM3 with peripheral neuropathy, learning disabilities and developmental delay; peripheral neuropathy only or no clinical symptoms (3,4). Therefore, the TUBB3 mutation may cause an isolated axonal sensorimotor polyneuropathy and detailed descriptions of the histopathology of sural nerve and lower extremity MRI results require further evaluation.
The present study evaluated individuals exhibiting an axonal sensorimotor polyneuropathy with the D417N mutation in TUBB3, which was identified by whole exome sequencing (WES), and reports the pathophysiological results observed.
Materials and methods
Patients
The present study enrolled five members of a Korean family which was severely affected by axonal Charcot-Marie-Tooth (CMT) (FC423), including two affected individuals. A healthy 45-year-old male was enrolled for nerve histology study. Written informed consent was obtained from all participants according to the protocol approved by the Institutional Review Board for Ewha Womans University, Mokdong Hospital (approval no. ECT 11-58-37; Seoul, Korea).
DNA preparation and whole exome sequencing
DNA from peripheral blood was isolated using a QIAamp blood DNA purification kit (Qiagen, Hilden, Germany). The isolated DNA was prescreened for duplication of 17p12 (PMP22) and mutations in the coding exons of GJB1, MPZ, NEFL and MFN2 as previously described (6). WES and subsequent filtering were performed in the affected members as previously described (7).
Capillary sequencing
Mutations in TUBB3 were analyzed by capillary sequencing. Exon four, harboring the mutation site, was amplified using the following primers (Macrogen, Seoul, Korea): Forward, 5′-CATCCAGAGCAAGAACAGCA-3′ and reverse, 5′-TTCGTACATCTCGCCCTCTT-3′. Polymerase chain reaction (PCR) amplifications was performed using Platinum PCR SuperMix High Fidelity kit (Life Technologies, Grand Island, NY, USA), with the above primers and the genomic DNA of the patients. Amplification was performed as follows: 32 cycles of 95°C for 30 sec, 56°C for 30 sec and 72°C for 60 sec. Following amplification, the sequences were analyzed using a BigDye terminator cycle sequencing kit and automatic genetic analyzer (ABI3130XL; Applied Biosystems, Life Technologies, Foster City, CA, USA).
Clinical assessments
Clinical assessments were performed by two independent neurologists as previously described (8). Patients were evaluated by taking a comprehensive medical history, including details of motor and sensory impairments, deep tendon reflexes, muscle atrophy and gait abnormalities, for example walking on heels or toes. The muscle strength of the flexor and extensor muscles was measured manually using the Medical Research Council scale (http://www.mrc.ac.uk). In order to determine physical disability, three scales were used: The functional disability scale (FDS) (9), CMT neuropathy score (CMTNS) (10) and sensory impairment, which was assessed with regard to the level and severity of pain, temperature, vibration and position.
Electrophysiological examination
Electrophysiological studies were performed using the Sierra Wave EMG system (Cadwell Laboratories, Inc., Kennewick, WA, USA) and the Toennies two-channel NeuroScreen system (Jaeger-Toennies, Hochberg, Germany). Motor nerve conduction velocities (MNCVs) of the median and ulnar nerves were determined by electrical stimulation at the elbow or wrist, while recording compound muscle action potentials (CMAPs) over the abductor pollicis brevis and adductor digiti quinti, respectively. Similarly, the MNCVs of the peroneal and tibial nerves were determined by stimulation at the knee and ankle, while recording CMAPs over the extensor digitorum brevis and adductor hallucis, respectively. CMAP amplitudes were measured from baseline to negative peak values. Sensory nerve conduction velocities were measured over the finger-wrist segment from the median and ulnar nerves by orthodromic scoring, and were also recorded for sural nerves. Sensory nerve action potential (SNAP) amplitudes were measured from positive to negative peaks. An electromyography (EMG) was performed in the bilateral proximal and distal limb muscles.
Lower extremity MRI
MRI study of the proband was performed using the Siemens Vision 1.5-T system (Siemens, Erlangen, Germany). MRI was performed in the axial (field of view, 24–32 cm; slice thickness, 10 mm and slice gap, 0.5–1.0 mm) and coronal planes (field of view, 38–40 cm; slice thickness, 4–5 mm and slice gap, 0.5–1.0 mm) using the following protocol: T1-weighted spin-echo (SE) [repetition time (TR)/echo time (TE) 570–650/14–20; 512 matrixes], T2-weighted SE (TR/TE 2800-4000/96-99; 512 matrixes) and fat-suppressed T2-weighted SE (TR/TE 3090-4900/85-99; 512 matrixes).
Histopathological studies
Histopathological analysis of the distal sural nerve was performed on the proband. Semi-thin sections were stained with 0.1% Toluidine blue solution (Sigma-Aldrich, St. Louis, MO, USA) for 3 min and then dehydrated with ethanol (Sigma-Aldrich). The density of myelinated fibers (MFs), axonal diameter and myelin thickness were determined using a computer-assisted image analyzer (AnalySIS 3.0; Soft Imaging System, Münster, Germany). Ultrathin sections were contrasted with uranyl acetate and lead citrate for ultrastructural study using a transmission electron microscope (H-7650; Hitachi, Tokyo, Japan).
Results
Genetic analysis
WES was performed in the two affected family members (Fig. 1A; III-7 and III-13). The total sequencing yields were 6.63 and 8.67 Gbp, respectively. Subsequent filtering identified 11 variants, which were shared by the affected family members (data not shown). Among the variants identified, the heterozygous D417N (c.1249G>A) mutation in TUBB3 was shown to be a cause of CFEOM3 (4). Since patients harboring the same mutation exhibited peripheral neuropathy, it was concluded that the D417N mutation was the putative cause of the clinical features observed in the patients examined in the present study. Capillary sequencing confirmed that the mutation was co-segregated within the family members (Fig. 1B).
Clinical manifestations
The proband (Fig. 1A, III-13; 48 years old) initially exhibited gait disturbance at 12 years of age. Progressive gait impairment was identified and the proband became wheelchair bound at 40 years of age (Fig. 1C). At 48 years, significant muscle weakness and atrophy of the distal lower limbs was observed. All sensory modalities were severely impaired and the vibration sense was more markedly affected than the pinprick sense. Deep tendon reflexes were absent and pathological reflexes were not present in all limbs. However, the proband did not demonstrate ophthalmoplegia, blepharoptosis or strabismus (Fig. 2). Mini-mental state examination and all cranial nerve examinations were normal.
By contrast, a 60-year-old cousin of the proband (Fig. 1A; III-7) began to experience gait disabilities when he was 45 years old and, unlike the proband, was still able to walk using ankle-foot devices. Neurological examination at 60 years of age revealed bilateral pes cavus and distal muscle atrophy of the upper and lower limbs. Based on the medical history notes taken, the deceased family members (I-2, II-2, II-5 and III-9) had exhibited distal lower limb weakness and gait disturbance. Furthermore, none had experienced any eye symptoms.
Axonal neuropathies identified by electrophysiological analysis
Nerve conduction studies were performed on the two patients aged 48 and 60 years, respectively (Table I). The values of the median MNCV were from 39.1–40.6 m/s, and those of the ulnar MNCV were from 40.0–53.7 m/s. Right CMAPs of the median nerves were below normal range, whereas those of the ulnar nerve were normal. Tibial and peroneal MNCVs were not elicited and SNAPs of the median, ulnar and sural nerves were absent. A needle EMG identified fibrillation potentials, positive sharp waves and neurogenic motor unit action potentials.
Lower extremity MRI results are length-dependent
MRIs of the lower extremities of the proband revealed hyperintense signal abnormalities in the lower leg muscles (Fig. 3A), whereas brain MRIs exhibited normal features (data not shown). T1-weighted images identified marked muscle atrophy with signal alterations observed in the lower leg muscles compared to those of the hip and thigh muscles. The predominant atrophy of the distal muscles was consistent with the length-dependent neuropathy hypothesis. At the hip (Fig. 3B) and thigh (Fig. 3C) levels, the fatty involvement of compartment muscles was not observed. By contrast, almost all muscles in the lower extremities demonstrated diffuse atrophies and fatty hyperintense signal changes (Fig. 3D).
Histopathological observations
Histopathological examination of the distal sural nerve of the proband (III-13) aged 48 years revealed an absence of large MFs, with the presence of small- or medium-sized and thinly scattered MFs (Fig. 4A). Regenerating axonal clusters and thick MFs were rarely identified. The number of remaining MFs (1,770/mm2) was markedly lower than that of a healthy 45-year-old male volunteer (7,300/mm2). The range and average MF diameter were smaller than age-matched controls and the histogram identified a unimodal distribution pattern (Fig. 4B). Electron microscopic examination revealed scattered myelinated and unmyelinated axons with swelling or vacuolization of the axoplasm as well as degenerated abnormal mitochondria and membranous structures (Fig. 4C). Degenerating MFs with the breakdown of myelin and axons were rarely noted (Fig. 4D). Although there were thinly scattered MFs, no further evidence of demyelination, including demyelinated axons or onion bulb formation, were present.
Discussion
The present study revealed that a Korean family with a TUBB3 mutation exhibited an axonal sensorimotor neuropathy. Histopathological analysis of the sural nerve biopsy of the proband revealed features characteristic of an axonal neuropathy, including a decreased number of MFs, the absence of large MFs and abnormally thick MFs. However, CFEOM3, ophthalmoplegia and intellectual impairment were not observed in the two patients. According to previous studies, the clinical features of the D417N mutation in TUBB3 are heterogeneous (4). Tischfield et al (4) compared the phenotypes of 15 individuals from four families harboring the mutation: Three patients exhibited CFEOM3 only, four patients exhibited CFEOM3 with peripheral neuropathy, two patients exhibited CFEOM3 with peripheral neuropathy and developmental delay, three patients exhibited peripheral neuropathy only, one patient exhibited CFEOM3 with peripheral neuropathy and learning disability, one patient exhibited CFEOM3 with learning disability and one patient was a non-penetrant carrier. Therefore, one-fifth of the patients exhibited a peripheral neuropathy phenotype only. An alternate amino acid change at the same site (D417H) also resulted in a heterogeneous phenotype within a family: All three affected family members exhibited CFEOM, wrist and finger contractures and facial weakness. However, one family member exhibited peripheral neuropathy, while another exhibited peripheral neuropathy with additional developmental delay and the third did not exhibit peripheral neuropathy (3). These results supported the hypothesis that mutations at the D417 position result in a broad phenotypic spectrum, suggesting that the clinical features may depend on the genetic background of each individual. In the present study, in comparison to the proband, his cousin experienced late-onset symptoms and a mild phenotype. Therefore, these patients exhibited clinical diversity within the family.
Application of WES was found to be a cost- and time-effective method used to reveal the underlying genetic causes of rare human diseases (11–13). Previous studies by our group have also applied WES and successfully isolated the causative mutations of patients with peripheral neuropathy (7,14–16). In the present study, the sole genetic cause of the symptoms observed was not isolated. However, certain evidence has suggested that the TUBB3 mutation may be the underlying causative gene of CMT in the patients examined in the present study. Only the TUBB3 D417N mutation has been reported to cause peripheral neuropathy and only four of those genes (APPL2, LMO7, TNRC6C and TUBB3) were expressed in the sural nerve according to transcriptome sequencing (data not shown). The TUBB3 D417N mutation was not found in the Korean controls (n=300) or reported in the Exome Variant Server (http://evs.gs.washington.edu/EVS/). Therefore, it was concluded that the TUBB3 mutation was the genetic cause of the axonal neuropathy observed.
To the best of our knowledge, to date, pathological features in the sural nerve have not been reported in patients with TUBB3 mutations. In the present study, a distal sural nerve biopsy was performed on a patient harboring a TUBB3 mutation. The results identified marked axonal loss, but no other evidence of demyelination, including demyelinated axons or onion bulb formation, were present. In addition, the nerve conduction studies indicated that the median and ulnar nerve conduction velocities were >38 m/s, and electromyography revealed neurogenic motor unit action potentials and fibrillation potentials. These results were compatible with those expected in axonal neuropathy.
MRI analysis revealed a distinct pattern of muscular involvement, where markedly greater muscle atrophy with hyperintense signal changes was identified in the lower leg muscles than that in the thigh or hip muscles. This was compatible with the hypothesis of length-dependent axonal degeneration. Previously, a type-specific pattern of fatty infiltration in muscles was reported: In demyelinating hereditary motor and sensory neuropathy (HMSN type 1A) the tibilalis anterior and peronei muscles were predominantly involved. However, late-onset axonal HMSN type 2A revealed predominant involvement of the soleus muscle, whereas early-onset HMSN type 2A exhibited whole calf and muscle involvements, including moderate to severe fatty changes in the thigh muscle (17). In the present study, the proband exhibited whole-compartment calf muscle involvement with no involvement of the thigh muscles. The proband therefore exhibited a pattern which differed from those identified in axonal HMSN type 2A. These findings are likely due to the variable pathophysiologies of axonal neuropathies.
In the future, specific molecular diagnoses will be important for the development of personalized therapies for axonal neuropathy. In this context, the results of the present study may aid in the development of genetic testing for axonal neuropathypatients.
Acknowledgements
The present study was supported in part by the Korean Health Technology R&D Project, Ministry of Health & Welfare (Sejong, Korea; no. A120182).
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