Novel FUS Deletion in a Patient With Juvenile Amyotrophic Lateral Sclerosis FREE

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disease affecting the upper and lower motor neurons. The mean age at symptom onset is 45 years for cases with a family history and 55 years for sporadic cases.¹ Affected individuals generally have a pure ALS phenotype, with degeneration affecting mainly neurons of the motor cortex, brainstem, and spinal cord. Patients usually die of respiratory failure after 3 to 5 years of disease progression; no effective treatment currently exists. Approximately 10% of patients with ALS have family members affected with the disease.² Mutations in the SOD1 gene explain approximately 2% of all ALS cases,³ while variants in TARDBP and FUS each explain about 1% to 3% of overall cases.⁴ On rare occasions, the first symptoms of motor neuron degeneration start before 25 years of age, usually with slower disease progression compared with later-onset typical ALS.⁵ This juvenile type of ALS has previously been associated with mutations in the SETX gene⁶ or the ALSIN gene for ALS2.⁷ Mutations in ALSIN are also known to cause juvenile primary lateral sclerosis and infantile-onset ascending spastic paralysis.⁸ After 2 reports in 2009 identified FUS mutations in familial ALS cases,^9- 10 a few research groups further identified FUS mutations in patients with juvenile ALS who experienced unusual and unexpected rapid progression.^11- 14 We report the case of a 19-year-old man with a deletion in exon 14 of the FUS gene, experiencing very rapidly progressing disease.

A 19-year-old man, with a history of mild learning difficulties who had been working in commercial toxic waste recycling for less than 1 year, first developed painful right shoulder and arm weakness at the end of August 2009, which progressed rapidly, and in less than 1 month included neck weakness. At the beginning of December, right arm muscle atrophy in the patient was noticed by members of his family, which prompted him to seek medical attention. A complete neurologic examination and workup was done including brain and spine magnetic resonance imaging, repeated electromyographic examination, toxicology screening, lumbar puncture, and biochemical blood tests. The general medical examination was normal. On neurologic examination, he had a normal mental status. Examination of the cranial nerves showed tongue atrophy and fasciculations; bilateral facial weakness, with the right side worse than the left; severe weakness and atrophy; and fasciculations of the sternocleidomastoids and trapezius, with the right side much worse than left. The remainder of the cranial nerves was normal. Motor examination showed atrophy and severe weakness of the right shoulder and upper arm including supraspinatus, infraspinatus, subscapularis, deltoid, pectoralis, and biceps. He had moderate weakness and atrophy of his right triceps and wrist and finger extensors and flexors, as well as interossei of the hands. There was normal bulk, tone, and strength of his left side and right leg. He was areflexic in the right arm, with mild hypereflexia in the left arm, knees, and ankles, with equivocal plantar responses. The cerebellar examination was normal, with the exception of the right arm, which was not testable. The sensory examination was normal as well as of his gait. The brain and spine magnetic resonance images were normal. Results from the lumbar puncture as well as the detailed biochemical tests and toxicology screen were normal. An electromyographic examination showed normal conduction velocities and normal needle examination of the left arm, left shoulder, and left leg. There was some denervation in muscles of the right leg and severe denervation in muscles of the right shoulder and arm. In March 2010, the patient was suspected of having juvenile-onset ALS and was prescribed treatment with riluzole. In May, he developed dysphagia and dysarthria, leading to anorexia and severe weight loss. In August, a control neurologic examination showed a 36-kg young man with diffuse severe muscle atrophy. Muscle weakness was asymmetric, with profound right upper limb weakness and, to a lesser degree, bilateral leg involvement. The patient rapidly progressed to respiratory failure requiring mechanical ventilation. Repeated neurologic examinations showed no cognitive or sensory symptoms or signs. Although follow-up visits were scheduled, the patient was lost to follow-up. In summary, the patient mostly displayed a lower motor neuron phenotype, with the upper limbs and bulbar regions principally affected. Upper motor neurons seemed to be mildly affected.

The patient's clinical history revealed that he was generally in good health for his first 17 years of life. He was seen for dysphasia at 6 years of age, was prescribed methylphenidate hydrochloride from 6 to 16 years of age, but he did not show any previous signs of paralysis, progressive weakness, or neurologic symptoms. However, he attended welding school when he was 16 years of age, and he worked with hazardous recycling material at 17 years of age.

While the patient had no apparent family history of motor neuron diseases, his mother was diagnosed with an Arnold-Chiari malformation at 13 years of age; she experienced equilibrium problems and 1 cerebellar ataxia episode at that time. The mother's parents were not reported to have developed any neurodegenerative symptoms. Other family members including the patient's father and brother are in good health.

DNA from the patient and his parents were extracted from peripheral blood using standard protocols. Primers for SOD1, TARDBP, and FUS were designed using the ExonPrimer software from the UCSC Human Genome Browser Web site (http://genome.ucsc.edu/cgi-bin/hgGateway). All 26 exons of the 3 genes were amplified for the patient for a total of 23 fragments, and the 1 fragment containing the mutation was amplified in his parents. Amplification was performed by polymerase chain reactions (PCRs) using the AmpliTaq Gold DNA Polymerase (Applied Biosystems) as per the manufacturer's instructions. The PCR product contained a minimum of 50 base pairs (bp) from each of the flanking introns. Products were directly sequenced in forward and reverse at the Genome Quebec Innovation Centre (Montreal, Quebec, Canada) using the 3730XL DNA analyzer (Applied Biosystems). Mutation surveyor software version 3.10 was used for mutation detection analyses (SoftGenetics). All exons of FUS were also amplified in 96 French-Canadian and 380 French control participants and have already been published.¹¹

A novel 1-bp deletion of a guanine in exon 14 of the FUS gene (c.1484delG) (Figure, A) was identified and is predicted to cause a shift in the reading frame, resulting in the inclusion of 33 new amino acids (p.R495QfsX527), which modifies the highly conserved C terminal of the protein (Figure, B) that is believed to play a crucial role in RNA processing.⁴ The final mutated protein is predicted to have 1 more amino acid than the wild type, resulting in a probable change in the C terminal function or interaction abilities. The variant was also identified in the patient's 47-year-old mother, who did not show any signs of motor neuron degeneration.

Interestingly, Elden et al¹⁵ found that intermediate-length polyglutamine tracts (range, 24-33 repeats) in ATXN2 confer an increased risk for developing ALS. We decided to test the length of the repeat in ATXN2 in the 19-year-old patient and his parents, the hypothesis being that the patient affected by juvenile ALS developed the symptoms earlier than his mother because he inherited an intermediate-length polyglutamine tract from his father, who does not carry the FUS mutation. We amplified the ATXN2 CAG repeats using PCR. We determined the CAG repeat sizes by capillary electrophoresis by incorporating a VIC-labeled M13 universal primer into the PCR reaction. Polymerase chain reaction products were then diluted (1:20),mixed with LIZ-500 size standard (Applied Biosystems), and processed for size determination on the ABI 3730 DNA analyzer (Applied Biosystems). Repeat sizes were determined using the GeneMapper Software version 4.0 (Applied Biosystems). All 3 individuals had both the (CAG)21 and (CAG)22 alleles, which are the most common in the general population. The experiment was repeated twice, yielding the same results.

A novel 1-bp deletion was detected in exon 14 of the FUS gene, leading to a frameshift and the integration of 33 new amino acids. The variant c.1484delG (p.R495QfsX527) is located in the highly conserved, extreme C terminal of the FUS protein, where most of the mutations in FUS have been identified. The variant was also identified in the patient's unaffected 47-year-old mother, who remains healthy. Separate deletions were already reported in familial ALS cases 1-bp upstream (c.1483delC) and 1-bp downstream (c.1485delA), producing p.R495EfsX527 and p.G497Afs527, respectively.¹² This confirms the importance of a conserved C terminal for the normal functioning of the FUS protein. Additionally, the same group published a nonsense substitution at position c.1483C>T that was shown to produce a truncated protein (p.R495X),¹² further reinforcing the importance of this region. What is particularly interesting is the age at onset and disease duration associated with the p.R495EfsX527-, p.G497Afs527-, and p.R495X-reported mutations. In fact, the 2 affected members of the family with the p.R495EfsX527 mutation displayed a significant difference in terms of age at onset and duration because 1 developed the first symptoms at 23 years of age and the disease progressed over a 48-month period, while the other developed ALS at 72 years of age and died 12 months after symptom onset. On the other hand, the 7 members of the family in whom the nonsense p.R495X mutation segregated had an early age at onset but varied in terms of disease duration. Five members of the family developed ALS at 14, 24, 27, 39, and 44 years of age, while 2 other members with the variant were still unaffected at 57 and 61 years of age. Moreover, the p.G497Afs527 mutation segregated in 3 members of another family, who experienced the first symptoms at 13, 29, and 29 years of age. The disease progressed very rapidly over 12-, 13-, and 18-month periods in those patients.

In addition, another group described a woman with sporadic ALS with a FUS p.G466VfsX14 splice mutation who experienced the first symptoms at 20 years of age; the disease progressed for a 22-month period.¹⁴ Interestingly, our group also previously published 1 French-Canadian sporadic ALS case with a FUS p.Q519X mutation. This patient started to develop motor neuron symptoms at 20 years of age, and the disease progressed rapidly during a 12-month period,¹¹ underlying the fact that a mutation in that specific region gave rise to an early ALS onset often characterized by rapid progression.

The research cases just described along with our reported juvenile case underscore the importance of the last 32 amino acids of the FUS protein and demonstrate that despite an existing variability in terms of age at onset, it seems that patients carrying such mutations frequently develop ALS at an earlier age while the disease progresses more rapidly. However, other genetic modifiers might influence the age at which the first ALS symptoms will appear. An intermediate length of the polyglutamine tracts in ATXN2 was not a modifier of the age at onset in our juvenile patient. On the other hand, the patient was highly exposed 2 years before developing the first symptoms to welding material as well as heavy metals, solvents, pesticides, and agricultural chemicals, all of which have been reported to be associated with ALS.^16- 18 While detailed biochemical tests and toxicology screen results were normal, unknown genetic variants already present in the patient could have led to a certain vulnerability to toxic exposition, or mutations caused by such environmental exposition could explain why the patient developed the first signs of ALS while his mother remains healthy. However, it cannot be ruled out that other susceptibility variants in other genes explain the early onset. It could also be hypothesized that the mother carries a protective variant, preventing the early onset of the disease.