Amyotrophic lateral sclerosis with long lasting disease course and SOD1 and TARDBP mutations: Report of two cases and overview of the literature.

Introduction Despite its canonical nosographic definition as a progressive neurodegenerative disorder typically involving both lower and upper motor neurons, leading to progressive muscle paralysis and death approximately 2–4 years after symptom onset, amyotrophic lateral sclerosis (ALS) is a heterogeneous disease, including clinical presentation and disease course. Forms with exclusive/predominant upper or lower motor neuron involvement are known, as well as subgroups of patients with a long disease course. ALS is sporadic in approximately 90% of cases, whereas about 10% of cases are familial (1). Mutations in the copper/zinc superoxide dismutase (SOD1) gene were first reported in familial ALS patients and account for approximately 20% of these patients (2); subsequently, a growing number of ALS-causing genes have been identified, among which is the TARDBP gene, coding for the TAR DNA-binding protein 43 (TDP-43) (3). In addition, several known familiar ALS mutations have also been reported in apparently sporadic ALS cases, underlying the complex genetic heterogeneity of ALS pathology. We describe here two cases of apparently sporadic ALS associated with mutations, respectively, in SOD1 and TARDP genes, characterized by predominant lower motor neuron limb involvement and long disease course. Cases description Case 1 A 70-years old Caucasian female came to our attention for a 12-year history of slowly progressive, mainly distal, hyposthenia and hypotrophy involving the four limbs; at first symptoms involved the left hand, mainly dorsal interosseous muscles, and roughly three years later left arm muscles; then, late in this interval, contralateral limb involvement, first the lower, then the upper one, began to occur. No family history of neuromuscular disorders was reported. Neurological evaluation showed diffuse and symmetric muscle weakness and hypotrophy at the four limbs, osteo-tendon hyporeflexia and indifferent cutaneous plantar reflex bilaterally; no sensitive, bulbar or respiratory impairment was documented, or bladder dysfunctions. Electrophysiological examinations were consistent with second motor neurons damage; however, motor evoked potentials demonstrated subtle abnormalities at upper motor neurons level, represented by a prolongation of the central conduction time at the four limbs. A subsequent extensive workout, including laboratory examinations, brain and spine MRI, cerebrospinal fluid analysis, was unremarkable. Molecular DNA analysis, performed by automatic sequencing, revealed a heterozygous mutation in the SOD1 gene (A4G transition in exon 5, position 365), this determining the aminoacid substitution p.E121G. DNA analysis searching for mutations of the FUS, TARDBP and SMN1 genes was negative. Case 2 A 55-years old Italian male, of Sardinian origin, came to our attention for a 13-year history of slowly progressive, symmetrical, mainly distal, hyposthenia involving the four limbs, with cramps and fasciculations. Neurological examination showed diffuse distal hypotrophy and weakness, brisk osteo-tendon reflexes at upper limbs, abolished osteo-tendon reflexes at lower limbs, mild flexor plantar reflexes, diffuse fasciculations; no sensitive, bulbar or respiratory impairment was present, or bladder dysfunctions. Electromyography was consistent with second motor neurons damage. Motor evoked potentials showed mild abnormal function of upper motor neurons: in detail, at upper limbs, motor evoked potentials were bilaterally reduced in amplitude, whereas at lower limbs, motor evoked potentials were reduced in amplitude, with a prolongation of the central conduction time. Additionally, a psychiatric history of bipolar disorder was present, without evidences of cognitive impairment. No family history of neuromuscular disorders was reported. Laboratory examinations, brain and spine MRI, and cerebrospinal fluid analysis, were unremarkable. Molecular DNA analysis showed a heterozygous c.1144G-4A mutation in the TARDBP gene (G-4A transition at nucleotide 1144, exon 6), determining the amino acid substitution p.A382T. Molecular analysis of SOD1, FUS and SMN1 genes was negative. Discussion Since its first reports and despite its monogenic nature, the great clinical heterogeneity of genetic ALS has been clear, also at the level of the same mutation in the same gene, this raising the fundamental question about the mechanisms underlying the observed phenotypic variability. This is particularly true when dealing with less common clinical forms as is the case of those with very long disease course. We here first describe an apparently sporadic case, disease onset at the age of 58 years, with predominant lower motor neuron involvement and slow disease course, carrying the p.E121G heterozygous missense mutation of the SOD1 gene. This mutation was first reported by Canosa et al. (4) in an Italian male patient, with disease onset at the age of 70 years and an apparently sporadic 14-year ALS course mainly involving the lower motor neurons, finally leading to dysarthria, dysphagia and respiratory failure. Subsequently, this mutation was reported by Dangoumau et al. (5) in a French patient with a slowly progressive ALS phenotype and disease onset at the age of 70 years; in addition, these authors carried out functional studies in the motor neuronal cell line NSC34 and in a primary culture of mouse motor neurons, revealing that the mutation p.E121G induced cytoplasm aggregates, as well as reduced cell viability under oxidative stress, thus supporting the hypothesis of a pathogenic role of the mutation. Also, the second patient, with a TARDBP p.A382T missense mutation, presented a clinical phenotype of long lasting disease course and predominant lower motor neuron involvement, although, as for the first case and due to the quite long time-elapse between disease onset and time of observation, we cannot exclude more subtle signs of upper motor neuron involvement as is the case for deep tendon hyperreflexia. It was previously reported that ALS patients of Sardinian ancestry have a higher frequency than expected of the TARDBP p.A382T missense mutation (6,7). Borghero et al. (8) reported in a cohort of 375 ALS patients of Sardinian origin a 25% occurrence of this mutation in familial cases and 19.3% in sporadic cases; regarding clinical phenotype, patients with a TARDBP p.A382T heterozygous mutation had the longest median survival up to 6.5 years and four patients showed extrapyramidal signs. Additionally, Cannas et al. (9) showed that the clinical presentation of the p.A382T TARDBP mutation may include forms of Parkinsonism in which the extrapyramidal signs are the crucial core of the disease at onset. Therefore, the present report seems to expand the clinical spectrum of the TARDBP p.A382T heterozygous mutation, this case being characterized by predominant lower limb involvement. In both cases family history resulted negative, although DNA analysis of other family members was not possible. Taken together, these case reports further underline the clinical heterogeneity of ALS disease and the need to investigate deep inside the pathophysiological mechanisms of ALS pathology, in particular to determine genetic and non-genetic factors modulating ALS phenotype.