The physiological basis of conduction slowing in ALS patients homozygous for the D90A CuZn-SOD mutation

Widespread CNS pathology in amyotrophic lateral sclerosis homozygous for the D90A SOD1 mutation

Mutations in the gene encoding the ubiquitously expressed free radical scavenging enzyme superoxide dismutase-1 (SOD1) are found in 2-6% of amyotrophic lateral sclerosis patients. The most frequent SOD1 mutation worldwide is D90A. Amyotrophic lateral sclerosis caused by this mutation has some unusual features: the heredity is usually recessive, the phenotype is stereotypic with slowly evolving motor symptoms beginning in the legs and may also include sensory, autonomic, and urinary bladder involvement. Furthermore, the mutant protein resembles the wild type, with normal content and enzymatic activity in the central nervous system. Here, we report neuropathological findings in nine patients homozygous for the D90A mutation. All nine had numerous small granular inclusions immunoreactive for misfolded SOD1 in motor neurons and glial nuclei in the spinal cord and brainstem. In addition to degeneration of the corticospinal tracts, all patients had degeneration of the dorsal columns. We also found intense gliosis in circumscribed cortical areas of the frontal and temporal lobes and in the insula. In these areas and in adjacent white matter, there were SOD1 staining neuropil threads. A few SOD1-immunopositive cytoplasmic neuronal inclusions were observed in cortical areas, as were glial nuclear inclusions. As suggested by the symptoms and signs and earlier neurophysiological and imaging investigations, the histopathology in patients homozygous for the D90A SOD1 extends beyond the motor system to include cognitive and sensory cortical areas. However, even in the patients that had a symptomatic disease duration of more than 2 or 3 decades and lived into their 70s or 80s, there were no SOD1-inclusion pathology and no typical dysfunction (apart from the musculature) in non-nervous organs. Thus, only specific parts of the CNS seem to be vulnerable to toxicity provoked by homozygously expressed mutant SOD1.

Corticomotoneuronal dysfunction in ALS patients with different SOD1 mutations

OBJECTIVE

To examine corticomotoneuronal function in amyotrophic lateral sclerosis (ALS) patients carrying superoxide dismutase 1 (SOD1) mutations using peristimulus time histograms (PSTH).

METHODS

Six I113T, 3 A4V, one G41D and one G114A patient were studied along with 21 healthy control subjects. Analyses included comparison with previously reported data from 8 D90A homozygous and 12 sporadic ALS (SALS) patients examined by the authors using identical methodology.

RESULTS

Cortical threshold was significantly reduced in A4V patients (41.3%) compared to I113T (58%), SALS (57%) and D90A (71%) patients, as well as healthy controls (49.7%). Estimated excitatory postsynaptic potentials (EPSPs) were significantly larger in A4V patients (4.39 mV) compared to healthy controls (2.95 mV), I113T (2.71 mV) and SALS (2.39 mV) patients. Clinical features and PSTH parameters in I113T were similar to SALS, however, PSTH primary peaks (PP) were significantly more dispersed, 9.5 ms compared to 4ms in SALS. PSTHs from single G41D and G114A patients were unremarkable, apart from large EPSP amplitudes in the G114A patient.

CONCLUSIONS

ALS patients with A4V and I113T SOD1 mutations have distinctive corticomotoneuronal changes that are different from those in D90A homozygous and SALS patients.

SIGNIFICANCE

PSTH studies should be considered for future in vivo studies of SOD1 pathophysiology in ALS.

A novel central motor conduction abnormality in D90A-homozygous patients with amyotrophic lateral sclerosis

Patients with amyotrophic lateral sclerosis (ALS) who are homozygous for the D90A SOD1 mutation have been noted to have central motor abnormalities distinct from those of patients with idiopathic ALS. We stimulated the motor cortex of ten patients homozygous for the D90A SOD1 mutation, using transcranial magnetic stimulation (TMS), and recorded the response evoked in the right first dorsal interosseous muscle when the muscle was at rest and when voluntarily active. A subgroup of patients had two distinct evoked responses when the cortex was stimulated at high intensity with the muscle at rest. When the muscle was modestly contracted, the first of these responses disappeared, whereas the second response was facilitated. Both fast and slow components of the corticospinal tract were usually intact and excited by TMS in these patients. We propose that there is an abnormality of intracortical or intraspinal inhibition in a subgroup of D90A SOD1 ALS patients, which suppresses fast-conducted activity when the muscle is active. Apart from further defining the phenotype of familial ALS, these findings may have importance in understanding the pathogenesis of central motor abnormalities in these patients.

Magnetic stimulation of the central and peripheral nervous systems

Since 1985, when the technique of transcranial magnetic stimulation (TMS) was first developed, a wide range of applications in healthy and diseased subjects has been described. Comprehension of the physiological basis of motor control and cortical function has been improved. Modifications of the basic technique of measuring central motor conduction time (CMCT) have included measurement of the cortical silent period, paired stimulation in a conditioning test paradigm, repetitive transcranial magnetic stimulation (rTMS), and peristimulus time histograms (PSTH). These methods allow dissection of central motor excitatory versus inhibitory interplay on the cortical motor neuron and its presynaptic connections at the spinal cord, and have proven to be powerful investigational techniques. TMS can be used to assess upper and lower motor neuron dysfunction, monitor the effects of many pharmacological agents, predict stroke outcome, document the plasticity of the motor system, and assess its maturation and the effects of aging, as well as perform intraoperative monitoring. The recent use of rTMS in the treatment of depression and movement disorders is novel, and opens the way for other potential therapeutic applications.