**1. Introduction**

626 Amyotrophic Lateral Sclerosis

Wang FC, Delwaide PJ (1998), Number and relative size of thenar motor units in ALS

Zhou C, Zhao CP, Zhang C, Wu GY, Xiong F, Zhang C (2007), A method comparison in

Electroencephalogr Clin Neurophysiol 109:36-43.

Scler 8:366-372.

patients: application of the adapted multiple point stimulation method.

monitoring disease progression of G93A mouse model of ALS. Amyotroph Lateral

Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease of motor neurones. There is a family history in approximately 10% percent of cases. Only 20% of such families have point mutations in the Cu, Zn superoxide dimutase 1 (SOD1) gene. Pre-symptomatic loss of motor neurons has been identified prior to the onset of symptoms in SOD1 mice. This loss was biphasic with initial loss in the pre-symptomatic phase followed by a period of stabilisation and then gradual loss at time of weakness to death. (Kong & Xu, 1998).

In order to determine the time course of motor neurone loss prior to symptomatic onset of disease, a longitudinal study of at-risk asymptomatic individuals (i.e. SOD1 mutation carriers with no neurological symptoms or signs as determined by a neurologist) was performed.

There was no detectable difference in the number of motor units in SOD1 mutation carriers compared to their SOD1 negative family controls. (Aggarwal & Nicholson, 2001). This may indicate that mutation carriers have undetectable loss of motor neurones until rapid and widespread cell death of motor neurones occurs, coinciding with the onset of symptomatic features. This implies that the disease is not the end result of the slow attrition of motor neurones. (Aggarwal, 2009).

The longitudinal study was extended on 20 asymptomatic carriers of the Cu, Zn superoxide dimutase 1 (SOD1) point mutation. In 2 of the 20 mutation carriers, there was a sudden reduction in MUNE, several months prior to the onset of weakness. (Aggarwal & Nicholson 2002), which also occurred in another 3 mutation carriers over the course of the study. (Aggarwal, 2009).

This suggests that gradual pre-clinical loss of motor neurones does not occur in asymptomatic SOD1 mutation carriers and supports the observation that sudden, catastrophic loss of motor neurones occurs immediately prior to the onset of symptoms and the development of the disease, rather than a gradual attrition of motor neurones over time. These results suggest that there may be a biological trigger initiating rapid cell loss, just prior to the onset of symptoms. This observation is an important contribution to the current understanding of the pathogenesis of MND.

Protection of Motor Neurons in Pre-Symptomatic Individuals Carrying

catalysing redox reactions. (Beckman et al., 1993).

symptoms.

1989).

**3. Familial ALS** 

SOD 1 Mutations: Results of Motor Unit Number Estimation (MUNE) Electrophysiology 629

Pre-symptomatic loss of motor neurones has been identified in an animal model of the disease (transgenic mice expressing mutant human SOD1-G93A). The initial loss in the presymptomatic phase related to severe motor axonal degeneration due to vacuolar changes in motor neurones and a slow decrease in CMAP amplitudes. After a period of stabilisation, there was a gradual loss of motor neurones and a rapid decrease in CMAP amplitude, at the onset of weakness due to myelin alteration. At this point, there was a striking loss of motor units. There was also decrease in evoked motor potentials (an indirect measure of the number of motor units), prior to the onset of symptoms. The onset of disease in transgenic G93A mice involves a sharp decline of muscle strength and a transient explosive increase in vacuoles derived from degenerating mitochondria, but little motor neurone death. These did not die until the terminal stage. (Kong & Xu, 1998). The decline exhibited kinetics consistent with both a constant and exponentially decreasing risk of neuronal death. An escalating risk forced by cumulative damage was not responsible for cell death. (Azzouz et al., 1997). It is possible that the high metabolic activity in motor neurones, combined with the toxic oxidative properties of the mutant SOD1, causes massive mitochondrial vacuolation in motor neurones, resulting in degeneration, earlier than other neurones, triggering the onset of weakness. The involvement of mitochondrial degeneration in the early stages is consistent with a direct effect of toxicity, mediated by properties gained by the mutant enzyme in

Until recently, it has not been possible to address this in humans, as pre-symptomatic diagnosis was not possible. Now, with the ability to identify Cu, Zn superoxide dismutase 1, (SOD1) mutation carriers, a group of human pre-symptomatic subjects can be studied to determine whether there was gradual lifelong pre-symptomatic loss of motor neurones or whether sudden catastrophic loss of motor neurones occurs just prior to the onset of clinical

The only forms of MND in which a clear cause has been established are the genetic variants. 20% of all familial cases are the dominantly inherited adult onset form of MND, which is clinically indistinguishable from the sporadic form of MND. These are due to a point mutation in the cytosolic Cu, Zn superoxide dismutase 1, (SOD1) gene on long arm of chromosome 21 (21q22.1). (Siddique & Deng, 1996). Mutations in other genes, alsin and the heavy subunit of neurofilament (NEFH) can also result in motor neurone degeneration in humans. Two other genes that have been investigated are the other isoforms of SOD. MnSOD (SOD2) maps to chromosome 6q25 and is primarily located in mitochondria and extracellular SOD (SOD3) maps to chromosome 4p15.2. Neither of these genes have yet to be linked to FALS. (Hand & Rouleau, 2002). There is however genetic heterogeneous and other causal genes remain to be found to explain the vast majority of FALS cases. (Siddique et al.,

The initial study to establish a causal link between the SOD1 gene and familial MND (FALS) identified a total of 11 missense mutations in two exons studied in 13 autosomal dominant MND families. (Rosen et al., 1993). This led to an explosion of SOD1 gene screening in MND pedigrees. To date 112 different mutations in the SOD1 have been found which can lead to changes throughout the protein. There have been 99 substitutions, 5 polymorphisms, 3 insertions, 4 deletions and 1 compound mutation types identified. Mutations have been identified in all five exons of the gene. These include 20 on exon 1, 13 on exon 2, 8 on exon 3,

Regular follow-up of SOD1 carriers with MUNE may lead to early diagnosis, creating an opportunity for future approaches and therapies aimed at preserving motor neurones rather than replacing lost motor neurones. Detecting the onset of motor neurone loss in asymptomatic individuals will identify those who may benefit from early institution of an active management program to improve their quality of life, until more effective treatment modalities are available for this devastating condition.
