Revisiting the concept of amyotrophic lateral sclerosis as a multisystems disorder of limited phenotypic expression.

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The current review will examine the contemporary evidence that amyotrophic lateral sclerosis (ALS) is a syndrome in which the unifying feature is a progressive loss of upper and lower motor neuron function. Although ALS is traditionally viewed as a neurodegenerative disorder affecting the motor neurons, there is considerable phenotypic heterogeneity and widespread involvement of the central nervous system. A broad range of both causative and disease modifying genetic variants are associated with both sporadic and familial forms of ALS. A significant proportion of ALS patients have an associated frontotemporal dysfunction which can be a harbinger of a significantly shorter survival and for which there is increasing evidence of a fundamental disruption of tau metabolism in those affected individuals. Although the traditional neuropathology of the degenerating motor neurons in ALS is that of neuronal cytoplasmic inclusions composed neuronal intermediate filaments, the presence of neuronal cytoplasmic inclusions composed of RNA binding proteins suggests a key role for RNA dysmetabolism in the pathogenesis of ALS. ALS is a complex multisystem neurodegenerative syndrome with marked heterogeneity at not only the level of clinical expression, but also etiologically.