We discovered mutations in the Senataxin gene (SETX) as the molecular basis of a juvenile-onset, autosomal dominant (AD) form of familial amyotrophic lateral sclerosis (ALS). This form of ALS, known as ALS4, is an essentially pure motor systems disorder characterized by limb weakness, severe muscle wasting, pyramidal signs, and slow disease progression. Intriguingly, recessive mutations of SETX cause a second disease, a form of ataxia known as Ataxia-Oculomotor Apraxia type 2 (AOA2). In AOA2, morbidity is also high with loss of ambulation occurring ~15-20 years following onset which normally ranges from 10-22 years of age. Together these findings demonstrate genotype-phenotype correlation for SETX mutations and confirm senataxin as an important neuronal protein. Senataxin is a large protein at 2677 amino acids and its precise function is largely unknown. It contains a conserved DNA/RNA helicase domain towards the C-terminal suggesting a function in RNA processing. Such important RNA processing functions have been characterized for the yeast orthologue, Sen1p. There is also a potential protein-protein interaction domain in the extreme N-terminal which is supported by studies in yeast. Our studies will focus on mutant forms of Senataxin associated with ALS4 and AOA2 and address the following broad aims: (1) We will characterize murine gene targeted and transgenic (Tg) models we have produced for ALS4 and test if they develop neuropathies allowing further detailed study of neuronal degeneration pathways;(2) Using gene trap technology we will develop a murine knock-out model for AOA2;and (3) we have hypothesized that ALS4 mutant Senataxin (L389S) may lead to toxic gain-of-function such as aberrant protein-protein interaction. To test this we undertook yeast two-hybrid (Y2H) screens with wt and ALS4 mutant N-terminal senataxin of a human brain expression library. We identified a ALS4 specific interaction with the kinesin, KIF1B, that was validated with further Y2H re-testing and mammalian 2-H assays. We will further examine this interaction and its potential pathogenic effects in ALS4.
The potential knowledge gained from this research is an improved understanding of the causes of neuronal death in familial forms of ALS and Ataxia. Murine models being developed as part of the study could also prove highly useful and would eventually be shared with other interested investigators. Since there is currently no cure or preventive treatment for these conditions, any knowledge gained has the potential to provide future diagnostic and therapeutic interventions for these patients.