Amyotrophic lateral sclerosis (ALS) is a complex fatal degenerative disease
selectively affecting motor neurones. The cause of the disease is still uncharacterised
for the sporadic cases (SALS), whereas several disease-linked genes, such as
SOD1 and VAPB, have been identified in families with ALS (FALS). However, since
the pathological features are indistinguishable between FALS and SALS, it is
generally believed that a common ALS-causing mechanism is responsible for both
forms of disease.
To gain a better understanding of ALS pathogenesis, which could also provide cues
for therapeutic treatment, studies on the characterisation of FALS-linked mutations
have been carried out. In this study, we screened for novel FALS-linked mutations
and found three TDP-43 mutations in five unrelated FALS families, that caused
marked changes at evolutional conserved amino acids and were absent in previously
screened control populations. In addition, functional characterisation of novel ALS linked
mutations in DAO (R199W) and VAPB (T46I) were undertaken whereby both
mutations were shown to disrupt the physiological properties of wild type proteins.
Whereas the T46I mutation in VAPB led to a sub-cellular redistribution and shift in
solubility of VAPB protein, and an impairment in the unfolded protein response (UPR)
and ubiquitin-proteasome system, R199W significantly abolished DAO enzyme
activity. Cell culture-based functional studies showed that the expression of both
mutant proteins triggered pathological features including ubiquitin-positive
aggregates and cell death. Furthermore, an association study was conducted to
investigate the cause of a dramatic VAPB down-regulation seen in SALS. An intronic
SNP close to exon 5 of VAPB, rs6100067, was found to be significantly linked to the
expression levels of VAPB in SALS. As the down-regulation of VAPB profoundly
affects the activation of IRE1/XBP1, the most characterised UPR pathway, this SNP
may present as a risk factor for ALS development.
From these results, we propose an ALS pathogenesis mechanism, whereby VAPB
plays a central role in sustaining multiple cellular events, such as UPR. Disruption or
deprivation of VAPB causes impairments in these cellular functions and leads to
motor neurone death.