<i>TBK1</i> mutations in Italian patients with amyotrophic lateral sclerosis: genetic and functional characterisation.

Abstract

<i>TANK-binding kinase 1</i> (<i>TBK1</i>) gene has been recently identified as a causative gene of amyotrophic lateral sclerosis (ALS). We sequenced the <i>TBK1</i> gene in a cohort of 154 Italian patients with ALS with unclear genetic aetiology. We subsequently assessed the pathogenic potential of novel identified <i>TBK1</i> variants using functional in vitro studies: expression, targeting and activity were evaluated in patient-derived fibroblasts and in cells transfected with mutated-<i>TBK1</i> plasmids. We identified novel genomic <i>TBK1</i> variants including two loss-of-function (LoF) (p.Leu59Phefs*16 and c.358+5G>A), two missense (p.Asp118Asn and p.Ile397Thr) and one intronic variant (c.1644-5_1644-2delAATA), in addition to two previously reported pathogenetic missense variants (p.Lys291Glu and p.Arg357Gln). Functional studies in patient-derived fibroblasts revealed that the c.358+5G>A causes aberrant pre-mRNA processing leading TBK1 haploinsufficiency. Biochemical studies in cellular models showed that the truncating variant p.Leu59Phefs*16 abolishes TBK1 protein expression, whereas the p.Asp118Asn variant severely impairs TBK1 phosphorylation activity. Conversely, the p.Ile397Thr variant displayed enhanced phosphorylation activity, whose biological relevance is not clear. The observed frequency of <i>TBK1</i> LoF variants was 1.3% (2/154), increasing up to 3.2% (5/154) by taking into account also the functional missense variants that we were able to classify as potentially pathogenic, supporting the relevance of <i>TBK1</i> in the Italian population with ALS.