TBK1 Mutation Spectrum in an Extended European Patient Cohort with Frontotemporal Dementia and Amyotrophic Lateral Sclerosis

Abstract

We investigated the mutation spectrum of the TANK‐Binding Kinase 1 (TBK1) gene and its associated phenotypic spectrum by exonic resequencing of TBK1 in a cohort of 2,538 patients with frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS), or FTD plus ALS, ascertained within the European Early‐Onset Dementia Consortium. We assessed pathogenicity of predicted protein‐truncating mutations by measuring loss of RNA expression. Functional effect of in‐frame amino acid deletions and missense mutations was further explored in vivo on protein level and in vitro by an NFκB‐induced luciferase reporter assay and measuring phosphorylated TBK1. The protein‐truncating mutations led to the loss of transcript through nonsense‐mediated mRNA decay. For the in‐frame amino acid deletions, we demonstrated loss of TBK1 or phosphorylated TBK1 protein. An important fraction of the missense mutations compromised NFκB activation indicating that at least some functions of TBK1 are lost. Although missense mutations were also present in controls, over three times more mutations affecting TBK1 functioning were found in the mutation fraction observed in patients only, suggesting high‐risk alleles (P = 0.03). Total mutation frequency for confirmed TBK1 LoF mutations in the European cohort was 0.7%, with frequencies in the clinical subgroups of 0.4% in FTD, 1.3% in ALS, and 3.6% in FTD‐ALS.

Analysis of known amyotrophic lateral sclerosis and frontotemporal dementia genes reveals a substantial genetic burden in patients manifesting both diseases not carrying the C9orf72 expansion mutation

“…This patient carried the p.N254Kfs*4 TBK1 mutation and a missense variant (p.P392L) in the sequestosome 1 ( SQSTM1 ) gene. Among TBK1 mutation carriers, three of them (cases carrying the p.K30_E76del, p.G272_T331del or p.T79del) have been recently reported in an independent study aimed at investigating the genetic contribution of TBK1 in an extended European cohort of patients belonging to the ALS/FTD disease continuum 18. These three patients were included in our exome sequencing project during the European cohort sample recruitment, and TBK1 mutation status was unknown at the time of exome sequencing.…”

Familial primary lateral sclerosis or dementia associated with Arg573GlyTBK1mutation

“…From a biological perspective, the pathogenicity of the Arg573Gly change was in doubt according to (1) SIFT program prediction as tolerated3; (2) in vitro protein activity studied through the NFκB/interferon pathway showing no reduction3; and (3) because of its location outside the kinase domain, the main functional region of the protein housing its catalytic activity 5. However, the presence of clear cosegregation supports pathogenicity and opens the possibility of other mutations located outside the kinase domain being pathogenic.…”

“…However, pathogenicity of many TBK1 missense mutations is difficult to establish in the absence of cosegregation data. A screening for TBK1 mutations carried out by the European Early-Onset Dementia (EU EOD) Consortium in a large series of FTD/ALS cases found one carrier of the Arg573Gly mutation 3. The index case belonged to a Spanish family with autosomal dominant disease manifesting in the sixth decade as either dementia or PLS, and in whom we were able to demonstrate cosegregation.…”

“…TBK1 screening included 2553 individuals with FTD and/or ALS and 2288 European controls, including 549 of Spanish origin, and was conducted by the EU EOD Consortium as previously described 3. The family with the Arg573Gly mutation comprises an affected father and nine offspring, of whom five have been affected by either dementia or PLS (figure 1).…”

Distinct Clinical Features and Outcomes in Motor Neuron Disease Associated with Behavioural Variant Frontotemporal Dementia

“…3d) and extramotor sites, also corresponding to Brettschneider stage 4. An extensive neuropathological report on patient 10 was previously published [18]. …”