Neuropathological characterisation of a novel TBK1 loss of function mutation associated with amyotrophic lateral sclerosis

Mutations in TANK binding kinase gene (TBK1) have been identified as causative in amyotrophic lateral sclerosis (ALS). Here, we examine the spectrum of TBK1 mutations in a cohort of ALS patients from Northern England, comparing missense and loss of function mutations with clinical phenotype. Analysis of 290 ALS cases identified seven variants, including one novel in-frame deletion (p.Ile85del). In silico analysis and review of the literature suggested that four variants, one nonsense mutation (p.Glu2Ter), two in-frame deletions (p.Ile85del, p.Glu643del) and one missense mutation (p.Gln565Pro) were pathogenic, whilst the remaining three missense mutations were variants of uncertain significance or benign. Post-mortem material was available from the patient with the novel in-frame deletion. Neuropathological examination established this individual had classical ALS pathology, with moderate phosphorylated TDP-43 neuronal and glial cytoplasmic inclusions in the motor cortex, skein-like inclusions in the lower motor neurons and “pre-inclusions” in the medulla. This corresponds to Type B FTLD-TDP pathology and is consistent with previously published literature on TBK1 mutants. In addition to demonstrating no changes in TBK1 staining, we are the first to show there was no differential expression of interferon regulatory factor IRF3, a downstream effector of TBK1 in the innate immunity pathway, in the TBK1-mutant tissue compared to controls. Comparison of clinical and neuropathological data, however, suggests that TBK1-ALS cases show classical ALS pathology but no specific phenotype.