Frontotemporal lobar degeneration with TAR DNA binding protein 43 inclusions (FTLD-TDP) is the most common pathology associated with frontotemporal dementia (FTD). Repeat expansions in chromosome 9 open reading frame 72 (C9ORF72) and mutations in progranulin (GRN) are the major known genetic causes of FTLD-TDP; however, the genetic etiology in the majority of FTLD-TDP remains unexplained. In this study, we performed whole-genome sequencing in 104 pathologically confirmed FTLD-TDP patients from the Mayo Clinic brain bank negative for C9ORF72 and GRN mutations and report on the contribution of rare single nucleotide and copy-number variants in 21 known neurodegenerative disease genes. Interestingly, we identified 5 patients (4.8%) with variants in optineurin (OPTN) and TANK-binding kinase 1 (TBK1) that are predicted to be highly pathogenic, including two double mutants. Case A was a compound heterozygote for mutations in OPTN, carrying the p.Q235* nonsense and p.A481V missense mutation in trans, while case B carried a deletion of OPTN exons 13–15 (p.Gly538Glufs*27) and a loss-of-function mutation (p.Arg117*) in TBK1. Cases C–E carried heterozygous missense mutations in TBK1, including the p.Glu696Lys mutation which was previously reported in two amyotrophic lateral sclerosis (ALS) patients and is located in the OPTN binding domain. Quantitative mRNA expression and protein analysis in cerebellar tissue showed a striking reduction of OPTN and/or TBK1 expression in 4 out of 5 patients supporting pathogenicity in these specific patients and suggesting a loss-of-function disease mechanism. Importantly, neuropathologic examination showed FTLD-TDP type A in the absence of motor neuron disease in 3 pathogenic mutation carriers. In conclusion, we highlight TBK1 as an important cause of pure FTLD-TDP, identify the first OPTN mutations in FTLD-TDP, and suggest a potential oligogenic basis for at least a subset of FTLD-TDP patients. Our data further adds to the growing body of evidence linking ALS and FTD and suggests a key role for the OPTN/TBK1 pathway in these diseases.

Frontotemporal lobar degeneration (FTLD) comprises a highly heterogeneous group of disorders clinically associated with behavioral and personality changes, language impairment, and deficits in executive functioning, and pathologically associated with degeneration of frontal and temporal lobes. Some patients present with motor symptoms including amyotrophic lateral sclerosis. Genetic research over the past two decades in FTLD families led to the identification of three common FTLD genes (microtubule-associated protein tau, progranulin, and chromosome 9 open reading frame 72) and a small number of rare FTLD genes, explaining the disease in almost all autosomal dominant FTLD families but only a minority of apparently sporadic patients or patients in whom the family history is less clear. Identification of additional FTLD (risk) genes is therefore highly anticipated, especially with the emerging use of next-generation sequencing. (FUS, EWS, TAF15); MAPT, microtubule-associated tau gene; CBD, corticobasal degeneration; PSP, progressive supranuclear palsy; GGT, globular glial tauopathies; AGD, argyrophilic grain disease; AD, Alzheimer's disease; TDP-43, TAR DNA-binding protein; FUS, fused in sarcoma; aFTLD-U, atypical-FTLD-U; EWS, ewing in sarcoma; TAF15, TATA-binding protein-associated factor 15; FTDP-17, frontotemporal dementia and parkinsonism linked to chromosome 17; cM, centimorgan; PiD, Pick's disease; MT, microtubule; NFTs, neurofibrillary tangles; iPSC, induced pluripotent stem cells; GRN, progranulin; CSF, cerebrospinal fluid; CBS, corticobasal syndrome; NCL, neuronal ceroid lipofuscinosis; SAHA, suberoylanilide hydroxamic acid; C9orf72, chromosome 9 open reading frame 72; DPRs, dipeptide repeat proteins; DENN, differentially expressed in normal and neoplasia; GEF, guanine nucleotide exchange factors; CHMP2B, charged multivesicular body protein 2B; ESCRT-III, endosome sorting complex required for transport 3; VCP, valosin-containing protein gene; IBM, inclusion body myopathy; PDB, paget disease of the bone; IBMPFD, inclusion body myopathy with Paget disease of the bone and frontotemporal dementia; MSP, multisystem proteinopathy; hnRNPA1, heterogeneous nuclear ribonucleoprotein A1; hnRNPA2B1, heterogeneous nuclear ribonucleoprotein A2/B1; SQSTM1, sequestosome 1; TBK1, TANK binding kinase 1; OPTN, optineurin; TRN1, transportin; CHCHD10, coiled-coil-helix-coiled-coil-helix domain containing 10; MICOS, mitochondrial contact site and cristae organization system; UBQLN2, ubiquilin 2; DCTN1, dynactin 1; CSF1R, colony stimulating receptor factor 1 gene; PRKAR1B, protein kinase A type I-beta regulatory subunit gene; TREM2, triggering receptor expressed on myeloid cells 2 gene; TMEM106B, the transmembrane protein 106B gene; GWAS, genome-wide association study; SNP, single nucleotide polymorphism; CTSC, cathepsin C; ATXN2, ataxin 2 gene; APOE, apolipoprotein E; PRNP, prion protein and UNC13A unc-13 homolog A, C. elegans; ANG, angiogenin; GENFI, genetic and frontotemporal dementia initiative; LEFFTDS, Longitudi...

In multipolar vertebrate neurons, action potentials (APs) initiate close to the soma, at the axonal initial segment. Invertebrate neurons are typically unipolar with dendrites integrating directly into the axon. Where APs are initiated in the axons of invertebrate neurons is unclear. Voltage-gated sodium (Na V ) channels are a functional hallmark of the axonal initial segment in vertebrates. We used an intronic Minos-Mediated Integration Cassette to determine the endogenous gene expression and subcellular localization of the sole Na V channel in both male and female Drosophila, para. Despite being the only Na V channel in the fly, we show that only 23 6 1% of neurons in the embryonic and larval CNS express para, while in the adult CNS para is broadly expressed. We generated a single-cell transcriptomic atlas of the whole third instar larval brain to identify para expressing neurons and show that it positively correlates with markers of differentiated, actively firing neurons. Therefore, only 23 6 1% of larval neurons may be capable of firing Na V -dependent APs. We then show that Para is enriched in an axonal segment, distal to the site of dendritic integration into the axon, which we named the distal axonal segment (DAS). The DAS is present in multiple neuron classes in both the third instar larval and adult CNS. Whole cell patch clamp electrophysiological recordings of adult CNS fly neurons are consistent with the interpretation that Na v -dependent APs originate in the DAS. Identification of the distal Na V localization in fly neurons will enable more accurate interpretation of electrophysiological recordings in invertebrates.