TARDBP and frontotemporal dementia: The serum TDP-43 levels did not associate with survival time after the diagnosis in the total FTD group (rs = − 0.126, p = 0.445), in the FTD-TDP subgroup (rs = − 0.236, p = 0.437), nor in the C9orf72 repeat expansion carrier group (rs = − 0.364, p = 0.272).