Future genetic studies need to identify genetic defects in at least two distinct familial forms of frontotemporal dementia (FTD) with unknown genetic defects: frontotemporal lobe degeneration with ubiquitin-positive inclusions with hippocampal sclerosis and frontotemporal lobe degeneration with motor neuron disease.

Frontotemporal lobar degeneration (FTLD) is a clinically, genetically and pathologically heterogeneous disorder. Within FTLD with ubiquitin-positive inclusions (FTLD-U), a new pathological subtype named FTLD-FUS was recently found with fused in sarcoma (FUS) positive, TDP-43-negative inclusions, and striking atrophy of the caudate nucleus. The aim of this study was to determine the frequency of FTLD-FUS in our pathological FTLD series, and to describe the clinical, neuroimaging and neuropathological features of FTLD-FUS, especially caudate atrophy. Demographic and clinical data collected prospectively from 387 patients with frontotemporal dementia (FTD) yielded 74 brain specimens. Immunostaining was carried out using a panel of antibodies, including AT-8, ubiquitin, p62, FUS, and TDP-43. Cortical and caudate atrophy on MRI (n = 136) was rated as normal, mild-moderate or severe. Of the 37 FTLD-U cases, 33 were reclassified as FTLD-TDP and four (0.11, 95%: 0.00–0.21) as FTLD-FUS, with ubiquitin and FUS-positive, p62 and TDP-43-negative neuronal intranuclear inclusions (NII). All four FTLD-FUS cases had a negative family history, behavioural variant FTD (bvFTD), and three had an age at onset ≤40 years. MRI revealed mild-moderate or severe caudate atrophy in all, with a mean duration from onset till MRI of 63 months (range 16–119 months). In our total clinical FTD cohort, we found 11 patients (0.03; 95% CI: 0.01–0.05) with bvFTD, negative family history, and age at onset ≤40 years. Caudate atrophy was present in 10 out of 136 MRIs, and included all four FUS-cases. The newly identified FTLD-FUS has a frequency of 11% in FTLD-U, and an estimated frequency of three percent in our clinical FTD cohort. The existence of this pathological subtype can be predicted with reasonable certainty by age at onset ≤40 years, negative family history, bvFTD and caudate atrophy on MRI.

Frontotemporal dementia is accompanied by motor neuron disease (FTD + MND) in approximately 10% of cases. There is accumulating evidence for a clinicopathological overlap between FTD and MND based on observations of familial aggregation and neuropathological findings of ubiquitin-positive neuronal cytoplasmatic inclusions (NCI) in lower motor neurons, hippocampus and neocortex in both conditions. Several familial forms exist with different genetic loci and defects. We investigated the familial aggregation and clinical presentation of FTD + MND cases in a large cohort of 368 FTD patients in The Netherlands. Immunohistochemistry of available brain tissue of deceased patients was investigated using a panel of antibodies including ubiquitin, p62 and TAR DNA-binding protein of 43 kDa antibodies. A total of eight patients coming from six families had a family history positive for FTD + MND (mean age at onset 53.2 +/- 8.4 years). Five patients presented with behavioural changes and cognitive changes followed by motor neuron disease, whereas symptoms of motor neuron disease were the presenting features in the remaining three patients. Other affected relatives in these families showed dementia/FTD, MND or FTD + MND reflecting the clinical interfamilial variation. No mutations were identified in any of the candidate genes, including Superoxide Dismutase 1, dynactin, angiogenin, Microtubule-Associated Protein Tau, valosin-containing protein and progranulin. Available brain tissue of five patients with familial FTD + MND showed NCI in hippocampus, neocortex and spinal cord in all, and neuronal intranuclear inclusions (NII) in two brains. TDP-43 antibody showed robust staining of neuronal inclusions similar in distribution and morphology to NCI and NII. Additionally, TDP-43 antibody also stained ubiquitin-negative glial inclusions in the basal striatum of one case. In conclusion, there exists considerable clinical variation within families with FTD + MND, which may be determined by other genetic or environmental factors. NII are also found in some cases of familial FTD + MND without Progranulin mutations. The observation of glial TDP-43 positive inclusions in one brain is very interesting, although their pathophysiological significance is yet unknown.

Frontotemporal dementia and parkinsonism linked to chromosome 17 have been associated with mutations in the microtubule associated protein tau (MAPT or tau) gene. This disorder is characterized by a large spectrum of neuronal and glial tau lesions in different brain regions. Pick bodies were found in a family with hereditary Pick's disease with the G272V mutation and in several families with other tau mutations in exons 9 and 11-13. The biochemical composition of Pick bodies varies between these mutations. Until recently, no detailed biochemical characterization of G272V brain material was done owing to unavailability of fresh frozen brain material. We now report a detailed study using the immunohistochemistry, western blots and electron microscopy of two brains with the G272V mutation that recently became available. Both brains showed severe neuronal loss in the temporal cortex, whereas in the frontal cortex the loss was less; and abundant Pick bodies in the dentate gyrus of the hippocampus, and caudate nucleus. The Pick bodies consisted exclusively of three-repeat (3R) isoforms, as was demonstrated by isoform-specific antibodies and supported by western blot analysis of sarkosyl-insoluble tau. These observations confirm that this family diagnosed with hereditary Pick disease meets all the criteria for this condition, including the presence of Pick bodies that are unphosphorylated at Ser262 and contain twisted filaments with long periodicity consisting only of 3R tau.

Tau mutations in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) are associated with changes in alternative splicing of exon 10. The DeltaK280 mutation in exon 10 is exceptional because in vitro observations suggest a dramatic effect on microtubule binding, enhanced self-aggregation, as well as a decrease of the 4R/3R ratio by the ablation of an exon splicing enhancer element. Using immunohistochemistry, Western blotting, and electron microscopy on brain material with the DeltaK280 mutation, we investigated which of these effects is most dominant in vivo. The brain showed abundant Pick bodies in several brain regions, which stained positive with 3-repeat-specific but not with 4-repeat-specific tau antibodies. Western blots of sarkosyl-insoluble tau showed exclusively three repeat (3R0N and 3R1N) tau in most regions, although some 4R1N could be detected in the frontal cortex. In addition, the sarkosyl-soluble tau fraction showed a significantly higher amount of 3-repeat tau. Because quantitative analysis of 4R and 3R mRNA transcripts showed a 4R/3R ratio of only 0.3, association between increased transcription and protein expression was observed. These observations confirm the postulated hypothesis that the DeltaK280 mutation abolishes a splice enhancer element, which overrules the decreased microtubule binding and enhanced self-aggregation.