Mutations in MAPT give rise to aneuploidy in animal models of tauopathy.

Tau is a major microtubule-associated protein in brain neurons. Its misfolding and accumulation cause neurodegenerative diseases characterized by brain atrophy and dementia, named tauopathies. Genetic forms are caused by mutations of microtubule-associated protein tau gene (MAPT). Tau is expressed also in nonneural tissues such as lymphocytes. Tau has been recently recognized as a multifunctional protein, and in particular, some findings supported a role in genome stability. In fact, peripheral cells of patients affected by frontotemporal dementia carrying different MAPT mutations showed structural and numerical chromosome aberrations. The aim of this study was to assess chromosome stability in peripheral cell from two animal models of genetic tauopathy, JNPL3 and PS19 mouse strains expressing the human tau carrying the P301L and P301S mutations, respectively, to confirm the previous data on humans. After demonstrating the presence of mutated tau in spleen, we performed standard cytogenetic analysis of splenic lymphocytes from homozygous and hemizygous JNPL3, hemizygous PS19, and relevant controls. Losses and gains of chromosomes (aneuploidy) were evaluated. We detected a significantly higher level of aneuploidy in JNPL3 and PS19 than in control mice. Moreover, in JNPL3, the aneuploidy was higher in homozygotes than in hemizygotes, demonstrating a gene dose effect, which appeared also to be age independent. Our results show that mutated tau is associated with chromosome instability. It is conceivable to hypothesize that in genetic tauopathies the aneuploidy may be present also in central nervous system, possibly contributing to neurodegeneration.

Missense mutations in progranulin gene associated with frontotemporal lobar degeneration: study of pathogenetic features.

GRN, the gene coding for the progranulin (PGRN) protein, was recognized as a gene linked to frontotemporal lobar degeneration (FTLD). The first mutations identified were null mutations giving rise to haploinsufficiency. Missense mutations were subsequently detected, but only a small subset has been functionally investigated. We identified missense mutations (C105Y, A199V, and R298H) in FTLD cases with family history and/or with low plasma PGRN levels. The aim of this study was to determine their pathogenicity. We performed functional studies, analyzing PGRN expression, secretion, and cleavage by elastase. GRN C105Y affected both secretion and elastase cleavage, likely representing a pathogenic mutation. GRN A199V did not alter the physiological properties of PGRN and GRN R298H produced only moderate effects on PGRN secretion, indicating that their pathogenicity is uncertain. In the absence of strong segregation data and neuropathological examinations, genetic, biomarker, and functional studies can be applied to an algorithm to assess the likelihood of pathogenicity for a mutation. This information can improve our understanding of the complex mechanisms by which GRN mutations lead to FTLD.

A mutation in the 5'-UTR of GRN gene associated with frontotemporal lobar degeneration: phenotypic variability and possible pathogenetic mechanisms.

Frontotemporal lobar degeneration (FTLD) is a very heterogeneous disorder. It is genetically linked to three major genes: microtubule-associated protein tau (MAPT), progranulin (GRN), and C9ORF72. In particular, mutations in GRN account for 5-10% of all cases and give rise to a wide spectrum of clinical phenotypes, ranging from behavioral frontotemporal dementia (bvFTD) to primary progressive aphasia, including progressive non-fluent aphasia (PNFA) and semantic dementia, and corticobasal syndrome (CBS). We studied a family affected by FTLD whose members showed three different phenotypes: bvFTD, PNFA, and CBS. We performed plasma progranulin measurement before any genetic analyses and, due to the low level detected, we sequenced GRN and found the new mutation EX0-5' splice site A > G in the 5'-UTR region, where no pathogenic mutations had been previously demonstrated. Genetic analyses of MAPT and C9ORF72 were normal. GRN mRNA expression showed about 50% reduction caused by this mutation, and similar results were found for progranulin level. Testing of nonsense mediated RNA decay gave negative results, suggesting a different mechanism of mRNA degradation. In summary, the EX0-5' splice site A > G mutation widens the GRN regions affected by null mutations, including the 5'-UTR, and confirms once more the large phenotypic variability linked to GRN mutations.