| Summary: | <p>TDP43 is a highly conserved ubiquitously expressed protein which performs multiple roles in RNA metabolism, including regulation of gene expression, pre-mRNA splicing and the regulation of micro-RNA biogenesis. It belongs to the hnRNP family and interacts extensively with other proteins and RNA to perform its known biological functions and, analogously to other proteins from the same family, is proposed to autoregulate its own protein levels.</p> <p>TDP43 biological dysfunction plays a pivotal role in the aetiology and pathophysiology of ALS and FTLD-TDP given that abnormally cleaved mislocalised fragments of TDP43, which are polyubiquitinated and hyperphosphorylated, constitute histopathological hallmarks of these diseases. Moreover, mutations in <em>TARDBP</em> (the gene encoding TDP43), most of which located in exon 6 that encodes the Glycine-rich C-terminal of the protein, are causative of ALS and FTLD-TDP. Additionally, widespread dysfunction in RNA metabolism involving TDP43 target genes is found in tissue samples from patients who suffered from these two neurodegenerative diseases.</p> <p>Mechanistic dissection between TDP43 dysfunction and downstream neurodegeneration has remained elusive given that both over-expressor cellular and mouse models and conditional deletion or knockdown models have been shown to develop behavioural dysfunction and, in some cases, neurodegeneration with associated dysfunction in RNA metabolism.</p> <p>In this thesis a reverse genetics approach was taken and two <em>Tardbp</em> ENU mutant mouse models were characterised, <em>Tardbp<sup>F210I</sup></em> (mutation in the RRM2 of TDP43) and <em>Tardbp<sup>M323K</sup></em> (mutation in the C-terminal domain of TDP43). Strikingly, both mutants showed significant phenotypes in TDP43-dependent splicing, with the F210I mutation leading to a "loss of normal splicing function" and the M323K mutation an "augmented alternative exon selection" leading to a splicing phenotype opposite to the observed as a consequence of the "loss of normal splicing function" caused by the F210I protein at the genome-wide level.</p> <p>Full behavioural characterisation of heterozygous mice for these two ENU mutations did not identify any phenotypes associated with neurodegeneration, but preliminary data suggests that homozygous mice for the M323K mutation may suffer from synaptic dysfunction. Therefore, despite developing genome-wide splicing changes as a consequence of TDP43 biological dysfunction, neither of the mutant mouse lines developed neurodegeneration.</p> <p>In conclusion, the characterisation of <em>Tardbp</em> ENU mouse mutants has revealed that TDP43 dysfunction leading to "loss of normal splicing function" or "augmented alternative exon selection" is not sufficient to cause, on its own, neurodegeneration in the laboratory mouse within its normal lifespan.</p>
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