Dissection of <i>TAF1</i> neuronal splicing and implications for neurodegeneration in X-linked dystonia-parkinsonism

Abstract

X-linked dystonia-parkinsonism (XDP) is a monogenic neurodegenerative disorder of the basal ganglia, which presents as a combination of hyperkinetic movements and parkinsonian features. The underlying genetic mechanism involves the insertion of a SINE-VNTR-Alu retrotransposon within the <i>TAF1</i> gene. Interestingly, alterations of <i>TAF1</i> have been involved in multiple neurological diseases. In XDP, the SINE-VNTR-Alu insertion in <i>TAF1</i> has been proposed to result in alternative splicing defects, including the decreased incorporation of a neuron-specific microexon annotated as 34'. This mechanism has become controversial as recent studies failed to provide support. In order to resolve this conundrum, we examined the alternative splicing patterns of <i>TAF1</i> mRNAs in XDP and control brains. The impact of the disease-associated SINE-VNTR-Alu on alternative splicing of microexon 34' was further investigated in cellular assays. Subsequently, microexon 34' incorporation was explored by RT-PCR and Nanopore long-read sequencing of <i>TAF1</i> mRNAs from XDP and control brains tissues. Using cell-based splicing assays, we demonstrate that presence of the disease-associated SINE-VNTR-Alu does not affect the inclusion of microexon 34'. In addition, we show that (1) microexon 34'-containing <i>TAF1</i> mRNAs are detected at similar levels in XDP as in controls and that (2) the architecture of <i>TAF1</i> transcripts is remarkably similar between XDP and controls brains. These results indicate that microexon 34' incorporation into <i>TAF1</i> mRNA is not affected in XDP brains. Our findings shift the current paradigm of XDP by discounting alternative splicing of <i>TAF1</i> microexon 34' as the molecular basis for this disease.