Dissection of TAF1 neuronal splicing and implications for neurodegeneration in X-linked dystonia-parkinsonism

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 TAF1 gene. Interestingly, alterations of TAF1 have been involved in multiple neurological diseases. In XDP, the SINE-VNTR-Alu insertion in TAF1 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 TAF1 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 TAF1 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 TAF1 mRNAs are detected at similar levels in XDP as in controls and that (2) the architecture of TAF1 transcripts is remarkably similar between XDP and controls brains. These results indicate that microexon 34′ incorporation into TAF1 mRNA is not affected in XDP brains. Our findings shift the current paradigm of XDP by discounting alternative splicing of TAF1 microexon 34′ as the molecular basis for this disease.

Promise and challenges of dystonia brain banking: establishing a human tissue repository for studies of X-Linked Dystonia-Parkinsonism

X-Linked Dystonia-Parkinsonism (XDP) is a neurodegenerative disease affecting individuals with ancestry to the island of Panay in the Philippines. In recent years there has been considerable progress at elucidating the genetic basis of XDP and candidate disease mechanisms in patient-derived cellular models, but the neural substrates that give rise to XDP in vivo are still poorly understood. Previous studies of limited XDP postmortem brain samples have reported a selective dropout of medium spiny neurons within the striatum, although neuroimaging of XDP patients has detected additional abnormalities in multiple brain regions beyond the basal ganglia. Given the need to fully define the CNS structures that are affected in this disease, we created a brain bank in Panay to serve as a tissue resource for detailed studies of XDP-related neuropathology. Here we describe this platform, from donor recruitment and consent to tissue collection, processing, and storage, that was assembled within a predominantly rural region of the Philippines with limited access to medical and laboratory facilities. Thirty-six brains from XDP individuals have been collected over an initial 4 years period. Tissue quality was assessed based on histologic staining of cortex, RNA integrity scores, detection of neuronal transcripts in situ by fluorescent hybridization chain reaction, and western blotting of neuronal and glial proteins. The results indicate that this pipeline preserves tissue integrity to an extent compatible with a range of morphologic, molecular, and biochemical analyses. Thus the algorithms that we developed for working in rural communities may serve as a guide for establishing similar brain banks for other rare diseases in indigenous populations.

SVA insertion in X-linked Dystonia Parkinsonism alters histone H3 acetylation associated with TAF1 gene

X-linked Dystonia-Parkinsonism (XDP) is a neurodegenerative disease linked to an insertion of a SINE-VNTR-Alu (SVA)-type retrotransposon within an intron of TAF1. This SVA insertion induces aberrant TAF1 splicing and partial intron retention, thereby decreasing levels of the full-length transcript. Here we sought to determine if these altered transcriptional dynamics caused by the SVA are also accompanied by local changes in histone acetylation, given that these modifications influence gene expression. Because TAF1 protein may itself exhibit histone acetyltransferase activity, we also examined whether decreased TAF1 expression in XDP cell lines and post-mortem brain affects global levels of acetylated histone H3 (AcH3). The results demonstrate that total AcH3 are not altered in XDP post-mortem prefrontal cortex or cell lines. We also did not detect local differences in AcH3 associated with TAF1 exons or intronic sites flanking the SVA insertion. There was, however, a decrease in AcH3 association with the exon immediately proximal to the intronic SVA, and this decrease was normalized by CRISPR/Cas-excision of the SVA. Collectively, these data suggest that the SVA insertion alters histone status in this region, which may contribute to the dysregulation of TAF1 expression.

Neuroinflammation and histone H3 citrullination are increased in X-linked Dystonia Parkinsonism post-mortem prefrontal cortex

Neuroinflammation plays a pathogenic role in neurodegenerative diseases and recent findings suggest that it may also be involved in X-linked Dystonia-Parkinsonism (XDP) pathogenesis. Previously, fibroblasts and neuronal stem cells derived from XDP patients demonstrated hypersensitivity to TNF-α, dysregulation in NFκB signaling, and an increase in several pro-inflammatory markers. However, the role of inflammatory processes in XDP patient brain remains unknown. Here we demonstrate that there is a significant increase in astrogliosis and microgliosis in human post-mortem XDP prefrontal cortex (PFC) compared to control. Furthermore, there is a significant increase in histone H3 citrullination (H3R2R8R17cit₃) with a concomitant increase in peptidylarginine deaminase 2 (PAD2) and 4 (PAD4), the enzymes catalyzing citrullination, in XDP post-mortem PFC. While there is a significant increase in myeloperoxidase (MPO) levels in XDP PFC, neutrophil elastase (NE) levels are not altered, suggesting that MPO may be released by activated microglia or reactive astrocytes in the brain. Similarly, there was an increase in H3R2R8R17cit₃, PAD2 and PAD4 levels in XDP-derived fibroblasts. Importantly, treatment of fibroblasts with Cl-amidine, a pan inhibitor of PAD enzymes, reduced histone H3 citrullination and pro-inflammatory chemokine expression, without affecting cell survival. Taken together, our results demonstrate that inflammation is increased in XDP post-mortem brain and fibroblasts and unveil a new epigenetic potential therapeutic target.