Understanding XDP through imaging, pathology, and genetics.

The X-linked dystonia-parkinsonism (XDP) is a severe, progressive, adult-onset, X-linked endemic disorder in Filipinos, which is characterized by dystonic movements that start in the third or fourth decade, and replaced by parkinsonism beyond the 10th year of illness. Understanding the pathophysiology of XDP and development of rational therapies will depend on observations from imaging, pathological, and genetic studies. In this paper we summarize the results of these studies on patients with XDP. The cranial magnetic resonance imaging shows hyperintense putaminal rim in both dystonic and parkinsonian stages, and atrophy of the caudate head or putamen in the parkinsonian stage. Neuropathological findings show atrophy of the caudate nucleus and putamen, with mild to severe neuronal loss and gliosis. In the neostriatum, the dystonic phase of XDP shows the involvement of striosomes and matrix sparing, while the later, i.e., parkinsonian phase, shows matrix involvement as well. In the dystonic phase, the loss of striosomal inhibitory projections lead to disinhibition of nigral dopaminergic neurons, perhaps resulting in a hyperkinetic state; while in the parkinsonian phase, severe and critical reduction of matrix-based projection may result in extranigral parkinsonism. Genetic sequencing of the XDP critical region in Xq13.1 has revealed an SVA retrotransposon insertion in an intron of TAF1. This may reduce neuron-specific expression of the TAF1 isoform in the caudate nucleus, and subsequently interfere with the transcription of many neuronal genes, including DRD2. Findings from imaging, pathology, and genetics studies are gradually shedding light on the pathophysiology of XDP, which hopefully will lead to more rational and directed therapies.