Lysosomal enzyme deficiency and GBA mutations in Dystonia

Glucocerebrosidase (GCase) deficiency due to mutations of the glucosidase acid beta (GBA) gene causes autosomal-recessive Gauchers disease, the most frequent lysosomal storage disorder. Over the past two decades, GBA mutations have been established as the most frequent genetic risk factor to develop Parkinsons Disease. In dystonia, the underlying aetiology in a relevant proportion of cases remains unknown, hampering the development of causative treatment strategies. Here, we explored the possible role of lysosomal dysfunction in clinical (n=130) and post mortem (n=10) patients with dystonia. As part of extensive diagnostic evaluations (screening for structural, acquired and degenerative causes of dystonia), lysosomal enzyme activity was measured in n=79 retrospectively collected cases of patients with combined dystonia and n=51 prospectively collected cases of patients with cervical dystonia using a clinically validated, fluorescence-based assay. Clinical information on all cases was extensively reviewed and an alternative aetiology of dystonia was identified in n=14 cases on follow-up. Of the remaining n=116 cases of dystonia of unknown origin, complete Sanger Sequencing of GBA exons 1-11 was performed using an established protocol in all n=97 of cases with available DNA. Where there was suspicion based on clinical examination or family history, nigro-striatal degeneration was excluded in n=19 (17.2%) cases with dystonia of unknown origin. Furthermore, lysosomal enzyme activity was measured in different brain regions of age-, sex- and post-mortem delay-matched cases with dystonia of unknown origin (n=10) and healthy controls (n=10) from the Queen Square brain bank. Among cases with dystonia of unknown origin, decreased white cell Glucocerebrosidase activity was measured in a range typical for homozygous (n=2; 1.7%) or heterozygous (n=23; 19.8%) GBA mutation carriers. The frequency of GBA mutations (5/80=6.25%) was significantly higher in patients than in controls (3/257=1.17%) of a historical control group from the same ethnic background (P=0.02; Odds Ratio=5.64, 95% Confidence Interval=1.44 - 21.58) - known pathogenic mutations E326K, T369M and N370S were found. We also identified lower Glucocerebrosidase activity in the cerebellar dentate nucleus (P=0.048) of dystonia patients than healthy controls. This study provides evidence for peripheral and central lysosomal dysfunction in a significant proportion and across the clinical spectrum of dystonia. As in Parkinsons disease, this was found irrespective of GBA mutation status, indicating a possible role of lysosomal dysfunction as a more general disease mechanism in dystonia.