GBA1 and Parkinson disease: However, post-mortem studies have demonstrated severe loss of glucosylceramidase enzyme activity in brain tissue from patients with Gaucher disease and in the substantia nigra of patients with Parkinson’s disease (Mazzulli et al., 2011; Gegg et al., 2012) and patients with Lewy body dementia (Kurzawa-Akanbi et al., 2012) with and without GBA mutations.