HDC and Parkinson disease: Unaltered or increased. Expression of HDC-mRNA in the TMN remains undisturbed and the enzymatic activity of HDC is preserved within normal range. Moreover, the concentration of the main metabolite of HA, t-MeHA, remains unchanged in the CSF. However, abnormally high HA concentration in the basal ganglia of the brains of PD patients has been shown postmortem in other studies.