SPP1 and Parkinson disease: Despite the lower levels of PTH produced in the PD subjects than the age-matched controls (p value= 0.003), the bone matrix glycoproteins, including osteoprotegerin (OPG) and osteopontin (OPN), were significantly upregulated (p value= 0.04 for OPG and p -value= 0.003 for OPN), as compared to the controls.