Platelet mitochondrial complex I and IV activities are not reliable stratification biomarkers in Parkinson's disease

Abstract

Background: Mitochondrial dysfunction, particularly complex I (CI) deficiency, is considered an integral feature of Parkinson's disease (PD). However, recent findings indicate that widespread neuronal CI deficiency in the brain is only present in a subpopulation of 20–30% of cases. This stratification may be relevant for selecting participants for clinical trials, emphasizing the need for clinically applicable biomarkers. We previously reported CI deficiency in skeletal muscle biopsies of a subpopulation of persons with PD (PwPs), suggesting potential for mitochondrial stratification using extra-neural tissues. Platelets are another tissue previously reported to exhibit mitochondrial respiratory defects in PD. However, studies have generally involved small sample sizes and reported variable results. Objective: To determine whether platelets exhibit impaired mitochondrial respiratory chain complex activity in PwPs, or in a subpopulation of PwPs. Methods: Using spectrophotometric activity assays, we assessed CI and complex IV (CIV) activities in platelet samples from 61 PwPs and 31 neurologically healthy controls from a well-characterized prospective cohort. The correlation between activities measured in platelets and skeletal muscle was also explored in 51 of the same individuals. Results: Platelet CI and CIV activities showed no difference between PwPs and controls at the group level, nor evidence of a subgroup with deficiency of either complex. There was no correlation between complex activities in platelet samples and skeletal muscle biopsies from the same individuals. Conclusions: Based on these results, we propose that platelet CI or CIV activities are not sensitive markers of mitochondrial dysfunction in PD.