Identification of BAG2 and Cathepsin D as Plasma Biomarkers for Parkinson’s Disease

The current diagnosis of Parkinson’s disease (PD) mostly relies on clinical rating scales related to motor dysfunction. Given that clinical symptoms of PD appear after significant neuronal cell death in the brain, it is required to identify accessible, objective, and quantifiable biomarkers for early diagnosis of PD. In this study, a total of 20 patients with idiopathic PD and 20 age‐matched patients with essential tremor according to the UK Brain Bank Criteria were consecutively enrolled to identify peripheral blood biomarkers for PD. Clinical data were obtained by clinical survey and assessment. Using albumin‐depleted and immunoglobulin G‐depleted plasma samples, we performed immunoblot analysis of seven autophagy‐related proteins and compared the levels of proteins to those of the control group. We also analyzed the correlation between the levels of candidate proteins and clinical characteristics. Finally, we validated our biomarker models using receiver operating characteristic curve analysis. We found that the levels of BCL2‐associated athanogene 2 (BAG2) and cathepsin D were significantly decreased in plasma of patients with PD (P = 0.009 and P = 0.0077, respectively). The level of BAG2 in patients with PD was significantly correlated with Cross‐Culture Smell Identification Test score, which indicates olfactory dysfunction. We found that our biomarker model distinguishes PD with 87.5% diagnostic accuracy (area under the curve (AUC) = 0.875, P < 0.0001). Our result suggests BAG2 and cathepsin D as candidates for early‐diagnosis plasma biomarkers for PD. We provide the possibility of plasma biomarkers related to the autophagy pathway, by which decreased levels of BAG2 and cathepsin D might lead to dysfunction of autophagy.

Study Highlights

• WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC?

☑ Although the current diagnostic method for Parkinson’s disease (PD) shows high accuracy, it is frequently inefficacious to diagnose early PD or predict PD onset. Several studies showed that the autophagy‐lysosomal pathway is altered in patients with early PD, suggesting autophagy‐related proteins could be potential biomarkers for early PD.

• WHAT QUESTION DID THIS STUDY ADDRESS?

☑ We aimed to identify plasma biomarkers for PD by quantitative analysis of proteins related to the autophagy‐lysosomal pathway.

• WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE?

☑ This study showed that decreased levels of BCL2‐associated athanogene and cathepsin D could be used as PD biomarkers with high accuracy.

• HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE?

☑ The diagnostic model using biomarkers identified in this study can be used for more accurate and convenient PD diagnosis. This study also supports that the autophagy‐lysosomal pathway is fundamentally linked to the pathogenesis of PD.

Parkinson’s disease (PD) is the second most common neurodegenerative disorder of insidious onset. PD is characterized by the presence of motor symptoms, including shaking, rigidity, bradykinesia, and postural disturbances, and non‐motor symptoms, including gait, speech, and swallowing difficulties. ¹ The motor symptoms of PD are caused by a significant decrease in dopamine levels in the brain due to the degeneration of dopaminergic (DA) neurons. ² Because the motor disturbance symptoms begin after a 60 to 80% loss of the DA neurons, it is critical to initiate the appropriate medical intervention at the early stage of disease progression. ³ Despite the rapid increase in PD prevalence, there are still no effective biological or imaging markers. The current diagnosis method of PD is made through the clinical criteria developed by the Brain Bank of the Parkinson’s Disease Society in the UK. ⁴ Even though these criteria are with a high degree of accuracy, it is still not effective to predict PD onset or diagnose patients with early PD without motor symptoms. Thus, the development of early PD biomarkers to predict PD is of importance.Identifying biomarkers is necessary as they can be administered in worldwide screening to predict PD progress and diagnose early PD. A biomarker should be applicable to all sexes and ages, easily accessible, noninvasive, and, most importantly, it should be a quantifiable value for clinical application. In this regard, using peripheral blood plasma is a promising way to develop biomarkers for PD. ⁵ Although several studies showed that the level of different types of α‐synuclein in the plasma of patients with PD could be used as a biomarker, it is still controversial whether the α‐synuclein level is a suitable biomarker for PD prediction or diagnosis, due to the inconsistency of the results. ⁶ , ⁷ Accordingly, the peripheral α‐synuclein level does not seem to have potential as a biomarker. Nonetheless, a promising finding is that the level of DJ‐1 decreased in the cerebrospinal fluid of patients with PD; however, DJ‐1 levels in the sera of patients with PD did not differ from those of control patients. ⁶ , ⁸ In addition, the levels of uric acid in sera and epidermal growth factor in plasma are reported to be decreased in patients with PD. ⁶ Thanks to the years of research on PD, it is now well‐known that several factors, including α‐synuclein, parkin, PINK1, LRRK2, and DJ‐1, are deeply related to the pathogenesis of PD. The α‐synuclein is a presynaptic neuronal protein, which is neuropathologically related to PD. ⁹ , ¹⁰ Several studies showed the implication of parkin and PINK1 in mitophagy that is thought to be one of the underlying pathogenic mechanisms of PD. ¹⁰ , ¹¹ , ¹² In addition, LRRK2 and DJ‐1 are reported to play important roles in autophagy‐mediated DA neuronal cell loss. ¹³ , ¹⁴ Most recently, Laperle et al. showed that lysosomal membrane proteins, such as LAMP1, were decreased in induced pluripotent stem cells of patients with young‐onset PD. ¹⁵ These studies, all together, suggest the deep implication of autophagy in PD.In this study, we aimed to identify autophagy‐related proteins as potential biomarkers for PD by quantitative analysis with patient plasma samples. In addition, we investigated the relationship between the potential biomarkers and clinical characteristics of the patients with PD.