α-Synuclein Biomarkers for Parkinson's Disease

α-Synuclein (α-syn) biomarkers show great promise as diagnostic tools for Parkinson's disease (PD). In recent years, a large body of evidence has validated their efficacy as diagnostic tools for PD and other synucleinopathies and has shown potential for use in patients with isolated prodromal symptoms of PD, such as rapid eye movement (REM) sleep behavior disorder and hyposmia, and further illuminates the pathophysiology of both idiopathic and genetic causes. Various detection methods have been deployed, predominantly immunohistochemistry and α-syn seed amplification assays. α-Syn has been shown to be detectable in many different tissues and biofluids in PD patients, each with benefits and limitations for practical use. α-Syn biomarker studies have shown sensitivities for diagnosis of PD and specificity against healthy controls up to 100%. However, lack of standardization of methods of detection currently limits interlaboratory validation of results. Verification of these assays could lead to more widespread inclusion of these modalities to detect α-syn into biological definitions of PD and provide frameworks for developing disease-modifying therapies. In this review, we discuss the current state of α-syn biomarkers and highlight their potential use in clinical practice and research settings, while identifying further work that is needed in this field.