CSF xanthine, homovanillic acid, and their ratio as biomarkers of Parkinson's disease

Abstract

Diminished nigrostriatal dopaminergic neurotransmission is a biochemical hallmark of Parkinson's disease. Despite this, a reliable trait biomarker of sporadic Parkinson's disease has not emerged from measurements of cerebrospinal fluid dopamine metabolites. Previous studies have highlighted strong neurochemical relationships between dopamine and various purine compounds. In this study, we analyzed cerebrospinal fluid concentrations of homovanillic acid (the major catabolite of dopamine) and the purine compound xanthine for a comparison of 217 unmedicated Parkinson's disease subjects and 26 healthy controls. These compounds were highly correlated for both the Parkinson's disease subjects (r = 0.68) and for controls (r = 0.73; both groups, p < 0.001). While neither homovanillic acid nor xanthine concentrations differentiated Parkinson's disease from controls, their ratio did. For controls, the mean [xanthine]/[homovanillic acid] quotient was 13.1 ± 5.5 as compared to the Parkinson's disease value of 17.4 ± 6.7 at an initial lumbar CSF collection (p = 0.0017), and 19.7 ± 8.7 (p < 0.001) at a second CSF collection up to 24 months later. The [xanthine]/[homovanillic acid] ratio in the Parkinson's disease subjects differed as a function of disease severity, as measured by the sum of Unified Parkinson's Disease Rating Scale Activities of Daily Living and Motor Exam ratings. The [xanthine]/[homovanillic acid] ratio also increased between the first and second CSF collections, suggesting that this quotient provides both a state and trait biomarker of Parkinson's disease. These observations add to other neurochemical evidence that links purine metabolism to Parkinson's disease.

Introduction

Based just on clinical history and examination, experienced clinicians can discern the clinical features of Parkinson's disease (PD) with a high degree of sensitivity and specificity (Hughes et al., 1992). Nonetheless, there is a continuing need for enhanced diagnostic capabilities, especially at the earliest stages of this disorder. Even when Parkinsonian signs and symptoms are relatively mild, the pathological impact of the disease is already advanced due to extensive loss of dopamine-synthesizing neurons in the substantia nigra pars compacta (SNpc) (Hornykiewicz and Kish, 1986). Biomarkers that can detect PD at very early or even pre-clinical stages are needed if effective neuroprotective strategies are to be utilized. Beyond its value as a diagnostic tool, a biomarker for PD is likely to offer insights into its pathophysiology.

Researchers have explored a diversity of clinical and laboratory tests in efforts to differentiate PD patients from a healthy population. Among these are transcranial sonography (Vlaar et al., 2009) and other applications of neuroimaging (Ravina et al., 2005, Vaillancourt et al., 2009). While radiotracer studies using positron or single photon emission computed tomography can demonstrate SNpc neuronal dropout through measurements of dopaminergic nerve terminal decline, these methods are impractical for screening purposes or for detecting the earliest stages of PD (Scherfler et al., 2007). Other evaluations yielding distinctive but non-specific changes in PD include testing of olfactory function (Verbaan et al., 2008), cardiac sympathetic innervation (Fujishiro et al., 2008), motor performance (de Frias et al., 2007), eye movements (Rivaud-Péchoux et al., 2007), and various motor reflexes and evoked responses (Meigal et al., 2009). Extensive biochemical analysis of cerebrospinal fluid (CSF) and blood has been conducted for dopamine metabolites, α-synuclein, and other CSF constituents offering diagnostic potential (Antoniades and Barker, 2008, Bogdanov et al., 2008, LeWitt and Galloway, 1990, Michell et al., 2008, Zhang et al., 2008). Though the search for PD biomarkers has led to some promising candidates, none has provided a reliable diagnostic test.

The CNS metabolism of purine compounds has garnered attention in PD research because of strong associations found between serum urate concentration and the risk for developing this disorder (Schlesinger and Schlesinger, 2008). Furthermore, both serum and CSF urate concentrations are inversely correlated with the rate of PD progression (Ascherio et al., 2009). These findings have been interpreted as evidence for a possible neuroprotective effect conferred by urate. As a strong anti-oxidant, urate in the PD patient might add to defenses against a disease mechanism acting through oxidative stress (Moore et al., 2005). On the other hand, the relationships observed between PD and systemic urate concentration might reflect an alteration of purine metabolism (especially that of adenosine) on the basis of its interplay with striatal dopamine neurotransmission (Stone et al., 1989). Adenosine receptors are involved in modulating striatal dopamine release (Jin et al., 1993, Okada et al., 1996). Other pharmacological implications of dopamine–adenosine relationships have been demonstrated by clinical trials showing enhanced anti-Parkinsonian effect of levodopa with co-administration of a selective adenosine receptor antagonist (LeWitt et al., 2008). Besides adenosine, other purine compounds also interact with dopamine metabolism (Loeffler et al., 1998, Loeffler et al., 2000). Understanding the particular link between dopamine neurotransmission and purines has been challenging because the latter compounds are abundant throughout the CNS and serve in a variety of roles (involving nucleic acids, energy transfer, and cellular signaling).

The relationship between striatal dopaminergic neurotransmission and purine metabolism led us to investigate for a PD biomarker associated with these neurochemical systems. Of particular interest has been xanthine (XAN), the second-to-last intermediate formed before the purine end-product in man, urate (Fig. 1). Toghi et al. (1993) reported that CSF XAN concentration was decreased by 19% in 11 PD subjects as compared to 14 controls. We sought to confirm these observations and hypothesized that indexing CSF XAN concentration to that of the dopamine metabolite homovanillic acid (HVA) might be informative as a PD biomarker, since high correlation between concentrations of these CSF constituents has been reported (Niklasson et al., 1983).