Urate as a predictor of the rate of clinical decline in Parkinson disease

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

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.

Serum urate and probability of dopaminergic deficit in early "Parkinson's disease"

The objective of this study was to investigate whether higher levels of urate, an antioxidant linked to a lower likelihood of developing Parkinson's disease, is also a predictor of having a dopamine transporter brain scan without evidence of dopaminergic deficit. In a cross-sectional study of 797 mildly affected, untreated parkinsonian subjects diagnosed with early Parkinson's disease in the Parkinson Research Examination of CEP-1347 Trial, we investigated the relationship at baseline between serum urate and striatal dopamine transporter density, determined by single-photon emission computed tomography of iodine-123-labeled 2-β-carboxymethoxy-3-β-(4-iodophenyl)tropane uptake. A scan without evidence of dopaminergic deficit was defined as lowest putamen iodine-123-labeled 2-β-carboxymethoxy-3-β-(4-iodophenyl)tropane > 80% age-expected putamen dopamine transporter density. The odds of having a scan without evidence of dopaminergic deficit rose across increasing quintiles of urate level, with an age- and sex-adjusted odds ratio of 3.2 comparing the highest to the lowest urate quintile (95% confidence interval, 1.5-7.2; P for trend = .0003), and remained significant after adjusting for potential confounding factors. The association was significant in men but not women, regardless of whether common or sex-specific quintiles of urate were used. Higher levels of urate were associated with a greater likelihood of a scan without evidence of dopaminergic deficit among subjects with early untreated parkinsonism in the Parkinson Research Examination of CEP-1347 Trial. The findings support the diagnostic utility of urate in combination with other determinants.

Penicillamine neurotoxicity: an hypothesis

Penicillamine, dimethyl cysteine, thiovaline, remains the drug of choice for the treatment of patience with Wilson disease. It is also of value in the treatment of cysteinuria and rheumatoid arthritis, it has also been suggested that it has value in the management of other rare diseases. It also has multiple toxicities. The majority of these can be explained as chemical toxicity, for instance its weak antipyridoxine action and its ability to interfere with lysyloxidea resulting in skin lesions. More important are its ability to induce immune reactions such as SLE, immune complex nephritis, the Ehlers Danlos syndrome and Goodpasture's syndrome. However the sudden increase in neurological signs which may occur in a small number of patients remains unexplained. The theory is proposed that this is due to lethal synthesis. In susceptible patients the–SH radical is liberated from penicillamine and will inhibit–SH dependent enzymes in the Krebs cycle leading to death in neurones. Other toxic metabolites may also be produced such as methyl mercaptan and ethyl mercaptan either of which could produce a similar metabolic block.

Avian renal proximal tubule urate secretion is inhibited by cellular stress-induced AMP-activated protein kinase

Urate is a potent antioxidant at high concentrations but it has also been associated with a wide variety of health risks. Plasma urate concentration is determined by ingestion, production, and urinary excretion; however, factors that regulate urate excretion remain uncertain. The objective of this study was to determine whether cellular stress, which has been shown to affect other renal transport properties, modulates urate secretion in the avian renal proximal tubule. Chick kidney proximal tubule epithelial cell primary culture monolayers were used to study the transepithelial transport of radiolabeled urate. This model allowed examination of the processes, such as multidrug resistance protein 4 (Mrp4, Abcc4), which subserve urate secretion in a functional, intact, homologous system. Our results show that the recently implicated urate efflux transporter, breast cancer resistance protein ( ABCG2), does not significantly contribute to urate secretion in this system. Exposure to a high concentration of zinc for 6 h induced a cellular stress response and a striking decrease in transepithelial urate secretion. Acute exposure to zinc had no effect on transepithelial urate secretion or isolated membrane vesicle urate transport, suggesting involvement of a cellular stress adaptation. Activation of AMP-activated protein kinase (AMPK), a candidate modulator of ATP-dependent urate efflux, by 5′-aminoimidazole-4-carboxamide 1-β-d-ribo-furanoside caused a decrease in urate secretion similar to that seen with zinc-induced cellular stress. This effect was prevented with the AMPK inhibitor compound C. Notably, the decrease in urate secretion seen with zinc-induced cellular stress was also prevented by compound C, implicating AMPK in regulation of renal uric acid excretion.

Epidemiology and etiology of Parkinson's disease: a review of the evidence

The etiology of Parkinson's disease (PD) is not well understood but likely to involve both genetic and environmental factors. Incidence and prevalence estimates vary to a large extent-at least partly due to methodological differences between studies-but are consistently higher in men than in women. Several genes that cause familial as well as sporadic PD have been identified and familial aggregation studies support a genetic component. Despite a vast literature on lifestyle and environmental possible risk or protection factors, consistent findings are few. There is compelling evidence for protective effects of smoking and coffee, but the biologic mechanisms for these possibly causal relations are poorly understood. Uric acid also seems to be associated with lower PD risk. Evidence that one or several pesticides increase PD risk is suggestive but further research is needed to identify specific compounds that may play a causal role. Evidence is limited on the role of metals, other chemicals and magnetic fields. Important methodological limitations include crude classification of exposure, low frequency and intensity of exposure, inadequate sample size, potential for confounding, retrospective study designs and lack of consistent diagnostic criteria for PD. Studies that assessed possible shared etiological components between PD and other diseases show that REM sleep behavior disorder and mental illness increase PD risk and that PD patients have lower cancer risk, but methodological concerns exist. Future epidemiologic studies of PD should be large, include detailed quantifications of exposure, and collect information on environmental exposures as well as genetic polymorphisms.

Development of Parkinson's disease biomarkers

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, affecting over 6 million people worldwide. It is anticipated that the number of affected individuals may increase significantly in the most populous nations by 2030. During the past 20 years, much progress has been made in identifying and assessing various potential clinical, biochemical, imaging and genetic biomarkers for PD. Despite the wealth of information, development of a validated biomarker for PD is still ongoing. It is hoped that reliable and well-validated biomarkers will provide critical clues to assist in the diagnosis and management of Parkinson's disease patients in the near future.

Uric acid as a potential disease modifier in patients with multiple system atrophy

BACKGROUND

Recent studies have suggested that mitochondrial dysfunction and oxidative stress play a key role in the pathogenesis of multiple system atrophy.

METHODS

We evaluated the influence of serum uric acid levels on disease progression in 52 patients with multiple system atrophy using changes in the annualized Unified Multiple System Atrophy Rating Scale scores.

RESULTS

The mean annualized Unified Multiple System Atrophy Rating Scale changes were significantly lower in patients with the highest uric acid quartile compared with those with the lowest quartile (8.4 ± 5.1 vs 20.2 ± 16.0, P = .038). Serum uric acid levels had a significant negative correlation with the annualized Unified Multiple System Atrophy Rating Scale changes (r = -0.40, P = .004). Multiple linear regression analysis showed that only serum uric acid concentration was significantly correlated with the annualized Unified Multiple System Atrophy Rating Scale changes (β = -2.687, P = .011).

CONCLUSIONS

These data suggest that serum uric acid may act as a potential disease modifier in multiple system atrophy.

Problems associated with fluid biomarkers for Parkinson's disease

This article focuses on biochemical markers that may be used in the diagnostics of Parkinson's disease and associated disorders, and to identify early cases and stratify patients into subgroups. We present an updated account of some currently available candidate fluid biomarkers, and discuss their diagnostic performance and limitations. We also discuss some of the general problems with Parkinson's disease biomarkers and possible ways of moving forward. It may be concluded that a diagnostically useful fluid biomarker for Parkinson's disease is yet to be identified. However, some interesting candidates exist and may prove useful in the future, alone or when analyzed together in patterns.

Discovering neuroprotection in Parkinson's disease, or getting to haphazard

Despite significant research efforts, a therapy to slow or halt the progression of Parkinson's disease ("neuroprotection") remains elusive. The discovery of such a therapy will likely require important observations from a number of perspectives: basic science investigation, clinical research, and careful observation in medical practice. Any possible insights will require confirmation in rigorous clinical trial testing. Combining the useful insights from all perspectives may be the most promising approach to discovering a neuroprotective therapy.

Urate: a novel biomarker of Parkinson's disease risk, diagnosis and prognosis

A growing number of studies have correlated higher urate levels with a lower risk of developing Parkinson's disease (PD) and with a favorable rate of disease progression, indicating that urate could be an important biomarker of the pathophysiology underlying PD. Dietary and genetic determinants of urate have also been linked to a reduced risk or delayed onset of PD. Based on the known antioxidant and metal complexing properties of urate, together with evidence for oxidative stress as a contributor to neurodegeneration in PD, urate may serve as an endogenous neuroprotectant that helps reduce the risk and rate of the disease. In this article we review the convergent biological, epidemiological and clinical data that identify urate as a promising biomarker of the risk, diagnosis and prognosis of PD.

Plasma urate and Parkinson's disease in women

Plasma urate has been consistently associated with a lower risk of Parkinson's disease in men, but it is less clear if this relation exists in women. Between 1990 and 2004, the authors conducted a nested case-control study among participants of the female-only Nurses' Health Study. In controls (n = 504), plasma urate was positively associated with age, body mass index, alcohol consumption, hypertension, and use of diuretics and was inversely associated with physical activity and postmenopausal hormone use, as expected. Mean urate levels were 5.04 mg/dL for cases (n = 101) and 4.86 mg/dL for controls (P = 0.17). The age-, smoking-, and caffeine-adjusted rate ratio comparing women in the highest (≥5.8 mg/dL) with those in the lowest (<4.0 mg/dL) quartile was 1.33 (95% confidence interval: 0.69, 2.57; P(trend) = 0.4). Further adjustment for body mass index, physical activity, history of hypertension, and postmenopausal hormone use did not change the results. Unlike in men, these findings do not support the hypothesis that urate is strongly associated with lower rates of Parkinson's disease among women.

Biosensor measurement of purine release from cerebellar cultures and slices

We have previously described an action-potential and Ca(2+)-dependent form of adenosine release in the molecular layer of cerebellar slices. The most likely source of the adenosine is the parallel fibres, the axons of granule cells. Using microelectrode biosensors, we have therefore investigated whether cultured granule cells (from postnatal day 7-8 rats) can release adenosine. Although no purine release could be detected in response to focal electrical stimulation, purine (adenosine, inosine or hypoxanthine) release occurred in response to an increase in extracellular K(+) concentration from 3 to 25 mM coupled with addition of 1 mM glutamate. The mechanism of purine release was transport from the cytoplasm via an ENT transporter. This process did not require action-potential firing but was Ca(2+)dependent. The major purine released was not adenosine, but was either inosine or hypoxanthine. In order for inosine/hypoxanthine release to occur, cultures had to contain both granule cells and glial cells; neither cellular component was sufficient alone. Using the same stimulus in cerebellar slices (postnatal day 7-25), it was possible to release purines. The release however was not blocked by ENT blockers and there was a shift in the Ca(2+) dependence during development. This data from cultures and slices further illustrates the complexities of purine release, which is dependent on cellular composition and developmental stage.

Treating gout with pegloticase, a PEGylated urate oxidase, provides insight into the importance of uric acid as an antioxidant in vivo

A high plasma urate concentration (PUA), related to loss of urate oxidase in evolution, is postulated to protect humans from oxidative injury. This hypothesis has broad clinical relevance, but support rests largely on in vitro data and epidemiologic associations. Pegloticase therapy generates H(2)O(2) while depleting urate, offering an in vivo test of the antioxidant hypothesis. We show that erythrocytes can efficiently eliminate H(2)O(2) derived from urate oxidation to prevent cell injury in vitro; during therapy, disulfide-linked peroxiredoxin 2 dimer did not accumulate in red blood cells, indicating that their peroxidase capacity was not exceeded. To assess oxidative stress, we monitored F2-Isoprostanes (F2-IsoPs) and protein carbonyls (PC), products of arachidonic acid and protein oxidation, in plasma of 26 refractory gout patients receiving up to five infusions of pegloticase at 3-wk intervals. At baseline, PUA was markedly elevated in all patients, and plasma F2-IsoP concentration was elevated in most. Pegloticase infusion rapidly lowered mean PUA to < or = 1 mg/dL in all patients, and PUA remained low in 16 of 21 patients who completed treatment. F2-IsoP levels did not correlate with PUA and did not increase during 15 wk of sustained urate depletion. There also was no significant change in the levels of plasma PC. Because refractory gout is associated with high oxidative stress in spite of high PUA, and profoundly depleting uric acid did not increase lipid or protein oxidation, we conclude that urate is not a major factor controlling oxidative stress in vivo.

Gout, hyperuricemia, and Parkinson's disease: a protective effect?

Oxidative stress is potentially involved in the pathogenesis of Parkinson's disease (PD). Serum uric acid, because of its antioxidant effect, might prevent the development of this neurodegenerative disorder. Indeed, a number of epidemiologic studies have consistently reported a strong inverse association between serum uric acid and the risk of PD. The risk of PD is also lower among individuals with gout. In addition, suggestive evidence from clinical studies shows high levels of uric acid as a marker of better prognosis in PD. Future research should determine the mechanisms underlying this association. This, in turn, could be used in the development of new preventive strategies and treatments for PD.

Biomarkers for prediction and targeted prevention of Alzheimer's and Parkinson's diseases: evaluation of drug clinical efficacy

Neurodegenerative diseases like Parkinson’s disease (PD) and Alzheimer’s disease (AD) are considered disorders of multifactorial origin, inevitably progressive and having a long preclinical period. Therefore, the availability of biological markers or biomarkers (BMs) for early disease diagnosis will impact the management of AD and PD in several dimensions; it will 1) help to capture high-risk individuals before symptoms develop, a stage where prevention efforts might be expected to have their greatest impact; 2) provide a measure of disease progression that can be evaluated objectively, while clinical measures are much less accurate; 3) help to discriminate between true AD or PD and other causes of a similar clinical syndrome; 4) delineate pathophysiological processes responsible for the disease; 5) determine the clinical efficacy of novel, disease-modifying (neuroprotective) strategies. In the long run the availability of reliable BMs will significantly advance the research and therapeutics of AD and PD.

Using 'omics' to define pathogenesis and biomarkers of Parkinson's disease

Although great effort has been put forth to uncover the complex molecular mechanisms exploited in the pathogenesis of Parkinson's disease, a satisfactory explanation remains to be discovered. The emergence of several -omics techniques, transcriptomics, proteomics and metabolomics, have been integral in confirming previously identified pathways that are associated with dopaminergic neurodegeneration and subsequently Parkinson's disease, including mitochondrial and proteasomal function and synaptic neurotransmission. Additionally, these unbiased techniques, particularly in the brain regions uniquely associated with the disease, have greatly enhanced our ability to identify novel pathways, such as axon-guidance, that are potentially involved in Parkinson's pathogenesis. A comprehensive appraisal of the results obtained by different -omics has also reconfirmed the increase in oxidative stress as a common pathway likely to be critical in Parkinson's development/progression. It is hoped that further integration of these techniques will yield a more comprehensive understanding of Parkinson's disease etiology and the biological pathways that mediate neurodegeneration.

Pathophysiological roles for purines: adenosine, caffeine and urate

The motor symptoms of Parkinson's disease (PD) are primarily due to the degeneration of the dopaminergic neurons in the nigrostriatal pathway. However, several other brain areas and neurotransmitters other than dopamine such as noradrenaline, 5-hydroxytryptamine and acetylcholine are affected in the disease. Moreover, adenosine because of the extensive interaction of its receptors with the dopaminergic system has been implicated in the pathophysiology of the disease. Based on the involvement of these non-dopaminergic neurotransmitters in PD and the sometimes severe adverse effects that limit the mainstay use of dopamine-based anti-parkinsonian treatments, recent assessments have called for a broadening of therapeutic options beyond the traditional dopaminergic drug arsenal. In this review we describe the interactions between dopamine and adenosine receptors that underpin the pre-clinical and clinical rationale for pursuing adenosine A(2A) receptor antagonists as symptomatic and potentially neuroprotective treatment of PD. The review will pay particular attention to recent results regarding specific A(2A) receptor-receptor interactions and recent findings identifying urate, the end product of purine metabolism, as a novel prognostic biomarker and candidate neuroprotectant in PD.

Metabolomic profiling in LRRK2-related Parkinson's disease

Background

Mutations in LRRK2 gene represent the most common known genetic cause of Parkinson's disease (PD).

Methodology/Principal Findings

We used metabolomic profiling to identify biomarkers that are associated with idiopathic and LRRK2 PD. We compared plasma metabolomic profiles of patients with PD due to the G2019S LRRK2 mutation, to asymptomatic family members of these patients either with or without G2019S LRRK2 mutations, and to patients with idiopathic PD, as well as non-related control subjects. We found that metabolomic profiles of both idiopathic PD and LRRK2 PD subjects were clearly separated from controls. LRRK2 PD patients had metabolomic profiles distinguishable from those with idiopathic PD, and the profiles could predict whether the PD was secondary to LRRK2 mutations or idiopathic. Metabolomic profiles of LRRK2 PD patients were well separated from their family members, but there was a slight overlap between family members with and without LRRK2 mutations. Both LRRK2 and idiopathic PD patients showed significantly reduced uric acid levels. We also found a significant decrease in levels of hypoxanthine and in the ratios of major metabolites of the purine pathway in plasma of PD patients.

Conclusions/Significance

These findings show that LRRK2 patients with the G2019S mutation have unique metabolomic profiles that distinguish them from patients with idiopathic PD. Furthermore, asymptomatic LRRK2 carriers can be separated from gene negative family members, which raises the possibility that metabolomic profiles could be useful in predicting which LRRK2 carriers will eventually develop PD. The results also suggest that there are aberrations in the purine pathway in PD which may occur upstream from uric acid.