Accuracy of a Panel of 5 Cerebrospinal Fluid Biomarkers in the Differential Diagnosis of Patients With Dementia and/or Parkinsonian Disorders

Cerebrospinal Fluid Biomarkers in Parkinson's Disease with Dementia and Dementia with Lewy Bodies

Inflammation has been implicated in the pathogenesis of Parkinson’s disease (PD). We here investigate levels of inflammatory biomarkers in cerebrospinal fluid (CSF) in PD and atypical parkinsonian disorders (APD) compared with neurologically healthy controls. We included 131 patients with PD and 27 PD with dementia (PDD), 24 with multiple system atrophy (MSA), 14 with progressive supranuclear palsy (PSP) and 50 controls, all part of the Swedish BioFINDER study. CSF was analyzed for CRP, SAA, IL-6, IL-8, YKL-40 and MCP-1 (CCL2) as well as α-synuclein (α-syn), tau, tau phosphorylated at Thr181 (P-tau), Aβ42 and NfL. In this exploratory study, we found higher levels of the inflammatory biomarker SAA in PDD and MSA compared with controls and PD and higher levels of CRP in PDD and MSA compared with PD. YKL-40 was lower in PD compared with controls. There were multiple positive correlations between the inflammatory markers, α-syn and markers of neuroaxonal injury (NfL and tau). In PD, higher levels of inflammatory biomarkers correlated with worse motor function and cognitive impairment. Thus, inflammatory biomarkers were increased in PDD and MSA. Furthermore, inflammatory biomarkers correlated with more severe disease regarding motor symptoms and cognitive impairment in PD, indicating an association between inflammation and more aggressive disease course. However, the results need confirmation in follow-up studies.

Dysarthria and dysphagia are known to occur in parkinsonian syndromes such as Parkinson disease (PD), dementia with Lewy bodies (DLB), corticobasal degeneration (CBD), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP). Differences in the evolution of these symptoms have not been studied systematically in postmortem-confirmed cases. To study differences in the evolution of dysarthria and dysphagia in postmortem-confirmed parkinsonian disorders. Eighty-three pathologically confirmed cases (PD, n = 17; MSA, n = 15; DLB, n = 14; PSP, n = 24; and CBD, n = 13) formed the basis for a multicenter clinicopathological study organized by the National Institute of Neurological Disorders and Stroke, Bethesda, Md. Cases with enough clinicopathological documentation for the purpose of the study were selected from research and neuropathological files of 7 medical centers in 4 countries (Austria, France, England, and the United States). Median dysarthria latencies were short in PSP and MSA (24 months each), intermediate in CBD and DLB (40 and 42 months), and long in PD (84 months). Median dysphagia latencies were intermediate in PSP (42 months), DLB (43 months), CBD (64 months), and MSA (67 months), and long in PD (130 months). Dysarthria or dysphagia within 1 year of disease onset was a distinguishing feature for atypical parkinsonian disorders (APDs) (specificity, 100%) but failed to further distinguish among the APDs. Survival time after onset of a complaint of dysphagia was similar in PD, MSA, and PSP (15 to 24 months, P =.7) and latency to a complaint of dysphagia was highly correlated with total survival time (rho = 0.88; P<.001) in all disorders. Latency to onset of dysarthria and dysphagia clearly differentiated PD from the APDs, but did not help distinguish different APDs. Survival after onset of dysphagia was similarly poor among all parkinsonian disorders. Evaluation and adequate treatment of patients with PD who complain of dysphagia might prevent or delay complications such as aspiration pneumonia, which in turn may improve quality of life and increase survival time.

Cerebrospinal fluid (CSF) biomarkers for Alzheimer's disease (AD) reflect brain biochemistry. Using combined immunoprecipitation and mass spectrometry, we have shown that amyloid beta 1-15 (Aβ1-15) is produced by concerted β- and α-secretase cleavage of amyloid precursor protein (APP) and that the relative levels of Aβ1-16 in AD compared to controls are increased. Furthermore, drug-induced γ-secretase inhibition enhances the relative levels of Aβ1-15 and Aβ1-16. Here, we investigate a novel immunoassay for Aβ1-15/16 in a broad range of neurodegenerative conditions. The CSF level of Aβ1-15/16 was measured by the bead-based amplified luminescent proximity homogeneous assay (Alpha technology). Concentrations of Aβ1-15/16 were analyzed in subjects with Parkinson disease (PD; n = 90), PD with dementia (PDD) (n = 32), dementia with Lewy bodies (DLB) (n = 68), AD (n = 48), progressive supranuclear palsy (PSP) (n = 45), multiple system atrophy (MSA) (n = 46), and corticobasal degeneration (CBD) (n = 12). The detecting antibody is specific to the C-terminal epitope of Aβ15. We found that a carboxypeptidase (CPB) present in fetal bovine serum (FBS), a component of the buffers used, degrades Aβ1-16 to Aβ1-15, which is then detected by the Aβ1-15/16 assay. Significantly, lower levels of Aβ1-15/16 were detected in PD, PDD, PSP, and MSA compared to other neurodegenerative diseases and controls. Using the specific Aβ1-15/16 assay, a reliable quantification of Aβ1-15 or Aβ1-15/16 in CSF samples is obtained. We found reduced levels of Aβ1-15 in parkinsonian disease groups. The molecular mechanism behind this reduction is at present unknown.

Striatal Aβ peptide deposition mirrors dementia and differentiates DLB and PDD from other Parkinsonian syndromes

Parkinsonian disorders consist mostly of Parkinson’s disease (PD); other forms are relatively rare, although dementia with Lewy bodies (DLB) seems to be, after Alzheimer’s disease, one of the most common causes of dementia. Atypical parkinsonisms (other than idiopathic PD) include progressive supranuclear palsy (PSP), multiple system atrophy (MSA), corticobasal degeneration (CBD) and DLB. In addition to parkinsonism symptoms the most characteristic features of these disorders, sometimes called parkinsonism-plus syndromes, are cerebellar ataxia in MSA, apraxia in CBD, gaze palsy in PSP or early in the course of dementia in DLB, and minimal, sometimes moderate (in MSA or PSP) or complete lack (in CBD) of response to L-dopa therapy. When appraising the condition, it is vital to separate it from those conditions that mimic the appearance of PD, but that are caused by another underlying pathology. Because of the lack of biological markers, differential diagnosis has to be based mostly on clinical findings. Nevertheless, neuroimaging studies, utilizing MRI, SPECT and PET examinations, seem to be helpful in establishing diagnosis. Nuclear medicine uses different tracers, specific for the dopamine transporting system or dopamine receptor ligands, which makes it possible to differentiate presynaptic dopaminergic system involvement (seen in PD) from pre- and postsynaptic involvement in MSA or PSP. Highly asymmetric tracer uptake reflects the asymmetry of cortical-subcortical atrophy and is characteristic for CBD. All of these conditions are accompanied by different psychiatric problems. In PD, depression is the most common, followed by dementia and psychosis at late stages. In PSP, subcortical dementia is typical with frontal behavior. Dementia is less frequent and late in CBD, but usually is the first or early symptom in DLB, followed by parkinsonian features. Functional neuroimaging may be useful for assessment of the preclinical period of PD (with potential early neuroprotective therapy), the rate of progression (evaluation of the influence of drugs and stereotaxic surgery on disease progression), the role of different structures in late complications (dyskinesias, fluctuations), and differential diagnosis of PD with essential tremor and atypical parkinsonian disorders. These techniques may also be helpful for investigation of pathogenesis and pathology underlying the non-motor symptoms of PD and atypical parkinsonisms as depression, dementia and psychosis. This review summarizes recent applications of SPECT, PET, but also MRI in the study of parkinsonian disorders, in terms of differential diagnosis and understanding concomitant neuropsychiatric phenomena.

The value of neurofilament light chain (NfL) levels as a biomarker for the diagnosis and differential diagnosis in patients with Parkinson's disease (PD) and atypical parkinsonian syndromes (APS) remains controversial. Furthermore, few studies have directly compared NfL levels among specific APS categories. This study aimed to compare cerebrospinal fluid (CSF) and blood NfL levels among PD, APS, other PD-related disorders, and controls, as well as rank NfL levels across these groups. PubMed, Embase, Web of Science, and the Cochrane Library were searched from the inception up to November 1st, 2024, to identify eligible studies reporting CSF or blood NfL concentrations in PD, PD dementia (PDD), multiple system atrophy (MSA), progressive supranuclear palsy (PSP), dementia with Lewy bodies (DLB), corticobasal syndrome (CBS), vascular parkinsonism (VP), essential tremor (ET), idiopathic rapid eye movement sleep behavior disorder (iRBD), and controls. The Bayesian approach was utilized to estimate the standardized mean difference (SMD) and the associated 95% credible intervals (CrIs) of NfL levels. The surface under the cumulative ranking curve (SUCRA) was employed to evaluate the ranking probabilities of NfL levels. Subgroup analysis and meta-regression were conducted to explore the sources of heterogeneity. The present network meta-analysis (NMA) included 78 studies with 13,120 participants (4050 controls, 5021 PD, 191 PDD, 1173 MSA, 887 PSP, 1254 DLB, 319 CBS, 160 ET, 65 iRBD, and 0 VP). Of these, the NMA of CSF NfL included 34 studies with 6,013 participants, while the NMA of blood NfL included 49 studies with 7,787 participants. Both CSF and blood NfL levels were significantly elevated in patients with PD and APS compared to controls. Compared to PD patients, CSF NfL levels were significantly elevated in MSA (SMD 1.85; 95% CrI 1.55-2.15), CBS (1.42; 1.08-1.75), PSP (1.35; 1.06-1.64), and DLB 0.52; 0.20-0.85) patients. Similarly, blood NfL levels were significantly higher in patients with MSA (1.36; 1.02-1.71), PDD (1.19; 0.65-1.72), PSP (1.15; 0.77-1.54), CBS (0.92; 0.11-1.72), and DLB (0.63; 0.14-1.12) compared to PD. Among APS, CSF NfL levels in MSA patients were significantly higher than those in PSP, DLB, and CBS patients, while blood NfL levels in MSA patients were significantly higher only compared to DLB. In both CSF and blood NfL, MSA patients exhibited the highest probability of ranking first for NfL level elevations (CSF: SUCRA = 0.998; blood: SUCRA = 0.925). Age significantly influenced the SMD of the comparison between MSA and PD in CSF NfL (β = -0.15; p = 0.016). CSF and blood NfL levels in PD and APS are higher than those in controls, and all APS categories show higher levels than PD, suggesting that NfL levels may serve as a potential biomarker for the differential diagnosis between PD and APS. However, caution is warranted when using NfL as a diagnostic biomarker for PD. Significant differences in NfL levels are also observed between certain APS categories. Patients with MSA exhibit the highest NfL levels among PD and related disorders.

Background: The relation between dementia with Lewy bodies (DLB) and Parkinson’s disease with dementia (PDD) is unknown. Objectives: To compare the cognitive profiles of patients with DLB and PDD, and...

Background The study is aimed at assessing the usefulness of 99m Tc-ECD SPECT in distinguishing patients with: Dementia with Lewy Bodies (DLB), Alzheimer's disease (AD) and Parkinson's disease with dementia (PD-D). Methods Fifty patients participated in the study: 13 patients with DLB (aged 74 ± 4 years), 19 patients with AD (aged 75 ± 7 years), 18 patients with PD-D (aged 72 ± 3 years) and 14 healthy individuals (aged 70 ± 6 years). A neuropsychological examination was conducted on all patients. Motor function of DLB and PD-D patients was assessed using the Hoehn-Yahr scale, Unified Parkinson's Disease Rating Scale and Schwab and England Activities of Daily Living Scale. 99m Tc-ECD SPECT was performed on all subjects. Statistical analysis of radiotracer uptake was performed for 16 regions of interest. To differentiate patients with DLB, PD-D and AD discriminate analysis was conducted. Results SPECT showed global hypoperfusion in all patient groups when compared to control group. In DLB perfusion was significantly lower within the left temporal region as compared to AD and within the right parietal region, left temporo-occipital junction and left thalamus as compared to PD-D. A significant decrease of perfusion in both thalami was found in PD-D in comparison to AD. Discriminate analysis allowed for the proper classification in 68% of all patients: PD-D was classified the most accurately (83%) and DLB with least precision (46%). Conclusions 99m Tc-ECD SPECT did not allow us to differentiate precisely among DLB, PD-D and AD. Overlapping neuropathological features of DLB, PD-D and AD may account for this pattern of perfusion.

Previous Cochrane reviews have considered the use of cholinesterase inhibitors in both Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB). The clinical features of DLB and PDD have much in common and are distinguished primarily on the basis of whether or not parkinsonism precedes dementia by more than a year. Patients with both conditions have particularly severe deficits in cortical levels of the neurotransmitter acetylcholine. Therefore, blocking its breakdown using cholinesterase inhibitors may lead to clinical improvement. To assess the efficacy, safety and tolerability of cholinesterase inhibitors in dementia with Lewy bodies (DLB), Parkinson's disease with dementia (PDD), and cognitive impairment in Parkinson's disease falling short of dementia (CIND-PD) (considered as separate phenomena and also grouped together as Lewy body disease). The trials were identified from a search of ALOIS, the Specialised Register of the Cochrane Dementia and Cognitive Improvement Group (on 30 August 2011) using the search terms Lewy, Parkinson, PDD, DLB, LBD. This register consists of records from major healthcare databases (MEDLINE, EMBASE, PsycINFO, CINAHL) and many ongoing trial databases and is updated regularly.Reference lists of relevant studies were searched for additional trials. Randomised, double-blind, placebo-controlled trials assessing the efficacy of treatment with cholinesterase inhibitors in DLB, PDD and cognitive impairment in Parkinson's disease (CIND-PD). Data were extracted from published reports by one review author (MR). The data for each 'condition' (that is DLB, PDD or CIND-PD) were considered separately and, where possible, also pooled together. Statistical analysis was conducted using Review Manager version 5.0. Six trials met the inclusion criteria for this review, in which a total of 1236 participants were randomised. Four of the trials were of a parallel group design and two cross-over trials were included. Four of the trials included participants with a diagnosis of Parkinson's disease with dementia (Aarsland 2002a; Dubois 2007; Emre 2004; Ravina 2005), of which Dubois 2007 remains unpublished. Leroi 2004 included patients with cognitive impairment and Parkinson's disease (both with and without dementia). Patients with dementia with Lewy bodies (DLB) were included in only one of the trials (McKeith 2000).For global assessment, three trials comparing cholinesterase inhibitor treatment to placebo in PDD (Aarsland 2002a; Emre 2004; Ravina 2005) reported a difference in the Alzheimer's Disease Cooperative Study-Clinical Global Impression of Change (ADCS-CGIC) score of -0.38, favouring the cholinesterase inhibitors (95% CI -0.56 to -0.24, P < 0.0001).For cognitive function, a pooled estimate of the effect of cholinesterase inhibitors on cognitive function measures was consistent with the presence of a therapeutic benefit (standardised mean difference (SMD) -0.34, 95% CI -0.46 to -0.23, P < 0.00001). There was evidence of a positive effect of cholinesterase inhibitors on the Mini-Mental State Examination (MMSE) in patients with PDD (WMD 1.09, 95% CI 0.45 to 1.73, P = 0.0008) and in the single PDD and CIND-PD trial (WMD 1.05, 95% CI 0.42 to 1.68, P = 0.01) but not in the single DLB trial.For behavioural disturbance, analysis of the pooled continuous data relating to behavioural disturbance rating scales favoured treatment with cholinesterase inhibitors (SMD -0.20, 95% CI -0.36 to -0.04, P = 0.01).For activities of daily living, combined data for the ADCS and the Unified Parkinson's Disease Rating Scale (UPDRS) activities of daily living rating scales favoured treatment with cholinesterase inhibitors (SMD -0.20, 95% CI -0.38 to -0.02, P = 0.03).For safety and tolerability, those taking a cholinesterase inhibitor were more likely to experience an adverse event (318/452 versus 668/842; odds ratio (OR) 1.64, 95% CI 1.26 to 2.15, P = 0.0003) and to drop out (128/465 versus 45/279; OR 1.94, 95% CI 1.33 to 2.84, P = 0.0006). Adverse events were more common amongst those taking rivastigmine (357/421 versus 173/240; OR 2.28, 95% CI 1.53 to 3.38, P < 0.0001) but not those taking donepezil (311/421 versus 145/212; OR 1.24, 95% CI 0.86 to 1.80, P = 0.25). Parkinsonian symptoms in particular tremor (64/739 versus 12/352; OR 2.71, 95% CI 1.44 to 5.09, P = 0.002), but not falls (P = 0.39), were reported more commonly in the treatment group but this did not have a significant impact on the UPDRS (total and motor) scores (P = 0.71). Fewer deaths occurred in the treatment group than in the placebo group (4/465 versus 9/279; OR 0.28, 95% CI 0.09 to 0.84, P = 0.03). The currently available evidence supports the use of cholinesterase inhibitors in patients with PDD, with a positive impact on global assessment, cognitive function, behavioural disturbance and activities of daily living rating scales. The effect in DLB remains unclear. There is no current disaggregated evidence to support their use in CIND-PD.

White matter hyperintensities on MRI in dementia with Lewy bodies, Parkinson's disease with dementia, and Alzheimer's disease

There are multiple cerebrospinal fluid (CSF) biomarkers with potential for distinguishing Parkinson's disease (PD) from atypical parkinsonian syndromes (APSs) and controls; however, consensus on their diagnostic performance remains limited. This study aims to systematically evaluate and rank the diagnostic accuracy of CSF biomarkers in differentiating PD from APS and controls through network meta-analysis, determining the clinical diagnostic yield of these biomarkers and whether they should be considered as first-line diagnostic and differentiating tools. A comprehensive research was performed on PubMed, Web of Science, Embase and Cochrane library from inception until December 31st, 2024. Random effects models for sensitivity, specificity, positive likelihood ratio (PLR) and negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and 95% CIs were used to calculate test accuracy. In addition, summary receiver operating characteristic (SROC) curves were used to summarize the overall diagnostic performance. The network meta-analysis based on the ANOVA model and surface under the cumulative ranking curve (SUCRA) scores were selected to rank the diagnostic performance of the included biomarkers by calculating the relative sensitivity, specificity, and diagnostic odds ratio (DOR). Seventy eligible studies containing 4925 PD patients, 698 multiple system atrophy (MSA), 177 progressive supranuclear palsy (PSP), 78 dementia with Lewy body (DLB) and 3072 healthy controls (HCs) or non-neurological controls (NNCs) were included in our meta-analysis. CSF Alpha-synuclein seed amplification assays (α-syn SAAs) showed high diagnostic accuracy with pooled sensitivity of 0.91 (95% CI 0.89-0.92) and specificity of 0.95 (95% CI 0.94-0.96) in distinguishing PD from HC or NNCs. The Area Under the Curve (AUC) of the SROC curve of CSF Neurofilament Light Chain (NfL) was 0.91, which effectively distinguished between PD and MSA. Both α-syn SAAs and NfL demonstrated similarly high diagnostic accuracy for differentiating PD from PSP. Only CSF Aβ1-42 has been evaluated for the differential diagnosis between PD and DLB, which demonstrated low pooled sensitivity (0.59, 95%CI 0.49-0.69) and specificity (0.68, 95%CI 0.56-0.78). Network meta-analysis using ANOVA model showed that among 13 CSF biomarkers, α-syn SAAs had the highest relative DOR (285.01, 95% CI 156.72-475.89), relative sensitivity (1.29, 95% CI 1.19-1.41) and specificity (1.54, 95% CI 1.39-1.72), while the SUCRA curvesalso reach the same conclusion, indicating the highest diagnostic accuracy in distinguishing PD from HCs or NNCs. Notably, these studies have general heterogeneities. CSF α-syn SAAs could serve as promising biomarkers for distinguishing PD from PSP and HCs or NNCs. Meanwhile, NfL is suitable for differentiating PD from MSA and PSP. In addition, the confirmation of these CSF biomarkers, as well as the discovery of new biomarkers, requires extensive study in large, independent cohorts. The main limitations of this analysis are the reliance on evidence derived from case-control studies, which are susceptible to bias, and the high level of heterogeneity observed for some biomarkers.

EEG abnormalities have been reported for both dementia with Lewy bodies (DLB) and Alzheimer's disease (AD). Although it has been suggested that variations in mean EEG frequency are greater in the former, the existence of meaningful differences remains controversial. No evidence is as yet available for Parkinson's disease with dementia (PDD). The aim of this study was to evaluate whether EEG abnormalities can discriminate between DLB, AD and PDD in the earliest stages of dementia and to do this 50 DLB, 50 AD and 40 PDD patients with slight cognitive impairment at first visit (MMSE > or = 20) were studied. To improve clinical diagnostic accuracy, special emphasis was placed on identifying cognitive fluctuations and REM-sleep behaviour disorder. EEG variability was assessed by mean frequency analysis and compressed spectral arrays (CSA) in order to detect changes over time from different scalp derivations. Patients' initial diagnoses were revised at a 2-year follow-up visit with neuroimaging evaluation. Initial diagnoses were confirmed in 36 DLB, 40 AD and 35 PDD patients. The most relevant group differences were observed between the AD and DLB patients in EEGs from posterior derivations (P<0.001). Dominant frequencies were 8.3 +/- 0.6 Hz for the AD group and 7.4 +/- 1.6 Hz for the DLB group, in which most of the patients (88%) exhibited a frequency band of 5.6-7.9 Hz. Dominant frequency variability also differed between the AD (1.1 +/- 0.4 Hz) and DLB groups (1.8 +/- 1.2 Hz, P<0.001). Of note, less than a half (46%) of the patients with PDD exhibited the EEG abnormalities seen in those with DLB. Graded according to the presence of alpha activity, five different patterns were identified on EEG CSA from posterior derivations. A pattern with dominant alpha bands was observed in patients with AD alone while, in those with DLB and PDD, the degree to which residual alpha and 5.6-7.9 bands appeared was related to the presence and severity of cognitive fluctuations. At follow-up, EEG abnormalities from posterior leads were seen in all subjects with DLB and in three-quarters of those with PDD. Of interest, in four patients initially labelled as having AD, in whom the occurrence of fluctuations and/or REM-sleep behaviour disorder during the 2-year follow-up had made the diagnosis of AD questionable, the initial EEG was characterized by the features observed in the DLB group. If revised consensus criteria for DLB diagnosis are properly applied (i.e. emphasizing the diagnostic weight of fluctuations and REM sleep behaviour disorder), EEG recording may act to support discrimination between AD and DLB at the earliest stages of dementia, since characteristic abnormalities may even precede the appearance of distinctive clinical features.

Accepted September 14, 1998. From the Neuropharmacology Unit, Defense and Veterans Head Injury Program, Henry M. Jackson Foundation, and National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; Veterans Administration Medical Center GRECC, Bedford, Massachusetts; and Departments of Neurology and Pathology, Boston University Medical School, Boston, Massachusetts. Address correspondence to Dr. Litvan, Neuropharmacology Unit, Defense and Veterans Head Injury Program, Henry M. Jackson Foundation, NINDS, NIH, Federal Building, Room 714, 7550 Wisconsin Avenue, Bethesda, MD 20892-9130; e-mail: litvan1@helix.nih.gov Copyright q 1999 American Psychiatric Press, Inc. Clinicopathologic Case Report

We compared the clinical and neuropsychological pattern of dementia with Lewy bodies (DLB) to Alzheimer's disease (AD) and Parkinson's disease with dementia (PD-d). Sixteen patients clinically diagnosed with DLB were compared with two groups of patients with PD-d (n = 15) and AD (n = 16) matched for level of dementia. Isolated cognitive impairment was the most common form of presentation in AD (93.8%) and DLB (31.3%) groups, while parkinsonism was in 100% of PD-d subjects. Psychoses associated with cognitive impairment at the beginning of the disease were more frequent in DLB patients (31.3%) than in AD (6.3%) and PD-d (0%) groups. There were no significant differences in Unified Parkinson Disease Rating Scale motor-subscale scores between DLB and PD-d patients. DLB and PD-d patients performed significantly worse on attentional functions and better on memory tests than AD. DLB patients also showed lower scores than AD subjects on visual memory, visuoperceptive, and visuoconstructive tests. No significant differences were found between PD-d group and DLB subjects on any neuropsychological test. We were unable to find any differences in cognitive tasks between PD-d and DLB subjects. Clinical features and neuropsychological deficiencies of DLB (attentional, visuoperceptive, and visuoconstructive deficits) and PD (attentional deficits) compared to AD (amnesic syndrome) can contribute to accurate identification of these entities and to the understanding of the neuropathological and neurochemical substrate underlying these diseases.