DNA Methylation Age Acceleration Is Not Associated with Age of Onset in Parkinson's Disease.

BACKGROUND: Epigenetic clocks using DNA methylation (DNAm) to estimate biological age have become popular tools in the study of neurodegenerative diseases. Notably, several recent reports have shown a strikingly similar inverse relationship between accelerated biological aging, as measured by DNAm, and the age of onset of several neurodegenerative disorders, including Parkinson's disease (PD). Common to all of these studies is that they were performed without control subjects and using the exact same measure of accelerated aging: DNAm age minus chronological age. OBJECTIVE: We aimed to assess the validity of these findings in PD, using the same dataset as in the original study, blood DNAm data from the Parkinson's Progression Markers Initiative cohort, but also including control samples in the analyses. METHODS: We replicated the analyses and findings of the previous study and then reanalyzed the dataset incorporating control samples to account for underlying age-related biases. RESULTS: Our reanalysis shows that there is no correlation between age of onset and DNAm age acceleration. Conversely, there is a pattern of overestimating DNAm age in younger and underestimating DNAm age in older individuals in the dataset that entirely explains the previously reported association. CONCLUSIONS: Our findings refute the previously reported inverse relationship between DNAm age acceleration and age of onset in PD. We show that these findings are fully accounted for by an expected over/underestimation of DNAm age in younger/older individuals. Furthermore, this effect is likely to be responsible for nearly identical findings reported in other neurodegenerative diseases. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Rare genetic variation in mitochondrial pathways influences the risk for Parkinson's disease.

BACKGROUND: Mitochondrial dysfunction plays a key role in PD, but the underlying molecular mechanisms remain unresolved. We hypothesized that the disruption of mitochondrial function in PD is primed by rare, protein-altering variation in nuclear genes controlling mitochondrial structure and function. OBJECTIVE: The objective of this study was to assess whether genetic variation in genes associated with mitochondrial function influences the risk of idiopathic PD. METHODS: We employed whole-exome sequencing data from 2 independent cohorts of clinically validated idiopathic PD and controls, the Norwegian ParkWest cohort (n = 411) and the North American Parkinson's Progression Markers Initiative (n = 640). We applied burden-based and variance-based collapsing methods to assess the enrichment of rare, nonsynonymous, and damaging genetic variants on genes, exome-wide, and on a comprehensive set of mitochondrial pathways, defined as groups of genes controlling specific mitochondrial functions. RESULTS: Using the sequence kernel association test, we detected a significant polygenic enrichment of rare, nonsynonymous variants in the gene-set encoding the pathway of mitochondrial DNA maintenance. Notably, this was the strongest association in both cohorts and survived multiple testing correction (ParkWest P = 6.3 × 10-3 , Parkinson's Progression Markers Initiative P = 6.9 × 10-5 , metaanalysis P = 3.2 × 10-6 ). CONCLUSIONS: Our results show that the enrichment of rare inherited variation in the pathway controlling mitochondrial DNA replication and repair influences the risk of PD. We propose that this polygenic enrichment contributes to the impairment of mitochondrial DNA homeostasis, thought to be a key mechanism in the pathogenesis of PD, and explains part of the disorder's "missing heritability." © 2018 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Familial aggregation of Parkinson's disease may affect progression of motor symptoms and dementia.

BACKGROUND: Familial aggregation has been described in PD of both early and late onset, but has not been studied in a true population-based sample. Moreover, little is known about its association with disease progression and endophenotypes. OBJECTIVES: The objectives of this work were to determine familial aggregation of idiopathic PD in a population-based cohort and study the association with clinical endophenotypes and disease progression. METHODS: We examined family history data from the Norwegian ParkWest study, a well-characterized, population-based cohort of incident PD patients and age-matched healthy controls. Family data were collected at baseline with a simplified questionnaire (192 cases and 193 controls) and after 3 years of longitudinal follow-up using an extended questionnaire (172 cases and 171 controls). RESULTS: Compared to the controls, the PD patients had an increased relative risk of having a first-degree relative with PD when using the extended questionnaire (relative risk = 1.988; P = 0.036), but not when using the simplified questionnaire (relative risk = 1.453; P = 0.224). There was no significant difference in age of onset or motor subtype (P = 0.801). However, cases with a family history of PD had reduced progression over 7 years as measured by UPDRS II (P = 0.008) and smaller rate of decrease of MMSE (P = 0.046). CONCLUSIONS: Our findings confirm familial aggregation in a population-based cohort of idiopathic PD. Moreover, we show that positive family history of PD in patients is associated with a slower progression of PD symptoms and cognitive decline. © 2016 International Parkinson and Movement Disorder Society.

Nicotinamide riboside supplementation is not associated with altered methylation homeostasis in Parkinson's disease.

Replenishing nicotinamide adenine dinucleotide (NAD) via supplementation of nicotinamide riboside (NR) has been shown to confer neuroprotective effects in models of aging and neurodegenerative diseases, including Parkinson's disease (PD). Although generally considered safe, concerns have been raised that NR supplementation could impact methylation dependent reactions, including DNA methylation, because of increased production and methylation dependent breakdown of nicotinamide (NAM). We investigated the effect of NR supplementation on DNA methylation in a double blinded, placebo-controlled trial of 29 human subjects with PD, in blood cells and muscle tissue. Our results show that NR had no impact on DNA methylation homeostasis, including individuals with common pathogenic mutations in the MTHFR gene known to affect one-carbon metabolism. Pathway and methylation variance analyses indicate that there might be minor regulatory responses to NR. We conclude that short-term therapy with high-dose NR for up to 30 days has no deleterious impact on methylation homeostasis.

Base excision repair causes age-dependent accumulation of single-stranded DNA breaks that contribute to Parkinson disease pathology.

Aging, genomic stress, and mitochondrial dysfunction are risk factors for neurodegenerative pathologies, such as Parkinson disease (PD). Although genomic instability is associated with aging and mitochondrial impairment, the underlying mechanisms are poorly understood. Here, we show that base excision repair generates genomic stress, promoting age-related neurodegeneration in a Caenorhabditis elegans PD model. A physiological level of NTH-1 DNA glycosylase mediates mitochondrial and nuclear genomic instability, which promote degeneration of dopaminergic neurons in older nematodes. Conversely, NTH-1 deficiency protects against α-synuclein-induced neurotoxicity, maintaining neuronal function with age. This apparent paradox is caused by modulation of mitochondrial transcription in NTH-1-deficient cells, and this modulation activates LMD-3, JNK-1, and SKN-1 and induces mitohormesis. The dependance of neuroprotection on mitochondrial transcription highlights the integration of BER and transcription regulation during physiological aging. Finally, whole-exome sequencing of genomic DNA from patients with idiopathic PD suggests that base excision repair might modulate susceptibility to PD in humans.

Meta-analysis of whole-exome sequencing data from two independent cohorts finds no evidence for rare variant enrichment in Parkinson disease associated loci.

Parkinson disease (PD) is a complex neurodegenerative disorder influenced by both environmental and genetic factors. While genome wide association studies have identified several susceptibility loci, many causal variants and genes underlying these associations remain undetermined. Identifying these is essential in order to gain mechanistic insight and identify biological pathways that may be targeted therapeutically. We hypothesized that gene-based enrichment of rare mutations is likely to be found within susceptibility loci for PD and may help identify causal genes. Whole-exome sequencing data from two independent cohorts were analyzed in tandem and by meta-analysis and a third cohort genotyped using the NeuroX-array was used for replication analysis. We employed collapsing methods (burden and the sequence kernel association test) to detect gene-based enrichment of rare, protein-altering variation within established PD susceptibility loci. Our analyses showed trends for three genes (GALC, PARP9 and SEC23IP), but none of these survived multiple testing correction. Our findings provide no evidence of rare mutation enrichment in genes within PD-associated loci, in our datasets. While not excluding that rare mutations in these genes may influence the risk of idiopathic PD, our results suggest that, if such effects exist, much larger sequencing datasets will be required for their detection.