Genetics of iron regulation and the possible role of iron in Parkinson's disease

F, Gögele M, Lill CM, Bertram L, Do CB, Eriksson N, Foroud T, Myers RH, Nalls M, Keller MF, Benyamin B, Whitfield JB, Pramstaller PP, Hicks AA, Thompson JR, Minelli C. Serum iron levels and the risk of Parkinson disease: a Mendelian randomization study

BACKGROUND

Although levels of iron are known to be increased in the brains of patients with Parkinson disease (PD), epidemiological evidence on a possible effect of iron blood levels on PD risk is inconclusive, with effects reported in opposite directions. Epidemiological studies suffer from problems of confounding and reverse causation, and mendelian randomization (MR) represents an alternative approach to provide unconfounded estimates of the effects of biomarkers on disease. We performed a MR study where genes known to modify iron levels were used as instruments to estimate the effect of iron on PD risk, based on estimates of the genetic effects on both iron and PD obtained from the largest sample meta-analyzed to date.

METHODS AND FINDINGS

We used as instrumental variables three genetic variants influencing iron levels, HFE rs1800562, HFE rs1799945, and TMPRSS6 rs855791. Estimates of their effect on serum iron were based on a recent genome-wide meta-analysis of 21,567 individuals, while estimates of their effect on PD risk were obtained through meta-analysis of genome-wide and candidate gene studies with 20,809 PD cases and 88,892 controls. Separate MR estimates of the effect of iron on PD were obtained for each variant and pooled by meta-analysis. We investigated heterogeneity across the three estimates as an indication of possible pleiotropy and found no evidence of it. The combined MR estimate showed a statistically significant protective effect of iron, with a relative risk reduction for PD of 3% (95% CI 1%-6%; p = 0.001) per 10 µg/dl increase in serum iron.

CONCLUSIONS

Our study suggests that increased iron levels are causally associated with a decreased risk of developing PD. Further studies are needed to understand the pathophysiological mechanism of action of serum iron on PD risk before recommendations can be made.

Imaging of iron

Magnetic resonance imaging (MRI) enables a noninvasive in vivo quantification of iron in various organs. Several techniques have been developed that detect signal alterations derived mainly from the magnetic properties of ferritin and hemosiderin, the major iron storage compounds. High magnetic susceptibility of ferritin shortens the transversal relaxation time of nearby water protons and thus induces a focal signal extinction of iron-rich areas in T2-weighted (T2w) MRI. T2w tissue contrast is additionally influenced by other factors such as water content, myelin density, and the presence of other metals. Therefore, more specific methods are needed with higher specificity to iron. These in vivo techniques can be divided into three groups: relaxometry, magnetic field correlation imaging and phase-based contrast covering susceptibility-weighted imaging, and quantitative susceptibility mapping. The differential diagnosis of various neurological disorders is aided by characteristic patterns of iron depositions. Reliable estimates of cerebral tissue iron concentration are equally important in studying physiological age-related as well as pathological conditions in neurodegenerative, neuroinflammatory, and vascular diseases. In the future, monitoring changes in iron storage and content may serve as sensitive biomarker for diagnosis as well as treatment monitoring.

Evaluation of brain iron content based on magnetic resonance imaging (MRI): comparison among phase value, R2* and magnitude signal intensity

BACKGROUND AND PURPOSE

Several magnetic resonance imaging (MRI) techniques are being exploited to measure brain iron levels increasingly as iron deposition has been implicated in some neurodegenerative diseases. However, there remains no unified evaluation of these methods as postmortem measurement isn't commonly available as the reference standard. The purpose of this study was to make a comparison among these methods and try to find a new index of brain iron.

METHODS

We measured both phase values and R2* in twenty-four adults, and performed correlation analysis among the two methods and the previously published iron concentrations. We also proposed a new method using magnitude signal intensity and compared it with R2* and brain iron.

RESULTS

We found phase value correlated with R2* in substantia nigra (r = -0.723, p<0.001) and putamen (r = -0.514, p = 0.010), while no correlations in red nucleus (r = -0.236, p = 0.268) and globus pallidus (r = -0.111, p = 0.605). And the new magnitude method had significant correlations in red nucleus (r = -0.593, p = 0.002), substantia nigra (r = -0.521, p = 0.009), globus pallidus (r = -0.750, p<0.001) and putamen (r = -0.547, p = 0.006) with R2*. A strong inverse correlation was also found between the new magnitude method and previously published iron concentrations in seven brain regions (r = -0.982, P<0.001).

CONCLUSIONS

Our study indicates that phase value may not be used for assessing the iron content in some brain regions especially globus pallidus. The new magnitude method is highly consistent with R2* especially in globus pallidus, and we assume that this approach may be acceptable as an index of iron content in iron-rich brain regions.

A mechanistic study of proteasome inhibition-induced iron misregulation in dopamine neuron degeneration

Ubiquitin proteasome system (UPS) impairment and iron misregulation have been implicated in dopamine (DA) neuron degeneration in Parkinson's disease. As previously shown, proteasome inhibition in a rodent model can cause nigral neuron degeneration accompanied by iron accumulation. To investigate the involvement of iron in DA neuron degeneration, we generated an in vitro model by applying proteasome inhibitor lactacystin in DAergic cell line MES23.5 culture. We found that lactacystin caused marked increase in labile iron, reactive oxygen species and ubiquitin-conjugated protein aggregation prior to cell injury. These effects were attenuated by iron chelators or antioxidants. Furthermore, we demonstrated that the iron regulatory protein (IRP)/iron response element system contributed to UPS impairment-mediated DA neuron injury. We documented that IRP2 disruption resulted in an increase in transferrin receptor 1 (TfR1), a decrease in ferritin heavy chain (H-Frt), and eventually cell death. These findings provide insight into the mechanistic interplay between UPS impairment and iron misregulation and suggest that the disturbances in IRP2, TfR1 and H-Frt may contribute to DA neuron degeneration.

Regulation of quinolinic acid neosynthesis in mouse, rat and human brain by iron and iron chelators in vitro

Several lines of evidence indicate that excess iron may play an etiologically significant role in neurodegenerative disorders. This idea is supported, for example, by experimental studies in animals demonstrating significant neuroprotection by iron chelation. Here, we tested whether this effect might be related to a functional link between iron and the endogenous excitotoxin quinolinic acid (QUIN), a presumed pathogen in several neurological disorders. In particular, the present in vitro study was designed to examine the effects of Fe(2+), a known co-factor of oxygenases, on the activity of QUIN's immediate biosynthetic enzyme, 3-hydroxyanthranilic acid dioxygenase (3HAO), in the brain. In crude tissue homogenate, addition of Fe(2+) (2-40 μM) stimulated 3HAO activity 4- to 6-fold in all three species tested (mouse, rat and human). The slope of the iron curve was steepest in rat brain where an increase from 6 to 14 μM resulted in a more than fivefold higher enzyme activity. In all species, the Fe(2+)-induced increase in 3HAO activity was dose-dependently attenuated by the addition of ferritin, the main iron storage protein in the brain. The effect of iron was also readily prevented by N,N'-bis(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid (HBED), a synthetic iron chelator with neuroprotective properties in vivo. All these effects were reproduced using neostriatal tissue obtained postmortem from normal individuals and patients with end-stage Huntington's disease. Our results suggest that QUIN levels and function in the mammalian brain might be tightly controlled by endogenous iron and proteins that regulate the bioavailability of iron.

The PLA2G6 gene in early-onset Parkinson's disease

BACKGROUND

The definite etiology of neurodegenerative disorders such as Parkinson's disease (PD) is still unknown. Because of its role in the generation of reactive oxygen species and its association with neurodegeneration with brain iron accumulation, a possible involvement of calcium-independent group VI phospholipase A(2) (iPLA(2)-VI) in the pathogenesis of PD has been proposed.

METHODS

In this study we analyzed all 17 exons of the PLA2G6 gene encoding iPLA(2)-VI in a group of 102 discordant pairs with early-onset Parkinson's disease (EOPD) and an additional sample of 166 EOPD patients and 155 unrelated controls.

RESULTS

The nonsynonymous single-nucleotide polymorphisms (SNPs) 2339A>G (n = 2) and 2341G>A (n = 1) in 2 neighboring codons were found in 3 patients with typical L-dopa-responsive sporadic EOPD and in none of our controls, indicating a possible role of PLA2G6 in the pathogenesis of EOPD in rare cases.

CONCLUSIONS

Future studies should investigate the prevalence of these SNPs in other PD populations and larger control groups and also address possible genetic alterations in the remaining parts of the PLA2G6 gene.

Insulin resistance impairs nigrostriatal dopamine function

Clinical studies have indicated a link between Parkinson's disease (PD) and Type 2 Diabetes. Although preclinical studies have examined the effect of high-fat feeding on dopamine function in brain reward pathways, the effect of diet on neurotransmission in the nigrostriatal pathway, which is affected in PD and parkinsonism, is less clear. We hypothesized that a high-fat diet, which models early-stage Type 2 Diabetes, would disrupt nigrostriatal dopamine function in young adult Fischer 344 rats. Rats were fed a high fat diet (60% calories from fat) or a normal chow diet for 12 weeks. High fat-fed animals were insulin resistant compared to chow-fed controls. Potassium-evoked dopamine release and dopamine clearance were measured in the striatum using in vivo electrochemistry. Dopamine release was attenuated and dopamine clearance was diminished in the high-fat diet group compared to chow-fed rats. Magnetic resonance imaging indicated increased iron deposition in the substantia nigra of the high fat group. This finding was supported by alterations in the expression of several proteins involved in iron metabolism in the substantia nigra in this group compared to chow-fed animals. The diet-induced systemic and basal ganglia-specific changes may play a role in the observed impairment of nigrostriatal dopamine function.

AMPA-receptor-mediated excitatory synaptic transmission is enhanced by iron-induced α-synuclein oligomers

Aggregated α-synuclein (α-syn) is a characteristic pathological finding in Parkinson's disease and related disorders, such as dementia with Lewy bodies. Recent evidence suggests that α-syn oligomers represent the principal neurotoxic species; however, the pathophysiological mechanisms are still not well understood. Here, we studied the neurophysiological effects of various biophysically-characterized preparations of α-syn aggregates on excitatory synaptic transmission in autaptic neuronal cultures. Nanomolar concentrations of large α-syn oligomers, generated by incubation with organic solvent and Fe(3+) ions, were found to selectivity enhance evoked α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor, but not NMDA-receptor, mediated synaptic transmission within minutes. Moreover, the analysis of spontaneous AMPA-receptor-mediated miniature synaptic currents revealed an augmented frequency. These results collectively indicate that large α-syn oligomers alter both pre- and post-synaptic mechanisms of AMPA-receptor-mediated synaptic transmission. The augmented excitatory synaptic transmission may directly contribute to nerve cell death in synucleinopathies. Indeed, already low micromolar glutamate concentrations were found to be toxic in primary cultured neurons incubated with large α-syn oligomers. In conclusion, large α-syn oligomers enhance both pre- and post-synaptic AMPA-receptor-mediated synaptic transmission, thereby aggravating intracellular calcium dyshomeostasis and contributing to excitotoxic nerve cell death in synucleinopathies.

Association study between four polymorphisms in the HFE, TF and TFR genes and Parkinson's disease in southern Italy

Iron overload may lead to neurodegenerative disorders such as Parkinson's disease (PD) and alterations of iron-related genes might be involved in the pathogenesis of this disease. The gene of haemochromatosis (HFE) encodes the HFE protein which interacts with the transferrin receptor (TFR), lowering its affinity for iron-bound transferrin (TF). We examined four known polymorphisms, C282Y and H63D in the HFE gene, G258S in the TF gene and S82G in the TFR gene, in 181 sporadic PD patients and 180 controls from Southern Italy to investigate their possible role in susceptibility to PD. No significant differences were found in genotype and allele frequencies between PD and controls for all the polymorphisms studied, suggesting that these variants do not contribute significantly to the risk of PD.

Genetic-based, differential susceptibility to paraquat neurotoxicity in mice

Paraquat (PQ) is an herbicide used extensively in agriculture. This agent is also suspected to be a risk factor for Parkinson's disease (PD) by harming nigro-striatal dopamine neurons. There is likely, genetic-based, individual variability in susceptibility to PQ neurotoxicity related PD. In this study, we measured the delivery of PQ to the brain after three weekly injections of PQ at 5 mg kg(-1), PQ-related neural toxicity after three weekly injections of PQ at 1 mg kg(-1)or 5 mg kg(-1), PQ-related iron accumulation and PQ-related gene expression in midbrain of DBA/2J (D2) and C57BL/6J (B6) inbred mouse strains after a single injection of PQ at 15 mg kg(-1) and 10 mg kg(-1), respectively. Results showed that compared to controls, PQ-treated B6 mice lost greater numbers of dopaminergic neurons in the substantia nigra pars compacta than D2 mice; however, distribution of PQ to the midbrain was equal between the strains. PQ also significantly increased iron concentration in the midbrain of B6 but not D2 mice. Microarray analysis of the ventral midbrain showed greater PQ-induced changes in gene expression in B6 compared to D2 mice. This is the first study to report genetically-based differences in susceptibility to PQ neurotoxicity and to understanding individual differences in vulnerability to PQ neurotoxicity and its relation to PD in humans.

Neurotoxic in vivo models of Parkinson's disease recent advances

Animal models have been invaluable to Parkinson's disease (PD) research. Of these, neurotoxin models have historically been the most widely utilized. The goal of this chapter is to give a brief historical description of classic PD models and then to identify the most recent important advances in modeling human PD in animals. Indeed, significant advances in modeling additional features of PD and expansion to new species have occurred in both older and newer models. The roles these new advances in modeling may have in future PD research are examined in this chapter.

Substantia nigra hyperechogenicity is a risk marker of Parkinson's disease: no

During the past two decades, transcranial sonography (TCS) has developed to an increasingly used brain imaging method that visualizes characteristic patterns of basal ganglia alterations in distinct movement disorders. Since the discovery of a characteristic abnormal hyperechogenic appearance of substantia nigra (SN) on TCS in Parkinson's disease (PD), which is stable during the course of the disease and probably present already in preclinical disease stages, the results of several studies have promoted the idea that this TCS finding in healthy subjects might be a risk marker of PD. The present view summarizes current scientific evidence favouring the idea that the TCS finding of SN hyperechogenicity alone may not be a (strong) risk marker of PD. Especially, it is discussed how reliable this TCS finding is, whether this TCS finding can be regarded as a progression marker or a risk marker of PD, how strongly it may indicate a risk of PD, what else if not an increased risk of PD could be indicated, and which role TCS of SN may finally play in the detection of subjects at risk of PD.

Decreased serum ceruloplasmin levels characteristically aggravate nigral iron deposition in Parkinson's disease

In vivo and post-mortem studies have demonstrated that increased nigral iron content in patients with Parkinson's disease is a prominent pathophysiological feature. However, the mechanism and risk factors associated with nigral iron deposition in patients with Parkinson's disease have not been identified and represent a key challenge in understanding its pathogenesis and for its diagnosis. In this study, we assessed iron levels in patients with Parkinson's disease and in age- and gender-matched control subjects by measuring phase values using magnetic resonance based susceptibility-weighted phase imaging in a 3T magnetic resonance system. Phase values were measured from brain regions including bilateral substantia nigra, globus pallidus, putamen, caudate, thalamus, red nucleus and frontal white matter of 45 patients with Parkinson's disease with decreased or normal serum ceruloplasmin levels, together with age- and gender-matched control subjects. Correlative analyses between phase values, serum ceruloplasmin levels and disease severity showed that the nigral bilateral average phase values in patients with Parkinson's disease were significantly lower than in control subjects and correlated with disease severity according to the Hoehn and Yahr Scale. The Unified Parkinson's Disease Rating Scale motor scores from the clinically most affected side were significantly correlated with the phase values of the contralateral substantia nigra. Furthermore, nigral bilateral average phase values correlated highly with the level of serum ceruloplasmin. Specifically, in the subset of patients with Parkinson's disease exhibiting reduced levels of serum ceruloplasmin, we found lowered nigral bilateral average phase values, suggesting increased nigral iron content, while those patients with normal levels of serum ceruloplasmin exhibited no changes as compared with control subjects. These findings suggest that decreased levels of serum ceruloplasmin may specifically exacerbate nigral iron deposition in patients with Parkinson's disease. Combining susceptibility-weighted phase imaging with serum ceruloplasmin determination is likely to be useful for the diagnosis and assessment of a subset of patients with Parkinson's disease.

Late-life hemoglobin and the incidence of Parkinson's disease

Brain iron promotes neurodegeneration in Parkinson's disease (PD). While hemoglobin (Hb) is the most abundant source of peripheral iron in humans, its relationship with PD is uncertain. This report examines the association between Hb in late life and PD incidence. From 1991 to 1993, Hb was measured in 3507 men in the Honolulu-Asia Aging Study. Men were aged 71-93 years and without PD. Participants were followed until 2001 for incident PD. Hb levels declined markedly with age. For men aged 71-75 years, 14.8% had levels < 14 g/dL versus 53.6% in those aged 86 and older (p < 0.001). During follow-up, 47 men developed PD (19.8/10,000 person-years). After age adjustment, PD incidence rose significantly from 10.3 to 34.9/10,000 person-years as Hb increased from < 14 to ≥ 16 g/dL (p = 0.024; relative hazard 3.2; 95% confidence interval, 1.2-8.9). Associations persisted after accounting for early mortality and adjustments for concomitant risk factors. While Hb declines with advancing age, evidence suggests that Hb that remains high in elderly men is associated with an increased risk of PD.

Val-9Ala and Ile+58Thr polymorphism of MnSOD in Parkinson's disease

OBJECTIVES

To investigate the polymorphism distribution of Val-9Ala and Ile+58Thr of the Mn-superoxide dismutase (Mn-SOD) gene among subjects with Parkinson's disease (PD) by analyses of genders and clinical severity.

DESIGN AND METHODS

We examined the DNA genotypes of Val-9Ala and Ile+58Thr from 295 PD subjects and 111 controls by nucleotide sequencing and BsaWI restriction.

RESULTS

Ala/Ala homozygosity was found in four PD subjects but not in the controls. All of the genotypes at codon +58 among the examined samples were Ile/Ile homozygotes. Although higher carrier rate of Ala allele among PD subjects than the controls, there were no differences by analyses of the genders and clinical severity.

CONCLUSION

The higher Ala-allele carrier rate among PD subjects may suggest a possible higher amount of mitochondrial Mn-SOD rendering higher intracellular stress in PD. In this study the polymorphisms at codons -9 and+58 did not give informative association evidences with PD.

Extended lifespan of Drosophila parkin mutants through sequestration of redox-active metals and enhancement of anti-oxidative pathways

The mechanisms underlying neuron death in Parkinson's disease are unknown, but both genetic defects and environmental factors are implicated in its pathogenesis. Mutations in the parkin gene lead to autosomal recessive juvenile Parkinsonism (AR-JP). Here we report that compared to control flies, Drosophila lacking parkin show significantly reduced lifespan but no difference in dopamine neuron numbers when raised on food supplemented with environmental pesticides or mitochondrial toxins. Moreover, chelation of redox-active metals, anti-oxidants and overexpression of superoxide dismutase 1 all significantly reversed the reduced longevity of parkin-deficient flies. Finally, parkin deficiency exacerbated the rough eye phenotype of Drosophila caused by overexpression of the copper importer B (Ctr1B). Taken together, our results demonstrate an important function of parkin in the protection against redox-active metals and pesticides implicated in the etiology of Parkinson's disease. They also corroborate that oxidative stress, perhaps as a consequence of mitochondrial dysfunction, is a major determinant of morbidity in parkin mutant flies.

T1rho and T2rho MRI in the evaluation of Parkinson's disease

Prior work has shown that adiabatic T(1rho) and T(2rho) relaxation time constants may have sensitivity to cellular changes and the presence of iron, respectively, in Parkinson's disease (PD). Further understanding of these magnetic resonance imaging (MRI) methods and how they relate to measures of disease severity and progression in PD is needed. Using T(1rho) and T(2rho) on a 4T MRI scanner, we assessed the substantia nigra (SN) of nine non-demented moderately affected PD and ten gender- and age-matched control participants. When compared to controls, the SN of PD subjects had increased T(1rho) and reduced T(2rho). We also found a significant correlation between asymmetric motor features and asymmetry based on T(1rho). This study provides additional validation of T(1rho) and T(2rho) as a means to separate PD from control subjects, and T(1rho) may be a useful marker of asymmetry in PD.

Analysis of settled dust with X-ray Fluorescence for exposure assessment of metals in the province of Brescia, Italy

Ferroalloy industries have been active for more than a century in the province of Brescia, Northern Italy. Air emission and water discharge have contaminated the environment in the surroundings of four plants with several metals including manganese. The presence of manganese in this region is especially interesting, because of the observed relationship between manganese exposure and Parkinsonism in a previous epidemiological survey. The aim of this study was represented by an initial screening of metal exposure in this area, using a geographic information system. X-ray fluorescence (XRF) was applied to identify heavy metals in deposited dust samples, collected in representative residential households throughout the province. The results were interpreted through a systematic mapping of all municipal districts of the Brescia province. A more frequent distribution of manganese and other metals was observed in the municipalities where the plants were located and differences in the geochemical and anthropogenic origin of metals were discussed, according to the point sources.

Systems genetics analysis of iron regulation in the brain

Iron imbalances in the brain, including excess accumulation and deficiency, are associated with neurological disease and dysfunction; yet, their origins are poorly understood. Using systems genetics analysis, we have learned that large individual differences exist in brain iron concentrations, even in the absence of neurological disease. Much of the individual differences can be tied to the genetic makeup of the individual. This genetic-based differential regulation can be modeled in genetic reference populations of rodents. The work in our laboratory centers on iron regulation in the brain and our animal model consists of 25 BXD/Ty recombinant inbred mouse strains. By studying naturally occurring variation in iron phenotypes, such as tissue iron concentration, we can tie that variability to one or more genes by way of quantitative trait loci (QTL) analysis. Moreover, we can conduct genetic correlation analyses between our phenotypes and others previously measured in the BXD/Ty strains. We have observed several suggestive QTL related to ventral midbrain iron content, including one on chromosome 17 that contains btbd9, a gene that in humans has been associated with restless legs syndrome and serum ferritin. We have also observed gene expression correlations with ventral midbrain iron, including btbd9 expression and dopamine receptor expression. In addition, we have observed significant correlations between ventral midbrain iron content and dopamine-related phenotypes. The following is a discussion of iron regulation in the brain and the contributions a systems genetics approach can make toward understanding the genetic underpinnings and relation to neurological disease.