Interaction between blood lead concentration and delta-amino-levulinic acid dehydratase gene polymorphisms increases the odds of essential tremor

Association of Diet With Essential Tremor: A Narrative Review

Essential tremor is a neurological disorder categorized by the rhythmic shaking of the upper limbs, lower limbs, neck, or head. The etiology of essential tremor is believed to be genetic variations, environmental factors, lifestyle, etc. Poor lifestyle and diet are important factors contributing to the onset of various disorders. Environment and lifestyle play a significant part in the dietary habits of an individual. Some diet components may probably be associated with the etiopathogenesis or progression of the essential tremor. Dietary habits may be a key influence on the commencement of tremors in healthy individuals. Typically, the diet of essential tremor patients is not supervised. It may also intensify the tremors in essential tremor patients. Association of the diet with the essential tremor can shed light on the root of tremor aggravating aspect and aid in diet modification in essential tremor patients. The aim of the review is to establish a relation between the diet with etiopathogenesis and the progression of essential tremor. The review includes studies providing information about essential tremor and correlating essential tremor with diet, lifestyle, environment, and genetic factors. Studies that did not provide a link to the association of essential tremor were excluded. The interpretation of the research indicated that genetic variations might be triggered due to enzymatic changes triggered by dietary patterns. Dietary components showed ambiguous, weak, strong, or no association. Essential tremor may be influenced by diet. Further research must be carried out on essential tremor patients in the nutritional domain. Physicians may monitor the diet of the essential tremor patients and record the progress of the disorder on its basis to manage the patients with essential tremor and provide better services.

Vitamin D Receptor and Binding Protein Gene Variants in Patients with Essential Tremor

Several studies have shown an association between some variants in the vitamin D receptor (VDR) and the GC vitamin D binding protein (GC) genes with the risk for Parkinson's disease or other neurological disorders. VDR rs2228570 has shown an association with essential tremor (ET) in a previous study. The aim of this study is to look for the association between several common variants in these genes and the risk for ET. We genotyped 272 patients diagnosed with familial ET and 272 age-matched controls using specific TaqMan assays for VDR rs2228570, VDR rs731236, VDR rs7975232, VDR rs739837, VDR rs78783628, GC rs7041, and GC rs4588 single nucleotide variants (SNVs). We found an association between GC rs7041 SNV and ET using recessive, codominant, and allelic models. Despite our results did not find an association between VDR rs2228570 and ET, the pooled data with those by a previous report suggest this association under recessive, codominant, and allelic models. None of the SNVs studied was related to the age at onset of tremor in ET patients. Data from the current study suggest an association between GC rs7041 and VDR rs2228570 SNVs and ET risk.

Genetic susceptibility of δ-ALAD associated with lead (Pb) intoxication: sources of exposure, preventive measures, and treatment interventions

Delta-aminolevulinic acid dehydratase (δ-ALAD) is involved in the synthesis of haem and exhibits a polymorphic nature. δ-ALAD polymorphism produces two alleles, namely δ-ALAD-1 and δ-ALAD-2, which in turn produce three different phenotypes, namely δ-ALAD1-1, δ-ALAD1-2, and δ-ALAD2-2. δ-ALAD gene is more susceptible to lead (Pb) toxicity than any other genes. Its genotype and phenotype frequencies change with respect to different geographical areas and extent of Pb exposure. The δ-ALAD-2 allele dominancy is linked with high concentration of lead in the body. It has also been thought that the δ-ALAD-2 allele can provoke Pb toxicity by producing a protein that binds more tightly with Pb than δ-ALAD-1 protein. However, few evidences suggest that δ-ALAD-2 may reduce harmful effects by increasing excretion of Pb from the body, thus producing its unavailability towards pathophysiologic alterations. However, the recent evidences have supported that the individuals who are heterozygote for the δ-ALAD-1 allele may be associated with a higher risk of long-term Pb toxicity. In this regard, the individuals who are exposed at occupational levels are among the most frequent study population. The main objective of our study was to explore the gene susceptibility associated with Pb poisoning. Moreover, this study also summarizes various sources of Pb exposure and thereafter outlined multiple strategies to minimize the Pb toxicity in order to save the exposed residential communities.

Genomic Markers for Essential Tremor

There are many reports suggesting an important role of genetic factors in the etiopathogenesis of essential tremor (ET), encouraging continuing the research for possible genetic markers. Linkage studies in families with ET have identified 4 genes/loci for familial ET, although the responsible gene(s) have not been identified. Genome-wide association studies (GWAS) described several variants in LINGO1, SLC1A2, STK32B, PPARGC1A, and CTNNA3, related with ET, but none of them have been confirmed in replication studies. In addition, the case-control association studies performed for candidate variants have not convincingly linked any gene with the risk for ET. Exome studies described the association of several genes with familial ET (FUS, HTRA2, TENM4, SORT1, SCN11A, NOTCH2NLC, NOS3, KCNS2, HAPLN4, USP46, CACNA1G, SLIT3, CCDC183, MMP10, and GPR151), but they were found only in singular families and, again, not found in other families or other populations, suggesting that some can be private polymorphisms. The search for responsible genes for ET is still ongoing.

Genetic Risk Factors for Essential Tremor: A Review

Highlights

In the current review, we thoroughly reviewed 74 identified articles regarding genes and genetic loci that confer susceptibility to ET. Over 50 genes/genetic loci have been examined for possible association with ET, but consistent results failed to be reported raising the need for collaborative multiethnic studies.

Background:

Essential tremor (ET) is a common movement disorder, which is mainly characterized by bilateral tremor (postural and/or kinetic) in the upper limbs, with other parts of the body possibly involved. While the pathophysiology of ET is still unclear, there is accumulating evidence indicating that genetic variability may be heavily involved in ET pathogenesis. This review focuses on the role of genetic risk factors in ET susceptibility.

Methods:

The PubMed database was searched for articles written in English, for studies with humans with ET, controls without ET, and genetic variants. The terms “essential tremor” and “polymorphism” (as free words) were used during search. We also performed meta-analyses for the most examined genetic variants.

Results:

Seventy four articles concerning LINGO1, LINGO2, LINGO4, SLC1A2, STK32B, PPARGC1A, CTNNA3, DRD3, ALAD, VDR, HMOX1, HMOX2, LRRK1,LRRK2, GBA, SNCA, MAPT, FUS, CYPsIL17A, IL1B, NOS1, ADH1B, TREM2, RIT2, HNMT, MTHFR, PPP2R2B, GSTP1, PON1, GABA receptors and GABA transporter, HS1BP3, ADH2, hSKCa3 and CACNL1A4 genes, and ETM genetic loci were included in the current review. Results from meta-analyses revealed a marginal association for the STK32B rs10937625 and a marginal trend for association (in sensitivity analysis) for the LINGO1 rs9652490, with ET.

Discussion:

Quite a few variants have been examined for their possible association with ET. LINGO1 rs9652490 and STK32B rs10937625 appear to influence, to some extent, ET susceptibility. However, the conflicting results and the lack of replication for many candidate genes raise the need for collaborative multiethnic studies.

Childhood Lead Exposure and Adult Neurodegenerative Disease

Millions of Americans now entering midlife and old age were exposed to high levels of lead, a neurotoxin, as children. Evidence from animal-model and human observational studies suggest that childhood lead exposure may raise the risk of adult neurodegenerative disease, particularly dementia, through a variety of possible mechanisms including epigenetic modification, delayed cardiovascular and kidney disease, direct degenerative CNS injury from lead remobilized from bone, and lowered neural and cognitive reserve. Within the next ten years, the generation of children with the highest historical lead exposures, those born in the 1960s, 1970s, and 1980s, will begin to enter the age at which dementia symptoms tend to emerge. Many will also enter the age in which lead stored in the skeleton may be remobilized at greater rates, particularly for women entering menopause and men and women experiencing osteoporosis. Should childhood lead exposure prove pro-degenerative, the next twenty years will provide the last opportunities for possible early intervention to forestall greater degenerative disease burden across the aging lead-exposed population. More evidence is needed now to characterize the nature and magnitude of the degenerative risks facing adults exposed to lead as children and to identify interventions to limit long-term harm.

Delta-amino-levulinic acid dehydratase gene and essential tremor

BACKGROUND

Several reports found a relationship between increased serum lead levels and the risk for essential tremor (ET), especially in carriers of the minor allele of the single nucleotide polymorphism (SNP) rs1800435 in the aminolevulinate dehydratase (ALAD) gene, which is involved in the synthesis of haem groups. Our group reported decreased risk for ET in carriers of the minor alleles of the rs2071746 and rs1051308 SNPs in the haem-oxygenases 1 and 2 (HMOX1 and HMOX2), respectively, involved in haem metabolism. We analysed whether ALAD rs1800435 alone and their interactions with the four common SNPs in the HMOX1 and HMOX2 genes are associated with the risk for ET.

MATERIALS AND METHODS

We analysed the genotype and allele variants frequencies of ALAD rs1800435 in 202 patients with familial ET and 218 healthy controls using a TaqMan method. We also analysed the role of the interaction between ALAD rs1800435 and the HMOX1 rs2071746, HMOX1 rs2071747, HMOX2 rs2270363 and HMOX2 rs1051308 with the risk of developing ET.

RESULTS

The frequencies of genotype and allelic variants of ALAD rs1800435 did not differ significantly between patients with ET and controls, and were not influenced by gender. Subjects carrying the ALAD rs1800435CC genotype (wild-type) and the HMOX2 rs1051308GG genotype or the HMOX2 rs1051308G allele had significantly decreased risk for ET.

CONCLUSIONS

These results suggest that the ALAD rs1800435 SNP is not related with the risk for ET, but its interaction with the HMOX2 rs1051308 SNP could be weakly associated with the risk for this disease.

Heme Oxygenase 1 and 2 Common Genetic Variants and Risk for Essential Tremor

Several reports suggested a role of heme oxygenase genes 1 and 2 (HMOX1 and HMOX2) in modifying the risk to develop Parkinson disease (PD). Because essential tremor (ET) and PD share phenotypical and, probably, etiologic factors of the similarities, we analyzed whether such genes are related with the risk to develop ET. We analyzed the distribution of allelic and genotype frequencies of the HMOX1 rs2071746, HMOX1 rs2071747, HMOX2 rs2270363, and HMOX2 rs1051308 single nucleotide polymorphisms, as well as the presence of copy number variations of these genes in 202 subjects with familial ET and 747 healthy controls. Allelic frequencies of rs2071746T and rs1051308G were significantly lower in ET patients than in controls. None of the studied polymorphisms influenced the disease onset. The present study suggests a weak association between HMOX1 rs2071746 and HMOX2 rs1051308 polymorphisms and the risk to develop ET in the Spanish population.

Lead exposure and tremor among older men: the VA normative aging study

BACKGROUND

Tremor is one of the most common neurological signs, yet its etiology is poorly understood. Case-control studies suggest an association between blood lead and essential tremor, and that this association is modified by polymorphisms in the δ-aminolevulinic acid dehydrogenase (ALAD) gene.

OBJECTIVE

We aimed to examine the relationship between lead and tremor, including modification by ALAD, in a prospective cohort study, using both blood lead and bone lead-a biomarker of cumulative lead exposure.

METHODS

We measured tibia (n = 670) and patella (n = 672) bone lead and blood lead (n = 807) among older men (age range, 50-98 years) in the VA Normative Aging Study cohort. A tremor score was created based on an approach using hand-drawing samples. ALAD genotype was dichotomized as ALAD-2 carriers or not. We used linear regression adjusted for age, education, smoking, and alcohol intake to estimate the associations between lead biomarkers and tremor score.

RESULTS

In unadjusted analyses, there was a marginal association between quintiles of all lead biomarkers and tremor scores (p-values < 0.13), which did not persist in adjusted models. Age was the strongest predictor of tremor. Among those younger than the median age (68.9 years), tremor increased significantly with blood lead (p = 0.03), but this pattern was not apparent for bone lead. We did not see modification by ALAD or an association between bone lead and change in tremor score over time.

CONCLUSION

Our results do not strongly support an association between lead exposure and tremor, and suggest no association with cumulative lead biomarkers, although there is some suggestion that blood lead may be associated with tremor among the younger men in our cohort.

Serious head trauma preceding essential tremor: A population-based study (NEDICES)

BACKGROUND

An association between head trauma and essential tremor (ET) has not been formally assessed. Our purpose was to assess the association between serious head trauma and ET.

METHODS

History of head trauma was assessed in 274 ET cases and 3201 controls in the Neurological Disorders in Central Spain (NEDICES) study, a population-based study in central Spain. Head trauma was defined as serious only if the subject reported that the trauma resulted in loss of consciousness, hospitalization, or a visit to the emergency department.

RESULTS

Thirty-two (11.7%) of 274 ET cases vs. 260 (8.1%) of 3201 controls reported a history of serious head trauma (p=0.04). In an adjusted logistic regression analysis, participants who reported serious head trauma were 52% more likely to have ET (odds ratio [OR] 1.52; 95% confidence interval [CI] 1.02-2.27, p=0.04). The odds of ET increased with increasing age of head trauma: adjusted ORs=1.23 (95% CI 0.43-3.46, p = 0.70) for serious head trauma that occurred before age 18 years, adjusted OR=1.49 (95% CI 0.62-3.55, p = 0.37) for serious head trauma between ages 18 to 39 years, and adjusted OR=1.61 (95% CI 1.00-2.57, p = 0.04) for serious head trauma at age 40 years or older.

CONCLUSIONS

Our study suggests that serious head trauma, especially when it occurs after 40 years, is associated with increased odds of ET. Additional studies are needed to reproduce this novel finding.

Tremor secondary to neurotoxic exposure: mercury, lead, solvents, pesticides

Lead, mercury, solvents, and pesticide exposures are common in certain occupations and may cause nervous system dysfunction. Tremors may be the herald manifestation among a constellation of acute toxicity signs and symptoms. However, since tremors may also be the only sign on clinical presentation and since tremors also occur in other diseases, relating tremors to a specific occupational exposure can be challenging. Diagnosis of tremor etiology must be based on other findings on physical exam, laboratory results, and/or imaging. Discerning whether the tremor resulted from the occupational environment versus other etiologies requires knowledge of potential exposure sources, additional detail in history taking, and support of other health and industrial professionals. Reduction or removal from the exposure source remains the key first step in treating patients suffering from tremor that had resulted from occupational exposure toxicity.

Semi-quantitative data on ethanol consumption in 354 ET cases and 370 controls

The notion that there is an association between essential tremor (ET) and higher ethanol consumption has crept into the literature; however, the data are limited and conflicted. A total of 354 ET cases and 370 matched controls were enrolled in a clinical-epidemiological study. Average current daily ethanol consumption was estimated using the Willett Semi-quantitative Food Frequency Questionnaire. The proportion of cases and controls who drank any ethanol was similar: 66.7% vs. 64.1%, p=0.46, as was the proportion who reported heavy ethanol consumption: 4.0% vs. 3.5%, p=0.74. The average daily ethanol intake was numerically higher in cases than controls (7.99 ± 12.39 [median=3.03] vs. 6.55 ± 10.62 [median=1.80] g), but this difference did not reach significance (p=0.15). Among cases, there was no correlation between average daily ethanol intake and tremor severity (r=0.008, p=0.88). These data, on more than 700 enrollees, do not support any sizable differences between ET cases and controls in terms of average daily ethanol consumption or ethanol overuse. The absence of a correlation in cases between ethanol consumption and tremor severity goes against the hypothesis that ET patients are self-medicating to a significant degree.

Lead exposure: a summary of global studies and the need for new studies from Saudi Arabia

Lead poisoning (plumbism) can cause irreversible genetic and reproductive toxicity, hematological effects, neurological damage, and cardiovascular effects. Despite many efforts to minimize lead poisoning, it continues to be a major health concern in many developing and developed countries. Despite efforts to control lead exposure and toxicity, serious cases of lead poisoning increasingly occur as a result of higher vehicular traffic and industrialization. The biomarkers for identification of genetic susceptibility to a particular disease are useful to identify individuals who are at risk for lead poisoning. Although many such studies have been taken up elsewhere, very few studies were performed in Saudi Arabia to assess susceptibility to lead poisoning. This indicates an urgent need for testing of susceptible individuals. The present paper was planned to understand the genetic susceptibility to lead toxicity in the various population studies conducted worldwide and also to correlate it with the current scenario in Saudi Arabia. Such studies are necessary for appropriate precautions in terms of diet and avoiding exposure to be used in order to prevent adverse health effects.

Update on genetics of essential tremor

Despite the research, few advances in the etiopathogenesis on essential tremor (ET) have been made to date. The high frequency of positive family history of ET and the observed high concordance rates in monozygotic compared with dizygotic twins support a major role of genetic factors in the development of ET. In addition, a possible role of environmental factors has been suggested in the etiology of ET (at least in non-familial forms). Although several gene variants in the LINGO1 gene may increase the risk of ET, to date no causative mutated genes have been identified. In this review, we summarize the studies performed on families with tremor, twin studies, linkage studies, case-control association studies, and exome sequencing in familial ET.

Sleep in essential tremor: a comparison with normal controls and Parkinson's disease patients

BACKGROUND

Recent studies have shed light on non-motor features of ET, such as depressive symptoms and cognitive changes, which might be attributed to pathophysiological changes in the brains of ET patients. Given these brain changes, we explored sleep abnormalities in ET patients.

METHODS

Sleep was assessed using the Epworth Sleepiness Scale (ESS) and the Pittsburgh Sleep Quality Index (PSQI) in 120 ET cases, 120 normal controls, and 40 PD cases.

RESULTS

The mean±SD (median) ESS score increased from normal controls (5.7±3.7 (5.0)), to ET cases (6.8±4.6 (6.0)), to PD cases (7.8±4.9 (7.0)), test for trend p=0.03. An ESS score >10 (an indicator of greater than normal levels of daytime sleepiness) was observed in 11 (9.2%) normal controls, compared to 27 (22.5%) ET cases and 10 (25.0%) PD cases (p=0.008 when comparing all three groups, and p=0.005 when comparing ET to normal controls). The global PSQI score was 7.8±2.8 (7.5) in controls, 8.0±3.3 (8.0) in ET cases, and 9.9±3.9 (10.0) in PD cases. The ET case-control difference was not significant (p=0.8), yet in a test for trend, PD cases had the highest PSQI score (most daytime sleepiness), followed by ET (intermediate), and lowest scores in controls (p=0.02).

CONCLUSIONS

Some sleep scores in ET were intermediate between those of PD cases and normal controls, suggesting that a mild form of sleep dysregulation could be present in ET.

Comprehensive analysis of 5-aminolevulinic acid dehydrogenase (ALAD) variants and renal cell carcinoma risk among individuals exposed to lead

Background

Epidemiologic studies are reporting associations between lead exposure and human cancers. A polymorphism in the 5-aminolevulinic acid dehydratase (ALAD) gene affects lead toxicokinetics and may modify the adverse effects of lead.

Methods

The objective of this study was to evaluate single-nucleotide polymorphisms (SNPs) tagging the ALAD region among renal cancer cases and controls to determine whether genetic variation alters the relationship between lead and renal cancer. Occupational exposure to lead and risk of cancer was examined in a case-control study of renal cell carcinoma (RCC). Comprehensive analysis of variation across the ALAD gene was assessed using a tagging SNP approach among 987 cases and 1298 controls. Occupational lead exposure was estimated using questionnaire-based exposure assessment and expert review. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using logistic regression.

Results

The adjusted risk associated with the ALAD variant rs8177796^CT/TT was increased (OR = 1.35, 95%CI = 1.05–1.73, p-value = 0.02) when compared to the major allele, regardless of lead exposure. Joint effects of lead and ALAD rs2761016 suggest an increased RCC risk for the homozygous wild-type and heterozygous alleles (^GGOR = 2.68, 95%CI = 1.17–6.12, p = 0.01; ^GAOR = 1.79, 95%CI = 1.06–3.04 with an interaction approaching significance (p_int = 0.06).. No significant modification in RCC risk was observed for the functional variant rs1800435^(K68N). Haplotype analysis identified a region associated with risk supporting tagging SNP results.

Conclusion

A common genetic variation in ALAD may alter the risk of RCC overall, and among individuals occupationally exposed to lead. Further work in larger exposed populations is warranted to determine if ALAD modifies RCC risk associated with lead exposure.

Essential tremor

Essential tremor (ET) is among the most common neurological disorders. This chapter reviews the epidemiology, clinical features, and pathophysiology of ET. The defining feature is a kinetic tremor of the arms. Patients often have a postural tremor as well. Other body regions may also be involved (especially the head). The severity of tremor may range from mild cases in population settings to more severe cases in treatment settings. Motor features aside from tremor have been described in ET, including tandem gait difficulty. Mild cognitive changes (especially executive dysfunction) have been documented in many studies as well. Despite being regarded as one of the most common hyperkinetic movement disorders, establishing a precise prevalence has been difficult, yet the prevalence among persons aged 40 and older seems to be 4% or higher. There are numerous examples of families in which the disease appears to be inherited yet genetic studies have not progressed to the point where ET genes have been identified. There is also a growing understanding that environmental factors are likely to contribute to the etiology of ET. More recent postmortem studies have helped localize the possible source of ET to structural alterations in the cerebellum and its connecting pathways.

Relationship between blood harmane and harmine concentrations in familial essential tremor, sporadic essential tremor and controls

INTRODUCTION

Harmane, a potent tremor-producing β-carboline alkaloid, may play a role in the etiology of essential tremor (ET). Blood harmane concentrations are elevated in ET cases compared with controls yet the basis for this elevation remains unknown. Decreased metabolic conversion (harmane to harmine) is one possible explanation. Using a sample of >500 individuals, we hypothesized that defective metabolic conversion of harmane to harmine might underlie the observed elevated harmane concentration in ET, and therefore expected to find a higher harmane to harmine ratio in familial ET than in sporadic ET or controls.

METHODS

Blood harmane and harmine concentrations were quantified by high performance liquid chromatography.

RESULTS

There were 78 familial ET cases, 187 sporadic ET cases, and 276 controls. Blood harmane and harmine concentrations were correlated with one another (Spearman's r=0.24, p<0.001). The mean (±SD) harmane/harmine ratio=23.4±90.9 (range=0.1-987.5). The harmane/harmine ratio was highest in familial ET (46.7±140.4), intermediate in sporadic ET (28.3±108.1), and lowest in controls (13.5±50.3) (p=0.03). In familial ET cases, there was no association between this ratio and tremor severity (Spearman's r=0.08, p=0.48) or tremor duration (Spearman's r=0.14, p=0.24).

CONCLUSION

The basis for the elevated blood harmane concentration, particularly in familial ET, is not known, although the current findings (highest harmane/harmine ratio in familial ET cases) lends support to the possibility that it could be the result of a genetically-driven reduction in harmane metabolism.

Interaction of the delta-aminolevulinic acid dehydratase polymorphism and lead burden on cognitive function: the VA normative aging study

OBJECTIVE

We evaluated the modifying influence of a delta-aminolevulinic acid dehydratase (ALAD) polymorphism on the relation between lead burden and cognition among older men.

METHODS

Information on ALAD genotype, lead measurements, potential confounders, and cognitive testing was collected from 982 participants. For each cognitive test and lead biomarker, we fit separate multiple linear regression models, which included an interaction term for ALAD genotype and the lead biomarker and adjusted for potential confounders.

RESULTS

With higher levels of tibia lead, ALAD 1-2/2-2 carriers exhibited worse performance on a spatial copying test in comparison with ALAD 1-1 carriers (P interaction = 0.03). However, there was no consistent pattern of an ALAD genotype-lead interaction for the other tests.

CONCLUSIONS

The results provide some evidence that ALAD genotype may modify the relation between lead and cognition among older men with low lead burden. However, future work in this area is needed to confirm these suggestive findings.

Environmental epidemiology of essential tremor

BACKGROUND

Essential tremor (ET) is one of the most common neurological disorders. Despite this, the disease mechanisms and etiology are not well understood. While susceptibility genotypes undoubtedly underlie many ET cases, no ET genes have been identified thus far. As with many other progressive, degenerative neurological disorders, it is likely that environmental factors contribute to the etiology of ET. Environmental epidemiology is the study in specific populations or communities of the effect on human health of physical, biologic and chemical factors in the external environment. The purpose of this article is to review current knowledge with regards to the environmental epidemiology of ET.

RESULTS

As will be discussed, a series of preliminary case-control studies in recent years has begun to explore several candidate toxins/exposures, including harmane (1-methyl-9H-pyrido[3,4-b]indole), lead and agricultural exposures/pesticides.

CONCLUSIONS

While several initial results are promising, as will be discussed, additional studies are needed to more definitively establish whether these exposures are associated with ET and if they are of etiological importance.

Dietary epidemiology of essential tremor: meat consumption and meat cooking practices

BACKGROUND/AIM

Harmane [1-methyl-9H-pyrido(3,4-b)indole] is a tremor-producing neurotoxin. Blood harmane concentrations are elevated in essential tremor (ET) patients for unclear reasons. Potential mechanisms include increased dietary harmane intake (especially through well-cooked meat) or genetic-metabolic factors. We tested the hypothesis that meat consumption and level of meat doneness are higher in ET cases than in controls.

METHODS

Detailed data were collected using the Lawrence Livermore National Laboratory Meat Questionnaire.

RESULTS

Total current meat consumption was greater in men with than without ET (135.3 +/- 71.1 vs. 110.6 +/- 80.4 g/day, p = 0.03) but not in women with versus without ET (80.6 +/- 50.0 vs. 79.3 +/- 51.0 g/day, p = 0.76). In an adjusted logistic regression analysis in males, higher total current meat consumption was associated with ET (OR = 1.006, p = 0.04, i.e., with 10 additional g/day of meat, odds of ET increased by 6%). Male cases had higher odds of being in the highest than lowest quartile of total current meat consumption (adjusted OR = 21.36, p = 0.001). Meat doneness level was similar in cases and controls.

CONCLUSION

This study provides evidence of a dietary difference between male ET cases and male controls. The etiological ramifications of these results warrant additional investigation.

Heritable essential tremor-idiopathic normal pressure hydrocephalus (ETINPH)

In this report, we identified a large five-generation distinctive kindred with essential tremor (ET) presenting during the teen years and the consequent appearance of idiopathic normal pressure hydrocephalus (iNPH) when elderly (>65 years), in an autosomal dominant fashion. Through clinical and genetic analysis, we defined this kindred as a new essential tremor-idiopathic normal pressure hydrocephalus (ETINPH) disorder. One of the most common neurological disorders, ET comprises uncontrollable tremor, most commonly the upper limbs. Molecular genetic studies in hereditary ET have been initiated, but only with negative results so far. iNPH is an adult-onset hydrocephalus characterized by ventricular enlargement in the absence of significant elevations of intracranial pressure. iNPH patients usually have a triad of clinical symptoms: gait impairment, incontinence, and dementia, which is among the most common medical problems in the older population. The genetic etiology of iNPH is totally unknown. We hypothesize that ET is the consequence of the abnormal function of a specific neuronal gene, and that the same gene causes tremor at an early age eventually leading to the development of iNPH later in life. An understanding of the genetic components of this disorder may offer us significant insights into the molecular pathogenesis of ET, iNPH, and other related neurological conditions. In our genetic analysis of this family, array-based comparative genomic hybridization (aCGH) was carried out, and we could not identify any possible copy number changes of the genomic fragment along the whole-genome in ETINPH patient. Candidate gene linkage analysis was also performed, and we excluded this disorder from several established loci associated with tremor. We conclude that the pedigree reported here is a new autosomal dominant genetic disorder ETINPH. The characterization of the gene that causes ETINPH will certainly enhance our understanding of motor diseases in general.

Late neurodevelopmental effects of early exposures to chemical contaminants: reducing uncertainty in epidemiological studies

Early exposures to environmental chemicals are reliably associated with late neurotoxicities in children. However, substantial scatter of observations exists around the estimated dose-effect relationships. This variability has many potential sources, one of which is interindividual differences in susceptibility. Such differences imply that the long-term impacts of exposure will not the same for all individuals, but will vary depending on a variety of factors that might either aggravate or mitigate contaminant effects. These include co-exposures, genetic polymorphisms and characteristics of the social environment. The context dependence of contaminant effects has implications both for study designs and analytical approaches. In addition, a systems approach to understanding the associations among contaminant exposures, covariates and health outcomes is necessary.

Lead burden and psychiatric symptoms and the modifying influence of the delta-aminolevulinic acid dehydratase (ALAD) polymorphism: the VA Normative Aging Study

The authors evaluated the association between lead burden and psychiatric symptoms and its potential modification by a delta-aminolevulinic acid dehydratase (ALAD) polymorphism. Lead measurements in blood or bone and self-reported ratings on the Brief Symptom Inventory from 1991 to 2002 were available for 1,075 US men participating in the Department of Veterans Affairs (VA) Normative Aging Study. The authors estimated the prevalence odds ratio for the association between interquartile-range lead and abnormal symptom score, adjusting for potential confounders. An interquartile increment in tibia lead (14 microg/g) was associated with 21% higher odds of somatization (95% confidence interval of the odds ratio: 1.01, 1.46). An interquartile increment in patella lead (20 microg/g) corresponded to a 23% increase in the odds of global distress (95% confidence interval of the odds ratio: 1.02, 1.47). An interquartile increment in blood lead (2.8 microg/dl) was associated with 14% higher odds of hostility (95% confidence interval of the odds ratio: 1.02, 1.27). In all other analyses, lead was nonsignificantly associated with psychiatric symptoms. The adverse association of lead with abnormal mood scores was generally stronger among ALAD 1-1 carriers than 1-2/2-2 carriers, particularly regarding phobic anxiety symptoms (p(interaction) = 0.004). These results augment evidence of a deleterious association between lead and psychiatric symptoms.

Association between delta-aminolevulinic acid dehydratase (ALAD) polymorphism and blood lead levels: a meta-regression analysis

A meta-regression analysis was undertaken to evaluate the association between delta-aminolevulinic acid dehydratase (ALAD) genotypes and blood lead levels obtained from data published in various journals. In total, 15 studies were included in the final analysis. Both fixed effects and random effects models were used to undertake the pooled analysis. Using a fixed effects model, pooled estimates of mean differences of various ALAD genotypes was significant at 0.61 microg/dl. Using a random effects model, the pooled estimate was also significant at 1.51 microg/dl. Data indicated that certain ALAD genotypes may affect the susceptibility of humans to lead.

Does age of onset in essential tremor have a bimodal distribution? Data from a tertiary referral setting and a population-based study

BACKGROUND/AIMS

The distribution of age of onset of essential tremor (ET) is unclear, with discrepancies in the literature. Some data suggest a bimodal distribution and other data 1 late-life peak. We studied age of ET onset in 2 distinct settings: a population-based study and a tertiary referral center.

METHODS

Age of onset data were collected.

RESULTS

In the population, there was only a small peak at the age of <or=30 years (14.1% of cases) but a clear peak in later life (85.9% of cases). In the tertiary referral center, a bimodal distribution was apparent with 1 large peak (42.2% of cases) at the age of <or=40 years and the second large peak (57.8% of cases) in later life. Familial cases accounted for only 52.6% of young-onset cases from the population, yet 82.7% from the tertiary center.

DISCUSSION

In the population-based study, a peak in later life was clearly present but a young-onset peak was barely discernable, comprising few cases. By contrast, in a tertiary referral center, age of onset was clearly bimodal. While age of ET onset is often said to be bimodal, this may be due to the preferential referral to tertiary centers of patients with young-onset, familial ET.

Elevated blood lead concentrations in essential tremor: a case-control study in Mersin, Turkey

BACKGROUND

Essential tremor (ET) is one of the most common neurologic disorders. Aside from underlying susceptibility genes, recent studies have also begun to focus on environmental toxic factors. Yet there remains a paucity of information on such factors, making studies of environmental factors important. A recent study in New York City found blood lead concentrations to be elevated in ET cases compared with matched controls. Chronic exposure to lead produces cerebellar damage, and this could predispose individuals to develop ET.

OBJECTIVE

The aim of this study was to determine whether the elevation in blood lead concentrations observed in a single study in New York was similarly present in ET cases sampled from a completely different geographic region.

METHODS

Blood lead concentrations were measured in 105 ET cases and 105 controls at Mersin University, Mersin, Turkey.

RESULTS

The median blood lead concentration was 2.7 microg/dL in ET cases compared with 1.5 microg/dL in controls (p < 0.001). In an unadjusted logistic regression model, blood lead concentration was associated with diagnosis: odds ratio (OR) = 4.01; 95% confidence interval (CI), 2.53-6.37; p < 0.001 (i.e., each 1-microg/dL increase in blood lead concentration was associated with a 4-fold increased odds of ET). This association was more robust when cases were compared with a subsample of controls who did not share the same home environment (OR = 8.13; 95% CI, 3.05-21.65; p < 0.001). In adjusted models, results were similar.

CONCLUSIONS

These data replicate those of a previous study in New York and demonstrate an association between the environmental toxicant lead and a common neurologic disorder.

Lead neurotoxicity in children: decomposing the variability in dose-effect relationships

BACKGROUND

Enormous progress has been made in recent decades in our understanding of lead neurotoxicology in children, but an important obstacle to additional progress is the striking variability that is evident in any plot of a lead biomarker versus a health endpoint.

METHODS

In this article, three potential sources of variability are identified: (1) errors or imprecision in characterizing dose (and/or outcome); (2) incomplete characterization of endpoint variance attributable to factors other than lead; and (3) inter-individual differences in susceptibility to lead neurotoxicity.

RESULTS

Strategies are suggested for reducing the variability contributed by these sources, including the development of validated PBPK models and biomarkers of early biological effects; the development of more comprehensive models of outcome variance and, specifically, the application of multi-level models that incorporate supra-individual and supra-family risk factors; and the use of study designs that permit assessments of the effect modifying influence of contextual factors on the form and severity of neurotoxicity.

CONCLUSION

Decomposing the variability in the distribution of observed scores around the best-fit lines that describe the dose-effect relationships for lead neurotoxicity in children is a major research need.

Update on pathogenesis and treatment of essential tremor

PURPOSE OF REVIEW

This review focuses on recent findings on the aetiological, clinical, pathological and genetic heterogeneity of essential tremor and new therapeutic approaches.

RECENT FINDINGS

Although essential tremor is one of the most common movement disorders, understanding of the causes and mechanisms of the disease is still very limited. Studies on the clinical presentation of essential tremor have expanded the clinical dimension, now including nontremor manifestations such as cerebellar signs, neuropsychological characteristics, distinct personality traits and behavioural symptoms. Results of neuropathologic and imaging studies are conflicting, with hints of neurodegeneration or a nondegenerative disturbance of functional circuits or receptors. Genetic heterogeneity of essential tremor has been demonstrated by linkage to three different chromosomal loci so far, and several negative genetic studies. New animal models are reinforcing previous hypotheses about gamma-aminobutyrate (GABA)-ergic mechanisms in essential tremor. New therapeutic agents for essential tremor have been tested and demonstrated to be partly effective.

SUMMARY

The traditional view of essential tremor as a single disease entity has been replaced with the concept that this disorder is a complex and heterogeneous disease. Heterogeneity of the condition, and lack of diagnostic criteria and objective diagnostic tests add to this problem. Many conflicting results may be due to differences in patient selection.

ICD-9 CM code 333.1 as an identifier of patients with essential tremor: a study of the positive predictive value of this code

BACKGROUND/AIMS

Health outcomes research often uses administrative databases. Patients with the diseases of interest are identified using International Classification of Diseases (ICD-9 CM) codes. The utility of the code for essential tremor (ET), 333.1, remains untested. We determined the positive predictive value (PPV) of the code 333.1.

METHODS

Patients with the ICD-9 CM code 333.1 were identified from billing records at the Neurological Institute of New York. Their medical records were reviewed to determine whether they met Consensus Criteria for ET.

RESULTS

Of 964 patients who carried the code 333.1, only 472 met diagnostic criteria for ET (i.e. PPV = 49.0%). The additional use of ICD 9-CM codes for parkinsonism and dystonia (as exclusionary criteria) only marginally improved this value (57.8%). Common diagnoses among the false positives were Parkinson's disease, dystonia, enhanced physiological tremor, drug-induced tremor, orthostatic tremor, and psychogenic tremor. Patients seen by general neurologists (vs. movement disorder specialists) were half as likely to meet diagnostic criteria for ET (34.6 vs. 51.9%, OR 0.50, 95% CI 0.23-0.70, p < 0.001).

CONCLUSION

The code 333.1 performed poorly when attempting to identify ET cases. Given the very high prevalence of ET, a unique diagnostic code would seem to be in order.

Essential tremor: emerging views of a common disorder

Essential tremor (ET) is the most prevalent tremor disorder. ET has traditionally been viewed as a monosymptomatic disorder characterized by a kinetic arm tremor, but this definition is gradually being replaced. The clinical spectrum has come to include several motor features, including tremor and ataxia, and several non-motor features, including possible cognitive impairment and personality disturbances. Postmortem studies are revealing several different patterns of pathology. The emerging view is that ET might be a family of diseases, unified by the presence of kinetic tremor, but further characterized by etiological, clinical and pathological heterogeneity. Effective pharmacological treatments for the disorder remain limited, although new insights into disease mechanisms might result in more-effective therapies. In addition, recent investigations of environmental toxicants that might be linked to ET open the way towards primary disease prevention through a reduction in exposure to these factors.

Interest in participating in clinical research: A study of essential tremor patients

Enrolling essential tremor (ET) patients in clinical research can be challenging. Investigators can maximize recruitment by targeting patient subgroups with greater interest in participation. Nothing has been published on factors that are associated with higher levels of interest in participation. The objective of this study was to identify factors associated with higher levels of interest in participating in clinical research on ET. A total of 149 ET patients were questioned about level of interest in participating in future research. Two questions were used, although one was of primary interest. Interest was rated from 0 to 10 (maximal). Data were collected on demographic factors, family history, and tremor-related disability. Tremor severity was assessed. The mean level of interest was 8.0 +/- 2.3. Level of interest was not related to age of tremor onset, tremor duration, tremor severity, extent of tremor-related disability, or use of tremor medication. Level of interest was related to family history of tremor (P < 0.05), concern that other family members might develop tremor (P < 0.05), >2 versus 0 live births in women (P < 0.05), the view that the tremor worsens with age (P < 0.05), and presence of head tremor (P = 0.05). A variety of factors were identified that were associated with greater interest in participating in clinical research. These observations should be assessed in additional patient samples. Investigators may use our observations to identify and target patients for clinical trials and other research.

Delta-aminolevulinic acid dehydratase polymorphism and the relation between low level lead exposure and the Mini-Mental Status Examination in older men: the Normative Aging Study

OBJECTIVE

To determine whether a polymorphism the in delta-aminolevulinic acid dehydratase (ALAD) gene modifies the neurotoxicity of lead in older adults.

METHODS

The authors studied men participating in the Department of Veterans Affairs' Normative Aging Study, assessing their recent exposure to lead by measuring blood lead (n = 915) at each triennial clinic visit, and, beginning in 1991, assessing their cumulative exposure by measuring lead levels in tibia (n = 722) and patella (n = 720), using K-shell x ray fluorescence. Starting in 1993 and again at each triennial visit, the authors administered the Mini-Mental State Examination (MMSE) to assess their cognitive functioning. The relation of the lead biomarkers to MMSE score was evaluated and this association was compared among men who carried the variant allele, ALAD-2, versus men without the allele.

RESULTS

Sixteen per cent of men carried the ALAD-2 allele. Median tibia and patella lead levels (first-third quartile) were 19 (13-28) and 27 (18-39) microg/g. Blood lead levels were consistent with non-occupational exposure: only 6% of men had levels > or =10 microg/dl. In multivariable adjusted analyses, higher levels of blood lead were associated with poorer performance on the MMSE. This association was most pronounced among ALAD-2 carriers, among whom a 3 microg/dl increment in blood lead (the interquartile range) was associated with a 0.26 point lower mean MMSE score (95% CI -0.54 to 0.01), compared with a 0.04 point lower score (95% CI -0.16 to 0.07) among non-carriers. The modest 0.22 point difference in these associations did not attain statistical significance, however (p(interaction) = 0.13). The associations between bone lead levels and MMSE score did not vary by ALAD-2 status.

CONCLUSIONS

Although not statistically significant, these findings suggest that ALAD genotype may modify blood lead's adverse association with cognition among older men who had community exposures to lead. However, despite a relatively large sample size and the use of sensitive methods for measuring lead burden, the evidence overall was fairly weak.

Exploring the relationship between caffeine intake and essential tremor

BACKGROUND

It has been suggested that environmental factors may be associated with essential tremor (ET). This study was carried out to evaluate the association of caffeine intake with ET.

METHOD

In a case control study, patients diagnosed with ET and healthy controls underwent a standardized questionnaire interview to evaluate the exposure to coffee and tea intake. A multivariate logistic regression analysis was carried out to evaluate the association of caffeine intake and other environmental factors with risk of ET.

RESULTS

179 subjects including 79 ET patients and 100 controls matched for age, gender and ethnicity were included in the analysis. Univariate analysis revealed that caffeine consumption in ET patients was higher than control group (median and 90th percentile range: 2300 (0, 9000) mg-years versus 1500 (0, 6090) mg-years, p=0.01). However, the multivariate logistic regression analysis demonstrated that caffeine was no longer a significant factor associated with ET (p=0.119). There was no significant correlation between amount of caffeine intake and disease duration (Spearman's r=0.194; p=0.202) or total tremor score (Spearman's r=0.045; p=0.771) in ET patients.

CONCLUSION

Caffeine consumption was not associated with risk of ET in our study population. Further studies are needed to investigate the significance of gene-environmental interaction in ET.

Interpretation of small effect sizes in occupational and environmental neurotoxicology: individual versus population risk

Some have questioned the importance of the small effect sizes generally reported in epidemiological studies of neurotoxicity. To some extent, this reflects a failure to appreciate the critical distinction between individual and population risk. In the first part of the paper, arguments are marshaled to support the contention that small shifts in the mean value of a health index within a study sample can, under some circumstances, carry substantial implications for the health status of the population from which the study sample was drawn. Under such circumstances, a population-based approach to prevention might be as effective as a patient-based approach. The second part clarifies conditions under which this will be true (e.g., a monotonic relationship between a health index and risk of disease) and conditions under which it might not (e.g., a J-shaped relationship). In the third part, the relative levels of uncertainty in characterizing individual versus population risk are explored. In neurotoxicological studies, uncertainty in characterizing individual risk could be reduced by adjusting, in addition to bone fide confounders, for covariates that are strong predictors of outcome and by more assiduous efforts to characterize major effect modifiers.

Jaw tremor: Prevalence and clinical correlates in three essential tremor case samples

The spectrum of involuntary movements seen in essential tremor (ET) is limited. Jaw tremor is one such movement. The prevalence and clinical correlates of jaw tremor have not been studied in detail. The objective of this study was to estimate the prevalence and examine the clinical correlates of jaw tremor in ET using ET cases from three distinct settings (population, tertiary-referral center, brain repository). All ET cases underwent a videotaped tremor examination in which tremors (including limb, head, voice, and jaw) were assessed. The prevalence [95% confidence interval (CI)] of jaw tremor was lowest in the population sample (7.5%; 3.9%-14.2%), intermediate in the tertiary-referral center (10.1%; 6.8%-14.7%), and highest in the brain repository (18.0%; 12.3%-25.5%; P = 0.03). Jaw tremor was associated with older age (P < 0.001), more severe action tremor of the arms (P < 0.001), and presence of head and voice tremor (P < 0.001). Jaw tremor was present in 4/14 (28.6%) ET cases with consistent rest tremor vs. 15/193 (7.8%) cases without rest tremor (odds ratio = 4.8; 95% CI = 1.3-7.0; P = 0.009). The prevalence of jaw tremor was 7.5% to 18.0% and was dependent on the mode of ascertainment, being least prevalent in a population-based sample. ET cases with jaw tremor had a more clinically severe and more topographically widespread disorder. The association in our study between jaw tremor and rest tremor, along with the published observation that jaw tremor can occur in Parkinson's disease (PD), raises the question whether jaw tremor in ET is a marker for subsequent conversion to PD.