Alzheimer's disease, Parkinson's disease and essential tremor: three common degenerative diseases with shared mechanisms?

Assessment of Three New Loci from Genome-wide Association Study in Essential Tremor in Chinese population

Essential tremor (ET) is the most common tremor disorder. Evidences indicated that genetics plays an essential role in the researches of etiology. A new genome-wide association study (GWAS) from European population identified three novel loci in ET, which were rs10937625 in STK32B, rs17590046 in PPARGC1A, and rs12764057, rs10822974 and rs7903491 in CTNNA3. Due to the different genetic background in different population, we performed a case-control study to investigate these variants in a cohort of 533 subjects in Chinese population. We found a significant difference in the distributions of genotypes and alleles frequencies between ET and control groups of rs10937625 (genotype p = 0.037, OR = 0.69[0.48–0.98]; allele p = 0.033, OR = 0.82[0.69–0.99]) and rs7903491 (genotype p = 0.030, OR = 1.34[1.03–1.74]; allele p = 0.029, OR = 1.16[1.02–1.32]) after adjusted for age and gender. And no associations were detected between rs17590046 (genotype p = 0.794; allele p = 0.791), rs12764057 (genotype p = 0.337; allele p = 0.337), rs10822974 (genotype p = 0.102; allele p = 0.100) and ET in Chinese population individually. Our research supports that C allele of rs10937625 in STK32B is a protective factor and G allele of rs7903491 in CTNNA3 is a risk factor for ET in Chinese population.

TREM2 R47H variant and risk of essential tremor: a cross-sectional international multicenter study

INTRODUCTION

Essential tremor (ET) is the most frequent movement disorder in adults. Its pathophysiology is not clearly understood, however there is growing evidence showing common etiologic factors with other neurodegenerative disorders such as Alzheimer's and Parkinson's diseases (AD, PD). Recently, a rare p.R47H substitution (rs75932628) in the TREM2 protein (triggering receptor expressed on myeloid cells 2; OMIM: *605086) has been proposed as a risk factor for AD, PD and amyotrophic lateral sclerosis (ALS). The objective of the study was to determine whether TREM2 p.R47H allele is also a risk factor for developing ET.

METHODS

This was a cross-sectional multicenter international study. An initial case-control cohort from Spain (n = 456 ET, n = 2715 controls) was genotyped. In a replication phase, a case-control series (n = 897 ET, n = 1449 controls) from different populations (Italy, Germany, North-America and Taiwan) was studied. Owed to the rarity of the variant, published results on p.R47H allele frequency from 14777 healthy controls from European, North American or Chinese descent were additionally considered. The main outcome measure was p.R47H (rs75932628) allelic frequency.

RESULTS

There was a significant association between TREM2 p.R47H variant and ET in the Spanish cohort (odds ratio [OR], 5.97; 95% CI, 1.203-29.626; p = 0.042), but it was not replicated in other populations.

CONCLUSIONS

These results argue in favor of population-specific differences in the allelic distribution and suggest that p.R47H (rs75932628) variant may contribute to the susceptibility of ET in Spanish population. However, taking into account the very low frequency of p.R47H, further confirmatory analyses of larger ET series are needed.

'Essential tremor' or 'the essential tremors': is this one disease or a family of diseases?

There is accumulating evidence that the entity referred to as 'essential tremor' (ET) is not a single disease. It may be a family of diseases better referred to as 'the ETs'. This review will summarize the following evidence: (1) the presence of etiological heterogeneity; (2) the heterogeneity of findings in postmortem studies, thus suggesting several diseases; (3) the recent discussion that age of onset may be an important marker of disease heterogeneity; (4) the clinical expansion of the concept of ET in recent years to include a broader range of tremor phenomenology, other motor features (gait ataxia), other involuntary movements (dystonia), and nonmotor features (cognitive problems, psychiatric problems), some of which could be primary; (5) the heterogeneity of pharmacological response profiles and clinical progression, and (6) the association of ET with Parkinson's disease, Alzheimer's disease, and possibly progressive supranuclear palsy, with the possibility that some ET patients are more predisposed to develop one of these. © 2013 S. Karger AG, Basel.

[Neuropsychiatric disturbances in essential tremor]

This review focuses on the neuropsychiatric manifestations (personality disturbances, depressive symptoms, cognitive impairment and dementia), which have been described in this last decade in patients with essential tremor. We compared the data derived from the Neurologic Disorders in Central Spain (NEDICES) study, a prospective population-based survey, with those derived from the literature. The traditional view of essential tremor as a mono-symptomatic condition characterized by action tremor is now changing. First, it is known that many patients also have other motor manifestations apart from tremor (e.g., ataxic gait). Second, in the last years, the presence of a variety of non-motor neuropsychiatric features has been described. Mild cognitive changes (especially executive dysfunction) have been documented in several independent studies. Further, two population-based studies have demonstrated an association between essential tremor with mild cognitive impairment and dementia. Psychiatric manifestations include specific personality traits, anxiety, social phobia, and depressive symptoms. Taking together, the neuropsychiatric dysfunction pattern and the new data on neuropathology of essential tremor suggest that this disease is a neurodegenerative cerebellar disorder and let us to question the classical concept of essential tremor as a benign mono-symptomatic disorder.