Clinical and genetic study of essential tremor in the Italian population

Expansion of GGC repeat in the human-specific NOTCH2NLC gene is associated with essential tremor

Essential tremor is one of the most common movement disorders. Despite its high prevalence and heritability, the genetic aetiology of essential tremor remains elusive. Up to now, only a few genes/loci have been identified, but these genes have not been replicated in other essential tremor families or cohorts. Here we report a genetic study in a cohort of 197 Chinese pedigrees clinically diagnosed with essential tremor. Using a comprehensive strategy combining linkage analysis, whole-exome sequencing, long-read whole-genome sequencing, repeat-primed polymerase chain reaction and GC-rich polymerase chain reaction, we identified an abnormal GGC repeat expansion in the 5′ region of the NOTCH2NLC gene that co-segregated with disease in 11 essential tremor families (5.58%) from our cohort. Clinically, probands that had an abnormal GGC repeat expansion were found to have more severe tremor phenotypes, lower activities of daily living ability. Obvious genetic anticipation was also detected in these 11 essential tremor-positive families. These results indicate that abnormal GGC repeat expansion in the 5′ region of NOTCH2NLC gene is associated with essential tremor, and provide strong evidence that essential tremor is a family of diseases with high clinical and genetic heterogeneities.

A Series of 211 Children with Probable Essential Tremor

Background

The objective of this study was to characterize the clinical features, course, and treatment of essential tremor (ET) in children.

Methods

A retrospective chart review was conducted over 25 years (1984-2011). Inclusion criteria were age <21 years and satisfying the core diagnostic criteria for ET. Primary exclusion criteria included other neurological findings, tremorogenic medications, sudden onset/stepwise progression, primary orthostatic tremor, and isolated task specificity; and secondary exclusion criteria were abnormal neuroimaging or metabolic/thyroid studies. Cases were reviewed for age, sex, family history, tremor characteristics, functional impairment, treatment, and follow-up.

Results

In total, 211 children had ET, including 130 males and 81 females. The mean ± standard deviation age was at diagnosis was 14.09 ± 5.0 years, the age of onset was 9.71 ± 5.62 years, and the age of onset was birth in 7 children. One hundred ninety-nine children had bilateral hand tremor, 34 had asymmetry, 9 had unilateral onset but later became bilateral. Twelve children had voice tremors, 13 had leg tremors, 5 had head tremors, and 7 had trunk tremors. Tremor at rest was present in 20 children. Thirty-five percent of the children had a family history of ET, including in a father (n = 21), mother (n = 13), brother (n = 6), sister (n = 3), and other family member (n = 28). Fifty-five percent of patients had functional disabilities, including writing (n = 66), eating (n = 28) drinking from a cup (n = 13), typing (n = 4), playing instruments (n = 6), buttoning (n = 6), and playing (n = 3). For treatment, 33 patients received propranolol, 1 received atenolol, 13 received primidone, 3 received metoprolol, and 1 received nadolol. In total, 99 patients were followed for a mean ± standard deviation of 1.82 ± 2.21 years.If left untreated, tremor remained unchanged in 33 patients, and 7 had an apparent short-term improvement. On propranolol, 15 of 20 patients significantly improved.

Conclusions

This is the largest series to date of ET in children. The current findings indicate that onset at birth is possible, family history is less common in children than in adults, and tremor at rest is possible. Functional disability was noted in 55% of children, and 29.4% required medication.

Key issues in essential tremor genetics research: Where are we now and how can we move forward?

Background

Genetics research is an avenue towards understanding essential tremor (ET). Advances have been made in genetic linkage and association: there are three reported ET susceptibility loci, and mixed but growing data on risk associations. However, causal mutations have not been forthcoming. This disappointing lack of progress has opened productive discussions on challenges in ET and specifically ET genetics research, including fundamental assumptions in the field.

Methods

This article reviews the ET genetics literature, results to date, the open questions in ET genetics and the current challenges in addressing them.

Results

Several inherent ET features complicate genetic linkage and association studies: high potential phenocopy rates, inaccurate tremor self-reporting, and ET misdiagnoses are examples. Increasing use of direct examination data for subjects, family members, and controls is one current response. Smaller moves towards expanding ET phenotype research concepts into non-tremor features, clinically disputed ET subsets, and testing phenotype features instead of clinical diagnosis against genetic data are gradually occurring. The field has already moved to considering complex trait mechanisms requiring detection of combinations of rare genetic variants. Hypotheses may move further to consider novel mechanisms of inheritance, such as epigenetics.

Discussion

It is an exciting time in ET genetics as investigators start moving past assumptions underlying both phenotype and genetics experimental contributions, overcoming challenges to collaboration, and engaging the ET community. Multicenter collaborative efforts comprising rich longitudinal prospective phenotype data and neuropathologic analysis combined with the latest in genetics experimental design and technology will be the next wave in the field.

Autosomal dominant essential tremor: a novel family with anticipation

Essential tremor (ET) is a common progressive movement disorder characterized by a clear genetic predisposition. In the last years, many efforts have been done to map susceptibility loci for ET. Here, we report a clinical and genetic study of a family with ET showing autosomal dominant inheritance and anticipation over three generations. The family has five affected members and exhibits a remarkable anticipation of age at onset of the disease along the generations. We excluded linkage to any of the three loci previously mapped in autosomal dominant ET families. Our data suggest the existence of an additional locus in which a repeat expansion is the possible genetic defect underlying ET.

Further evidence of genetic heterogeneity in familial essential tremor

Familial essential tremor (FET) is a common hereditary movement disorder with phenotypic variability and genetic heterogeneity. To date, linkage analyses revealed three loci associated to essential tremor (ET) (ETM1 on 3q13, ETM2 on 2p22-25, and a locus on 6p23). We performed a genetic analysis of these candidate chromosomal regions in a fifth-generation Italian kindred with autosomal-dominant ET. Of the 22 clinically evaluated family members, nine were affected by ET. The genetic study indicates that the ET in this family is not associated to any of the known ET loci. These findings support evidence of further genetic heterogeneity for such disease.

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.

Changes at the CYP2C locus and disruption of CYP2C8/9 linkage disequilibrium in patients with essential tremor

To identify low-penetrance genes related to sporadic essential tremor (ET) at the CYP2C locus, located in chromosome 10 q23.33. Leukocytary DNA from 200 ET patients and a control group of 300 unrelated healthy individuals with known CYP2C19 genotypes was studied for common CYP2C8 and CYP2C9 allelic variants by using amplification-restriction analyses. Patients with ET showed the following differences compared with healthy subjects: a 1.6-fold reduction in the frequency for CYP2C8*3 (p=0.006), a 1.35-fold reduction of CYP2C9*2 (p=0.05) and a 1.52-fold reduction in the frequency for CYP2C9*3 (p=0.07). The frequency for patients with ET carrying at least one defective allele was 1.33-fold reduced as compared with healthy subjects (p=0.002). In addition, a disruption of the CYP2C8*3/CYP2C9*2 linkage disequilibrium was observed in ET patients, with a 2.1-fold reduction in the percentage for carriers of the haplotype CYP2C8*3 plus CYP2C9*2 in ET patients (p=0.0001). These findings were independent of gender, age, age of onset, or clinical symptoms. These results suggest that alterations at the CYP2C gene locus are associated with the risk for ET.

Exploring the relationship between essential tremor and Parkinson's disease

Although essential tremor (ET) and Parkinson's disease (PD) are considered distinct disorders, there is overlap in some clinical features. In some PD patients, a long-standing postural tremor in the hands may precede the onset of parkinsonian features by several years or decades. Furthermore, large families with both ET and PD phenotypes have been described and autopsy studies have demonstrated Lewy body pathology in brains of ET patients. Functional neuroimaging suggests that some ET patients have dopaminergic deficit. We examine here the evidence for and against an association between ET and PD, and critically review data supporting the notion that a subset of ET patients is predisposed to developing PD.

HS1-BP3 gene variant is common in familial essential tremor

Essential tremor (ET) is a movement disorder characterized by a postural or kinetic tremor of the hands, head, or voice. It is typically a familial condition and affects 1% to 4% of the general population. The trait is genetically linked to chromosome 2p in some families. A variant (828C-->G) in exon 7 of the hematopoietic-specific protein 1 binding protein 3 gene (HS1-BP3) on chromosome 2p recently has been found to segregate with ET in 2 families. To determine the frequency of this variant in a larger series, we studied patients with ET, Parkinson disease (PD), and controls without tremor. Affected singletons representing 73 families from the United States with dominantly inherited ET, 35 individuals with PD, and 304 healthy controls older than age 60 were tested for the 828C-->G variant in exon 7 of the HS1-BP3 gene by a BseYI restriction enzyme digest of the polymerase chain reaction product. Heterozygous carriers of the mutant allele were identified in 12 individuals with ET (16.4%) and in 1 individual with PD and postural tremor (3%). All of the healthy controls (608 chromosomes) were homozygous for the wild-type allele. The 828C-->G genetic variant in the HS1-BP3 gene occurs relatively frequently in subjects with ET. The variant may also be found in some individuals with PD and postural tremor. The HS1-BP3 gene plays a putative role in regulating catecholamine and serotonin metabolism, but the functional consequences of the amino acid substitution (A265G) caused by this genetic variant is unknown.

Increased tremor severity in bilineal essential tremor: A report of two families

Up to 6% of the general population have essential tremor (ET). In a number of couples both partners may have ET. The clinical profile of ET in children and the parents where both parents and the child have ET remains to be established. We report on two families where both parents and one child have ET. The severity of ET was greater in the children than in either parent. Such families could provide special opportunity to determine relation between genotype and phenotypic expression of ET.

Report from a U.S. conference on essential tremor

Seventy researchers met in Washington, DC, on 20-21 October 2005 to identify and discuss the most pressing research issues in essential tremor (ET). The conference attendees concluded that the following six objectives are of immediate and overriding importance: (1) a collaborative network of research centers; (2) an international committee for developing a standard protocol for the diagnosis and quantification of ET; (3) the identification of one or more genes for ET; (4) a centralized repository of DNA and, ideally, immortalized cell lines from well-characterized ET families and healthy controls; (5) a reliable and efficient repository of optimally prepared and categorized brain samples for hypothesis-driven neuropathological examinations in well-characterized ET patients; and (6) animal models of ET for screening promising drugs. The conference attendees hope that this statement from the United States will engender international collaboration in finding a cure for ET.

A variant in the HS1-BP3 gene is associated with familial essential tremor

BACKGROUND

Genetic linkage studies have identified two susceptibility loci for essential tremor (ET) on chromosomes 3q13 (ETM1) and 2p24.1 (ETM2). Linkage disequilibrium studies in separate population samples from the United States and Singapore suggest an association between ET and loci at ETM2.

METHODS

Fine mapping studies were conducted on multiplex and singleton US families linked to ETM2 using newly detected loci within the candidate interval to establish the minimal critical region (MCR) harboring an ET gene. The genes and transcripts within this interval were systematically analyzed by single-strand conformational polymorphism analysis and DNA sequencing.

RESULTS

A 464-kb region between loci D2S2150 and etm1231 was defined as the MCR. The coding regions and flanking intronic splice sites of two genes and seven transcripts in this interval were evaluated for mutations. A missense mutation (828C-->G) in the transcript FLJ14249 (HS1-BP3) was identified in one US family. This mutation was found in another apparently unrelated US family with ET and was absent in 150 control samples (300 chromosomes). The 828C-->G mutation causes a substitution of a glycine for an alanine residue in the HS1-BP3 protein. The HS1-BP3 protein binds to proteins that are highly expressed in motor neurons and Purkinje cells and regulate the Ca2+/calmodulin-dependent protein kinase activation of tyrosine and tryptophan hydroxylase.

CONCLUSIONS

A rare variant in the HS1-BP3 gene that is associated with essential tremor (ET) in two families is reported. This finding will facilitate research on the functional role of this gene and related genes in the pathogenesis of ET.

Frequent sequence variation at the ETM2 locus and its association with sporadic essential tremor in Korea

Essential tremor (ET) is the most common but a complex neurological movement disorder. ET usually affects hands, but it may also affect head, neck, face, jaw, tongue, voice, trunk and, rarely, legs and feet. Although two susceptibility loci were identified on chromosome 2p24 (ETM2) and 3q13 (ETM1 or FET1), the exact transcript(s) has not been cloned. We analyzed unrelated Korean individuals with ET for a genetic association with three reported polymorphic loci (STS-etm1240, STS-etm1231, and STS-etm1234) in a candidate region on chromosome 2p24.1. We investigated sequence polymorphisms at these three loci in 30 ET patients and 30 controls using polymerase chain reaction (PCR) amplification followed by sequence analysis. Eight different sequence variants (5 at etm1234, 2 at etm1240, and 1 at etm1231) were detected from 7 patients. Of interest, sequence variants were found only in classic ET patients but not in nonclassic ET patients and healthy individuals. Additionally, we also observed that a decrease in the number of short tandem repeats within etm1234 locus is more frequent in ET patients compared to controls. Our data thus support that ET development would be linked with the ETM2 locus and will facilitate the search for the ETM2 gene transcript.

Mutational analysis of parkin gene by denaturing high-performance liquid chromatography (DHPLC) in essential tremor

OBJECTIVES

To evaluate the relationship between point mutations within the parkin gene and essential tremor (ET).

BACKGROUND

Essential tremor, the most common movement disorder, has long been recognised as an autosomal dominant disease. To date the genes involved in ET pathogenesis are still unknown. Several authors reported the association of ET with Parkinson's disease (PD).

PATIENTS AND METHODS

One hundred and ten unrelated ET patients were analysed for point mutations within the parkin gene. Experimental conditions for DHPLC mutational analysis of the coding region of the parkin gene were set up.

RESULTS

Neither obvious disruptive mutations, nor mutations previously described in patients with Parkinson's disease were identified in the cohort of patients analysed. DHPLC analysis detected two already reported polymorphisms [c.1138G>C (V380L) and c.1180G>A (D394N)], and four novel rare variants (frequency <1%) [c.645C>A (H215Q); c.847C>T (H279H); c.1393G>A (V465M) and c.2695A>G] located within exonic regions. Four new polymorphisms [c.413-20T>C; c.872-35G>A; c.872-68C>G; c.1286-117A>G], and one rare variant (c.934-3C>T) were also found within intronic regions.

CONCLUSION

Causative sequence variants in the parkin gene have not been identified in this cohort of Italian ET patients.

Haplotype analysis at the ETM2 locus in a Singaporean sample with familial essential tremor

An ancestral haplotype on chromosome 2p24.1 described in an American sample with familial essential tremor (ET) was analyzed in a different ethnic sample from Singapore. Six polymorphic loci (etm1240, etm1231, etm1234, APOB, etm1241, and etm1242) in a 274-kb interval within an ET gene candidate region (ETM2) were analyzed in Singaporean individuals with a family history of ET (n = 52) and compared to Singaporean controls older than age 65 (n = 49). The allele frequencies were significantly different between cases and controls for the loci etm1234 (p = 0.0001) and APOB (p = 0.0320). An extended haplotype formed by the loci etm1231, etm1234, and APOB occurred with a frequency of 31% in Singaporean cases and in 1.8% of elderly Singaporean controls (p = 0.0005). Haplotype studies in two different population samples suggest that a disease locus for ET lies near or within the 100-kb interval between the loci etm1231 and APOB.

Muscle loading as a method to isolate the underlying tremor components in essential tremor and Parkinson's disease

Tremor is clinically evaluated and classified on the basis of its response to limb posture (resting, postural, and kinetic tremor), but the mechanisms underlying this powerful influence remain unclear and no satisfactory method exists to identify or quantify underlying tremor subtypes. Postural change is closely linked to changes in gravitational load. We therefore assessed the effect of changes in muscle load on essential tremor (ET) and parkinsonian tremor (PT) independently of postural change. A motor accurately delivered a series of constant (0.2-1.2 Nm) flexion and extension torques about the affected wrist while subjects maintained a constant wrist angle by isometrically contracting wrist flexors or extensors against the applied loads. Linear regression of tremulous electromyogram (EMG) spectral peak amplitude against the applied loads estimated the magnitudes of the load-dependent (LDT) and load-independent (LIT) tremor components. The amplitude of ET was linearly related to increase in gravitational load. It thus contained a large LDT component and a small or absent LIT component. Muscle loading revealed significant LDT and LIT components in PT. LIT was dominant at zero load (classic rest tremor) but both components were present during loading (classic postural tremor). Muscle loading more clearly identifies tremor subtypes than postural effects alone. The method could be applied in clinical and pathophysiological studies.

Essential tremor is not associated with alpha-synuclein gene haplotypes

A specific allele of the NACP-Rep1 polymorphism within the alpha-synuclein promoter was found to be associated both with Parkinson's disease and essential tremor. We repeated the association study on a large series of Italian patients with essential tremor using a panel of polymorphisms within the alpha-synuclein gene. Our results did not confirm the association reported previously and failed to identify a alpha-synuclein specific haplotype as susceptibility factor for essential tremor.