No evidence of association between CAG expansions and essential tremor in a large cohort of Italian patients

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.

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.

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.

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.

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.

Association of the C677T and A1298C polymorphisms of methylenetetrahydrofolate reductase gene in patients with essential tremor in Turkey

Essential tremor (ET) is a most common human movement disorder of unknown etiology. Previous reports have shown that the C677T polymorphism of methylenetetrahydrofolate reductase gene has been associated with neurodegenerative disorders. To investigate the role of methylenetetrahydrofolate reductase gene polymorphisms in essential tremor, we analyzed the alleles and genotypes of methylenetetrahydrofolate reductase (MTHFR) C677T and MTHFR A1298C in a total of 158 unrelated essential tremor patients and compared them with those of 246 unrelated healthy control subjects, using a polymerase chain reaction restriction fragment length polymorphism method. The allele frequency of MTHFR 677T was 35.76% in the essential tremor cases and 30.08% in the controls. We obtained statistically significant results for MTHFR677 and also for MTHFR1298. The MTHFR T677T genotype was overrepresented and was statistically significant. The T677T/A1298A and C677C/C1298C compound genotypes were similarly statistically significant. The C677C/A1298A compound genotype provided protection for essential tremor. In conclusion, the MTHFR 677T, 1298C alleles and MTHFR T677T genotype and T677T/A1298A, and C677C/C1298C compound genotypes are genetic risk factors for essential tremor in Turkey.

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.

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.