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

Claudia M. Testa

doi:10.5334/tohm.160

Figures & Tables

Table 1.

Association Study Cohort Sizes in Essential Tremor (ET) Genetics Research

Study	ET n	Control n	ET Cohort Source	Overall Result
ETM1
HS1-BP3 828C→G variant	49	92	United States	Negative
Shatunov et al, 200563	49	92	United States	Negative
HS1-BP3 828C→G variant	73	304	United States	Positive
Higgins et al, 200661	73	304	United States	Note initial 2005 report from same group with G variant in ET cases in 2 of 21 US ET families, not ET or unaffecteds in the other families, and not 150 US controls or 73 ET Singaporean cases
HS1-BP3 828C→G variant	222	132	United States	Negative
Deng et al, 200562	222	132	United States	Negative
ETM2
ETM2 sequence variants	30	30	Korean	Positive
Kim et al, 200537	30	30	Korean	Positive
ETM2 polymorphisms	61	68	Czech	Negative
Zahorakova et al, 201038	61	68	Czech	Negative
DRD3 312A→G variant	104	116	Latvian	Negative
Inashkina et al, 200839	104	116	Latvian	Negative
DRD3 312A→G variant	116	158	Italian	Negative
Vitale et al, 200870	116	158	Italian	Negative
DRD3 312A→G variant	163	192	Singaporean	Negative
Tan et al, 200768	163	192	Singaporean	Negative
DRD3 312A→G variant	201	282	Spanish	Positive
Garcia-Martin et al, 200966	201	282	Spanish	Positive
DRD3 312A→G variant	276	184	United States	Positive
Jeanneteau et al, 200665	30	50	French	Positive
DRD3 312A→G variant	299	528	German, Danish, French	Negative
Lorenz et al, 200969	299	528	German, Danish, French	Negative
DRD3 312A→G variant	433	272	United States	Negative
Blair et al, 200867	433	272	United States	Negative
LINGO genes
LINGO1 rs9652490 and rs11856808	109	430	Chinese Han	Negative
Zuo et al, 201054	109	430	Chinese Han	Negative
LINGO1 rs9652490	117	160	Chinese	Negative
Wu et al, 201157	117	160	Chinese	Note combining with Tan et al53 plus unpublished data into 307 ET and 804 control, meta-analysis positive
LINGO1 rs9652490	141	130	Latvian	Positive
Radovica et al, 201252	141	130	Latvian	9 other SNPs including rs11856808 negative
LINGO4 variants	150	300	Chinese Han	Negative
Liang et al, 201259	150	300	Chinese Han	Negative
LINGO1 rs9652490	190	733	Singaporean	Positive
Tan et al, 200953	190	733	Singaporean	Positive
LINGO1 rs9652490 and rs11856808	226	1117	Spanish	Negative
Lorenzo-Betancor et al, 201155	226	1117	Spanish	Negative
LINGO1 SNPs	257	265	United States	Positive for rs9652490 and others
Clark et al, 201051				Negative for rs11856808 and others out of 15 SNPs total
Clark et al, 201051				Note meta-analysis combining 3 or 4 published cohorts46, 50, 51, 53 also positive
LINGO1 rs9652490 and rs11856808	259	479	French Canadian	Negative
Bourassa et al, 201156	259	479	French Canadian	Negative
LINGO2 variants	327	499	Chinese, Singaporean	Positive for rs7033345 and rs10812774 of 8 variants
Wu et al, 201158	327	499	Chinese, Singaporean	Positive for rs7033345 and rs10812774 of 8 variants
LINGO1 SNPs	332	574	German, French	Positive for rs9652490 and 1 other
Thier et al, 201048	332	574	German, French	Negative for rs11856808 and others out of 10 SNPs total
LINGO1 rs9652490	356	428	North American	Positive
Vilarino-Guell et al, 201050	356	428	North American	Positive
GWAS	452	14,394	Icelandic	Positive for LINGO1 rs9652490 and rs11856808 in initial cohort; rs9652490 only in confirmatory cohorts
Stefansson et al, 200946	281	1188	Austrian, German, United States
LINGO1 and LINGO2 variants	1247	642	North American	Positive for LINGO1 rs9652490 and 4 others of 16 total, and LINGO2 rs1412229 alone of 21.
Vilarino-Guell et al, 201049	1247	642	North American
*SNCA*
REP1 length	46	100	United States	Positive
Tan et al, 200075	46	100	United States	Positive
SNCA variants	106	90	Italian	Negative
Pigullo et al, 200390	106	90	Italian	Negative
SNCA variants	647	1285	North American	Negative
Ross et al, 201191	647	1285	North American	Negative
Other variants
GABAAR subunit genes variants	200	250	Spanish	Negative
Garcia-Martin et al, 201194, 95	200	250	Spanish	Negative
GABAAR subunits and GABA transporter genes variants	503	818	German, Dutch	Negative
Thier et al, 201193	503	818	German, Dutch	Negative
HNMT rs11558538	204	295	Spanish	Positive
Ledesma et al, 200896	204	295	Spanish	Positive
HNMT rs11558538	338	409	North American	Negative
Keeling et al, 201097	338	409	North American	Negative

Studies are grouped under genetic categories, and listed by first author and year of publication. Within hypothesis groups, studies are ordered by ET n. If the study combined cohorts in one analysis then the full ET n is used. If the same study used different cohorts in different analyses, the larger ET n is listed first. ET cohort source is geographical; some papers have further details on ethnicity. North American = United States and Canada. Overall result considers the main thrust of the study, with a few further notations; see text for comments on age of onset influences, and cited references for full study results details including specific genetic variants tested, haplotype analyses, methods.

Abbreviations: DRD3, dopamine D3 receptor gene; ET, essential tremor; ETM, Hereditary Essential Tremor, Online Mendelian Inheritance of Man (OMIM) locus designation; GABA, gamma-aminobutyric acid; GABAAR, gamma-aminobutyric acid A receptor; HNMT, histamine N-methyltransferase gene; HS1-BP3, HS1 binding protein 3 gene; LINGO, leucine-rich repeat- and Ig domain-containing Nogo receptor-interacting protein; numbered italicized LINGOs are genes encoding LINGO proteins; REP1, mixed dinucleotide repeat sequence in the SNCA promoter region; SNCA, gene encoding α-synuclein; SNPs, single nucleotide polymorphisms; US, United States.

Table 2.

Consequences of Basic Essential Tremor (ET) Features Create Challenges for ET Genetics Research. The current ET clinical definition has a direct impact on ET research. Ways to move forward acknowledge and address these challenges.

Current ET Feature	Consequences	Possible Responses	Challenges	Moving Forward
Phenocopies:High prevalenceMultiple genetic lociUnknown genotypes	Creates false positivesLowers LOD scoreIncreased type 1 errorOne genetic variant may have large phenotypic range (genocopies).	Test different phenotype features or subsets against genotype data (see also Table 3).Use inclusive varied phenotype as outcome rather than narrow ET definition.	How to make phenotype groups more genetically homogeneous is currently unknown.Expanding or narrowing possible phenotypes may or may not increase genetic homogeneity within groups.	Research definitions, specify ET subsets, accept potential even “non-ET” phenotype variables as part of hypotheses generation and testing, use phenotype variables across clinical diagnoses.Experimental design goals are to broaden testable hypotheses, thus move past the potential stalemate of as yet unknown phenotype-genotype connections.
Increasing prevalence with age	Creates false negativesLowers LOD scoreIncreased type 2 error	Consider all familial unaffected subjects as unknown.Restrict controls to older ages.	Strict affected status assignments can greatly impact power.Lowers impact of this error type but does not eliminate it.	Prospective studies of age of onset may change perspective on this ET feature.Large collaborative efforts particularly on control data and family member data help improve power and get past this ET feature.
ET is a clinical diagnosis:Tremor self-reportingLack of any tremor data particularly for controlsET misdiagnoses:Other disorders;mild tremor versus normal	False negativesFalse negativesFalse positives and false negatives	Prospective longitudinal primary exam dataDirect exams of all subjects: case, control, family memberLarger sample sizes to improve ability to detect associationsUtilizing possible / probable / definite categories, restricting analysis to definite cases	Detailed longitudinal phenotyping requires high investment in time and funds.Both subject and researcher resources impact feasibility of extended direct exams.Large sample sizes are expensive and time consuming to collect.There is always uncertainty in a clinical diagnosis, especially with mild tremor, and comparing to other clinically defined movement disorders.Some level of mechanistic heterogeneity likely in all case and control groups.	Collaborative efforts between: multiple centers; movement disorders experts and genetics experts; researchers and ET research participants.Collaborative efforts to define minimal and additional phenotype data, minimal and additional biosample amounts and time points.Explicit review of feasibility versus data quality tradeoffs to understand choices for each experiment: Non-blinded exams may cause ascertainment bias; time burden for subjects may limit phenotype data; scoring videos rather than in person exams may lower some phenotype data quality but improve blinding and allow multiple examiners; definite-only ET case definition may strengthen result but impact power.Use experimental design process to broaden range of testable hypotheses as above.

Abbreviations: ET, essential tremor; LOD, logarithm base 10 of odds.

Table 3.

Moving Past Underlying Assumptions in Essential Tremor (ET) Genetics Research. Many research groups are responding to early assumptions about ET with new approaches and theories. This process uncovers further challenges to advancing ET genetics research.

Assumption	Possible Responses	Challenges	Moving Forward
ET is a simple, known phenotypeBasic binary ET / no ET phenotype groups are adequate for genetic analyses	Collect range of motor and non-motor phenotype data“Sporadic” versus “familial”Early age of onsetClassic ET versuscomplicated ETET-PDET-parkinsonismET-dystonia	ET is tremor in isolation by clinical definitionMay not be currently useful, often ill defined.Retrospective data, large differences in actual tremor age of onset versus bothersome increased tremor symptoms reported as age of onset.ET is tremor in isolation by clinical definitionET and PD may or may not be related disorders; ET and PD are common mutual misdiagnoses.ET is tremor in isolation by clinical definition; ET and PD may or may not be related disordersIsolated head tremor is ET by clinical definition, but could be cervical dystonia; ET and limb dystonia misdiagnoses versus ET plus dystonia in non-tremor area.	Use of research phenotype features rather than clinical diagnosis criteriaIf “familial” subset used, clear definition with how family history data obtained.Longitudinal prospective data would strengthen this considerably.Use of research phenotype features rather than clinical diagnosis criteria.Record ET, PD, and dystonia exam features; then able to exclude ET-PD or dystonia cases from genetics studies, or focus on an ET-PD, ET-parkinsonism, or ET-dystonia subset depending on hypothesis.Longitudinal prospective studies with neuropathology will best address research questions.
ET is a family of autosomal dominant disorders caused by a small number of common genetic variants	Many rare genetic variants, alone and in combination, are behind much of ET.Epigenetic or other novel hypotheses for disease transmission	Detecting different types of genetic risk factors and causal mutationsAddressing non-mendelian inheritance at a complex biological data level	Collaborative phenotype-genotype studies with multiple research groups to achieve large sample numbers and rich prospective longitudinal phenotype data.High-throughput next generation genetic sequencing (exome, genome), epigenetic (methylome) and data analysis techniques.
ET research subjects do not want to or need to understand genetic research at its current level, as no causal mutations are known.	ET community research concerns may strengthen phenotyping and genetics experiment approaches.Return genetic research results information to research subjects and ET community.	ET community concerns may not be relevant to phenotype-genotype experiment design.How to return complex genetic data information to research subjects is not straightforward.	Understand ET community research goals, ET subject observations about ET phenotype and inheritance.Improve education of ET community on goals and results of genetics research, to increase motivated informed participation in genetics research studies.

Abbreviations: ET, essential tremor; PD, Parkinson’s disease.