Many genetics studies report no information on how ET is determined, little information on subject cohort characteristics beyond number of ET versus controls, and little to no information on how terms like “familial” and “sporadic” ET are defined. Linkage experiments vacillate between lack of solid data for accurate subject status assignments and loss of power; association studies often use very small sample sizes and less expensive but less informative techniques like studying a single SNP. These issues, discussed above, impact the ability to detect ET causal mutations and genetic risk associations in studies to date (). We can do better on the basics: many groups already are. Is that enough? Conceptualizing ET as a family of primarily genetic disorders and improving the quality of basic work has moved the field forward, but only so far. Past ET genetics studies largely work under two sets of assumptions: definition choices framing ET as a phenotypically homogeneous disorder; and genetics experiment structures based on straightforward Mendelian autosomal dominant inheritance of common genetic variants. Current key challenges involve shifting fundamental assumptions to create further progress in the field ().
From the movement disorders side: defining the ET phenotype
Even the best ET work contends with assumptions underlying all clinically defined complex disorders: the clinical definition is known, and the clinically based definition relates to underlying pathology. An excellent treatment by MacMahon and Pugh114
notes “The disease entities … have been selected, from the innumerable possibilities … on the basis of usefulness for prevention or treatment or on the basis of medical tradition.” To begin to organize clinical care, definition decisions must be made based on disease manifestation, which is sometimes a matter of tradition rather than data. One only assumes that “arrangements of ill persons by their manifestations may identify groups that have at least some degree of homogeneity with respect to causal factors … a useful basis for investigation of cause.”114
The spectrum of ET phenotype and pathology are still open questions, creating opportunities to re-examine current clinical ET definitions, particularly that of a monosymptomatic homogeneous clinical picture.18, 115, 116
Even a clinically useful definition is not necessarily useful for research on disease mechanism. Phenotype definition issues relevant to ET genetics research include whether and how to use tremor age of onset, motor symptoms beyond action tremor, non-motor symptoms, and phenotypes closely overlapping with other clinically defined movement disorders.
Starting with monosymptomatic kinetic tremor as ET, a proposed ET research variable is early age of onset. As an example, LINGO gene variants may influence ET age of onset, or be more strongly associated with early age of onset cases.14, 49, 51
However, retrospective age of onset represents fairly weak data in ET. Patients often report the age when tremor became noticeable or bothersome as their age of onset, discounting the “nervousness” or mild tremor noted from a young age.10
Prospective age of onset studies will require extended longitudinal follow-up of large varied cohorts. In the meantime, clarifying ways to obtain age of onset as research data, or working within the small subset of cases with moderate to severe tremor at an early age, could strengthen the use of this proposed characteristic.
The debate becomes more interesting when research moves away from monosymptomatic kinetic tremor. Are there other motor, and non-motor, ET features? Are there mechanistic connections between ET and other disorders? These are areas of intense opinion. They are considered here in the context of ET genetics research design, advocating moving past binary clinical label phenotype data ().
The idea of formally dividing ET into classic and “complicated” cases, usually ET–parkinsonism or ET–dystonia, dates back to at least the 1800s.10, 115, 116
Early attempts to clearly separate ET and PD also acknowledge “exceptional cases” that could represent an ET–parkinsonism overlap.10
Researchers may therefore have to make definition choices discordant from clinical ones, or be more agnostic about which features are “allowed” in research phenotypes. Exploring ET in a research context may require using ET subsets within or beyond clinical diagnoses. Whether or not clinical ET exists in subsets, use of ET subsets and broader phenotyping parameters may be useful for research.
The ongoing attempts to untangle ET and PD117–119
can be viewed from a genetics research perspective. Pragmatically, cases with both ET and PD (ET–PD) are well reported. Whether ET–PD represents individuals fortunate enough to have two common entities by chance, or one underlying mechanism evolving from an ET to an ET–PD phenotype, specifying how parkinsonism and PD is determined in ET cases and eliminating ET–PD cases from ET groups is important (see Ross et al91
for one example) but far from standard in ET genetics work.118
Focusing specifically on ET–PD compared with ET or PD alone, or ET–PD families rather than cases,120
may help settle points of debate. Research groups are already looking for genetic connections between ET and PD, at both association and family study levels, with mixed results.15,49,91,118–122
Specific families with apparent coinheritance of ET and other defined disorders, such as PD,120
PD and restless leg syndrome,123
or idiopathic normal pressure hydrocephalus,124
may yield rare variant information.
Features beyond kinetic tremor may be part of the ET phenotype itself, not an indication of ET plus a second disorder. Some level of parkinsonism, not PD, may be clinically acceptable in ET. Minimal parkinsonian signs without clinical PD are frequently reported, including mild changes in tone with cogwheel rigidity, and mild arm swing decrease.23, 25, 112, 115, 116
Severe kinetic tremor may break up fine motor tasks, creating clumsiness symptoms and impaired fine motor testing. Rest tremor is documented in ET, notably in individuals with longstanding severe classic ET in whom the rest tremor component is less severe than kinetic/posture tremor.112,125–127
Rest tremor in ET may be observed without any other parkinsonian signs.112,125–127
Specifying parkinsonism features as part of minimal ET phenotyping allows investigators to test a range of hypotheses: that all ET cases share an underlying cause and should be included in the same cohort; that cases with any parkinsonism features should be excluded to improve phenotype and presumably genotype homogeneity; that ET–parkinsonism cases are an informative subset with distinct causal mechanism(s).
A related approach is to use phenotype variables instead of disease diagnosis as the outcome against genetic predictors. For example, a genetic variant could be tested against a tremor feature, within ET or regardless of ET or PD clinical diagnosis. As an illustration of principle, asymmetric tremor severity is often observed in PD, although not all PD cases have any tremor. The ET clinical definition includes both upper extremity tremor in isolation, and head tremor in isolation.18, 115, 128
ET versus PD genetic papers rarely comment on whether PD cases were tremor predominant, or if ET cases include those with head tremor alone. Current clinical definitions stress bilateral upper extremity tremor in ET with unilateral tremor as a red flag indicating an alternative diagnosis, but tremor severity in ET may be asymmetric,24, 129
and many descriptions of ET note frequent unilateral onset with eventual (2–3 years) spread to both upper extremities.10, 19, 23, 25
Asymmetric severity and unilateral tremor onset may predict greater ET progression.130
Asymmetric tremor severity, or even just the presence of upper extremity tremor, can be tested against genotypes within or between ET and PD cohorts. As above, outward tremor features may or may not indicate underlying mechanistic connections: when hypotheses are structured around phenotype variables instead of diagnoses, the door is open to either outcome.
Non-tremor features of ET are also rarely explored in genetics research. Clinical scales focus on tremor, often biased to upper extremity kinetic tremor. Mild imbalance manifesting as impaired tandem gait is common in ET.131–134
ET subjects have significant impairments on multiple measures of postural control and functional mobility, independent of tremor body area (head or no head tremor) and tremor severity.135
Dystonia as an ET motor feature is considered below. Non-motor features such as mood and cognitive changes are under ongoing study but are not considered a clear part of clinically defined ET.136
Genetic risks could be tested against any of these outcomes, again within ET or across ET and other clinically defined groups. This adds both richer information and more flexibility to research designs. Major challenges to this approach are agreeing on phenotype variables across research groups, having sufficient power to address multiple testing issues, and addressing subject burden as research participation time increases. Regarding agreement on phenotype datasets, testing hypotheses in this type of structure does not commit the clinical ET definition to change, or imply that phenotype variables are clearly part of ET. It improves the ability to explore a range of possible outcomes, whether ET and other disorders share mechanistic connections or not—or whether phenotype-variable subsets do a better job representing genotypes than diagnosis subsets, or not.
Dystonia and ET is another contested area of potential mechanistic connection, with some genetics data behind it. Concurrent dystonia is an exclusion criterion in the current ET clinical diagnosis.18, 115, 128
On the other hand, head tremor in isolation is considered clinical ET, but cervical dystonia often causes head tremor. As above, ET genetics studies rarely record whether ET cases include head tremor, arm tremor, or both; nor do they generally specify if dystonia was specifically queried or examined. Dystonia is therefore a source of ET misdiagnoses, i.e., false positives. More interesting is considering dystonia as a potentially useful phenotype variable. Tremor and dystonia can occur in separate body areas. A relevant example is ET-like arm tremor with cervical dystonia, a phenotype of highly contested classification (see Schiebler et al137
for review). Dystonia in a non-tremor area may develop long after the tremor, reinforcing how longitudinal information about disease course is important in forming ET subsets.137
Outside of the ET clinical definition debate, there is movement towards recognizing (by any label) ET and dystonia, or recording specific variables such as arm tremor, head/neck tremor, and cervical dystonia. These arguments parallel the PD and tremor examples above. For dystonia, motivation to change ET research approaches also comes from genetics.
Phenotypes can be considered at the pedigree as well as the individual case level. In the ETM3 linkage study, dystonia was not only specifically recorded for all family members; the 6p23 locus was only linked in families with a mix of dystonia and ET.36
A detailed ET phenotype study utilizing in-person movement disorders examinations of all probands and family members observed multiple cases of dystonia, with or without ET in each individual case, in 28% of 97 ET kindreds.138
This provides further motivation for an ET–dystonia subset. As in ET–PD or ET–parkinsonism, the hypothesis is that pedigree-level ET subsets can reflect underlying genotypes in some cases.
What if kinetic arm tremor plus cervical dystonia isn’t “real” ET? We do not know what real ET will be in a future, more causal mechanism-driven definition set. Currently we assume ET clinical definitions match underlying disease mechanisms. Any ET research phenotype choice could increase rather than decrease genetic heterogeneity compared with an ET clinical diagnosis group. Conversely, one genotype may cause a wide range of phenotypes ( and ). This challenge can be experienced as a stalemate, where any phenotype choice is equally wrong, as we do not yet know phenotype–genotype connections. Instead, using a rich, open phenotyping approach broadens the range of testable hypotheses, including whether or not a given phenotype is related to a genotype, or ultimately to a clinical diagnosis. A shift from binary ET diagnosis outcome based on limited clinical criteria to research-oriented phenotypes generates progress by opening up possibilities in genetics experiment design.
The conceptual shift in ET phenotyping is reflected in research techniques. In addition to more videotaped detailed examination data, and use of validated scales across tremor and non-tremor features, investigators are incorporating objective measures such as digitized spiral analysis, electromyography, and accelerometry recording (see Shatunov et al36
for example). Calls for longitudinal detailed ET phenotype cohort studies with neuropathological follow-up are well founded:117, 139
such studies would greatly enhance ET genetics work by providing the level of phenotype data needed, tied to neuropathological diagnosis as well as clinical diagnosis. Phenotypically homogeneous criteria are driven clinically, not by biomarkers or other reliable gold standards. Neuropathology is the gold standard diagnosis for PD, although not ET or dystonia; still this is a way to greatly enhance data used for decision making in genetic sample analyses.
Improving ET phenotyping includes basic issues such as direct examination of research subjects. Redefining ET phenotyping encompasses incorporating a range of motor, non-motor, examination, objective measure, prospective, longitudinal, and neuropathological data into a rich, fruitful resource. New approaches to ET phenotyping will enable ET research to fully exploit advancing genetics technologies.
From the genetics side: mechanism of inheritance
Working within a genetic framework, the ET field is already moving into new ways to detect and analyze patterns of disease-associated change. ET was originally conceptualized as a phenotypically homogeneous familial entity, caused by common genetic variants. ET is better characterized as a complex trait;14, 121
pathophysiology could therefore involve rare genetic variants in combination, instead of a few common variants.121,140–143
As reflected by the association studies in ET (), the field is moving from single genetic variant analysis to more detailed, comprehensive sequencing approaches of candidate areas or the genome. While ET genetics thinking has already started to shift, integrating new phenotyping approaches and use of different phenotyping datasets against genotype data remain rare. This section outlines key ET challenges from the genetics side: whether and how to distinguish familial and sporadic ET; ET mechanisms of inheritance; and moving into new genetics technologies.
Genetic research reports often distinguish between “familial” and “sporadic” ET, even though this distinction is considered supporting not primary data for clinical definition,18
and is not currently clinically useful. Is the assumption that ET can be divided into familial and sporadic cases useful for research? A mix of familial and sporadic disease in a cohort could certainly affect attempts at genetics research: LINGO1
results may be stronger in familial ET.14
However, it is often unclear what these terms mean in ET. There is little consensus on whether studies should focus in on subjects with an extensive, clear family history, or not—or what constitutes extensive or clear. Many issues make ET family history challenging to interpret: “senile tremor” is dismissed by patients and providers, mild tremor may be unknown to other family members, ET is often misdiagnosed, direct examinations of all family members may be appropriate but not feasible. Given these issues plus the tremendous percentage of positive family history in studies discussed above, one valid approach is to disregard sporadic and familial subsets as not useful at this time.26
An alternative is to rank the quality of family history, where a conservative definition of positive family history may hinder acquiring large sample sizes and exclude informative cases, but improve overall data quality and help focus genetics work on a potentially powerful ET subset.
Even with the above caveats, the well-reported high rate of positive family history is an obvious starting point for ET research. There is varied and convincing evidence that much of ET is inherited in an autosomal dominant fashion,45, 144
although complex multigenetic modes of inheritance cannot yet be excluded.22, 36, 45, 116, 145
The field has already moved into deep sequencing both coding and non-coding DNA stretches, or utilizing full exome strategies: in the first case with mixed results, and in the latter with conspicuously silent results. This could reasonably be blamed on the unfortunate wealth of inherent ET phenotype issues discussed above (), as well as natural shaking out of replication attempts from early smaller studies. Another factor may be a main feature of ET: assumptions based on pedigree appearances of a highly prevalent and penetrant autosomal dominant disorder ().
The high number of affected individuals in ET families was initially attractive to research groups. The high prevalence and high rate of affected family members, while potentially challenging for genetics studies, were not considered an indication of primary mechanism.26
For example, the ET parish study concluded that chance variations in previous generations when the parish population was very small were enough to account for the high phenotype and thus assumed high genetic variant frequency25
. As more and more familial linkage studies returned negative results, some suspected “too many” affecteds for a straightforward autosomal dominant mechanism.145
A combination of phenocopies, ascertainment bias, and non-ET tremors (physiologic tremor, PD, medications) could explain the high reported affected numbers, as above. A non-Mendelian inheritance pattern presents an alternate explanation.98
Epigenetics encompasses mechanisms that change gene expression or activity without changing DNA sequences: DNA methylation and histone modification are classic examples. Epigenetic states can, at least in part, be inherited.146
This type of inheritance can occur in humans, through unclear mechanisms (reviewed in Zimprich98
). Epigenetic variation would not be detected by genome sequencing experiments. In a recent hypothesis paper, Zimprich details how epigenetic inheritance could explain observations in ET.98
An epigenetic feature may be transmitted from one allele to another in a parent cell prior to meiosis; thus in the gametes both the disease-causing allele and the originally benign allele become disease causing. This is termed paramutation.147
Long described in plants, the role of paramutation in mammals, particularly humans, is postulated from observations in diabetes mellitus but is uncertain.98, 148
Zimprich puts forward a framework of primarily epigenetic and paramutation inheritance, rather than genetic state, accounting for ET pedigree phenotype patterns.98
This intriguing theory pushes the field to consider diverse mechanism options. Pursuing this theory will require advances in detecting and analyzing epigenetic changes; for example, “methylome” analysis.149
Another alternate explanation of ET inheritance remains within genetics: ET may be a complex trait, with work to date identifying only a limited amount of the heritable component of ET.15, 22, 36, 45, 116, 121, 145
Uncovering “missing heritability” in ET may require researchers to pursue rare rather than common causal genetics variants, and/or many genetic risk factors.15, 121, 140
As in epigenetics, progress in genetics is often made through advancing technologies. Next-generation sequencing allows sequencing the whole genome, for example from members of one pedigree in an attempt to identify a causal mutation. High-throughput sequencing can be restricted to exons (exome sequencing).150
New sequencing technology holds great promise for testing the theory that ET is a complex trait caused by combinations of rare genetic variants, rather than a family of disorders caused by a small number of common causal mutations. Experiments utilizing next generation sequencing to achieve detailed analysis across coding, intronic, and regulatory areas could help detect rare genetic variants contributing to ET.15,121,140–142
Advances in analyzing large datasets will also be crucial. This can be considered across a spectrum of genetic data: rare variants with a high impact on disease causation, low-frequency genetic variants with intermediate effects, and combinations of relatively common variants acting as genetic risk factors each with a small effect size.140, 151
Already, meta-analyses leveraging multiple independent GWAS datasets in PD have set an example for detecting genetic risk factors with small effect sizes.151
Considering genetic and environmental risk factor data together is another potentially fruitful approach152
that demands advances in large dataset analyses.
New sequencing technologies are also necessary for detecting the full range of genetic variants that may contribute to ET. Structure variants are not single nucleotide (SNP) changes; instead the term encompasses various changes such as insertions, deletions, and DNA sequence inversions.140
One structural variant form is copy number variants: DNA stretches that are usually unique are repeated, in duplicate or triplicate. Their size in base pairs varies widely. Efforts focusing on specific genetic regions have uncovered copy number variant effects in movement disorders such as PD and chorea–acanthocytosis.153, 154
New array technologies are greatly expanding the ability to detect copy number variants.155
Detecting copy number and other structural variants on a large scale is hampered by sequencing technology limitations, cost, statistical power issues, and challenges in interpreting the clinical significance of observed variants.140, 155
Genetic approaches will benefit from lowering costs and advances in epigenetic methods, high-throughput sequencing, and high-dimensional data analysis. For ET genetics research, progress will come as much from evolving phenotype work as new “-omics”. Contributions from both the genetics and the movement disorders sides position the field to meet ET research challenges.