Familial Aggregation of the Cerebellar Signs in Familial Essential Tremor

Cerebellar oscillations driven by synaptic pruning deficits of cerebellar climbing fibers contribute to tremor pathophysiology

Essential tremor (ET) is one of the most common movement disorders and the prototypical disorder for abnormal rhythmic movements. However, the pathophysiology of tremor generation in ET remains unclear. Here, we used autoptic cerebral tissue from patients with ET, clinical data, and mouse models to report that synaptic pruning deficits of climbing fiber (CF)-to-Purkinje cell (PC) synapses, which are related to glutamate receptor delta 2 (GluRδ2) protein insufficiency, cause excessive cerebellar oscillations and might be responsible for tremor. The CF-PC synaptic pruning deficits were correlated with the reduction in GluRδ2 expression in the postmortem ET cerebellum. Mice with GluRδ2 insufficiency and CF-PC synaptic pruning deficits develop ET-like tremor that can be suppressed with viral rescue of GluRδ2 protein. Step-by-step optogenetic or pharmacological inhibition of neuronal firing, axonal activity, or synaptic vesicle release confirmed that the activity of the excessive CF-to-PC synapses is required for tremor generation. In vivo electrophysiology in mice showed that excessive cerebellar oscillatory activity is CF dependent and necessary for tremor and optogenetic-driven PC synchronization was sufficient to generate tremor in wild-type animals. Human validation by cerebellar electroencephalography confirmed that excessive cerebellar oscillations also exist in patients with ET. Our findings identify a pathophysiologic contribution to tremor at molecular (GluRδ2), structural (CF-to-PC synapses), physiological (cerebellar oscillations), and behavioral levels (kinetic tremor) that might have clinical applications for treating ET.

Genetic Testing Preferences of Individuals in Families with Essential Tremor

Background

The search for essential tremor (ET) genes is active, and it is only a matter of time before genetic tests become available. Genetic testing preferences in families have been studied in numerous other neurological disorders but there are no published data about ET.

Methods

We surveyed 34 ET probands and their relatives (43 affected, 28 unaffected) enrolled in our Family Study of Essential Tremor to assess their interest in genetic testing. We examined whether clinical factors influenced their interest in testing. Clinical utility (“Your physician will be able to use the information obtained to improve your care”) and penetrance (“How likely an individual who carries an ET gene is to develop ET”) were defined for participants.

Results

Interest in genetic testing was high in ET families (90/105 [85.7%]). There was a significant difference between affected (including probands and affected relatives) and unaffected relatives in terms of their interest in genetic testing, with the former being more interested (70/77 [90.9%] vs. 20/28 [71.4%] p = 0.04). Participants were more likely to want testing in the scenarios with high clinical utility; disease penetrance was not a determining factor (all p < 0.05). Sixteen hypothetical factors were identified that might influence a participant’s decision to undergo genetic testing for ET.

Discussion

Interest in genetic testing was high in ET families. While genetic testing is not currently available for ET, the hunt for ET genes is ongoing, and this is a highly familial disorder. Understanding genetic testing preferences will greatly aid clinicians once a genetic test becomes available.

Comparative Burden of Subclinical Tremor in a Cohort of Normal Individuals Stratified by Familial Risk for Essential Tremor

BACKGROUND

The burden of mild (i.e., subclinical) tremor within essential tremor (ET) families is not fully understood. We assessed the burden of mild tremor in a cohort of 287 adults, none of whom reported tremor or were diagnosed with ET.

METHODS

We recruited adults in 2 groups based on the familial risk for ET: 244 high-risk individuals (i.e., reporting one or more first-degree relative with ET) and 43 low-risk individuals (i.e., reporting no relatives with ET). Tremor was objectively assessed on 4 hand-drawn spirals (total spiral score = 0-12). Mild tremor was defined using 3 different cut points.

RESULTS

The prevalence rates of mild tremor among high-risk individuals ranged from 41.4 to 98.4% and were highly dependent on the cut point. Above a certain threshold (i.e., a total spiral score ≥5), 1-in-5 (i.e., 19.7%) high-risk individuals exhibited mild tremor, whereas no low-risk individuals did. High-risk individuals were 3.09-4.50 times more likely than low-risk individuals to exhibit mild tremor.

CONCLUSION

The burden of ET extends beyond the boundaries of the clinically defined disease, and partially expressed forms of ET are abundant in ET families. This fact greatly complicates gene-finding studies and epidemiological studies whose goal is to detect disease-linked associations.