Autosomal dominant essential tremor: a novel family with anticipation

Genome-wide estimates of heritability and genetic correlations in essential tremor

INTRODUCTION

Despite considerable efforts to identify disease-causing and risk factors contributing to essential tremor (ET), no comprehensive assessment of heritable risk has been performed to date.

METHODS

We use GREML-LDMS to estimate narrow-sense heritability due to additive effects (h²) and GREMLd to calculate non-additive heritability due to dominance variance (δ²) using data from 1,751 ET cases and 5,311 controls. We evaluate heritability per 10 Mb segments across the genome and assess the impact of Parkinson's disease (PD) misdiagnosis on heritability estimates. We apply genetic risk score (GRS) from PD and restless legs syndrome (RLS) to explore its contribution to ET risk and further assess genetic correlations with 832 traits by Linkage disequilibrium score regression.

RESULTS

We estimated ET narrow-sense heritability to be h² = 75.5% (s.e = ±0.075). In contrast, dominance variance showed insignificant effect on the overall estimates. Heritability split by 10 Mb regions revealed increased estimates at chromosomes 6 and 21. The proportion of genetic variance due to PD misdiagnosed cases was estimated to be 5.33%. PD and RLS GRS were not significantly predictive of ET case-control status.

CONCLUSIONS

We show for the first time that ET is a highly heritable condition in which additive common variability plays a prominent role. Chromosomes 6 and 21 may contain causative risk variants influencing susceptibility to ET. Despite overlapping symptomatology, ET does not seem to share genetic etiologies with PD or RLS. Our study suggests that most of ET genetic component is yet to be discovered and future GWAS will reveal additional risk factors contributing to ET.

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.

What is essential tremor?

Classic essential tremor is a clinical syndrome of action tremor in the upper limbs (at least 95 % of patients) and less commonly the head, face/jaw, voice, tongue, trunk, and lower limbs, in the absence of other neurologic signs. However, the longstanding notion that essential tremor is a monosymptomatic tremor disorder is being challenged by a growing literature describing associated disturbances of tandem walking, personality, mood, hearing, and cognition. There is also epidemiologic, pathologic, and genetic evidence that essential tremor is pathophysiologically heterogeneous. Misdiagnosis of essential tremor is common because clinicians frequently overlook other neurologic signs and because action tremor in the hands is caused by many conditions, including dystonia, Parkinson disease, and drug-induced tremor. Thus, essential tremor is nothing more than a syndrome of idiopathic tremulousness, and the challenge for researchers and clinicians is to find specific etiologies of this syndrome.