Characterizing the Transcriptomic Landscapes of Essential Tremor

IETF Funded Research

Principal Investigator:
Dr. Guy Rouleau, OC, OQ, MD, PhD, FRCPC, FRSC
Director, The Neuro
Chair, Department of Neurology and Neurosurgery, McGill University

PROJECT SUMMARY

Specific aims: Essential tremor (ET) is one of the most common neurological disorders affecting nearly 1% of the worldwide population. It causes uncontrollable shaking of multiple body parts including the hands, arms, head and legs. The disorder is quite debilitating, having an effect on the capacity of patients to perform daily activities such as eating and dressing.

Not much is known about why and how ET arises in certain individuals. We know that the main brain region responsible for ET is the cerebellum, the brain region that is required to control the coordination of movements. However, we have limited information on the role that the different cells in the cerebellum play in the disease. We previously used RNA sequencing to investigate gene expression changes in the cerebellum of ET patients. This method is unable to study gene expression changes in specific cell populations and lacks the required resolution to understand the role of each cell type in the disease.

We also know that certain drugs are better than others at treating essential tremor. Propranolol is the most prescribed drug in treating ET, but the mechanism behind its efficiency is unknown. We previously screened the effects of propranolol on gene expression in human neural stem cells as well as cerebellar cancer cells. Although we found that it affected the expression of genes related to ET, these cells do not accurately represent cells found in the cerebellum of ET patients.

We would like to better resolve these issues by using more relevant techniques and cellular models to understand 1) the role of different cerebellar cells in ET, 2) the effects that genetic risk factors for ET have on the expression of certain genes in the cerebellum and 3) the effects of propranolol on cerebellar cells derived from ET patients.

Aim 1: Assess the transcriptomic effects of propranolol on ET patient-derived cerebellar cells

Human induced pluripotent stem cells (hiPSCs) from ET patients that have positive responses to propranolol will be differentiated into cerebellar cell cultures containing mature PCs and other cerebellar neurons and treated with propranolol. Single-cell RNA sequencing will be used to compare the effects of propranolol on gene expression in the different cell populations of the cerebellum. Changes to cell morphology and electrophysiological properties will also be assessed. We aim to identify genes that explain positive responses to propranolol.

Aim 2: Characterize the cellular landscape of the essential tremor cerebellum

We will use single-cell RNA sequencing to study gene expression changes in all cell populations of the cerebellum. We aim to sequence close to 1 million cells from cerebelli of 16 ET patients and 80 healthy patients. We hope to identify gene expression changes in relevant cell types in ET such as PCs. We will also investigate the role of non-neuronal cells, such as oligodendrocytes and astrocytes, in the pathophysiology of ET. We hope to learn about the pathological features of different cell types in the cerebellum of ET patients.

Aim 3: Evaluating the effects of genetic risk factors on gene expression in cerebellar cells

Small changes to the genetic code can have a large effect on the expression of genes. These effects are mostly cell-type specific. Recent advances in single-cell RNA sequencing and population genetics have allowed us to link genetic events to expression events (called expression quantitative trait loci (eQTLs)) and infer the effects of genetic risk factors on gene expression in specific cells. We plan to leverage our large-scale single-cell cerebellar data set coupled with genotyping data to first map these single-cell eQTLs, and second assess how genetic risk factors for ET (identified in our last genome-wide association study) can affect the expression of certain genes in different cell types of the cerebellum. We hope to characterize the progression from genetic risk factor to functional effects on the cerebellum and thus better understand the pathophysiological mechanisms driving risk for ET.