Towards gene therapy approaches in a rare and orphan disease: Charcot Marie-Tooth (CMT) peripheral neuropathy.

Project Leader: Robert Burgess, Ph.D.

Co-investigator: Scott Harper, Ph.D. (Nationwide Children’s Hospital)

The long-term goal of this project is to develop a therapeutic approach for Charcot-Marie-Tooth Type 2D (CMT2D), caused by autosomal dominant mutations in the GARS gene. This objective of this project is to reduce mutant Gars gene expression using disease allele-specific RNAi in mouse models of CMT2D that are humanized for the GARS gene. We expect to demonstrate pre-clinical proof-of-principle for the effectiveness of this approach and develop an in vivo platform for testing therapeutic vectors for the treatment of CMT2D.

Background & Rationale

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous collection of disorders in which peripheral motor and sensory axons degenerate. It is the most common inherited condition in the peripheral nervous system, and approximately 1:2500 people are affected. There is still no cure for any form of CMT and treatment options are limited to supportive care such as surgery to treat foot deformities, and braces and therapy to improve mobility. Clinically, CMT is broadly divided into demyelinating (type 1) and axonal (type 2) forms, as well as intermediate forms that have both demyelinating and axonal features. Genetically, at least eighty loci in the human genome can cause CMT, but particular genetic forms are often quite rare. The identification of disease-associated genes has helped categorize the pathophysiological bases of CMT, such as structural proteins of myelin (type 1) and the axonal cytoskeleton, axonal transport, and mitochondrial dysfunction (type 2), but many forms do not clearly fit these categories, suggesting there are many cellular and molecular routes to axon degeneration. As such, compelling molecular pathways for therapy remain elusive, particularly for the axonal neuropathies. Furthermore, the genetic heterogeneity makes it unlikely that a single therapy will be effective for all forms of CMT. Together, these factors conspire against the development of effective therapies for CMT. In the absence of a small number of good drug targets for development, personalized genetic approaches may offer sensible alternative therapeutic strategies. We are beginning to explore such a genetic approach as a possible treatment for CMT2D, a dominant axonal neuropathy (type 2) caused by mutations in GARS. Several years of work in mouse models of CMT2D have demonstrated a toxic effect of the mutant Gars protein and most importantly, that an allele-specific knockdown of the mutant transcript or protein should be beneficial.

We are currently working with Dr. Scott Harper of the Center for Gene Therapy at Nationwide Children’s Hospital to establish the feasibility of mutant Gars allele-specific knockdown using AAV-RNAi gene therapy vectors Once these proof-of-concept studies are completed, we will introduce the human variants in GARS associated with CMT2D into the mouse. These experiments will allow initial evaluation of therapeutic effectiveness of this approach to gene therapy for CMT2D. Once completed positive results can be translated to clinical applications.