CMT4B1 is a severe form of Charcot-Marie-Tooth disease caused by mutations in the gene that makes Myotubularin-related protein 2 (MTMR2). Dr. Alessandra Bolino, PhD, discovered that mutations in this gene affect the production of fats, called phospholipids. Among other cellular roles, phospholipids help build and maintain Schwann cell myelin sheaths; the insulation that wraps around healthy nerves.

In people with CMT4B1, phospholipids are produced in excess, which leads to an altered structure of the outer coating of the nerves, called the myelin sheath. The misshapen myelin sheath in CMT4B1 causes the nerves to function improperly and leads to damage.

In this CMTRF co-funded project with the Muscular Dystrophy Association and Dr. Alessandra Bolino from San Raffaele Hospital in Italy, Dr. Bolino aims to understand the biological pathways impacted in CMT4B1 and identify potential treatment approaches that might restore normal myelin structure and nerve function in CMT4B1 by utilizing known drugs that target these biological pathways.

Dr. Bolino is investigating whether three signaling pathways are impacted by the MTMR2 mutations and are overactive in CMT4B1, specifically mTORC1, RhoA and Arf6. These pathways are involved in many cellular functions including regulating cell growth, metabolism, cell migration membrane trafficking and survival.

Dr. Bolino is utilizing small molecule drugs, Rapamycin and Fasudil, that modulate the mTORC1 and RhoA pathways, to study the genetic removal of Arf6 in these pathways.

Rapamycin is an immunosuppressant drug that is commonly used for anticancer therapies and organ transplant therapies. Similarly, Fasudil is used for treatment following certain types of strokes and is being tested as a potential therapeutic for other neurological conditions and high blood pressure.

Using cell models of CMT4B1, preliminary results suggested that targeting the mTORC1 and RhoA pathways could have benefits on CMT4B1 characteristics and warranted further study in a CMT4B1 animal model. However, when tested in a mouse model, the drug that reduces the mTORC1 pathway, Rapamycin, surprisingly made the disease symptoms worse. This finding was unexpected but still provides valuable information in understanding how the pathway can be modified to lead to therapeutic benefits. Dr. Bolino is currently conducting further investigations into what changes Rapamycin had in the nerves of these animals.

Then, Dr. Bolino tested the RhoA-targeting drug, Fasudil, in the CMT4B1 mouse model. This treatment did not lead to any beneficial effects in the nerves of the CMT4B1 mouse model — although in this case there was no worsening as was seen with Rapamycin treatment. These findings suggest that targeting the RhoA pathways, specifically using Fasudil, may not be an ideal therapeutic approach for CMT4B1, but this requires further investigation.

While these results suggest that Rapamycin and Fasudil may not be good candidates for treating CMT, it’s important to understand why this is occurring and how it can be prevented in other instances. It’s also incredibly important knowledge for clinicians that may have considered treating CMT4B1 patients with drugs targeting these pathways. Gaining this knowledge is important for determining what other drugs may work for CMT treatments.

Overall, the concluding year of this project will provide clues for how to treat CMT4B1 and what pathways could be druggable targets for CMT therapies.