Research News

LAY SUMMARY- PI3 Kinase Inhibition as a Novel Treatment Strategy for MTM

Myotubular myopathy (MTM) is one of the most severe neurologic disorders of childhood. It is characterized by significant lifelong morbidity (including ventilator and wheelchair dependence), and early mortality. Currently, no treatments or disease modifying therapies exist for MTM.  Much current research effort is focused on developing gene therapy for MTM.  However, while gene therapy may represent the ultimate treatment for MTM, complementary and/or alternative therapeutic strategies have the potential for significantly improving quality and length of life for all MTM patients.  The goal of our research program is to identify and develop small molecule based complementary therapies for this devastating disorder.

MTM is caused by mutations in the MTM1 gene. MTM1 (or myotubularin) is an enzyme responsible for removing phosphates from lipids called phosphoinositides.  We have shown that loss of MTM1 results in the accumulation of a specific phosphoinositide called PI(3)P.  Based on this, we hypothesize that (a) this accumulation of PI(3)P is responsible for many of the problems that affect the muscle in myotubular myopathy and (b) reducing (or “rebalancing”) levels of PI(3)P may reverse or prevent these problems and thus result in clinical benefit.  We tested these hypotheses using a genetic strategy, where we genetically inactivated in the mouse an enzyme that can make PI(3)P (a PI3 kinase).  Inactivating this PI3 kinase in the mouse model of MTM resulted in mice that are completely healthy AND have no problems with their muscles.  This exciting result suggests the possibility that inhibiting PI3 kinase in patients may be a meaningful treatment strategy.

In year 1 of our MTM Trust grant, we have accomplished the following.  We have verified that the genetic interaction between the PI3 kinase and MTM1 holds true in zebrafish.  We did this using a technology called morpholino knockdown to genetically change the PI3 kinase in our zebrafish model of MTM.  We then did preliminary testing of a set of drugs known to inhibit PI3 kinases.  The initial studies with these drugs show that they improve the MTM zebrafish phenotype, thus supporting our hypothesis that inhibitors of this pathway may be viable therapies for MTM.  Finally, we demonstrated using our mouse experiments that the impact of PI3 kinase inactivation appears beneficial even after the disease has started.  This suggests the very exciting potential that drugs that target this pathway may have benefit even after the disease has developed.

In year 2 of the project, we completed testing of 6 PI3 kinase inhibitors and showed that 3 of them (with preferential action against PIK3C2B) are able to improve aspects of the mtm zebrafish phenotype. We went on to test one of these drugs (wortmannin) in the MTM mouse model. Using a placebo-controlled design, we demonstrated that wortmannin increases muscle strength and prolongs survival of the MTM mouse by 13 days (from 37 days to 50 days). These very promising observations support the concept that PI3 kinase inhibition is a viable treatment strategy for MTM. These data, along with the genetic studies in the mouse mentioned above, were recently accepted for publication in the Journal of Clinical Investigation, a “high impact” journal that reflects the important nature of our findings. http://www.jci.org/articles/view/86841

In the second 6 months of year 2 of the project, we have worked to identify more effective PI3 kinase inhibitors for MTM. The reason for this is that the effect of wortmannin is relatively modest, and wortmannin acts to inhibit some proteins that we know are important for normal muscle function and for muscle health in the MTM mouse model. To find new PIK3C2B inhibitors, we are using a novel prediction methodology (in collaboration with Cyclica) based on known drug structures and activities. From this, we have a prioritized list of 15 drugs that are predicted to act specifically to inhibit PIK3C2B and not inhibit other similar proteins. We are now verifying that these drugs have activity against PIK3C2B using non-cell based methods, and at the same time testing these drugs for their ability to improve the mtm zebrafish phenotype. The most promising candidate(s) from this group will then be trialed in the MTM mouse during year 3 of our MTM trust grant.

In all, we believe we have met all key milestones of our MTM Trust grant through the first 2 years of the project, and are on (or ahead of) target for year 3 goals. In fact, we are on pace to potentially identify two new therapeutics suitable for clinical translation and clinical trials by the end of the 3 year MTM Trust grant cycle, which we would view as a considerable achievement. The ultimate goal therefore is to have at least one candidate drug ready for translation to clinical trial by the end of year 3 of the grant.