A recent review article makes the case that DM is a brain disease and that better understanding of and treatment strategies for the neurological consequences of DM are essential.
Considerable Gaps Exist in Understanding of the CNS in DM
The central nervous system (CNS) consequences are arguably among the least understood aspects of myotonic dystrophy (DM) and certainly have received only modest attention in drug development.
- Grant applications—or at least successful ones—in this niche are few. The National Institutes of Health’s (NIH) categorical spending database reports that approximately $1.6 million of the $8.8 million awarded for DM in fiscal 2016 is for grants focused solely on the CNS (just one R01 and one P01 that is in its last year);
- Savvy industry researchers recognize the considerable contribution that CNS sequela make toward the overall burden of disease;
- Exploration of the CNS in has produced sparse natural history data and few insights into biomarkers that can provide early indications of target engagement and modulation and clinically meaningful endpoints.
A Status Report
Drs. Genevieve Gourdon (French National Institute of Health and Medical Research) and Giovanni Meola (University of Milan) have published a provocative review article addressing current status of understanding of CNS-related symptoms and the development of tools and therapeutic strategies to address them in DM.
An overall premise of this review, while acknowledging the considerable skeletal and cardiac muscle involvement, is that DM is a brain disease. The authors do acknowledge an essential barrier in moving forward toward CNS-targeted therapies—that the current level of understanding of how repeat expansion-triggered molecular changes link to CNS phenotypes in DM is only a shadow of the mechanism-to-phenotype understanding that we have for skeletal muscle.
The authors review current knowledge of the neuropsychological, cognitive and other CNS signs in CDM, DM1 and DM2, based on available functional and imaging methodology. A gap in longitudinal data, understanding how CNS symptoms progress with age, was identified as particularly acute. To drive interventional studies, it is necessary to establish strong correlations between CNS functioning and endpoints (such as imaging) that can be easily and reproducibly assessed in clinical trials of manageable duration. While we are not yet to this point, the authors note that small studies linking skeletal muscle and neurologic changes are suggestive of common disease mechanisms and highlight the potential that further data may support the linkage of muscle and CNS endpoints in the same clinical trial.
While the invasive splicing biomarkers developed for skeletal muscle-targeted drugs are not available for the CNS, assessment of proteins or exosomal RNA in blood or CSF may provide insights into patient neurological status. The authors’ literature review suggests that correlations between biomarkers and CNS functional status may be weak, at least for studies reported thus far. Instead they point to the value of cell and animal-based models as a means to develop sufficient scientific rationale to drive human clinical trials.
Finally, Drs. Gourdon and Meola give their assessment of putative strategies to therapeutically target the CNS in DM: the primary DNA mutation, toxic RNA, mediator proteins, or the variety of downstream targets arising from specific gene mis-splicing events. Nearly all of the data supporting these various strategies has been developed in studies of skeletal muscle, while conceptually applicable to the CNS, but will encounter both drug delivery and potentially different toxicity questions.
Taken together, the difficulties of assessing the CNS in DM, identifying meaningful endpoints for clinical trials, and ultimately establishing the effectiveness of CNS targeted therapies is captured in the author’s observation of the complex interrelationship of neurological, psychological and social factors in DM. Yet it is essential that we find ways to address the critically important neurological sequela of DM.
Steps to Address the Problem
The authors point to knowledge gaps—longitudinal natural history studies, linkage of imaging and other biomarkers to CNS phenotypes, and better-powered studies focused on more homogeneous cohorts—as important to the path forward. For its part, MDF has organized a forum, Bringing the Patient Voice to CNS-Targeting Drug Development in Myotonic Dystrophy, to bring in the invaluable patient perspective at the IDMC-11/MDF Annual Conference in San Francisco, on September 9, 2017.
Myotonic Dystrophies: State of the Art of New Therapeutic Developments for the CNS
Gourdon G, Meola G. Front Cell Neurosci. 2017 Apr 20;11:101. doi: 10.3389/fncel.2017.00101. eCollection 2017.