Looking at DUX4
Scientific breakthroughs have revealed that the cause of FSHD lies in the undesired production of the DUX4 protein in skeletal muscle cells, causing them to waste away. DUX4 sets in motion a cascade of events involving hundreds of genes whose exact roles and relative contributions to FSHD pathology have not yet been fully elucidated. What is known, however, is that FSHD is characterized by sustained inflammation that damages muscle cells. Further damage is caused by a process named oxidative stress that involves the formation of highly reactive oxygen atoms. Finally, a process of programmed cell death causes progressive muscle loss.
DUX4 is sporadically produced in just a fraction of FSHD-affected muscle cells, but even these small amounts result in muscle dystrophy. Given the central role of DUX4 in FSHD, we believe that to stop the progression of FSHD we have to prevent DUX4 protein production at the source.
The cause of DUX4 expression
Regulatory mechanisms are needed to activate or silence the right genes in the right tissue at the right time. The human body has control mechanisms to do so. These mechanisms can turn genes on or off over time, without altering the information of the gene itself. In fact, during the last few years it has become evident that FSHD is associated with changes in this control system at the DUX4 gene on chromosome 4, leading to its undesired activation in muscle cells. So FSHD is not just about having the DUX4 gene; more importantly, it is about having an unduly activated DUX4 gene.
Our approach to FSHD drug discovery
Because the regulatory system of gene expression in the cell is very complex and consists of many interacting proteins, we used a so-called phenotypic approach. This means that we looked for molecules that prevented the production of the DUX4 protein without placing any restriction on the underlying mechanisms. In other words, our drug discovery program is unbiased. By using skeletal muscle cells derived from FSHD patients, our phenotypic approach captures the biological complexity of FSHD and, importantly, maximizes our discovery potential. The very low and variable expression levels of DUX4 have hampered such an approach in the past, but together with our partners, we managed this challenge.
We have established a platform for the reliable quantification of DUX4 protein levels in primary patient-derived FSHD skeletal muscle cells. To the best of our knowledge, we are the first to overcome this challenge. Using this unique DUX4 screening platform, we screened the effects of over 34,000 small molecule compounds and identified more than 300 so-called “hits” acting through several novel mechanisms that repress the production of the muscle-toxic DUX4. ‘Small molecules’ are relatively simple organic compounds that are manufactured by chemical synthesis.
Towards a causal therapy
These hits - with various underlying mechanisms of action - open doors to a therapy, however they are no medicines for FSHD yet. Thereto we carefully evaluate how these hits work in FSHD, to prioritize and optimize the avenues with the most promising safety and efficacy profile. In addition, we prepare for the development of lead compounds for preclinical and clinical development. This way, we are working towards a much needed treatment for FSHD, without cutting corners.