Unravelling the mechanisms of transcriptional dysregulation in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease affecting adult males worldwide, resulting in progressive and yet untreatable motor dysfunction and severe disability. Previous studies from my group and others have shown that skeletal muscle is primarily affected in this disease and therapies exclusively directed at restoring the muscle atrophy are sufficient to rescue the disease phenotype in animal models. SBMA is caused by mutations in the Androgen Receptor (AR), a protein that is critical in maintaining skeletal muscle young and strong and whose functions is to bind DNA and turn specific genes on or off. Proteins like AR are called 'Transcription Factors'. How this transcription factor exerts such effects and why muscles become weak and atrophic in SBMA are key unanswered and interconnected questions: such lack of understanding of these mechanisms of control of muscle function in health and disease is precisely the reason why no treatments are yet available for SBMA patients and, as a matter of fact, for most diseases characterised by skeletal muscle loss. During the course of this Fellowship, my plan is to understand how the SBMA mutation hijacks AR ability to instruct the cell (i.e. coordinate transcription), leading to muscle atrophy and weakness. To achieve this goal, we will use relevant disease models, such as skeletal muscle cells and muscle biopsies derived from patients, which is key to generate meaningful data, and employ state-of-the-art molecular biology tools which allow the investigation of these very specific functions. In particular, we have recently set up in the lab a powerful and innovative microscopy technique, called single-molecule tracking, which enables the study of transcription factor dynamics in living cells and at high resolution, giving important information regarding the diffusion and binding behaviour of these proteins in their own environment.
What's most exciting about this research is its potential for therapeutic impact. Using genetic approaches to either enhance or suppress AR ability to regulate transcription, we aim to test whether restoration of mutant AR transcriptional activity is sufficient to treat SBMA. To perform these experiments, we have devised a platform to grow muscle in a 3D bundle (muscle-on-a-chip), which recapitulates the architectural and structural complexities of muscle, effectively representing a replica of skeletal muscle outside of the body. Lastly, key findings will be functionally validated in vivo in a SBMA mouse model.
Through application of these innovative and fully integrated cell and molecular biology techniques in humans and model systems, these studies will provide a substantial advancement in the understanding of the mechanisms of disease and provide proof-of-principle evidence of targeting transcriptional activity as a treatment strategy for SBMA and other diseases characterized by skeletal muscle loss.

Spinal and bulbar muscular atrophy (SBMA) is a severe and progressive adult-onset neuromuscular condition caused by a polyglutamine expansion (polyQ) in the transcription factor Androgen Receptor (AR). Understanding the mechanisms of pathogenesis is paramount to develop effective treatments for this condition, which are currently not available. Previous work from my and other labs has shown that: 1) binding of AR to DNA is a necessary step to trigger SBMA, 2) transcriptional dysregulation occurs early in the disease, and 3) skeletal muscle is a primary driver of SBMA pathogenesis and a target for therapy. These premises represent the foundation of my research goal, which is to understand how dysregulation in AR transcriptional activity leads to the skeletal muscle loss in SBMA. Our preliminary data show that in skeletal muscle AR cooperates with another transcription factor, SMAD4, to activate a set of DNA regulatory regions, named enhancers, which are of critical importance in coordinating gene expression in mammalian cells. Combining transcriptional analysis in patient-derived skeletal muscle, a state-of-the-art 3D muscle-on-a-chip model, and our recently established experimental set-up of advanced imaging, which allows the study of transcription factor dynamics in live cell and at single molecule resolution, I will explore here the central hypothesis that alteration in AR-SMAD4 cooperativity leads to SBMA by perturbing enhancer activity. Here I plan to address the following questions: What are the effects of polyQ AR on SMAD4 chromatin binding and transcription dynamics? How disruption in enhancer activity determines transcriptional dysregulation in SBMA? And lastly, is targeting hijacked enhancers sufficient to reverse SBMA muscle pathology and rescue the disease? Successful completion of this research programme will set the stage for targeting transcriptional activity as a strategy to treat SBMA and other muscle wasting diseases.