MUC1 kidney disease (MKD) is caused by a frame-shift mutation in the MUC1 gene (MUC1-fs). Affected individuals develop kidney failure and no treatment is currently available. The main goal of this study was to investigate the cellular and molecular mechanisms by which MUC1-fs alters the epithelial cell function, and to develop a mechanism-based therapy for this disease. To achieve these goals, we have used a high content imaging approach as follows: (i) a thorough colocalization study had identified the accumulation of the mutated protein MUC1-fs in TMED9-enriched COP vesicles, where it induced ER stress by activating the unfolded protein response. (ii) a CRISPR-Cas9 arrayed study was used to discover the cargo receptor TMED9 as a novel mediator of the retention of the misfolded protein MUC1-fs. TMED9 was shown to bind MUC1-fs, prevent its forward trafficking along the secretory pathway and promote the misfolded protein accumulation. (iii) a drug screen, composed of the drug repurposing library was developed to identify BRD4780 which, by binding TMED9, reroutes the misfolded protein into the lysosome where it is ultimately degraded. BRD4780 was shown to clear MUC1-fs from patient cells, kidneys of knock-in mice and patient kidney organoids. Importantly, BRD4780 showed no overt toxicity and in fact, it rescued cells from ER stress-induced cell death.
In summary, using a high content imaging approach we have elucidated the molecular mechanism underlying MKD and discovered a small molecule with exciting and promising potential as a therapeutic lead for misfolded protein disorders.