This is a pretty interesting paper on several levels. It sheds light on Mucin I kidney disease (MKD), on protein degradation pathways (a hot topic these days, as those in the industry well know), and it also provides a small molecule lead compound. It’s a large multicenter team, starting off with the Broad Institute, but bringing in several other associated groups around Boston and Cambridge, as well as Wake Forest, Charles University (Prague), Pittsburgh, and the University of Cyprus. As you’d imagine from that, there’s a lot to be told, and I definitely won’t be covering every detail in this post.
MKD, a form of (fortunately rare) tubulointerstitial kidney disease, was already known to be localized to the MUC1 gene, via various frameshift mutations that end up in pretty much the same place in patients with the disease. The aberrant protein is missing part or all of important transmembrane and intracellular domains, so it’s no wonder it doesn’t work. It ends up secreted and presumably clogs up glomerular function somehow, but the details aren’t very clear (although as the disease progresses they can be seen histologically). There are no treatments, partly because the disease are quite rare and partly because of the lack of any mechanisms to get a handle on. From what I can see, most people don’t know that they even have MKD (or the other diseases in its class) until they start showing major kidney problems – and in fact, they may not even realize that they have a family history of it.
This new paper fills in the details, though. It is indeed a proteinopathy disease, although accumulation of the mutant Muc-1 protein takes a while to cause trouble. The unfolded protein response can handle it, up to a point (this is similar to the nucleolus stuff I blogged about last week – the cell has a lot of repair mechanisms, backups, and mitigation strategies, but they can only go so far). Eventually, slow loss of UPR function with aging and/or other environmental stress allows damage to build up. The team shows that a protein called TMED9 is a crucial part of the disease etiology – it’s a cargo receptor, part of a class of proteins that seem to sort proteins into forming vesicles inside the cell. And in this case, the mutant Muc-1 protein gets stuck in these TMED9-containing vesicles in between the Golgi and the endoplasmic reticulum, and they just pile up there.
The med-chem part of the paper comes in via a screen for small molecules that might disrupt this process, and the paper identifies BRD4780 as an active (shown at right). I’m not sure what I would have expected to come out of such a screen, but that isn’t it. That really looks like it should be hitting an ion channel, or maybe an amine GCPR. As a matter of fact, it was originally developed as a selective imidazoline receptor ligand, back in the 1990s at Allergan (where it was known as AGN-192403). The idea then was that such ligands could be antihypertensives, but to the Allergan teams’s surprise, this potent and selective I-1 compound seem to have no cardiovascular effects whatsoever. In years since, it’s turned up as having protective effects on mitochondria and as an inhibitor of fin regeneration in a zebrafish screen, but in neither of those was an imidazoline ligand expected to be a hit a priori. As in this paper as well!
Its mechanism of action on this protein trafficking problem is completely unknown. Something about it causes the mutant mucin to move on from the stuck vesicles, heading into endosomes and ultimately being degraded by the lysosome, which is just where such misfolded nonfunctional junk is supposed to end up. Update: as you can see from the comments, the paper tries to make a case for direct targeting of TMED9 by the compound, but I’m not really convinced by that, either. There’s no real SAR in this paper from what I can see, not even to the point of resolving the enantiomers of the compound, so there would seem to be some real room to step in and do something about some rare proteinopathies. There appear to be up to 20 different proteins that get stuck in a similar way on their way to the ER in various rare diseases, and the paper demonstrates that it works for two of these besides Muc-1.
The screen was a high-content cell-based one, using fluorescent markers to determine mucin protein localization. So discovery of this compound wasn’t waiting on the mechanistic details at all; this was a phenotypic screen that has both provided a compound and illuminated some of those details by doing so, which is just what a good screen is supposed to do for you. I hope to see follow up on this work, both out of scientific curiosity and in the hopes that this might lead to a useful therapy for a number of patients who currently have nothing available to them at all.
To that second point, though, one can also see how something like MKD has not attracted as much interest from the rare-disease-development crowd. One of the tricky parts of that business is that you have to be able to find your patients in the first place, and you also have to find them in time to be able to help them. A lot of potential targets drop off the list of prospects when you dig into those, and as mentioned above, this could be a tough one. People are born with the protein handling problem, but they don’t start showing symptoms for decades – and those symptoms are usually signs of rather serious kidney damage by that point. The cellular protein-clearing machinery (and the kidney as a whole) can take a lot of abuse, but that abuse occurs pretty silently, which means that the people who are likely to be helped the most by a new drug in this area don’t even know that they have a disease in the first place. Anyone wanting to do drug development in this field is going to have to find a way out of that dilemma. . .