The Maternal Embryonic Leucine zipper Kinase (MELK) is definitely an interesting enzyme. It’s been implicated in a number of cancer cell lines, and it also has important roles in the normal cell cycle, in embryogenesis, and other functions. It’s one of those proteins that’s found all across mammalian (and non-mammalian) species, with a lot of sequence conservation, but its roles in different species seem to vary more than you’d think.
The drug industry is interested because of the oncology connection – aggressive breast cancer cell lines and others are reported to have enhanced MELK activity, and their proliferation can apparently be slowed by siRNA knockdown. It’s also been implicated in glioblastoma, colon cancer, ovarian cancer and others. Several small-molecule inhibitors have been reported, and there are reports that they may ultimately work through a p53-based mechanism downstream. The one that made it to the clinic first is OTSSP167, from OncoTherapy Science in Japan, although its progress has not been as fast as anticipated, from the looks of it.
There was a report last year that the compound could well be hitting other kinase targets at the concentrations that it reaches in vivo, and that these might be responsible for some of its effects. That has certainly happened several times in the kinase-inhibitor world, although to be sure it happened more in earlier years. But now there’s a paper out that calls into question the whole idea of inhibiting MELK in the first place.
A group at Cold Spring Harbor has found that knocking MELK down (update – to be more accurate, completely knocking it out) via CRISPR treatment in a whole list of different cancer cell lines has no effect on their growth. Considering the earlier siRNA work, that’s something to sit down and think about. What’s more, these modified cell lines are still sensitive to the OTS compound, which certainly lends force to the earlier reports of off-target effects. The off-target effects aren’t just important, apparently – they’re the only thing the compound has to offer.
So at the very least, MELK monotherapy doesn’t look promising. But hey, the compound in the clinic isn’t a MELK monotherapy agent anyway, so it has that going for it. This is an embarrassing situation for OncoTherapy Science, I’d have to say, and doesn’t make their web site’s presentation on the compound look very robust. On the other hand, though, this is a very good example of target (de)validation through CRISPR, and I congratulate the Sheltzer group at Cold Spring for doing it. (Here’s an article with some background on the work).
It certainly raises a number of questions – for one, what was going on with those earlier siRNA experiments? That technique certainly can have some off-target effects of its own, and in this case those may have been the whole reason for the effects on cell growth. Another thing to consider is that there’s an X-ray structure of the OTS compound bound to MELK, which just goes to show you that that says nothing at all about what’s happening in vivo. The whole MELK field is going to have to figure out what’s going on, because I’d be willing to bet that many groups would have thought that CRISPR-ing it out would have been lethal all by itself. But apparently not, so it’s back to the drawing board for everyone.