i work with a USC grad who's taking a year off from columbia med to do the howard hughes fellowship at NIH...PM me if you want to get in touch with her!
I'm a USC undergrad. Yeah, there's only like 4 SC people on mdapps from the last few years or so, but I think it's fair to assume that whether or not you're accepted by columbia, like every other school, will depend on a lot more than your undergrad university...if that's what you're wondering.