Wow, absolutely terrible news. Truly a towering figure of Chicago house music (and as 3rd says, house in general)
My favorite Paul Johnson memory is seeing him at a long-gone spot in Wicker Park about 15 years ago (i.e. recently enough that a few vestigial remnants of pre-gentrification WP were still around) play to like, half a dozen people, after a screening of a short documentary about DJs that a friend of a friend had produced. W/no crowd he was in full-on play whatever the fuck I want mode, an endless barrage of personal favorite 80s Chicago house + pre-house classics, disco, Italo, EBM, whatever. I didn't know enough to recognize it at the time, but probably exactly the kind of set he grew up listening to from the Hot Mix 5 etc. One of the key moments that got me interested in dance music in the first place.