Second-row has emerged as a real problem area for the floundering Grand Slam champions, with Alun-Wyn Jones, Luke Charteris and Bradley Davies all ruled out of next weekend's opening clash with Ireland, and Ian Evans a major doubt.
Interim head coach Howley could switch Ryan Jones from the back-row, but the former captain is recovering from a dislocated thumb.
So 6ft 7in Kohn, who started five of Quins' Heineken Cup pool matches this season in their march to the No 1 seeding for the quarter-finals, becomes the third uncapped lock in the Wales squad, joining Ospreys' James King and Dragons' Andrew Coombs.
"It is a really proud moment for me and a great opportunity to show what I can do," he said.
Kohn has made more than 130 appearances for Aviva Premiership champions Quins since joining them from Bristol in 2006.
"He is a strong ball-carrier and dominant at the set-piece, and it will be good to see him in the international environment."
Howley is due to announce his team for the Ireland game next Thursday. Other injured players out of the equation include fly-half Rhys Priestland, flanker Dan Lydiate and centre Ashley Beck.