Keenan Cahill

YouTube powerhouse Keenan Cahill, famous for his lip-synch videos with celebrities, has died. Keenan's manager, David Graham, tells TMZ ... Keenan had open heart surgery on Dec. 15 but there were complications from the procedure and he was put on life support. We're told Keenan died Thursday…

He's already been through 50 Cent and Katy Perry ... but now YouTube powerhouse Keenan Cahill -- that kid who lip-syncs with the stars -- is facing off with Pauly D ... with fist pumps of FURY!!!! 15-year-old Keenan was diagnosed with a rare genetic disorder when he was a baby -- but he's burned…