Very roughly...
John Komnenos lives for a few years longer, and is able to totally wipe the Turks out of Asia Minor by 1150. His son Manuel inherits the throne in 1152, and, Manuel being Manuel, goes on a millitary adventure a few years later, to regain Southern Italy from the Normans, which is done in alliance with the Holy Roman Emperor. With Byzantine garrisons in Naples, the Pope becomes to feel increasingly insecure, and attempts to organise a crusade against the "heretical Greeks". Manuel is in no mood to be Crusaded against though, and, in alliance with the Holy Roman Emperor, marches on Rome. The city falls in 1168, and is subject to a brutal sacking, when the Byzantines' Turkish foederati troops run amok.
Any good?