(Reuters) -AstraZeneca said on Friday its Alexion unit had agreed to buy U.S. drugmaker Pfizer's early-stage rare disease gene therapy portfolio for up to $1 billion, plus royalties on sales, as the British drugmaker bets on new genetic therapies. The British company also said Sharon Barr, head of research and development at Alexion, would succeed Mene Pangalos, the long-time biopharmaceuticals head of research at AstraZeneca.