Novartis has entered an agreement to buy AveXis Inc., a gene therapy company, for $8.7 billion dollars as part of its neuroscience strategy.   The deal follows the recent sale of Novartis’s stake in its consumer health-care joint venture to GlaxoSmithKline PLC (GSK.LN) for $13 billion.    Novartis will pay $218 a share of AveXis in cash, an 88% premium to its closing price on April 6, and intends to fund the acquisition with available cash and short-term borrowing.   ...