Biogen Inc. didn't have to look far beyond the revenue stream for its spinal muscular atrophy (SMA) blockbuster, Spinraza (nusinersen), to conclude that keeping partner Ionis Pharmaceuticals Inc. close at hand was in its best interest. With the five-year neuroscience deal that spawned Spinraza set to expire in the next year, Biogen sweetened the pot by paying $1 billion for a 10-year extension.