Plano-based drug developer Reata Pharmaceuticals Inc. raised more than $505 million in a stock sale Monday to speed its transition from a clinical-stage biotech startup to a company that makes treatments for life-threatening diseases. The publicly-traded company will use the money to register and commercialize its first two treatments: one for Alport syndrome, a genetic disease that can cause kidney disease and failure, and the other for Friedreich’s ataxia, a genetic disease that often begins in childhood and leads to brain and nervous system damage.