Dr. Nancy A. Simonian took the helm at Syros Pharmaceuticals Inc. in Watertown in 2013, the latest step on a path that has taken her from practicing medicine at Massachusetts General Hospital to the executive ranks of biotechs Biogen Inc. and Millennium Pharmaceuticals Inc. Two-year-old Syros was founded by Flagship Ventures and Arch Venture Partners based on science from the Dana-Farber Cancer Institute and the Massachusetts Institute of Technology. Simonian, 54, is working with her team to develop drugs that regulate genes that cause cancer and other diseases.