Jeffrey M. Besterman, Ph.D., has been appointed executive vice president of R&D and chief scientific officer at MethylGene, Inc. Joe Walewicz, vice president of business and corporate development, and Klaus B. Kepper, vice president finance and chief financial officer, will be leaving the company.   Dr. Charles Baum, chief executive officer, will take over Dr. Besterman’s responsibilities and Mark Gergen, chief operating officer has assumed the responsibilities of Mr. Kepper and Mr. Walewicz...