In a deal reminiscent of Genzyme's $700 million collaboration with Cambridge neighbor Alnylam Pharmaceuticals a year ago, the rare drug giant today announced a deal worth up to up to $845 million with gene therapy startup Voyager Therapeutics. The agreement between 35-employee Voyager, located on Sydney Street in Cambridge, came out of a natural overlap between what the two companies have been working on, according to Mark Barrett , Genzyme's vice president of Strategy & Business Development for Rare Diseases.