Arsanis (NASDAQ: ASNS) and X4, two Boston-area biotech firms, agreed Tuesday morning to merge in a deal that will enable X4 to become a publicly traded company. The combined company will keep the X4 name and be led by X4’s president and CEO (and former Genzyme vet) Paula Ragan. Arsanis president and CEO Rene Russo has resigned to “pursue other opportunities,” the companies said in a statement.