Intellia Therapeutics Inc., a gene-editing start-up that filed for a proposed $120 million IPO on Monday, added a new partnership with Regeneron Pharmaceuticals Inc. The six-year deal, focused on discovery and development of CRISPR/Cas9-based products, carries a $75 million up-front fee for Cambridge, Mass.-based Intellia, plus a promised $50 million equity investment, potential milestone payments of up to $320 million per target and possible royalties.