Targeting rare cardiomyopathy conditions with no approved disease-modifying therapies, Atrium Therapeutics Inc. started operations in San Diego, assuming the public listing and technology once held by Avidity Biosciences Inc., an RNA company acquired by Novartis AG for about $12 billion. Novartis closed the M&A, announced last October, as Atrium unveiled its pipeline and a $270 million cash position.