Mega deals morph into mega problems for Wall Street

By Carl O'Donnell and Pamela Barbaglia NEW YORK/LONDON (Reuters) - If 2015 was a dream year for Wall Street's top dealmakers, 2016 is starting to take a nightmarish turn. Some of the mega transactions that had champagne corks popping in boardrooms are running into antitrust problems and, in the case of pharmaceutical firm Pfizer Inc's $160 billion takeover of rival Allergan PLC , political opposition to a deal that envisaged the biggest drug company in the United States moving to Ireland to lowe