Fannie Mae has completed its first major securitization of subprime mortgages. But officials say the $975 million deal, backed by loans originated by Countrywide Home Loans, doesn’t necessarily mark the start of a trend. "We are looking to respond to the needs of our lender partners and are evaluating deals on a case-by-case basis," said William Senhauser, Fannie’s vice president and deputy general counsel. The Countrywide transaction also includes about $108 million in prime second