Heavy refinance volume pushed both Fannie Mae and Freddie Mac single-family mortgage securitization up appreciably during the fourth quarter of 2012, helping to close out a post-crisis record year for GSE mortgage-backed security business, according to a new Inside The GSEs analysis.Fannie and Freddie issued $352.51 billion in single-family MBS during the fourth quarter, a 5.2 percent increase from the previous period and the biggest quarter in over three years.