A shift from refinances to purchase mortgages has helped push combined loan-to-value ratios on jumbo mortgage-backed securities to the highest levels since the financial crisis. The higher LTV ratios are prompting an increase in credit enhancement requirements. JPMorgan Chase this week issued a $355.64 million jumbo MBS with an average combined LTV ratio of 74.0 percent. And WinWater Home Mortgage is preparing a $276.91 million jumbo MBS with an average combined LTV ratio of ...