* Singapore mortgage market is weak spot - DBS CEO (Recasts with new comments from CEO, details of results): SINGAPORE (Reuters) - Southeast Asia’s biggest lender DBS Group Holdings Ltd beat market estimates to post a record quarterly profit, as strong lending income offset weakness in wealth management, brokerage and investment banking fees. DBS, the first Singapore bank to kick off the sector’s results, posted an 8.5 percent rise in first-quarter net profit from a year earlier, and said the macro-economic environment had stabilized. “By and large, I’m relatively sanguine about the business momentum,” CEO Piyush Gupta told a news conference.