Subprime lending could help topple more thrifts in the months ahead, the head of one regulatory agency warned last week. According to Ellen Seidman, director of the Office of Thrift Supervision, 17 of the 1,045 institutions regulated by the agency have low-ratings. "One of those institutions could fail within the next 12 months," she said. Seidman characterized six of those thrifts as "aggressive subprime lenders."