BUZZ-MBIA Inc: BTIG downgrades on exposure to Puerto Rico debt
** Bond insurer's shares down 4 percent at $11 premarket
** BTIG LLC analysts downgrade stock to "neutral" from "buy"
** Cite uncertainty regarding impact of restructuring of debt of Puerto Rico Electric Power Authority (PREPA)
** "MBI has $1.531 billion of insured exposure to PREPA's debt as of March 31," analysts said in a note to investors
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