The purpose of this study is to evaluate the safety and effectiveness of an experimental device called the AltaValve System in people with moderate-to-severe and severe mitral regurgitation. The study aims to determine whether the device can help treat a leaky valve; it involves a procedure that is less invasive than open heart replacement of the mitral valve. Study procedures include transcatheter mitral valve replacement procedure and clinic visits. A smaller study to assess the safety of the AltaValve System was already completed; this study is being conducted to confirm AltaValve System’s long-term effectiveness. The AltaValve is a self-expandable device created to fit the patient’s heart. The tissue valve is a man-made valve that imitates the patient’s own mitral valve leaflets (leaf-like structures that help control the blood flow in the heart). The fabric and tissue are intended to reduce or stop the leakage of the mitral valve.