MMP9 and Alzheimer disease: On the other hand, the role of MMPs in chronic neurodegeneration has not been elucidated, but postmortem analysis of the cerebral cortex of AD patients and controls without dementia demonstrated that tissue from AD patients carrying the APOE4 isoform had increased permeability to fibrin and immunoglobulin G, which correlated with the degeneration of pericytes and the accumulation of the pro-inflammatory cytokines cyclophilin A and MMP-9 [29].