Investigation of the role of matrix metalloproteinases in the genetic etiology of Alzheimer's disease

Matrix metalloproteinases (MMPs) are a multigene family of proteinases regulating the functions of a large number of signaling and scaffolding molecules that are involved in neuro-inflammation, synaptic dysfunction and neuronal death. MMPs have been associated with neurological conditions, such as Alzheimer's disease (AD), through a sudden and massive upregulation of particular members of the MMP family. Evidence for this hypothesis can be found in the clinical observation of increased MMP1 and MMP3 expression levels in plasma of AD patients compared to control individuals and in the pro-amyloidogenic effects that have been described for additional MMP family members like MMP13, MT1-MMP, and MT5-MMP. Consequently, we investigated the role of MMP1, 3, 13, MT1-MMP, and MT5-MMP in the genetic etiology of AD. We performed full exonic resequencing of these 5 MMPs in 1278 AD patients (mean age at onset [AAO]: 74.88 ± 9.10, range: 29-96) and 797 age-matched control individuals (mean age at inclusion [AAI]: 74.92 ± 6.48, range: 65-100) from Flanders-Belgium and identified MMP13 as most promising candidate gene. We identified 6 ultra-rare (≤0.01%) MMP13 missense mutations in 6 patients that were absent from the control cohort. We observed in one control individual a frameshift mutation (p.G269Qfs*2) leading to a premature termination codon. Based on previously described functional evidence, suggesting that MMP13 regulates BACE1 processing, and our genetic findings, we hypothesize a gain-of-function disease mechanism for the missense mutations found in patients. Functional experimental studies remain essential to assess the effect of these mutations on disease related processes and genetic replication studies are needed to corroborate our findings.