ATP-binding Cassette Transporter A7 (ABCA7) Loss of Function Alters Alzheimer Amyloid Processing

The ATP-binding cassette transporter A7 (ABCA7) has been identified as a susceptibility factor of late onset Alzheimer disease in genome-wide association studies. ABCA7 has been shown to mediate phagocytosis and affect membrane trafficking. The current study examined the impact of ABCA7 loss of function on amyloid precursor protein (APP) processing and generation of amyloid-beta (A beta). Suppression of endogenous ABCA7 in several different cell lines resulted in increased beta-secretase cleavage and elevated A beta. ABCA7 knock-out mice displayed an increased production of endogenous murine amyloid A beta 42 species. Crossing ABCA7-deficient animals to an APP transgenic model resulted in significant increases in the soluble A beta as compared with mice expressing normal levels of ABCA7. Only modest changes in the amount of insoluble A beta and amyloid plaque densities were observed once the amyloid pathology was well developed, whereas A beta deposition was enhanced in younger animals. In vitro studies indicated a more rapid endocytosis of APP in ABCA7 knock-out cells that is mechanistically consistent with the increased A beta production. These in vitro and in vivo findings indicate a direct role of ABCA7 in amyloid processing that may be associated with its primary biological function to regulate endocytic pathways. Several potential loss-of-function ABCA7 mutations and deletions linked to Alzheimer disease that in some instances have a greater impact than apoE allelic variants have recently been identified. A reduction in ABCA7 expression or loss of function would be predicted to increase amyloid production and that may be a contributing factor in the associated Alzheimer disease susceptibility.