iPSC-derived cortical neurons with a novel frameshift PSEN2 mutation increase the ratio of aggregate prone amyloid beta
Jacquelyn E. Braggin1, Stephanie A. Bucks1, Refugio Martinez2, Carole Smith1, Bryce Sopher1, Leah Osnis1, Kevin Green1, Debby Tsuang1,4, Jim Leverenz7, Paul Valdmanis4, Michael Dorschner2,3,5, Tom Bird1,4,6, Jessica Young2, and Suman Jayadev1*
1Neurology; 2Pathology; 3UW Medicine Center for Precision Diagnostics; 4Department of Medicine, Division of Medical Genetics; 5Psychiatry & Behavioral Sciences, University of Washington; Seattle, WA; 6Geriatric Research, Education, and Clinical Center, VA Puget Sound Health Care System; Seattle, WA; 7Cleveland Lou Ruvo Center for Brain Health; Cleveland, OH.
Heterozygous mutations in Presenilin 2 (PSEN2) cause nearly fully penetrant autosomal dominant Alzheimer Disease (AD). PSEN2, an essential component of the gamma-secretase complex of proteins, catalyzes a crucial step in the amyloidogenic amyloid precursor protein (APP) cleavage cascade to generate amyloid-beta (Aβ) peptides. Accumulation of aberrantly-produced insoluble Aβ isoforms is an important component of AD pathogenesis. While most driver PSEN2 mutations are missense, we have discovered a novel heterozygous PSEN2 two-basepair deletion frameshift mutation (PSEN2K115fsX) in two unrelated individuals with AD. To determine whether this mutation exhibits molecular hallmarks of AD, we generated induced pluripotent stem cells (iPSCs) from patient fibroblasts to study cell type-specific mutational effects. Following differentiation of control and mutant iPSCs into cortical neurons, we measured secreted Aβ isoforms in conditioned media and found that the ratio of insoluble to soluble Aβ is increased from mutant cells. To validate this PSEN2 mutation as a molecular driver of these observations, we are testing whether correction of the mutation in genome-edited isogenic iPSC lines rescues the Aβ phenotype. Finally, this frameshift mutation offers a unique way to probe the impact of major biochemical changes to the expression and function of one PSEN2 allele. We are analyzing the impact of mutation on PSEN2 RNA splice isoforms as well as exploring the greater role of PSEN2 AD mutations in human microglia through iPSC differentiation.