Aberrant Splicing in the Presenilin-1 Intron 4 Mutation Causes Presenile Alzheimer's Disease by Increased A 42 Secretion

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Abstract
We previously described a splice donor site mutation in intron 4 of presenilin-1 (PSEN1) in two patients with autopsy-confirmed early-onset Alzheimer's disease (AD). Here we provide evidence that the intron 4 mutation is present in four additional unrelated early-onset AD cases, that the mutation segregates in an autosomal dominant manner and that all cases have one common ancestor. We demonstrate that the intron 4 mutation produces three different transcripts, two deletion transcripts (Δ4 and Δ4cryptic) and one insertion transcript (insTAC), by aberrant splicing. The deletion transcripts result in the formation of C-truncated (∼7 kDa) PSEN1 proteins while the insertion transcript produces a full-length PSEN1 with one extra amino acid (Thr) inserted between codons 113 and 114 (PSEN1 T113–114ins). The truncated proteins were not detectable invivo in brain homogenates or lymphoblast lysates of mutation carriers. In vitro HEK-293 cells overexpressing Δ4, Δ4cryptic or insTAC PSEN1 cDNAs showed increased Aβ42 secretion (∼3.4 times) only for the insertion cDNA construct. Increased Aβ42 production was also observed in brain homogenates. Our data indicate that in the case of intron 4 mutation, the AD pathophysiology results from the presence of the PSEN1 T113–114ins protein comparable with cases carrying dominant PSEN1 missense mutations.