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 (Delta4 and Delta4cryptic) and one insertion transcript (insTAC), by aberrant splicing. The deletion transcripts result in the formation of C-truncated (approximately 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 in vivo in brain homogenates or lymphoblast lysates of mutation carriers. In vitro HEK-293 cells overexpressing Delta4, Delta4cryptic or insTACPSEN1 cDNAs showed increased Abeta42 secretion (approximately 3.4 times) only for the insertion cDNA construct. Increased Abeta42 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.

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The c-FOS gene product, a putative transacting transcriptional regulator of the amyloid precursor protein (APP) gene, is a candidate locus for the familial Alzheimer's disease (FAD) mutation on chromosome 14 (FAD14). In light of this functional relationship, we investigated the nucleotide sequence and segregation of c-FOS and the nucleotide sequence of the 5' APP promoter. Single-stranded conformational polymorphisms (SSCPs) in the c-FOS gene revealed that c-FOS closely cosegregates with the FAD14 gene but does not show allelic association with FAD. A conservative third-position T→C mutation was demonstrated in exon 2 (codon 84) of c-FOS, and a C→G substitution was detected at—209 bp in the 5' promoter of APP. Neither were unique to FAD and are unlikely to be pathogenic or secondary modifiers of the FAD phenotype. We conclude that the c-FOS open reading frame is probably not the site of the FAD14 locus, but we cannot exclude the existence of modifier loci on chromosome 21.