The nonsense-mediated decay pathway and mutually exclusive expression of alternatively spliced FGFR2IIIb and -IIIc mRNAs

Exons IIIb and me of the FGFR2 gene are alternatively spliced in a mutually exclusive manner in different cell types. A switch from expression of FGFR 2IIIb to FGFR2IIIc accompanies the transition of nonmalignant rat prostate tumor epithelial cells (DTE) to cells comprising malignant AT3 tumors. Here we used transfection of minigenes with and without alterations in reading frame and with and without introns to examine how translation affects obser ved FGFR2 splice products. We observed that nonsense mutations in other tha n the last exon led to a dramatic reduction in mRNA that is abrogated by re moval of downstream introns in both DTE and AT3 cells. The mRNA, devoid of both IIIb and me exons (C1-C2), is a major splice product from minigenes la cking an intron downstream of the second common exon C2. From these observa tions, we suggest that repression of exon me and activation of exon mb incl usion in DTE cells lead to the generation of both IIIb-C2 and C1-C2 product s. However, the C1-C2 product hom the native gene is degraded due to a fram eshift and a premature termination codon caused by splicing C1 and C2 toget her. Derepression of exon me and repression of exon IIIb lead to the genera tion of both C1-IIIc-C2 and C1-C2 products in AT3 cells, but the C1-C2 prod uct is degraded. The C1-IIIb-IIIc-C2 mRNA containing a premature terminatio n codon in exon me was present, but at apparently trace levels in both cell types. The nonsense-mediated mRNA decay pathway and cell type-dependent ra tes of inclusion of exons IIIb and me result in the mutually exclusive expr ession of FGFR2IIIb and IIIc.