Molecular characterization of in-frame and out-of-frame alternative splicings in coagulation factor XI pre-mRNA

Alternative splicing of pre-mRNAs is a central process to the generation of proteome complexity. However, many alternative mRNA isoforms carry premature termination codons (PTCs) rendering them possible targets for the nonsense-mediated mRNA decay (NMD) pathway. The F11 gene consists of 15 exons spanning approximately 23 kb on chromosome 4q35 and codes for coagulation factor XI (FXI), a 160-kDa dimeric zymogen composed of 4 apple domains and a serine protease domain. Here, we characterized the F11 splicing pattern in human liver and platelets identifying multiple in-frame and out-of-frame splicing events. Inhibition of NMD resulted in the up-regulation of all unproductively spliced F11 transcripts, thus providing evidence that these PTC-containing mRNAs are under the control of NMD. Among in-frame alternatively spliced transcripts, the one skipping exons 6 and 7 would lead to the synthesis of a FXI protein lacking 1 apple domain (FXI-Delta6/7). Ex vivo expression in mammalian cells demonstrated that FXI-Delta6/7 is mostly retained intracellularly, and secreted only in low amounts. Traces of this FXI isoform were detectable in human plasma. Our results suggest that the coupling of alternative splicing and NMD may play a role in regulating F11 expression, and point to the existence of a novel FXI isoform.