Absence of Runx3 expression in normal gastrointestinal epithelium calls into question its tumour suppressor function

The Runx3 transcription factor regulates cell fate decisions during embryonic development and in adults. It was previously reported that Runx3 is strongly expressed in embryonic and adult gastrointestinal tract (GIT) epithelium (Ep) and that its loss causes gastric cancer. More than 280 publications have based their research on these findings and concluded that Runx3 is indeed a tumour suppressor (TS). In stark contrast, using various measures, we found that Runx3 expression is undetectable in GIT Ep. Employing a variety of biochemical and genetic techniques, including analysis of Runx3-GFP and R26LacZ/Runx3^Cre or R26tdTomato/Runx3^Cre reporter strains, we readily detected Runx3 in GIT-embedded leukocytes, dorsal root ganglia, skeletal elements and hair follicles. However, none of these approaches revealed detectable Runx3 levels in GIT Ep. Moreover, our analysis of the original Runx3^LacZ/LacZ mice used in the previously reported study failed to reproduce the GIT expression of Runx3. The lack of evidence for Runx3 expression in normal GIT Ep creates a serious challenge to the published data and undermines the notion that Runx3 is a TS involved in cancer pathogenesis.

Translation regulation of Runx3

Runx3 protein products that are translated from the distal (P1)- and proximal (P2)-promoter transcripts appear on Western blots as a 47-46kDa doublet corresponding to full-length proteins bearing the P1- and P2-N-termini respectively. An additional 44kDa protein band, the origin and nature of which was unclear, is also detected. Transfection of full-length Runx3 cDNA bearing the P2 N-terminus (P2-cDNA) into HEK293 cells resulted in expression of both 46 and 44kDa proteins. Sequence analysis of the P2-cDNA revealed an in-frame ATG 90bp downstream (+90ATG) of the proximal +1ATG. Insertion of an N-terminal HA-tag into P2-cDNA immediately downstream of the +1ATG produced HA-tagged 46kDa and untagged 44kDa proteins, consistent with the possibility that the latter was translated through initiation at the internal +90ATG site. Deleting or blocking the activity of the +1ATG, the natural cap-dependent translation initiation site in P2-cDNA, abrogated production of the 46kDa Runx3 protein while facilitating production of the 44kDa product. These findings supported the notion that Runx3 44kDa protein resulted from internal translation initiation at the +90ATG. Northern blot and RT-PCR analyses performed on RNA from P2-cDNA transfected cells showed a single transcript and product respectively, of the expected size, ruling out the possibility that the 44kDa protein was translated from transcripts originating at a cryptic promoter or produced by alternative splicing. Taken together, the data indicate that the 44kDa protein results from translation initiation at the internal ATG and that Runx3, like its family members Runx1 and Runx2, contains a mechanism for internal mRNA translation initiation.