Runx3 knockouts and stomach cancer

Abstract

Gene targeting often results in knockout mice that show several phenotypes, some of which may not directly relate to the intrinsic function of the disrupted gene. Hence, to study the biological function of genes using knockout mice, one must identify the defects that are directly due to the loss of the targeted gene. Runx3 is a transcription factor that regulates lineage-specific gene expression in developmental processes. Recently, two groups produced Runx3 knockout mice. Two comparable defects were identified in both knockout strains, one involved neurogenesis and the other thymopoiesis. In addition, a stomach defect pertaining to gastric cancer was observed in one of the mutant strains, but not in the other. Here, we assess the differences between the two Runx3 mutant strains and discuss further studies that could reconcile these discrepancies. This article highlights the difficulties of inferring gene function through the interpretation of knockout phenotypes.

Figure 1

The mammalian RUNX genes: structure and mode of function. (A) The three mammalian RUNX genes have similar genomic organization with two promoters (P1 and P2) and a very large first intron. The two promoters give rise to two biologically distinct 5′ untranslated regions (UTRs) (yellow and orange). In humans and mice, each gene resides on different chromosomes (human 21, 6 and 1, and mouse 16, 17 and 4, respectively). The highly conserved runt domain is encoded by the three exons marked in green. Exons comprising the transactivation domain are shown in black and grey and the 3′ UTR in blue. Runx3 is the smallest and simplest of the three genes. (B) The runt domain directs binding to the RUNX DNA-motif PyGPyGGT at the promoter of target genes, and protein–protein interactions withcore-binding factor-β (CBF-β). The RUNX proteins bind to the same DNA motif and either activate or repress transcription through interactions with other transcription factors (blue ellipse) and co-activators (arrows), or co-repressors (blocked line). Of note, due to lack of space, only a few examples of RUNX transcriptional co-modulators are indicated.

Figure 2

The Runx3 genomic locus including the neighbouring genes and the potential lacZ messenger RNA and protein products. (A) Genomic organization showing the two promoters (P1 and P2) and six exons. The different targeting sites in exons 2 and 4 used by Levanon et al. (2002) and Li et al. (2002), respectively and the corresponding targeting cassettes are indicated, as are the distances between Runx3 and the Dsi-1 and Clic4 loci. The positions of the CpG-rich islands at the 5′ and 3′ regions of Runx3 are marked. The various lacZ mRNAs that are transcribed from the P1 and P2 promoters in both type I and type II knockout mice are shown in (B) as are the corresponding protein products (LacZ and runt domain–LacZ) in (C). The yellow and orange segments represent the 5′ untranslated regions of P1 and P2 respectively. The green segments denote the runt domain (RD) and the blue segments represent the LacZ protein. Of note, the genomic organization of Runx3 (Bangsow et al. 2001) which is shown here including the number of Runx3 exons (1–6) differs from that in Li et al., (2002) due to the omission of exon 1 in the latter.