Mammalian NDR kinases : tumor suppressors with essential functions in embryonic development

NDR kinases are highly conserved from yeast to man. Loss-of-function models of Ndr homologs in yeast and fly demonstrate essential functions of the respective kinases. Mammalian Ndr1 and Ndr2 are widely expressed and share a high degree of sequence identity. Human NDR kinases function in centriole duplication, mitotic chromosome alignment, apoptosis and proliferation. Mice that lack functional NDR1 protein are phenotypically normal, but protein levels of NDR2 are up-regulated in Ndr1-null tissues suggesting a compensatory link between both isoforms. Aged Ndr1 knock-out (KO) mice develop T-cell lymphoma, indicating a tumor suppressive function of mammalian NDR kinases. Several reports describe deregulated Ndr transcript levels in human cancers but the functional relevance of the expression changes has not been addressed.
The present study reveals that mice carrying a targeted deletion of Ndr2 are phenotypically normal but show an up-regulation of NDR1 protein levels. Combined loss of Ndr1 and Ndr2 results in embryonic lethality, demonstrating that NDR kinases play essential roles in mammalian development. Ndr-null embryos are small and developmentally delayed at embryonic day (E) 8 and die around E10. Transcript levels of the CDK inhibitors p21 and p27 are up-regulated in Ndr-null embryos at E8.5, suggesting that NDR kinases positively regulate proliferation in vivo. Mutant somites are small and irregularly shaped. Asymmetric expression of the somite-clock genes Lunatic Fringe and Hes7 in mutant embryos indicates that NDR kinases contribute to ensure bilateral symmetry in the embryo. In the absence of NDR kinases, heart development arrests at the linear heart tube stage and does not proceed to cardiac looping. Proper establishment of the left / right axis is a prerequisite for rightward cardiac looping. Cardiac malformation is most likely the primary cause for embryonic lethality of Ndr-null embryos. Asymmetric gene expression and impaired cardiac looping might reflect a general symmetry defect in NDR-deficient embryos.
Embryonic lethality precludes the analysis of in vivo functions of NDR kinases in the adult mouse. To address the role of NDR in the context of tumorigenesis, I have generated an intestinal epithelium specific Ndr1/2 double KO (Ndr1-/-Ndr2Δ/ΔVilCre) mouse line. Ndr1-/-Ndr2Δ/ΔVilCre mice develop rectal prolapse, a symptom of chronic inflammation of the colon. Importantly, patients suffering from chronic colitis are at increased risk of developing colorectal cancer (CRC). Although Ndr1-/-Ndr2Δ/ΔVilCre mice do not spontaneously develop colon cancer, initial studies indicate that Ndr1-/-Ndr2Δ/ΔVilCre mice are more susceptible to Azoxymethane (AOM)-induced colon carcinogenesis. Therefore, Ndr1-/-Ndr2Δ/ΔVilCre mice could provide a new model system to study the molecular mechanisms that underlie the increased risk of CRC formation in patients with chronic colonic inflammation.
In summary, this study demonstrates that mammalian NDR kinases are essential for embryonic development. They positively regulate growth, somitogenesis and heart development. Whether the defect in bilateral symmetry and the cardiac phenotype are causally connected remains to be addressed. Complete loss of NDR kinases in the intestinal epithelium causes rectal prolapse and increased susceptibility to AOM-induced CRC formation. Lastly, the conditional Ndr double KO mouse line represents a valuable tool to address additional in vivo functions of mammalian NDR kinases in normal physiology and disease.