C-terminal deletion of NOTCH1 intracellular domain (N1) increases its stability but does not amplify and recapitulate N1-dependent signalling.

Since the generation of a mouse strain conditionally expressing the active intracellular domain of Notch1 (N1), many laboratories have exploited this model (Rosa) to assess the impact of constitutive Notch1 signalling activation in normal and pathological processes. It should be underscored that Cre-recombination leads to the expression of a C-terminally truncated form of N1 (N1) in the Rosa mutant mice. Given that no studies were undertaken to delineate whether deletion of this region leaves intact N1 function, stable cell lines with single targeted integration of inducible N1 and N1 were generated. We found that C-terminal deletion of N1 stabilized the protein but did not promote the activity of Notch responsive promoters. Furthermore, despite higher expression levels, N1 failed to phenocopy N1 in the promotion of anchorage-independent growth. Our results thus suggest that the C-terminal region of N1 plays a role in shaping the Notch response. Therefore, it should be taken into consideration that N1 is truncated when interpreting phenotypes of Rosa mutant mice.