Two distinct Notch1 mutant alleles are involved in the induction of T-cellleukemia in c-myc transgenic mice

We have previously characterized a large panel of provirus insertion Notch1 mutant alleles and their products arising in thymomas of MMTVD/myc transge nic mice. Were, we show that these Notch1 mutations represent two clearly d istinct classes. In the first class (type I), proviral integrations were cl ustered just upstream of sequences encoding the transmembrane domain. Type I Notch1 alleles produced two types of mutant Notch1 RNA, one of which enco ded the entire Notch1 cytoplasmic domain [N(IC)] and the other of which enc oded a soluble ctodomain [N(EC)(Mut)] which, in contrast to the processed w ild-type ectodomain [N(EC)(WT)], did not reside at the cell surface and bec ame secreted in a temperature-dependent manner. A second, novel class of mu tant Notch1 allele (type II) encoded a Notch1 receptor with the C-terminal PEST motif deleted (Delta CT). The type II Notch1(Delta CT)protein was expr essed as a normally processed receptor [N(EC)(WT) and N(IC)(Delta CT)] at t he cell surface, and its ectodomain was found to be shed into the extracell ular medium in a temperature- and calcium-dependent manner. These data sugg est that both type I and type II mutations generate two structurally distin ct Notch1 N(EC) and N(IC) proteins that may participate in tumor formation, in collaboration with the c-myc oncogene, through distinct mechanisms. Con stitutive type I N(IC) and type II N(IC)(Delta CT) expression may enhance N otch1 intracellular signaling, while secreted or shed type I N(EC)(Mut) and type II N(EC) proteins may differentially interact in an autocrine or para crine fashion with ligands of Notch1 and affect their signaling.