A noncoding mutation in the NOTCH1 gene initiates oncogenic NOTCH signaling via wild-type NICD stabilization in CLL.

Chronic Lymphocytic Leukemia (CLL) is the most common chronic blood cancer in adults. Active NOTCH signaling in CLL is associated with poorer prognosis. Importantly, CLL patients with NOTCH1 non-coding mutations in the 3-prime untranslated region (3'UTR) manifested with a more aggressive disease course even compared to those with mutations in the NOTCH1 coding region. Here, we comprehensively characterize a cryptic splice acceptor site in 3'UTR of the NOTCH1 gene being converted into a stronger site. The functional consequences of the resulting NOTCH1 protein variants depend on the exact localization of the splice site, the used open reading frame and the appearance of the next STOP codon. The most frequent 3'UTR mutation (g.139390152, A>G) generates a novel NOTCH1 protein, lacking the PEST domain but expressing an altered C-terminus consisting of 68 amino acids. Mechanistically, we show that this splice variant (NOTCH1 152) is transcriptionally less active and dysregulates the regular ubiquitination dependent degradation of the wild type NOTCH1 intracellular domain (NICD) in trans. Thus, the NOTCH1 152 variant acts as a "sponge" protein in a novel mechanism of oncogenic NOTCH signaling activation, explaining the detrimental disease outcome of CLL patients with non-coding NOTCH1 mutations. We propose that the detection of NOTCH1 152 protein by specific antibodies is a useful prognostic marker for CLL patients.