Development of a Notch pathway assay and quantification of functional Notch pathway activity in T-cell acute lymphoblastic leukemia

The Notch signal transduction pathway is pivotal for various physiological processes including immune responses, and has been implicated in the pathogenesis of many diseases including T-cell acute lymphoblastic leukemia. Various targeted drugs are available that inhibit Notch pathway signaling, but their effectiveness varies due to variable Notch pathway activity among individual patients. Quantitative measurement of Notch pathway activity is therefore essential to identify patients who could benefit from targeted treatment. We here describe a new assay that infers a quantitative Notch pathway activity score from mRNA levels of conserved direct NOTCH target genes. Following biological validation, we assessed Notch pathway activity in a cohort of TALL patient samples and related it to biological and clinical parameters including outcome. High Notch pathway activity was not limited to T-ALL samples harbouring strong NOTCH1 mutations, including juxtamembrane domain mutations or hetero-dimerization combined with PEST-domain or FBXW7 mutations, indicating that additional mechanisms may activate NOTCH signaling. The measured Notch pathway activity related to intracellular NOTCH levels, indicating that the pathway activity score more accurately reflects Notch pathway activity than predicted on the basis of NOTCH1 mutations. Importantly, patients with low Notch pathway activity had a significantly shorter event-free survival compared to patients showing higher activity.