NOTCH1 Aberrant Activation Is a Common Nonmutational Early Event in Chronic Lymphocytic Leukemia Hematopoietic Stem-Cells

Chronic lymphocytic leukemia (CLL) is a B cell disorder characterized by constitutive NOTCH1 activation. NOTCH1 mutations are recurrently associated with CLL providing a new clonal marker to track CLL origin. To investigate CLL-initiating cells, we assessed NOTCH1 mutational status and signaling in hematopoietic stem (HSCs) and progenitor cells from CLL bone marrow. We collected a total of 29 bone marrow (BM) samples including 22 CLL patients and 7 healthy donors (HDs). BM cells were sorted into CD34+CD38- HSCs (purity 94.23% ± 3.04% ) and CD34+CD38+ progenitor fraction (98.12% ± 1.34%). While Sanger sequencing analysis of the NOTCH1 failed to detect alterations in CD34+/CD38- HSCs, AS-PCR and Droplet digital PCR (ddPCR) indicated the presence of small HSCs mutated clones in 57% of cases. Altogether, these data confirm that NOTCH1 mutation is an early event in CLL hematopoiesis. The analysis of CD34+/CD38+ progenitors detected the NOTCH1 mutation in the majority of samples (66%). The NOTCH1 mutational burden progressively increased along specific stages of HSC differentiation (6.4%± 4.7 in CD34+CD38- to 14.9%±11.3 in CD34+CD38+CD10+CD19+ cells, 22.7%±6.5 in CD34-CD38+CD10+CD19+ cells and 40.5%±4.3 in neoplastic CD5+CD19+ cells). This suggests that the NOTCH1 lesion is selected and expands during HSC differentiation toward a B neoplastic cell, thus strengthening the hypothesis that the genetic alteration is an initial event associated with the stepwise malignant transformation of CLL. Conversely, CD34+ cells (1x10⁶) from CLL patients carrying the mutation, when transplanted into NOD/SCID/IL2Rgnull (NSG) mice generated a mature CLL phenotype (the median percentage of CD5+CD19+ cells in hCD45+ cells was 38.5% and 43.8% in BM and spleen respectively) lacking NOTCH1 mutation (as showed by AS-PCR assay performed on enriched hCD45 cells). This data is more in line with the hypothesis that this genetic abnormality is acquired at the mature B cell stage as an additional leukemogenic event to transform into clinical CLL. Thus, we analyzed the NOTCH1 signaling status in HSCs and progenitor cells of NOTCH1-mutated and unmutated CLL samples. The flow-cytometric analysis of the active NOTCH1-ICN protein level showed unmutated and mutated CLL has a significantly higher NOTCH1-ICN level than HDs samples in both CD34+/CD38- HSCs and CD34+CD38+ (73.4%±22.9 and 83%±16.4 vs 33.3%±14.8; 94.4%±7.3 and 92.8%±4.3 vs 47.9%±13.8, p<0.01 and p<0.001, respectively). c-MYC expression was found significantly higher in HSCs cells from NOTCH1 mutated and unmutated CLL samples compared to HD (3.5±0.7 and 2.6±0.08 vs 1.3±0.1). Western blot analysis of the expression levels of the NOTCH1-TM subunit revealed that HSCs from mutated and unmutated CLL patients always expressed the NOTCH1-TM protein, compared to HDs where NOTCH1-TM was either absent or expressed at lower levels. The present study indicated that the pool of CD34+ cells, including HSC and progenitor compartments, have NOTCH1 aberrantly expressed and activated in CLL patients compared to HDs demonstrating a common nonmutational NOTCH1 activation occurring early in CLL hematopoiesis. This selective pressure might contribute to the onset of specific NOTCH1 mutations in a DNA context that is prone to spontaneous microdeletion. These data represent a rationale for the use of therapies targeting the NOTCH1 signaling in CLL aimed to inhibit the survival of CLL-initiating cells.