NOTCH1 Aberrations in Chronic Lymphocytic Leukemia

Abstract

Chronic lymphocytic leukemia (CLL) is an incurable B-cell neoplasm characterized by highly variable clinical outcomes. In recent years, genomic and molecular studies revealed a remarkable heterogeneity in CLL, which mirrored the clinical diversity of this disease. These studies profoundly enhanced our understanding of leukemia cell biology and led to the identification of new biomarkers with potential prognostic and therapeutic significance. Accumulating evidence indicates a key role of deregulated NOTCH1 signaling and NOTCH1 mutations in CLL. This review highlights recent discoveries that improve our understanding of the pathophysiological NOTCH1 signaling in CLL and the clinical impact of NOTCH1 mutations in retrospective and prospective trials. In addition, we discuss the rationale for a therapeutic strategy aiming at inhibiting NOTCH1 signaling in CLL, along with an overview on the currently available NOTCH1-directed approaches.

Keywords: NOTCH1; chronic lymphocytic leukemia; gene mutation; prognostic biomarker; targeted therapy.

Figure 1

NOTCH1 protein structure and signaling activation. (A) The mature NOTCH1 receptor is a heterodimer composed of an extracellular subunit (NOTCH1-EC) and a transmembrane and intracellular subunit (NOTCH1-TMIC). The NOTCH1-EC includes epidermal growth factor (EGF)-like repeats, involved in ligand binding, three LIN-12/NOTCH repeats (LNR), which prevent receptor activation in the absence of ligands, and the heterodimerization domain (HD) involved in non-covalent interactions between the NOTCH1-EC and NOTCH1-TMIC. The NOTCH1-TMIC consists of the transmembrane domain (TM) and the intracellular domain (ICD) (NOTCH1-ICD). NOTCH1-ICD comprises an RBPJ-associated molecule (RAM) domain, seven ankyrin (ANK) repeats, nuclear localization signals (NLS), a transactivation domain (TAD), and a PEST domain, which is involved in proteasomal degradation of active NOTCH1-ICD. (B) Newly synthesized NOTCH1 precursor is cleaved by a furin-like convertase (Furin) in the Golgi apparatus to generate the mature receptor. NOTCH1 signaling initiates when a JAGGED or DELTA ligand expressed on a signal sending cell interacts with NOTCH1 on a signal receiving cell. This interaction triggers two sequential cleavages of NOTCH1: the first, by an a disintegrin and metalloproteinase (ADAM) metalloproteinase, generates the substrate for the second cleavage by γ-secretase, which releases the active NOTCH1-ICD. NOTCH1-ICD translocates to the nucleus where it forms a transcriptional activation complex by interacting with the transcription factor CSL/RBP-Jk, mastermind-like proteins, and others coactivators (CoA), leading to the expression of NOTCH1 target genes. In physiological conditions, NOTCH1 signal attenuation is mediated by ubiquitination and proteasomal degradation of NOTCH1-ICD.

Figure 2

The distribution of NOTCH1 mutations in chronic lymphocytic leukemia (CLL). (A) Schematic diagram of the human NOTCH1 gene consisting of 34 exons (vertical bars) and spanning 50 kb. (B) The NOTCH1 protein includes an extracellular domain (EC) and an intracellular domain (ICD) linked by a transmembrane portion (TM). The NOTCH1-ICD consists of RBPJ-associated molecule (RAM) and ankyrin (ANK) domains involved in CSL/RBP-Jk binding, a transactivation domain (TAD) and a PEST (polypeptide enriched in proline, glutamate, serine and threonine) domain, which is a hotspot for mutation in CLL. (C) The ANK/TAD (upper panel) and PEST (lower panel) domains are magnified, and the color-coded shapes indicate the position of the in frame deletion/insertion (green), non-sense (yellow), frameshift (blue), missense (red), and non-coding (black dot) mutations found in CLL. The F-box and WD repeat domain-containing 7 (FBXW7) and WSSSSP degron are indicated within the PEST domain.

Figure 3

Frequency of different types of NOTCH1 mutations in chronic lymphocytic leukemia. Pie charts describing the incidence of NOTCH1 mutations according to the type of genetic variant (A) and domains (B) involved.

Figure 4

NOTCH1 mutation frequency varies according to the clinical time point of the disease. Box–whisker plots indicate the prevalence of mutated NOTCH1 in monoclonal B-cell lymphocytosis (MBL), in chronic lymphocytic leukemia (CLL) at diagnosis, in relapsed/refractory (R/R) CLL, and in Richter syndrome.

Figure 5

Association between NOTCH1 mutations and other genetic features in chronic lymphocytic leukemia. The patterns of co-occurrence and mutual exclusivity between NOTCH1 mutations and other genetic features are shown.

Figure 6

Schematic representation of NOTCH1 involvement during chronic lymphocytic leukemia (CLL) development. NOTCH1 activation and mutation are early oncogenic events acquired within hematopoietic stem cells (HSCs) and maintained in their hematopoietic progeny. As suggested by xenograft models, CLL-HSCs are skewed toward B-cell differentiation giving rise to premalignant B cells that resemble human MBL, the pre-leukemic state of CLL. We hypothesize that NOTCH1 alterations, together with other oncogenic events, contribute to the generation of pre-leukemic mature B cells by favoring the expansion of pro-B cells and by cooperating with pre-BCR signaling during the antigen-driven selection of pre-B cells. In lymphoid tissues, antigenic stimulation of specific stereotyped BCRs, in combination with NOTCH1-related microenvironment interactions and additional genetic lesions, might have a role in the transformation process of pre-leukemic mature B cells into CLL.

Figure 7

Biological consequences of deregulated NOTCH1 in chronic lymphocytic leukemia.