Notch2 Increases the Resistance to Venetoclax-Induced Apoptosis in Chronic Lymphocytic Leukemia B Cells by Inducing Mcl-1

Abstract

Chronic lymphocytic leukemia (CLL) has experienced a clinical revolution—thanks to the discovery of crucial pathogenic mechanisms. CLL is still an incurable disease due to intrinsic or acquired resistance of the leukemic clone. Venetoclax is a Bcl-2 inhibitor with a marked activity in CLL, but emerging patterns of resistance are being described. We hypothesize that intrinsic features of CLL cells may contribute to drive mechanisms of resistance to venetoclax. We analyzed the expression of Interferon Regulatory Factor 4 (IRF4), Notch2, and Mcl-1 in a cohort of CLL patients. We evaluated CLL cell viability after genetic and pharmaceutical modulation of Notch2 expression in patients harboring trisomy 12. We tested venetoclax in trisomy 12 CLL cells either silenced or not for Notch2 expression or in combination with an inhibitor of Mcl-1, AMG-176. Trisomy 12 CLL cells were characterized by low expression of IRF4 associated with high levels of Notch2 and Mcl-1. Notch2 and Mcl-1 expression determined protection of CLL cells from spontaneous and drug-induced apoptosis. Considering the involvement of Mcl-1 in venetoclax resistance, our data demonstrated a contribution of high levels of Notch2 and Mcl-1 in a reduced response to venetoclax in CLL cells carrying trisomy 12. Furthermore, reduction of Mcl-1 expression by silencing Notch2 or by treatment with AMG-176 was able to restore the response of CLL cells to venetoclax. The expression of Notch2 identifies a subset of CLL patients, mainly harboring trisomy 12, characterized by high levels of Mcl-1. This biological mechanism may compromise an effective response to venetoclax.

Mechanisms of venetoclax resistance and solutions

“…The lack of the transcriptional regulator IRF4, a critical regulator of NOTCH signaling pathway, leads to high NOTCH2 expression, which in turn mediates the upregulation of intracellular MCL-1 expression and ultimately leads to the development of primary resistance to venetoclax in trisomy 12 CLL cells. The reduced resistance of cells to venetoclax after knockdown of NOTCH2 expression further supports this conclusion ( 42 , 43 ), but the specific relevance of venetoclax resistance to trisomy 12 still requires further exploration.…”

“…Furthermore, the effect of MEK inhibitors on inhibiting phosphorylation of BIM is to reverse drug resistance ( 45 ). For drug resistance in CLL due to MCL-1 upregulation induced by chromatin alterations such as trisomy 12 mutations, the combination of MCL-1 inhibitors with BCL-2 inhibitors produced some results, but further studies are needed to evaluate whether patients’ benefits outweighed the toxicity with these combinations ( 43 ). We propose that the development of drugs designed to directly overcome chromosome number alterations, such as drugs that target the chromosome replication process of tumor cells, may be one of the solutions for this mechanism.…”

“…Clinical trials have also been conducted in combination with gilteritinib (NCT03625505). In addition, CLL cells with trisomy 12 potentially develop primary resistance to venetoclax due to specific alterations in the IRF4/NOTCH2 axis, and the initial treatment phase has been unsatisfactory ( 42 , 43 ), suggesting that we should choose another appropriate drug if trisomy 12 of CLL cells occurs.…”

SOHO State of the Art Updates and Next Questions | Mechanisms of Resistance to BCL2 Inhibitor Therapy in Chronic Lymphocytic Leukemia and Potential Future Therapeutic Directions

“…CLL harbouring trisomy 12 are noted to have reduced levels of the transcriptional factor IRF4, elevated NOTCH2 expression, and consequently increased expression of MCL1. 33 In similar fashion, overexpression of BCL-XL in mantle cell lymphoma is observed as consequence of aberrations in the SWI-SNF chromatin remodelling complex. 34 Increased expression of alternative BCL2 family proteins is linked with venetoclax resistance.…”

Role of Notch2 pathway in mature B cell malignancies

“…In fact, the +12 CLL cells are characterized by a reduced response to pro-apoptotic treatments in relation to abnormal expression of Mcl-1 mediated by Notch2. As a consequence, oncogene Mcl-1 overexpression correlates with poor prognosis and both apoptosis and chemo-resistance in CLL patients ( 97 ). At the same time, rare CLL cases which harbors IRF4 activating mutations lead to direct upregulation of oncogene Myc conferring to leukemic cells a proliferative advantage ( 98 ).…”