Biological Impact of Monoallelic and Biallelic BIRC3 Loss in Del(11q) Chronic Lymphocytic Leukemia Progression

Chronic lymphocytic leukemia (CLL) patients harboring 11q22.3 deletion, del(11q), are characterized by a rapid disease progression. One of the suggested genes to be involved in the pathogenesis of this deletion is BIRC3, a negative regulator of NF-κB, which is monoallelically deleted in ~80% of del(11q) CLL cases. In addition, truncating mutations in the remaining allele of this gene can lead to BIRC3 biallelic inactivation, which accounts for marked reduced survival in CLL. Nevertheless, the biological mechanisms by which monoallelic or biallelic BIRC3 lesions could contribute to del(11q) CLL pathogenesis, progression and therapy response are partially unexplored.

We used the CRISPR/Cas9 system to model monoallelic and biallelic BIRC3 loss in vitro. First, we generated an isogenic HG3 CLL cell line harboring monoallelic del(11q) - HG3-del(11q) - by the introduction of 2 guide RNAs targeting 11q22.1 and 11q23.3 (~17 Mb). Loss-of-function BIRC3 mutations (MUT) were introduced in the remaining allele, generating 3 HG3-del(11q) BIRC3^MUT clones. In addition, single BIRC3^MUT were introduced in HG3 and MEC1 CLL-derived cells for experimental validation (n = 3 clones/cell line).

We first questioned whether monoallelic and biallelic BIRC3 loss had an impact in the DNA-binding activity of NF-κB transcription factors. Interestingly, HG3-del(11q) had higher p52 and RelB (non-canonical NF-κB signaling) activity than HG3^WT cells (P = 0.005; P = 0.007), being this activity further increased in HG3-del(11q) BIRC3^MUT cells (P < 0.001; P < 0.001). In depth analysis of the non-canonical signaling components by immunoblot revealed that HG3-del(11q) and, to a greater extent, HG3-del(11q) BIRC3^MUT cells presented NF-κB-inducing kinase (NIK) cytoplasmic stabilization, high p-IKKα levels and p52-RelB nuclear translocation. Besides, HG3-del(11q) BIRC3^MUT cells showed increased levels of the anti-apoptotic proteins BCL2 and BCL-xL.

We next assessed this pathway ex vivo in stroma and CpG-stimulated primary CLL cells with or without BIRC3 deletion (n = 22; 11 each group). Remarkably, stimulated BIRC3-deleted primary cells showed higher p52 and RelB activity than BIRC3^WT cases (P = 0.01; P = 0.07), and the percentage of BIRC3-deleted cells correlated with p52 activity in del(11q) cases (P = 0.04). We further performed western blot analyses in a homogenous cohort of del(11q) cases including (n = 4) or not including (n = 3) BIRC3 within the deleted region. Interestingly, del(11q)/BIRC3 deleted cases presented high levels of stabilized NIK, which correlated with higher p52 processing (P = 0.003). These patients also showed higher BCL2 levels than those del(11q)/BIRC3 undeleted, and we could further observe a correlation between p52 and BCL2 levels (P = 0.01). Given this p52-dependent BCL2 upregulation, we treated the CRISPR/Cas9 edited clones with venetoclax, demonstrating that HG3-del(11q) BIRC3^MUT cells were more sensitive upon BCL2 inhibition than HG3^WT clones (mean IC₅₀ 3.5 vs. 5.75 μM; P = 0.005).

In vitro proliferation assays were performed to interrogate the impact of BIRC3 loss in CLL cell growth, revealing that HG3 BIRC3^MUT cell lines had higher growth rates than BIRC3^WT cells (P = 0.001). HG3-del(11q) BIRC3^MUT cells also showed enhanced proliferation in comparison to HG3-del(11q) clones (P = 0.009). We further determined the clonal dynamics of del(11q) and/or BIRC3^MUT cell lines in clonal competition experiments, showing that HG3 BIRC3^MUT and HG3-del(11q) BIRC3^MUT cells progressively outgrew HG3^WT and HG3-del(11q) cells, respectively, overtime (P = 0.02; P = 0.006). Furthermore, we injected these edited cell lines into NSG mice (n = 20) in vivo, showing that mice xenografted with HG3 BIRC3^MUT and HG3-del(11q) BIRC3^MUT cells presented, by flow cytometry, an increase of human CD45+ cells in spleen 14 days after injection, compared to HG3^WT and HG3-del(11q) cells (P = 0.02; P = 0.015).

In summary, this work demonstrates that biallelic BIRC3 deletion through del(11q) and mutation triggers non-canonical NF-κB signaling, driving BCL2 overexpression and conferring clonal advantage, which could account for the negative predictive impact of BIRC3 biallelic inactivation in CLL. Taken together, our results suggest that del(11q) CLL patients harboring BIRC3 mutations should be considered as a CLL subgroup at a high risk of progression that might benefit from venetoclax-based therapies.

Funding: PI18/01500