LMO2 Facilitates Synthetic Lethality after PARP Inhibition (PARPi) in Diffuse Large B-Cell Lymphoma (DLBCL)

Novel therapies for DLBCL are needed to improve patients' outcomes. LIM domain only 2 (LMO2) protein is ubiquitously expressed and plays important roles in endothelial and hematopoietic cell development. LMO2 protein expression is upregulated in germinal center B (GCB) cells, the cell of origin of DLBCL. 73% of GCB and 45% of ABC DLBCLs express LMO2 protein at levels of reactive GCB cells. Although the function of LMO2 in B cells and DLBCL is unknown, expression of LMO2 serves as one of the best prognostic markers of longer survival following R-CHOP therapy. Additionally, LMO2 expression in DLBCL cells results in genomic instability. These observations suggest that LMO2 may decrease DNA repair efficiency. Indeed, here we demonstrate that primary DLBCL tumors and cell lines expressing high levels of LMO2 protein (LMO2^HIGH) are defective in DNA double-strand break (DSB) repair via the homologous recombination (HR) pathway. We found that LMO2^HIGH DLBCL cells and LMO2^LOW DLBCL cells expressing a GFP-LMO2 fusion protein via a doxycycline-inducible system have a defective accumulation of the HR proteins BRCA1 and RAD51 to damaged chromosomes as visualized by immunofluorescence (IF) and Western blot assays. Furthermore, LMO2^HIGH DLBCL exposed to ionizing radiation showed decreased levels of phosphorylated (S4 and S8) replication protein A (RPA32 subunit), a marker of DSB end-resection activity and an essential step for HR-dependent DSB repair. Consequently, LMO2^HIGH DLBCL showed decreased HR activity as assessed via a DR-GFP reporter system and scoring the frequency of HR-dependent sister chromatid exchanges. Also, in LMO2^HIGH vs LMO2^LOW DLBCLs, we observed higher levels of ionizing radiation-induced foci (IRIF) for 53BP1 and RIF1 - non-homologous end-joining (NHEJ) core factors playing a critical role in defining DSB repair pathway choice. Similarly, we found a higher accumulation of 53BP1 and RIF1 chromatin-enriched fraction after DNA damage in LMO2^HIGH than LMO2^LOW DLBCLs. Furthermore, we show that LMO2 forms a complex with 53BP1 but not BRCA1 after DNA damage, as demonstrated by Co-IP, GST pull-down assays and spatial co-localization in IRIFs. This suggests that LMO2 functionally interacts with 53BP1 during DSB repair. Indeed, knockdown of 53BP1 in LMO2^HIGH cells increased the levels of BRCA1 and RAD51 IRIF to values similar to LMO2^LOW cells without affecting LMO2 levels, revealing that LMO2 depends on 53BP1 to inhibit HR activity.

Since LMO2^HIGH DLBCL cells exhibit a defective HR-pathway, we next explored the therapeutic potential of PARPi in DLBCL. We found that in LMO2^HIGH but not LMO2^LOW DLBCL cell lines the PARPi olaparib induced a significant decrease in cell proliferation and colony formation and an increase in cell death via apoptosis. The sensitivity to olaparib directly correlated with LMO2 protein levels. The proliferation defect in LMO2^HIGH DLBCL cells was due to the increased DNA damage caused by exposure to PARPi, as observed by an increase in γH2AX foci. Induction of LMO2 expression in LMO2^LOW DLBCL cell lines led to sensitivity to olaparib, demonstrating that the proliferation defect induced by olaparib was dependent on LMO2 expression. Silencing of LMO2 via shRNA or CRISP/Cas9 in LMO2^HIGH cell lines rescued the proliferation defect induced by olaparib. The proliferation deficiency induced by olaparib was synergistic with doxorubicin in LMO2^HIGH but not in LMO2^LOW DLBCL cell lines and untreated patient-derived primary DLBCL tumors.

We also examined the in vivo efficacy of olaparib in DLBCL mice models. Olaparib treatment prolonged survival of mice harboring LMO2^HIGH but not LMO2^LOW DLBCL xenograft tumors. Olaparib in combination with RCHOP significantly prolonged survival of mice harboring LMO2^HIGH DLBCL tumors compared to cohorts treated with either olaparib or RCHOP alone. Further, analysis of tumors excised from OCI-LY1 bearing animals treated with olaparib for 3 days revealed increased cell death and a higher sensitivity to PARPi in OCI-LY1 LMO2 cells compared to the OCI-LY1 LMO2 negative-GFP control cells, indicating in vivo survival advantage for LMO2^LOW cells upon olaparib treatment.

In summary, high expression of LMO2 results in HR-dysfunction phenocopying the BRCA1/2 mutations observed in breast and ovarian tumors. In LMO2^HIGH DLBCL, PARPi-induced killing is synergistic with doxorubicin, thus providing a clear path for therapeutic development of PARPi in DLBCL.

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