Low LIM-domain only 2 (LMO2) expression in aggressive B cell lymphoma correlates with MYC and MYC/BCL2 rearrangements, especially in germinal center cell-type tumors

LIM-domain only 2 (LMO2) is a highly conserved cysteine-rich protein that is a critical regulator of hematopoietic cell development, stem cell maintenance, and is implicated in angiogenesis [1,2]. We have previously reported that LMO2 expression is increased in diffuse large B cell lymphoma (DLBCL) of germinal center (GC) cell origin, and that high expression (LMO2) is associated with improved patient survival [3,4]. LMO2 increases genomic instability due to accumulation of double stranded DNA breaks (DSBs) via defective homologous recombination of DNA repair, making LMO2expressing tumors more sensitive to chemotherapy and thus providing an explanation for improved survival [4–6]. More recently, we reported that the inhibition of homologous recombination DNA repair by LMO2 occurs through its interaction with 53BP1 and inhibition of BRCA1 recruitment to DSB foci. This specific mechanism of DNA repair inhibition makes LMO2 expressing tumors more sensitive to poly(ADP-ribose) polymerase 1 and 2 inhibitors [7]. Based on the association of LMO2 expression with sensitivity to chemotherapy in general and targeted therapies specifically, our understanding of LMO2 expression and its association with other prognostic histopathologic features is of clinical significance. Although data are limited, few previous reports have indicated a significant positive correlation between LMO2 expression and IGH-BCL2 but not BCL6 translocations. This is in agreement with the understanding that LMO2 expressing tumors are more frequently of GC type where IGH-BCL2 rearrangements are more common [8]. Conversely, LMO2-negative DLBCL and Burkitt lymphomas have been reported to more commonly harbor MYC translocations [9–11]. We collected consecutive aggressive large B-cell lymphomas including tumors with morphologic features of diffuse large B cell lymphoma, NOS (DLBCL) and highgrade B-cell lymphoma (HGBCL). All cases were reviewed by an expert Hematopathologist (JC). Cases with morphology and immunophenotype typical of Burkitt lymphoma were excluded. Standard immunohistochemistry (IHC) was performed in all cases for clinical diagnostic purposes and reviewed for the purposes of this study. Immunohistochemistry performed for routine diagnostic purposes included: CD20, CD3, CD5, CD10, BCL6, BCL2, MUM1, and MYC (Leica Biosystems Ltd. Newcastle, UK). For the purpose of this study, immunohistochemistry for LMO2 (Ventana, Tuscon, Arizona, United States) was performed in all cases in our clinical immunohistochemistry laboratory after appropriate clinical validation. LMO2 and MYC expression were considered positive if more than 30% (LMO2) and 40% (MYC) of lymphoma cells showed nuclear positivity, respectively [12,13]. Therefore, lymphomas classified as ‘negative’ for LMO2 or MYC expression were either negative or low expressers of the respective proteins and tumors considered positive were high expressers of the respective proteins. Tumors were classified as GCB or non-GCB by Hans algorithm [14]. Fluorescence in situ hybridization (FISH) was performed in our clinical cytogenetics laboratory on formalin-fixed, paraffin-embedded tissue sections using protocols recommended by the manufacturers for each of the following probes: MYC break-apart probe (Abbott Molecular/Vysis, Downers Grove, IL, USA), BCL-6 breakapart probe (Abbott Molecular/Vysis), and dual-color fusion probes for IGH-MYC and IGH-BCL2 (Abbott Molecular/Vysis). A total of 200 interphase nuclei were analyzed for each probe in areas of tumor and stored using the Applied Imaging/Cytovision system. A neoplasm was considered positive for translocation if observed signals were clearly >10% for IGH-BCL2 and IgH-MYC fusion and >15% for BCL6 and MYC break-apart probes, based on validation studies performed in our laboratory.