CD72 Expression Patterns in ZAP70 Positive and Negative Chronic Lymphocytic Leukemia: Potential Regulatory Role in BCR Signalling

Introduction: Absence of mutations in IgV_H genes or higher number of ZAP70+ cells (as a surrogate marker) in chronic lymphocytic leukemia (CLL) B-cells defines a patient group with a poorer clinical course. These features relate to the role of BCR signalling in the proliferation and survival of CLL B-cells, and establish a link between these markers and the biology of CLL prognostic subgroups. The identification of additional players in this context may help to better understand the molecular basis of this disease and contribute to develop new therapeutic approaches. A search for genes potentially related to BCR signalling, when comparing mutated and unmutated CLL cases using serial analysis of gene expression, revealed a 4-fold increase of CD72 tags in unmutated samples, a specific B cell surface glycoprotein known to transmit both positive and negative signals in BCR signalling.

Objective: This finding lead us to explore the potential role of CD72 on BCR signalling in distinct CLL prognostic subgroups, as defined by ZAP70 expression.

Methods: Percentage of ZAP70+ and CD72+ cells were evaluated by flow cytometry on gated CD19+CD5+ cells in 25 CLL samples. Positive cases for ZAP70 and CD72 were defined using a cut-off of 35% and 40% positive cells, respectively. Real time PCR was used to quantify the expression levels of 3 genes related to proliferation and survival, RELB, Beta-Catenin (CTNNB1) and AKT1, on 16 CD19+ enriched (purity > 90%) CLL samples.

Results: Samples were classified as 11 ZAP70+ and 14 ZAP70−. Median percentage of CD72+ cells in ZAP70+ was significantly higher than for ZAP70− cases (82% compared to 39%, respectively, P=0.0029). Furthermore, percentages of CD72 and ZAP70 were positively correlated (r=0.5930 and P=0.0009). Interestingly, ZAP70+ cases were restricted to CD72+ cases (n=11, CD72+ZAP70+ [+/+]), whereas six CD72+ cases were ZAP70− (ZAP70−CD72+ [−/+]). Finally, there were 8 cases CD72−ZAP70− [−/−]. No differences among these 3 groups were observed in regard to laboratory parameters (white blood cells, total lymphocytes, lymphocyte percentage, haemoglobin, haematocrit and platelet number). Despite the reduced number of samples analysed (6 +/+, 6 −/− and 4 −/+), transcripts for RELB (P<0.05), CTNNB1 (P<0.05), and AKT1(P=0.057) were expressed at higher levels in ZAP70+CD72+ than in ZAP70−CD72+ samples. Additionally, the transcripts were expressed at higher levels in ZAP70−CD72− than in ZAP70−CD72+ samples, and this difference was statistically significant (P<0.05) for CTNB1 and AKT1, but not for RELB (P=0.054).

Conclusion: Our data indicate that higher percentages of ZAP70+ cells are associated with higher expression levels of transcripts related to proliferation and survival of CLL B-cells. In the absence of ZAP70 expression, CD72 may act as a negative regulator of the BCR pathway, as indicated by the lowest levels of transcripts on ZAP70−CD72+ cases.