Role Of Lyn Kinase In The Pathogenesis Of Chronic Lymphocytic Leukemia

The Src family kinase Lyn is predominantly expressed in B cells and plays a central role in initiating B cell receptor (BCR) signaling. Lyn is associated with BCR complexes and is renowned for its role in B cell activation and proliferation. Active Lyn contributes to positive regulation of signalling through tyrosine phosphorylation of components of the BCR. Intriguingly, Lyn was also shown as a negative regulator of BCR signal transduction. Lyn plays an essential role in negative regulation of signalling through its unique ability to phosphorylate immunoreceptor tyrosine based inhibition motifs (ITIM) in inhibitory cell surface receptors. ITIM phosphorylation induces the recruitment of inhibitory phosphatases such as SHP-1/2 and SHIP-1, which attenuate BCR signalling. Lyn-deficient mice have reduced number of B cells and increased numbers of myeloid progenitors.

It was reported that expression and activity of Lyn in human chronic lymphocytic leukemia (CLL) is elevated compared to healthy B cells. Besides, higher levels of Lyn are associated with a shorter treatment-free survival of CLL patients. This rises up a hypothesis about Lyn’s significant role in B cell tumorigenesis, malignant transformation of B cells, and the balance between myeloid cells and B lymphocytes.

We generated Eµ-TCL1 transgenic LYN-deficient mice (TCL1^+/wtLYN^-/-) and monitored them in order to identify the population of malignant B cells and to characterize the development of malignant cells in these mice in comparison with Eµ-TCL1 transgenic mice (TCL1^+/wtLYN^wt/wt). In comparison to TCL1^+/wtLYN^wt/wt mice, TCL1^+/wtLYN^-/- mice show a significantly reduced number of malignant B cells in the peripheral blood, as well as a reduced leukocyte count. Besides, TCL1^+/wtLYN^-/- mice have significantly decreased infiltration of malignant B cells in lymphoid tissues such as spleen, liver, lymph node and bone marrow. This result is also resembled in a hepato-splenomegaly in the TCL1^+/wtLYN^wt/wt mice. These mice develop severe splenomegaly and hepatomegaly due to infiltration of malignant cells, while TCL1^+/wtLYN^-/- mice do not develop hepatomegaly. The non-transgenic LYN^-/- control mice develop splenomegaly due to infiltration of myeloid cells.

Although TCL1^+/wtLYN^-/- mice have hindered development of TCL1-induced CLL, preliminary data suggest it is not only due to LYN-deficiency in B cell compartment of these mice. Indeed, B cell of TCL1^+/wtLYN^-/- mice show enhanced proliferation and better survival ex vivo compared to TCL1^+/wtLYN^wt/wt mice. Notably, TCL1^+/wtLYN^-/- mice developed a skewed microenvironment which might contribute to CLL down regulation. LYN^-/- microenvironment, particularly in aged mice, does not support engraftment of TCL1-induced leukemic B cell as well as LYN^wt/wt mice in our transplantation model. These results point to a complex regulation of Lyn signalling in CLL involving not only leukemic cells but also cells of the micromillieu, that needs further investigation.