Overexpression of Syk tyrosine kinase in peripheral T-cell lymphomas

Abstract

Peripheral T-cell lymphomas (PTCLs) are fatal in the majority of patients and novel treatments, such as protein tyrosine kinase (PTK) inhibition, are needed. The recent finding of SYK/ITK translocations in rare PTCLs led us to examine the expression of Syk PTK in 141 PTCLs. Syk was positive by immunohistochemistry (IHC) in 133 PTCLs (94%), whereas normal T cells were negative. Western blot on frozen tissue (n=6) and flow cytometry on cell suspensions (n=4) correlated with IHC results in paraffin. Additionally, western blot demonstrated that Syk-positive PTCLs show tyrosine (525/526) phosphorylation, known to be required for Syk activation. Fluorescence in situ hybridization showed no SYK/ITK translocation in 86 cases. Overexpression of Syk, phosphorylation of its Y525/526 residues and the availability of orally available Syk inhibitors suggest that Syk merits further evaluation as a candidate target for pharmacologic PTK inhibition in patients with PTCL.

Figure 1

Immunohistochemical staining for Syk in reactive and neoplastic T lymphocytes. (a) Benign lymph node (×10). Reactive follicles contain CD20-positive B cells; most lymphocytes are positive for Syk (arrowheads). Paracortical regions contain CD3-positive T cells; most lymphocytes are negative for Syk (arrows and inset, ×100). (b) Angioimmunoblastic T-cell lymphoma (×40). The tumor cells are negative for CD20 and positive for CD3. Nearly all cells present are positive for Syk, including the atypical medium-sized lymphoid cells (inset, ×100). (c) Peripheral T-cell lymphoma, unspecified (×40; see also Figure 2c). The tumor cells are negative for CD20 and positive for CD3 and Syk (inset, ×100). (d) Anaplastic lymphoma kinase (ALK)-negative anaplastic large-cell lymphoma (×40). The tumor cells are negative for CD20 and positive for CD30 and Syk (inset, ×100; see also Figure 2b, lane 5). (e) ALK-negative anaplastic large-cell lymphoma (×40). The tumor cells are positive for CD30 and negative for CD20 and Syk (inset, ×100). (f) Hepatosplenic T-cell lymphoma (×40). CD3-positive T cells expressing the cytotoxic marker TIA-1 infiltrating the hepatic sinusoids. They are negative for Syk (inset, ×100).

Figure 2

Syk expression in reactive and neoplastic T lymphocytes. (a) Western blot of lysates from flow-sorted reactive splenic lymphocytes shows no Syk expression in αβ or γδ T cells. Reactive B cells show Syk expression, as do Raji B-cell lymphoma cells. (b) Western blots of lysates from frozen peripheral T-cell lymphoma (PTCL) specimens show Syk expression similar to that shown by immunohistochemistry (IHC). Cases positive for Syk were positive for phospho-Syk (Tyr525/526) as well. Cases include PTCL-Us (lanes 1, 2 and 6), angioimmunoblastic T-cell lymphomas (lanes 3 and 4) and anaplastic lymphoma kinase-negative anaplastic large-cell lymphomas (lane 5; see also Figure 1e). (c) Flow cytometry results from an Syk-positive PTCL-U (see also Figure 1c). The neoplastic T cells show diminished expression of CD3 and CD5, allowing selective gating on both neoplastic and normal T-cell populations (middle panel). The CD3-dim neoplastic T cells (purple) are positive for cytoplasmic Syk with an intensity of staining between that of the normal T cells (blue, Syk-negative) and CD3-negative B cells (green, Syk-positive and CD19-positive (not shown)). (d) Flow cytometry results from an Syk-negative hepatosplenic T-cell lymphoma (see also Figure 1f). The neoplastic T cells show loss of CD5. By selective gating on the neoplastic and normal T-cell populations, both are shown to be Syk-negative. FITC, fluorescein isothiocyanate; PE, phycoerythrin; PerCp, peridinin chlorophyll protein.