Potentially oncogenic B-cell activation induced smaller isoforms of FOXP1 are highly expressed in the activated B cell like subtype of DLBCL

Abstract

The FOXP1 forkhead transcription factor is targeted by recurrent chromosome translocations in several subtypes of Bcell non-Hodgkin lymphomas, where highlevel FOXP1 protein expression has been linked to a poor prognosis. Western blotting studies of diffuse large B-cell lymphoma (DLBCL) cell lines unexpectedly identified the atypical high-level expression of 2 smaller, 60 to 65 kDa, FOXP1 isoforms in all 5 of those with the activated B cell (ABC)like DLBCL subtype and in a subgroup of primary DLBCL. The anti-FOXP1 (JC12) monoclonal antibody cannot distinguish FOXP1 isoforms by immunohistochemistry, a finding that may be clinically relevant as high-level expression of the full-length FOXP1 protein was observed in some germinal center derived DLBCLs. ABC-like DLBCLderived cell lines were observed to express 2 novel, alternatively spliced FOXP1 mRNA isoforms, encoding N-terminally truncated proteins. These transcripts and the smaller protein isoforms were induced as a consequence of normal B-cell activation, which thus represents an additional mechanism for up-regulating FOXP1 expression in lymphomas. The expression of potentially oncogenic smaller FOXP1 isoforms may resolve the previously contradictory findings that FOXP1 represents a favorable prognostic marker in breast cancer and an adverse risk factor in B-cell lymphomas. (Blood. 2008;111:2816-2824). © 2008 by The American Society of Hematology.