The
forkhead transcription factor FOXP1 is generally regarded as an oncogene in activated B cell-like
diffuse large B-cell lymphoma. Previous studies have suggested that a small
isoform of FOXP1 rather than full-length FOXP1, may possess this oncogenic activity. Corroborating those studies, we herein show that activated B cell-like
diffuse large B-cell lymphoma cell lines and primary activated B cell-like
diffuse large B-cell lymphoma cells predominantly express a small FOXP1
isoform, and that the 5'-end of the Foxp1 gene is a common insertion site in murine
lymphomas in
leukemia virus- and transposon-mediated insertional mutagenesis screens. By combined mass spectrometry, (quantative) reverse transcription polymerase chain reaction/sequencing, and small interfering
ribonucleic acid-mediated gene silencing, we determined that the small FOXP1
isoform predominantly expressed in activated B cell-like
diffuse large B-cell lymphoma lacks the N-terminal 100
amino acids of full-length FOXP1. Aberrant overexpression of this FOXP1
isoform (ΔN100) in primary human B cells revealed its oncogenic capacity; it repressed apoptosis and plasma cell differentiation. However, no difference in potency was found between this small FOXP1
isoform and full-length FOXP1. Furthermore, overexpression of full-length FOXP1 or this small FOXP1
isoform in primary B cells and
diffuse large B-cell lymphoma cell lines resulted in similar gene regulation. Taken together, our data indicate that this small FOXP1
isoform and full-length FOXP1 have comparable oncogenic and transcriptional activity in human B cells, suggesting that aberrant expression or overexpression of FOXP1, irrespective of the specific
isoform, contributes to lymphomagenesis. These novel insights further enhance the value of FOXP1 for the diagnostics, prognostics, and treatment of
diffuse large B-cell lymphoma patients.