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N-terminally truncated FOXP1 protein expression and alternate internal FOXP1 promoter usage in normal and malignant B cells.

ABSTRACT: Strong FOXP1 protein expression is a poor risk factor in diffuse large B-cell lymphoma and has been linked to an activated B-cell-like subtype, which preferentially expresses short FOXP1 (FOXP1S) proteins. However, both short isoform generation and function are incompletely understood. Here we prove by mass spectrometry and N-terminal antibody staining that FOXP1S proteins in activated B-cell-like diffuse large B-cell lymphoma are N-terminally truncated. Furthermore, a rare strongly FOXP1-expressing population of normal germinal center B cells lacking the N-terminus of the regular long protein (FOXP1L) was identified. Exon-targeted silencing and transcript analyses identified three alternate 5' non-coding exons [FOXP1-Ex6b(s), FOXP1-Ex7b and FOXP1-Ex7c], downstream of at least two predicted promoters, giving rise to FOXP1S proteins. These were differentially controlled by B-cell activation and methylation, conserved in murine lymphoma cells, and significantly correlated with FOXP1S protein expression in primary diffuse large B-cell lymphoma samples. Alternatively spliced isoforms lacking exon 9 (e.g. isoform 3) did not encode FOXP1S, and an alternate long human FOXP1 protein (FOXP1AL) likely generated from a FOXP1-Ex6b(L) transcript was detected. The ratio of FOXP1L:FOXP1S isoforms correlated with differential expression of plasmacytic differentiation markers in U-2932 subpopulations, and altering this ratio was sufficient to modulate CD19 expression in diffuse large B-cell lymphoma cell lines. Thus, the activity of multiple alternate FOXP1 promoters to produce multiple protein isoforms is likely to regulate B-cell maturation.

SUBMITTER: Brown PJ

PROVIDER: S-EPMC5004466 | biostudies-literature | 2016 Jul

REPOSITORIES: biostudies-literature

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N-terminally truncated FOXP1 protein expression and alternate internal FOXP1 promoter usage in normal and malignant B cells.

Brown Philip J PJ Gascoyne Duncan M DM Lyne Linden L Spearman Hayley H Felce Suet Ling SL McFadden Nora N Chakravarty Probir P Barrans Sharon S Lynham Steven S Calado Dinis P DP Ward Malcolm M Banham Alison H AH

Haematologica 20160407 7

Strong FOXP1 protein expression is a poor risk factor in diffuse large B-cell lymphoma and has been linked to an activated B-cell-like subtype, which preferentially expresses short FOXP1 (FOXP1S) proteins. However, both short isoform generation and function are incompletely understood. Here we prove by mass spectrometry and N-terminal antibody staining that FOXP1S proteins in activated B-cell-like diffuse large B-cell lymphoma are N-terminally truncated. Furthermore, a rare strongly FOXP1-expres ...[more]

PMID: 27056922

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Dataset Information

N-terminally truncated FOXP1 protein expression and alternate internal FOXP1 promoter usage in normal and malignant B cells.

Publications

N-terminally truncated FOXP1 protein expression and alternate internal FOXP1 promoter usage in normal and malignant B cells.

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