IRF8 is a transcriptional regulator of CD37 expression in diffuse large B-cell lymphoma
Diffuse large B-cell lymphoma (DLBCL) represents the most common form of non-Hodgkin lymphoma that is still incurable in a large fraction of patients. Transmembrane protein CD37 is highly expressed on mature B lymphocytes, but loss of CD37 expression is observed in ~ 50% of DLBCL patients which correlates with inferior treatment outcome. However, the underlying molecular mechanisms that control CD37 expression in malignant B cells are still unknown.
Here we investigated regulation of the CD37 gene in human DLBCL at the (epi-)genetic and transcriptional level.
No differences were observed in DNA methylation patterns and single nucleotide variants within the promoter region of the CD37 gene between CD37-positive and CD37-negative DLBCL samples. On the contrary, CD37-negative DLBCL cell lines lacked CD37 promoter activity in transient reporter assays, suggesting differential transcription regulation between CD37-negative and CD37-positive DLBCL. Using an unbiased SILAC-based quantitative proteomic approach, we detected significantly higher expression levels of transcription factor IRF8 in nuclear extracts of CD37-positive as compared to CD37-negative DLBCL. Direct binding of IRF8 to the promoter region of CD37 was confirmed by DNA pull-down combined with mass spectrometry and targeted chromatin immunoprecipitation. Functional analysis showed that overexpression of IRF8 enhanced CD37 protein expression, while CRISPR/Cas9 knockout of IRF8 decreased CD37 levels in DLBCL cell lines. Immunohistochemical analysis in a large cohort of primary DLBCL (n=206) revealed significant more loss of IRF8 in CD37-negative versus CD37-positive DLBCL.
Together, this study provides new insight into the molecular mechanism underlying differential CD37 expression, and reveals IRF8 as key transcriptional regulator of CD37 in human DLBCL.