SOX11 is a novel binding partner and endogenous inhibitor of SAMHD1 ara-CTPase activity in mantle cell lymphoma.

ABSTRACT: Sterile alpha motif and histidine-aspartate (HD) domain-containing protein 1 (SAMHD1) is a deoxynucleoside triphosphate triphosphohydrolase with ara-CTPase activity that confers cytarabine (ara-C) resistance in several hematological malignancies. Targeting SAMHD1's ara-CTPase activity has recently been demonstrated to enhance ara-C efficacy in acute myeloid leukemia. Here, we identify the transcription factor SRY-related HMG-box containing protein 11 (SOX11) as a novel direct binding partner and first known endogenous inhibitor of SAMHD1. SOX11 is aberrantly expressed not only in mantle cell lymphoma (MCL), but also in some Burkitt lymphomas. Coimmunoprecipitation of SOX11 followed by mass spectrometry in MCL cell lines identified SAMHD1 as the top SOX11 interaction partner, which was validated by proximity ligation assay. In vitro, SAMHD1 bound to the HMG box of SOX11 with low-micromolar affinity. In situ crosslinking studies further indicated that SOX11-SAMHD1 binding resulted in a reduced tetramerization of SAMHD1. Functionally, expression of SOX11 inhibited SAMHD1 ara-CTPase activity in a dose-dependent manner resulting in ara-C sensitization in cell lines and in a SOX11-inducible mouse model of MCL. In SOX11-negative MCL, SOX11-mediated ara-CTPase inhibition could be mimicked by adding the recently identified SAMHD1 inhibitor hydroxyurea. Taken together, our results identify SOX11 as a novel SAMHD1 interaction partner and its first known endogenous inhibitor with potentially important implications for clinical therapy stratification.

SOXC transcription factors in mantle cell lymphoma: the role of promoter methylation in SOX11 expression.

The related transcription factors SOX11, SOX4 and SOX12 (classified as the SOXC family) compete for the same target genes. SOX11 is expressed in most mantle cell lymphomas (MCL) but a small subset is, like normal lymphocytes, SOX11 negative. Here we report the variable expression of SOX4 and high expression of SOX12 in MCL compared to non-malignant tissue. Our results show that the expression of the SOXC genes is highly correlated in SOX11 positive MCL. SOX11 expression is epigenetically regulated but there are partly conflicting results regarding the underlying mechanisms. Here we report that the SOX11 promoter region is hypomethylated in both MCL and normal B-lymphocytes. Methylation at other sites is important for sustaining high SOX11 in MCL since treatment with 5-azacytidine decreased SOX11 levels in SOX11 positive MCL cell lines: Granta519 and Rec1. Furthermore, 5-azacytidine treatment of the SOX11 negative MCL cell line, JVM2, induced SOX4 but not SOX11.