The Novel Tumor Suppressor SAMHD1 Is Differentially Expressed and Partly Regulated By MYC in Peripheral T-Cell Lymphomas (PTCL)

Introduction: The SAM domain and HD domain 1 (SAMHD1) protein is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase, which has been initially described to restrict human immunodeficiency virus type 1 (HIV-1) in certain cell types through depletion of intracellular dNTP substrates required for HIV-1 reverse transcription. Mutations of SAMHD1 gene have been linked to Aicardi-Goutières syndrome (AGS) and have been identified as putative drivers of chronic lymphocytic leukemia resulting in decreased SAMHD1 mRNA and protein levels. More recently, SAMHD1 mutations have been reported in T-prolymphocytic leukemia (T-PLL). Based on these findings and the fact that SAMHD1 limits the dNTP pool in the cell, it may play a role in oncogenesis as a tumor suppressor. In addition, SAMHD1 may confer resistance to nucleoside-based chemotherapies by hydrolysing their active triphosphate metabolites, with cytarabine in acute myeloid leukemia being an example (Herold et al, Nat Med 2017; 23(2):256-263). The expression patterns and the potential role of SAMHD1 in the pathogenesis of peripheral T-cell lymphomas (PTCL) are not yet known.

Methods: The patient cohort included 64 PTCLs of various histologic types which were diagnosed and treated at Karolinska University Hospital (Sweden). A control group of 4 reactive lymph nodes and 2 reactive tonsils was included in the study for comparison. All tissue samples were obtained prior to therapy. SAMHD1 expression was assessed by immunohistochemistry performed on a PTCL tissue microarray (TMA) with duplicate tumor cores from each case or full tissue sections using dual immunostaining (SAMHD1 / CD68) and a monoclonal antibody against SAMHD1 (Bethyl Laboratories, San Antonio, TX). At least 500 lymphoma cells were counted to calculate the percentage of SAMHD1-positive tumor cells. Overall survival (OS) was defined as time from diagnosis to death or last follow-up. Event-free survival (EFS) was defined as time from diagnosis to relapse, death, or last follow-up. Survival analyses were performed using the Kaplan-Meier method (log-rank test) and Cox regression models. Two T-cell lymphomas cell lines (Mac1, Mac2A) were used as an in vitro system. As our preliminary findings from in silico analysis revealed potential binding sites for MYC on the SAMHD1 gene promoter, we hypothesized that MYC might regulate SAMHD1 expression. Therefore, the T-cell lymphoma cell lines were treated with the selective BET / MYC inhibitor JQ-1 or transiently transfected with a MYC-overexpressing plasmid or MYC gene-specific siRNA constructs, respectively. Western blot analysis was used to assess the protein levels.

Results: SAMHD1 protein was expressed in reactive T-cells and histiocytes (CD68+) in all reactive lymphoid tissues (lymph nodes and tonsils) with strong staining intensity. SAMHD was differentially expressed among PTCL subtypes generally with weaker staining intensity as compared to normal T-cells and histiocytes, thus being positive in all (100%) angioimmunoblastic T-cell lymphomas (AILT), 67% PTCL-NOS, 45% ALK+ ALCL, 20% of ALK+ ALCL, and none (0%) of T-lymphoblastic lymphomas (p=0.0017, chi-square test). Among the SAMHD1- positive cases, the percentage of positive lymphoma cells ranged from 0 to 100% and its highest median was observed in AILT. SAMHD1 expression inversely correlated with CD30 expression (% CD30+ positive lymphoma cells) (p=0.0025, Mann-Whitney test). No significant associations between SAMHD1 levels and other clinicopathologic parameters or clinical outcome (EFS or OS) were found, however, the number of patients analyzed in each histologic subtype was limited. Inhibition of MYC activity by JQ-1 or MYC gene silencing with specific siRNA resulted in a substantial increase in the SAMHD1 protein level in T-cell lymphoma cell lines. Inversely, transient transfection of the cell lines with a MYC overexpressing plasmid resulted in decreased levels of SAMHD1. Taken together, the in vitro data suggest a possible MYC-associated regulation (repression) of SAMHD1 gene expression in T-cell lymphoma.

Conclusions: SAMHD1 is shown for the first time to be differentially expressed among PTCL types and its regulation may involve MYC. Preliminary survival analysis shows no significant associations of SAMHD1 expression with EFS and OS in this cohort of PTCL, however, analysis of a larger PTCL study group is underway to draw definite conclusions.