Downregulation of the Tumor Suppressor SAMHD1 in Classical Hodgkin Lymphoma

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 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. Therefore, a line of evidence suggests that SAMHD1 may play a role in oncogenesis as a tumour suppressor. In addition, SAMHD1 may confer resistance to nucleoside-based chemotherapies by hydrolysing their active triphosphate metabolites, and thus, it may lead to resistance to certain chemotherapeutic agents with cytarabine in acute myeloid leukemia being a recent example (Herold et al, Nat Med 2017; 23(2):256-263). The expression patterns and the potential role of SAMHD1 in the pathogenesis of classical Hodgkin lymphoma (HL) are not yet known.

Methods: SAMHD1 protein levels were assessed by Western blot analysis in 20 human lymphoma and leukemia cell lines including 5 classical HL (L1236, L428, HDML2, HDMyZ, MDA-V) and 3 diffuse large B-cell lymphoma (DLBCL) (RC-K8, OCI-Ly3, U2932) cell lines. As our findings from in silico analysis revealed potential binding sites for MYC on the SAMHD1 gene promoter, HL cell lines were treated with selective MYC inhibitors to assess possible MYC-associated regulation of SAMHD1 expression. The patient cohort included 212 cases of classical HL diagnosed and treated at the University of Texas MD Anderson Cancer Center (USA), University of Athens (Greece) and University of Crete (Greece). A control group of 4 reactive lymph nodes and 76 B-cell non-Hodgkin lymphomas (58 DLBCLs) was included in the study for comparison. All tissue samples were obtained prior to therapy. SAMHD1 expression was assessed by immunohistochemistry performed on TMAs with duplicate tumor cores from each case or full tissue sections using a monoclonal antibody (Bethyl Laboratories, San Antonio, TX). An arbitrary 10% cutoff was used to define SAMHD1 positivity in the neoplastic 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.

Results: In 4 out of 5 HL cell lines, SAMHD1 protein levels were dramatically reduced as compared to all 3 DLBCL and other lymphoma and leukemia cell lines tested. Inhibition of MYC activity resulted in a substantial increase in the SAMHD1 protein level suggesting a possible MYC-associated gene regulation. Similarly, the neoplastic Hodgkin-Reed Sternberg (HRS) cells of classical HL were SAMHD1-negative in 132 of 212 (62.3%) patients. Among the SAMHD1-positive cases, the percentage of positive HRS cells ranged from 10 to 95% with a median 40%. By contrast, SAMHD1 was negative in only 15% of the control group of DLBCLs. In addition, most of the germinal center (GC) cells of the reactive lymph nodes were SAMHD1-positive with consistently higher intensity of immunostaining as compared to HRS cells of classical HL. No significant associations between SAMHD1 levels and clinicopathologic parameters were observed in the HL cohort. In a subset of 56 HL patients with long follow up period (median 195 months), FFS at 10 years was 79% vs. 71% for patients with SAMHD1-negative vs. SAMHD1-positive HL, respectively. Similarly, OS at 15 years was 71% vs. 63% for SAMHD1-negative vs. SAMHD1-positive HL, respectively. These differences did not reach statistical significance (p>0.05, log-rank) likely because of the relatively small number of patients included in this preliminary survival analysis.

Conclusions: SAMHD1 is shown for the first time to be downregulated in HRS cells of classical HL further supporting its role as a tumor suppressor and its regulation may involve MYC. Preliminary survival analysis shows a statistical trend towards inferior FFS and OS for patients with SAMHD1-positive HRS cells, however, analysis of the entire cohort of HL patients is underway to draw definite conclusions.