The role of SAMHD1 and cGAS-STING associated anti-tumor response mechanisms in lymphomas

Lymphomas are a phenotypically and genetically heterogeneous group of malignant diseases of the lymphocytes, accounting for 4.4% of all cancers. They are broadly categorized into Hodgkin lymphoma (HL) and non-Hodgkin lymphomas (NHL), the latter comprising about 90% of all lymphoma types. The doctoral project aimed at investigating the emerging role of SAMHD1 and cGAS-STING anti-tumor immune response mechanisms in the biology and prognosis of lymphomas, focusing on HL and T-cell NHLs. In particular for T-cell NHL biology, the hypothesis was that the neoplastic T-cells, which derive from T lymphocytes, may retain the ability to produce critical mediators of anti- tumor immune responses, including interferons (IFNs), cytokines and chemokines. Therefore, stimulation of mechanisms triggering immune responses in the neoplastic T- cells may activate the normal natural killer (NK) and T-cells of the tumor microenvironment resulting in immunogenic cell death of the neoplastic T-cells. The overall goal of this research was to uncover potential new targets for immunotherapy and novel biomarkers that could better stratify the patients for optimal treatment options.

Intensive research over the last decades has uncovered the dynamic interactions between the cancer cells and the host immune cells, leading to development of novel immunotherapy strategies. Although standard chemotherapy remains the first line treatment for most aggressive lymphomas, ongoing clinical trials are currently investigating new strategies with promising results, including combination of immune checkpoint immunotherapy with small-molecule-based precision therapeutics. In the present doctoral project, we hypothesized that the SAMHD1 and cGAS-STING pathway regulate anti-tumor immune responses in certain lymphoma types, and could represent novel therapeutic targets. In addition, expression of SAMHD1 and critical components of the cGAS-STING pathway assessed in clinical tumor samples may have prognostic and predictive value. The hypotheses are clinically relevant since pharmacologic modulators of SAMHD1 and cGAS-STING pathway activity are already available for clinical trials.

First, in this doctoral research, SAMHD1 expression by the neoplastic Hodgkin cells was found to be an independent adverse prognostic factor in patients with HL uniformly treated with standard chemotherapy protocols. These results may contribute to better stratification of the patients for optimal treatment options, and also might have additional therapeutic implications since modulation of SAMHD1 activity is currently available using SAMHD1 inhibitors.

Second, the results of the doctoral research showed that STING gene and protein expression are restricted to T- and NK-cell NHLs, with anaplastic lymphoma kinase-positive (ALK+) anaplastic large cell lymphoma (ALCL) showing the highest frequency of positive tumors. By contrast, STING seems to be downregulated in normal and neoplastic B-cells, likely due to decreased STING1 gene transcription; however, the mechanisms remain to be investigated. The data suggest that therapeutic targeting of the cGAS-STING pathway may be more efficient in T- and NK-cell NHLs.

Since recent evidence suggests that the natural compound sulforaphane (SFN) may inhibit SAMHD1 phosphorylation, the biologic effects of SFN were investigated in vitro in classic HL. SFN significantly inhibited cell growth and viability of HL cells, and more importantly, SFN substantially enhanced anti-tumor immune responses and NK cell- mediated killing of the co-cultured neoplastic HL cells. NK cell-mediated killing of HL cells was associated with induction of IFN-ß gene expression, production of NK ligands MICA/B and release of numerous cytokines and chemokines in part through cGAS-STING- dependent mechanisms. These results indicate that pharmacologic modulation of the cGAS-STING pathway by SFN or STING agonists may contribute to novel investigational strategies for patients with refractory or relapsed cHL.

Using an in vitro study model for ALK+ ALCL, the findings of the doctoral project also showed the suppressive effects of NPM-ALK oncogenic kinase on anti-tumor immune responses, partially through inhibition of the cGAS-STING pathway. STAT3, one of the most important downstream targets of NPM-ALK, likely mediates the suppressive effects of NPM-ALK on anti-tumor immune responses by controlling gene and protein expression of type I IFNs, cytokines and chemokines. Moreover, the cGAS-STING pathway is functional and responds to STING stimulation. These results suggest that modulation of the cGAS-STING activity may contribute to immunotherapy and targeted therapy in addition to ALK inhibitors and anti-CD30 antibodies currently used in clinical trials.

In the last doctoral study, a novel patient-derived cell line, namely BIA-XR1, and xenograft model (PDX) derived from a patient with BIA-ALCL was established and characterized. This new cell line showed a unique KRAS mutation (in addition to a common STAT3 mutation). Novel in vitro systems, including PDX for uncommon types of T-cell lymphomas, such as BIA-ALCL, represent essential experimental tools to uncover the molecular pathogenesis and novel therapeutic targets.

In summary, the results of the doctoral study enriched our knowledge on the clinical and biologic involvement of SAMHD1 and cGAS-STING anti-tumor immune response pathways in HL and T-NHLs with potential therapeutic implications, but also raised scientific questions that merit further investigation.

List of scientific papers

I. Xagoraris I*, Vassilakopoulos TP*, Drakos E, Angelopoulou MK, Panitsas F, Herold N, Medeiros LJ, Giakoumis X, Pangalis GA, Rassidakis GZ. Expression of the novel tumour suppressor sterile alpha motif and HD domain-containing protein 1 is an independent adverse prognostic factor in classical Hodgkin lymphoma. Br J Haematol. 2021 May;193(3):488-496. https://doi.org/10.1111/bjh.17352

II. Xagoraris I, Farrajota Neves da Silva P, Kokaraki G, Stathopoulou K, Wahlin B, Österborg A, Herold N, Ng SB, Medeiros LJ, Drakos E, Sander B, Rassidakis GZ. STING is Commonly and Differentially Expressed in T- and NK-cell But Not B-cell Non-Hodgkin Lymphomas. Cancers (Basel). 2022 Feb 24;14(5):1186. https://doi.org/10.3390/cancers14051186

III. Xagoraris I, Yang Y, Bougka E, Trogrlic D, Xyderou P, Stathopoulou K, Herold N, Lundqvist A, Rassidakis GZ. Sulforaphane Promotes NK Cell- Mediated Anti-Tumor Immune Responses Partially Via cGAS-STING Pathway in Classical Hodgkin Lymphoma. [Accepted]

IV. Xagoraris I, Yang Y, Trogrlic D, Månsson A, Plastira C, Leventaki V, Qian H, Lundqvist A, Rassidakis GZ. NPM-ALK Oncoprotein Suppresses Anti- Tumor Immune Responses Partially Through STAT3 and cGAS-STING Pathway in ALK+ Anaplastic Large Cell Lymphoma. [Manuscript]

V. Xagoraris I, Stathopoulou K, Aulerio R, He M, Ketscher A, Jatta K, de Flon FH, Barbany G, Rosenquist R, Westerberg LS, Rassidakis GZ. Establishment and characterization of a novel breast implant-associated anaplastic large cell lymphoma cell line and PDX model (BIA-XR1) with a unique KRAS mutation. Curr Res Transl Med. 2023 Jun 20;71(3):103401. https://doi.org/10.1016/j.retram.2023.103401

* Denotes equal first author contribution