Abstract 3999: HLA-G, SPAG9 and STAT3 signalling: An alliance that promotes early-stage brain metastases
Authors: Bassey-Archibong Blessing I. , Chokshi Chirayu R. , Aghaei Nikoo , Kieliszek Agata , Tatari Nazanin , McKenna Dillon , Singh Mohini , Subapanditha Minomi , Parmar Arun , Savage Neil , Lu Yu , Venugopal Chitra , Singh Sheila 
Published: June 2022

Abstract
Brain metastases (BM) are the most common brain tumours in adults and a prominent cause of cancer-related mortality globally. Leading sources of BM are cancers of the lung, breast and melanoma, which together account for approximately 80% of all BM. Unfortunately, current clinical modalities for BM including surgery, radiation therapy and chemotherapy still offer limited efficacy and median survival times of 4 - 12 months in treated patients, emphasizing the need for more effective therapeutic strategies and generally a better understanding of the disease. We recently identified the presence of stem-like cells termed “brain metastasis-initiating cells” or BMICs in patient-derived BM from lung, breast and melanoma cancers that are able to recapitulate the complete brain metastatic cascade in pre-clinical models of BM. Through these models, we serendipitously captured lung, breast and melanoma BMICs at the “pre-metastatic” stage of BM - a stage where circulating metastatic cells have seeded the brain, but not yet formed full-blown (macro-metastatic) brain lesions. Transcriptomic analysis of pre-metastatic and macro-metastatic lung, breast and melanoma BMICs revealed a unique genetic profile in pre-metastatic BMICs that was distinct from their macro-metastatic counterparts. Further analysis identified several genes commonly up-regulated in all pre-metastatic BMIC cohorts irrespective of their primary tumour of origin. Intriguingly, we found that inhibition of the non-classical human leukocyte class I antigen-G or HLA-G gene (one of the top up-regulated genes in the pre-metastatic cohorts), reduced the ability of BMICs to form mature brain lesions. Correspondingly, HLA-G over-expression increased the capacity of BMICs to establish secondary brain tumours. Mechanistically, we discovered that over-expressing HLA-G levels in BMICs (to simulate the high levels that occurs in pre-metastatic BMICs), increased the activation of STAT3 signalling and this was mediated in part via a novel HLA-G binding partner - SPAG9. Our work thus uncovered a potential cooperative role between HLA-G, SPAG9 and STAT3 signalling during the early stages of BM. Indeed, attenuation of SPAG9 protein levels or STAT3 signalling in HLA-G over-expressing BMICs using CRISPR knockout and a STAT3 inhibitor respectively obstructed the ability of high HLA-G levels to promote mature brain lesions. This is the first study to reveal a role for an HLA-G-SPAG9-STAT3 axis in BM and highlights the potential of targeting this axis to inhibit BM, which will markedly extend patient survival.
Citation Format: Blessing I. Bassey-Archibong, Chirayu R. Chokshi, Nikoo Aghaei, Agata Kieliszek, Nazanin Tatari, Dillon McKenna, Mohini Singh, Minomi Subapanditha, Arun Parmar, Neil Savage, Yu Lu, Chitra Venugopal, Sheila Singh. HLA-G, SPAG9 and STAT3 signalling: An alliance that promotes early-stage brain metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3999.