The Role of SAMSN1 in Multiple Myeloma

Abstract

Multiple myeloma (MM) is a haematological malignancy characterised by the uncontrolled clonal proliferation of neoplastic plasma cells (PCs) within the bone marrow (BM). Our group previously identified a homozygous deletion of Samsn1, a gene encoding a putative adaptor protein, in the genome of the MM-prone C57BL/KaLwRij mouse strain. In addition, the re-expression of Samsn1 in the C57BL/KaLwRij-derived 5TGM1 MM PC line was shown to inhibit tumour growth in vivo. Furthermore, SAMSN1 expression was found to be down-regulated in the PCs of MM patients compared to healthy controls. Collectively, these data suggested that SAMSN1 may be a novel tumour suppressor gene in MM. The fact that Samsn1-/- C57BL/KaLwRij mice develop MM with late onset and incomplete penetrance suggested that Samsn1 loss may co-operate with another lesion to drive disease development. In this thesis, bioinformatic analysis revealed that the down-regulation of the tumour suppressor GLIPR1 was significantly associated with reduced SAMSN1 expression in the PCs of MM patients. Glipr1 expression was found to be absent in 5TGM1 cells and its re-introduction reduced tumour growth in vivo, although this did not reach statistical significance. In addition, Samsn1 and Glipr1 double knockout mice were generated and monitored for clonal PC expansions for one year. These mice were not found to display enhanced PC abnormalities compared to wildtype mice, suggesting that the concomitant loss of Samsn1 and Glipr1 is insufficient to promote MM development within this timeframe. Although Samsn1 was previously shown to negatively regulate the proliferation and cytoskeletal remodelling of activated B cells, the mechanism(s) by which it inhibited 5TGM1 tumour formation was yet to be determined. In this thesis, Samsn1 was found to inhibit the growth of metastatic, but not primary, 5TGM1 tumours following intratibial injection into C57BL/KaLwRij mice but had no effect on the BM homing of 5TGM1 cells in vivo. These data suggest that Samsn1 may promote the action of anti-tumour factors from within the BM microenvironment. In addition, Samsn1 was found to not inhibit 5TGM1 tumour growth in immunodeficient mice and to promote the cytotoxicity of C57BL/KaLwRij-derived CD8+ T cells towards 5TGM1 cells. These findings suggest that Samsn1 may enhance immune system-mediated targeting of MM PCs. Notably, Samsn1 was found to inhibit 5TGM1 tumour growth in immunocompetent C57BL/Samsn1-/- mice, but not wildtype C57BL/6 mice, and anti-Samsn1 antibodies were detected in the serum of a Samsn1-/- mouse. These data suggest that the increased immunogenicity of Samsn1- expressing 5TGM1 cells in C57BL/KaLwRij mice may be due to the presence of adaptive immune cells that recognise Samsn1 as a foreign antigen in this Samsn1-/- mouse strain. As SAMSN1-specific immune cells will be deleted by tolerance processes in patients, the findings from the 5TGM1/KaLwRij model no longer support a potential tumour suppressor role for SAMSN1 in human MM. Given that SAMSN1 levels were found to have no effect on the growth of human MM cell lines in vitro or in vivo, the current weight of evidence suggests that SAMSN1 is unlikely to be an important tumour suppressor gene in MM.