Identification of FAM46C As a Multiple Myeloma Repressor

Introduction: Partial loss of chromosome arm 1p frequently occurs in multiple myeloma (MM), and is associated with a poor prognosis. Several minimally altered regions on 1p have been identified, including 1p32.3, 1p31.3, 1p22.1-1p21.3, and 1p12. Cytoband 1p12 was deleted in 19% of cases, and this deletion was associated with shorter overall survival (OS) in univariate analysis. The target of homozygous deletion 1p12 was FAM46C. In addition, mutations of FAM46C were identified in 3.4% to 13% of primary MM tumors and 25% of 16 human myeloma cell lines (HMCLs), implying its potential pathogenic relevance. In other work we have suggested that FAM46C mutation is a progression event and have shown that it is rarely seen in newly diagnosed del17 patients, inferring some overlap in function. However, there is no published functional annotation of FAM46C and its role in MM remains unknown. In the present study, we aimed to identify the biological role of FAM46C in myeloma cells.

Materials/Method: The expression of FAM46C in HMCLs was analyzed by western blot. Lentiviral constructs expressing wild type and mutated FAM46C were generated and transduced into HMCLs, followed by cell viability assay and cell cycle analysis. Cells were harvested and processed to measure gene expression and cell signaling changes after introduction of FAM46C by mRNAseq, pathway analysis and immunoblotting assay.

Results: The expression of FAM46C protein is generally low in most HMCLs. Introduction of wild type FAM46C in HMCLs induced a substantial cell growth inhibition and apoptosis, especially in two HMCLs including MM1.S and KMS11. Cell viability of KMS11 and MM1.S was reduced by 50% to 80% at day 6 after introduction of FAM46C, compared to 0-30% growth retardation detected in HMCLs and non-myeloma cell lines that do not carry FAM46C deletion. We identified 88 genes whose mRNA expression was significantly altered after enforced expression of FAM46C in MM1.S cells. Pathway analysis revealed that FAM46C-regulated genes are enriched in the canonical pathways associated with unfolded protein response, cell cycle control and DNA damage repair. Critical MM genes that are downregulated by FAM46C expression include IRF4 and MYC, which are also downstream targets of immunomodulatory drugs (IMiDs). Consistently, some HMCLs such as KMS11 and OPM2 show an enhanced sensitivity to lenalidomide after introduction of FAM46C. Next, lentiviral constructs expressing various FAM46C mutants were generated in order to understand the consequence of FAM46C mutation. The mutant constructs mimic mutations identified in MM patients or HMCLs. Those mutants and wild type FAM46C were transduced and tested together in MM1.S cells. We found that three published misssense mutations, one frame-shift mutation and deletion of the sequence between aa172 and aa186 of FAM46C (which has been found in previous studies as a hot spot of mutation) all abolished FAM46C-mediated anti-myeloma activity, thus would be expected to confer a MM cell survival advantage.

Conclusion: Our data demonstrated that enforced FAM46C expression in myeloma cells induced myeloma growth inhibition and apoptosis. Mutations in FAM46C and TP53 in newly diagnosed patients seem mutually exclusive but not in relapsed patients from our patients sequencing studies, suggesting it may associate with disease progression. Together, these studies suggest that FAM46C may function as a tumor suppressor in myeloma. We also found that published mutations of FAM46C confer a survival advantage to MM cells, and that FAM46C overexpression downregulates IRF4 and MYC and is thus associated with loss of myeloma cell survival.