FAM46C/Tent5c, a non-canonical polyA-polymerase expressed by plasma cells, is recurrently mutated, or genetically deleted, in 20% of multiple myeloma (MM) patients but rarely in other cancers. While the role of FAM46C as a tumor suppressor has been defined in MM cell lines, a comprehensive mechanistic investigation of its physiological and pathological function in immunocompetent in vivo models is lacking. Here, we generated isogenic mouse MM cell lines with Tent5c loss by CRISPR-Cas9 editing, which promoted tumor cell proliferation upon in vivo transplantation that shortened survival of C57BL/6 immunocompetent mice. Reintroduction of full-length wild-type Tent5c, but not of several mutant Tent5c isoforms, decreased MM cell proliferation and clinical aggressiveness, concordant with the loss-of-function nature of common FAM46C mutations. To further explore the role of FAM46C, we engineered mice using Cre-LoxP recombination to conditionally delete Tent5c in mature B lymphocytes and plasma cells by a cγ1-cre allele upon T cell driven-immunization with SRBCs. Tent5c-KO mice exhibited a higher number of germinal center B cells that shifted towards activated B-cell differentiation at the expense of memory B lymphocytes, promoting formation of aberrant plasma cells in spleen and bone marrow (BM) with increased proliferation and expression of B-cell markers. To determine FAM46C/Tent5c oncogenic function in MM, Tent5c-KO mice were crossed with BIcγ1 mice, which progressively develop human-like MM preceded by asymptomatic conditions (Larrayoz et al, Nat. Med. 2023). Tent5c-BIcγ1 mice showed marked acceleration of MM development from precursor stages with respect to BIcγ1 mice (median OS, 187 vs 296 days; p<0,0001), exhibiting an augmented number of MM cells infiltrating the BM and circulating in peripheral blood. Transcriptional analyses of MM cells at single-cell resolution revealed enrichment in cell proliferation signatures and high expression of MYC and its target genes, leading to MYC protein stabilization. To validate mouse data in patients, a series of 658 newly diagnosed MM cases from the CoMMPass study with available genomic and transcriptomic data was characterized, with 41 (6%) with FAM46C somatic mutation, 128 (20%) with chromosome 1(p12) deletion, and 24 (4%) with both mutation and deletion. MM cells with FAM46C mutation and/or deletion showed enrichment in proliferation signatures and increased MYC expression, as well as an association with elevated circulating tumor cells (p=0.05). In a second series of 87 untreated smoldering MM patients, cases with FAM46C genetic loss and/or mutation exhibited accelerated progression into clinically active MM (progression rate at 2 years, 50% vs 25%; p=0.00072). Notably, increased degradation of immunoglobulin mRNAs caused by reduced polyadenylation was found in MM cells from Tent5c-BIcγ1 mice, which accordingly exhibited marked reduction of Ig secretion. To evaluate whether Ig production/secretion defects could be MYC-dependent, MYC expression was genetically induced in MM cells from genetically engineered mice with Tent5c loss, called the Tent5c-MYC-Icγ1 model. While MM with an aggressive, proliferative phenotype was found in Tent5c-MYC-Icγ1 mice, which was similar to that of Tent5c-BIcγ1 mice, full restoration of Ig production and secretion was observed. Further transcriptional investigations revealed that MM cells with Tent5c loss showed reduced expression of antigen presentation machinery and MHC-I/II genes, and increased expression of IFN pathway genes, suggesting aberrant tumor immunogenicity. Accordingly, major changes in the BM immune microenvironment were found, with prominent infiltration of CD4 and CD8 T cells with effector-memory phenotypes and expression of co-inhibitory molecules PD1, TIGIT and LAG3, and abundant mature LAG3+TIGIT- NK cells. Overall, our study uncovered a new role of FAM46C/Tent5c in regulating B-cell differentiation from the germinal centers. In addition, these results highlight that FAM46C/Tent5c inactivation promotes a MYC-driven proliferative and non-secretory MM phenotype with profound remodeling of the immune microenvironment, which collectively accelerate disease progression from early stages.

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2025
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