A FAM46C-Dependent Molecular Circuit Modulates Antibody Synthesis and Proliferation in Plasma Cell Dyscrasias

Plasma cells (PCs) are the terminal effectors of B lymphocyte differentiation in charge of antibody production. Such intensive task requires rapid endoplasmic reticulum (ER) expansion and the concerted induction of the machinery affording ER import, folding, and trafficking along the secretory route. Moreover, the by-products of massive immunoglobulin (Ig) production cause a peculiar dependence of PCs on protein degradative pathways with a crucial clinical consequence: theextraordinary success of proteasome inhibitors against multiple myeloma (MM) and systemic light chain amyloidosis (AL). Notably, an essential player in the proteomic reshaping occurring during PC differentiation is the non-canonical poly(A)polymerase FAM46C/TENT5C. Upon B cell activation, FAM46C is induced to polyadenylate and stabilize mRNAs encoding Igs and ER-targeted proteins, thus promoting humoral immune responses (Mroczek et al., 2017; Bilska et al., 2020). Interestingly, FAM46C locus on chromosome 1p12 is mutated or deleted in up to 20% MM patients (Chapman et al., 2011; Boyd et al., 2011, Barbieri et al., 2016), suggesting that MM cells are subject to a selective pressure to restrict FAM46C expression and activity, to ultimately curb Ig synthesis and secretion in favor of cell survival and tumor growth. Accordingly, exogenous re-expression of FAM46C in mutated MM cells raised the secretory capacity beyond sustainability, resulting in intracellular ATP reduction, ROS accumulation and increased apoptotic susceptibility (Fucci et al., 2020). Our research aims to discover the precise molecular circuits, centered on FAM46C, regulating PC secretory capacity in efficient coordination with degradative pathways, to discover unprecedented vulnerabilities of PCs and new therapeutic opportunities against PC dyscrasias.

To clarify the molecular mechanisms underlying the effects of FAM46C on Ig secretion we combined cutting-edge protein biochemistry, imaging and unbiased proteomic assays and adopted in vitro and in vivo models of MM and AL. In these models, we disclosed that FAM46C concertedly promotes the expression of ER and Golgi proteins, potently expanding the secretory apparatus and light chain (LC) secretion. To assess the impact of FAM46C on PC proliferation and disease progression in vivo, we silenced or overexpressed FAM46C in AMLC-2 cells that bear a hemizygous loss in 1p12 region (Arendt et al., 2008). Following intravenous injection into Rag2^-/- IL2rg^-/- mice, we found remarkably reduced tumor growth and longer survival in recipients of FAM46C-overexpressing cells, despite higher initial circulating LC levels, as compared to other groups. Moreover, we observed intermediate disease progression in mice receiving mock-transduced ALMC-2 cells, attesting to dose-dependent impact of FAM46C. The in vivo association of higher secretory activity with drastically decreased PC proliferation reveals that FAM46C uncouples monoclonal component levels from disease burden.

Finally, among FAM46C-modulated proteins, we found SRP components and RNA methyltransferases involved in ribosome assembly and tRNA processing. The data unveil a new role played by FAM46C in PCs, in addition to its mRNA stabilizing activity, indicating the existence of a previously unrecognized integrated network coordinating ribosome biogenesis, protein translation, and ER expansion, potentially crucial to harmonize intensive protein secretion with cellular homeostasis.

Altogether, our studies disclose an unexpected molecular circuit orchestrating Ig synthesis and secretion, and regulating PC proliferation and survival that may be exploited to design new therapeutic strategies aimed to reduce pathogenic LC production, to eliminate MM and AL cells, and to improve the efficacy of current therapeutic regimens.

Nuvolone:Oncopeptides AB: Research Funding; Jansenn-Cilag: Other: Travel Support; Gate Bioscience: Research Funding; Pfizer: Research Funding.