OAB-028: FAM46C-dependent tuning of endoplasmic reticulum capacity in Multiple Myeloma

Background Protein secretion is a driving force in evolution and tissue specialization. Plasma cells (PC), responsible for intense antibody (Ab) production, are paradigmatic professional secretors whose differentiation entails rapid endoplasmic reticulum (ER) expansion and concerted expression of the protein translocation, folding and trafficking machinery. A novel player in PC differentiation, recently implicated in Ab immunity is the non-canonical poly(A) polymerase FAM46C/TENT5C. Methods In developing PCs, FAM46C is induced to polyadenylate and enhance the translation of a number of mRNAs encoding Igs and ER-targeted proteins [1,2]. Being deleted or mutated in up to 20% multiple myeloma (MM) patients, but intact in other cancers, the FAM46C gene is among the most frequent genomic hits in MM and a myeloma-specific onco-suppressor [3,4]. Results We have recently shown that the ER-specific action of FAM46C depends on its interaction with the ER transmembrane FNDC3 proteins. As a result, FAM46C concertedly boosts the expression of ER and Golgi proteins, potently upsizing the secretory apparatus and Ig secretion [5]. Our data suggest that in MM the FAM46C gene is under selective pressure to contain the proteosynthetic, oxidative and metabolic stress associated with Ig secretion, favoring myeloma cell survival and growth. In keeping with this, exogenous re-expression of FAM46C in mutated MM cells raised Ig secretory capacity beyond sustainability, causing ATP shortage, ROS accumulation and apoptosis. Interestingly, although able to increase the secretory capacity across different cell types, FAM46C overexpression induced apoptosis exclusively in MM cells, being well tolerated in non-professional secretors, suggesting a key role of the secretory cargo in FAM46C toxicity. Moreover, we discovered a broader function of FAM46C in ER homeostasis beyond mRNA stabilization. Indeed, we found that FAM46C is part of a novel integrated network coordinating the biogenesis of ribonucleoprotein complexes, ER protein translation and import, and ER expansion, to harmonize massive Ab production with protein homeostasis. Finally, our proteomic analysis unveiled a surprising opposite modulation of signal recognition particle (SRP) proteins mediated by FAM46C. Indeed, while 4 out of 6 SRP proteins were positively regulated by FAM46C, the heterodimeric members SRP9 and SRP14, mediating translational arrest, increased upon FAM46C loss. Conclusions We infer that PCs have the ability to sense FAM46C-induced ER expansion and modulate SRP composition to minimize translational pausing and maximize Ab manufacture. This novel integrated network offers a framework to identify unprecedented highly specific therapeutic targets against PC dyscrasias.