Overexpression of the Cytokine BAFF and Autoimmunity Risk
Authors: Maristella Steri, Valeria Orrù, M. Laura Idda, Maristella Pitzalis, Mauro Pala, Ilenia Zara, Carlo Sidore, Valeria Faà, Matteo Floris, Manila Deiana, Isadora Asunis, Eleonora Porcu, Antonella Mulas, Maria G. Piras, Monia Lobina, Sandra Lai, Mara Marongiu, Valentina Serra, Michele Marongiu, Gabriella Sole, Fabio Busonero, Andrea Maschio, Roberto Cusano, Gianmauro Cuccuru, Francesca Deidda, Fausto Poddie, Gabriele Farina, Mariano Dei, Francesca Virdis, Stefania Olla, Maria A. Satta, Mario Pani, Alessandro Delitala, Eleonora Cocco, Jessica Frau, Giancarlo Coghe, Lorena Lorefice, Giuseppe Fenu, Paola Ferrigno, Maria Ban, Nadia Barizzone, Maurizio Leone, Franca R. Guerini, Matteo Piga, Davide Firinu, Ingrid Kockum, Izaura Lima Bomfim, Tomas Olsson, Lars Alfredsson, Ana Suarez, Patricia E. Carreira, Maria J. Castillo-Palma, Joseph H. Marcus, Mauro Congia, Andrea Angius, Maurizio Melis, Antonio Gonzalez, Marta E. Alarcón Riquelme, Berta M. da Silva, Maurizio Marchini, Maria G. Danieli, Stefano Del Giacco, Alessandro Mathieu, Antonello Pani, Stephen B. Montgomery, Giulio Rosati, Jan Hillert, Stephen Sawcer, Sandra D’Alfonso, John A. Todd, John Novembre, Gonçalo R. Abecasis, Michael B. Whalen, Maria G. Marrosu, Alessandra Meloni, Serena Sanna, Myriam Gorospe, David Schlessinger, Edoardo Fiorillo, Magdalena Zoledziewska, Francesco Cucca
Journal: New England Journal of Medicine (2017)

Genomewide association studies of autoimmune diseases have mapped hundreds of susceptibility regions in the genome. However, only for a few association signals has the causal gene been identified, and for even fewer have the causal variant and underlying mechanism been defined. Coincident associations of DNA variants affecting both the risk of autoimmune disease and quantitative immune variables provide an informative route to explore disease mechanisms and drug-targetable pathways.Using case-control samples from Sardinia, Italy, we performed a genomewide association study in multiple sclerosis followed by TNFSF13B locus-specific association testing in systemic lupus erythematosus (SLE). Extensive phenotyping of quantitative immune variables, sequence-based fine mapping, cross-population and cross-phenotype analyses, and gene-expression studies were used to identify the causal variant and elucidate its mechanism of action. Signatures of positive selection were also investigated.A variant in TNFSF13B, encoding the cytokine and drug target B-cell activating factor (BAFF), was associated with multiple sclerosis as well as SLE. The disease-risk allele was also associated with up-regulated humoral immunity through increased levels of soluble BAFF, B lymphocytes, and immunoglobulins. The causal variant was identified: an insertion-deletion variant, GCTGT→A (in which A is the risk allele), yielded a shorter transcript that escaped microRNA inhibition and increased production of soluble BAFF, which in turn up-regulated humoral immunity. Population genetic signatures indicated that this autoimmunity variant has been evolutionarily advantageous, most likely by augmenting resistance to malaria.A TNFSF13B variant was associated with multiple sclerosis and SLE, and its effects were clarified at the population, cellular, and molecular levels. (Funded by the Italian Foundation for Multiple Sclerosis and others.).