Abstract
Multiple myeloma, also known as plasma cell myeloma, is a cancer of plasma cells in the bone marrow. New Zealand has the highest global incidence of myeloma, with higher incidence in Māori and Pacific populations. This study focused on a single nucleotide polymorphism (SNP), rs34562254 in TNFRSF13B, that associates with an increased risk of multiple myeloma and has a high allele frequency in Polynesians. The gene TNFRSF13B encodes the protein transmembrane activator and CAML interactor (TACI), which functions as a B cell receptor and signals through NF-κB pathway. TACI has a long and a short isoform. The rs34562254 variant results in a proline to leucine substitution in TACI; therefore, this variant can also be described as TACI P251L in the long isoform, and P205L in the short isoform. We aimed to explain the association of the TACI P251L/P205L variant and myeloma risk through investigating the functional impact of the variant, specifically in its NF-κB signalling compared to TACI wild type.
A luciferase assay was designed and optimised to quantify NF-κB signalling of different forms of TACI by testing their effect on the transcriptional response of an NF-κB reporter construct. In addition, an alternate immunofluorescence method was used to visualise cellular p65 location to measure the NF-κB activities of the different forms of TACI.
The key finding of this study was a decreased NF-κB signalling activity of TACI P205L variant in the short isoform, compared to TACI wild type in the short isoform, in both no stimulation and BAFF-stimulated conditions. In the luciferase assay, the NF-κB activity of TACI P205L variant was 51% of the wild type without BAFF, and 29% with BAFF stimulation. For the immunofluorescent p65 localisation assay, the NF-κB activity of P205L variant was 74% of the wild type without BAFF stimulation, and 67% with BAFF stimulation (for the repeat experiment, 52% without BAFF, and 60% with BAFF stimulation).
These results suggest a mechanism for the effect of the TACI P205L variant on the increased risk of myeloma. Hypotheses, relating to the role of TACI in B cell development, non canonical NF-κB pathways, and other TACI signalling pathways are discussed.