Novel Insights into the Multiple Sclerosis Risk Gene ANKRD55

Abstract

An intronic variant in ANKRD55, rs6859219, is a genetic risk factor for multiple sclerosis, but the biological reasons underlying this association are unknown. We characterized the expression of ANKRD55 in human PBMCs and cell lines. Three ANKRD55 transcript variants (Ensembl isoforms 001, 005, and 007) could be detected in PBMCs and CD4⁺ T cells but were virtually absent in CD8⁺, CD14⁺, CD19⁺, and CD56⁺ cells. Rs6859219 was significantly associated with ANKRD55 transcript levels in PBMCs and CD4⁺ T cells and, thus, coincides with a cis-expression quantitative trait locus. The processed noncoding transcript 007 was the most highly expressed variant in CD4⁺ T cells, followed by 001 and 005, respectively, but it was not detected in Jurkat, U937, and SH-SY5Y cell lines. Homozygotes for the risk allele produced more than four times more transcript copies than did those for the protective allele. ANKRD55 protein isoforms 005 and 001 were predominantly located in the nucleus of CD4⁺ T cells and Jurkat and U937 cells. ANKRD55 was produced by primary cultures of murine hippocampal neurons and microglia, as well as by the murine microglial cell line BV2, and it was induced by inflammatory stimuli. ANKRD55 protein was increased in the murine mouse model of experimental autoimmune encephalomyelitis. Flow cytometric analysis of CNS-infiltrating mononuclear cells showed that CD4⁺ T cells and monocytes expressed ANKRD55 in experimental autoimmune encephalomyelitis mice, with the higher fluorescence intensity found in CD4⁺ cells. A low percentage of microglia also expressed ANKRD55. Together, these data support an important role for ANKRD55 in multiple sclerosis and neuroinflammation.