A novel class of somatic mutations in blood detected preferentially in CD8+ cells.

Somatic mutations have a central role in cancer but their role in other diseases such as autoimmune disorders is poorly understood. Earlier work has provided indirect evidence of rare somatic mutations in autoreactive T-lymphocytes in multiple sclerosis (MS) patients but such mutations have not been identified thus far. We analysed somatic mutations in blood in 16 patients with relapsing MS and 4 with other neurological autoimmune disease. To facilitate the detection of somatic mutations CD4+, CD8+, CD19+ and CD4-/CD8-/CD19- cell subpopulations were separated. We performed next-generation DNA sequencing targeting 986 immune-related genes. Somatic mutations were called by comparing the sequence data of each cell subpopulation to other subpopulations of the same patient and validated by amplicon sequencing. We found non-synonymous somatic mutations in 12 (60%) patients (10 MS, 1 myasthenia gravis, 1 narcolepsy). There were 27 mutations, all different and mostly novel (67%). They were discovered at subpopulation-wise allelic fractions of 0.2%-4.6% (median 0.95%). Multiple mutations were found in 8 patients. The mutations were enriched in CD8+ cells (85% of mutations). In follow-up after a median time of 2.3years, 96% of the mutations were still detectable. These results unravel a novel class of persistent somatic mutations, many of which were in genes that may play a role in autoimmunity (ATM, BTK, CD46, CD180, CLIP2, HMMR, IKFZF3, ITGB3, KIR3DL2, MAPK10, CD56/NCAM1, RBM6, RORA, RPA1 and STAT3). Whether some of this class of mutations plays a role in disease is currently unclear, but these results define an interesting hitherto unknown research target for future studies.