Unbiased examination of genome-wide human endogenous retrovirus transcripts in MS brain lesions

Background: Human endogenous retrovirus (HERV) expression in multiple sclerosis (MS) brain lesions may contribute to chronic inflammation, but expression of genome-wide HERVs in different MS lesions is unknown. Objective: We examined the HERV expression landscape in different MS lesions compared to control brains. Methods: Transcripts from 71 MS brain samples and 25 control WM were obtained by next-generation RNA sequencing and mapped against HERV transcripts across the human genome. Differential expression of mapped HERV-W and HERV-H reads between MS lesion types and controls was analysed. Results: Out of 6.38 billion high-quality paired end reads, 174 million reads (2.73%) mapped to HERV transcripts. There was no difference in HERVs expression level between MS and control brains, but HERV-W transcripts were significantly reduced in chronic active lesions. Of the four HERV-W transcripts exclusively present in MS, ERV3633503 located on chromosome 7q21.13 close to the MS genetic risk locus had the highest number of reads. In the HERV-H family, 75% of transcripts located to nearby 7q21-22 were overrepresented in MS, and ERV3643914 was expressed more than 16 times in MS compared to control brains. Conclusion: Novel HERV-W and HERV-H transcripts located at chromosome 7 regions were uniquely expressed in MS lesions, indicating their potential role in brain lesion evolution.