Ancient Viral DNA in the Human Genome Linked to Neurodegenerative Diseases

Background: Human endogenous retroviruses (HERVs) are sequences in the human genome that originated from infections with ancient retroviruses during our evolution. Previous studies have linked HERVs to neurodegenerative diseases, but defining their role in aetiology has been challenging. Here, we used a retrotranscriptome-wide association study (rTWAS) approach to assess the relationships between genetic risk for neurodegenerative diseases and HERV expression in the brain, calculated with genomic precision. Methods: We analysed genetic association statistics pertaining to Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis, and Parkinson's disease, using HERV expression models calculated from 792 cortical samples. Robust risk factors were considered those that survived multiple testing correction in the primary analysis, which were also significant in conditional and joint analyses, and that had a posterior inclusion probability above 0.5 in fine-mapping analyses. Results: The primary analysis identified 12 HERV expression signatures associated with neurodegenerative disease susceptibility. We found one HERV expression signature robustly associated with amyotrophic lateral sclerosis on chromosome 12q14 (MER61_12q14.2) and one robustly associated with multiple sclerosis on chromosome 1p36 (ERVLE_1p36.32a). A co-expression analysis suggested that these HERVs are involved in homophilic cell adhesion via plasma membrane adhesion molecules. Conclusions: We found HERV expression profiles robustly associated with amyotrophic lateral sclerosis and multiple sclerosis susceptibility, highlighting novel risk mechanisms underlying neurodegenerative disease, and offering potential new targets for therapeutic intervention.