Atypical Teratoid Rhabdoid Tumor (AT/RT) is a rare pediatric central nervous system cancer often characterized by deletion or mutation of SMARCB1, a tumor suppressor gene. In this study, we found that SMARCB1 regulates Human Endogenous Retrovirus K (HERV-K, subtype HML-2) expression. HML-2 is a repetitive element scattered throughout the human genome, encoding several intact viral proteins that have been associated with stem cell maintenance and tumorigenesis. We found HML-2 env expression in both the intracellular and extracellular compartments in all AT/RT cell lines (n = 4) and in 95% of AT/RT patient tissues (n = 37) evaluated. SMARCB1 knock-down in neural stem cells (NSCs) led to an upregulation of HML-2 transcription. We found that SMARCB1 binds adjacent to the HML-2 promoter, repressing its transcription via chromatin immunoprecipitation; restoration of SMARCB1 expression in AT/RT cell lines significantly downregulated HML-2 expression. Further, targeted downregulation of HML-2 transcription via CRISPR-dCas9 coupled with suppressor proteins led to cellular dispersion, decreased proliferation, and cell death in vitro. HML-2 knock-down with shRNA, siRNA, and CRISPR-dCas9 significantly decreased Ras expression as measured by qRT-PCR, suggesting that HML-2 modulates MAPK/ERK signaling in AT/RT cells. Overexpression of NRAS was sufficient to restore cellular proliferation, and MYC, a transcription factor downstream of NRAS, was bound to the HERV-K LTR significantly more in the absence of SMARCB1 expression in AT/RT cells. We show a mechanism by which these undifferentiated tumors remain pluripotent, and we demonstrate that their formation is aided by aberrant HML-2 activation, which is dependent on SMARCB1 and its interaction with MYC.

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Objectives Reactivation of HERV‐K(HML‐2) has been found in subsets of individuals with amyotrophic lateral sclerosis (ALS). This study examines the antibody response against HML‐2 in ALS and analyzes its clinical relevance. Methods Antibodies to HML‐2 envelope (env) were analyzed using a peptide array for epitope mapping and by a peptide enzyme‐linked immunosorbent assay (ELISA) in 242 healthy donors, and 243 ALS and 85 multiple sclerosis (MS) individuals. Extracellular levels of HML‐2 were analyzed by digital polymerase chain reaction (PCR). Results Antibodies in the sera of ALS individuals recognized more HML‐2 env peptides compared to healthy controls (p < 0.0001). ALS individuals had higher levels of HML‐2 than healthy donors (p = 0.02) and higher antibody levels against a select HML‐2 env peptide compared to healthy donors or individuals with multiple sclerosis (p < 0.0001). 55.14% of ALS compared to 21.16% of healthy donors and 13.10% of MS individuals had antibodies against the HML‐2 peptide (AUC = 0.769, p < 0.0001). Levels of extracellular HML‐2 DNA in serum (p = 0.02) and the number of HML‐2 env peptides recognized by ALS sera (p = 0.02) correlated with disease duration. Among ALS individuals, lower levels of HML‐2 antibodies were associated with a definite diagnosis per EL Escorial criteria (p = 0.03), and with a lower predicted (p = 0.02) and observed survival (p = 0.03). Interpretation There is a differential antibody response against specific epitopes of HML‐2 env in ALS and controls, suggesting epitope spreading, likely due to persistent antigenic exposure following reactivation of the viral genes. Low levels of antibodies to HML‐2 env in ALS are associated with poor prognosis and decreased survival probability. ANN NEUROL 2022;92:782–792