The cancer testis antigen, HORMAD1, promotes genomic stability in lung adenocarcinoma

Cancer-testis antigens (CTAs) are proteins whose expressions are typically restricted to the testes but are aberrantly expressed in cancer. There are over 200 known CTAs, most of which have no known function in cancer. In the testis, HORMA Domain Containing Protein 1 (HORMAD1) is expressed in meiotic cells, and recruits proteins to sister chromosomes for the exchange of genetic information during crossing over. HORMAD1 is necessary for crossing over, and mice lacking HORMAD1 are healthy, yet infertile. HORMAD1 is aberrantly expressed in approximately 50% of lung adenocarcinoma patient samples. Patient tumor expression data reveal that high HORMAD1 expression correlates with poor overall patient survival and increased mutational burden. HORMAD1-positive tumors are enriched for genes essential for the DNA replication stress response and DNA repair, including MRE11 and RAD51. HORMAD1 promotes the formation of RAD51 filaments, which accumulate in response to replication stress. Cancer cells undergo chronic replication stress and are able to proliferate while evading catastrophic DNA damage. To determine if HORMAD1 is directly associated with proteins involved in DNA metabolism, we conducted immunoprecipitation mass spectrometry and found that endogenous HORMAD1 associates with a subset of proteins involved in the replication stress response. We found that cells lacking HORMAD1 undergo nascent DNA strand degradation, which is mediated by the MRE11-DNA2-BLM pathway. Moreover, we found that HORMAD1 protects stalled replication forks that are remolded by F-Box DNA Helicase 1 (FBH1) to maintain fork stability. FBH1 negatively regulates the activity of RAD51 via displacement from DNA and/or through ubiquitination for degradation. We found that loss of HORMAD1 leads to single strand DNA gaps during replication. Moreover, we found that HORMAD1 protects FBH1-mediated gaps during replication, but the mechanism as to how HORMAD1 protects these gaps is unknown. We hypothesize that HORMAD1 recruits RAD51 to FBH1-mediated single strand gaps during replication to protect these gaps from degradation and promote genomic stability. Our data suggest that aberrant HORMAD1 expression reduces replication stress-induced DNA damage accumulation to prevent genomic instability. This research gives insight into the mechanism by which HORMAD1 promotes genomic stability and how this process can be targeted for lung adenocarcinoma treatment.