The HERV-K Np9 Protein Regulates Viability of Teratocarcinoma Cells

Abstract

Human endogenous retroviruses are remnants of ancient germline infections that make up approximately 8% of the modern human genome. Most HERVs entered primates approximately 40 million years ago, and have been rendered inactive due to the accumulation of mutations. However, one of the most recent entrants into the human germline is the HERV-K (HML-2) subfamily, most having only integrated between 200,000 and 5 million years ago. The HML-2 subfamily is the most conserved and transcriptionally active, and some members have retained functional open reading frames (ORFs) to code for all of their viral proteins. HERV elements exist in the human genome as retroviral genes (gag, pol, and env) flanked by two long terminal repeats (LTRs). There are approximately 117 full-length copies and around 2500 solitary LTRs of HERV-K (HML-2) spanning multiple chromosomes. Even though no infectious HERV particle has been detected to date, HERV-K (HML-2) have been found to produce viral like particles in breast cancer, leukemia, lymphoma, melanoma, and teratocarcinoma. HERVs have been implicated in human biology, autoimmune diseases, and malignancies. There are two types of HERV-K (HML-2): type I is characterized by a 292 bp deletion at the boundary of the pol and env genes, while type II contains the full sequence for pol and env. The deletion in the type I virus causes an alternative splicing event that results in the expression of the accessory protein Np9, while type II expressed the accessory protein Rec.

In our previous study, we have detected the activation of HERV-K type I proviruses in the blood of patients with HIV-1 infection. The HIV-1 infection activates the expression of a novel HERV-K (HML-2) provirus, which we termed K111, and it is present in multiple copies in the centromeres of chromosomes throughout the human genome. At the time of detection, the K111 virus was not annotated in the most recent human genome assembly. While a few copies of the K111 provirus appear in the genomes of the extinct Neanderthal and Denisovan, modern humans have a least 100 copies of K111 spread across the centromeres of fifteen chromosomes, which suggests the expansion of K111 during the evolution of hominins. Most of the viral genes of K111 are mutated and cannot produce viable viral products, with one exception: the K111 virus contains an intact ORF for the Np9 protein and variants of Np9. For this reason, we investigated the effects of retaining the coding sequence for this accessory protein.

To this end, we decided to study the role of HERV-K Np9 in teratocarcinoma cells. Teratocarcinoma cells have been shown to express HERV-K mRNA, proteins and produce viral like particles. Also, NCCIT teratocarcinoma cells have been shown to express K111 in their viral particles. We show that decreasing the expression of Np9 in NCCIT teratocarcinoma cells increases the sensitivity of NCCIT cells to chemical stresses (bleomycin and cisplatin) and environmental stress (serum starvation). Further, we determined that Np9 is essential to the migration of NCCIT teratocarcinoma cells. These findings support the implication that the HERV-K accessory protein Np9 has oncogenic potential.