Characterization of DNA methylation and promoter activity of long terminal repeat elements of feline endogenous retrovirus RDRS C2a

Endogenous retroviruses (ERVs) are genomic elements derived from retroviral infections in ancestral germ lines. Most ERVs are inactivated by genetic or epigenetic mechanisms, such as DNA methylation. RD-114-virus-related sequence (RDRS) C2a is a feline endogenous retrovirus present in all domestic cats; however, its expression and function are not clearly known. DNA methylation at CpG dinucleotides is a hallmark of silenced ERVs. This study aimed to investigate whether long terminal repeats (LTRs) of RDRS C2a function as a gene regulatory region. The DNA methylation status of RDRS C2a was examined by bisulfite sequencing, and CpG sites in 5ʹ LTR of RDRS C2a were found hypomethylated, whereas those in 3ʹ LTR were hypermethylated in feline cells. Several transcription factor-binding sites were identified in LTRs of RDRS C2a. Luciferase assay suggested that 5ʹ LTR of RDRS C2a exhibited strong transcriptional activity, which was suppressed by in vitro DNA methylation. The study indicates that 5ʹ LTR of RDRS C2a possibly functions as a promoter for itself or neighboring genes.