Identification of an Active Reverse Transcriptase Enzyme Encoded by a Human Endogenous HERV-K Retrovirus

Abstract

Of the numerous endogenous retroviral elements that are present in the human genome, the abundant HERV-K family is distinct because several members are transcriptionally active and coding for biologically active proteins. A detailed phylogeny of the HERV-K family based on the partial sequence of the reverse transcriptase (RT) gene revealed a high incidence of an intact RT open reading frame within the HML-2 subgroup of HERV-K elements. In this study, we report the cloning of six full-length HML-2 RT genes, of which five contain an uninterrupted open reading frame. The RT enzymes were expressed as glutathione S-transferase fusion proteins inEscherichia coli, and several HERV-K RT enzymes demonstrated polymerase as well as RNase H activity. Several biochemical properties of the RT polymerase were analyzed, including the template requirements and optimal reaction conditions (temperature, type of divalent cation). Inspection of the nucleotide sequence of the HERV-K RT genes demonstrated a mosaic structure, suggesting that a high level of genetic recombination has occurred in this virus family, which is a hallmark of replication by means of reverse transcription. The selective pressure to maintain the RT coding potential is illustrated by the sequence of a particular HERV-K isolate that contains three 1-nucleotide deletions within a small RT segment, thus maintaining the open reading frame. These combined results may suggest that these endogenous RT enzymes still have a biological function. It is possible that the RT activity was involved in the spread of this major class of retroelements by retrotransposition, and in fact it cannot be excluded that this retrovirus group is still mobile. The endogenous RT activity may also have been involved in the shaping of the human genome, e.g., by formation of pseudogenes.

Monocyte activation and differentiation augment human endogenous retrovirus expression: Implications for inflammatory brain diseases

“…Moreover, this effect was observed only with HERV‐K and HERV‐H, HERV families shown previously to be the most transcriptionally active or to contain a large percentage of members with intact genomes 39, 40. It is not surprising that our findings suggest production of HERV‐K particles; earlier reports have demonstrated that several members of this family encode functional RT enzymes and exhibit the capacity to form particles under similar conditions 41–43. However, the ability to detect HERV‐H sequences is intriguing, because particles containing RNA from the RGH subgroup of this family have been found to be more prevalent in patients with MS 12, 44.…”

Identification of active loci of a human endogenous retrovirus in neurons of patients with amyotrophic lateral sclerosis

“…15,16 Conversely 7q36.1, which belongs to the HML-3 subfamily of HERV-K, has a putative ORF for RT, with a unique insertion of 2 codons that would potentially translate to 2 amino acids inserted prior to the conserved RT motif LPQG (see Supplementary Fig). 18,19 The HML-3-derived 7q36.1 HERV-K ORF for RT resides in the probable 14.3cM candidate interval (between D7S2511 and D7S798) for MND, 17 and may thus represent a putative disease marker in ALS and other types of MND. These observations show a disease-specific pattern of expression of HERV-K in ALS from novel loci in the human genome and intact large ORFs in the motor cortex, whereas patients with systemic illness had a nonspecific pattern of expression with truncated transcripts, and there was no expression of HERV-K in patients with Parkinson disease or those with accidental death.…”

Endogenous retroviruses and human evolution

“…HERV transcripts commonly contain many mutations in their ORFs and do not code for any functional proteins. However, some proviruses have intact ORFs, which are evidenced by the presence of retroviral proteins or the detection of their enzymatic activities in some human tissues 2, 3, 18, 26. Furthermore, in addition to retroviral proteins, virus‐like particles (VLPs) have been found in human tissues, which were shown to be able to bud from the cell membrane, but to be unable to infect cells 18.…”