Regulation of PERVs
Although in most normal healthy cells of pigs, the release of PERV
particles (PERV-A, -B, -C) was not
observed, PERVs were expressed in all tissues, with the highest
expression in the lung, followed by the ovary, blood, liver and placenta
(Matousková et al., 2013). And a higher expression of PERV mRNA was
found in melanomas than that in normal skin (Dieckhoff et al., 2007). It
is of great interest that the integrated PERV proviral copy number
differs among several organs of the same pig (Fiebig et al., 2018). As
PERVs are transmitted vertically, like normal cellular genes, the number
should be exactly the same in all organs of the same animal. These
indicate that the expression and replication of PERVs are
highly-regulated both temporally and spatially during pig development.
LTRs play a vital role in the
regulation of the PERVs within the host genome. In the LTR U3 region,
there are directly repeated nucleotide sequences. A majority of these
repeat boxes exist as one of the two types, which are distinguished by a
39bp (PERV-A and -B) or 37bp (PERV-C)
of the repeated nucleotides. And the multiplications of these
repeat boxes also correlate with
relatively late viral infection event, higher viral production, and
higher infectious viral titer (Denner et al., 2003). After PERVs
integrate into the host genome, these repeat boxes may increase the
transcription of viral genes and flanked cellular genes. In response to
the deleterious effects of the replicating PERVs, the number of these
repeat boxes is restricted by natural instability and the constraints
imposed by virion packaging limits (Łopata et al., 2018).
Heritable changes in PERV expression without changing the DNA sequence
can occur through altering DNA methylation or histone modification. In
mice, the endogenous retroviruses (ERVs) cause an estimated 10% of
germline mutations in current inbred laboratory strains. Transcription
of ERVs is generally repressed by epigenetic marks such as DNA
methylation and histone modification (Schorn, Gutbrod, LeBlanc, &
Martienssen, 2017). Likewise, treatment of cells with 5‑azacytidine,
which promotes the demethylation of genomic DNA, results in PERV
induction. It indicates the transcription of PERVs is sensitive to DNA
methylation, and most PERV proviruses in pig genomes are currently
silenced by DNA methylation on PERV 5’ LTRs CpG island (Matousková et
al., 2013; Wolf, Nielsen, Mikkelsen, & Pedersen, 2013). And the PK-15
cell line, which has a very low DNA methylation level, keeps a high PERV
transcriptional profile. And viral production also can exemplify the
relation between the DNA methylation and the PERV mRNA transcription
(Matousková et al., 2013).
Transcriptional silencing can also be achieved via establishment of a
heterochromatic structure, which maintains an inactive form of
chromatin. The highly condensed conformation prevents the access of
transcriptional machinery and eventually suppresses transcriptional
expression. Heterochromatin formation is associated with several histone
marks in the genome, such as trimethylation of H3K9 (H3K9me3) and H4K20
(H4K20me3) (Lawrence, Daujat, & Schneider, 2016). Primarily the H3K9me3
has been found to be involved in inactivating viral chromatin of
ERVs(Groh & Schotta, 2017). Meanwhile, knock-out experiments of
methyltransferase, which generates the H3K9me3, have shed light on their
involvement in ERVs silencing (Maksakova, Mager, & Reiss, 2008).
Likewise, PERVs are marked by the H3K9me3 and H4K20me3, and these marks
were not restricted in porcine embryonic germ cells (pEGCs), but also
apparent in PK15 cells. Interestingly, the younger and more active PERV
subfamilies are generally marked by higher histone methylation levels
than the ancient inactivated PERV subtypes (Wolf et al., 2013).