Classification of PERVs
Endogenous Retrovirus (ERVs) discovery occurred in the late 1960s and
early 1970s (Weiss, 2006). The first reports on Avian Leukosis Virus
(ALV), Murine Leukaemia Virus (MLV), Murine Mammary Tumor Virus (MMTV)
and PERVs were almost at the same time (Garcia-Montojo, Doucet-O’Hare,
Henderson, & Nath, 2018). Breese et al. first described PERVs in
1970 as virus-like particles, same as those seen in the baby hamster
kidney (BHK-21) cell line infected with MLV (Breese, 1970). And
according to the morphological properties, some investigators speculated
that these viral particles belonged to the Type C Virus (Armstrong,
Porterfield, & De Madrid, 1971). In 1974, George et al. further
characterized the biochemical and immunological properties of the viral
particles and demonstrated that it was a typical type C virus (Todaro,
Benveniste, Lieber, & Sherr, 1974).
Two sets of human-tropic PERVs were first reported in 1997. By using the
3’ Rapid Amplification of Cloned Ends (3’RACE), the Weiss team obtained
two classes of sequences (PERV-A and
PERV-B) from the PK15 and human embryonic kidney 293 cells (HEK-293)
infected with PERVs (Le Tissier, Stoye, Takeuchi, Patience, & Weiss,
1997). The distinct third class of PERV is PERV-C, isolated from a swine
lymphoma infected only with pig cells (Akiyoshi et al., 1998; Takeuchi
et al., 1998). The PERV A and B are found to be polytropic, and the PERV
C is ecotropic only infecting pig cells (Wilson, Wong, VanBrocklin, &
Federspiel, 2000).
In 2001, in addition to the PERV-A, -B, and -C, classified together as
gamma genera of retroviruses
(gamma-1), Patience et al.detected the DNAs of four novel groups of gamma-retrovirus (gamma-2 to
gamma-5) and four novel groups of beta-retrovirus (beta-1 to beta-4) in
the genomes of domestic pigs (Patience et al., 2001). And so far, only
the gamma-1 PERVs remain replication-competent as an active provirus.
PERV-A, -B and -C subfamilies are mainly different in env gene
sequence which encodes envelop protein, while the other genes encoding
internal proteins, including gag and pol , are highly
conserved among the three subfamilies. The PERV-A and PERV-C could form
a hybrid virus named as PERV-A/C through a recombination of theenv gene (Denner, 2008). As one part of the env gene is
originated from PERV-A, the recombinant
PERV-A/C virus can infect human
cells. And associated with alterations in the LTR sequences, the
recombinant virus exhibits an increased replication capacity (Denner,
Specke, Thiesen, Karlas, & Kurth, 2003). Fortunately, by using PERV-C
free pigs, the risk of PERV-A and PERV-C recombination in
xenotransplantation can be eliminated.
Molecular structure of PERVs
PERVs are enveloped-linear ssRNA retroviruses containing two copies of
the genomic RNA. Infectious PERVs (PERV-A, -B, and -C) belong to the
Retroviridae family, Orthoretrovirinae subfamily, Gammaretrovirus genus.
The transcript of PERVs has three open reading frames (ORFs) which
respectively encode structural proteins (Gag), polymerases (Pol) and
glycoproteins (Env) (Fig.1a).
The Gag proteins consist of three major domains: the matrix (MA), the
capsid (CA), and the nucleocapsid (NC). The MA domain initially binds to
the membranes and detaches from the host cell during budding.
The CA domain mediates the crystal lattice-forming protein interactions
in capsids. And the NC domain is responsible for the packaging of the
viral RNA genome (Denner & Tönjes, 2012; Pornillos & Ganser-Pornillos,
2019).
In PERVs, the pol gene encodes the protease (PR), the integrase
(IN), and the reverse transcriptase (RT). The PR can be auto-activated
to cleave Gag and Pol at specific sites to initiate PERV maturation
(Blusch, Seelmeir, & von der Helm, 2002). The RT is a key enzyme in
viral propagation, generating complementary DNAs (cDNAs) from the PERV
ssRNA templates. With the help of the IN, the PERV cDNAs can be
integrated into the host genome, from which the new viral copies can be
generated via host transcription (Łopata, Wojdas, Nowak, Łopata, &
Mazurek, 2018).
The Env protein plays a critical role in the entry of PERV particles
into the host cells during viral replication cycle. Env protein is
synthesized as a trimeric precursor in the endoplasmic reticulum, which
is subsequently cleaved into a surface subunit (SU) and a transmembrane
subunit (TM) by a cellular furin-like PR(Fig.1b). The SU subunit
contains a receptor-binding domain (RBD) which binds to a specific
receptor on the target cell. Through the hydrophobic domain, the TM
subunit is buried in the lipid bilayer of the viral particle. And the
cytoplasmic tail of the TM subunit contains an R peptide, which will be
cleaved during virion maturation to activate the fusion capacity
(Bobkova, Stitz, Engelstädter, Cichutek, & Buchholz, 2002) (Fig.1c).
The CxxC motif is localized at the C-terminus of the SU and covalently
links the CXnCC motif on the TM to form an inter-subunit
disulfide bond (Johnson, 2019). The disulfide bond anchors the SU
subunit to the surface of PERV particles (Fig.1b, c).
In the proviral form of PERVs, the coding sequences of gag ,pol , and env are flanked by a 5’ and a 3’ long terminal
repeats (LTRs). And each LTR contains a unique 3 (U3), repeat region
(R), and unique 5 (U5) regions in 5’ to 3’ direction: U3-R-U5 (Denner,
2021). The transcription of PERVs starts at the U3–R junction in the 5ʹ
LTR, and ends at the R–U5 junction in the 3ʹ LTR. LTRs play an
essential role in the replication cycle of PERVs (Łopata et al., 2018).
Moreover, LTRs possess promoters, enhancers and multiple transcription
factor binding sites (Jung et al., 2013), which drives the transcription
of PERVs within the host genome.