Page 361 - Avian Virology: Current Research and Future Trends

352 | Lupiani et al.
pp38/pp24 ICP4 protein was identified as a 1415 amino acid long protein
MDV unique phosphoprotein, pp38 (encoded by MDV073) that shows similar size to other alphaherpesviruses (Anderson
and pp24 (encoded by MDV008) are located at the junction of et al., 1992). However, later sequence information revealed that
UL/IRL and UL/TRL, respectively, and share the 65 amino- there is a much larger ORF that encodes 2323 amino acid long
terminal amino acids. The pp38, which localizes in the cytoplasm, protein in this region, and a similar pattern was found in HVT
is an early viral protein that is readily detectable in tumour cells indicating that MDV ICP4 is much larger than other alphaher-
during virus reactivation. The pp38 and pp24 share the common pesviruses (Tulman et al., 2000). Later study reported that MDV
promoter-enhancer of the transformation-related 1.8-kb gene ICP4 is involved in the maintenance of transformation, as ICP4
family (Bradley et al., 1989), and pp38 has been suggested to be antisense oligodeoxynucleotides transfected into MSB1 cells
involved in MDV tumorigenicity (Cui et al., 1991). The pp38 showed reduced colony formation in soft agar (Xie et al., 1996).
was shown to be involved in the maintenance of transformation Other studies suggested that transfection of ICP4 into MSB1
state of MSB1 (an MDV tumour cell line) using oligodeoxynu- cells increased transcription of pp38, pp24 and endogenous ICP4
cleotides antisense to the predicted pp38 mRNA sequence (Xie indicating MDV ICP4 serves as a transcriptional transactivator
et al., 1996). Inoculation of chickens with a pp38 deletion mutant similar to the ICP4 protein of other alphaherpesviruses (Endoh,
virus revealed that pp38 is involved in early cytolytic infection in 1996; Pratt et al., 1994). A most recent study reported that the
lymphocytes but not in the induction of tumours (Reddy et al., MDV ICP4 gene as a 9438 nucleotides long and utilizes four
2002). Further animal studies suggested that pp38 is essential for alternative poly(A) signals and is controlled by two alternative
cytolytic infection of B cells, while is dispensable for cytolytic promoters, a distal and a proximal promoter (Rasschaert et al.,
infection of FFE and horizontal transmission (Gimeno et al., 2018). In this study, with the application of RACE-PCR and tran-
2005b). The pp38 has also been shown to be a substrate of MDV script analysis, it was shown that the MDV ICP4 gene consists
encoded protein kinase, US3, but the exact phosphorylation site of three exons separated by two introns, and there are five ICP4
within pp38 remains unknown (Schumacher et al., 2008). ORFs in this region due to the alternative splice mechanism.
Similar to pp38, pp24 is expressed in lytic phase as well as in Further, it was also shown that the expression of ICP4 was under
MDV tumour cells. It has been showed that pp24 interacts with different transcriptional and post-transcriptional regulation in the
pp38 in MDV infected cells and the heteropolymer of pp24/pp38 lytic, latent and reactivation phases of infection (Rasschaert et al.,
can enhance the bi-directional promoter activity between pp38 2018).
and 1.8-kb family of RNAs (Ding et al., 2008). The biological In addition to the ICP4 gene, this region of MDV genome
significance of such interaction is not fully understood. also codes a group of latency associated transcripts (LATs) that
contains several small RNAs and a 10-kb RNA which map anti-
vTR sense to the ICP4 gene and have been suggested to be expressed
MDV encodes a viral telomerase RNA (vTR), located within the in MDV induced T lymphoma and in lytically infected cells
a-like sequences that are present at the end of the TRL and IRL (Cantello et al., 1997; Osterrieder et al., 2006). A recent study
regions. MDV vTR shares 88% sequence identity with chicken identified a total of 22 LAT variants due to the alternative splicing
TR (chTR) (Trapp et al., 2006). Deletion of both copies of vTR of 15 exons (Strassheim et al., 2012). Furthermore, the cluster
significantly impaired the ability of MDV mutants to induce T-cell 3 MDV miRNA (MDV-miR-6 to -8) is located within the LAT
lymphomas, without affecting lytic replication in vivo, indicating region and may play a role in MDV oncogenesis by regulating
its essential role in MDV tumour formation (Trapp et al., 2006). host gene expression through miRNA pathways (Burnside et al.,
Surprisingly, an in vitro functional assay showed that chicken 2006), and will be discussed in detail later.
telomerase reverse transcriptase (TERT) is more efficient in its
interactions with vTR than chTR (Fragnet et al., 2005). But a later 1.8-kb gene family (pp14 and RLORF9)
study demonstrated that the interaction of vTR with TERT is not There are two identical copies of a bi-directional promoter in the
required for MDV oncogenesis as disruption of that interaction IRL and TRL (at the junction with UL) which are responsible
still resulted in the induction of T-cell lymphomas (Kaufer et al., for the expression of pp38 and the 1.8-kb family of transcripts
2010). The MDV genome encodes telomere sequence at the edge (Tahiri-Alaoui et al., 2009a). In the 1.8-kb transcripts family,
of linear genomic sequence (Kheimar et al., 2017). Deletion of pp14 and RLORF9 are the representative transcripts. There are
vTR impaired the ability of integration of MDV genome into host two splice variants of pp14 that have different 5′ exon and share a
telomere genomic region leading to a hypothesis that vTR might 3′ exon (Tahiri-Alaoui et al., 2009a). It has been shown that there
be evolved to add telomere repeat to the end of the viral genome are internal ribosome entry sites (IRES) in the 5′ leader of a vari-
to promote the integration (Osterrieder et al., 2006). ant bicistronic mRNA that encodes pp14 and RLORF9 and in
the intercistronic region (ICR) region of the bicistronic mRNA
ICP4 and latency-associated transcripts (LATs) (Tahiri-Alaoui et al., 2009a,b). The IRES within ICR can control
MDV ICP4, encoded by MDV084 and MDV100 which are the translation of RLORF9 when the cap-dependent translation
located within IRS and TRS regions of MDV genome, is the is inhibited, while the 5′ leader IRES controls the translation of
homologue of HSV-1 ICP4 and VZV ORF62. In all alphaher- pp14 to produce two different pp14 isoforms with different 5′
pesviruses, ICP4 is an immediate early transcriptional regulator exon (Tahiri-Alaoui et al., 2009a,b). Deletion of RLORF9 did
that is robustly expressed during lytic replication. Initially, MDV not significantly affect the viral pathogenesis (Tahiri-Alaoui et al.,