Integration of Multiple Chicken Retroviruses into Multiple Chicken Herpesviruses: Herpesviral gD as a Common Target of Integration

255

221

Here we present the first complete genomic sequence, with analysis, of a very virulent strain of Marek's disease virus serotype 1 (MDV1), Md5. The genome is 177,874 bp and is predicted to encode 103 proteins. MDV1 is colinear with the prototypic alphaherpesvirus herpes simplex virus type 1 (HSV-1) within the unique long (UL) region, and it is most similar at the amino acid level to MDV2, herpesvirus of turkeys (HVT), and nonavian herpesviruses equine herpesviruses 1 and 4. MDV1 encodes 55 HSV-1 UL homologues together with 6 additional UL proteins that are absent in nonavian herpesviruses. The unique short (US) region is colinear with and has greater than 99% nucleotide identity to that of MDV1 strain GA; however, an extra nucleotide sequence at the Md5 US/short terminal repeat boundary results in a shorter US region and the presence of a second gene (encoding MDV097) similar to the SORF2 gene. MD5, like HVT, encodes an ICP4 homologue that contains a 900-amino-acid amino-terminal extension not found in other herpesviruses. Putative virulence and host range gene products include the oncoprotein MEQ, oncogenicity-associated phosphoproteins pp38 and pp24, a lipase homologue, a CxC chemokine, and unique proteins of unknown function MDV087 and MDV097 (SORF2 homologues) and MDV093 (SORF4). Consistent with its virulent phenotype, Md5 contains only two copies of the 132-bp repeat which has previously been associated with viral attenuation and loss of oncogenicity.Marek's disease (MD) is a lymphoproliferative disease of chickens caused by the highly infectious cell-associated alphaherpesvirus MD virus serotype 1 (MDV1) (18). Yearly economic losses from MD total $1 billion worldwide (18). MDV1 infection results in a rapid onset of malignant T-cell lymphomas within several weeks of infection. Tumor infiltration results in a neural form of disease, which causes progressive paralysis, or a visceral form of disease, which is usually very acute and accompanied by high mortality. Productive virus replication in the skin and feather follicle epithelia with subsequent virus shedding is responsible for disease transmission (18).MD is controlled by vaccination and good management practices (18). Naturally occurring nonpathogenic strains of MDV1, MDV2, and herpesvirus of turkey (HVT or MDV3) have been used individually or together in bivalent vaccines (18,40). Recent increases in MD-related mortality and condemnations among vaccinated poultry have occurred in the United States. These increases in disease have occurred approximately 6 years after the introduction of new vaccines (99). In the late 1970s, following the introduction of HVT vaccines, and since 1992, after the introduction of bivalent MDV2-HVTbased vaccines, new MDV1 strains of greater virulence (very virulent [vv] and very virulent plus [vv ϩ ] MDV1) were isolated. These viruses are characterized by higher cytolytic activity, unusual tissue tropism, increased atrophy of lymphoid organs, immunosuppression, enhanced capacity to transform T cells, and earlier host death (7...

Section: Resultsmentioning

confidence: 95%

The Genome of a Very Virulent Marek's Disease Virus

Tulman

Afonso

et al. 2000

255

221

“…This event, however, has usually resulted in only a solitary REV LTR or one having a 23-bp repeated sequence at its 5Ј terminus that is retained during subsequent replication of the altered herpesviruses (10,12,14). In contrast, when the recipient was the herpesvirus of turkeys, a stable integrant having an intact provirus was found in addition to ones containing individual proviruses with internal deletions (11). In each instance, the last two nucleotides of the 3Ј LTR were absent and a 5-bp host sequence adjacent to the point of insertion was duplicated.…”

Section: Discussionmentioning

confidence: 99%

Reticuloendotheliosis Virus Sequences within the Genomes of Field Strains of Fowlpox Virus Display Variability

Singh

Schnitzlein

Tripathy

2003

Nine field strains of fowlpox virus (FPV) isolated during a 24-year span from geographically diverse outbreaks of fowlpox in the United States were screened for the presence of reticuloendotheliosis virus (REV) sequences in their genomes by PCR. Each isolate appeared to be heterogeneous in that either a nearly intact provirus or just a 248-or 508-nucleotide fusion of portions of the integrated REV 5 and 3 long terminal repeats (LTRs) was exclusively present at the same genomic site. In contrast, four fowlpox vaccines of FPV origin and three originating from pigeonpox virus were genetically homogeneous in having retained only the 248-bp LTR fusion, whereas two other FPV-based vaccines had only the larger one. These remnants of integrated REV presumably arose during homologous recombination at one of the two regions common to both LTRs or during retroviral excision from the FPV genome. Loss of the provirus appeared to be a natural event because the tripartite population could be detected in a field sample (tracheal lesion). Moreover, the provirus was also readily deleted during propagation of FPV in cultured cells, as evidenced by the detection of truncated LTRs after one passage of a plaque-purified FPV recombinant having a "genetically marked" provirus. However, the deletion mutants did not appear to have a substantial replicative advantage in vitro because even after 55 serial passages the original recombinant FPV was still prevalent. As to the in vivo environment, retention of the REV provirus may confer some benefit to FPV for infection of poultry previously vaccinated against fowlpox.

“…As with other herpesviruses, the GϩC content in the repeat regions is higher than in unique regions (55% in repeats versus 45% in unique regions) (21,32,73). HVT contains no retrovirus long terminal repeat sequences as have been described for cell culture-adapted MDV1 strains (36,38,39,45). The assembled HVT sequence contains a 251-bp sequence that is identical at each terminus and at the IRL/IRS junction.…”

Section: Resultsmentioning

confidence: 84%

The Genome of Turkey Herpesvirus

Afonso

Tulman

et al. 2001

122

Here we present the first complete genomic sequence of Marek's disease virus serotype 3 (MDV3), also known as turkey herpesvirus (HVT). The 159,160-bp genome encodes an estimated 99 putative proteins and resembles alphaherpesviruses in genomic organization and gene content. HVT is very similar to MDV1 and MDV2 within the unique long (UL) and unique short (US) genomic regions, where homologous genes share a high degree of colinearity and their proteins share a high level of amino acid identity. Within the UL region, HVT contains 57 genes with homologues found in herpes simplex virus type 1 (HSV-1), six genes with homologues found only in MDV, and two genes (HVT068 and HVT070 genes) which are unique to HVT. The HVT US region is 2.2 kb shorter than that of MDV1 (Md5 strain) due to the absence of an MDV093 (SORF4) homologue and to differences at the UL/short repeat (RS) boundary. HVT lacks a homologue of MDV087, a protein encoded at the UL/RS boundary of MDV1 (Md5), and it contains two homologues of MDV096 (glycoprotein E) in the RS. HVT RS are 1,039 bp longer than those in MDV1, and with the exception of an ICP4 gene homologue, the gene content is different from that of MDV1. Six unique genes, including a homologue of the antiapoptotic gene Bcl-2, are found in the RS. This is the first reported Bcl-2 homologue in an alphaherpesvirus. HVT long repeats (RL) are 7,407 bp shorter than those in MDV1 and do not contain homologues of MDV1 genes with functions involving virulence, oncogenicity, and immune evasion. HVT lacks homologues of MDV1 oncoprotein MEQ, CxC chemokine, oncogenicity-associated phosphoprotein pp24, and conserved domains of phosphoprotein pp38. These significant genomic differences in and adjacent to RS and RL regions likely account for the differences in host range, virulence, and oncogenicity between nonpathogenic HVT and highly pathogenic MDV1.Turkey herpesvirus (HVT) is a ubiquitous, nonpathogenic virus of domestic turkeys (15,(78)(79)(80), and it is classified as the third serotype within the Marek's disease virus (MDV) group of antigenically and genetically related lymphotropic avian herpesviruses (15). This group also includes MDV serotype 1 (MDV1), which is the etiologic agent of the globally and economically significant Marek's disease in chickens (15). MDV1 is pathogenic in chickens, causing cytolytic infection in B cells, latent infection in T cells, and induction of T-cell lymphoma. MDV2 is nonpathogenic or of low pathogenicity in chickens (15).HVT is nonpathogenic in chickens, but it does induce a viremia which is associated with induction of protective immune responses against MDV1 (15). Chickens infected with HVT become persistently infected and maintain long-lasting immunity (15,29,44,53,80). HVT appears to replicate less efficiently in skin than does MDV1, a phenotype that may be involved in the relatively infrequent transmission of HVT among chickens compared to attenuated strains of .HVT and combinations of HVT and nonpathogenic strains of MDV1 and MDV2 have been used extensively and e...