Comparative genomic sequence analysis of the Marek's disease vaccine strain SB-1

Marek's disease virus (MDV), an oncogenic alphaherpesvirus, induces a rapid onset T-cell lymphoma and demyelinating disease in chickens. Since the 1970s the disease has been controlled through mass vaccination with herpesvirus of turkeys [meleagrid herpesvirus type 1 (MeHV-1)]. Over time this vaccine's efficacy decreased, and in the 1980s a bivalent vaccine consisting of MeHV-1 and a non-oncogenic gallid herpesvirus type 3 (GaHV-3) strain known as SB-1 was introduced. The complete DNA sequence (165,994 bp) of this GaHV-3 strain was determined using 454 pyrosequencing. A total of 524 open reading frames (ORFs) were examined for homology to protein sequences present in GenBank using BLAST (E-values <0.9). Of the 128 ORF hits, 75 ORFs showed homology to well-characterized alphaherpesviral proteins. Phylogenetically, this strain partitions in its own branch along with the GaHV-3 strain HPRS24 and shows more relatedness to MeHV-1 than gallid herpesvirus type 2 (GaHV-2, Marek's disease virus). When comparing the GaHV-3 ORFs to their homologues in MeHV-1 and GaHV-2, a greater percentage of amino acid similarity was found with homologous ORFs in the genome of SB-1 than with those in the HPRS24 genome. Overall, twice as many of the 75 ORFs within the SB-1 genome showed greater sequence identities and similarities to homologous ORFs in the Marek's disease genome than those within the HPRS24 genome. This paper describes the sequence difference between the two GaHV-3 genomes. Overall 19 ORFs differ in the number of predicted amino acids; of these, eight (U(L)3.5, U(L)5, U(L)9, U(L)28, U(L)30, U(L)36, U(L)37, and U(L)50) encode well-characterized alphaherpesviral proteins A sequence within the unique short region of the SB-1 genome exhibited significant sequence homology to long terminal repeat (LTR) sequences of avian retroviruses. This sequence was only found in the SB-1 genome and not the HPRS24 genome.