Functional characterization of the 40 bp internal repeat of murine gamma herpesvirus-68.

Gamma herpesvirus genomes have been observed to contain different kinds of repeat sequences, the functions of which range from immune evasion to being a part of the lytic origin of replication. The MHV 68 genome also contains two such internal repeats. In this study, we addressed the role of the 40 bp internal repeat and the hypothetical ORF M6 (which contains the 40 bp repeat) in in vitro replication and pathogenesis in mice. The 40 bp internal repeat and/or ORF M6 of MHV-68 are dispensable for lytic replication both in vitro and in vivo, however, it might be possible that the 40 bp internal repeat contains sequences involved in lytic replication, i.e., an origin of lytic replication. The 40 bp internal repeat plays an important role for the amplification of latency, as demonstrated by the reduction in the extent of splenomegaly and reduced reactivation. This phenotype is dependent on the number of 40 bp internal repeat units present in the genome, and is not associated with expression of ORF M6. During latency, the expression of the MHV-68 ORF MK3, known for its role in immune evasion, is reduced in the absence of the 40 bp repeat, suggesting that the 40 bp repeat is involved in the establishment of latency via regulation of the expression of ORF MK3 in a tissue specific/infection cycle specific fashion. For the first time, we have demonstrated the importance of an internal repeat in latency of a gamma herpesvirus and in the pathogenesis. Our data suggest that internal repeats could act as enhancer regions for certain virus specific genes, thus playing a role in affecting the phenotype of virus-induced disease