Murine gammaherpesvirus 68 (MHV-68), a widely used small-animal model for the analysis of gammaherpesvirus pathogenesis, encodes the MHV-68-specific ORFs M12 and M13. The function of M12 and M13 has not been investigated so far. Therefore, we constructed and analysed recombinant MHV-68 with mutations in either M12, M13 or M12/M13. Both the M12 and M13 mutants did not display any phenotype in vitro or in vivo. However, although the M12/13 double mutant showed similar lytic growth in fibroblasts in vitro and in the lungs of infected mice as wild-type MHV-68, it was significantly attenuated in vivo during latency. This phenotype was completely restored in a revertant of the M12/13 double mutant. Thus, it appears that M12 and M13 might have redundant functions that are only revealed if both genes are lacking. The observation that M12/13 have a function during latency not only contributes to the further understanding of the pathogenesis of MHV-68 infection but might also be of interest considering that M12/13 are located at a genomic position similar to that of LMP2A and K15. The latter are important proteins of their respective human gammaherpesviruses EBV and KSHV that contribute to cellular survival, cell activation and proliferation, which was deduced from in vitro studies.
Keywords: MHV-68; ORF M12; ORF M13; gammaherpesvirus.