Analysis of genomic homology of murine gammaherpesvirus (MHV)-72 to MHV-68 and impact of MHV-72 on the survival and tumorigenesis in the MHV-72-infected CB17 scid/scid and CB17+/+ mice

Murine gammaherpesvirus (MHV)-68-infected mice are well-known as models for Epstein-Barr virus (EBV)-related lymphoproliferative diseases. MHV-72 may be a relative of MHV-68, but any genetic comparison between the two (except for the M7 gene) has never been reported. The genetic compositions of MHV-72 and MHV-68 were compared and the pathology of MHV-72 infection studied in CB-17 severe combined immunodeficiency (scid/scid; SCID) and CB17 wild-type (CB17+/+) mice. The MHV-72 DNA sequence was almost identical to MHV-68 except for approximately 7000 bp corresponding to the MHV-68 M1-M3 genes. Twenty-seven of 30 MHV-72-infected SCID mice (90%) died from generalized infection with intranuclear viral inclusions for approximately 1 month, while MHV-72-infected CB17+/+ mice recovered from acute infection. Long observation and pathological study of 68 MHV-72-infected mice for up to 24 months revealed that the survival rate (29.4%) and survival time (21.3 months) of MHV-72-infected CB17+/+ mice were significantly lower (P = 0.0127) and shorter (P = 0.0065) than those of the controls (61.1% and 22.9 months), respectively. The malignancy development rate (60.3%) of the infected CB17+/+ mice was also significantly higher (P = 0.004) than those of the controls (22.2%). However, no MHV-72 DNA was detected in the tumors of infected mice. MHV-72 may have some tumor-promoting effects but the tumorigenesis in infected CB17+/+ mice is different from EBV-associated tumors.