RIII/Sa mice with a high incidence of mammary tumors express two exogenous strains and one potential endogenous strain of mouse mammary tumor virus.

The inbred mouse strain RIII has long been known for shedding large amounts of mouse mammary tumor virus (MMTV) particles in milk and for the development of hormone-dependent early mammary tumors at a very high incidence (>90%). We have established one RIII subline (RIII/Sa) that shows a pattern of virus expression and tumor incidence similar to that in RIII mice. In the present study, we analyzed the milk and mammary tumors of RIII/Sa mice for virus characterization by reverse transcriptase PCR (RT-PCR) cloning and sequencing of the open reading frame (ORF) of the MMTV long terminal repeats (LTRs). Our results show that these mice express a mixture of at least three different MMTV strains, two of which, designated here as RIII/Sa MMTV-1 and RIII/Sa MMTV-2, are exogenous. The third virus, RIII/Sa MMTV-3, appears to carry the signature of an endogenous provirus, Mtv-17. Similar studies done with the milk and mammary glands of another subline, RIIIS/J, revealed that they do not express MMTV in their milk. The RIII/Sa and RIIIS/J mice also exhibited differences in their endogenous proviral contents. Twelve spontaneously developed mammary tumors of RIII/Sa mice were examined for possible Wnt-1 and/or int-2/Fgf3 mutations that are usually found to occur in most mouse mammary tumors as a consequence of MMTV proviral integration. This work led to the isolation of one MMTV-Wnt-1 junction fragment and one MMTV-int-2/Fgf3 junction fragment from 2 of the 12 tumors. Further analyses showed that both junction fragments contained the RIII/Sa MMTV-2-specific LTR ORF, indicating that this virus was involved in the development of both tumors. Whether RIII/Sa MMTV-1 and/or RIII/Sa MMTV-3 plays any role in mammary tumor development in RIII/Sa mice remains to be established. Overall, the present study demonstrates, to our surprise, that (i) RIII/Sa mice express, unlike other native mouse strains, three strains of MMTVs; and (ii) the virions are completely different from the virus expressed by another subline of RIII mice, the BR6 mice.