Viewpoint on Emv2, the onlhy endogenous ecotropic murine leukemia virus of C57BL/6 mice

The paper by Lee and colleagues [1] and their additional comment on this work [2] argue that C57BL strain mice carry an endogenous ecotropic murine leukemia virus (E-MLV) with intact coding potential that may be distinct from the previously described Emv2. However, a review of the published literature and analysis of the sequenced genome do not support this view, as pointed out by Young and colleagues [3]. It is apparent that some confusion has resulted from the fact that the sequenced mouse genome is poorly annotated for retroviral sequences, from observations that the Emv2 provirus is replication incompetent, and because the original map location of Emv2 on the Chromosome (Chr) 8 linkage map is not precisely identical to its location on the database map Build. It is worthwhile elaborating on several points relevant to this issue: the fact that mice carrying only Emv2 can, in fact, produce infectious virus, and the evidence supporting the map location of this functional E-MLV provirus on distal Chromosome (Chr) 8 of C57BL mice.Three different subtypes of E-MuLVs have been found in inbred and wild house mice, but only the AKV subtype is present in the common strains of laboratory mice. The different strains carry different numbers of these Emv proviruses inserted in different chromosomal locations [4,5]; this explains the detection of Emv2 sequences in many inbred strains by Lee et al. [1] using primers that are E-MuLV-, but not locus-specific, and also explains the presence of related sequences in M. m. molossinus, a wild mouse species previously shown to carry AKV-type Emv loci [6].As pointed out by Young and colleagues [3], Southern blot analysis of C57BL DNA and examination of its sequenced genome identify one and only one ecotropic MLV provirus in this mouse strain that is found in the BAC RP23-214K5 (GenBank No. AC158362, position 97057-105700); this full-length provirus, Emv2, is 99.5% identical to the infectious AKV MLV derived from Emv11 (J01998). A pol def