Sequence and insertion sites of murine melanoma-associated retrovirus

We previously showed that B16 melanoma cells produce ecotropic melanoma-ass ociated retrovirus (MelARV) which encodes a melanoma-associated antigen rec ognized by MM2-9B6 monoclonal antibody. The biological significance of MelA RV in melanoma formation remains unknown, We found that infection of normal melanocytes with MelARV resulted in malignant transformation. It is likely that MelARV emerged from the defective Emv-2 provirus, a single copy of ec otropic provirus existing in the genome of C57BL/6 mice. In the present stu dy, me cloned and sequenced the full-length MelARV genome and its insertion sites and we completed sequencing of the Emv-2 provirus, Our data show tha t MelARV has a typical full-length retroviral genome with high homology (98 .54%) to Emv-2, indicating a close relationship between both viruses. MelAR V probably emerged as a result of recombination between Emv-2 and an endoge nous nonecotropic provirus. Some observed differences in the gag and pol re gions of MelARV might account for the restoration of productivity and infec tivity of a novel retrovirus that somatically emerged during melanoma forma tion. MelARV does not contain any oncogene and therefore might induce trans formation by insertional mutagenesis. We sequenced two insertion sites of M elARV. The first insertion site represents the 3' coding region of the c-ma f proto-oncogene at 67.0 centimorgans (cM) on chromosome 8, The c-maf proto -oncogene encodes a basic leucine zipper protein homologous to c-fos and c- jun. Insertion of MelARV in BL6 melanoma cells resulted in the up-regulatio n of c-maf, It is noteworthy that the Emv-2 provirus is also inserted into a noncoding region at 61.0 cM on the same chromosome 8. The second insertio n site is the 3' noncoding region of the DNA polymerase gamma (PolG) gene o n chromosome 7, The expression of PolG was not affected by the MelARV inser tion. Further investigation of the biological significance of MelARV in mel anoma formation is being undertaken.