Loss of leukemogenicity caused by mutations in the membrane glycoprotein structural gene of Friend spleen focus-forming virus.

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Abstract
Friend virus infection of mice causes progressive leukemogenesis, a rapid splenic erythroblastosis that develops weeks later into a disseminating erythroleukemia. The replication-defective Friend spleen focus-forming virus (F-SFFV) encodes a membrane glycoprotein with an apparent MW of 55,000 (designated gp55), which is structurally and immunologically related to the membrane envelope glycoproteins of dual tropic murine leukemia viruses. Three spontaneous F-SFFV mutants that encode abnormally sized gp55-related glycoproteins with apparent MW of 40,000, 54,000 and 58,000, respectively, were isolated. RNA blot and Southern blot analyses indicate that the mutant nucleic acids do not have substantial deletions or insertions in their glycoprotein gene regions. Protein fragmentation patterns indicate that the mutations affect nonoverlapping domains of the glycoprotein. These mutant glycoproteins seem to be defective in their processing to the plasma membranes. Although transmitted efficiently between cultured cells, the mutants have dramatically reduced leukemogenicities compared with the same titers of wild-type F-SFFV. Evidently, the gp55 structural gene is necessary for initiating the erythroblast proliferative phase of Friend disease and changes in membranes can be primary causes rather than only secondary consequences of tumor progression.

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