Development of transforming function during transduction of proto-ras into Harvey sarcoma virus

Oncogenic retroviruses are generated by transduction of the coding region of a protooncogene and acquire genetic changes during subsequent replication. Critical genetic events which occurred during and after transduction of rat proto-ras-1Ha into Harvey sarcoma virus were identified by evaluating the transforming activity of plausible synthetic progenitor proviruses encompassing the complete proto-ras genomic region with or without various 5' deletions. All progenitor proviruses induced phenotypic transformation of mouse NIH 3T3 cells, although with a 5- to 10-fold lower frequency than Harvey sarcoma provirus. Although no tumor formation was observed in vivo after inoculation in the absence of helper murine retrovirus, both wild-type and progenitor viruses inoculated in the presence of helper virus induced tumors in newborn BALB/c mice. No critical alterations of the p21ras coding region and no deletion of 5' genomic elements were detected in a progenitor virus encompassing the complete proto-ras genomic region that had been isolated from tumors. However, one progenitor virus that included all proto-ras exons induced tumors with a decreased latency. This virus contained a mutation in codon 12 (glycine to valine), which had apparently been selected during tumorigenesis in vivo. During the genesis of Harvey sarcoma virus, critical steps conferring transforming function are therefore transduction of coding proto-ras exons and enhancement of their transforming function by specific amino acid changes in p21ras.