Mutagenesis of avian carcinoma virus MH2: only one of two potential transforming genes (delta gag-myc) transforms fibroblasts.

Abstract

Most directly oncogenic retroviruses contain single, autonomous transforming (onc) genes (1). However, three oncogenic retroviruses contain two genes with oncogenic potential, namely the avian carcinoma viruses MH2 (2-5) and OK10 (6, 7) and avian erythroblastosis virus (AEV) (8, 9). The 5.5-kilobase (kb) RNA genome of MH2 (10) has the genetic structure Agag-mht-myc (2-5). One of the two genes with potential transforming function encoded by MH2 is a 3-kb Agag-mht gene, defined by a p100 protein product (11). A close structural relative of this gene (5, 12) is the only transforming gene of murine sarcoma virus MSV 3611 (13).The other MH2 gene is a myc-related gene, termed 8gag-myc, which is discontiguous. This gene includes a gag-derived 5' exon of only six gag codons (therefore 8gag) and a 3' 1.6-kb myc exon that is colinear with the two 3'-terminal exons of chicken proto-myc (5, 14). This gene is expressed via a subgenomic 2.6-kb mRNA as a 57,000-dalton (p57) protein product (15-18). The 7.5-kb RNA genome of OK10 virus (19) contains two overlapping myc-related genes (7); a contiguous gag-Apol-myc gene, which encodes a p200 protein; and a discontiguous 8gag-myc gene, which as in MH2 is also expressed as a subgenomic 3.5-kb mRNA encoding a p57protein (6,17,18 Here we analyze MH2 to determine whether it could serve as a model for multigene carcinogenesis. Several lines of evidence indicate that the 8gag-myc gene of MH2 is necessary for oncogenicity and that it transforms fibroblasts without Agag-mht as helper gene. (1) The 8gag-myc gene of MH2 is shared by MH2 and OK10 (7). Since both viruses have similar oncogenic spectra yet each has a specific second gene with potential transforming function, it follows that 8gag-myc is essential for common oncogenic properties (7).(ii) A study that has isolated MH2-transformed fibroblasts that express high levels of the 8gag-myc product p57 but no detectable Agag-mht product pl00 has concluded that p57 is sufficient for transforming function (15 6389The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Cancer genes: rare recombinants instead of activated oncogenes (a review).

“…Indeed, a complete 5' proto-myc exon was missing from the viral Agag-myc gene (18). This was apparently not an accident, since the same 5' proto-myc exon was also missing in the three other myc-containing avian carcinoma viruses MH2 (40,41), CMII, and OK10 (7,93). Thus, a viral and a cellular gene functioned as progenitors or proto-onc genes of each of the viral recombinant myc genes (Fig.…”

“…Group 3 includes those that are colinear with a reading frame of a proto-onc gene. The ras genes of Harvey and BALB murine sarcoma virus (39) and the myc gene of avian carcinoma virus MH2 (40,41) are examples. Group 4 includes those with an amino-terminal domain from a proto-onc gene and a carboxyl-terminal domain from the virus.…”

“…The transforming function of the recombinant onc genes of group 3, which encode transforming proteins that are colinear with proteins encoded by proto-onc genes, cannot be explained in this fashion. However, all viral onc genes of this group each lack at least one proto-onc-specific 5' exon, like the avian carcinoma viruses with myc genes (7,18,40,41,93) or the murine sarcoma viruses with ras genes (39). Conceivably, elimination of transcribed or untranscribed suppressors or elimination of an upstream proto-ras cistron (39) or protomyc cistron (42) and recombination with viral promoters are the mechanisms that generate transforming function (Fig.…”

“…Examples are the onc genes of avian carcinoma virus MH2 (7,40,41), of avian erythroblastosis and sarcoma virus AEV (8), of avian erythro-and myeloblastosis virus E26 (95), of feline sarcoma virus GR-FeSV (8,99), and of RSV (6,8). Certainly the odds against transduction of rare rearranged proto-onc genes instead of normal proto-onc genes are overwhelming.…”

Characterization of murine A-raf, a new oncogene related to the v-raf oncogene.

“…A homologous gene, v-mil, was subsequently found in the genome of the avian carcinoma virus MH2 (12,13,36), which also carries the oncogene v-myc. The avian homolog of v-raf in MH2 appears to be biologically active (25), although it may not have been maintained as an effective oncogene in this virus (1,39). In contrast, 3611-MSV efficiently transforms established epithelial (14) and primary as well as established fibroblastic cells in vitro (22a), immortalizes interleukin 3-dependent primary hematopoietic cells (11,23), and induces predominantly fibrosarcomas and erythroleukemias in newborn mice (25).…”