The nature and expression of int-5, a novel MMTV integration locus gene in carcinogen-induced mammary tumors

Our previous studies have resulted in the identification and cloning of int-5, a novel site of mouse mammary tumor virus (MMTV) integration, from BALB/c D2 precancerous hyperplastic alveolar nodules (HAN). This paper presents a detailed characterization of the int-5 locus from both D2 HAN and normal genome and expression of the unique gene from the MMTV integration site. Our results show that the cellular gene at the MMTV integration site in the int-5 locus is identical to the gene encoding aromatase (CYP19), a member of the cytochrome P-450 gene superfamily. MMTV is integrated within exon 10 in the 3′ untranslated region of the aromatase gene. This gene (int-5/aromatase) is expressed in normal mammary gland and overexpressed in mammary tumors. These results suggest that the overexpression of this gene may be responsible for mammary tumorigenesis. This is the first demonstration of integration of MMTV in a cellular gene that plays a role in hormone-dependent breast cancers.