Activation of a novel proto-oncogene, Frat1, contributes to progression of mouse T-cell lymphomas

Abstract

Acceleration of lymphomagenesis in oncogene‐bearing transgenic mice by slow‐transforming retroviruses has proven a valuable tool in identifying cooperating oncogenes. We have modified this protocol to search for genes that can collaborate effectively with the transgene in later stages of tumor development. Propagation of tumors induced by Moloney murine leukemia virus (M‐MuLV) in Eμ‐Pim1 or H2‐K‐myc transgenic mice by transplantation to syngeneic hosts permitted proviral tagging of ‘progression’ genes. Molecular cloning of common proviral insertion sites that were detected preferentially in transplanted tumors led to the identification of a novel gene, designated Frat1. The initial selection for integrations near Frat1 occurs in primary tumor cells that have already acquired proviruses in other common insertion sites, yielding primary lymphomas that contain only a minor fraction of tumor cells with an activated Frat1 allele. Transplantation of such primary lymphomas allows for a further expansion of tumor cell clones carrying a proviral insertion near Frat1, resulting in detectable Frat1 rearrangements in 17% of the transplanted Eμ‐Pim1 tumors and 30% of the transplanted H2‐K‐myc tumors, respectively. We have cloned and sequenced both the mouse Frat1 gene and its human counterpart. The proteins encoded by Frat1 and FRAT1 are highly homologous and their functions are thus far unknown. Tumor cell lines with high expression of Myc and Pim1 acquired an additional selective advantage in vivo upon infection with a Frat1‐IRES‐lacZ retrovirus, thus underscoring the role of Frat1 in tumor progression, and the ability of Frat1 to collaborate with Pim1 and Myc in lymphomagenesis.

Overexpression of Frat1 in transgenic mice leads to glomerulosclerosis and nephrotic syndrome, and provides direct evidence for the involvement of Frat1 in lymphoma progression

“…The o spring developed spontaneous lymphomas signi®cantly faster than Em- Pim1 single-transgenic animals. The elevated Myc expression in all tumors from bitransgenic animals is in concordance with the concerted proviral activation of Myc and Frat1 in several transplanted lymphomas from M-MuLV-infected Em-Pim1 transgenic mice (Jonkers et al, 1997), and provides further evidence for the collaborative action of Pim1, Myc and Frat1 in lymphomagenesis.…”

“…In M-MuLV-induced T cell lymphomas, activating proviral insertions in Frat1 occur only in tumor cells that have already acquired two or more oncogenic mutations, suggesting that the selective advantage of Frat1 is limited to lymphoma progression (Jonkers et al, 1997). This implies that Em-pp-Frat1 transgenic mice, although they are not predisposed to spontaneously occurring tumors, might develop tumors with increased frequency when the initiating oncogenic lesions are induced by retorviral or chemical agents.…”

“…In contrast, only a limited number of genes have been isolated that may be important for lymphoma progression in vivo (Jonkers and Berns, 1996). Recently, we have described the identi®cation of a novel gene, Frat1, that is primarily involved in progression of mouse T cell lymphomas (Jonkers et al, 1997). Activation of Frat1 via retroviral insertions is preferentially detected in transplanted tumors of MMuLV-infected Em-Pim1 or H2-K-Myc transgenic mice.…”

“…Further support for a role of Frat1 in tumor progression came from transplantation experiments with cell lines derived from spontaneous lymphomas in Em-Pim1 transgenic mice. These cell lines, which already overexpress Myc and Pim1, gained an additional selective advantage in vivo upon transduction with a Frat1-IRES-lacZ retrovirus (Jonkers et al, 1997).…”

“…Retroviral activation of the Frat1 tumor progression gene was found to be invariably preceded by the activation of at least two other loci, suggesting that this gene can only confer an additional selective advantage to cells that are already (partially) transformed (Jonkers et al, 1997). Therefore, overexpression of Frat1 in transgenic animals should have little or no e ect on the incidence of spontaneous lymphomas.…”