Mrvil, a common MRV integration site in BXH2 myeloid leukemias, encodes a protein with homology to a lymphoid-restricted membrane protein Jaw1 John D Shaughnessy Jr* ,1 , David A Largaespada 2 , Erming Tian 1 , Colin F Fletcher 3 , Brian C Cho 4 , Paresh Vyas 5 , Nancy A Jenkins 3 and Neal G Copeland 3 1 Division of Hematology and Oncology, Department of Medicine, University of Arkansas for Medical Sciences, Little Rock, Arkansas, AR 72205, USA; 2 Department of Laboratory Medicine and Pathology, University of Minnesota Cancer Center, Minneapolis, Minnesota, MN 55455, USA; 3 Mammalian Genetics Laboratory, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland, MD 21702, USA; 4 Department of Microbiology and Immunology, Thomas Jeerson University Cancer Institute, Philadelphia, Pennsylvania, PA 19107, USA; 5 Department of Hematology and Oncology, Children's Hospital, Boston, Massachusetts, USA Ecotropic MuLVs induce myeloid leukemia in BXH2 mice by insertional mutagenesis of cellular proto- oncogenes or tumor suppressor genes. Disease genes can thus be identi®ed by viral tagging as common sites of viral integration in BXH2 leukemias. Previous studies showed that a frequent common integration site in BXH2 leukemias is the Nf1 tumor suppressor gene. Unexpect- edly, about half of the viral integrations at Nf1 represented a previously undiscovered defective none- cotropic virus, termed MRV. Because other common integration sites in BXH2 leukemias encoding proto- oncogenes contain ecotropic rather than MRV viruses, it has been speculated that MRV viruses may selectively target tumor suppressor genes. To determine if this were the case, 21 MRV-positive BXH2 leukemias were screened for new MRV common integration sites. One new site, Mrvi1 was identi®ed that was disrupted by MRV in two of the leukemias. Ecotropic virus did not disrupt Mrvi1 in 205 ecotropic virus-positive leukemias, suggesting that Mrvi1 is speci®cally targeted by MRV. Mrvi1 encodes a novel protein with homology to Jaw1, a lymphoid restricted type II membrane protein that localizes to the endoplasmic reticulum. MRV integration occurs at the 5' end of the gene between two dierentially used promoters. Within hematopoietic cells, Mrvi1 expression is restricted to megakaryocytes and some myeloid leukemias. Like Jaw1, which is down- regulated during lymphoid dierentiation, Mrv1 is downregulated during monocytic dierentiation of BXH2 leukemias. Taken together, these data suggest that MRV integration at Mrvi1 induces myeloid leukemia by altering the expression of a gene important for myeloid cell growth and/or dierentiation. Experi- ments are in progress to test whether Mrvi1 is a tumor suppressor gene. Keywords: myeloid leukemia model; retroviral inser- tional mutagenesis; common sites of integration Introduction BXH2 mice represent an important model for the identi®cation of myeloid leukemia disease genes. Not only do these mice have the highest spontaneous incidence of myeloid leukemia of any known inbred mouse strain, but the leukemias in these mice are retrovirally-induced and the proviruses in the tumors can thus be used as insertional tags to identify the disease genes. BXH2 is one of 12 recombinant inbred (RI) BXH strains derived from crossing C57BL/6J and C3H/HeJ mice. Unlike the parental strains, or the 11 other BXH R1 strains, BXH2 mice spontaneously express a B-ecotropic murine leukemia virus (MuLV) beginning early in life and 495% of these mice die of myeloid leukemia by 1 year of age (Bedigian et al., 1981, 1984). The BXH2 B-ecotropic virus is not transmitted through the germline (Jenkins et al., 1982), rather the virus is horizontally transmitted via transplacental infection of implantation stage embryos (Bedigian et al., 1993). The BXH2 B-ecotropic virus is thought to have been generated by a rare recombina- tion event between two defective endogenous ecotropic proviruses, Emv1 and Emv2 inherited from the C3H/ HeJ and C57BL/6J parents, respectively (Jenkins et al., 1982). The susceptibility of BXH2 mice to myeloid leukemia is genetically determined as infection of other BXH RI strains with the BXH2 B-ecotropic virus produces either no leukemias or primarily B-cell leukemias (Bedigian et al., 1993). Several disease genes have been identi®ed by proviral tagging in BXH2 mice. The genes include a tumor suppressor gene (Nf1) (Buchberg et al., 1990; Largaespada et al., 1995), the Myb proto-oncogene (Buchberg et al., 1990; our unpublished results), two class I homeobox genes (Hox7 and Hox9) (Nakamura et al., 1996), and a Pbx1-related homeobox gene, Mesi1 (Moskow et al., 1995; Nakamura et al., 1996a). Two of these genes, Nf1 and Hoxa9 are also involved in human myeloid leukemia (Shannon et al., 1994; Nakamura et al., 1996b; Borrow et al., 1996). As expected, nearly all of the viral integrations at Myb, Hoxa7, Hoxa9, and Meis1 loci are B-ecotropic viruses. This is not the case, however, for the viruses located at Nf1. In this case, approximately half of the viral insertions represent a novel defective non-ecotropic virus (Cho et al., 1995). Sequence analysis showed that *Correspondence: JD Shaughnessy Jr Received 15 August 1998; accepted 25 August 1998 Oncogene (1999) 18, 2069 ± 2084 ã 1999 Stockton Press All rights reserved 0950 ± 9232/99 $12.00 http://www.stockton-press.co.uk/onc