MOLECULAR ANALYSIS OF FACTORS AFFECTING LYMPHOMA SUSCEPTIBILITY OF AKXD RECOMBINANT INBRED MOUSE STRAINS

Abstract

Recombinant inbred (RI) strains of mice result from the inbreeding of an F­­­₂ generation which itself was produced by crosses between two existing inbred strains. The genomes of individual RI strains represent stable segregant populations resulting from the reassortment of the two progenitor genotypes. The AKXD RI strains have been derived from progenitor strains AKR/J and DBA/2J that differ significantly in lymphoma incidence. As a result of how the genes affecting lymphoma susceptibility have segregated and been fixed in the RI strains, expression of different levels of lymphoma susceptibility, different ages of onset of disease and involvement of different cell types in the tumors have been observed among the AKXD strains. Ten independently derived AKXD RI strains were analyzed for lymphoma susceptibility. Nine strains were found to be highly susceptible to lymphomas although the level of susceptibility varied greatly. Southern blot analysis of the ten different AKXD strains using molecular probes representing the joint regions of the immunoglobulin heavy chain, kappa light chain and T-cell receptor ­­­β-chain genes was performed to classify the strains according to predominant lymphoma type. Additionally a histological classification based on the cell morphology scheme of Pattengale and Taylor was used to verify the molecular classifications of stem, Pre-B, B, T and myeloid cell lineages. Of the nine highly lymphomatous strains analyzed, three died primarily of T cell tumors, two died primarily of B-cell tumors and four were susceptible to both B and T cell tumors. Using molecular probes specific for ecotropic proviruses or ecotropic and oncogenic class I mink cell focus forming (MCF) proviruses it was shown that 91% of the 135 tumors analyzed contained detectable somatically acquired proviruses. Generally MCF proviruses were associated with T-cell lymphomas whereas ecotropic proviruses were associated with B-cell lymphomas. The segregation of a dominant gene, Rmcf, which alters the sensitivity of cells to infection by MCF MuLVs, was recently identified by Hartley et al. on mouse chromosome 5. The allele of this gene carried by each strain also correlated with the predominant lymphoma types seen in these tumors. The three strains that died predominantly of MCF virus -associated T cell lymphomas inherited the Rmcf­­­ˢ (sensitive) allele from the parental AKR/J strain. AKXD-2 inherited the Rmcf­­­ʳ (resistant) allele from the DBA/2J parental strain and lymphomas from this strain contained no detectable MCF proviruses. Using probes homologous to Myc, Pvt-1, Pim-1 and Fis-1, rearrangements were detected in lymphomas and shown to result from viral integration in the respective regions. Proviral integration near Myc, Pvt-1, Fis-1 and Pi -1 were detected primarily in T-cell lymphomas. In three cases rearrangements near Fis-1 and Pim-1 were detected in the same tumor tissue as rearrangements near Myc. Proviral integration near Myb was not detected in any of the lymphomas analyzed. The significance of the genetic loci analyzed will be discussed with respect to their role in the molecular basis of the disease process.