Charles L. Sidman

Genetically determined murine models of immunodeficiency.

We have discussed more than 30 mutant genes known to cause abnormalities in the development and regulation of the immune system. The loci defined by these deleterious alleles have been assigned to 13 different autosomal chromosomes in addition to X and Y. It is important to note that these single genes do not act alone but function in concert with the background genome. Studies of these mutations on different inbred strain backgrounds are contributing important information on the influence of background modifying genes. The development of stocks of mice carrying multiple mutations on an inbred strain background enables the use of a well-characterized mutation to explore a less-well-understood genetic model. Investigators are urged to assure proper conditions for studies with immunological mutants by using the appropriate methods of animal husbandry. A detailed guide for maintaining immunologically compromised rodents has been prepared. These experiments performed by nature provide a valuable resource for investigating the immune system in normal and pathologic states. As the gene products of the loci defined by these mutations become known, the information obtained will provide additional insight into mechanisms underlying normal immune function as well as immunologic disease processes in man.

Fatal Acute Lymphoblastic Leukemia in Mice Transgenic for B Cell-Restricted bcl-xL and c-myc

Expression of the c-myc gene is frequently dysregulated in malignant tumors and translocations of c-myc into the Ig H chain locus are associated with Burkitt’s-type lymphoma. There is indirect evidence that bcl-x, an anti-apoptotic member of the bcl-2 gene family, may also contribute to a variety of B lymphoid tumors. In this study, we show that mice transgenic for both B cell-restricted c-myc and bcl-xL developed aggressive, acute leukemias expressing early B lineage and stem cell surface markers. Of interest, the tumor cells proliferated and differentiated down the B cell developmental pathway following in vitro treatment with IL-7. Analysis of sorted leukemic cells from spleen indicated constitutive expression of sterile μ and κ transcripts in combination with evidence for D-JH DNA rearrangements. Several B cell-specific genes were either not expressed or were expressed at low levels in primary tumor cells and were induced following culture with IL-7. IL-7 also increased V-Jκ and V-DJH rearrangements. These data demonstrate oncogenic synergy between c-myc and bcl-xL in a new mouse model for acute lymphoblastic leukemia. Tumors in these animals target an early stage in B cell development characterized by the expression of both B lineage and stem cell genes.

Upregulation of endogenousp53 and induction of in vivo apoptosis in B-lineage lymphomas ofEμ-myc transgenic mice by deregulated c-myc transgene

Eμ‐myc transgenic mice carry a constitutively overexpressed c‐myc oncogene and develop B‐lineage lymphomas. Previous studies have shown that c‐myc overexpression can lead to in vitro apoptosis. Here, we investigated the in vivo effects of altered c‐myc expression on cell proliferation versus death in spontaneously arising Eμ‐myc tumors. Eμ‐myc tumors display extensive in vivo apoptosis confined to small clusters of cells with greatly increased expression of both the c‐myc transgene and the endogenous p53 gene as compared with that in normal, pretumor, or surrounding tumor tissue. This restricted overexpression of both the c‐myc transgene and the endogenous p53 gene in small clusters of apoptotic tumor cells indicates that overexpression of these genes and apoptosis are not obligatory or uniform during tumor development and suggests that further somatic mutations or microenvironmental influences may be responsible for these properties. Nevertheless, the clear ability of tumor cells to undergo apoptosis in vivo may be exploitable for therapeutic purposes. Mol. Carcinog. 18:66–77, 1997.

Heterogeneity of B-lymphoid tumors in E mu-myc transgenic mice.

The clinically important issue of tumor heterogeneity was studied in C57BL/6-E mu-myc transgenic mice, which provide a genetically uniform model system in which all animals eventually develop B cell lymphomas after additional genetic changes beyond enforced expression of the transgenic oncogene. Three different approaches were compared for discerning the cellular and genetic homogeneity of these tumors. Analysis of Igh gene rearrangement showed mainly monoclonality and only infrequent oligoclonality in the tumors from a given animal. In contrast, cytogenetic examination indicated a substantial degree of heterogeneity in the tumors from a given animal and showed that a wide variety of secondary genetic changes occur in E mu-myc transgenic mice. Flow cytometry of DNA content also revealed a high degree of heterogeneity within and among the tumor masses from single E mu-myc mice. Estimates of tumor heterogeneity revealed by these three techniques often did not coincide, indicating that these different approaches reflect distinct cellular parameters. Transgenic E mu-myc mice additionally homozygous for the scid mutation displayed enhanced levels of secondary genetic changes that were valuable for the methodological comparisons performed here, and demonstrated that the extent of tumor heterogeneity can be influenced by specific genes other than the primary E mu-myc transgene. In summary, a combination of methodologies appears to be required to reveal the full extent of tumor heterogeneity within a single individual.