W. Y. Langdon

The c-myc oncogene perturbs B lymphocyte development in Eμ-myc transgenic mice

Transgenic mice bearing a c-myc oncogene subjugated to the lymphoid-specific immunoglobulin heavy chain enhancer (E mu) develop clonal B lymphoid malignancies, but most young E mu-myc mice lack malignant clones. Their prelymphomatous state has allowed us to examine how constitutive c-myc expression influences B cell development. We find that early stages are overrepresented, even before birth. Pre-B cells of polyclonal origin increase greatly, while B cells develop in reduced number. Both the pre-B and the B cells appear to be in an active state, since they are larger than normal and a greater fraction are in the cell cycle. Enforced myc expression has thus favored proliferation over maturation. Hence, a normal function of c-myc may be to regulate differentiation as well as to promote cell cycling.

Transgenic mouse models for hematopoietic tumorigenesis.

The development of transgenic mice carrying specific cancer-promoting genes represents a major advance in molecular oncology (for reviews, see Cory and Adams, 1988; Hanahan, 1987). In principle, the effect of a particular oncogene can be assessed for every cell type within the animal by this approach. The transgene can either be linked to a catholic regulatory sequence and expressed throughout all tissues, or its expression can be confined to a particular cell lineage by fusing it to a tissue-specific promoter/enhancer control element. Our own interests center on hematopoietic neoplasia, so we have produced transgenic mice harboring oncogenes targeted for constitutive expression in hematopoietic cells. The principal regulatory sequence we have used is the immunoglobulin (Ig) heavy chain enhancer (Eµ). While the function of this sequence is to direct expression of Ig heavy chain (IgH) genes in B lymphoid cells, it may also be active in at least some T lymphoid and even myeloid cells since the IgH locus is transcriptionally active in certain T and myeloid cell lines (Kemp et al, 1980a) and thymocytes (Kemp et al, 1980b) and an immunoglobulin µ transgene can be expressed in T as well as B cells (Grosschedl et al, 1984).

c-myc-induced lymphomagenesis in transgenic mice and the role of the Pvt-1 locus in lymphoid neoplasia.

Dysregulated expression of the c-myc proto-oncogene has been strongly implicated in lymphoid neoplasia, as reviewed by Leder et al (1984), Klein and Klein (1985), Adams and Cory (1985) and Cory (1986). In plasmacytomas of the mouse and Burkitt lymphomas of man, a predominant form of chromosomal translocation couples the c-myc gene to the IgH constant region locus, presumably bringing c-myc under the control of factors that regulate immunoglobulin expression. In someT lymphomas induced by retroviruses, a provirus has inserted near c-myc (e. g. Corcoran et al 1984). Thus strong circumstantial evidence links altired regulation of myc with lymphomagenesis. Typically, the alterations occur in the immediate vicinity of c-myc, but another class may involve long-range effects within the myc-bearing chromosome, as exemplified by the changes within the pvt-1 locus discussed later.

The myc Oncogene and Lymphoid Neoplasia: From Translocations to Transgenic Mice

The c-myc proto-oncogene encodes a nuclear phosphoprotein which probably plays a crucial role in growth control [4]. The protein has DNA-binding activity in vitro, but its function remains unknown. While avian retroviruses carrying the closely related v-myc sequence rapidly transform myeloid cells, the cellular myc gene has been strongly implicated in several types of lymphoid neoplasia. The fundamental mechanism releasing the oncogenic potential of c-myc is believed to be deregulation of its expression. Most chicken bursal lymphomas resulting from infection with avian leukosis virus, which does not itself bear an oncogene, carry a provirus near or within the c-myc gene [6]. About a quarter of T lymphomas with a retroviral aetiology also bear a c-myc-associated provirus [3].

Molecular Characterization of a Transforming Retrovirus Involved in Pre-B Cell Lymphomas

Cas-Br-M is a biologically cloned ecotropic murine leukemia virus [MuLV] originally isolated from wild mice of the Lake Casitas region of California. Inoculation of Cas-Br-M into newborn NFS/N mice induces hematopoietic neoplasms. The tumors are evident at 18–30 weeks and include T- and B-cell lymphomas, myeloid leukemias and erythroleukemias [Fredrickson et al, 1984]. it is unclear how Cas-Br-M is able to generate such a wide spectrum of hematopoietic neoplasms, although envelope-recombinant mink cell focus-forming [MCF] MuLVs are involved in some of these tumors [Holmes et al, 1986]. This was determined by preparing cell-free extracts from a range of tumors and injecting the extracts into newborn NFS/N mice. It was reasoned that if Cas-Br-M was the etiological agent then the mice should develop a range of tumor phenotypes with long latencies. If, however, MCF viruses were involved, the mice should develop tumors with the same phenotype as the extracted tumor and with reduced latency. Of 12 extracts examined, three appeared to support the latter hypothesis [Holmes et al, 1986]. On further examination one of these extracts, called Cas NS-1, was found to contain a virus capable of transforming mouse fibroblasts.

Oncogene Cooperation and B-Lymphoid Tumorigenesis in Eµ-myc Transgenic Mice

Alteration of the cellular myc oncogene has been strongly implicated in several types of lymphoid tumors. Although its normal role remains largely unknown, c-myc seems likely to play a crucial role in the control of cellular proliferation and appears to become oncogenic when genetic alterations deregulate its expression. The gene is activated by retroviral insertion in most avian bursal lymphomas and several mammalian T lymphomas and by chromosomal translocation to the active immunoglobulin heavy-chain (IgH) locus in human Burkitt’s lymphomas, mouse plasmacytomas, and rat immunocytomas (reviewed by [3]). Direct evidence that deregulated myc expression causes malignancy has been provided by studies with transgenic mice. Mice bearing myc coupled to immunoglobulin enhancers [1,12] or a retroviral long terminal repeat [8,11] succumb to tumors. Particularly striking are the Eµ-myc transgenic mice, which bear c-myc driven by the IgH enhancer and invariably develop B-lymphoid tumors.

Lessons from Translocations and Transgenic Mice: Constitutive c-myc Expression Predisposes to Neoplasia

The predominant chromosome translocations associated with Burkitt lymphomas of man and plasmacytomas of the mouse result from recombination of the c-myc protooncogene with the immunoglobulin heavy chain locus. The crucial outcome of this event is deregulation of c-myc expression. The hypothesis that constitutive myc expression highly predisposes to tumorigenesis has now been amply vindicated by studies with transgenic mice. Animals bearing a c-myc gene coupled to the immunoglobulin μ or κ enhancer frequently develop a fatal lymphoma, Eμ being particularly effective. The lymphoid regulatory elements play an essential role because transgenic mice carrying either a “normal” c-myc gene or a gene lacking the putative 5′ regulatory region fail to develop tumors. The tumors in Eμ-myc mice are all B lymphoid in origin and monoclonal. Tumorigenesis is preceded by a benign polyclonal proliferation of early B lineage cells which is evident as early as 18 days of gestation. Characterization of this preneoplastic phase induced by the deregulated c-myc gene suggests that the level of c-myc expression is a crucial factor in determining the balance between self-renewal and maturation within a differentiation lineage.