Jim Battey

The human c-myc oncogene: Structural consequences of translocation into the igh locus in Burkitt lymphoma

We have determined the sequence of the normal human c-myc gene and compared it to portions of a c-myc gene that has been translocated into the immunoglobulin heavy chain locus in a Burkitt lymphoma cell. The normal c-myc gene is encoded in three discrete exons divided by two large intervening sequences. Its mRNA is transcribed from two active promoters located about 150 nucleotides from one another. Each promoter initiates transcription of a long (approximately 550 bp) untranslatable leader sequence encoding the entire first exon. This exon and additional 5 flanking sequences are tightly conserved between mouse and man. In the Burkitt cell BL22, the rearranged c-myc gene retains both promoters and is unchanged in its amino acid coding domains. Translocation of this gene joins it to the immunoglobulin heavy chain switch region at a point approximately 1000 bp 5 to the dual c-myc promoters. These genes are joined in opposite transcriptional orientation. The structure of the translocated gene and the nature of its linkage to the immunoglobulin locus and the presence of two c-myc promoters and consequently two long leader sequences raise novel possibilities for the activation of an oncogene.

Chromatin structure and protein binding in the putative regulatory region of the c-myc gene in burkitt lymphoma

A chromosomal myc gene displays one of three patterns of activity depending upon the arrangement of the gene and its allelic partner. In nonmalignant B cells both myc alleles are normally expressed. In Burkitt lymphoma cells carrying both a translocated and a nontranslocated myc allele, the translocated allele is inappropriately expressed, while the nontranslocated allele is virtually inactive. Here we examine the chromatin structure of these genes using DNAase I hypersensitivity in nonmalignant lymphoblastoid cells and in the Burkitt lymphoma, BL31 . Three hypersensitivity patterns emerge that correlate with the state of the gene and reveal sites associated with putative regulatory structures. One region is associated with the two myc promoters, one with a specific nuclear protein binding site, and one--which is markedly enhanced in the inactive germline gene in the Burkitt cell--with a putative negative control region. The perturbation of the normal pattern in this particular Burkitt cell may be due to the action of an immunoglobulin enhancer.

Activation and somatic mutation of the translocated c-myc gene in Burkitt lymphoma cells

In contrast to other human tumors in which the c-myc gene and its transcript are greatly amplified, careful analysis of t(8;14) Burkitt cell lines indicates that the c-myc transcript is marginally, and in some cases not at all, increased by comparison to control lymphoblastoid cell lines. Instead, there is a more subtle alteration in the expression of the translocated c-myc gene characterized by a shift in promoter utilization and an apparent insensitivity to the regulation that inactivates the normal c-myc allele within these same cells. In some Burkitt cell lines, such deregulation might be because of the loss of a putative control region through removal of the large dual promoter/leader segment of the c-myc gene. In other cell lines, however, this deregulation may be explained by somatic mutations that occur within the putative control region even though it is located many hundreds of bases from the translocation breakpoint.

Duplication and deletion in the human immunoglobulin ϵ genes

The human IgE gene encodes a polypeptide chain that is involved in allergic reactions and in the immune response to parasitic disease in man. We have cloned three chromosomal regions corresponding to this sequence and find that two of them derive from curiously duplicated gene segments that also encode IgA constant-region genes. One of the IgE sequences corresponds to the active gene, and its structure defines a complete amino acid sequence of the human IgE constant region. The other cloned segment is a pseudogene from which the first two IgE coding domains have been deleted and replaced by a switch-like sequence that also occurs close to the normal IgE gene. The third IgE segment remains unlinked to the other heavy-chain genes. Evidently, the epsilon-alpha locus has been the site of several complicated genetic rearrangements during recent evolutionary time.

A novel alteration in the structure of an activated c-myc gene in a variant t(2;8) burkitt lymphoma

We have characterized a variant Burkitt lymphoma in which translocation joins the immunoglobulin kappa locus on chromosome 2 to the c-myc gene on chromosome 8. This Burkitt lymphoma is especially interesting because, in contrast to the more common lymphomas that carry 8;14 translocations, it carries a translocation that involves a light chain locus and occurs 3 to and at least 20 kb downstream of the c-myc gene. Furthermore, the c-myc gene from the translocated chromosome is abnormally expressed in that there is a characteristic shift in c-myc promoter utilization and an increase in c-myc transcript. These disturbances could be explained by novel structural alterations that occur in the c-myc gene and include a duplication of a 2.5 kb segment of DNA containing the two c-myc promoters and their untranslated leader exons. Interestingly, these alterations arise at a considerable distance from the translocation breakpoint.

Chromatin Structure of the Human c- myc Oncogene: Definition of Regulatory Regions and Changes in Burkitt’s Lymphomas

Chromosomal translocations of the myc oncogene are a consistent feature of all Burkitt’s lymphomas and are also observed in many murine plasmacytomas. These translocations of myc occur into the immunoglobulin loci and they result in a general increase in myc transcription, but this increase in myc is variable [2, 6, 7, 10, 11, 21, 22]. Since myc may be regulated during the cell cycle (see [9]), deregulation may mean expression at the inappropriate time, which in turn may result in only a modest overall increase in transcription of myc in Burkitt’s lymphomas. On the other hand, the true (and unidentifled) precursor cell of Burkitt’s lymphomas may have a very low level of myc transcription and we are as yet unable to assess properly the true increase in transcription as a consequence of translocations.

Chromatin structural changes in the putative regulatory region of c-myc accompany the translocation in a Burkitt lymphoma.

Several DNAase I hypersensitive sites mark the putative regulatory region immediately 5′ of the myc gene. A sequence near at least one site binds to a protein(s) from nuclear extracts in vitro. Three patterns of chromatin structure exist, one associated with the translocated myc allele in a Burkitt lymphoma (BL 31), one associated with the non-translocated (germline) allele in the same Burkitt cell and one associated with the germline myc allele in non-malignant B cells. The non-translocated and transcriptionally silent myc allele in BL 31 shows only one strong hypersensitive site, a site which may mediate negative control over myc. The heavy chain immunoglobulin enhancer that is juxtaposed with myc on the translocated allele in BL 31 may be responsible for the chromatin structure on this deregulated allele. These data have novel implications for the activation of the myc oncogene by translocations.