Antibody Genes, Oncogenes and Antisense Genes

There are a number of mechanisms involved in producing a diversity of antibodies including multiple germline genes, somatic gene rearrangement, somatic hypermutation and combinatorial association. By the process of somatic hypermutation, one immunoglobulin gene in the germline can be mutated to produce many different genes in B cells. In chapter 2, this process is characterized. It was found that phosphorylcholine binding antibodies are encoded by one germline VH gene segment. In B cells, this VH gene segment may have extensive point mutations, many of which are silent, indicating the presence of some somatic hypermutational mechanism. Only the VH gene was found to be mutated indicating that the mutational mechanism was specific for VH genes. One way to study somatic immunoglobulin gene rearrangements, presented in chapter 3, might be to characterize rearrangements which are not easily explained. Immunoglobulin gene rearrangement was thought to exclusively involve immunoglobulin genes. However, some immunoglobulin genes can reproducibly rearrange with other DNA sequences. Insight into the basis of these rearrangements was uncovered by identifying the chromosomal origin of the nonimmunoglobulin rearranging DNA. This DNA originated on chromosome 15 whereas the immunoglobulin gene originated on chromosome 12. The juxtaposition of these sequences is common in plasmacytomas but rare or absent in normal B cells suggesting that it is involved in tumorigenesis. For example, it may be that aberrant immunoglobulin rearrangements can activate a cellular oncogene resulting in a plasmacytoma. This possibility was supported by results from other laboratories when it was found that the non-immunoglobulin rearranging DNA contained the cellular homologue of the myc oncogene. To understand lymphocyte tumorigenesis, it would be useful to understand the function of the c-myc gene product in normal and transformed cells. One way to begin is to determine which types of cells express the c-myc gene. This approach was employed in chapter 5 and it was found that the c-myc gene is expressed in dividing, but not resting, lymphocytes. One possible function for the c-myc gene product is that it functions in cellular proliferation. Another way to study the function of the c-myc gene product would be to prevent expression of the rearranged c-myc gene in plasmacytomas. For example, it may be possible to inhibit the synthesis of the c-myc gene product by antisense c-myc RNA. If the antisense RNA can hybridize to the c-myc RNA in vivo, synthesis of myc protein may be prevented. A test case, in which antisense TK RNA is used to inhibit TK expression, is presented in chapter 6. In L cells, high levels of antisense TK RNA expression were capable of inhibiting TK activity. The mechanism of inhibition involves RNA:RNA hybridization since double stranded RNA was formed. If this test case can be applied to other instances, it may be possible to use antisense RNA to inhibit the synthesis of a particular gene product and thus study its cellular function.