Neoplastic transformation of B lineage cells constitutes a large proportion of human leukemias and lymphomas. Development of genetic and other therapies for these tumors relies on an understanding of the molecular mechanisms of transformation as well as molecular mechanisms of normal development. We have been studying molecular mechanisms governing B cell differentiation and neoplasia using several approaches.(1) Human preB cell growth and differentiation: We are attempting to develop a tissue culture system for isolation, propagation and cloning of normal human B-cell precursors for use in the study of human B cell differentiation. These experiments have shown that certain mouse stromal cell-lines can provide long-term support to populations of cells expressing human hematopoietic stem cell antigen, CD34, and that limited differentiation of erythroid, myeloid, and lymphoid lineages can be obtained using these conditions. This culture system has potential use in ex vivo expansion of human CD34 cells which could be used for gene therapy studies as well as for in vitro B-lineage differentiation studies. (2) B-cell tumorigenesis in myc-oncogene transgenic mice: We have been studying the process of neoplastic transformation in B-lineage tumors that arise in transgenic mice which overexpress the myc oncogene under transcriptional regulation of mutant or wild type immunoglobulin heavy chain enhancer constructs. Using RT-PCR, we have found that the p53 gene frequently is also abnormal in that it is mutated or not expressed in 30% of the tumors. We have found that lack of p53 expression sometimes correlates with DNA rearrangements of the p53 gene. In order to identify other genes that may also contribute to transformation with the inappropriately expressed myc gene, we use an RT-PCR assay capable of quantifying expression of 20 candidate oncogenes. These studies may lead to development of gene therapy approaches for lymphoid lineage tumors. (3) Models of B-cell differentiation: Tumors arising in these mice have also been used to investigate normal molecular events governing B cell differentiation. Cell surface expression of the surrogate immunoglobulin complex (consisting of heavy chain plus surrogate light chains lambda-5 and VpreB) correlates with down-regulated RAG gene expression and subsequent inability to catalyze V(D)J recombination. These cells may represent an important stage in which preB cells undergo allelic exclusion at the immunoglobulin heavy chain locus. These surrogate light chains might signal down-regulation of the RAG genes. These cells express the surrogate light chains and RAG genes but have not yet completely rearranged the immunoglobulin heavy chain gene. Therefore, transfectants expressing the immunoglobulin heavy chain are being isolated and examined for RAG gene expression. The surrogate light chains were found to be expressed in human preB acute lymphocytic leukemias and could constitute a target for therapeutic approaches to ALL.
