06-465 Sigma-AldrichAnti-Abl SH2 Domain Antibody

Mammalian c-Abl belongs to an evolutionary conserved family of non-receptor tyrosine kinases. It is distributed both in the cytoplasm in association with F-actin, and in the nucleus where it binds chromatin. The normal function of c-Abl is poorly understood. Nevertheless, there has been rapid progress in the characterization of the structural features, signal transduction pathways, substrates and ligands involved in the action of c-Abl and Abl-derived oncogenes. These developments suggest that several mechanisms co-operate to allow regulation of normal cell growth by c-Abl and induction of leukemias by Bcr-abl.

The abl oncogene was originally discovered in Abelson virus, a murine retrovirus. This virus and the protein tyrosine kinase encoded by abl are well known for their ability to transform B lymphocyte progenitors. Most of the transformed cells resemble a normal B lineage progenitor called a pre-B cell and appear to be arrested in differentiation at the stage of immunoglobulin light chain gene rearrangement. Recent evidence obtained using temperature-sensitive Abelson virus mutants provides direct support for this idea. Lymphoid cells transformed by one such virus undergo light chain rearrangement soon after shift to the nonpermissive temperature. This event is accompanied by several changes classically associated with light chain gene rearrangement including increased activity of the NF-kappa B transcription factor, expression of light chain RNAs and increased levels of expression of the RAG-1 and RAG-2 genes. Although the mechanism by which abl protein blocks the activity of these factors is not yet known, these data suggest that tyrosine phosphorylation may be intimately connected to regulation of early B cell development and expression of genes that are central to all phases of antigen receptor gene rearrangement.

Although the biological function of the c-Abl tyrosine kinase remains unsolved, potentially productive avenues towards the elucidation of that function have been identified by recent progress. An F-actin binding and a sequence-specific DNA-binding domain have been discovered in c-Abl, and DNA binding has been shown to be cell-cycle regulated. Deletion of those two domains in the mouse c-Abl results in a loss of biological function despite the production of an active tyrosine kinase. These findings suggest a role for c-Abl in the regulation of processes occurring on F-actin and on specific DNA elements.

The process of malignant transformation can be ascribed to a series of characteristics and definable mutations of genes which encode proteins that control cell growth and differentiation. During the course of malignant transformation the cancer-related genes are altered by a variety of mechanisms including translocations, deletions, and point mutations which commonly result in the expression of aberrant proteins. Our laboratory has focused on determining the extent to which cancer-specific proteins expressed by aberrant cancer-related genes can function as tumor-specific antigens. The current paper reviews our studies with two prototype cancer-specific proteins, mutated p21ras protein and chimeric p210bcr-abl protein. Ras protooncogenes are activated by point mutation in approximately 20% of human malignancies. The mutations occur primarily at codons 12 or 61 and result in the expression of p21ras proteins with single substituted amino acids. Only a limited number of amino acid substitutions occur. Murine studies demonstrate that immunization with synthetic peptides corresponding to the mutated segment can elicit both class II restricted CD4+ helper/inducer T-cell responses and class I restricted CD8+ cytotoxic T-cell responses specific for mutated p21ras protein. In addition, the existence in vivo of tumors expressing mutated ras proteins can be detected by assaying for T-cell immunity to the mutated segment of ras protein. Preliminary human studies show that some patients with colon cancer have existent antibody responses to p21ras protein, implying the possible existence of autochthonous T-cell immunity to mutated ras proteins in those patients. In chronic myelogenous leukemia the human c-abl protooncogene from chromosome 9 is translocated to the specific breakpoint cluster (bcr) region on chromosome 22. The translocation results in the formation of a bcr-abl fusion gene that encodes at 210-kD chimeric protein. The joining region segment of chimeric bcr-abl protein is composed of a unique combination of c-abl and bcr amino acids and is expressed only by malignant cells. Studies demonstrate that immunization of mice with synthetic peptides corresponding to the joining region segment can elicit class II restricted CD4+ T-cell responses to p210bcr-abl proteins. Preliminary studies show that bcr-abl peptides can bind in the groove of both murine and human class I MHC molecules and can elicit bcr-abl peptide-specific cytotoxic T lymphocytes (CTL). Whether bcr-abl peptide-specific CTL can lyse cells expressing bcr-abl protein is a yet unknown. In summary, the results of the studies reviewed confirm that cancer-specific oncogenic proteins can serve as tumor-specific antigens.