What is Myb?
"Myb" is an acronym derived from "myeloblastosis", an old-fashioned name for a type of leukemia, a cancer of blood cells. Our laboratory studies the Myb gene family. This family was first recognized in the form of the v-Myb oncogene of the avian myeloblastosis virus (AMV). This acutely transforming retrovirus causes a rapidly fatal monoblastic leukemia in chickens. We now know that v-Myb was created by the insertion of a retrovirus into a highly conserved cellular proto-oncogene (c-Myb), followed by recombination between viral and cellular sequences, and rescue by a replication-competent "helper" virus.
The second member of the Myb gene family to be identified was the C1 locus of maize. This gene was among the first targets of transposon-induced mutagenesis discovered in Barbara McClintock’s pioneering studies of maize kernel color mutants, which revealed the presence of "jumping genes" (transposable elements). More recently members of the Myb gene family have been discovered in even more distantly related inhabitants of the barnyard including fungi and the cellular slime mold Dictyostelium.
The Myb proteins bind to DNA and regulate gene expression. These proteins are defined by the Myb domain, a sequence of approximately 50 amino acids with a structure similar to the helix-turn-helix motif of prokaryotic transcriptional repressors and eukaryotic homeodomains. Multiple copies of the Myb domain are frequently present as tandem repeats within a single protein. For example, c-Myb, the cellular homolog of v-Myb, contains three adjacent Myb repeats near its amino terminus. Three tandem repeats closely related to those of c-Myb are present in two other Myb-related proteins in vertebrates (A-Myb and B-Myb), in Drosophila Myb, and in Dictyostelium Myb. Surprisingly, the intensively studied nematode worm C elegans appears to have lost the Myb gene during the course of evolution. All three-repeat Myb proteins appear to bind to the same core DNA sequence motif (PyAACNG). The first, second and third Myb repeats fall into distinct families. Although the last common ancestor of these diverse eukaryotes is generally believed to have lived approximately one billion years ago, each Myb repeat is more closely related to other members of the same family than to other repeats within the same protein.
These data imply that the duplications that generated these tandem repeats occurred prior to the divergence of these distantly related species. Furthermore, the conservation of the Myb domain in fungi and slime molds suggest that this domain is at least as old as the eukaryotic homeodomain, zinc finger, and bZIP DNA-binding domains. Interestingly, some proteins with distantly related Myb domains do not appear to bind directly to DNA, but rather act as components of multi-protein machines that modify histones, the protein spools on which the DNA in eukaryotic cells is wound and compacted.
Our laboratory initially focused on understanding the structure and function of v-Myb and c-Myb using retroviruses, cell culture, and the avian embryo as model systems. We identified specific domains of v-Myb that are required for leukemic transformation of normal blood cells. We also showed that truncation of the c-Myb protein is required for efficient oncogenic transformation, that different truncations cause transformation of different cell types, and that these truncations alter DNA binding and transcriptional regulation. However, at present the animal Myb genes have not been placed in a genetic pathway. Specifically, many key upstream regulators, interacting proteins, and downstream target genes remain to be identified. Therefore, in addition to biochemical and viral genetic approaches to these problems, we have used the fruit fly Drosophila melanogaster as a model genetic system for understanding the Myb pathway in animals.
We have created and analyzed null mutants of Drosophila Myb (Dm-Myb). We found that: (i) in the absence of Dm-Myb protein, there is an increase in mitotic arrest with abnormal chromosome number and aberrant spindle formation; (ii) Dm-Myb is localized on recently replicated DNA in both mitotically cycling and endocycling cells; (iii) Dm-Myb is required for chorion gene amplification in ovarian follicle cells, an intensively studied model for DNA replication in higher eukaryotes; (iv) Dm-Myb is required for the progression of chromosome condensation in mitotically cycling cells; (v) Myb acts in opposition to the E2F2-RBF tumor suppressor proteins in the epigenetic regulation of genes critical for G2/M progression of the cell cycle. In addition, we have shown that vertebrate B-Myb, but neither A-Myb nor c-Myb, can complement Dm-Myb null mutants in Drosophila hemocytes.