Expression of p89(c-Mybex9b), an alternatively spliced form of c-Myb, is required for proliferation and survival of p210BCR/ABL-expressing cells.

The c-Myb gene encodes the p75(c-Myb) isoform and less-abundant proteins generated by alternatively spliced transcripts. Among these, the best known is p(c-Mybex9b), which contains 121 additional amino acids between exon 9 and 10, in a domain involved in protein-protein interactions and negative regulation. In hematopoietic cells, expression of p(c-Mybex9b) accounts for 10-15% of total c-Myb; these levels may be biologically relevant because modest changes in c-Myb expression affects proliferation and survival of leukemic cells and lineage choice and frequency of normal hematopoietic progenitors. In this study, we assessed biochemical activities of p(c-Mybex9b) and the consequences of perturbing its expression in K562 and primary chronic myeloid leukemia (CML) progenitor cells. Compared with p75(c-Myb), p(c-Mybex9b) is more stable and more effective in transactivating Myb-regulated promoters. Ectopic expression of p(c-Mybex9b) enhanced proliferation and colony formation and reduced imatinib (IM) sensitivity of K562 cells; conversely, specific downregulation of p(c-Mybex9b) reduced proliferation and colony formation, enhanced IM sensitivity of K562 cells and markedly suppressed colony formation of CML CD34(+) cells, without affecting the levels of p75(c-Myb). Together, these studies indicate that expression of the low-abundance p(c-Mybex9b) isoform has an important role for the overall biological effects of c-Myb in BCR/ABL-transformed cells.

Isolation and functional assessment of common, polymorphic variants of the B-MYB proto-oncogene associated with a reduced cancer risk.

The B-MYB proto-oncogene is a transcription factor belonging to the MYB family that is frequently overexpressed or amplified in different types of human malignancies. While it is suspected that B-MYB plays a role in human cancer, there is still no direct evidence of its causative role. Looking for mutations of the B-MYB gene in human cell lines and primary cancer samples, we frequently isolated two nonsynonymous B-MYB polymorphic variants (rs2070235 and rs11556379). Compared to the wild-type protein, the B-MYB isoforms display altered conformation, impaired regulation of target genes and decreased antiapoptotic activity, suggesting that they are hypomorphic variants of the major allele. Importantly, the B-MYB polymorphisms are common; rs2070235 and rs11556379 are found, depending on the ethnic background, in 10-50% of human subjects. We postulated that, if B-MYB activity is important for transformation, the presence of common, hypomorphic variants might modify cancer risk. Indeed, the B-MYB polymorphisms are underrepresented in 419 cancer patients compared to 230 controls (odds ratio 0.53; (95%) confidence interval 0.385-0.755; P=0.001). This data imply that a large fraction of the human population is carrier of B-MYB alleles that might be associated with a reduced risk of developing neoplastic disease.

Coding sequence and intron-exon junctions of the c-myb gene are intact in the chronic phase and blast crisis stages of chronic myeloid leukemia patients.

The c-myb gene encodes a transcription factor required for proliferation, differentiation and survival of normal and leukemic hematopoietic cells. c-Myb has a longer half-life in BCR/ABL-expressing than in normal cells, a feature which depends, in part, on PI-3K/Akt-dependent regulation of proteins interacting with the leucine zipper/negative regulatory region of c-Myb. Thus, we asked whether the stability of c-Myb in leukemic cells might be enhanced by mutations interfering with its degradation. We analyzed the c-myb gene in 133 chronic myeloid leukemia (CML) patients in chronic phase and/or blast crisis by denaturing-high performance liquid chromatography (D-HPLC) and sequence analysis of PCR products corresponding to the entire coding sequence and each exon-intron boundary. No mutations were found. We found four single nucleotide polymorphisms (SNPs) and identified an alternatively spliced transcript lacking exon 5, but SNPs frequency and expression of the alternatively spliced transcript were identical in normal and CML cells. Thus, the enhanced stability of c-Myb in CML blast crisis cells and perhaps in other types of leukemia is not caused by a genetic mechanism.