Large Scale Copy Number Variation Upregulates the Expression of MYB in Human T-ALL.

Using comparative genome hybridization (array CGH), we have discovered a small area of increased copy number on the long arm of chromosome 6 in 8 out of 20 (40%) T-ALL cell lines. The region of increased copy number is very small, containing only one gene, MYB, the cellular homolog of the avian oncogene v-myb. By performing fiber-FISH on these cell lines, we have shown that the increased copy number results from a discrete tandem duplication of the MYB gene on one allele. Although myb is a frequent target of retroviral insertional activation in screens for oncogenes whose overexpression accelerates the onset of murine T-ALL, its overexpression and increased copy number has not previously been implicated in human T-ALL. Using gene expression profiling and Western blotting, we have demonstrated that the duplication in human T-ALL results in increased levels of MYB expression. Furthermore, using quantitative PCR we have confirmed that this tandem duplication occurs in primary human T-ALL samples. In our studies to date, MYB tandem duplication and overexpression appears to occur as part of the major multistep molecular pathway in T-ALL that affects a majority of cases, in which the leukemic cells also have TAL1/SCL and LMO1/2 overexpression, and NOTCH1 gene activating mutations, together with homozygous deletion of the INK4A/ARF locus. We are currently determining the mechanism through which MYB overexpression contributes to the pathogenesis of T-ALL by siRNA knockdown in T-ALL cell lines. Our finding of MYB tandem gene duplication differs from classic forms of oncogene amplification involving double minutes or homogenously staining regions. It is possible that MYB copy number is increased through a novel somatic mechanism of allele-specific, tandem duplication of a small genomic region during the process of malignant transformation. Another possibility is suggested by recent studies documenting that inherited large-scale copy number variation (CNV) accounts for much of the phenotypic diversity within human populations. Further studies will be needed to determine whether MYB tandem duplication is present in the germline DNA of T-ALL patients, which if identified, would provide the first example of an inherited CNV functioning as a mechanism of cancer susceptibility.