Abstract
D-type cyclins are key regulators of progression through G1 phase of the cell cycle. Strong expression of at least one of the three D cyclins is common in human cancers. However, while the cyclin D1 and D3 genes (CCND1 and CCND3) are recurrently involved in genomic rearrangements, especially in mantle-cell lymphoma and multiple myeloma, no clear involvement of the cyclin D2 gene (CCND2) has been reported to date in human malignancies. In T-cell acute lymphoblastic leukemia (T-ALL), the T-cell receptor genes TCRA/D and TCRB are frequently involved in chromosomal rearrangements. In order to identify new genomic rearrangements and oncogenes in human T-ALL, we performed an interphasic FISH screening of T-ALL cases using TCR flanking probes. Using this approach, we identified two new chromosomal translocations: t(7;12)(q34;p13) and t(12;14)(p13;q11), involving the TCRB and TCRA/D loci, respectively. Molecular analysis of the breakpoint sequences demonstrated the involvement of the CCND2 locus at 12p13. Expression analysis using RQ-PCR and immunoblotting demonstrated dramatic cyclin D2 overexpression in the translocated cases (n=3) compared to other T-ALLs (total, n=86), whereas other genes located near the translocation breakpoints were not deregulated on microarray analysis. To further evaluate expression in T-ALL with respect to normal T-cell differentiation, we analyzed CCND2 expression in purified subpopulations from normal human thymus. CCND2 levels were downregulated through progression from the early stages of normal human T-cell differentiation and transition to beta-selection. In the most immature T-ALLs, a moderate CCND2 expression was observed, consistent with their differentiation stage, while low expression was found in other T-ALL. By contrast, the massive and sustained expression in the CCND2-rearranged T-ALL cases strongly suggested an oncogenic role due to the TCR translocation. T-ALL oncogenesis is a multi-step process; we here found that the TCR-CCND2 translocations were associated with other oncogene expression (TAL1, HOXA, or TLX3/HOX11L2), NOTCH1 activating mutations, and/or CDKN2A/p16/ARF deletion, showing that cyclin D2 dysregulation could contribute to multi-event oncogenesis in various T-ALL groups. In conclusion, this report is the first clear evidence of a direct involvement of cyclin D2 in human cancer due to recurrent somatic genetic alterations. This reinforces the view that the strong expression of cyclin D2 which is detected in various types of cancer can contribute to oncogenesis, and points to cyclin D2 as a potential target for therapy in these tumors.
B cell origin of non-T cell acute lymphoblastic leukemia. A model for discrete stages of neoplastic and normal pre-B cell differentiation.