Mutually Exclusive Cdk4-Cyclin D1 and Cdk6-Cyclin D2 Pairing Inactivates Rb and Promotes Cell Cycle Dysregulation in Multiple Myeloma.

Multiple myeloma (MM), the second most common hematopoietic cancer, ultimately becomes refractory to treatment when self-renewing MM cells begin unrestrained proliferation by unknown mechanisms. Cell cycle reentry and progression is modulated by the balance between positive cell cycle regulators [cyclins and cyclin-dependent kinases (Cdk)] on the one hand and Cdk inhibitors (CKI) on the other. Here we show that one, but not more than one, of the three early G1 D cyclins is elevated in each case of MM. However, cyclin D1 or D3 overexpression rarely leads to phosphorylation of the retinoblastoma protein Rb and G1 cell cycle progression, unless cyclin-dependent kinase 4 (Cdk4) is correspondingly elevated in the absence of Cdk6. By contrast, cyclin D2 and Cdk6 are coordinately increased, thereby overriding inhibition by Cdk inhibitors, p18INK4c and p27Kip1, and phosphorylating Rb in conjunction with Cdk4. Thus, Cdk4 pairs exclusively with cyclin D1 whereas Cdk6 pairs exclusively with cyclin D2, although Cdk4 can also pair with cyclin D2 in MM cells. The basis for the exclusive Cdk4-cyclin D1 and Cdk6-cyclin D2 pairing is coordinated transcriptional activation. Cyclin D1 or D3 expression is scattered or uniform among bone marrow MM cells and at a constant frequency in the clinical course. By contrast, phosphorylation of Rb by Cdk6-cyclin D2 occurs in discrete foci of bone marrow MM cells early in the clinical course before cell proliferation, then is heightened with proliferation and disease progression. Mutually exclusive Cdk4-cyclin D1 and Cdk4/6-cyclin D2 pairing, therefore, critically controls cell cycle re-entry and G1 progression, which underlies the expansion of self-renewing MM cells in MM progression.