Mutually exclusive cyclin-dependent kinase 4/cyclin D1 and cyclin-dependent kinase 6/cyclin D2 pairing inactivates retinoblastoma protein and promotes cell cycle dysregulation in multiple myeloma

Multiple myeloma, the second most common hematopoietic cancer, ultimately becomes refractory to treatment when self-renewing multiple myeloma cells begin unrestrained proliferation by unknown mechanisms. Here, we show that one, but not more than one, of the three early G₁ D cycling is elevated in each case of multiple myeloma. Cyclin D1 or D3 expression does not vary in the clinical course, but that alone is insufficient to promote cell cycle progression unless cyclin-dependent kinase 4 (cdk4) is also elevated, in the absence of cdk6, to phosphorylate the retinoblastoma protein (Rb). By contrast, cyclin D2 and cdk6 are coordinately increased, thereby overriding the inhibition by cdk inhibitors p18^INK4c and p27^Kip1 and phosphorylating Rb in conjunction with the existing cdk4. Thus, cyclin D1 pairs exclusively with cdk4 and cdk6 pairs only with cyclin D2, although cyclin D2 can also pair with cdk4 in multiple myeloma cells. The basis for this novel and specific cdk/D cyclin pairing lies in differential transcriptional activation. In addition, cyclin D1- or cyclin D3-expressing multiple myeloma cells are uniformly distributed in the bone marrow, whereas cdk6-specific phosphorylation of Rb occurs in discrete foci of bone marrow multiple myeloma cells before proliferation early in the clinical course and is then heightened with proliferation and disease progression. Mutually exclusive cdk4/cyclin D1 and cdk6/cyclin D2 pairing, therefore, is likely to be a critical determinant for cell cycle reentry and progression and may play a pivotal role in the expansion of self-renewing multiple myeloma cells.