CDK6 protein expression is associated with disease progression and treatment resistance in multiple myeloma

Multiple myeloma (MM) is a heterogeneous malignancy of plasma cells. Despite improvement in the prognosis of MM patients after the introduction of many new drugs in the past decades, MM remains incurable since most patients become treatment‐resistant. Cyclin‐dependent kinase 6 (CDK6) is activated in many types of cancer and has been associated with drug resistance in MM. However, its association with disease stage, genetic alterations, and outcome has not been systematically investigated in large cohorts. Here, we analyzed CDK6 expression using immunohistochemistry in 203 formalin‐fixed paraffin‐embedded samples of 146 patients and four healthy individuals. We found that 61.5% of all MM specimens express CDK6 at various levels. CDK6 expression increased with the progression of disease with a median of 0% of CDK6‐positive plasma cells in monoclonal gammopathy of undetermined significance (MGUS) (n = 10) to 30% in newly diagnosed MM (n = 78) and up to 70% in relapsed cases (n = 55). The highest median CDK6 was observed in extramedullary myeloma (n = 12), a highly aggressive manifestation of MM. Longitudinal analyses revealed that CDK6 is significantly increased in lenalidomide‐treated patients but not in those who did not receive lenalidomide. Furthermore, we observed that patients who underwent lenalidomide‐comprising induction therapy had significantly shorter progression‐free survival when their samples were CDK6 positive. These data support that CDK6 protein expression is a marker for aggressive and drug‐resistant disease and describes a potential drug target in MM.