MLL fusion-driven activation ofCDK6potentiates proliferation inMLL-rearranged infant ALL

Abstract

Acute lymphoblastic leukemia in infants (< 1 year-of-age) is characterized by a high incidence of MLL rearrangements. Recently, direct targets of the MLL fusion protein have been identified. However, functional validation of the identified targets remained unacknowledged. In this study, we identify CDK6 as a direct target of the MLL fusion protein and an important player in the proliferation advantage of MLL-rearranged leukemia. CDK6 mRNA was significantly higher expressed in MLL-rearranged infant ALL patients compared with MLL wild-type ALL patients (P < 0.001). Decrease of MLL-AF4 and MLL-ENL fusion mRNA expression by siRNAs resulted in downregulation of CDK6, affirming a direct relationship between the presence of the MLL fusion and CDK6 expression. Knockdown of CDK6 itself significantly inhibited proliferation in the MLL-AF4-positive cell line SEM, whereas knockdown of the highly homologous gene CDK4 had virtually no effect on the cell cycle. Furthermore, we show in vitro sensitivity of MLL-rearranged leukemia cell lines to the CDK4/6-inhibitor PD0332991, inducing a remarkable G 1 arrest, and downregulation of its downstream targets pRB1 and EZH2. We therefore conclude that CDK6 is indeed a direct target of MLL fusion proteins, playing an important role in the proliferation advantage of MLL-rearranged ALL cells.

Therapeutic Targeting the Cell Division Cycle 25 (CDC25) Phosphatases in Human Acute Myeloid Leukemia — The Possibility to Target Several Kinases through Inhibition of the Various CDC25 Isoforms

“…CDK6 seems to have a similar function also for other translocations involving 11q23. The same seems to be true in acute lymphoblastic leukemia with 11q23 fusion genes [159]. CDK6 inhibition by the combined CDK4/6-inhibitor PD0332991 seems to have similar effects [159].…”

“…The same seems to be true in acute lymphoblastic leukemia with 11q23 fusion genes [159]. CDK6 inhibition by the combined CDK4/6-inhibitor PD0332991 seems to have similar effects [159]. CDC25 is a regulator of the CDK4/CDK6/cyclin D complex (Figure 2), and the possible use of CDC25 inhibitors alone or in combination with CDK targeting treatment should therefore be further explored.…”

CDK6 as a key regulator of hematopoietic and leukemic stem cell activation

“…5,7 CDK6 has also been shown to be critical in acute myeloid leukemia (AML) and acute lymphoblastic leukemia driven by mixed lineage leukemia fusion proteins. 9,10 There is considerable interest in targeting CDK4/6 in cancer therapy, and the Food and Drug Administration nominated CDK4/6 inhibitors as the "breakthrough therapeutic advance" in 2013.…”

Targeting CDK6 in cancer: State of the art and new insights

“…A separate knock-down experiment confirmed that CDK6, but not CDK4, was required for the proliferation of MLL-ALL cells. 62 Taken together, these findings suggest that CDK6 is a major oncogenic target of MLL-AML and MLL-ALL, and that CDK6 inhibitors are highly likely to find an application in treating patients with the disease.…”