G1/S-specific cyclin-D1
G1/S transition of mitotic cell cycle
CRL4<sup>AMBRA1</sup> is a master regulator of D-type cyclins.
D-type cyclins are central regulators of the cell division cycle and are among the most frequently deregulated therapeutic targets in human cancer<sup>1</sup>, but the mechanisms that regulate their turnover are still being debated<sup>2,3</sup>. Here, by combining biochemical and genetics studies in somatic cells, we identify CRL4<sup>AMBRA1</sup> (also known as CRL4<sup>DCAF3</sup>) as the ubiquitin ligase that targets all three D-type cyclins for degradation. During development, loss of Ambra1 induces the accumulation of D-type cyclins and retinoblastoma (RB) hyperphosphorylation and hyperproliferation, and results in defects of the nervous system that are reduced by treating pregnant mice with the FDA-approved CDK4 and CDK6 (CDK4/6) inhibitor abemaciclib. Moreover, AMBRA1 acts as a tumour suppressor in mouse models and low AMBRA1 mRNA levels are predictive of poor survival in cancer patients. Cancer hotspot mutations in D-type cyclins abrogate their binding to AMBRA1 and induce their stabilization. Finally, a whole-genome, CRISPR-Cas9 screen identified AMBRA1 as a regulator of the response to CDK4/6 inhibition. Loss of AMBRA1 reduces sensitivity to CDK4/6 inhibitors by promoting the formation of complexes of D-type cyclins with CDK2. Collectively, our results reveal the molecular mechanism that controls the stability of D-type cyclins during cell-cycle progression, in development and in human cancer, and implicate AMBRA1 as a critical regulator of the RB pathway.