Ubiquitin-dependent degradation of p27Kip1 and p21Waf1/Cip1 by AMBRA1 ensures G1 and S phase progression and limits replication stress.

The cell cycle orchestrates the events that lead to cell replication and division. AMBRA1 interacts with the E3 ubiquitin ligase CRL4DDB1 complex to regulate the stability of D-type cyclins, key regulators of the G1-S phase transition. However, whether AMBRA1 has a role in the S phase remains to be elucidated. Here, we show that AMBRA1 affects the turnover of p21Waf1/Cip1 and p27Kip1 by coupling the CRL4DDB1 complex to these proteins. In the absence of AMBRA1, the increased stability of p21Waf1/Cip1, rather than p27Kip1, resulted in the accumulation of replication stress. Mechanistically, the excess of p21Waf1/Cip1 during the S phase resulted in more PCNA-bound p21Waf1/Cip1, negatively affecting the binding of FEN1 to PCNA, which left under-replicated DNA. Consequently, AMBRA1-depleted cells are sensitive to FEN1 inhibition. Aberrantly low levels of AMBRA1 occurring in Sonic Hedgehog-type medulloblastomas correlate with high levels of p21Waf1/Cip1 and a worse prognosis. Finally, AMBRA1 and p21 levels may serve as biomarkers for patient stratification and treatment in medulloblastoma.