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.