Regulation of mitotic entry by microcephalin and its overlap with ATR signalling
Gemma K Alderton1, Laura Galbiati, Elen Griffith
1Genome Damage and Stability Centre, University of Sussex, East Sussex, BN1 9RQ, UK.
Abstract:
Ataxia-telangiectasia mutated and Rad3 related (ATR)-Seckel syndrome and autosomal recessive primary microcephaly (MCPH) syndrome share clinical features. RNA interference (RNAi) of MCPH1 have implicated the protein it encodes as a DNA-damage response protein that regulates the transcription of Chk1 and BRCA1, two genes involved in the response to DNA damage. Here, we report that truncating mutations observed in MCPH-syndrome patients do not impact on Chk1 or BRCA1 expression or early ATR-dependent damage-induced phosphorylation events. However, like ATR-Seckel syndrome cells, MCPH1-mutant cell lines show defective G2-M checkpoint arrest and nuclear fragmentation after DNA damage, and contain supernumerary mitotic centrosomes. MCPH1-mutant and ATR-Seckel cells also show impaired degradation of Cdc25A and fail to inhibit Cdc45 loading onto chromatin after replication arrest. Additionally, microcephalin interacts with Chk1. We conclude that MCPH1 has a function downstream of Chk1 in the ATR-signalling pathway. In contrast with ATR-Seckel syndrome cells, MCPH1-mutant cells have low levels of Tyr 15-phosphorylated Cdk1 (pY15-Cdk1) in S and G2 phases, which correlates with an elevated frequency of G2-like cells displaying premature chromosome condensation (PCC). Thus, MCPH1 also has an ATR-independent role in maintaining inhibitory Cdk1 phosphorylation, which prevents premature entry into mitosis.
Insights
Microcephalin (MCPH1) mutations cause Seckel syndrome by disrupting DNA damage response pathways, leading to cell cycle defects and premature mitosis. MCPH1 functions downstream of ATR signaling and independently regulates Cdk1 phosphorylation.
Area of Science:
- Genetics
- Cell Biology
- Molecular Biology
Background:
- Ataxia-telangiectasia mutated and Rad3 related (ATR)-Seckel syndrome and autosomal recessive primary microcephaly (MCPH) syndrome share clinical features.
- MCPH1 mutations are linked to DNA damage response.
- Previous studies implicated MCPH1 in regulating Chk1 and BRCA1 transcription.
Purpose of the Study:
- Investigate the role of MCPH1 in DNA damage response and cell cycle regulation.
- Clarify the relationship between MCPH1, ATR signaling, and Seckel syndrome.
- Determine the precise function of MCPH1 in preventing premature mitosis.
Main Methods:
- Analysis of MCPH1 mutations in patient-derived cell lines.
- Assessment of DNA damage response pathways, including Chk1 and BRCA1 expression.
- Evaluation of cell cycle checkpoint control (G2-M arrest) and centrosome duplication.
- Examination of protein degradation (Cdc25A) and chromatin loading (Cdc45).
- Investigation of Cdk1 phosphorylation status and premature chromosome condensation.
Main Results:
- MCPH1 mutations do not affect Chk1/BRCA1 expression or early ATR-dependent phosphorylation.
- MCPH1-mutant cells exhibit defective G2-M checkpoint arrest, nuclear fragmentation, and supernumerary mitotic centrosomes.
- Impaired Cdc25A degradation and Cdc45 chromatin loading observed in MCPH1-mutant cells.
- MCPH1 interacts with Chk1, suggesting a role downstream in the ATR pathway.
- MCPH1-mutant cells show reduced inhibitory Cdk1 phosphorylation (pY15-Cdk1) and increased premature chromosome condensation.
Conclusions:
- MCPH1 functions downstream of Chk1 within the ATR signaling pathway.
- MCPH1 plays a critical role in maintaining the G2-M DNA damage checkpoint.
- MCPH1 has an ATR-independent function in regulating Cdk1 phosphorylation to prevent premature entry into mitosis.
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