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Updated: Aug 20, 2026
Capturing Common Fragile Site Breaks by Native γH2A.X ChIP
Published on: January 24, 2025
SMC1 involvement in fragile site expression
Antonio Musio1, Cristina Montagna, Tullio Mariani
1Istituto di Tecnologie Biomediche, Consiglio Nazionale delle Ricerche, Via Fratelli Cervi, 93, 20090 Segrate, Milan, Italy. antonio.musio@itb.cnr.it
Abstract:
Common fragile sites have been involved in neoplastic transformation, although their molecular basis is still poorly understood. Here, we demonstrate that inhibition of the SMC1 by RNAi is sufficient to induce fragile site expression. By investigating normal, ATM- and ATR-deficient cell lines, we provide evidence that the contribution of SMC1 in preventing the collapse of stalled replication fork is an Atr-dependent pathway. Using a fluorescent antibody specific for gamma-H2AX, we show that very rare discrete nuclear foci appear 1 and 2 h after exposure to aphidicolin and/or RNAi-SMC1, but became more numerous and distinct after longer treatment times. In this context, fragile sites might be viewed as an in vitro phenomenon originating from double-strand breaks formed because of a stalled DNA replication that lasted too long to be managed by physiological rescue acting through the Atr/Smc1 axis. We propose that in vivo, following an extreme replication block, rare cells could escape checkpoint mechanisms and enter mitosis with a defect in genome assembly, eventually leading to neoplastic transformation.
Insights
Inhibition of SMC1 protein triggers fragile site expression, a key step in cancer development. This occurs via an ATR-dependent pathway that prevents replication fork collapse, highlighting a novel mechanism in genome instability.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- Common fragile sites are implicated in cancer, but their underlying molecular mechanisms remain unclear.
- Understanding fragile sites is crucial for elucidating pathways leading to neoplastic transformation.
Purpose of the Study:
- To investigate the role of SMC1 protein in fragile site expression and genome stability.
- To elucidate the molecular pathways, particularly ATR-dependent mechanisms, involved in preventing replication fork collapse at fragile sites.
Main Methods:
- RNA interference (RNAi) to inhibit SMC1 expression.
- Analysis of normal and ATM/ATR-deficient cell lines.
- Immunofluorescence staining for gamma-H2AX to detect DNA double-strand breaks.
- Treatment with aphidicolin to induce replication stress.
Main Results:
- SMC1 inhibition by RNAi induced fragile site expression.
- SMC1's role in preventing stalled replication fork collapse is dependent on ATR.
- Increased gamma-H2AX foci, indicative of double-strand breaks, were observed after aphidicolin and/or RNAi-SMC1 treatment.
Conclusions:
- Fragile sites may arise from unrepaired DNA double-strand breaks due to prolonged replication stalling, managed by the ATR/SMC1 axis.
- In vivo, replication blocks can lead to mitotic entry with genome assembly defects, potentially causing neoplastic transformation.
- The ATR/SMC1 pathway is critical for maintaining genome integrity during replication stress.
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