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Restarting Stalled Replication Forks02:37

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DNA replication is initiated at sites containing predefined DNA sequences known as origins of replication. DNA is unwound at these sites by the minichromosome maintenance (MCM) helicase and other factors such as Cdc45 and the associated GINS complex.The unwound single strands are protected by replication protein A (RPA) until DNA polymerase starts synthesizing DNA at the 5’ end of the strand in the same direction as the replication fork. To prevent the replication fork from falling apart,...
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Organisms are capable of detecting and fixing nucleotide mismatches that occur during DNA replication. This sophisticated process requires identifying the new strand and replacing the erroneous bases with correct nucleotides. Mismatch repair is coordinated by many proteins in both prokaryotes and eukaryotes.
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The double-stranded structure of DNA has two major advantages. First, it serves as a safe repository of genetic information where one strand serves as the back-up in case the other strand is damaged. Second, the double-helical structure can be wrapped around proteins called histones to form nucleosomes, which can then be tightly wound to form chromosomes. This way, DNA chains up to 2 inches long can be contained within microscopic structures in a cell. A double-stranded break not only damages...
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The basic reaction of homologous recombination (HR) involves two chromatids that contain DNA sequences sharing a significant stretch of identity. One of these sequences uses a strand from another as a template to synthesize DNA in an enzyme-catalyzed reaction. The final product is a novel amalgamation of the two substrates. To ensure an accurate recombination of sequences, HR is restricted to the S and G2 phases of the cell cycle. At these stages, the DNA has been replicated already and the...
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Identifying the Effects of BRCA1 Mutations on Homologous Recombination using Cells that Express Endogenous Wild-type BRCA1
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Replication Gaps Underlie BRCA Deficiency and Therapy Response.

Nicholas J Panzarino1, John J Krais2, Ke Cong1

  • 1University of Massachusetts Medical School, Worcester, Massachusetts.

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Summary
This summary is machine-generated.

Single-stranded DNA replication gaps, not DNA double-strand breaks, explain the chemotherapy sensitivity of BRCA-deficient cancers. Restoring fork stability or filling these gaps can overcome treatment resistance.

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Identifying the Effects of BRCA1 Mutations on Homologous Recombination using Cells that Express Endogenous Wild-type BRCA1
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Area of Science:

  • Molecular Biology
  • Cancer Biology
  • Genetics

Background:

  • Tumors with BRCA1/BRCA2 (BRCA) gene defects are sensitive to chemotherapy.
  • This sensitivity was attributed to DNA repair defects and replication fork instability.
  • Recent studies challenge this, showing chemotherapies like cisplatin do not initially cause DNA double-strand breaks (DSB) in BRCA-deficient tumors.

Purpose of the Study:

  • To investigate the underlying mechanism of hypersensitivity in BRCA-deficient cancers.
  • To determine if single-stranded DNA (ssDNA) replication gaps, rather than DSBs, homologous recombination (HR), or fork protection (FP), are responsible for this sensitivity.
  • To explore therapeutic strategies targeting these mechanisms.

Main Methods:

  • Utilized BRCA-deficient cancer cell models.
  • Assessed the role of ssDNA replication gaps, HR, and FP in response to genotoxic stress.
  • Investigated the impact of restoring fork restraint and gap filling on therapy resistance.
  • Examined DSB formation in the presence of apoptosis inhibition.

Main Results:

  • BRCA-deficient cells exhibit hypersensitivity due to ssDNA replication gaps arising from unrestrained replication under stress.
  • Defects in HR or FP alone do not explain this hypersensitivity.
  • Restoring fork restraint or gap filling confers therapy resistance, independent of HR or FP status.
  • DSBs are not directly induced by genotoxic agents but by cell death nucleases, and are not fundamental to genotoxic agent action.

Conclusions:

  • ssDNA replication gaps are the primary cause of the "BRCAness" phenotype and hypersensitivity to genotoxic chemotherapies.
  • Targeting ssDNA gaps offers a promising strategy for resensitizing BRCA-deficient tumors to treatment.
  • This finding provides potential biomarkers and therapeutic targets for refractory cancers.