121: G-quadruplexes, BRCA2, and a Helicase Weak Spot

Keahia DL et al., Proceedings of the National Academy of Sciences (PNAS) - This study shows that G-quadruplex (G4) DNA structures are hotspots of replication stress and mutagenesis in BRCA2-deficient cerebellar granule cell progenitors (GCPs), driving SHH-subgroup medulloblastoma, and identifies upregulation of the G4-resolving helicase PIF1 in tumors. Key terms: BRCA2, G-quadruplexes, PIF1, medulloblastoma, granule cell progenitors.

Study Highlights:
Deletion of Brca2 exons 3–4 combined with Trp53 loss in mice produced fully penetrant SHH medulloblastomas that resemble proliferating GCPs. Whole-genome sequencing of tumors revealed structural variant breakpoints significantly overlapping putative and experimentally mapped G4-forming sequences. BRCA2-deficient GCPs show reduced replication speed and increased cell death upon G4 stabilization with pyridostatin, and tumor cells upregulate Pif1. Genetic loss of Pif1 in primary tumor cells increases genomic instability and fork stalling when G4s are stabilized, implicating PIF1 as a tumor maintenance factor and potential therapeutic target.

Conclusion:
G4 DNA structures are an endogenous source of replication stress in proliferating GCPs that, without BRCA2, lead to genome instability and medulloblastoma; tumor cells compensate by upregulating PIF1, which may be exploitable therapeutically.

Music:
Enjoy the music based on this article at the end of the episode.

Article title:
G-quadruplexes as a source of vulnerability in BRCA2-deficient granule cell progenitors and medulloblastoma

First author:
Keahia DL

Journal:
Proceedings of the National Academy of Sciences (PNAS)

DOI:
10.1073/pnas.2503872122

Reference:
Keahia DL, Sanders MA, Paul MR, Webster ALH, Hatten ME, Smogorzewska A, et al. G-quadruplexes as a source of vulnerability in BRCA2-deficient granule cell progenitors and medulloblastoma. PNAS. 2025;122(35):e2503872122. doi:10.1073/pnas.2503872122

License:
This episode is based on an open-access article published under the Creative Commons Attribution 4.0 International License (CC BY 4.0) – https://creativecommons.org/licenses/by/4.0/

Support:
Base by Base – Stripe donations: https://donate.stripe.com/7sY4gz71B2sN3RWac5gEg00

Official website https://basebybase.com

On PaperCast Base by Base you'll discover the latest in genomics, functional genomics, structural genomics, and proteomics.

Episode link: https://basebybase.com/episodes/g-quadruplexes-as-a-source-of-vulnerability-in-brca2-deficient-granule-cell-progenitors-and-medulloblastoma

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2025-08-29.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Audited the central experimental results and claims described in the transcript that map to the article's main findings: BRCA2 deficiency in granule cell progenitors, G4-related replication stress, PIF1 upregulation, Ptch1 overlap with G4s, PDS experiments, and therapeutic implications of PIF1 inhibition.
- transcript topics: BRCA2 function in granule cell progenitors; G-quadruplex structures and replication stress; Brca2Δex3-4; Trp53−/− mouse medulloblastoma model; Structural variant breakpoints and Ptch1 overlap with G4s; PIF1 upregulation and dependency in BRCA2-deficient MB; Pyridostatin (PDS) stabilization of G4s

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 5
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- BRCA2 deficiency (BRCA2Δex3-4) with TP53 loss leads to SHH-subgroup medulloblastoma with complete penetrance in mice.
- G4 structures cause replication fork stalling in BRCA2-deficient GCPs; BRCA2 promotes fork protection at G4s; stabilizing G4s with pyridostatin (PDS) blocks replication and induces
- Ptch1 mutations in MB overlap with G4-dense loci; SVs/CNVs in MBs co-localize with predicted G4-forming sequences in both mouse and human samples.
- PIF1 helicase is highly upregulated in BRCA2-deficient MBs and supports replication through G4s; PIF1 knockout impairs replication and increases genomic instability, suggesting a t
- Targeting PIF1 could selectively kill BRCA2-deficient MB cells (synthetic lethality) when combined with other therapies; limitations exist (p53 status, incomplete mechanistic detai
- PDS experiments, EdU incorporation, fork speed measurements, and micronuclei formation collectively support a BRCA2-G4-PIF1 axis in MB.

QC result: Pass.