PSIP1 keeps R-loops off the genes it sits on, and removing it causes transcription-replication conflict, DNA damage and PARP-inhibitor sensitivity
also called PSIP1 R-loop · LEDGF genome integrity
When genes are switched on, the new RNA can stick back onto the DNA and jam the machinery. PSIP1 stops that happening. Take PSIP1 away and the jams cause DNA breaks, which is a reason to be careful about drugging it and also a possible way to exploit it.
The same thing, for a scientist
PSIP1 interacts with R-loops and with proteins involved in R-loop homeostasis including PARP1. Genome-wide mapping of PSIP1, R-loops and gamma-H2AX in PSIP1-depleted human and mouse cell lines showed accumulation of **R-loops and DNA damage at gene promoters** in the absence of PSIP1. The accumulation causes local transcriptional arrest and transcription-replication conflict, which produces the damage. Repair pathway choice shifts - 53BP1 foci up, RAD51 foci down - and PSIP1 depletion increases the sensitivity of cancer cells to **PARP1 inhibitors** and to DNA-damaging agents that induce R-loop-dependent damage. For this beat the entry does two jobs. It supplies a second, independent mechanism by which LEDGF loss destabilises the genome, which compounds the bite-back in `mechanism-ledgf-dna-end-resection-and-homologous-recombination`. And it supplies the most concrete combination hypothesis on the axis: if LEDGF loss sensitises to PARP inhibition, then a LEDGF-directed agent plus a PARP inhibitor is a synthetic-lethal pairing that nobody has tested in KMT2A-rearranged leukaemia.
What was mapped
PSIP1 ChIP, DRIP-style R-loop mapping and gamma-H2AX mapping, genome-wide, in PSIP1-depleted human and mouse cells 1. R-loops and damage accumulate at promoters, which is where the menin-KMT2A-LEDGF module also sits, so the two functions are colocalised as well as co-carried by the same protein.
The PARP1 interaction matters twice over: PARP1 is in the PSIP1 R-loop interactome, and PARP inhibition is the sensitisation the depletion produces. That is a coherent story rather than a coincidence, and it makes the combination prediction sharper than a generic 'HR-deficient cells are PARPi-sensitive' argument.
A KMT2A-specific angle nobody has taken
R-loops are a plausible bridge between this beat and the break-machinery work running in parallel in this project. KMT2A breakpoints cluster in a defined region and the mechanism of the break is a live question. LEDGF sits on transcribed gene bodies, suppresses R-loops there, and its loss produces transcription-replication conflict. Whether LEDGF status modulates R-loop formation at the KMT2A breakpoint cluster region itself is an unasked question that connects an oncoprotein cofactor to the genesis of the translocation that creates the oncoprotein.
Stated as a prediction: if LEDGF suppresses R-loops at highly transcribed gene bodies, then LEDGF-deficient progenitors should show increased R-loop signal and increased DNA damage at KMT2A, and would be more, not less, prone to the initiating break. That is testable in the existing conditional knockout mouse with DRIP-seq, and it would be an uncomfortable result for a prevention-stage LEDGF intervention.
Limits
R-loop mapping methods are notoriously sensitive to the antibody and the RNase H control, and a genome-wide accumulation after depleting a chromatin protein is compatible with an indirect transcriptional effect. The paper's own causal claim - accumulation causes arrest and conflict, leading to damage - is supported by the co-mapping and the foci data but not by a rescue with RNase H overexpression reported in the abstract. Everything here is in cancer cell lines; none of it is haematopoietic.
| interactor named | PARP1 |
|---|---|
| combination hypothesis | LEDGF-directed agent plus PARP inhibitor in KMT2A-rearranged leukaemia - untested |
| untested prediction | LEDGF-deficient progenitors show increased R-loop signal and DNA damage at the KMT2A breakpoint cluster region |
phenotype of loss
[ "R-loop accumulation at promoters", "local transcriptional arrest", "transcription-replication conflict", "gamma-H2AX accumulation", "53BP1 up / RAD51 down", "PARP inhibitor sensitisation" ]
- A LEDGF-directed agent would raise R-loops and transcription-replication conflict genome-wide, which is a mutagenic state rather than a merely cytostatic one.
- In a paediatric patient expected to be cured and to live sixty years, a drug that transiently destabilises the genome carries a second-malignancy risk no preclinical model in this field measures.
- The PARP-inhibitor synergy that makes this attractive as a combination is the same phenotype that makes it dangerous as a monotherapy in a normal tissue.
- Does LEDGF loss increase R-loops and DNA damage at the KMT2A breakpoint cluster region? If it does, LEDGF deficiency would promote the initiating translocation while blocking the leukaemia it causes, which is a genuinely strange and testable combination.
- Is the PARP-inhibitor sensitisation reproducible in KMT2A-rearranged leukaemia lines? It is one plate and it would be the first combination hypothesis on this axis with a mechanistic basis.
- Are the R-loop and CtIP accounts of LEDGF genome-stability one mechanism or two? Nobody has run an RNase H rescue against the resection phenotype.
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
PSIP1 interacts with R-loops and with R-loop-homeostasis proteins including PARP1; genome-wide mapping in PSIP1-depleted human and mouse cells shows accumulation of R-loops and DNA damage at gene promoters; the accumulation causes local transcriptional arrest and transcription-replication conflict leading to DNA damage; PSIP1 depletion increases 53BP1 foci, reduces RAD51 foci and sensitises cancer cells to PARP1 inhibitors and to DNA-damaging agents.
PSIP1/LEDGF reduces R-loops at transcription sites to maintain genome integrity. · Jayakumar S, Patel M, et al. · Nat Commun 2024What this evidence cannot show
Cancer cell lines, human and mouse, not haematopoietic and not KMT2A-rearranged. R-loop mapping is method-sensitive. Open access in Europe PMC (PMC10774266).
Evidence that argues against this (1)
- An independent account of LEDGF's genome-stability role runs through CtIP tethering and end resection rather than through R-loops, and the two mechanisms have not been reconciled; if the R-loop phenotype is downstream of the repair defect, the PARPi sensitisation has a different and less specific explanation. Daugaard M et al. 2012
- E2moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
The chromatin context in which PSIP1 suppresses R-loops is the same H3K36-methylated, actively transcribed chromatin at which PSIP1 associates with DNA repair proteins.
H3K36me3 and PSIP1/LEDGF associate with several DNA repair proteins, suggesting their role in efficient DNA repair at actively transcribing loci. · Sundarraj J, Taylor GCA, et al. · Wellcome Open Res 2017What this evidence cannot show
An association study; the colocalisation of the two functions is inferred from shared chromatin context rather than demonstrated at single loci.
Evidence that argues against this (1)
- H3K36me3 is a very common mark on transcribed gene bodies, so shared occupancy is weak evidence that two functions are mechanistically linked. Li L et al. 2017
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
The leukaemic function of LEDGF is exercised at the same transcribed, H3K36-methylated chromatin: the PWWP domain reads H3K36me2/3 nucleosomes with a multivalent contact that also engages nucleosomal DNA.
Structure of H3K36-methylated nucleosome-PWWP complex reveals multivalent cross-gyre binding. · Wang H, Farnung L, et al. · Nat Struct Mol Biol 2020What this evidence cannot show
Structural work on the PWWP-nucleosome contact; cited here to establish that the leukaemic and genome-integrity functions occupy the same chromatin, not to make a claim about R-loops.
Evidence that argues against this (1)
- Occupying the same chromatin does not mean the same molecules perform both functions at the same time, and a drug might in principle separate them if the two depended on different surfaces. Koutná E et al. 2023
second independent genome-integrity function of this protein
two unreconciled accounts of the same genome-stability role
both functions are exercised on the same H3K36-methylated chromatin
the R-loop route to the same combination prediction