Which repair pathway glues the fusion together: classical NHEJ vs alternative end joining
also called c-NHEJ vs alt-EJ · MMEJ translocation · microhomology-mediated end joining · Poltheta translocation
Two different repair crews can join the wrong chromosome ends. One works fast and blunt; the other looks for a few matching letters on each side and trims everything in between, leaving a tell-tale scar. Reading those scars in patient DNA tells us which crew made the fusion, and in human cells the answer is mostly the fast, blunt one.
The same thing, for a scientist
Two non-homologous pathways can ligate ends from different chromosomes: classical NHEJ (Ku70/80, DNA-PKcs, XRCC4/LIG4) and alternative/microhomology-mediated end joining (PARP1, MRN, POLQ, LIG1/LIG3). Their junction signatures differ: c-NHEJ produces blunt or minimally processed joins with short or absent microhomology and small indels, whereas alt-EJ produces longer deletions and 2-20 bp microhomology. In mouse cells, c-NHEJ factors suppress translocations and alt-EJ makes them. Ghezraoui et al. showed the opposite in human cells: designer-nuclease-induced translocations were altered in LIG4- or XRCC4-deficient human cells, with junctions showing significantly longer deletions and more microhomology (i.e. a shift toward alt-EJ), demonstrating that in wild-type human cells translocations are made by c-NHEJ. KMT2A t(4;11) junctions carry short duplications, deletions and inversions of under a few hundred nucleotides plus microhomology, and apoptosis-initiated KMT2A translocations require DNA-PKcs. The weight of evidence therefore places c-NHEJ as the primary joiner for human KMT2A fusions, which is an important negative for Poltheta-inhibitor-based prevention strategies.
The species trap1 deeper
Most of the textbook claim that translocations are alt-EJ products comes from mouse work, where c-NHEJ factors suppress translocations. Using ZFNs, TALENs and CRISPR/Cas9 to make paired DSBs on two chromosomes in human cells, Ghezraoui et al. found that translocations were altered rather than abolished in LIG4- or XRCC4-deficient cells, and the residual junctions had significantly longer deletions and more microhomology, the signature of alt-EJ taking over. The conclusion was explicit: unlike mouse cells, translocations in human cells are generated by c-NHEJ 1. They also showed paired-Cas9-nick-induced translocations depended on c-NHEJ despite having distinct joining characteristics.
Reading this correctly
The result does not say alt-EJ never joins a human translocation. It says that when c-NHEJ is available, it wins. Microhomology at a patient junction is therefore weak evidence for alt-EJ, because c-NHEJ also uses short microhomology opportunistically. Diagnosing pathway from a handful of patient junctions is less decisive than the older literature implies.
What the KMT2A junctions actually show1 deeper
Reciprocal breakpoints from 14 primary t(4;11) leukaemias all carried duplications, deletions and inversions of fewer than a few hundred nucleotides, interpreted as the product of several DNA strand breaks on both participating chromosomes followed by error-prone repair 2. Separately, apoptosis-initiated KMT2A translocations produced junctions containing microhomology, DNA-PKcs was found at the cleavage site, and pharmacological suppression of DNA-PKcs eliminated the translocations 3. DNA-PKcs dependence is a c-NHEJ hallmark.
Multiple breaks per event, not one clean cut
The presence of inversions and duplications, not just deletions, in every one of the 14 cases is the most underappreciated detail in this literature 2. It implies the founder event was not two tidy DSBs but a small local cluster of breaks on both chromosomes, resolved messily. If true, that raises the bar for any model, including the clean TOP2-poison model, that predicts a single defined cut.
Consequences for Poltheta inhibitors
Potent, selective allosteric inhibitors of the Poltheta polymerase domain exist, notably ART558, which blocks theta-mediated end joining without targeting NHEJ and is synthetic-lethal in BRCA1/2-mutant cells 4. It is tempting to propose them as anti-translocation agents. The human-cell genetics argue this would not work for preventing KMT2A fusions, because it inhibits the pathway that is not doing the joining, and might even divert ends toward the pathway that is. I rate this strategy as unpromising and say so in hyp-block-kmt2a-break-machinery-upstream.
| mechanism class | DNA double-strand break repair / end joining |
|---|---|
| kmt2a junction signature | short duplications, deletions and inversions under a few hundred bp plus microhomology; DNA-PKcs dependent in the apoptotic model |
| verdict | classical NHEJ is the most likely joiner of human KMT2A fusions; alt-EJ/Poltheta inhibition is therefore a poor anti-translocation target |
pathways
[
{
"name": "classical NHEJ",
"factors": [
"XRCC6/Ku70",
"XRCC5/Ku80",
"PRKDC/DNA-PKcs",
"XRCC4",
"LIG4",
"NHEJ1/XLF"
],
"junction_signature": "blunt or short microhomology, small indels",
"human_translocation_role": "primary joiner"
},
{
"name": "alternative / microhomology-mediated EJ",
"factors": [
"PARP1",
"MRE11-RAD50-NBN",
"POLQ",
"LIG1",
"LIG3"
],
"junction_signature": "longer deletions, 2-20 bp microhomology",
"human_translocation_role": "backup; dominates only when c-NHEJ is disabled"
}
]- Inhibiting the joining pathway does not prevent the break; it converts a fusion into a persistent unrepaired DSB, which is likely worse for a fetal progenitor.
- Because c-NHEJ is absolutely required for V(D)J recombination, any c-NHEJ inhibitor would cause severe combined immunodeficiency. This pathway cannot be drugged preventively in a child.
- Poltheta inhibition, the only clinically advancing alt-EJ drug class, is aimed at the wrong pathway for this purpose and might redirect ends toward the pathway that actually makes the fusion.
- Nobody has systematically classified junction signatures across a large modern KMT2A breakpoint cohort against a c-NHEJ vs alt-EJ classifier. This is a cheap, high-value computational analysis (see `ana-junction-microhomology-signature-mining`).
- Does the junction signature differ between de novo infant and therapy-related KMT2A-r cases? If the break mechanisms differ, the joining signatures might too.
- Are the inversions and duplications seen in all 14 t(4;11) cases reproducible at NGS resolution, and do they indicate clustered breaks or template-switching during repair?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In human cells, chromosomal translocations are generated by canonical NHEJ; LIG4 or XRCC4 deficiency alters rather than abolishes them and shifts junctions toward longer deletions and more microhomology.
Chromosomal translocations in human cells are generated by canonical nonhomologous end-joining · Ghezraoui H, Piganeau M, Renouf B, et al. · Molecular Cell 2014What this evidence cannot show
Designer-nuclease-induced DSBs at engineered loci in transformed human cell lines. Nuclease-induced breaks are clean and simultaneous, unlike the messy clustered breaks inferred at KMT2A, so the pathway assignment may not transfer perfectly.
Evidence that argues against this (1)
- Mouse studies show the opposite dependence, with c-NHEJ suppressing and alt-EJ generating translocations, so pathway assignment is species-specific and any single system can mislead. Roukos V et al. 2014
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
All 14 sequenced primary t(4;11) reciprocal breakpoints contained duplications, deletions and inversions of under a few hundred nucleotides, indicating multiple strand breaks on both chromosomes resolved by error-prone repair.
A DNA damage repair mechanism is involved in the origin of chromosomal translocations t(4;11) in primary leukemic cells · Gillert E, Leis T, Repp R, et al. · Oncogene 1999What this evidence cannot show
n=14; junction complexity could partly reflect PCR and cloning artefacts of the era, though the consistency across all cases argues against that.
Evidence that argues against this (1)
- Complex junctions are the generic signature of error-prone end joining, not evidence for a KMT2A-specific multi-break mechanism. Genomic junctions from nine patients with the other paradigm prenatal fusion show the same repertoire — microhomologies at the end points with small deletions and duplications — and no consistent V(D)J signal sequences, topoisomerase II consensus sites or other recombination motifs; the authors read them as products of imprecise, constitutive repair. Observing duplications, deletions and inversions in all 14 t(4;11) junctions therefore establishes that the ends were processed before ligation, which is not the same as establishing that multiple independent strand breaks occurred on both chromosomes. Wiemels JL et al. 1999
- E3moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
Apoptosis-initiated KMT2A translocation junctions contain microhomology, DNA-PKcs localises to the cleavage site, and suppressing DNA-PKcs eliminates the translocations.
Apoptotic triggers initiate translocations within the MLL gene involving the nonhomologous end joining repair system · Betti CJ, Villalobos MJ, Diaz MO, Vaughan AT · Cancer Research 2001What this evidence cannot show
Wortmannin is not DNA-PKcs-selective; TK6 cell line only; single laboratory and never independently replicated.
Evidence that argues against this (1)
- Microhomology is not a reliable marker of which pathway joined the ends, so 'microhomology plus DNA-PKcs' is not the coherent picture it appears to be. In B cells lacking KU70, KU80, or BOTH Ku plus DNA ligase 4, class switch recombination still occurred substantially, demonstrating an alternative end-joining pathway genuinely distinct from classical NHEJ — and although those cells were biased toward microhomology-mediated joins, they nonetheless generated SUBSTANTIAL NUMBERS OF DIRECT JOINS, in contrast to XRCC4- or Lig4-deficient cells. Direct joins arise without classical NHEJ and microhomology-mediated joins arise with it, so junction microhomology at a KMT2A breakpoint does not assign the event to alt-EJ, and the DNA-PKcs dependence reported pharmacologically in one cell line does not assign it to classical NHEJ either. Boboila C et al. 2010
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Selective allosteric Poltheta polymerase inhibitors (ART558) block theta-mediated end joining without inhibiting NHEJ and elicit BRCA-gene synthetic lethality.
Poltheta inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance · Zatreanu D, Robinson HMR, Alkhatib O, et al. · Nature Communications 2021What this evidence cannot show
Developed and validated as a synthetic-lethal oncology agent in HR-deficient tumours. There is no published evidence that Poltheta inhibition reduces the frequency of chromosomal translocations in human cells, and I could not find any. Citing it as an anti-translocation strategy would be an unsupported extrapolation.
Evidence that argues against this (1)
- Using a Poltheta inhibitor to PREVENT fusion formation would probably do the opposite. Polq-null murine cells are selectively hypersensitive to DNA strand-breaking agents, POLQ's polymerase activity is required for that resistance, POLQ performs end joining of long 3' single-stranded overhangs independently of Ku70, and — decisively for this entry — Polq-defective mice showed a MARKED INCREASE in Myc/IgH translocations. The authors state explicitly that their findings 'depart from the prevailing view that alternative end joining processes are generically translocation-prone' and that POLQ SUPPRESSES genomic instability and genome rearrangements originating at double-strand breaks. ART558-class agents are therefore not a candidate prophylaxis against translocation; if anything they are a hazard, and that should be stated wherever this evidence item is used. Yousefzadeh MJ et al. 2014
for TOP2 break chemistry, NHEJ via TDP2 SUPPRESSES translocation, the opposite direction to nuclease-induced breaks
the cohort-scale junction classification that entry's open question asks for
independently reaches the same verdict that POLQ inhibition is the wrong pathway and the wrong direction
multiple breaks on both chromosomes with deleted, inverted or duplicated fragments; reciprocal but not balanced
Counter-evidence context (Reichel 2001, PMID 11420702): the junction-based exclusion of recombinase involvement at KMT2A rests on n=14 Sanger-era sequencing, while breakpoint distribution at this locus is known to differ significantly between infants and older children - a small negative is least informative where the mechanism is age-heterogeneous
shared microhomology-biased alt-EJ joining chemistry
paired induced breaks are handed to the same error-prone joining that made the disease
the repair pathway that joins the wrong ends
- related to →Apoptotic nucleases cut KMT2A, and cells that survive the attempt can carry a translocation
Counter-evidence (Yousefzadeh 2014, PMID 25275444): Polq-defective mice show a MARKED INCREASE in Myc/IgH translocations and the authors state their findings depart from the view that alt-EJ is generically translocation-prone -- POLQ-dependent end joining SUPPRESSES rearrangement, so supplying more breaks to it does not by itself supply more fusions, and Poltheta inhibitors are not prophylaxis
Counter-evidence (Wiemels and Greaves 1999, PMID 10463610): genomic junctions from nine patients with the other paradigm in-utero fusion showed NHEJ signatures and NO consistent V(D)J signal sequences, topo II consensus sites or other recombination motifs -- the cryptic-RSS footprint demonstrated at BCL2 is absent where a RAG mechanism would matter most