Experiment: degrade TOP2A and TOP2B separately in a fetal-like human progenitor and map DNA breaks at the KMT2A locus without a poison
also called TOP2 isoform degron break map · poison-free KMT2A break mapping · dTAG TOP2A/TOP2B END-seq at the KMT2A BCR
Nobody has ever asked which of the two nearly identical DNA-cutting enzymes breaks the KMT2A gene when no drug is present, because the standard way to see the enzyme working is to jam it with a drug. This experiment removes each enzyme on demand and counts the breaks directly, so the drug is never needed.
The same thing, for a scientist
**The question.** Which topoisomerase II isoform generates double-strand breaks at the KMT2A breakpoint cluster region in a human haematopoietic progenitor, and does it do so in the absence of a topoisomerase poison? Every isoform assignment in this literature was made under etoposide [`gene-top2b` E2] or on naked DNA with purified enzyme [`gene-top2a` E3], and the fetal disease has no poison in it. The circularity is methodological and this design breaks it. **The design.** Knock a degron tag (dTAG or auxin-inducible) into the endogenous *TOP2A* and *TOP2B* loci of human CD34+ cells - cord blood for feasibility, fetal liver HSC/LMPP for the claim - in four arms: untagged, TOP2A-degron, TOP2B-degron, and both. Deplete acutely (hours, not days, so the cell cycle does not redistribute), then map DSBs genome-wide at single-nucleotide resolution by END-seq or sBLISS, with and without a low-dose etoposide arm as a positive control. Score three readouts: genome-wide break landscape, breaks within the 8.3 kb KMT2A BCR and specifically the intron 11/exon 12 and intron 10/exon 11 hotspots, and KMT2A rearrangement frequency by inverse PCR in a parallel long-culture arm. **Why now.** `ana-dsb-mapping-assays-at-the-kmt2a-locus` in this graph records that three good break-mapping methods exist and none has ever been published on KMT2A in a blood progenitor. `exp-human-cd34-kmt2a-breakage-assay` records that KMT2A rearrangement happens in untreated cytokine-stimulated CD34+ cells with no genotoxin - so the poison-free break this experiment would map is already known to exist as an outcome; what is missing is the enzyme. And Atkin et al. have now shown that a low-input adaptation of DSBCapture works on ~83,000 primary human cells [E4], which removes the material objection for the fetal arm.
Arms, readouts and the decision each one drives
| arm | readout | what a positive means |
|---|---|---|
| untagged, no drug | baseline break map at the KMT2A BCR | establishes that poison-free breaks at this locus are detectable at all; a flat result falsifies the fetal hypothesis at its root |
| TOP2B-degron, no drug | change in BCR breaks | the fetal-hypothesis prediction: TOP2B loss reduces them |
| TOP2A-degron, no drug | change in BCR breaks | the control; a reduction here inverts the isoform assignment |
| both degrons, no drug | change in BCR breaks | if breaks persist with both isoforms gone, TOP2 is not the poison-free source and the field should look at apoptotic nucleases and replication-transcription conflict |
| untagged + low-dose etoposide | positive control | reproduces the published TOP2B-dependence and validates the assay |
| each degron + etoposide | isoform apportionment under poison | answers gene-top2a's open question about which end each isoform supplies |
| long culture, inverse PCR | KMT2A rearrangement frequency | connects break rate to rearrangement rate - the step this graph repeatedly says is not rate-limiting |
The single most informative cell is row 4. This graph's break-machinery beat concludes that KMT2A breakage is common and the rare step is downstream. If both isoforms can be removed and the poison-free breaks persist, the TOP2 account of the de novo disease is finished and a whole class of hypotheses - including hyp-top2b-transcription-coupled-fetal-kmt2a-break, which this experiment is written to test - can be retired for the cost of one arm.
The three ways this experiment fails, stated in advance
- Degron toxicity. TOP2A is required for mitosis; acute TOP2A degradation in a proliferating progenitor will arrest cells, and an arrested cell has a different break landscape for reasons unrelated to the question. Mitigation: read out within hours, include a cell-cycle-matched comparison, and treat the TOP2A arm as interpretable only for the non-cycling fraction.
- Sensitivity at one locus. END-seq detects at least one DSB per cell among 10,000 cells not harbouring DSBs, which is the specification that makes a single-locus question tractable; but the expected poison-free break rate at an 8.3 kb window is unknown and could be below any assay's floor. Mitigation: power the design on the positive-control arm first and report the detection limit explicitly, so a negative is a bounded negative rather than a silent one.
- The fetal cell. Human fetal liver HSC/LMPP are scarce, ethically constrained and difficult to engineer with a knock-in degron. Cord-blood CD34+ is the feasible substitute and is not the same cell. Mitigation: run the full matrix in cord blood, and in fetal liver run only the antibody and occupancy questions - is TOP2B there, is it at the BCR - which need no engineering.
| design | Four-arm acute degron matrix (untagged / TOP2A-degron / TOP2B-degron / both) in human CD34+ progenitors - cord blood for the feasible version, fetal liver HSC-LMPP for the claim - crossed with poison-free and low-dose-etoposide conditions, plus a TDP2- or proteasome-inhibited poison-free arm to raise lesion persistence without adding a poison. |
|---|---|
| readout | Genome-wide DSB map at single-nucleotide resolution (END-seq in nuclei preferred; low-input DSBCapture as the primary-material fallback), scored genome-wide, across the 8.3 kb KMT2A BCR, and at the intron 10/exon 11 and intron 11/exon 12 hotspots; plus KMT2A rearrangement frequency by inverse PCR in a parallel long-culture arm. |
| controls | Untagged cells with and without etoposide as assay positive control reproducing the published TOP2B-dependence; cell-cycle profiling in every arm because TOP2A degradation arrests mitosis and an arrested cell has a different break landscape; a matched non-fragile locus of equal length and expression as a within-sample negative; reported detection limit so that any negative is a bounded negative. |
| cost tier | high |
| timeline | 18-30 months: 6-9 months for degron knock-in and validation in cord-blood CD34+, 6 months for the break-mapping matrix, 6-12 months for the fetal-liver occupancy arm subject to tissue access. |
| why nobody has done it | TOP2 engagement is conventionally assayed by trapping the enzyme with a poison, which is the exact perturbation the fetal question needs absent. Acute degron technology plus single-nucleotide break mapping dissolves that circularity and neither was routine when the foundational isoform papers were written. |
| prior art gap | ana-dsb-mapping-assays-at-the-kmt2a-locus records that no modern break-mapping method has ever been published on KMT2A in a blood progenitor. |
| smallest useful version | Cord-blood CD34+, TOP2B-degron only, poison-free plus etoposide control, END-seq, scored at the KMT2A BCR. Two arms and one locus; it answers whether the isoform assignment survives removal of the poison. |
decision it drives
{
"TOP2B_degron_reduces_poison_free_BCR_breaks": "supports hyp-top2b-transcription-coupled-fetal-kmt2a-break; the fetal break has an enzyme and the infant age peak becomes a measurable expression question",
"TOP2A_degron_reduces_them_instead": "the isoform assignment in this whole literature is inverted for the unpoisoned state",
"neither_degron_changes_them": "TOP2 is not the poison-free source; retire the fetal TOP2 hypothesis and redirect to apoptotic nucleases, replication-transcription conflict and programmed-break side reactions",
"no_poison_free_breaks_detectable": "a bounded negative - report the detection limit, because this is the result most easily mistaken for a failed experiment"
}- This experiment produces no therapeutic option under any outcome. Its value is entirely in retiring or sharpening a hypothesis, and a high-cost experiment whose best case is a better explanation should be argued for on those terms rather than dressed as translational.
- The most likely result is the least useful one: a small, noisy reduction in the TOP2B arm that is consistent with TOP2B being one of several routes. Anyone funding this should require the apportionment analysis to be specified in advance, because a partial answer here is indistinguishable from the current state of the field.
- TOP2A degron arms will arrest proliferating cells, and an arrested progenitor's break landscape differs for reasons that have nothing to do with the question. If the cell-cycle control is not built in from the start, the TOP2A arm will generate a confident artefact.
- Is there a poison-free assay for TOP2 catalytic engagement on chromatin? If one existed it would be more informative than this whole design, because it would measure the enzyme rather than inferring it from a subtraction.
- What is the expected poison-free DSB rate in an 8.3 kb window in a primary progenitor? Without a prior, the design cannot be powered and the negative cannot be bounded - this is the first calculation anyone running it must do.
- Can a degron knock-in be made in human fetal liver HSC at all, or is the fetal arm permanently limited to antibody and occupancy measurements? The answer determines whether the hypothesis this experiment tests is testable in the right cell in this decade.
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
END-seq monitors DNA double-strand breaks and end resection genome-wide at base-pair resolution in vivo and can detect at least one DSB per cell among 10,000 cells not harbouring DSBs.
DNA Breaks and End Resection Measured Genome-wide by End Sequencing. · Canela A, Sridharan S, Sciascia N, Tubbs A, Meltzer P, Sleckman BP, et al. · Mol Cell 2016What this evidence cannot show
The sensitivity figure is the method paper's own specification under its validation conditions. Applying it to a single 8.3 kb window in a primary human progenitor is an extrapolation and the experiment must establish its own detection limit rather than quote this one.
Evidence that argues against this (1)
- A lower-input alternative now exists and may be the better choice for primary material: a low-input adaptation of DSBCapture has been run successfully on 500 ng of genomic DNA, roughly 83,000 cells, from patient samples. Method choice should follow the material available rather than the citation count. Atkin ND et al. 2021
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Most etoposide-induced chromosome breaks in the MLL/KMT2A locus and the overall genotoxicity of etoposide are TOP2B-dependent, established by genetic depletion in human cells.
Model for MLL translocations in therapy-related leukemia involving topoisomerase IIβ-mediated DNA strand breaks and gene proximity. · Cowell IG, Sondka Z, Smith K, Lee KC, Manville CM, Sidorczuk-Lesthuruge M, et al. · Proc Natl Acad Sci U S A 2012What this evidence cannot show
Transformed human cell lines, chronic depletion rather than acute degradation, and always in the presence of etoposide. Reproducing this result is the positive control for the design and its reproduction in a primary progenitor would itself be new.
Evidence that argues against this (1)
- Fusion formation at the recurrent hotspots requires both TOP2 isoforms and transcriptional elongation, so the positive control should be expected to show TOP2B-dependence of BREAKS and a different, possibly two-isoform, dependence of REARRANGEMENT frequency. The design separates those readouts for exactly this reason. Gothe HJ et al. 2019
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Conversion of trapped TOP2 cleavage complexes into irreversible TOP2-linked double-strand breaks is accelerated about two-fold at transcribed loci and depends on proteasomal degradation and TDP2 phosphodiesterase activity, while TOP2 chromatin localisation and trapping require pre-existing cohesin binding and are transcription-independent.
Topoisomerase II-Induced Chromosome Breakage and Translocation Is Determined by Chromosome Architecture and Transcriptional Activity. · Canela A, Maman Y, Huang SN, Wutz G, Tang W, Zagnoli-Vieira G, et al. · Mol Cell 2019What this evidence cannot show
All under etoposide. This is the evidence that a TDP2 or proteasome arm belongs in the design, because it predicts that those manipulations change break persistence without changing break initiation - a dissociation the assay can see.
Evidence that argues against this (1)
- If persistent breaks require a poison to trap the complex in the first place, the poison-free arms may simply find nothing, and the experiment's central negative would be uninformative about whether TOP2B ever cuts KMT2A in a fetus - only about whether the cut persists long enough to be captured. The design should therefore include a TDP2-inhibited or proteasome-inhibited poison-free arm to raise persistence without adding a poison. Gómez-Herreros F et al. 2017
- E4moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
A low-input adaptation of the DSBCapture break-mapping method works on 2 ug (about 330,000 cells) and 500 ng (about 83,000 cells) of purified genomic DNA from primary patient samples, and detected a 3.2-fold group-average increase in breakage at the KMT2A intron 10/exon 11 region in KMT2A-rearranged remission patients relative to non-fusion AML patients.
Assessing acute myeloid leukemia susceptibility in rearrangement-driven patients by DNA breakage at topoisomerase II and CCCTC-binding factor/cohesin binding sites. · Atkin ND, Raimer HM, Wang Z, Zang C, Wang YH · Genes Chromosomes Cancer 2021What this evidence cannot show
n=3 per arm, adult, treated remission samples. Cited here for feasibility - the input requirement - rather than for the biological result. Full text read via the PMC HTML route.
Evidence that argues against this (1)
- Their method maps breaks in purified genomic DNA rather than in nuclei, which the authors argue improves ligation efficiency but which also means breaks introduced during DNA purification are indistinguishable from biological ones. For a poison-free experiment where the expected signal is small, that is the dominant systematic error, and an in-nuclei method such as END-seq may be the safer choice despite the higher input. Canela A et al. 2016
the degron experiment written to falsify it
executes the gap that analysis names: no modern break-mapping method has been published on KMT2A in a blood progenitor
the same cell system, with the enzyme identified rather than assumed
the TOP2A degron arm answers which end each isoform supplies
a TDP2-inhibited poison-free arm raises lesion persistence without adding a poison
low-dose etoposide is the assay positive control reproducing the published TOP2B dependence