The clearance clock has machinery, and the machinery says the clock does NOT stop in a non-dividing cell - a postmitotic heart replaces its histones with a half-life of about two weeks
also called histone chaperone network as the clearance clock · replication-independent turnover in quiescent cells · who clears H3K79 when the cell is not dividing
This project argues that the chemical tag its main drug target writes can only disappear when the histone carrying it is swapped out, so a slowly dividing cell should hold on to the tag and resist the drug. The machinery that does the swapping turns out to run even in cells that never divide at all: in adult heart muscle, histones are replaced with a half-life of about two weeks.
The same thing, for a scientist
`mechanism-h3k79-turnover-and-dilution` establishes that H3K79 methylation has one writer and no eraser, so it is lost only through histone replacement, and draws the clinical consequence that a quiescent leukaemia-initiating cell should retain the mark longest - its 'sharpest bite-back in the whole beat'. **This entry supplies the machinery for that claim and, in doing so, qualifies it.** Four things the chaperone literature adds: 1. **Replication-independent turnover is fast in a cell that never divides.** In postmitotic adult mouse cardiomyocytes, H2B-GFP pulse-chase gives a histone **half-life of approximately two weeks**, which the authors describe as contrasting dramatically with the long lifetime of the cell; rapid-turnover regions correspond to cis-regulatory domains of heart genes [E1]. **A non-dividing cell is not a cell with frozen chromatin.** 2. **Turnover is actively regulated rather than passive.** Polycomb EED augments turnover at enhancers by engaging the BAF complex for nucleosome exchange from the free histone pool [E1], and H3K79 valency tracks turnover rate genome-wide [E2]. 3. **The writer is itself a chaperone.** Yeast Dot1p has intrinsic histone chaperone activity, regulates nucleosome dynamics via histone exchange, can assemble core histones into nucleosomes, and is **particularly important for histone exchange and chromatin accessibility in the transcribed regions of long genes**; a methylation-independent Dot1p function is required for cryptic transcription control [E3]. **DOT1 does not merely read turnover - it causes some of it.** 4. **There is no known eviction chaperone for H3.3**, while H2A.Z has one (ANP32E). If that asymmetry is real, H3.3 removal in a gene body is driven by transcription itself, which makes the clock at a fusion target gene a function of how hard the fusion is driving elongation. **Net effect on the graph's argument: the clearance-clock objection to DOT1L inhibitors survives, but the quiescence version of it is weaker than stated.**
What survives, what needs qualifying, and what is now a new prediction
Survives.
- H3K79 methylation is lost only by histone replacement. Nothing in the chaperone literature contradicts this.
- Replacement rate, not enzyme inhibition, sets how fast the mark disappears once the enzyme is blocked.
- Slow-turnover chromatin clears the mark last. Turnover rate varies by locus class, and cis-regulatory domains turn over fastest 12.
Needs qualifying.
- "A quiescent cell replaces its histones rarely and therefore retains H3K79 methylation longest." The best direct measurement in a postmitotic tissue gives a histone half-life of ~2 weeks 1. Quiescence removes the replication-coupled route and leaves the replication-independent route running. The correct statement is not 'the clock stops' but 'the clock loses one of its two hands', and how much that slows it is a number nobody has measured in a leukaemic cell.
- The word 'dilution' in
mechanism-h3k79-turnover-and-dilutionimplies the replication-coupled route. Half the clock is exchange, which is not dilution.
New prediction, which is testable and cheap. If DOT1L itself contributes chaperone activity at long transcribed genes 3, then a SAM-competitive DOT1L inhibitor (pinometostat) and a DOT1L degrader are predicted to differ: the inhibitor blocks methylation while leaving the protein, and therefore its exchange activity, in place; a degrader removes both and would be expected to slow the very turnover that clears the mark. That is a direct, falsifiable difference between two drug modalities on the same target, and it has never been tested. Yeast Dot1p lacks a SET domain and so does human DOT1L, so the structural basis for the transfer is at least not absent - but the chaperone activity itself has only been shown in yeast.
Which chaperone runs which hand of the clock
| route | rate set by | machinery | what stops it |
|---|---|---|---|
| replication-coupled dilution | cell division rate | CAF-1 (gene-chaf1a, gene-chaf1b) with PCNA; ASF1 upstream | quiescence |
| replication-independent exchange, gene bodies | transcription rate; local turnover | gene-supt6h with Pol II; FACT; protein-huca-complex; gene-asf1b in blood; possibly DOT1L itself | transcriptional silence |
| replication-independent exchange, heterochromatin | not characterised | gene-atrx/gene-daxx | not characterised |
| supply of new histone | soluble pool size | gene-nasp; Cyclin E/CDK2 burst in S phase | quiescence (no S-phase burst) |
The fourth row is the one that could rescue the original quiescence argument. A quiescent cell has no S-phase histone biosynthesis burst, so even if its exchange machinery runs, its supply of new histone may be limited. Nobody has measured the soluble histone pool in a quiescent leukaemic cell. That is the experiment that would decide whether cell-state-quiescence and cell-state-relapse-initiating-persister are genuinely protected from a DOT1L inhibitor.
| mechanism class | rate mechanism / drug scheduling |
|---|---|
| extends | mechanism-h3k79-turnover-and-dilution |
| falsifiable prediction | a DOT1L degrader should clear H3K79me2 more SLOWLY than a SAM-competitive DOT1L inhibitor at long transcribed genes, if DOT1L's intrinsic chaperone activity transfers from yeast to human |
measured numbers
[
{
"quantity": "histone half-life in postmitotic adult mouse cardiomyocytes",
"value": "approximately 2 weeks",
"method": "H2B-GFP pulse-chase",
"pmid": "31104571",
"title": "Replication-Independent Histone Turnover Underlines the Epigenetic Homeostasis in Adult Heart.",
"authors": "Li Y, Ai S, Yu X, et al.",
"venue": "Circ Res",
"year": 2019,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
}
]unmeasured numbers
[ "nucleosome turnover rate at KMT2A-fusion target genes in any leukaemic cell", "soluble H3-H4 pool size in a quiescent leukaemic cell", "H3-specific (as opposed to H2B) turnover half-life in any non-dividing cell", "whether H3K79me2 clearance under DOT1L inhibition differs between a SAM-competitive inhibitor and a degrader" ]
machinery
{
"replication_coupled": [
"gene-chaf1a",
"gene-chaf1b",
"gene-asf1a",
"gene-asf1b"
],
"replication_independent_gene_body": [
"gene-supt6h",
"protein-fact-complex",
"protein-huca-complex",
"gene-asf1b",
"gene-hira"
],
"replication_independent_heterochromatin": [
"gene-atrx",
"gene-daxx"
],
"supply": [
"gene-nasp"
],
"eviction": [
"gene-anp32e (H2A.Z only; no H3.3 equivalent known)"
]
}- If replication-independent turnover runs in non-dividing cells, then the reassuring version of this argument - that a DOT1L inhibitor simply needs longer in a quiescent cell - is not available either: the mark also cannot be re-established quickly in recovering normal marrow, so toxicity has the same long time constant as efficacy.
- A degrader-based DOT1L programme, which the field has moved toward for other targets, could be WORSE than an inhibitor on this argument, because removing the protein removes its exchange activity as well as its catalysis.
- Anything that slows nucleosome turnover is a resistance mechanism requiring no mutation in DOT1L: quiescence, chromatin compaction, differentiation arrest and reduced transcription all qualify, and all are things a leukaemia does under treatment.
- Measure nucleosome turnover at HOXA9 in a KMT2A-rearranged line and relate it to the rate of H3K79me2 loss under DOT1L inhibition. This is the number the graph's dosing argument rests on and it returns 0 records.
- Does a quiescent leukaemic cell have a soluble histone pool large enough to support exchange? If not, the original quiescence bite-back is rescued by supply rather than by division rate.
- Do a DOT1L inhibitor and a DOT1L degrader clear H3K79me2 at different rates? If DOT1L is itself a chaperone at long genes, they should - and this graph's whole argument about schedule would then depend on modality.
- Is there an H3.3 eviction chaperone? If not, H3.3 removal in a gene body is transcription-driven and the fusion sets its own clock.
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
In postmitotic cardiomyocytes from adult mouse heart, H2B-GFP pulse-and-chase measurement gives a short histone half-life of approximately two weeks, which contrasts dramatically with the long lifetime of the cardiomyocyte; rapid histone-turnover regions correspond to cis-regulatory domains of heart genes; recruitment of chromatin modifiers including Polycomb EED correlates positively with histone turnover rate at enhancers; and EED augments histone turnover to restrain enhancer overactivation by directly interacting with and engaging the BAF complex for nucleosome exchange from the free histone pool.
Replication-Independent Histone Turnover Underlines the Epigenetic Homeostasis in Adult Heart. · Li Y, Ai S, Yu X, et al. · Circulation Research 2019What this evidence cannot show
Abstract read from efetch 2026-09-20; full text not read (no PMCID in the record). **Mouse cardiomyocytes, not leukaemia and not a quiescent haematopoietic cell.** A postmitotic cardiomyocyte and a quiescent leukaemia-initiating cell are both non-dividing and are otherwise nothing alike - the cardiomyocyte is a huge, transcriptionally active, metabolically extreme cell. The half-life is bulk H2B-GFP, so it is an H2A-H2B measurement rather than an H3-H4 one, and H3-H4 tetramers are the more stable species; the number should not be transferred to H3.3 without saying so. This entry uses it to establish that replication-independent turnover is substantial in a non-dividing cell, which is what it shows.
Evidence that argues against this (1)
- H2A-H2B dimers exchange far more readily than H3-H4 tetramers, so a two-week H2B half-life is an upper bound on the speed of H3.3 replacement and could substantially overstate how fast an H3K79-bearing H3 is replaced in the same cell. The H3-specific measurement in a postmitotic cell has not been made. Arrieta A et al. 2022
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Using chemical-induced proximity, the valency of H3K79 methylation is determined by nucleosome turnover rates, propagation of the mark across the genome is predicted by nucleosome turnover simulations, and turnover accounts for the asymmetric distribution of H3K79me toward the transcriptional unit.
Nucleosome Turnover Regulates Histone Methylation Patterns over the Genome. · Chory EJ, Calarco JP, Hathaway NA, et al. · Molecular Cell 2019What this evidence cannot show
Abstract read from efetch 2026-09-20; full text not read (PMC6510544 exists). Identifiers re-fetched for this entry rather than copied from `mechanism-h3k79-turnover-and-dilution`, which already carries this paper - per this project's rule that an identifier in the graph is unverified until this session fetched it. Chemical-induced proximity is an artificial recruitment regime and the propagation conclusions rest on simulation fitted to those data. Not a leukaemic system and no KMT2A fusion.
Evidence that argues against this (1)
- A turnover-only model cannot explain a locus where the enzyme is tethered at high local concentration by an oncogenic fusion: there, re-methylation after exchange is near-immediate and the state distribution should decouple from turnover rate. Graded weakening of the DOT1L-AF9 interaction changes the mark at target genes without changing nucleosome turnover. Kuntimaddi A et al. 2015
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Yeast Dot1p, which lacks a SET domain and does not methylate free histones, displays histone chaperone activity and regulates nucleosome dynamics via histone exchange; a methylation-independent function of Dot1p is required for control of cryptic transcription within transcribed regions following disruption of the Set2-Rpd3S pathway; Dot1p can assemble core histones into nucleosomes and facilitate ATP-dependent chromatin-remodelling activity through its nucleosome-binding domain in vitro; and global analysis indicates Dot1p is particularly important for histone exchange and chromatin accessibility in the transcribed regions of long genes.
Dot1 regulates nucleosome dynamics by its inherent histone chaperone activity in yeast. · Lee S, Oh S, Jeong K, et al. · Nature Communications 2018What this evidence cannot show
Abstract read from efetch 2026-09-20; full text not read (PMC5770421 exists). **Saccharomyces cerevisiae throughout, with in vitro reconstitution.** Human DOT1L also lacks a SET domain, but no chaperone activity has been demonstrated for the human protein and this entry does not claim one. The prediction drawn in the sections - that a DOT1L inhibitor and a DOT1L degrader should differ in how fast H3K79me clears - rests on this transfer holding, and it is recorded as a prediction for exactly that reason.
Evidence that argues against this (1)
- Yeast Dot1p and human DOT1L differ in the property this graph cares about most: recruitment. Human DOT1L is recruited to KMT2A-fusion targets through AF9/ENL and AF4-family partners that have no yeast counterpart, so a yeast result about intrinsic chaperone activity in long genes may be irrelevant at a fusion-occupied locus where the enzyme's concentration is set by tethering. Kuntimaddi A et al. 2015
- E4established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.database
As of 2026-09-20, PubMed returns 0 records for '"nucleosome turnover"[tiab] AND (quiescent OR quiescence OR non-dividing)' and 6 for '("histone turnover"[tiab] OR "histone exchange"[tiab]) AND (quiescent OR quiescence OR postmitotic OR non-dividing)', of which the relevant one is the adult-heart study; '"nucleosome turnover"[tiab] AND (MLL OR KMT2A)' returns 0.
NCBI E-utilities esearch over PubMed for nucleosome and histone turnover in non-dividing cells and against KMT2A, run 2026-09-20 with query translations recorded · US National Library of Medicine 2026What this evidence cannot show
Query translations captured; no [UID] terms. **Both singular and plural forms were not run for every phrase**, and this project has documented that a multi-word [tiab] term is matched as an unstemmed phrase so singular and plural are different queries - 'nucleosome turnovers' was not searched. The six hits were title-checked. The zero for nucleosome turnover in KMT2A is the load-bearing absence: **the number the graph's dosing argument needs has not been measured in this disease.**
Evidence that argues against this (1)
- A phrase-bounded zero cannot see a study that measured turnover without using the phrase - for instance by calibrated ChIP of newly synthesised histone, by SNAP-tag pulse-chase, or by CATCH-IT - and those are the methods a turnover study would most likely use. Li Y et al. 2019
supplies the machinery and qualifies the quiescence claim
replication-independent turnover continues in non-dividing cells; histone half-life ~2 weeks in postmitotic heart
the compartment a turnover-limited drug reaches last, qualified
yeast Dot1p has intrinsic histone chaperone activity at long transcribed genes
- related to →Pinometostat (EPZ-5676) — the DOT1L inhibitor, and the most instructive failure in this field
predicts a degrader and a SAM-competitive inhibitor clear the mark at different rates
soluble H3-H4 supply is the fourth term in the turnover rate
replication-coupled arm of the clock
replication-independent gene-body arm of the clock
supplies the unmeasured number the clearance-clock argument rests on