The 7SK elongation brake is not an off-chromatin reservoir: 7SK occupies enhancers and super-enhancers genome-wide, recruits BAF and repositions nucleosomes - which means NOT APPLICABLE is the wrong epigenetics annotation for LARP7, MEPCE and HEXIM2, and BOUNDED ABSENCE is the right one
also called 7SK-BAF axis · q181 the brake has a chromatin address
7SK is a small RNA best known as a parking space that holds the transcription accelerator in reserve, which made it look like something with no chromatin biology. In fact 7SK sits on enhancers all across the genome, pulls in a nucleosome-moving machine, and changes how DNA is packaged there.
The same thing, for a scientist
This beat's coverage triage found that three of the four 7SK snRNP subunits - LARP7, MEPCE and HEXIM2 - carry no epigenetics annotation of any kind, and the tempting resolution was to record a reasoned NOT APPLICABLE on the grounds that they act on an RNA rather than on a nucleosome. That resolution is wrong. Flynn and colleagues showed that 7SK occupies enhancers and super-enhancers genome-wide in mouse and human cells, physically interacts with the BAF chromatin-remodelling complex, recruits BAF to enhancers, and inhibits enhancer transcription by modulating nucleosome position; 7SK occupancy coincides with Brd4 and is sensitive to JQ1. So the particle these three proteins constitute has a documented, genome-wide chromatin address and a nucleosome-level mechanism. The correct annotation for all three is therefore a BOUNDED ABSENCE - the chromatin readout exists for the complex and has not been measured for the individual subunit in any KMT2A-rearranged system - which is a materially different statement from NOT APPLICABLE, and one that names an experiment.
What was shown
7SK inhibits enhancer transcription by modulating nucleosome position 1. Specifically:
- 7SK occupies enhancers and super-enhancers genome-wide in mouse and human cells.
- It is required to limit enhancer-RNA initiation and synthesis, in a manner distinct from promoter pausing -- so this is not the P-TEFb reservoir function re-described.
- Clustered elements at super-enhancers uniquely require 7SK to prevent convergent transcription and DNA-damage signalling.
- 7SK physically interacts with BAF and recruits it to enhancers.
- 7SK occupancy coincides with Brd4 and is exquisitely sensitive to JQ1.
The last point matters for this project directly: BET inhibition is an active modality in KMT2A-rearranged leukaemia, and it moves 7SK occupancy at enhancers.
Why this changes the annotation, and why I was about to get it wrong
ana-q181-the-only-chromatin-machinery-with-genuinely-nothing-is-the-ptefb-brake identifies eight entries as the sampling-free real gap, seven of them on the P-TEFb reserve/release axis. Its own closing section proposes that "for at least four of them the honest entry is a reasoned NOT-APPLICABLE: LARP7 and MEPCE act on an RNA, not on a nucleosome."
That is the error this entry corrects, and it is mine. The reasoning was about the subunit's direct enzymatic substrate, and the dimension asks about chromatin context. A protein whose complex occupies enhancers genome-wide and repositions nucleosomes there has a chromatin context whatever its own substrate is.
The distinction the corpus already draws on the localization dimension applies exactly [ana-q181-the-not-applicable-convention-exists-for-localization-and-not-for-epigenetics]:
| state | what it says | correct here? |
|---|---|---|
| RECORDED | we know the mark/locus | no -- not for the individual subunits |
| BOUNDED ABSENCE | the measurement exists to be made and has not been | yes |
| NOT APPLICABLE | there is nothing to measure | no -- and this was my first instinct |
This is the generalisable lesson of the beat. A NOT APPLICABLE is a strong claim about the literature and is exactly as falsifiable as a positive one. Reaching for it because a protein's direct substrate is not a histone is the same shortcut as reading a protein's compartment off its enzyme class. The three-state metric this beat recommends would be actively harmful if the third state were assigned this way.
What is NOT claimed
- No KMT2A-rearranged system. Flynn's work is in mouse and human cells, not leukaemia lines carrying a KMT2A fusion. This corpus's own
noncoding-rn7skentry already poses the matching open question -- "Is 7SK itself a chromatin-associated RNA at KMT2A fusion target genes... No 7SK CHART or ChIRP exists in any KMT2A-rearranged cell" -- and this entry does not answer it. - The occupancy is 7SK's, not each subunit's. No ChIP or CUT&RUN for LARP7, MEPCE or HEXIM2 at enhancers in any system was found in the searches behind this entry. Assigning the RNA's address to its protein partners is an inference, and it is the inference
mechanism-q181-larp7s-epigenetic-readout-is-ptefb-release-not-a-histone-markis careful to label as one. - No histone mark is claimed for any of the three. The mechanism is nucleosome positioning and BAF recruitment, not a writing, reading or erasing activity.
| epigenetic readout | Nucleosome repositioning at enhancers and super-enhancers, via recruitment of the BAF (SWI/SNF) remodelling complex by the 7SK snRNA scaffold. Not a histone mark: no writing, reading or erasing activity is involved. |
|---|---|
| epigenetic bounded absence | No ChIP, CUT&RUN or CHART/ChIRP has placed LARP7, MEPCE or HEXIM2 - or 7SK itself - at chromatin in any KMT2A-rearranged cell. The corpus's own noncoding-rn7sk entry poses this as an open question and it remains open. |
| epigenetics not applicable | false |
| not applicable rejected because | The tempting NOT APPLICABLE - 'these proteins act on an RNA, not a nucleosome' - is refuted by PMID 26878240, which gives the particle a genome-wide enhancer address and a nucleosome-level mechanism. The correct state is BOUNDED ABSENCE. |
| shown for fusion | NO - nobody has looked in any KMT2A fusion. Flynn 2016 is mouse and human non-leukaemic cells; no 7SK CHART or ChIRP exists in any KMT2A-rearranged line. |
marks named
[]
measurements
[
{
"quantity": "KMT2A-rearranged systems in which 7SK chromatin occupancy has been measured",
"value": 0,
"units": "systems",
"subject": "7SK CHART/ChIRP or LARP7/MEPCE/HEXIM2 ChIP in a KMT2A-fusion cell",
"universe": "the literature reachable by the PubMed searches recorded in this entry, 2026-09-24",
"null": null,
"note": "a bounded absence, not a negative result; the assay exists and has been run in other systems"
}
]- BET inhibitors are an active modality in KMT2A-rearranged leukaemia and JQ1 strips 7SK from enhancers. If 7SK's enhancer occupancy is part of what restrains pervasive enhancer transcription, a BET inhibitor could simultaneously reduce Brd4-driven elongation and release the 7SK brake - two effects with opposite signs on P-TEFb availability. Nobody has measured the net direction in this disease, and a compound whose two mechanisms partly cancel would look like a dose-response failure rather than a mechanistic one.
- A therapeutic strategy aimed at sequestering P-TEFb into 7SK would, on this evidence, also load BAF onto enhancers genome-wide. BAF subunits are recurrently mutated tumour suppressors and this corpus already holds SWI/SNF dependency entries for MLL-AF9 cells, so the off-target consequence of stabilising the brake is a global remodelling change, not a clean loss of elongation.
- Reading this entry as evidence that the 7SK subunits are now 'covered' would be the escape-hatch failure inverted: the complex has a chromatin address and the subunits still have no measurement in this disease. The bounded absence is the finding.
- Does 7SK occupy the KMT2A::AFF1 target enhancers - the HOXA cluster, MEIS1, IRX1 - in SEM or RS4;11? A CHART or ChIRP in one line would answer it and no such dataset exists. This is the corpus's own standing question in noncoding-rn7sk, restated here because this entry supplies the reason it is worth running.
- Is BAF occupancy at those enhancers 7SK-dependent with Brd4 held constant? The published causal direction rests on 7SK depletion without that control, and BET inhibition is an active modality in this disease, so the two are confounded exactly where it matters clinically.
- Do LARP7, MEPCE or HEXIM2 individually occupy chromatin, or is the address the RNA's alone? No subunit-level ChIP was found in any system.
- If 7SK occupancy at super-enhancers is JQ1-sensitive, does BET inhibition in a KMT2A-rearranged line release P-TEFb from 7SK as a side effect, and would that partly oppose the intended effect of the drug?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
7SK snRNA occupies enhancers and super-enhancers genome-wide in mouse and human cells, is required to limit enhancer-RNA initiation and synthesis in a manner distinct from promoter pausing, physically interacts with the BAF chromatin-remodelling complex and recruits BAF to enhancers, and inhibits enhancer transcription by modulating nucleosome position; 7SK occupancy at enhancers coincides with that of Brd4 and is exquisitely sensitive to the bromodomain inhibitor JQ1.
7SK-BAF axis controls pervasive transcription at enhancers · Flynn RA, Do BT, Rubin AJ, Calo E, Lee B, Kuchelmeister H, Rale M, Chu C, Kool ET, Wysocka J, Khavari PA, Chang HY · Nature Structural & Molecular Biology 23(3):231-8 2016What this evidence cannot show
Read at abstract level with the MeSH record; the full text was not retrieved. expected_absent_terms names KMT2A and MLL deliberately - the paper is NOT in a KMT2A-rearranged system and the claim explicitly does not extend there. Mouse and human cell lines, not primary leukaemia. The nucleosome-position result and the BAF interaction are separate experiments and the abstract does not state that BAF recruitment is necessary for the nucleosome effect.
Evidence that argues against this (1)
- The JQ1 sensitivity cuts against reading 7SK occupancy as an independent chromatin address: if 7SK is at enhancers because Brd4 is, then the occupancy is a property of the acetyl-reader network and 7SK is a passenger, and the causal arrow in 'recruits BAF to enhancers' would need the reverse experiment - BAF occupancy after 7SK depletion with Brd4 held constant - which the abstract does not describe. No paper contradicting the 7SK-BAF interaction was found in the searches run for this entry (PubMed, '7SK snRNA BAF chromatin remodeling enhancer pervasive transcription', 1 record), but a single-laboratory result with no independent replication located is weaker than the confidence label alone conveys.
gives the particle a genome-wide enhancer address and a nucleosome-level mechanism
supplies the non-leukaemic evidence behind that entry's open question on 7SK as a chromatin-associated RNA
refutes that entry's own proposal that LARP7/MEPCE should be annotated NOT APPLICABLE; the correct state is BOUNDED ABSENCE
the rejection of NOT APPLICABLE for LARP7 rests on the particle's enhancer address
- related to →Plain-English: chromatin and epigenetics — the sticky notes on the DNA, and which ones matter here
an enhancer-level, nucleosome-positioning mechanism with no histone mark involved