Junction RT-PCR is the one routine assay in this disease that IS fusion-specific - and it is structurally blind to KMT2A*
also called fusion transcript RT-PCR attribution · MRD RT-PCR attribution · RNA-level attribution
The test used every day in clinics to find and track this leukaemia reads the exact point where the two genes are joined, so a positive result can only come from the fusion. Its blind spots are the things that have no join at all - including a shortened version of KMT2A made from a promoter inside the gene.
The same thing, for a scientist
Against the reagent failures catalogued across this beat, junction RT-PCR stands out: **it is genuinely fusion-specific, and it is the standard of care.** A primer pair spanning the fusion junction amplifies a template that exists only if the rearrangement is transcribed; neither wild-type KMT2A mRNA nor wild-type AFF1 mRNA carries both primer sites. The European BIOMED-1 Concerted Action standardised exactly this for nine aberrations including t(4;11) with MLL-AF4, with external, nested and independently shifted primer sets and a designed confirmation step [E1]. It is the RNA-level analogue of the junction peptide this beat computed, and it has the properties the peptide has: uniqueness by construction, no cross-reactivity to worry about, and an assay design that is cheap and already deployed. **What it can attribute.** The presence of a specific fusion transcript, its partner identity, its breakpoint-defined variant, and - quantitatively - its abundance relative to a control gene. This is why the KMT2A recombinome is a transcript-level resource: partner assignment and the spliced-versus-reciprocal distinction are RT-PCR and sequencing calls [E2]. **What it cannot attribute, in four parts.** 1. **KMT2A\* has no junction at all.** The gene-internal promoter that coincides with breakpoint cluster region hot spot II drives an mRNA encoding an N-terminally truncated MLL protein retaining the C-terminal regions including the SET domain [E3]. There is no partner sequence in it, so no junction primer pair can amplify it. Junction RT-PCR is **structurally blind** to KMT2A\*, not merely insensitive - and KMT2A\* is a species whose promoter sits inside the very region the breakpoints cluster in, so its expression is plausibly altered by the rearrangement itself. Nobody measures it. 2. **A targeted panel returns negative for every partner it was not designed around.** BIOMED-1 covers t(4;11) with MLL-AF4; the recombinome now lists well over a hundred partners [E2]. An absence of testing is not a negative result. 3. **RNA is not protein.** A transcript-positive patient is not thereby shown to make fusion protein, and none of the protein-level attribution problems in this beat are touched by an RNA assay. 4. **Reciprocal status is a transcript call.** The spliced-fusion classification that this graph uses as a natural experiment - 241 of 460 KMT2A::MLLT1 cases expressing no reciprocal transcript [E2] - is defined on RT-PCR, so a low-abundance or alternatively spliced reciprocal message could be missed and scored as absent.
Why KMT2A* is the interesting blind spot rather than a technicality
The other three limitations are the ordinary limitations of any targeted assay. KMT2A* is different because of where its promoter is.
The gene-internal promoter colocalises with breakpoint cluster region hot spot II, and the paper that found it proposes that the genomic instability of KMT2A is linked to transcription through that region 3. So the same transcriptional activity that may help create the translocation also creates a protein species that every assay in this disease is blind to: junction RT-PCR cannot see it because it has no junction, and an anti-MLL-C antibody cannot separate it from wild-type MLL-C or from the reciprocal product (concept-the-mll-c-ambiguity-across-fusion-families-and-kmt2a-star).
KMT2A* carries the SET domain. A catalytically competent, N-terminally truncated KMT2A species in a rearranged blast is not a small thing to be unable to count.
What would make KMT2A* countable
- RNA: a 5'-RACE or a promoter-specific RT-qPCR using a forward primer in the gene-internal first exon. No junction needed, because the species is defined by its transcription start site rather than by a join. This is a primer-design exercise, not a new technology.
- Protein: the species is N-terminally truncated, so it has a unique N-terminus and therefore a unique tryptic peptide at that end - exactly the same logic that makes the fusion junction peptide unique. A targeted PRM method for the KMT2A* N-terminal peptide would count it directly. This beat did not compute that peptide; it is the obvious extension of
ana-junction-peptide-detectability-kmt2a-fusions. - Neither requires a new antibody, which is the recurring finding of this beat.
| reagent class | junction-spanning RT-PCR of fusion transcripts |
|---|---|
| fusion attributable | yes, at RNA level, by construction |
can attribute
[ "presence of a specific fusion transcript", "partner identity", "breakpoint-defined transcript variant", "relative transcript abundance for MRD" ]
cannot attribute
[ "KMT2A* - no junction exists, so it is structurally invisible", "any partner the primer panel was not designed around", "fusion protein presence or abundance", "a reciprocal transcript with unexpected splice structure, which is scored as absent" ]
kmt2a star
{
"origin": "gene-internal promoter coinciding with breakpoint cluster region hot spot II",
"product": "N-terminally truncated MLL retaining the C-terminus including the SET domain",
"junction": "none",
"source_pmid": "16983345",
"countable_by": [
"promoter-specific RT-qPCR or 5' RACE from the gene-internal first exon",
"targeted PRM on the unique N-terminal tryptic peptide of the truncated protein"
]
}standardisation
{
"study": "BIOMED-1 Concerted Action",
"aberrations": 9,
"includes_t_4_11": true,
"laboratories": 14,
"countries": 8,
"source_pmid": "10602411"
}- MRD monitoring by junction RT-PCR would not detect a relapse driven by a KMT2A-derived species that has no junction, so a patient could be scored MRD-negative while carrying an expanding clone that the assay is structurally unable to see.
- If KMT2A* is SET-competent and is present in blasts, a therapy that removes the fusion leaves a catalytically active KMT2A species behind, which is a resistance route nobody is monitoring.
- A menin inhibitor acts on the fusion's N-terminal menin-binding motif; KMT2A* lacks that motif entirely, so it is intrinsically drug-resistant by construction, and its contribution to residual disease is unmeasured in every trial run so far.
- How much KMT2A* transcript and protein is there in a KMT2A-rearranged blast, and does the rearrangement change it? Its promoter sits inside the breakpoint cluster region, so a rearrangement could plausibly alter its output, and no assay in routine or research use can see it.
- Is there a unique tryptic peptide at the KMT2A* N-terminus, and is it in the detectable mass window? The same computation that produced the fusion junction peptide would answer it, and it would make the third KMT2A-derived species countable for the first time.
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
The European BIOMED-1 Concerted Action standardised RT-PCR for nine well-defined chromosomal aberrations with fusion gene transcripts, including t(4;11) with MLL-AF4, designing external, nested and independently shifted primer sets so that a positive result can be confirmed or excluded by a second amplification, and validated them across 14 laboratories in eight countries using 17 cell lines and patient samples.
Standardized RT-PCR analysis of fusion gene transcripts from chromosome aberrations in acute leukemia for detection of minimal residual disease. Report of the BIOMED-1 Concerted Action: investigation of minimal residual disease in acute leukemia. · van Dongen JJ, Macintyre EA, Gabert JA, et al. · Leukemia 1999What this evidence cannot show
Abstract read in full via the Europe PMC core record; no PMCID exists and the full text was not retrieved, so the specific t(4;11) primer positions and the sensitivity achieved for that aberration are not quoted here. A 1999 standard; assay chemistry has moved to real-time and digital PCR, but the junction-spanning design principle is unchanged.
Evidence that argues against this (1)
- A junction-spanning primer pair is specific but not universal: breakpoint heterogeneity means a primer set designed around the commonest KMT2A intron 9-11 breakpoints can fail on a patient whose fusion joins different exons, producing a false negative that looks like disease clearance. The recombinome's breakpoint distributions are the resource for bounding that risk. Meyer C et al. 2023
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Partner assignment, breakpoint variant and the spliced-versus-reciprocal distinction in KMT2A leukaemia are transcript-level calls, with about 241 of 460 KMT2A::MLLT1 fusions classified as spliced fusions expressing only the direct fusion transcript and no reciprocal one.
The KMT2A recombinome of acute leukemias in 2023. · Meyer C, Larghero P, Almeida Lopes B, et al. · Leukemia 2023What this evidence cannot show
The 241-of-460 figure is recorded in this graph's concept-the-mll-c-ambiguity-across-fusion-families-and-kmt2a-star as read verbatim from PMC10169636; the identifiers here were re-fetched from PubMed rather than copied from that entry. A transcript-level classification cannot establish the absence of a protein.
Evidence that argues against this (1)
- A reciprocal-negative call made by RT-PCR is an absence of amplification, and an absence of amplification has the same status as any other counted absence: it is bounded by the primer set. A reciprocal transcript with an unexpected splice structure would be scored negative, and the reciprocal-negative arm this graph relies on as a natural experiment would then be contaminated. van Dongen JJ et al. 1999
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
A gene-internal promoter coinciding with KMT2A breakpoint cluster region hot spot II drives an mRNA encoding an N-terminally truncated MLL protein retaining the C-terminal functional regions including the SET domain, and etoposide-induced double-strand breaks colocalise with the RNA polymerase II binding site and transcription initiation region rather than with a nearby topoisomerase II consensus.
Transcription linked to recombination: a gene-internal promoter coincides with the recombination hot spot II of the human MLL gene. · Scharf S, Zech J, Bursen A, et al. · Oncogene 2007What this evidence cannot show
Abstract read in full via the Europe PMC core record; no PMCID and the full text was not reached, so the abundance of the KMT2A* transcript or protein in leukaemic blasts, and whether it is altered by the rearrangement, are not established. The structural claim used here - that the transcript contains no partner sequence and therefore no junction - follows from its being a KMT2A-internal transcript and does not require the full text.
Evidence that argues against this (1)
- If KMT2A* is expressed at trivial levels in blasts, its invisibility to junction RT-PCR matters to nobody. The counter to that is that nobody has measured it, precisely because every assay in the disease is blind to it - so the claim that it is trivial is currently as unevidenced as the claim that it is important. Meyer C et al. 2023
adds the RNA-level half: junction RT-PCR is structurally blind to KMT2A* because KMT2A* has no junction
the one routine assay in this disease that attributes to this fusion by construction
- analogous to →In-silico digest of 81 modelled KMT2A fusion junctions: the peptide that would prove the fusion protein exists
the protein-level analogue: uniqueness by construction at the join, in peptide rather than amplicon
KMT2A* is a C-only species that biases the MLL-N minus MLL-C subtraction and is uncountable by RNA junction assays
KMT2A* lacks the menin-binding motif entirely, so it is drug-resistant by construction and is not monitored in any trial
- ← related toMRD by fusion transcript: fast, standardised, partner-limited - and structurally blind to KMT2A*
same assay, attribution side; both structurally blind to KMT2A*