SIAH1/SIAH2: the disposal route that destroys wild-type AF4 and that the reciprocal fusion escapes
also called SIAH1 AF4 degradation · AF4.MLL proteasomal protection · SIAH2 and AF4-MLL
Normally the cell keeps AF4 levels down by tagging it for destruction through a pair of enzymes called SIAH1 and SIAH2. The mirror-image fusion protein carries the tag-binding piece but somehow avoids being destroyed, so it accumulates.
The same thing, for a scientist
The AF4 N-terminus - exactly the fragment AFF1 contributes to the reciprocal product - is the region that binds the E3 ubiquitin ligases SIAH1 and SIAH2, and the AF4.MLL fusion protein is protected against proteasomal degradation by that route while wild-type AF4 is not; both wild-type AF4 and AF4.MLL showed growth-transforming potential in the same study [E1]. This is one of two published stability mechanisms for the reciprocal product, the other being Taspase1-dependent complex formation [E2], and the two are not obviously the same story: one says the protein is protected from an E3 ligase, the other says it is degraded unless a protease cuts it. Nobody has reconciled them in one system. A third, newer layer says the abundance of both t(4;11) transcripts is set post-transcriptionally by IGF2BP3, which lowers KMT2A::AFF1 protein and raises AFF1::KMT2A [E3] - so reciprocal protein dose is a regulated variable, not a consequence of allele presence.
Why the disposal route matters for a protein nobody can detect in patients
Every clinical reciprocal-status measurement is RT-PCR on transcript. If the protein's abundance is set by a degradation switch - SIAH binding, Taspase1 cleavage, IGF2BP3 - then transcript-positive and protein-positive are different populations, and the prognostic association measured on transcript is measuring the wrong variable. This is a testable, unglamorous explanation for why a reciprocal-positive patient might do better or worse than the transcript predicts, and it has never been checked.
Three stability mechanisms, not one, and they have never been put in the same experiment
SIAH1/2 protection 1; Taspase1-dependent assembly into a long-lived complex 2; IGF2BP3-mediated control of transcript and protein abundance with opposite signs for the two fusion products 3. Each is from a different group with a different system. The honest summary is that reciprocal protein level is known to be regulated and not known to be predictable.
| kind | protein stability control |
|---|---|
| binding region | AFF1 N-terminus (the fragment retained in the reciprocal product) |
| measured in patients | false |
e3 ligases
[ "SIAH1", "SIAH2" ]
competing mechanisms
[ "Taspase1-dependent complex stabilisation", "IGF2BP3 post-transcriptional control" ]
- Anything that stabilises or destabilises the two t(4;11) products moves them in opposite directions on the published IGF2BP3 data, so a drug lowering the direct oncoprotein may raise the reciprocal one.
- SIAH1/2 are general E3 ligases with many substrates; targeting them for a fusion-stability effect is not a selective proposition.
- Every mechanism in this entry is measured on transfected protein. None has been demonstrated on an endogenous reciprocal product, because no reagent distinguishes it.
- Is SIAH-mediated degradation of wild-type AF4 actually operating in t(4;11) blasts, where wild-type AFF1 is also present from the untranslocated allele?
- Do the SIAH and Taspase1 accounts describe the same stabilisation event or two different ones? One experiment expressing AF4-MLL with SIAH knockdown and with Taspase1 knockdown in parallel would settle it.
- If IGF2BP3 raises the reciprocal product and lowers the direct one, does IGF2BP3 inhibition convert a favourable reciprocal-positive phenotype into an unfavourable one? The therapeutic implication has the opposite sign to the one usually assumed.
- E1moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
The oncogenic mechanism of AF4 and AF4.MLL involves the E3 ubiquitin ligases SIAH1 and SIAH2 acting through the N-terminal portion of AF4, with the AF4.MLL fusion protein protected against proteasomal degradation; both AF4 wild-type and AF4.MLL show growth-transforming potential.
Interaction of AF4 wild-type and AF4.MLL fusion protein with SIAH proteins: indication for t(4;11) pathobiology? · Bursen A, Moritz S, Gaussmann A, et al. · Oncogene 2004What this evidence cannot show
2004 transfection work in non-haematopoietic cells with growth-based transformation readouts; no measurement in a t(4;11) patient sample and no in vivo model. The mechanism is asserted as an indication for t(4;11) pathobiology in the paper's own title.
Evidence that argues against this (1)
- A study that removed the reciprocal transcript from two human t(4;11) lines found no growth, cell-cycle or apoptotic phenotype, which is hard to reconcile with a protein whose accumulation is supposed to be transforming. Kumar AR et al. 2011
- E2moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
A separate stability mechanism is reported for the same protein: uncleaved AF4-MLL is subject to proteasomal degradation and Taspase1 cleavage converts it into a stable multiprotein complex with a very long half-life.
Unraveling the Activation Mechanism of Taspase1 which Controls the Oncogenic AF4-MLL Fusion Protein. · Sabiani S, Geppert T, Engelbrecht C, et al. · EBioMedicine 2015What this evidence cannot show
Transfected-construct biochemistry; the two stability accounts (SIAH protection versus cleavage-dependent stabilisation) have never been tested against each other in one system, and both come from the same laboratory group.
Evidence that argues against this (1)
- Whether either mechanism operates on an endogenous reciprocal protein is unaddressed, because no assay distinguishing the reciprocal-derived MLL-C fragment from the wild-type one exists; the one immunoprecipitation study assigning an AF4-MLL band did so on apparent molecular weight. Wilkinson AC et al. 2013
- E3weak confidence 2 of 5: Preliminary, small, indirect, or not yet peer reviewed.paper
IGF2BP3 binds both t(4;11) fusion transcripts and reduces KMT2A::AFF1 protein abundance while increasing AFF1::KMT2A levels, so the ratio of the two products is set post-transcriptionally.
Influence of IGF2BP3 on t(4;11) fusion mRNA and protein abundance. · Hanewald T, Benz T, Marschalek R · Leuk Res 2026What this evidence cannot show
Engineered model systems rather than primary blasts, and a 2026 report whose independent replication does not yet exist. Direction of effect on the reciprocal product is the opposite of the direction on the direct product, which is the part that should be replicated before it is relied on.
Evidence that argues against this (1)
- No reagent distinguishes the reciprocal-derived protein from wild-type MLL-C in a t(4;11) cell, so a reported change in AFF1::KMT2A protein level rests on engineered systems; the one immunoprecipitation study assigning an AF4-MLL band in SEM and RS4;11 did so on apparent molecular weight. Wilkinson AC et al. 2013
Sets the abundance of the reciprocal oncoprotein through the AFF1 N-terminal fragment it retains.
The SIAH-binding region is the AFF1 N-terminus.
A second, unreconciled account of the same protein's stability.