TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion-dependent transcription

Abstract

Gene rearrangement of the mixed lineage leukemia (MLL) gene causes leukemia by inducing the constitutive expression of a gene subset normally expressed only in the immature haematopoietic progenitor cells. MLL gene rearrangements often generate fusion products of MLL and a component of the AF4 family/ENL family/P-TEFb (AEP) complex. MLL-AEP fusion proteins have the potential of constitutively recruiting the P-TEFb elongation complex. Thus, it is hypothesized that relieving the promoter proximal pausing of RNA polymerase II is the rate-limiting step of MLL fusion-dependent transcription. AEP also has the potential to recruit the mediator complex via MED26. We recently showed that AEP activates transcription initiation by facilitating TBP loading to the TATA element through the SL1 complex. In the present study, we show that the key activity responsible for the oncogenic property of MLL-AEP fusion proteins is the TBP loading activity, and not the mediator recruitment or transcriptional elongation activities. Thus, we propose that TBP loading by AF4 through SL1 is the major rate-limiting step in MLL fusion-dependent transcription.

Transcriptional activation by MLL fusion proteins in leukemogenesis

“…Each subdomain contains an evolutionarily conserved motif: the DLXLS motif, the SDE motif, and the NKW motif. MED26 binds to the DLXLS motif, the SDE motif is responsible for binding to SL1, and the NKW motif is required for SL1-mediated transcriptional activation, presumably by loading TBP to the TATA element [82,92]. Although the artificial MTM-pSER fusion protein activates HOXA9 expression constitutively to immortalize hematopoietic progenitors, deletion of either the SDE motif or the NKW motif resulted in loss of transforming ability (Fig.…”

“…Because of known transcriptional elongation functions of P-TEFb [74,78,79], it was presumed that the recruitment of P-TEFb through AF4 triggers the expression of MLL target [71]. Their definition was slightly modified because the DLXLS motif, which was originally included in the ALF domain, was included in the pSER domain in our reports [82,92]. (B) Functions of various subdomains in the pSER domain depicted schematically.…”

“…The mediator complex facilitates the PIC formation of RNAP2 [91]. MED26 binds to the pSER domain of AF4 [92], which can be divided into three subdomains (Fig. 3B).…”

Cooperative gene activation by AF4 and DOT1L drives MLL-rearranged leukemia

“…The AF4 family proteins associate with the phosphorylated transcription elongation factor b (P-TEFb) complex and the ELL protein family, both of which demonstrate transcription elongation activities (18,19). Additionally, AF4 family proteins associate with selectivity factor 1 (SL1), containing TATA box-binding protein (TBP) and TBP-associated factor RNA polymerase I subunits A through D (TAF1A/B/C/D), to initiate RNA polymerase II-dependent (RNAP2-dependent) transcription, which is the critical step activated by MLL-AEP fusion proteins in leukemic transformation (20,21). Thus, AEP is a multifunctional coactivator that can facilitate both the initiation and elongation of transcription.…”

Differential expression of cyclin-dependent kinases in the adult human retina in relation to CDK inhibitor retinotoxicity

“…One drug, palbociclib (a CDK4/6 inhibitor), has been FDA approved, albeit with restricted clinical indications (Zhang et al 2017). Highly selective CDK9 inhibitors are also of interest as cancer therapeutics (Franco et al 2018), with proposed mechanisms via the regulation of RNA Polymerase II (Huang et al 2014;Lu et al 2015;Okuda et al 2016;Rahl et al 2010;Samarakkody et al 2015). NVP-1 and NVP-2 are selective CDK9 inhibitors (Barsanti 2011) which, interestingly, display differential neurotoxicity (Sutton 2014).…”