Runt-related transcription factor 1 (RUNX1) also known asacute myeloid leukemia 1 protein (AML1) orcore-binding factor subunit alpha-2 (CBFA2) and it is aprotein that is encoded by theRUNX1gene, in humans.[5][6]
In humans, the gene RUNX1 is 260 kilobases (kb) in length, and is located on chromosome 21 (21q22.12). The gene can betranscribed from 2 alternativepromoters, promoter 1 (distal) or promoter 2 (proximal). As a result, variousisoforms of RUNX1 can be synthesized, facilitated byalternative splicing. The full-length RUNX1 protein is encoded by 12exons. Among the exons are two defined domains, namely the runt homology domain (RHD) or therunt domain (exons 2, 3 and 4), and thetransactivation domain (TAD) (exon 6). These domains are necessary for RUNX1 to mediate DNA binding and protein-protein interactions respectively. The transcription of RUNX1 is regulated by 2enhancers (regulatory element 1 and regulatory element 2), and these tissue specific enhancers enable the binding oflymphoid orerythroid regulatory proteins, therefore the gene activity of RUNX1 is highly active in thehaematopoietic system.
The protein RUNX1 is composed of 453 amino acids. As a transcription factor (TF), its DNA binding ability is encoded by the runt domain (residues 50 – 177), which is homologous to thep53 family. The runt domain of RUNX1 binds to the coreconsensus sequence TGTGGNNN (where NNN can represent either TTT or TCA).[11] DNA recognition is achieved by loops of the 12-strandedβ-barrel and theC-terminus "tail" (residues 170 – 177), which clamp around the sugar phosphate backbone and fits into themajor and minor grooves of DNA. Specificity is achieved by making direct or water-mediated contacts with the bases. RUNX1 can bind DNA as amonomer, but its DNA binding affinity is enhanced by 10 fold if it heterodimerises with the core binding factor β (CBFβ), also via the runt domain. In fact, the RUNX family is often referred to as α-subunits, together with binding of a common β-subunit CBFβ, RUNX can behave as heterodimeric transcription factors collectively called thecore binding factors (CBFs).
The consensus binding site for CBF has been identified to be a 7 bp sequence PyGPyGGTPy. Py denotespyrimidine which can be eithercytosine orthymine.[12]
Christiane Nüsslein-Volhard andEric F. Wieschaus discovered the transcription factor RUNX in a screen that was conducted to identify mutations that affect segment number and polarity inDrosophila melanogaster.[13] The mutation that led to presegmentation patterning defects and runted embryos was namedrunt. Following this discovery, theDrosophila segmentation generunt was cloned by Gergen et al. Although the protein encoded byrunt was demonstrated to exhibit nuclear translocation, it was not yet established that this protein is a transcription factor.[14] Subsequently, in 1991, Ohki et al. cloned the humanRUNX1 gene; RUNX1 was found to be rearranged in the leukemic cell DNAs from t(8;21)(q22;q22)acute myeloid leukemia patients.[15] However, the function of human RUNX1 was not established. Soon after the discovery of the drosophila runt protein and the human RUNX1 protein, RUNX1's function was discovered. RUNX1 was purified as a sequence-specificDNA-binding protein that regulated the disease specificity of the Moloney murine Leukemia virus.[16] Furthermore, Ito et al. purifiedRUNX2, the homolog of RUNX1.[17] Purified transcription factors consisted of two subunits, a DNA binding CBFα chain (RUNX1 or RUNX2) and a non-DNA-binding subunit called core binding factor β (CBFβ); the binding affinity of RUNX1 and RUNX2 was significantly increased by association with CBFβ.[17][18][19]
Mice embryos with homozygous mutations on RUNX1 died at about 12.5 days. The embryos displayed lack of fetal liver hematopoiesis.[20]
Similar experiments from a different research group demonstrated that the knockout embryos die between embryonic days 11.5 and 12.5 due to hemorrhaging in the central nervous system (CNS).[21]
RUNX1 plays a crucial role in adult (definitive)haematopoiesis during embryonic development. It is expressed in all haematopoietic sites that contribute to the formation of haematopoietic stem and progenitor cells (HSPCs), including the yolk sac,[22]allantois, placenta, para-aortic splanchnopleura (P-Sp; (the visceralmesodermal layer),[23] aorta-gonad-mesonephros (AGM) and the umbilical andvitelline arteries.[24] HSPCs are generated via thehemogenic endothelium, a special subset of endothelial cells scattered within blood vessels that can differentiate into haematopoietic cells. The emergence of HSPCs is often studied in mouse and zebrafish animal models, in which HSPCs appear as "intra-aortic" clusters that adhere to the ventral wall of the dorsal aorta. RUNX1 or CBF takes part in this process by mediating the transition of an endothelial cell to become a haematopoietic cell.[25] There is increasing evidence that RUNX1 may also be important during primitive haematopoiesis.[26] This is because in RUNX1 knockout mice, primitive erythrocytes displayed a defective morphology and the size of blast cell population was substantially reduced, apart from the absence of HSPCs which would result in embryonic lethality by Embryonic day (E) 11.5 – 12.5.
At a molecular level, expression of the gene RUNX1 is upregulated by the RUNX1 intronic cis-regulatory element (+23 RUNX1 enhancer).[27] This +23 RUNX1 enhancer contains conserved motifs that encourage binding of various haematopoiesis related regulators such asGata2,ETS factors (Fli-1, Elf-1, PU.1) and the SCL / Lmo2 / Ldb1 complex, as well as RUNX1 itself acting in an auto-regulatory loop. As mentioned before, the main role of RUNX1 is to modulate the fate of haematopoietic cells. This can be achieved by binding to thethrombopoietin (TPO) receptor/ c-Mpl promoter, followed by the recruitment of transcription activators or repressors in order to promote transition of the hemogenic endothelium to HSCs, or differentiation into lineages of lower haematopoietic hierarchies. RUNX1 can also modulate its own level by upregulating the expression ofSmad6 to target itself forproteolysis.[28]
A broad range of heterozygousgermline mutations in RUNX1 have been associated with Familial Platelet Disorder, a mild bleeding disorder associated with a high rate of myeloid leukemia.[29] At least 39 forms of somatic RUNX1 mutation are implicated in various myeloid malignancies. Examples range from RUNX1 point mutations acquired from low-dose radiation leading tomyelodysplastic neoplasms or therapy-related myeloid neoplasms, to chromosomal translocation of the RUNX1 gene with the ETO / MTG8 /RUNX1T1 gene located on chromosome 8q22, t(8; 21), generating a fusion protein AML-ETO, categorized asacute myeloid leukemia (AML) M2.
In t(8; 21), breakpoints frequently occur atintron 5 – 6 of RUNX1 and intron 1b – 2 of ETO, creatingchimeric transcripts that inherit the runt domain from RUNX1, and all Nervy homology regions (NHR) 1-4 from ETO. As a consequence, AML-ETO retains the ability to bind at RUNX1 target genes whilst acting as a transcription repressor via the recruitment ofcorepressors andhistone deacetylases, which is an intrinsic function of ETO. Oncogenic potential of AML-ETO is exerted because it blocks differentiation and promote self-renewal in blast cells, resulting in massive accumulation of blasts (>20%) in the bone marrow. This is further characterized histologically by the presence ofAuer rods andepigenetically bylysineacetylation on residues 24 and 43. Other actions of AML-ETO that could induce leukemogenesis include downregulation of the DNA repair enzyme 8-oxoguanine DNA glycosylase (OGG1) and increase in the level of intracellularreactive oxygen species, making cells that express AML-ETO more susceptible to additional genetic mutations.
Role in T-cell acute lymphoblastic leukemia (T-ALL)
Around 15% of T-ALL patients have RUNX1 mutations which are clustered around the DNA binding domain of RUNX1. Those mutations are proposed to cause loss-of-function and might play a tumor suppressor role.[30]
Runx1 was first discovered to be expressed in mouse embryonic skin.[31] It is expressed in theepithelial compartment to control hair follicle activation fromtelogen to anagen through activating Wnt signaling and Lef1 levels[32] At the same time it is expressed in thedermis where it suppresses the same targets to allow for embryogenic development of hair shaft and follicles.[33] In the human hair follicle the expression patterns are similar to the mouse - indicating that it plays a similar role.[34] In addition to hair follicle development, Runx1 is also implicated in skin and epithelial cancer development.[34][35] Thus there are similarities across tissue in Runx1 behavior.
High expression of RUNX1 is associated with adverse survival ofpancreatic cancer patients and has tumor promoting potential in pancreatic cancer.[36] The most common cause of resistance to therapeutic treatments is the suppression of the programmedcell death (apoptosis) of pancreatic cancer cells. A key factor inapoptosis initiation is the proteinNOXA, which is suppressed in a particularly aggressive form of pancreatic cancer. Genetic suppression of theNOXA gene is mediated by the transcription factor RUNX1. Pharmacological or genetic inhibition of RUNX1 de-represses theNOXA gene and inducesapoptosis in pancreatic cancer cells.[36]
^Avramopoulos D, Cox T, Blaschak JE, Chakravarti A, Antonarakis SE (October 1992). "Linkage mapping of the AML1 gene on human chromosome 21 using a DNA polymorphism in the 3' untranslated region".Genomics.14 (2):506–7.doi:10.1016/S0888-7543(05)80253-8.PMID1427868.
^Okuda T, Nishimura M, Nakao M, Fujita Y (October 2001). "RUNX1/AML1: a central player in hematopoiesis".International Journal of Hematology.74 (3):252–7.doi:10.1007/bf02982057.PMID11721959.S2CID5918511.
^Asou N (February 2003). "The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications".Critical Reviews in Oncology/Hematology.45 (2):129–50.doi:10.1016/S1040-8428(02)00003-3.PMID12604126.
^Ogawa E, Inuzuka M, Maruyama M, Satake M, Naito-Fujimoto M, Ito Y, Shigesada K (May 1993). "Molecular cloning and characterization of PEBP2 beta, the heterodimeric partner of a novel Drosophila runt-related DNA binding protein PEBP2 alpha".Virology.194 (1):314–331.doi:10.1006/viro.1993.1262.PMID8386878.
Perry C, Eldor A, Soreq H (March 2002). "Runx1/AML1 in leukemia: disrupted association with diverse protein partners".Leukemia Research.26 (3):221–8.doi:10.1016/S0145-2126(01)00128-X.PMID11792409.
Imai O, Kurokawa M, Izutsu K, Hangaishi A, Maki K, Ogawa S, Chiba S, Mitani K, Hirai H (March 2002). "Mutational analyses of the AML1 gene in patients with myelodysplastic syndrome".Leukemia & Lymphoma.43 (3):617–21.doi:10.1080/10428190290012155.PMID12002768.S2CID45854670.
Hart SM, Foroni L (December 2002). "Core binding factor genes and human leukemia".Haematologica.87 (12):1307–23.PMID12495904.
Ganly P, Walker LC, Morris CM (January 2004). "Familial mutations of the transcription factor RUNX1 (AML1, CBFA2) predispose to acute myeloid leukemia".Leukemia & Lymphoma.45 (1):1–10.doi:10.1080/1042819031000139611.PMID15061191.S2CID10770839.
Yamada R, Tokuhiro S, Chang X, Yamamoto K (September 2004). "SLC22A4 and RUNX1: identification of RA susceptible genes".Journal of Molecular Medicine.82 (9):558–64.doi:10.1007/s00109-004-0547-y.PMID15184985.S2CID9156168.
Harada H, Harada Y, Kimura A (September 2006). "Implications of somatic mutations in the AML1/RUNX1 gene in myelodysplastic syndrome (MDS): future molecular therapeutic directions for MDS".Current Cancer Drug Targets.6 (6):553–65.doi:10.2174/156800906778194595.PMID17017876.
1cmo: IMMUNOGLOBULIN MOTIF DNA-RECOGNITION AND HETERODIMERIZATION FOR THE PEBP2/CBF RUNT-DOMAIN
1co1: FOLD OF THE CBFA
1e50: AML1/CBF COMPLEX
1ean: THE RUNX1 RUNT DOMAIN AT 1.25A RESOLUTION: A STRUCTURAL SWITCH AND SPECIFICALLY BOUND CHLORIDE IONS MODULATE DNA BINDING
1eao: THE RUNX1 RUNT DOMAIN AT 1.25A RESOLUTION: A STRUCTURAL SWITCH AND SPECIFICALLY BOUND CHLORIDE IONS MODULATE DNA BINDING
1eaq: THE RUNX1 RUNT DOMAIN AT 1.25A RESOLUTION: A STRUCTURAL SWITCH AND SPECIFICALLY BOUND CHLORIDE IONS MODULATE DNA BINDING
1h9d: AML1/CBF-BETA/DNA COMPLEX
1hjb: CRYSTAL STRUCTURE OF RUNX-1/AML1/CBFALPHA RUNT DOMAIN AND C/EBPBETA BZIP DIMERIC BOUND TO A DNA FRAGMENT FROM THE CSF-1R PROMOTER
1hjc: CRYSTAL STRUCTURE OF RUNX-1/AML1/CBFALPHA RUNT DOMAIN BOUND TO A DNA FRAGMENT FROM THE CSF-1R PROMOTER
1io4: CRYSTAL STRUCTURE OF RUNX-1/AML1/CBFALPHA RUNT DOMAIN-CBFBETA CORE DOMAIN HETERODIMER AND C/EBPBETA BZIP HOMODIMER BOUND TO A DNA FRAGMENT FROM THE CSF-1R PROMOTER
1ljm: DNA recognition is mediated by conformational transition and by DNA bending
