Runt-related transcription factor 1 (RUNX1) also known as acute myeloid leukemia 1 protein (AML1) or core-binding factor subunit alpha-2 (CBFA2) and it is a protein that is encoded by the RUNX1 gene, in humans. RUNX1 is a transcription factor that regulates the differentiation of hematopoietic stem cells into mature blood cells. In addition it plays a major role in the development of the neurons that transmit pain. It belongs to the Runt-related transcription factor (RUNX) family of genes which are also called core binding factor-α (CBFα). RUNX proteins form a heterodimeric complex with CBFβ which confers increased DNA binding and stability to the complex. Chromosomal translocations involving the RUNX1 gene are associated with several types of leukemia including M2 AML. Mutations in RUNX1 are implicated in cases of breast cancer.
Gene and protein In humans, the gene RUNX1 is 260 kilobases (kb) in length, and is located on chromosome 21 (21q22.12). The gene can be transcribed from 2 alternative promoters, promoter 1 (distal) or promoter 2 (proximal). As a result, various isoforms of RUNX1 can be synthesized, facilitated by alternative splicing. The full-length RUNX1 protein is encoded by 12 exons. Among the exons are two defined domains, namely the runt homology domain (RHD) or the runt domain (exons 2, 3 and 4), and the transactivation 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 2 enhancers (regulatory element 1 and regulatory element 2), and these tissue specific enhancers enable the binding of lymphoid or erythroid regulatory proteins, therefore the gene activity of RUNX1 is highly active in the haematopoietic 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 the p53 family. The runt domain of RUNX1 binds to the core consensus sequence TGTGGNNN (where NNN can represent either TTT or TCA). DNA recognition is achieved by loops of the 12-stranded β-barrel and the C-terminus "tail" (residues 170 – 177), which clamp around the sugar phosphate backbone and fits into the major and minor grooves of DNA. Specificity is achieved by making direct or water-mediated contacts with the bases. RUNX1 can bind DNA as a monomer, 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 the core binding factors (CBFs). The consensus binding site for CBF has been identified to be a 7 bp sequence PyGPyGGTPy. Py denotes pyrimidine which can be either cytosine or thymine.
Discovery and characterization of RUNX1 Christiane Nüsslein-Volhard and Eric F. Wieschaus discovered the transcription factor RUNX in a screen that was conducted to identify mutations that affect segment number and polarity in Drosophila melanogaster. The mutation that led to presegmentation patterning defects and runted embryos was named runt. Following this discovery, the Drosophila segmentation gene runt was cloned by Gergen et al. Although the protein encoded by runt was demonstrated to exhibit nuclear translocation, it was not yet established that this protein is a transcription factor. Subsequently, in 1991, Ohki et al. cloned the human RUNX1 gene; RUNX1 was found to be rearranged in the leukemic cell DNAs from t(8;21)(q22;q22) acute myeloid leukemia patients. 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-specific DNA-binding protein that regulated the disease specificity of the Moloney murine Leukemia virus. Furthermore, Ito et al. purified RUNX2, the homolog of RUNX1. 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.
Mouse knockout Mice embryos with homozygous mutations on RUNX1 died at about 12.5 days. The embryos displayed lack of fetal liver hematopoiesis. 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).
Participation in haematopoiesis
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