Each year, three to four new cases of acute myeloid leukemia (AML) are reported per 100,000 individuals. The prognosis of AML is highly variable despite intensive research for new markers and therapies [1]. Relapse is the most frequent cause of therapeutic failure [2, 3], less than 50% of patients have a5 yearoverall survival rate (OS) andonly 20% of elderly survive 2 years [1].
Genetic and epigenetic alterations in the hematopoietic stem and progenitor cells causing their aberrant proliferation and block of differentiation (HSPCs) leading to different clinical types of AML [4]. Several studies have recognized various genes affected by the somatic mutations due to different AML subtypes [5,6]. Mutations in transcriptional regulator, additional sex combs like 1, tumor protein 53, and FMsrelated tyrosine kinase 3 (FLT3) genes and certain chromosomal translocations (Breakpoint cluster regionAbelson murine leukemia viral oncogene homolog 1) were reported to predispose to AML [7]. The data from previous studies has helped in elucidation of AML biology, which could facilitate better risk assessment while determining novel drug targets and therapeutic strategies [5, 6].
The RUNX1 gene has emerged as anovel therapeutic target for AML [8].It belongs to atranscriptional regulator family called Runx that comprise three members: RUNX1,RUNX2 and RUNX3 [9]. It is located on chromosome 21. As shown in a previous study in a mouse model, RUNX1 gene is vital for the process of hematopoiesis [8]. It consists of a “Runt homology domain” (RHD) that facilitates the formation of the heterodimer of RUNX1 and PEBP2β that acts as a DNA binding and transcription factor [10].
RUNX1 gene activity is tightly regulated via several mechanisms, such as translational regulation, posttranslational modifications (PTM), and alternative splicing [11, 12]. The PTMs, such as methylation, acetylation, and phosphorylation promote RUNX1 transcription [13]. Perturbation in the activity of RUNX1 leads to development of several hematopoietic neoplasms [9].
Several studies have shown the tumor suppressor activity of RUNX1 against myeloid neoplasms. AML patients often exhibit chromosomal translocations involving RUNX1 and its cofactor CBFB. The most common AML subtype, also referred to as CBF-AML, is characterized by the chromosomal aberrations inv(16) and t(8;21) that lead to the formation of CBFB-MYH11 and RUNX1-RUNX1T1 (AML1-ETO) fusion genes, respectively. Around 15 to 20% of adult de novo AML cases suffer from CBF-AML [14]. Another aberration t(3;21) leads to the formation RUNX1-MECOM (also called AML1-EVI1) fusion gene that retains the N-terminal of RUNX1 [15]. This fusion gene is found in chronic myeloid leukemia with blastic or accelerated phase, therapy-related myeloid neoplasms, and, rarely, in de novo AML. The formation of the abovementioned fusion genes disrupts the normal function of RUNX1-CBFB. In addition, RUNX1 is itself mutated in several myeloid neoplasms [16]. Germline RUNX1 mutations lead to familial platelet disorder with predisposition to AML [17, 18]. Around 15% of cytogenetically normal AML [19–21] and 6–11% of myelodysplastic syndromes [22–24], 10% of chronic myelomonocytic leukemia [25] and 20% of systemic mastocytosis cases exhibit somatic mutations in RUNX1 gene [26]. In general, such mutations affect the transcription activation domain or the Runt domain. They are usually frameshift or nonsense mutations that adversely affect the transcriptional activity of RUNX1 [27, 28]. Due to these mutations, RUNX1 is unable to participate in various crucial events like early hematopoietic development and myeloid maturation [29]. Several previous studies have shown the tumor suppressor role of RUNX1 via various mouse models [30–33].
Other studies have shown that, in CBF-MLL fusion leukemia, RUNX activity is needed to maintain the phenotype of leukemogenic cell [9]. RUNX1 down regulation led to apoptosis and cell-cycle arrest in human cord blood cells that expressed the RUNX1-RUNX1T1 and MLL-AF9 fusion genes [9]. In another study, a rapid development of leukemia was observed in a knock-in mice model that expressed a mutant CBFB-MYH11 fusion product that lacked the RUNX1 high-affinity binding domain and caused an inadequate RUNX1 suppression [34]. Similarly, in a mouse bone marrow transplant model, a truncated C-terminal version of RUNX1-RUNX1T1, RUNX1-RUNX1T1-9a, exhibited weak RUNX1suppression but high leukemogenic potential [35]. In addition, RUNX1 knockdown in ME-1 and Kasumi-1 cell (expressing CBFB-MYH11 and RUNX1-RUNX1T1, respectively) led to abnormal cell cycle and apoptosis [36] (Fig. 1).
In addition, phosphorylated RUNX1 was reported to conjugate with FLT3-ITD and induce AML [4]. Internal tandem duplication of the FLT3 gene (FLT3-ITD) is one of the commonest AML mutations that cause constitutive activation of FLT3 receptor tyrosine (Tyr) kinase [4]. Studies on mouse model have revealed that FLT3-ITD cannot induce AML on its own [36]. A previous study has reported an association between expression of RUNX1 and FLT1-ITD signaling in AML cells; both entities synergistically participate in AML development [4]. AML patients with FLT3-ITD mutations exhibit overexpression of RUNX1 RNA and its downstream target, HHEX. In addition, downregulation of RUNX1 adversely affects the leukemogenic activity of AML cells [4].
RUNX1 could act as both tumor promoter as well as suppressor depending on several factors, including its expression levels. Thus, regulation of RUNX1 expression could act as a potential therapeutic strategy against cancer [9].
It has been previously observed that RUNX1 downregulation promotes the differentiation of AML cells expressing FLT3-ITD. It indicates that RUNX1 downregulation did not induce any potential mutations that could adversely affect the differentiation potential of these cells [4]. Therefore, introduction of such mutations could act as a potential therapeutic strategy for AML patients with FLT3-ITD mutations [4].
Our study is aimed at assessing RUNX1 gene expression level and its association with other genetic markers and clinical outcome in Egyptian De-novo AML patients.