**2. Molecular characterization of CML**

The *BCR-ABL* fusion protein with strong tyrosine kinase activity is one of the molecular biological bases of leukemia [32]. The Ph chromosome is generated by the translocation t (9; 22) (q34; q11). The normal *c-ABL* gene is located on chromosome 9 and has 11 exons (1b, 1a, a2–a11). During chromosomal translocation, *c-ABL* gene breakdown can occur at three points: upstream of 1b, between 1b and 1a, and between 1a and a2. Regardless of the breakpoint, the first two alternative exons (1b and 1a) are always separated [21].

The normal *BCR* gene is located on chromosome 22 and consists of 23 exons (e1– e23). In generation of the *BCR-ABL* fusion gene, there are three breakpoints that can occur in the *BCR* gene: major (*M-BCR*), minor (*m-BCR*), and micro (*μ-BCR*). The M-*BCR* break can generate junctions e14a2, e13a2, e14a3, and e13a3; m-BCR breakdown can induce junctions e1a2 and e1a3; and the breakdown in μ-BCR can give rise to e19a2 junction [21].

Despite the many junctions that exist, in Ph-positive hematological neoplasms, the junctions e14a2 and e13a2 are the most frequent. These two junctions generate the same transcript encoding the p210 BCR-ABL protein. The junction e19a2 produces a transcript encoding the p230 BCR-ABL protein. And junction e1a2 creates a transcript that encodes p190 BCR-ABL [21, 29].

All domains of c-ABL protein—SRC-homology-2 (SH2) domain, SH3 domain, tyrosine kinase domain, nuclear localization signal (NLS), nuclear export signal (NES), DNA-binding domain (DBD), and actin-binding domain (ABD)—are present in the three BCR-ABL proteins. However, the three BCR-ABL proteins have different BCR domains [33].

The SH2 and SH3 regulatory domains mediate protein-protein interactions and control activation of transduction signals. The SH3 domain is known as a negative

regulator of kinase activity, acting as a counterpoint to the SH2 domain, inactivating its tyrosine kinase-activating and receptor potential [34]. In BCR-ABL products, a partial or complete deletion of the SH3 domain occurs, losing negative control. In contrast, the SH2 domain is eventually activated by the presence of a tyrosine kinase-activating component [35].

The BCR-ABL fusion protein acts as an oncoprotein by activating several signaling pathways that lead to transformation. Myc, Ras, c-Rafn MAP/ERK, SAPK/JNK, STAT, NFKB, PI-3kinase, and c-Jun are included as signal cascade molecules. Many signaling proteins have been shown to interact with BCR-ABL through various functional domains and/or to become phosphorylated in BCR-ABL-expressing cells. In brief, BCR-ABL activates the main signal pathways, such as RAS/MAPK, PI-3kinase, c-ABL pathways and CRKL pathways, and JAK-STAT, and the Src pathway to play a major role in transformation and proliferation. Inhibition of apoptosis is thought to result from activation of the PI-3 kinase and RAS pathways with induction through AKT of Myc and BCL-2 [36].

Activation of these signaling pathways leads to deregulation of cellular processes such as proliferation, differentiation, DNA repair, decreased adhesion of leukemic cells to bone marrow stroma, and reduced apoptotic response to mutagenic stimulation, leading to uncontrolled clonal proliferation [37].
