**2. Is there any specific reason underlying the generation of the Ph chromosome?**

Basically, every chromosomal translocations require DNA Double-strand breaks (DSBs) in two different locations and that the broken ends of nonhomologous chromosomes are fused together. DNA double-strands breaks might be due to different causes (*e.g.* ionizing radiation, reactive oxygen species, DNA replication across a nick, malfunctioning of DNA metabolic enzymes such as type II DNA topoisomerase or RAG complex during illegitimate V(D)J recombination). Cells to preserve their genome integrity upon DNA damage respond by activating a repair machinery that should catalyzes the joining of the broken ends [3]. However, the outcome of the joining process leads to a variety of rearrangement. For instance, precise joining of broken ends can generate a normal chromosome. Inversions, deletions and duplications can occur when joining involves two broken ends on the same chromosome. Non-Homologous End Joining (NHEJ) is often imprecise; thus some nucleotides may be lost during the joining process. Eventually, translocations may occur when the broken ends of two nonhomologous chromosomes are joined together thus leading to novel chromosomes containing part of normal chromosomes [4].

Aside these notions, currently our knowledge regarding the molecular mechanisms responsible for the reciprocal chromosomal translocation occurring between the chromosome 9 and the 22, t(9;22), generating the Philadelphia chromosome (Ph), remain still rather elusive. Fundamentally, it has been speculated that there are two plausible hypotheses. One view prefers to lean towards an entirely random "breaking and re-ligating process" occurring with relatively similar frequency between any two chromosomes within a cell. Chromosomal translocations that give and adaptive advantage are pretty rare and associated with negative consequences (*e.g*. cancer). The success of the t(9;22) can be explained by the fact that, the resulting fusion gene encodes for a protein with transforming properties conferring selective fitness advantages to the host cell. Conversely, by virtue of this, any

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*The Paradigm of Targeting an Oncogenic Tyrosine Kinase: Lesson from BCR-ABL*

other chromosomal rearrangement that does not satisfy the requisite for survival and expansion will be handicapped and thus will soon disappear. Though, at this time there are no experimental indications ruling out this view it is worthwhile to notice that the juxtaposition of the ABL and BCR genes has been observed in nuclei of human hematopoietic cells over the S/G2 phases and through the whole G2 phase up to the middle of the M phase (*i.e.* metaphase stage) [5]. Ultimately, there is no conclusive evidence that DNA sequences potentially relevant to chromosomal translocations, such as the Alu repeats or Chi-like octamers, are present around the BCR-ABL rearrangement [6]. Hence, by now the only trustable conviction is that the exchange between two chromosomal positions implies that they must be physically close to each other at the time the event occurs. However, besides the external triggering events (*e.g.* ionizing radiation) the details about what are the molecular players and how they cooperate to the birth of such aberrant chromosome remain to

The results of the chromosomal translocation occurring between the chromosome 9 and 22, t(9;22), are a longer chromosome 9 (9q+) and a smaller derivative chromosome 22, the Ph [7, 8]. By the eighties of the past century the molecular characterization of the Ph led to the identification of a novel chimeric gene, BCR-ABL, which later on has been found to encode for a chimeric protein with a constitutive tyrosine kinase activity and with potent oncogenic properties [9, 10]. The c-ABL and Breakpoint of Cluster Region (BCR) loci are localized on the long arm of the chromosome 9 and 22, respectively [11]. Depending on the different breakpoints occurring on the two chromosomes resulting in different BCR-ABL variants. Though, all BCR and c-ABL DNA breakpoints fall within intronic regions those occurring in the BCR gene are highly variable and thus responsible for defining the major differences among the different variants. The variation in the BCR part of the fusion transcript contrasts with the constant c-ABL part. Indeed, all the breakpoints so far identified within the c-ABL gene occur in a large (300-kb) region in the 5′ portion of the gene, localized upstream of the exon 2, and generally falling in the intron sequences restricted between the two alternative first exons (1b and 1a). Regardless the structure of the different fusion genes the BCR exons directly fuse to the second c-ABL exon (a2). The most frequent BCR-ABL fusion variant is the p210 in which the BCR exon 13, or 14, is fused downstream of the alternative exons 1 of the c-ABL gene and thus leading to a fusion protein with approximately the first half from BCR and the remaining second half from ABL. Mostly this variant is found in CML patients accounting for approximately 95% of the BCR-ABL fusion gene in all the CML cases. A second frequent variant, p190, is found in approximately 20–30% of adult patients with Acute Lymphoblast Leukemia (ALL) [12] and, very rarely, also in Acute Myeloid Leukemia (AML) [13]. When compared to the p210 variant, in this case the breakpoint within the BCR locus is localized in the 3′ half of the first BCR intron, thus encoding for a shorter BCR portion (approximately 425 aminoacids). The third most common BCR-ABL variant, p230, is the largest and is defined by a breakpoint cluster region encompassed between the exons 19 and 21. Whereas the p190 characterizes a more acute form of leukemia usually of lymphoid origin, the latter variant is peculiar of neutrophilic CML. Besides, there are additional BCR-ABL variants, though they have been observed less frequently. Interestingly, some of them are peculiar because they are the results of alternative splicing leading to truncated chimeric proteins that are all lacking tyrosine kinase activity [14]. Furthermore, in hematopoietic malignancies, the BCR gene has been identified

*DOI: http://dx.doi.org/10.5772/intechopen.97528*

be mechanistically elucidated.

**3. What is the consequence of the t(9;22)?**

*The Paradigm of Targeting an Oncogenic Tyrosine Kinase: Lesson from BCR-ABL DOI: http://dx.doi.org/10.5772/intechopen.97528*

other chromosomal rearrangement that does not satisfy the requisite for survival and expansion will be handicapped and thus will soon disappear. Though, at this time there are no experimental indications ruling out this view it is worthwhile to notice that the juxtaposition of the ABL and BCR genes has been observed in nuclei of human hematopoietic cells over the S/G2 phases and through the whole G2 phase up to the middle of the M phase (*i.e.* metaphase stage) [5]. Ultimately, there is no conclusive evidence that DNA sequences potentially relevant to chromosomal translocations, such as the Alu repeats or Chi-like octamers, are present around the BCR-ABL rearrangement [6]. Hence, by now the only trustable conviction is that the exchange between two chromosomal positions implies that they must be physically close to each other at the time the event occurs. However, besides the external triggering events (*e.g.* ionizing radiation) the details about what are the molecular players and how they cooperate to the birth of such aberrant chromosome remain to be mechanistically elucidated.
