Chronic myeloid leukemia (CML) is a myeloproliferative disease that originates in abnormal pluripotent bone marrow stem cells and is consistently associated with the Philadelphia (Ph) chromosome which normally leads to BCR/ABL gene fusion (1). The Ph chromosome created as a result of t(9;22) (q34;q11) is observed in more than 90% of CML patients. The BCR-ABL fusion gene is formed by the transposing of the 3′ portion of the ABL oncogene from 9q34 to the 5′ portion of the BCR gene on chromosome 22, and this fusion gene encodes a constitutive active tyrosine kinase (2). Masked or variant Ph translocations characterize 5–10% of CML cases. A masked Ph chromosome is found in cases with a normal karyotype, as a result of a cryptic rearrangement, or in patients with complex changes where the typical t(9;22) (q34;q11) is not detectable by G-banding (3). These rearrangements are detected by fluorescence in situ hybridization (FISH) (4). The variant Ph translocation is cytogenetically classified as involving chromosomes 9 and 22, as well as one or more other chromosomes (5,6). Imatinib mesylate (Glivec, formerly STI571) was designed specifically to inhibit the tyrosine kinase activity of the BCR/ABL protein and other tyrosine kinases such as cABL, c-KIT and PDGF (platelet-derived growth factor receptor). By binding to an active site of the tyrosine kinase, Glivec switches off downstream signaling, cells stop proliferating and apoptosis ensues (7). Various studies showed a high efficiency of imatinib therapy to achieve a complete or major cytogenetic response, i.e., a reduction to 0–34% Ph-positive cells. This positive effect is achieved in cases with a simple t(9;22) combined with complex translocations resulting in BCR/ABL gene fusion, as well as in cases with cytogenetic clonal evolution (8,9).

This study investigated a novel Ph chromosome-positive CML case with a new complex rearrangement formed by four chromosomes and new complex aberrations involving four chromosomal breakpoints. Treatment with imatinib proved successful. In this case, the high-resolution array-proven multicolor banding (aMCB) technique was crucial in the detection of genetic changes.

Karyotyping was performed following the initiation of chemotherapy treatment, with various karyotypic changes. A complex karyotype 46,XX,t(9;12;16;22)/46,XX was determined in the GTG-banding (Fig. 1) and was further specified by molecular cytogenetic studies (Figs. 2 and 3). A dual-color FISH using a probe specific for BCR and ABL revealed a typical Ph chromosome with the BCR/ABL fusion gene (Fig. 2A). Dual-color FISH using WCP- and CEP-specific probes was applied to evidence further rearrangements (Figs. 2B and D). Thus, the chromosomes 9, 12, 16 and 22 were found to be involved in the karyotypic changes. aMCB using probes for the corresponding chromosomes was performed as previously reported (14). A complex translocation among the four chromosomes was detected (Fig. 3), and the final karyotypes obtained were: 46,XX,t(9;12;16;22)(q34;q24.2~24.31;p11.2;q11)[12]/ 46,XX[8].

We described a novel Philadelphia (Ph) chromosome-positive CML case with the new complex variant translocation t(9;12;16;22)(q34;q24.2–24.31;p11.2;q11). To the best of our knowledge, this translocation has never previously been observed in CML (15).

In 5–10% of Ph chromosome-positive CML cases, complex translocations are noted in both chromosomes 9 and 22 and other chromosomes, such as 12 and 16 (3). At present, it appears that variant translocations are able to affect any chromosome. However, it has been suggested that the distribution of the breakpoints is non-random, with the chromosomal bands most susceptible to breakage being 1p36, 3p21, 5q31, 6p21, 9q22, 10q22, 11q13, 12p13, 17p13, 17q21, 17q25, 19q13, 21q22, 22q12 and 22q13 (1).

Two potential mechanisms for variant translocation formation have been suggested. The first is a single-event rearrangement via the simultaneous breakage of a number of chromosomes followed by mismatched joining (6). Nacheva et al proposed a classical Ph translocation followed by further translocation between chromosomes 9 and 22, in addition to a third chromosome (16). The mechanism of the formation of a variant Ph translocation may have prognostic importance in that a two-event mechanism indicates clonal evolution, whereas a variant translocation occurring via a single genomic rearrangement may confer a similar prognosis to the classical Ph translocation (17).

In conclusion, we report a novel case of a Ph chromosome-positive CML in CP with a new complex variant Ph translocation involving the four chromosomal aberrations of 9q34, q24.2–24.31, 16p11.2 and 22q11. Notably, the reported patient had a beneficial response to imatinib, although she succumbed to unknown causes.