[Research advance on molecular genetics of CML blast crisis]

Philadelphia (Ph) chromosome at (9; 22) reciprocal chromosomal translocation producing BCR-ABL fusion gene, emerges in almost all patients with chronic myeloid leukemia (CML). The protein product of BCR-ABL is a constitutively active tyrosine kinase that drives the abnormal proliferation of CML cells. Blast crisis (BC) is the terminal phase of CML, which is often associated with additional chromosomal and molecular secondary changes. Although the mechanisms responsible for transition of CML chronic phase (CP) into BC remain poorly understood, ample evidence suggests that it depends on synergy of BCR/ABL with other genes dysregulated during disease progression, and signaling pathways are abnormally activated by BCR/ABL. With the application of imatinib, a ABL-specific tyrosine kinase inhibitor, its remarkable therapeutic effects suggest that blast crisis transition will be postponed in most patients with CML. Rate of cumulative best response in CML-CP patients from the IRIS trial after 5 years are 98% for complete hematologic response, 92% for major cytogenetic response and 87% for complete cytogenetic response. However, a minority of CML-CP patients and most patients in progression either fail or respond suboptimally to imatinib. There are many distinct patterns of resistance, and ABL kinase mutations is a common finding associated with clinical resistance. Dasatinib and nilotinib can restore hematologic and cytogenetic remission in the majority of patients with primary failure or acquired resistance in chronic phase. This review illustrates the molecular mechanisms underlying transition to CML-BC, also addresses oneself to how and why imatinib resistance occurs.