t(9;22)

t(9;22) — oncology-focused definition

t(9;22), also called the Philadelphia chromosome (Ph), is a reciprocal translocation between chromosome 9 and 22 that creates the BCR-ABL1 fusion gene.

• Chromosomes involved:
  • 9q34 → ABL1 (tyrosine kinase gene)
  • 22q11 → BCR (breakpoint cluster region)

Oncology relevance:

• BCR-ABL1 fusion protein has constitutive tyrosine kinase activity, leading to uncontrolled proliferation of myeloid cells.
• Found in:
  • Chronic myeloid leukemia (CML) – hallmark diagnostic feature
  • Philadelphia-positive acute lymphoblastic leukemia (Ph+ ALL)

Clinical significance (for oncology pharmacists):

• Diagnostic marker: Confirms CML or Ph+ ALL
• Prognostic marker: BCR-ABL1 levels used to monitor treatment response
• Therapeutic target:
  • Tyrosine kinase inhibitors (TKIs): imatinib, dasatinib, nilotinib, bosutinib, ponatinib
  • Therapy requires monitoring for resistance mutations (e.g., T315I)

Key takeaway:

t(9;22) creates a driver fusion gene (BCR-ABL1) that defines disease, guides targeted therapy, and is central to treatment monitoring in CML and Ph+ ALL.