Philadelphia chromosome
| Philadelphia chromosome | |
|---|---|
 |
|
| A metaphase cell positive for the bcr/abl rearrangement using FISH | |
| Classification and external resources | |
| Specialty | oncology |
| ICD-10 | C92.1 |
| ICD-9-CM | 205.1 |
| ICD-O | 9875/3 |
| MeSH | D010677 |
The Philadelphia chromosome or Philadelphia translocation is a specific genetic abnormality in chromosome 22 of leukemia cancer cells (particularly chronic myelogenous leukemia (CML) cells). This chromosome is defective and unusually short because of reciprocal translocation of genetic material between chromosome 9 and chromosome 22, and contains a fusion gene called BCR-ABL1. This gene is the ABL1 gene of chromosome 9 juxtaposed onto the BCR gene of chromosome 22, coding for a hybrid protein: a tyrosine kinase signalling protein that is "always on", causing the cell to divide uncontrollably.
The presence of this translocation is a highly sensitive test for CML, since 95% of people with CML have this abnormality (the remainder have either a cryptic translocation that is invisible on G-banded chromosome preparations, or a variant translocation involving another chromosome or chromosomes as well as the long arm of chromosomes 9 and 22). However, the presence of the Philadelphia (Ph) chromosome is not sufficiently specific to diagnose CML, since it is also found in acute lymphoblastic leukemia (ALL, 25–30% in adult and 2–10% in pediatric cases) and occasionally in acute myelogenous leukemia (AML).
The chromosomal defect in the Philadelphia chromosome is a translocation, in which parts of two chromosomes, 9 and 22, swap places. The result is that a fusion gene is created by juxtaposing the ABL1 gene on chromosome 9 (region q34) to a part of the BCR (breakpoint cluster region) gene on chromosome 22 (region q11). This is a reciprocal translocation, creating an elongated chromosome 9 (termed a derivative chromosome, or der 9), and a truncated chromosome 22 (the Philadelphia chromosome). In agreement with the International System for Human Cytogenetic Nomenclature (ISCN), this chromosomal translocation is designated as t(9;22)(q34;q11). The symbol ABL is derived from Abelson, the name of a leukemia virus which carries a similar protein.
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