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Philadelphia chromosome (Ph), named after the city where it was first described, was described for the first time by Nowell and Hungerford in 1960, in two patients who had lost the long arm of chromosome 22.[1][2][3] As the Giemsa staining improved over the decade, Rowley showed that this truncated chromosome was generated as a result of the translocation of the long arm of chromosomes 9 and 22.[2][3][4] The presence of the abnormal Ph chromosome in cells of myeloid, megakaryocytic, along with erythroid lineages pointed to the origination of the disorder in a pluripotential stem cell.[2] Philadelphia chromosome is the hallmark of chronic myeloid leukemia (CML) along with some other leukemias including acute lymphoblastic leukemia (ALL) (mostly B cell ALL, rarely T cell ALL), acute myeloid leukemia (AML), chronic neutrophilic leukemia (CNL), and mixed phenotype acute leukemia (MPAL).[4][5][6][7][8][9][10][11]

This oncogene leads to constitutive activation of tyrosine kinase, which is responsible for more proliferation and better viability of the myeloid lineage cells.[4] The translocation results in decreased requirements for growth factors and provides freedom from their regulatory control.[2] This oncogene also affects cellular differentiation and apoptosis.[4]