Detection of t(9;22) by PCR amplification
The t(9;22), termed the Philadelphia chromosome, was described in 1960 by Nowell and Hungerford, and represented the first non-random chromosomal abnormality shown to be associated with a specific neoplasm, namely CML (although it is found in other disorders). The t(9;22) is formed by the fusion of the BCR gene on chromosome 22 with the ABL proto-oncogene on chromosome 9 and occurs in the vast majority of patients with CML and in up to 20% of adult patients with ALL. The chronic myeloid leukemic cells transcribe an 8.5-kb chimeric mRNA that is translated into a 210-kDa protein (p210) with tyrosine kinase activity. The breakpoints at the ABL gene can occur at any point up to 200
kb upstream in the intron and therefore cannot easily be amplified by PCR using genomic DNA as described earlier in this chapter. In contrast, the chimeric mRNA will usually be of two possible types. It is therefore possible to amplify the chimeric mRNA by first reverse-transcribing to cDNA. Using this technique, it is possible to detect one leukemic cell in up to 106 normal cells (see Chapter 7).
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