Significant Discrepancy In Peripheral Blood Compared to Bone Marrow BCR-ABL Transcript Levels In Chronic Phase Chronic Myeloid Leukemia (CML)

Abstract 4471

Chronic myeloid leukemia (CML) is associated with the reciprocal t(9;22)(q34;q11) translocation, which generates the BCR-ABL fusion oncogene and is the most common myeloproliferative disease affecting adults. The clinical outcome in this disease has been revolutionized with the use of imatinib mesylate (Gleevec), a targeted tyrosine kinase inhibitor, and molecular surveillance, with the development of quantitative PCR (qPCR) approaches to measure BCR-ABL transcript levels. A number of guidelines outlining follow-up strategies for patients with chronic phase CML on imatinib therapy have been established. Once a patient is stable, a typical recommendation includes peripheral blood (PB) monitoring by qPCR of BCR-ABL levels every 3–6 months to determine response or relapse, with consideration of annual bone marrow (BM) examinations to assess for cytogenetic evolution. At the Queen Elizabeth II Health Sciences Centre and IWK Health Centre in Halifax, Nova Scotia, 34 patients with chronic phase CML on imatinib were identified from 2006 to 2008, with 36 paired samples, where transcript levels were assessed in both PB and BM within one week of each other. In 24 of the cases, the BCR-ABL transcript levels in PB and BM were within 0.5 log values of each other. In the remaining 12 cases, BCR-ABL transcript levels differed by greater than 0.5 log. Three cases had higher BM levels, but surprisingly, 9 patients had a higher BCR-ABL transcript level in the PB. In all cases, BCR-ABL levels were assessed by Q-RT-PCR using the ABI7500 instrument and primers and probe designed to detect p210 and p190 breakpoints. Results were recorded as a ratio of %BCR-ABL to GAPDH that was amplified as an internal control. There was no significant difference in clinical, morphological or laboratory parameters between these patients and others who had comparable PB and BM BCR-ABL levels. These findings highlight the need to compare BCR-ABL transcript levels derived from the same tissue during longitudinal monitoring. Moreover, while potentially due to stochastic factors, the striking observation of higher PB BCR-ABL transcript levels raises the question of which tissue represents the most accurate source for monitoring of BCR-ABL transcript levels and whether there is value in confirming a significant change in PB transcript level with BM evaluation. The discrepant levels in PB and BM could not be attributed to technical issues; the timing of sample processing from collection and quality of mRNA were comparable and no variability was observed in GAPDH levels to account for the difference. Without a technical explanation, the mechanism underlying this phenomenon remains uncertain. We speculate that it may reflect CML stem cell geography with one possibility being that the CML niche may be located external to the BM. Further studies are needed to confirm these observations. If corroborated, then revision of surveillance approaches for chronic phase patients may be indicated.