Increased MSI2 expression Is Associated with Aggressive CML and AML

Abstract 2516

The events triggering arrested differentiation and a more aggressive disease in chronic myeloid leukaemia (CML) patients are unclear. Dysregulation of MSI2 has been suggested as a causal event in the transformation of chronic phase (CP), a relatively indolent disease phase, to blast crisis (BC), which is usually fatal. The Musashi gene family, regulated by HOXA9, has been shown to control critical cell fate decisions by binding to the 3'untranslated region of target mRNAs, thereby inhibiting translation. This results in dysfunction of the regulatory pathway, leading to hematopoietic stem cell (HSC) proliferation, impaired myeloid differentiation and worse clinical prognosis in CML and AML. In this study we attempt to verify these published results and test the utility of using MSI2 as a prognostic marker in CML and AML.

To assess if MSI2 expression levels might be prognostic in CML, we screened 65 heterogeneous patients [median age; 45 years (19–75); M:35; F:30] of whom 54 were in CP and 11 in advanced disease, treated with different modalities. Of these 64 were administered one or more of the tyrosine kinase inhibitors, of these 2 underwent haematopoietic stem cell transplant. One patient was managed with interferon plus cytarabine only. BCR-ABL1 Kinase domain mutations were documented in 8 of the 11 patients in advanced disease, of which 3 were T315I in combination with another mutation and 4 mapped to the P-loop. Of the 30 CP patients screened 10 had KD mutation, of which one was T315I and 2 were within the P-loop. The overall median survival for 11 patients in advanced disease [2 accelerated phase; 9 BC (4 myeloid, 5 lymphoid)] is 3.9 years (1.1–19.0), 7 of the BC had died (median survival 4.0 years). All the CP patients are alive with an overall median survival of 3.54 years (0.46–16.4). In addition we screened 89 diagnostic samples from acute myeloid leukaemia (AML) patients (M:53; F:36) with a median age of 61 years (8–85) and 27 normal control blood donor samples. The MSI2, BCR-ABL1 and GUSß endogenous control gene) mRNA expression were measured by TaqMan real time quantitative polymerase chain reaction (RQ-PCR) in separate assays. The MSI2 and GUSß mRNA were measured in duplicate and BCR-ABL1 in triplicate as were the standards for the three genes assayed. The data were expressed as % ratio of the control gene, only samples with >5500 GUSß copies were included in the data presented here.

Msi2 expression was detected in all samples by RQ-PCR but was significantly increased (p=<0.0001) in advanced disease CML patients [median 6.7 (1.3–22.9)], irrespective of lymphoid or myeloid transformation, when compared with CML CP subjects [median 2.2 (0.2–6.3)]. BCR-ABL1 was detectable in all CML samples. The median for BCR-ABL1 copies in the 11 advanced disease patients was 101.4 (0.3–325.8). Remarkably, when patients in CP were classified as less and greater than the 2.16 MSI2 median, the BCR-ABL1 mRNA median values were 19.4 (0.1–1000) and 1.31 (0.02–393) transcripts, respectively, i.e. higher MSI2 expression was associated with lower BCR-ABL1 transcripts (p=0.023). Furthermore, there was no significant difference in MSI2 expression between the normal control population [median 2.16 (1.33–7.53)] and CP patients (p=0.204). The 89 AML samples had a median MSI2 value of 3.67 (0.41–40.17). Outcome data was available for 86 and these were classified into tertiles (low, intermediate and high MSI2). Kaplan-Meier survival analysis revealed a p value of 0.088 when comparing the outcome of patients in the low and high groups and 0.091 between low and intermediate/high. Although this is a small cohort, the difference in outcome was due to only 4 out of 26 (15.4%) patients in the low group who had died with a mean survival of 525 days (182–840); compared to 17 out of 53 (32%) in the intermediate/High group with a mean survival of 52 days (1–120).

In summary, our data identified an inverse relationship between MSI2 and BCR-ABL1 expression levels in CP patients. These observations may reflect the finding of lower expression of HOXA family and MSI2 in quiescent CML stem cells compared to normal stem cells. A longitudinal study among the CP patients might indicate how it effects their overall survival. We also provide evidence that increased MSI2 expression correlates with an aggressiveness of CML and early death in AML.