Bone Marrow WT1 Levels In Long-Term Survivors Of Core-Binding Factor AML and Acute Promyelocytic Leukemia

Quantitation of WT1 mRNA levels may be used in minimal residual disease studies in AML cases lacking a suitable molecular target. The normal thresholds of bone marrow WT1 levels at different time–points have not been clearly stated. Difficulties arise as a consequence of the relatively high WT1 expression in normal bone marrow and the unknown influence of chemotherapy.

To establish the reference range value of bone marrow WT1 levels in treated CBF-AML and APL in complete remission.

Patients with bone marrow samples obtained  from ≥2 years (2-11 y) after initial diagnosis of CBF-AML (AML1-ETO n:21 patients CBFb-MYH11 n:21 patients) or APL(n:22 patients) were included in the study (120 samples). Ages at diagnosis ranged from 1 to 76 years. Mean age was: AML1-ETO 36.8(2-76y),CBFb-MYH11 32.4(1-55y), PML-RARa 47.3(21-76y). Peripheral blood, reticulocyte count and a bone marrow aspirate were also available in 86 cases. WT1 was analyzed following the primers and conditions of the ELN group. Specific chimeric real time PCR was simultaneously performed in each sample in accordance with the BIOMED protocol. Each WT1 or chimeric PCR was analyzed in triplicate whereas the control gene (abl) was analyzed  in duplicate.

All patients with an available BM study were in morphologic complete remission. Two samples corresponded to an anemic patient (2/86). Six cases had a reticulocyte count above  2%. Platelet counts were below 120x10⁹/l in 3 cases (3/86). Leukocyte counts below 3.5x10⁹/l were observed in 6 samples (6/86). The calculated mean WT1 copy number from the 120 bone marrow samples was 47.23 (AML1-ETO:61.33, CBFb-MYH11:43.51, PML-RARa:36.4 p:ns). The mean chimeric copy number was 0.75 (AML1-ETO:0.82, CBFb-MYH11:0.72, PML-RARa:0.69 p:ns).  Only 5 samples had a specific copy number of chimeric transcripts above 5 (3 AML1-ETO,1 CBFb-MYH11 and one PMAL-RARa patient who had a molecular relapse one month later and was succesfully treated). In 6 samples, the specific chimeric copy number  was between 1 and 5 copies ( 4 AML1-ETO and 2 CBFb-MYH11). In the remaining 109 BM samples, the specific copy number was <1.  In 9 BM samples, the WT1 levels were in the range of 100-300 copies (7 AML1-ETO and 2 CBFb-MYH11). All these 9 samples had specific chimeric gene number  below 1.5 copies. In three samples (1 AML1-ETO, 1 CBFb-MYH11 and 1 PML-RARa) peaks of WT1 levels of ≥300 copies were detected. In these three samples the specific chimeric gene was undetectable (<0.01).

Bone marrow WT1 copy number in treated CBF-AML and APL was 47.23. In ten percent of cases, the WT1 copy number was greater than 100 copies. Occasionally, WT1 peaks (≥300 ) were detected. The meaning of the differential bone marrow WT1 transcriptional activity in long-term survivors of AML remains to be investigated.

No relevant conflicts of interest to declare.