BMI-1 and SALL4 are stem cell genes that modulate stem cell pluripotency and play a role in leukemogenesis. Dysregulated expression of both genes may have a cooperative effect in leukemogenesis . Patients with RA and RARS who have a higher percentage of BMI-1+ cells showed disease progression to RAEB, suggesting that BMI-1 is a novel molecular marker that predicts the progression and prognosis of MDS . In this study, we analyzed BMI-1 and SALL4 expression in primary AML and CML at diagnosis and those in complete remission.
It has been shown that BMI-1 overexpression occurs in a variety of cancers including several types of leukemias and lymphomas . In this study, BMI-1 was found to be overexpressed in AML and chronic phase CML patient groups; and its expression level was lower in patients who achieved complete remission. Similar results were reported by Sawa, M et al. who found that moderate to high BMI-1 expression was detected in AML patients, and the AML-M0 subtype showed higher relative expression of the BMI-1 transcript . In addition, Merkerova, M et al. demonstrated that BMI-1 and its significantly higher BMI-1 transcript level in CML cells seem to play a secondary role in CML transformation . Our results also indicate that a decreased BMI-1 expression level is associated with complete disease remission. Interestingly, the BMI-1 expression level in the CML-BC group appeared to be low in comparison with the de novo CML group, although the difference was not significant. Further investigation is needed using a larger patient cohort to extend our findings. Preliminary results indicate that BMI-1 may have potential as a therapeutic target for myeloid leukemia. It has been reported that BMI-1 depletion by RNA interference leads to reduced U937 cell growth and proliferation and increased apoptosis , and an antisense BMI-1 gene can inhibit the growth of K562 cells and upregulate p16 expression in K562 cells .
Using immunohistochemistry and real-time PCR, SALL4 was demonstrated to be constitutively expressed in human primary acute myeloid leukemia . In this study, we found that SALL4 was overexpressed in different primary AML subtypes, and its expression was lower in the AML-CR patient group. These results are similar to the findings of Jeong, HW et al. who showed that AML patients who responded to treatment had decreasing SALL4 expression throughout the course of treatment, while AML patients with disease relapse or drug resistance had increasing SALL4 expression, which was correlated with disease progression . Interestingly, unlike the SALL4 expression characteristics in AML, the SALL4 expression level in the CML-CP and CML-CR groups was lower. Moreover, the SALL4 expression level in patients with chronic phase CML was significantly lower than that in the CML-CR group. There is no direct evidence demonstrating the SALL4 expression level in CML-CP and comparing the expression feature to healthy individuals; however, Lu and colleagues have found that the SALL4 protein was overexpressed in CML samples in blast crisis but not those in chronic phase by FACS . Our results also demonstrated that SALL4 expression was higher in the CML-BC group in comparison with the CML-CP and CML-CR groups; however, there was no significant difference in comparison with the HI group. Is it possible that SALL4 is preferentially expressed in leukemic blasts? These results are similar to a report by Cui W et al. who demonstrated that only precursor B-cell lymphoblastic leukemias/lymphomas and AML had detectable SALL4 in neoplastic tissues . The different SALL4 expression patterns in AML and CML suggest that these two disease entities may have different biological characteristics and/or mechanisms of leukemogenesis, at least for the association between SALL4 and pathogenesis. However, there are not reports comparing the data of SALL4 expression level in CML-CP to healthy individuals, it is difficult to evaluate the significance of this finding. Recently, research from Zhu et al. showed that hematopoietic transcription factor PU.1 expression was significantly lower in newly diagnosed APL patient samples as compared to normal hematopoietic cells, which may relate to the expression level of PML-RARα, and they found that suppression of PU.1 expression occurred concurrently with PML-RARα expression, the authors suggested that low PU.1 expression in APL patients is required for disease initiation and progression . This finding might provide a direction in farther analysis the correlation of SALL4 with BCR-ABL in the pathogenesis of CML and to address this question.
In principle, the BMI-1 and SALL4 gene expression level should be positively correlated in stem cells . Little is known about the expression pattern and differences in the SALL4 and BMI-1 genes in patients with AML and CML. In this study, we analyzed the correlation between the relative expression levels of BMI-1 and SALL4. A positive expression level correlation was found for both genes in HI, AML, chronic phase CML, CML-BC and CML-CR patient groups; however, there was no significant correlation between these genes in patients with AML-CR, leaving their role in this group an open question. These results indicate that a positively correlated expression pattern is a common feature in patients with myeloid leukemia and healthy individuals, and both genes may cooperate during cell proliferation and differentiation.
Based on the different expression features of SALL4 in AML and CML, we further analyzed its regulating gene ABCA3, which is a member of the ATP-binding cassette (ABC) family of transport proteins [30, 31]. Unlike the description by Wult, Norwood and Steinbach groups, who showed that ABCA3 is highly expressed in acute meyloid leukemia samples and is associated with unfavorable clinical treatment outcome [24, 33, 45], in the present study, lower expression level of ABCA3 was found not only in AML but also in CML groups, especially in CML-CP and CR groups. Moreover, the expression level of ABCA3 lost the correlation with SALL4 expression in leukemia patients. To determine whether these results relate to favorable clinical outcome, further investigation is needed. Additionally, detection of ABCA2, ABCB2 and ABCC10, which were found overexpressed in childhood AML, may be worthy to build the gene regulation network in proliferation of myeloid leukemia cells.
In conclusion, we determined the expression characteristics of the SALL4, ABCA3 and BMI-1 genes in different phases of AML and CML. Further studies will be needed to determine whether BMI-1 and SALL4 are novel therapeutic targets for leukemic stem/initiation cells in primary myeloid leukemia.