Expression Of Axl And Its Ligand Gas6 In Acute Leukemia And Their Clinical Significances

BACK GROUND While acute leukemia (AL) is among the most chemotherapy-sensitive cancers. The treatment of AL has gained many advanced development during these years, however, multidrug resistance (MDR) decreases the curative effect of chemotherapy drastically. Axl is a member of the family of receptor tyrosine kinase,it was initially described as a transforming gene isolated from patients with chronic myeloid leukemia (CML) and chronic myeloproliferative disease, it was located at chromosome 19 q13.1 and has a mass of 140KD. Axl overexpression has also been reported in several types of human cancers, including prostatic, thyroid, breast, lung and gastric. Gas6, the biological ligand of Axl tyrosine kinase receptor, is so named by virtue of the initial finding that its encoding gene (growth arrest-specific gene 6) is highly expressed in growth-arrest cells. During recent years, Axl.combining with its ligand Gas6, can activate its own tyrosine kinase and the downstream signaling pathways. They play a role in cell proliferation, adhersion, cell transformation and anti-apoptosia. In U937 cells ectopically overexpressing Axl, addition of Gas6 induced Axl phosphorylation and activation of the PI3K/Akt and Raf/MEK/ERK pathways. The Gas6-Axl activation pathway of drug resistance was associated with increased expression of Bcl-2,Bcl-xL and Twist. How ever, little has been known about Gas6-Axl until now. Gas6-Axl may become an attractive and broadly applicable target for immunotherapeutic strategies and a new biochemical marker which can detecte minimal residual disease (MRD) in AML after getting complete remission (CR) and can estimate their prognosis.OBJECTIVE To investigate the expression of Axl,Gas6 and Bcl-2 gene in acute leukemia (AL) and to explore the role of Gas6-Axl in multidrug resistance and their clinical significance.METHODS1. Bone marrow mononuclear cells from 47 newly diagnosed,27 relapse/ refractory AL (including AML and ALL) patients and 12 normal controls were analyzed, Real-time reverse transcription polymerase chain reaction (RQ-RT-PCR) was used to detect the expressions of Axl,Gas6,Bcl-2 mRNA.2. All the statistical analyses were carried out using SPSS 16.0. The significance of different groups were evaluated by medians and P<0.05 was considered statistically signigicant.3. Spearman correlation coefficient was used to analyze the relevance between Axl and Gas6.4. Spearman correlation coefficient was used to analyze the relevance between Axl and Bcl-2.5. According to their response of chemotherapy, the newly diagnosed AML patients was divided into two groups:complete remission group (CR) and non-complete remission group (PR+NR), levels of Axl,Gas6 mRNA expression between the two groups were compared by Mann-Whitney U Test, respectively.6. Spearman correlation coefficient was used to analyze the relevance of Axl expression and clinical variables (including gender, age, peripheral WBC (white blood cell) counts,bone marrow blast percentage, chromosome karyotypes and immunophenotype).RESULTS1. No statistic difference of the median expression of Axl mRNA was found between the ALL group and normal control group (Z=-1.603, P>0.05), but the median expression of Axl mRNA in AML group were statistically higher than those in normal controls (Z=-2.800, P=0.005); The levels of Axl mRNA expression in the newly diagnosed AML group were higher than those in normal controls (Z=-2.124, P<0.05) and were significantly higher in the relapse/refractory AML group than in the newly diagnosed AML group (Z=-2.581, P<0.05). 2. The median expression of Gas6 mRNA in the ALL group and the AML group were significantly higher than those in normal controls ((Z=-3.621,-2.121, P<0.05); but there are statistically difference between the AML group and the ALL group (Z=-2.347, P<0.05); There was no statistic difference between the newly diagnosed ALL group and the relapse/refractory ALL group (Z=-0.707, P>0.05); The levels of Gas6 mRNA expression in the newly diagnosed AML group were higher than those in normal controls (Z=-3.180, P<0.05), and were significantly higher in the relapse/ refractory AML groups than in the newly diagnosed AML group (Z=-2.414, P =0.017).3. The median expression of Bcl-2 mRNA in the ALL group and the AML group were significantly higher than those in normal controls (Z=-3.145,-2.489, P<0.05); but there are statistically difference between the AML group and the ALL group (Z= 2.421, P<0.05); The levels of Bcl-2 mRNA expression in the newly diagnosed AML/ALL group were higher than those in normal controls P<0.05); and were significantly higher in the relapse/refractory AML/ALL groups than in the newly diagnosed AML/ALL group (P<0.05).4. The expressions of Axl and Gas6 were positively correlated in AML patients (r=0.514,P<0.01).5. The expressions of Axl and Bcl-2 were positively correlated in AML patients (r=0.613, P<0.01)6. Observation of the relationship between Ax1,Gas6 expression and therapeutic efficacy in AML:Compared to non-complete remission group, the complete remission group had lower expressions of Axl and Gas6(P<0.05).7. The relationship between Axl expression and progress of AML:The expression of Axl was followed in 4 AML patients in different phases of disease. The expression of Axl decrease in the first and second paients when they get CR. The level is high continually in third patient and he is in a NR condition and die in his disease. For the forth patient, the Axl turns low when she is CRand high when she relapses.8. The level of Axl mRNA expression had no correlation with peripheral WBC counts,bone marrow blast percentage, gender, age, chromosome karyotypes and immunophenotype(P>0.05).CONCLUSIONS1. No statistic difference of the median expression of Axl mRNA was found between the ALL group and normal control group. Axl maybe no related to the development and prognosis of ALL.2. The expressions of Axl as well as Gas6 were related to the multidrug resistance of AML cells, and these two genes were positively correlated.3. The expression of Bcl-2 in the ALL group and the AML group were significantly higher than those in normal controls; but Bcl-2 expression in ALL group were higher than in AML group, which might provide explain to the causes of lower remission rates and higher relapse rates in ALL patients.4. The expressions of Axl and Bcl-2 were positively correlated, which show that Bcl-2 and Gas6-Axl were closely related to the multidrug resistance development and prognosis of AML. But wether Bcl-2 was caused by Gas6-Axl, it need to other evidence.5. Over-expression of Gas6-Axl may lead to poor remission rate of AML pations; These two genes were important factors in poor therapeutic effect and unfavourable prognosis.6. The expression of Axl decreases when getting CR and turns higher when relapse. Therefore, detecting the expression of Axl can be used as detecting minimal residual disease. When Axl expression change from low to high, the early relapse can be monitored with molecular biology diagnosis.7. The level of Axl mRNA expression had no correlation with peripheral WBC counts,bone marrow blast percentage, gender, age, chromosome karyotypes and immunophenotype, which demonstrate Gas6-Axl may be predictors independent these clinical variables.