Variable but Consistent Pattern of Meningioma 1 Gene (MN1) Expression in Different Genetic Subsets of Acute Myelogenous Leukaemia and Its Potential Use as Marker for Minimal Residual Disease Detection.

Meningioma 1 (MN1) gene overexpression has been reported in acute myeloid leukaemia (AML) patients and identified as a negative prognostic factor. In order to characterize the patients presenting the gene overexpression and to verify if MN1 transcript could be a useful marker for minimal residual disease detection, MN1 has been quantified by RQ-PCR in 136 AML patients of different cytogenetic groups and in 50 normal controls. In 20 patients bearing a fusion gene transcript (FG) suitable for MRD assessment, we performed a simultaneous analysis of the MN1 and of the FG transcript during follow-up. Sequential MN1 and WT1 analysis was also performed in 10 AML patients lacking other molecular markers. The MN1 levels were extremely low in normal samples: the median of 2^−ΔΔCt is 4,6 ±2,9 (range 3–10) in PB and 16 ±19,6 (range 6–50) in BM and 12,9 ±4,8 (range 11–19) in normal CD34+ cells. Conversely, about 50% of the AML samples with normal karyotype (NK) showed high expression of the MN1 gene with a median value of 2^−ΔΔCt =111±590 (range 52–2352) in BM and 101± 399 (range 12–1136) in PB. All samples carrying the CBFβ-MYH11 FG expressed a significantly higher amount of MN1 transcript as compared to controls (p<0,0001 in both BM and PB): median =1176±1180 (range 362–2272) in BM and 588±401 (range 17–1060) in PB. About 50% of the samples with AML1-ETO FG abnormally expressed MN1: median of 89±58 (range 55–181) in BM and 54±29 (range 21–81) in PB. Finally, the APL samples expressed MN1 values comparable to those of healthy subjects in both BM (p= 0,05) and PB (p=0,08). Interestingly, the paired analysis established a remarkable correlation between MN1 expression in PB and BM with a r value of 0,9627. Stratification of patients according to the presence of FLT3 mutation or ITD demonstrated no significant association between the two abnormalities. In contrast, MN1 overexpression is typically present in patients with mutations in NPM1. 36 out of 47 patients presenting NPM1 mutations were characterized by abnormal expression of MN1. Finally, we were unable to find any significant correlation between EVI-1 and MN1 expression (r= 0,06). To assess the significance of MN1 as a marker for MRD detection in AML, the MN1 transcript was quantified during follow-up of 20 AML patients characterized by the presence of FG (15 CBFβ-MYH11 and 5 AML1-ETO) and 10 patients lacking additional markers monitored by WT1 quantitative assessment, In all cases characterized by FG transcript, the longitudinal pattern of MN1 expression always paralleled that of the FG. Furthermore, MN1 strictly paralleled WT1 in patients without any FG. In all the cases MN1 rose at least two months before relapse. In conclusion, the data obtained show that high levels of MN1 expression are present in 47% of patients with NK primarily in those with wild type NPM1, and in all cases with inv(16). The MN1 levels during follow-up were found to follow the pattern of the other molecular markers (fusion gene transcripts and WT1). Increased MN1 expression in the BM during follow up was always found to be predictive of an impending hematological relapse.