Promotor Demethylation as a Mechanism of HOX11 Activation in Adult T-ALL?

Promotor demethylation of oncogenes has been associated with transcriptional activation in cancer cells. The proto-oncogene HOX11/TLX1 has been found to be aberrantly expressed in up to 30% of adult T-ALL patients. In few, a translocation between the HOX11 locus at 10q24 and the T-cell receptor locus has been identified. In the majority of cases the mechanism leading to HOX11 reactivation remains unclear. It had been proposed that an epigenetical modification by demethylation of the proximal HOX11 promotor could be responsible for an aberrant expression of HOX11. To test this hypothesis we have correlated the methylation status of CpG residues in the proximal HOX11 promotor with the gene expression status of HOX11 in adult T-ALL samples from the German Multicenter ALL Study (GMALL) 5/93 and 6/99. HOX11 expression was measured in 286 pretreatment peripheral blood and bone marrow blasts by comparative real-time RT-PCR as described previously. The methylation status was then randomly analyzed after bisulphite treatment by methylation-specific PCR (MSP) in 53 T-ALL samples with HOX11 expression (HOX11 positive) and 102 samples without HOX11 expression (HOX11 negative). Of the 150 analyzed patients only 4% of HOX11 positive patients (n=53) and 10% of HOX11 negative patients were methylated (M) in the analyzed promotor area. HOX11 negative patients were significantly more common associated with an unmethylated status (U) then HOX11 positive patients (57% vs 32%, p=0.003). The most prominent methylation phenotype in HOX11 positive patients compared to HOX11 negative samples was a mixed (MU) methylation status (55% vs. 27%, p=0.001). Interestingly, remission duration was significantly higher in pt. with the MU methylation status (M=49%, U=54% vs. MU=76%, p-logrank=0.0362). This translated also into a significant difference in the overall survival (M=55%, U=49%, MU=75%, p-logrank= 0.0202). However, in a multivariate analysis the methylation status could not be confirmed as an independent prognostic factor. The promotor-associated CpG methylation status was found to be remarkably heterogenous in the analyzed adult T-ALL patients with a predominantly unmethylated status in HOX11 negative samples and a mixed methylated/unmethylated picture in HOX11 positive samples. These findings contrast with our initial hypothesis that HOX11 expression is silenced in normal tissue by a promotor-associated CpG methylation and aberrantly reexpressed in leukemia cells by demethylation. However, various CpG residues associated with the HOX11 promotor might be of variable importance for the gene expression and intrinsic limitations of methylation-specific PCR might have influenced our analysis. Bisulphite sequencing techniques as well as the newly developed genome-wide cytosine methylation array could help overcome these issues. Understanding the role of promotor-associated methylation in HOX11 expression could clarify pathways in leukemogenesis and provide a valuable tool for better risk stratification in T-ALL.