Genetic and Epigenetic Silencing of Mesoderm Specific Transcript (MEST) In Acute Myelogenous Leukemia.

Abstract 3639

About 10 % of patients with either myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) present either with deletions on the long arm of chromosome 7 or monosomy 7. These chromosomal aberrations are associated with a poor prognosis. Following biallelic inactivation as proposed by Knudsen, a “second hit” of the remaining allele might be required for loss of gene function. Epigenetic silencing might display such a hit in particular since no additional genetic hits could be identified so far. The role of epigenetic regulation might be of special importance in patients with -7/7q-, since several studies have shown that patients harbouring a chromosome 7 abberrations do more benefit from a therapy with demethylating agents as compared to patients with other chromosomal changes.

To address the issue of epigenetic silencing in these AML cases we utilized DNA methylation profiling to identify the potential tumor suppressor genes on chromosome 7.

We used MBD2 based enrichment of methylated DNA from 4 AML patient samples with monosomy 7 and from 23 patient samples with with other chromosomal changes including normal karyotype AML. For validation we analyzed regional DNA methylation using quantitative MassArray technology on DNA from 115 del(7q) AML or monosomy 7 patients as well as normal karyotype patients (n=20) and CD34 + cells from healthy individuals (n=5).

We could identify the gene MEST to be silenced by hypermethylation (> 30 %) of a CpG island on the remaining allele in 20% of the patients with -7/7q- and 40% of patients with normal karyotype. Thus, preferentially in patients that do not have a deletion already.

MEST is an imprinted gene located on 7q32.2. However, silencing is correlated with hypermethylation of a CpG island located at an alternative promoter of MEST, independent of the methylation status of the imprinting control region.

In two patient samples with monosomy 7 and hypermethylation of MEST (> 80%), DNA methylation was erased after 5-Aza-2′-deoxycytidine (DAC) treatment to less than 10 %. In concordance, also AML cell lines with hypermethylation of the MEST locus loose methylation after sublethal DAC treatment which leads to a reexpression of the gene.

Thus, we hypothesize that MEST functions as a tumor suppressor in AML and is genetically as well as epigenetically silenced AML. Reactivation of MEST by demethylating treatment may contribute to the mechanism by which demethylating drugs display their therapeutic potential in leukemia.