EZH2 in MPNs and MDS

Thomas Ernst, MD

Barcelona—New research indicates mutations of the histone methyltransferase EZH2 confer poor prognosis in myeloproliferative neoplasms (MPNs) and myelodysplastic syndrome (MDS), which suggests EZH2 acts as a tumor suppressor in myeloid malignancies.

Thomas Ernst, MD, of Wessex Regional Genetics Laboratory in the United Kingdom, discussed the research that led to these conclusions at the Presidential Symposium of the 15th Congress of the European Hematology Association, which took place June 10-13.

Dr Ernst said he and his fellow researchers began by performing genome-wide single nucleotide polymorphism analysis on 148 cases of MDS or MPN. Twelve of these cases had 7q acquired uniparental disomy (aUPD), a characteristic that was recently reported to be common in MPNs and MDS but uncommon in acute myeloid leukemia.

When searching for microdeletions, the investigators found a 400kb deletion at 7q36.1 that encompassed C7orf33, CUL1, and EZH2. Subsequent sequencing uncovered a frameshift mutation in the residual EZH2 allele of the deleted case as well as homozygous EZH2 mutations in 9 of the 12 cases with 7q aUPD.

“This means that 7q aUPD is associated with EZH2 mutations,” Dr Ernst said. “And we found subclonal mutations as well as missense mutations, as well as nonsense mutations in these 7q aUPD patients.”

The researchers then attempted to ascertain the prevalence of EZH2 variants in 645 samples from patients with MPN, MDS, or MDS/MPN. The team identified EZH2 mutations in 44% of these patients, most commonly in MDS/MPN and myelofibrosis. However, when they screened 54 cases of acute myeloid leukemia with chromosome 7 loss, Dr Ernst and his colleagues uncovered no EZH2 mutations.

Once they had determined the prevalence of EZH2 mutations, the researchers decided to examine the mutations more closely in MDS/MPN cases. This analysis revealed an association between EZH2 mutations and poor prognosis. Overall survival was 39 months in cases without EZH2 mutations and 13 months in those with the mutations. Similarly, progression-free survival was 30 months in cases without EZH2 mutations and 17 months in those with them.

“Of course, this data is very preliminary and needs to be confirmed in prospective studies,” Dr Ernst said.

Once they had described the role of EZH2 mutations in MDS and MPNs, the researchers’ final step was to examine the role of EZH2 itself. So they infected Sf9 cells with baculoviruses expressing FLAG-tagged EZH2 mutants as well as EED and SUZ12.

Immunoprecipitation and H3K27 methyltransferase assays revealed that all of the mutants abrogated or greatly reduced EZH2 catalytic activity. This finding, along with their other results, prompted Dr Ernst and his colleagues to conclude that EZH2 acts as a tumor suppressor in MDS and MPNs.