First name
Valentina
Last name
Petrocelli
Year of Study
Research Center
Thesis Title
Role of Ezh2 methyltransferase activity in the maintenance of MYC-driven B cell lymphomas
Thesis Abstract
The Polycomb group protein Ezh2 catalyzes the Histone H3 lysine-27 trimethylation
(H3K27me3) within the Polycomb Repressive Complex 2 (PRC2). PRC2 exerts a
critical control over the expression of a large set of target genes controlling important
biological functions, including cell proliferation, differentiation and stem cell selfrenewal.
Aberrant Ezh2 function is commonly observed in several cancer types and is
due to deregulated enzymatic activity and/or expression of the Polycomb protein.
Studies in preclinical models have started to reveal the importance of Ezh2 in B cell
lymphomagenesis. In contrast, little is known about the effects of Ezh2 deregulated
function/constitutive expression in B cell tumor maintenance and progression.
The present study addresses this issue taking advantage of a MYC-driven mouse
lymphoma model, featuring high Ezh2 expression as a result of malignant B cell
transformation. Conditional, genetic inactivation of Ezh2 methyltransferase activity in
aggressive primary Burkitt-like mouse B cell lymphomas led to the identification of
two classes of tumors, differentially responding to the loss of Polycomb function. In
type-1 lymphomas, Ezh2 inactivation impaired clonal tumor growth starting from
single lymphoma cells. Instead, type-2 lymphomas were largely resistant to the loss of
Ezh2 catalytic function, giving rise to a substantial number of Ezh2 mutant clones.
Transcriptome analyses allowed the identification of a molecular signature
discriminating type-1 from type-2 lymphomas, including genes controlling cell cycle
progression, DNA replication and cell survival, which were more expressed in type-2
tumors. These results correlated with a more aggressive behavior of type-2
lymphomas when transplantated into immunoproficient hosts.
The growth of rare Ezh2 mutant subclones, established from type-1 lymphomas, was
impaired by the treatment with an Ezh1/2 small molecule inhibitor, identifying the
Ezh2 paralog, Ezh1, as a determinant of resistance of tumor cells to Ezh2 inactivation.
Ezh2 inhibition led to genome wide loss of H3K27me3, which was comparable
between lymphoma types. However, while the loss of H3K27me3 at target genes in
type-1 lymphomas failed to alter their expression, in type-2 lymphomas Ezh2 targets
were in most cases deregulated following the loss of the histone mark. Based on these
results, we propose that Ezh2 mutant subclones from type-1 lymphomas select an
H3K27me3-independent mechanism to ensure correct regulation of Ezh2 target
genes, which is needed for tumor growth. We also find that residual H3K27me3 is
deposited at the promoter of new genes by a non-canonical PRC2/Ezh1, in Ezh2
mutant subclones from type-1 lymphomas. This activity alters the expression of target
genes contributing to tumor growth.
We finally report the isolation of clonal variants from type-1 lymphomas that acquire
secondary resistance to pharmacological Ezh1/2 inhibition. The latter tumors
(together with type-2 lymphomas) will be instrumental to unravel the genetic bases of
resistance of MYC-driven lymphomas to PRC2 inhibition.
Anti-Ezh2 inhibitors are currently being tested in phase-1 and -2 clinical trials for the
treatment of both solid and blood cancers including B cell lymphomas. Our studies
highlight the importance of understanding in more detail the mechanisms of action of
Ezh2/PRC2 in tumors, in order to identify those that may benefit from anti-Ezh2
therapies. Our results also provide evidence for mechanisms of lymphoma resistance
to Ezh2 inhibition and suggest strategies to circumvent such resistance.
(H3K27me3) within the Polycomb Repressive Complex 2 (PRC2). PRC2 exerts a
critical control over the expression of a large set of target genes controlling important
biological functions, including cell proliferation, differentiation and stem cell selfrenewal.
Aberrant Ezh2 function is commonly observed in several cancer types and is
due to deregulated enzymatic activity and/or expression of the Polycomb protein.
Studies in preclinical models have started to reveal the importance of Ezh2 in B cell
lymphomagenesis. In contrast, little is known about the effects of Ezh2 deregulated
function/constitutive expression in B cell tumor maintenance and progression.
The present study addresses this issue taking advantage of a MYC-driven mouse
lymphoma model, featuring high Ezh2 expression as a result of malignant B cell
transformation. Conditional, genetic inactivation of Ezh2 methyltransferase activity in
aggressive primary Burkitt-like mouse B cell lymphomas led to the identification of
two classes of tumors, differentially responding to the loss of Polycomb function. In
type-1 lymphomas, Ezh2 inactivation impaired clonal tumor growth starting from
single lymphoma cells. Instead, type-2 lymphomas were largely resistant to the loss of
Ezh2 catalytic function, giving rise to a substantial number of Ezh2 mutant clones.
Transcriptome analyses allowed the identification of a molecular signature
discriminating type-1 from type-2 lymphomas, including genes controlling cell cycle
progression, DNA replication and cell survival, which were more expressed in type-2
tumors. These results correlated with a more aggressive behavior of type-2
lymphomas when transplantated into immunoproficient hosts.
The growth of rare Ezh2 mutant subclones, established from type-1 lymphomas, was
impaired by the treatment with an Ezh1/2 small molecule inhibitor, identifying the
Ezh2 paralog, Ezh1, as a determinant of resistance of tumor cells to Ezh2 inactivation.
Ezh2 inhibition led to genome wide loss of H3K27me3, which was comparable
between lymphoma types. However, while the loss of H3K27me3 at target genes in
type-1 lymphomas failed to alter their expression, in type-2 lymphomas Ezh2 targets
were in most cases deregulated following the loss of the histone mark. Based on these
results, we propose that Ezh2 mutant subclones from type-1 lymphomas select an
H3K27me3-independent mechanism to ensure correct regulation of Ezh2 target
genes, which is needed for tumor growth. We also find that residual H3K27me3 is
deposited at the promoter of new genes by a non-canonical PRC2/Ezh1, in Ezh2
mutant subclones from type-1 lymphomas. This activity alters the expression of target
genes contributing to tumor growth.
We finally report the isolation of clonal variants from type-1 lymphomas that acquire
secondary resistance to pharmacological Ezh1/2 inhibition. The latter tumors
(together with type-2 lymphomas) will be instrumental to unravel the genetic bases of
resistance of MYC-driven lymphomas to PRC2 inhibition.
Anti-Ezh2 inhibitors are currently being tested in phase-1 and -2 clinical trials for the
treatment of both solid and blood cancers including B cell lymphomas. Our studies
highlight the importance of understanding in more detail the mechanisms of action of
Ezh2/PRC2 in tumors, in order to identify those that may benefit from anti-Ezh2
therapies. Our results also provide evidence for mechanisms of lymphoma resistance
to Ezh2 inhibition and suggest strategies to circumvent such resistance.
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