The results herein indicate that inactivation of CREBBP/EP300
represents a common event in the two most frequent forms of B-NHL, namely FL and DLBCL. Previous extensive surveys in malignancies of epithelial origin have reported inactivating mutations of EP300
in exceedingly rare cases (<2% of primary biopsies)26–28
. Considering their virtual absence in solid tumors, and the finding of recurrent mutations in B-ALL34
, our results point to a specific role for CREBBP/EP300
inactivation in the pathogenesis of malignancies derived from B-lymphocytes. Overall, CREBBP/EP300
lesions are among the most frequent structural alterations yet detected in FL and DLBCL, thus representing an important feature of the pathogenesis of these common diseases. Moreover, the observation of significantly reduced CREBBP and EP300 expression levels in a sizeable fraction of DLBCL samples, independent of genetic lesions ( and Fig. S12
), suggests that additional epigenetic mechanisms may cause reduction of HAT dosage in a larger number of tumors.
One key observation of this study is that CREBBP/EP300
lesions are mostly detected in heterozygosity, suggesting a haploinsufficent role in tumor suppression. This notion is supported by several observations. First, congenital heterozygous mutations of CREBBP/EP300
are sufficient to cause significant pathologic and developmental phenotypes, including tumorigenesis, in patients with RTS23–25
, thereby confirming the deleterious effect of reduced HAT activity. Second, a fraction of mice with conditional deletion of Crebbp
in mature B-cells showed reduced survival past 12-months of age41
. Hematologic malignancies were also reported in 22% of constitutive 10–21 month-old Crebbp
heterozygous mice and ~30% of chimeric animals after bone marrow or spleen cell transplantation from Crebbp
. Third, in contrast with the abundantly expressed histone deacetylases (HDACs), HATs are limiting in the cell, suggesting that small dosage variations can have severe biological consequences43
. Overall, these data provide direct evidence for CREBBP (and, more rarely, EP300) as haploinsufficient tumor suppressors, whose specific role in lymphomagenesis will have to be tested by conditional deletion of these alleles in GC B-cells.
Given the global involvement of CREBBP/EP300 on gene transcriptional regulation, it is difficult to predict which cellular targets/pathways may be critically affected by HAT reduction in lymphomagenesis. At this stage, our results demonstrate that mutant CREBBP and EP300 proteins are deficient in acetylating BCL6 and p53, leading to constitutive activation of the oncoprotein and to decreased p53 tumor suppressor activity. The balance between the activities of these two genes is critical for the regulation of DNA damage responses in mature GC cells during immunoglobulin gene remodeling36,44
. Thus, the consequences of BCL6 activity overriding p53 would be an increased tolerance for DNA damage in the context of diminished apoptotic and cell cycle arrest responses.
These results have important therapeutic implications in view of current attempts to utilize a variety of HDAC inhibitors as anti-cancer drugs. While the benefits of these compounds have been proven in certain malignancies, such as mature T-cell lymphoproliferative disorders, their efficacy in other common cancers, including B-NHL, is uncertain at this stage45
. The findings of this study suggest that the use of HDAC inhibitors has a rational basis in B-NHL, as it may contribute to re-establishing physiologic acetylation levels. On the other hand, their efficacy should be re-evaluated by stratifying patients based on the presence of HAT defects as well as by testing the numerous HDAC and sirtuin inhibitors with target specificity.