Abstract
Homeobox genes (HOX) encode transcription factors that are frequently deregulated in leukemias. Our previous findings described that HOX gene expression differs among genetically characterized subtypes of pediatric AML with PML-RARa+ patients having the lowest overall HOX gene expression. We observed that HOX gene expression positively correlated with expression of histone 3 lysine 27 (H3K27) demethylases JMJD3 and UTX and negatively with DNA methyltransferase DNMT3b. Interestingly, it has been shown that JMJD3 is a direct target of PML-RARa protein (Martens, JH et al, 2010, Cancer Cell). These findings led us to postulate the hypothesis that reduced levels of HOX genes in PML-RARa+ AML can be caused by the suppressed expression of histone demethylases, such as JMJD3 and UTX, resulting in increased H3K27 methylation and transcription inhibition.
We chose PML-RARa+ NB4 cell line to study the role of PML-RARa fusion gene in the regulation of HOX gene expression. To inhibit the effect of PML-RARa we used all-trans retinoic acid (ATRA; 1 uM, 10 uM) which was described to release the block caused by this fusion protein. Expression of particular HOX genes (e.g., HOXA1, HOXA3, HOXA5, HOXA7) together with that of JMJD3 and UTX assessed by qPCR was significantly elevated after ATRA treatment, while gene expression of DNMT3b was decreased.
To test whether the reduction in HOX gene expression is directly related to the levels of JMJD3 and UTX, we cultured NB4 cells with a specific inhibitor of these histone demethylases, GSK-J4 (1 uM, 10 uM), in combination with ATRA. This co-treatment led to inhibition of JMJD3 and UTX proteins, followed by significant reduction of HOX genes expression (e.g., HOXA1, HOXA3, HOXA5, HOXA7). This result supports our hypothesis that HOX genes expression is directly related to JMJD3/UTX activity.
To determine the effect of ATRA and GSK-J4 on histone marks we have isolated histones by acid extraction and detected the levels of histones by western blot in NB4 ATRA or GSK-J4/ATRA treated cells. We observed that the level of repressive histone methylation mark (trimethylated H3K27; H3K27me3) was decreased after ATRA treatment (activation of JMJD3/UTX) and increased after GSK-J4/ATRA co-treatment (inhibition of JMJD3/UTX). The opposite effect was observed in active histone methylation marks where di- and tri-methylated H3K4 (H3K4me2, H3K4me3) increased after ATRA treatment and decreased after GSK-J4/ATRA co-treatment. H3K9 dimethylated (another repressive histone methylation mark) levels did not change.
Next, to investigate the histone code directly in particular HOX genes regions we performed chromatin immunoprecipitation (ChIP) assays. We studied the presence of H3K27me3 and H3K4me2 in 5´UTR genomic region of particular HOX genes (HOXA1, HOXA2, HOXA3, HOXA5, HOXA7) in cells treated with ATRA alone or in the combination with GSK-J4. Preliminary results showed reduction in repressive marks (H3K27me3) upon ATRA treatment, whereas addition of GSK-J4 prevented this decrease. Accordingly, we observed that ATRA/GSK-J4 co-treatment reduced active histone mark H3K4me2.
To evaluate the role of DNA methylation in observed expression changes after ATRA treatment we performed bisulfite sequencing of particular promoter sites of HOX genes (e.g., HOXA7, HOXA5). Although we detected decreased DNMT3b gene expression after ATRA treatment there was no change in DNA methylation of CpGs in studied regions.
Our results demonstrate that changes in chromatin activity correspond with changes in HOX gene expression. Moreover, ChIP data show direct binding of the modified histones and HOX 5´UTR sites. Our data implicate histone demethylases in regulation of HOX gene expression in PML-RARa+ leukemic blasts. DNA methylation in these particular HOX genes is not involved in the regulation. Elucidating the mechanism of regulation of HOX genes expression can help to understand their role in the leukemogenic process.
Supported by GACR P304/12/2214 and GAUK 568213.
Disclosures
No relevant conflicts of interest to declare.
DNA methylation pathways and their crosstalk with histone methylation