SUMMARY The recognition of modified histones by “reader” proteins constitutes a key mechanism regulating gene expression in the chromatin context. Compared with the great variety of readers for histone methylation, few protein modules that recognize histone acetylation are known. Here we show that the AF9 YEATS domain binds strongly to histone H3K9 acetylation and, to a lesser extent, H3K27 and H3K18 acetylation. Crystal structural studies revealed that AF9 YEATS adopts an eight-stranded immunoglobin fold and utilizes a serine-lined aromatic “sandwiching” cage for acetyllysine readout, representing a novel recognition mechanism that is distinct from that of known acetyllysine readers. ChIP-seq experiments revealed a strong co-localization of AF9 and H3K9 acetylation genome-wide, which is important for the chromatin recruitment of the H3K79 methyltransferase DOT1L. Together, our studies identified the evolutionarily conserved YEATS domain as a novel acetyllysine-binding module and established a direct link between histone acetylation and DOT1L-mediated H3K79 methylation in transcription control.

Recognition of modified histones by “reader” proteins plays a critical role in the regulation of chromatin1. H3K36 trimethylation (H3K36me3) is deposited onto the nucleosomes in the transcribed regions following RNA polymerase II (Pol II) elongation. In yeast, this mark in turn recruits epigenetic regulators to reset the chromatin to a relatively repressive state thus suppressing cryptic transcription2. However, much less is known about the role of H3K36me3 in transcription regulation in mammals. This is further complicated by the transcription-coupled incorporation of the histone variant H3.3 in gene bodies3. Here we show that the candidate tumor suppressor ZMYND11 specifically recognizes H3K36me3 on H3.3 (H3.3K36me3) and regulates Pol II elongation. Structural studies reveal that in addition to the trimethyl-lysine binding by an aromatic cage within the PWWP domain, the H3.3-dependent recognition is mediated by the encapsulation of the H3.3-specific “Ser31” residue in a composite pocket formed by the tandem bromo-PWWP domains of ZMYND11. ChIP-sequencing analyses reveal a genome-wide colocalization of ZMYND11 with H3K36me3 and H3.3 in gene bodies, and its occupancy requires the pre-deposition of H3.3K36me3. Although ZMYND11 is associated with highly expressed genes, it functions as an unconventional transcription corepressor via modulating Pol II at the elongation stage. ZMYND11 is critical for the repression of a transcriptional program that is essential for tumor cell growth; low expression level of ZMYND11 in breast cancer patients correlates with worse prognosis. Consistently, overexpression of ZMYND11 suppresses cancer cell growth in vitro and tumor formation in mice. Together, this study identifies ZMYND11 as an H3.3-specific reader of H3K36me3 that links the histone variant-mediated transcription elongation control to tumor suppression.

MicroRNA-34a (miR-34a), a transcriptional target of p53, is a well-known tumor suppressor gene. Here, we identified Fra-1 as a new target of miR-34a and demonstrated that miR-34a inhibits Fra-1 expression at both protein and messenger RNA levels. In addition, we found that p53 indirectly regulates Fra-1 expression via a miR-34a-dependant manner in colon cancer cells. Overexpression of miR-34a strongly inhibited colon cancer cell migration and invasion, which can be partially rescued by forced expression of the Fra-1 transcript lacking the 3'-untranslated region. The expression of matrix metalloproteinase (MMP)-1 and MMP-9, two enzymes involved in cell migration and invasion, was decreased in miR-34a-transfected cells, and this can be rescued by Fra-1 overexpression. Moreover, we found that miR-34a was downregulated in 25 of 40 (62.5%) colon cancer tissues, as compared with the adjacent normal colon tissues and that the expression of miR-34a was correlated with the DNA-binding activity of p53. Unexpectedly, the DNA-binding activity of p53 was not inversely correlated with Fra-1 expression, and a significant statistical inverse correlation between miR-34a and Fra-1 expression was only observed in 14 of 40 (35%) colon cancer tissues. Taken together, our in vitro data suggest that p53 regulates Fra-1 expression, and eventually cell migration/invasion, via a miR-34a-dependent manner. However, in vivo data indicate that the p53-miR-34a pathway is not the major regulator of Fra-1 expression in human colon cancer tissues.

Level II.

Increased periductal echogenicity and floating echogenic foci in the gallbladder were identified as the 2 most significant findings for the sonographic diagnosis of active C. sinensis infection.

Hepatitis B surface antigen (HBsAg) seropositivity is an important risk factor for hepatocellular carcinoma (HCC), and HBsAg-transgenic mice have been reported to spontaneously develop HCC. The major histocompatibility complex class I-related molecules A and B (MICA and MICB) are NKG2D ligands that play important roles in tumor immune surveillance. In the present study, we found that HBsAg overexpression in HepG2 cells led to upregulation of 133 and downregulation of 9 microRNAs (miRNAs). Interestingly, several HBsAg-induced miRNAs repressed the expression of MICA and MICB via targeting their 3'-untranslated regions. In addition, the expression of MICA and MICB was significantly reduced upon HBsAg overexpression, which was partially restored by inhibiting the activities of HBsAg-induced miRNAs. Moreover, HBsAg-overexpressing HCC cells exhibited reduced sensitivity to natural killer cell-mediated cytolysis. Taken together, our data suggest that HBsAg supresses the expression of MICA and MICB via induction of cellular miRNAs, thereby preventing NKG2D-mediated elimination of HCC cells.

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