Hepatocellular carcinoma (HCC) is a frequent form of cancer with a poor prognosis and with limited possibilities for medical intervention. Recent evidence has accumulated that long noncoding RNAs (lncRNAs) are important regulators of disease processes including cancer. Chromatin remodeling in cancer cells may result in an unusual expression of lncRNAs and indeed it has been shown that more than 7000 unannotated lncRNAs are expressed in HCCs. We identified a novel long intergenic noncoding RNA, Linc00176, that plays a role in proliferation and survival of HCC. Linc00176 regulates expression of more than 200 genes by the sponge function for tumor suppressor miRNAs, miR-9 and miR-185. Linc00176 is expressed at a high level only in HCC, and is activated by Myc, Max and AP-4 transcription regulators. Myc also upregulates miR-9 and miR-185. In Linc00176-depleted HCC, these miRNAs were released from Linc00176 and downregulated their target mRNAs. Thus, depletion of Linc00176 disrupted the cell cycle and induced necroptosis in HCC via released tumor suppressor miRNAs. These data indicate that atypically expressed lncRNAs may be useful targets for cancer therapy.

The TREX (transcription/export) complex has been conserved throughout evolution from yeast to man and is required for coupled transcription elongation and nuclear export of mRNAs. The TREX complex in mammals and Drosophila is composed of the THO subcomplex (THOC1, THOC2, THOC5, THOC6, and THOC7), THOC3, UAP56, and Aly/THOC4. In human and Drosophila, various studies have shown that THO is required for the export of heat shock mRNAs, but nothing is known about other mRNAs. Our previous study using conditional THOC5 (or FMIP ) knockout mice revealed that the presence of THOC5 is critical in hematopoietic cells but not for terminally differentiated cells. In this study, we describe the establishment of a mouse embryo fibroblast cell line (MEF), THOC5 flox/flox. Four days after infection of MEF THOC5 flox/flox with adenovirus carrying Cre-recombinase gene (Ad-GFP-Cre), THOC5 is down-regulated >95% at the protein level, and cell growth is strongly suppressed. Transcriptome analysis using cytoplasmic RNA isolated from cells lacking functional THOC5 reveals that only 2.9% of all genes were down-regulated more than twofold. Although we examined these genes in fibroblasts, one-fifth of all downregulated genes (including HoxB3 and polycomb CBX2) are known to play a key role in hematopoietic development. We further identified 10 genes that are spliced but not exported to the cytoplasm in the absence of THOC5. These mRNAs were copurified with THOC5. Furthermore, Hsp70 mRNA was exported in the absence of THOC5 at 37°C, but not under heat shock condition (42°C), suggesting that THOC5 may be required for mRNA export under stress and/or upon signaling-induced conditions.

Deficiencies in DNA mismatch repair (MMR) result in increased mutation rates and cancer risk in both humans and mice. Mouse strains homozygous for knockouts of either the Pms2 or Mlh1 MMR gene develop cancer but exhibit very different tumor spectra; only Mlh1 ؊/؊ animals develop intestinal tumors. We carried out a detailed study of the microsatellite mutation spectra in each knockout strain. Five mononucleotide repeat tracts at four different chromosomal locations were studied by using single-molecule PCR or an in vivo forward mutation assay. Three dinucleotide repeat loci also were examined. Surprisingly, the mononucleotide repeat mutation frequency in Mlh1 ؊/؊ mice was 2-to 3-fold higher than in Pms2 ؊/؊ animals. The higher mutation frequency in Mlh1 ؊/؊ mice may be a consequence of some residual DNA repair capacity in the Pms2 ؊/؊ animals. Relevant to this idea, we observed that Pms2 ؊/؊ mice exhibit almost normal levels of Mlh1p, whereas Mlh1 ؊/؊ animals lack both Mlh1p and Pms2p. Comparison between Mlh1 ؊/؊ animals and Mlh1 ؊/؊ and Pms2 ؊/؊ double knockout mice revealed little difference in mutator phenotype, suggesting that Mlh1 nullizygosity is sufficient to inactivate MMR completely. The findings may provide a basis for understanding the greater predisposition to intestinal cancer of Mlh1 ؊/؊ mice. Small differences (2-to 3-fold) in mononucleotide repeat mutation rates may have dramatic effects on tumor development, requiring multiple genetic alterations in coding regions. Alternatively, this strain difference in tumor spectra also may be related to the consequences of the absence of Pms2p compared with the absence of both Pms2p and Mlh1p on as yet little understood cellular processes.