Condensin in Chromatid Cohesion and Segregation

Fragile sites, chromosomal lesions, tandem repeats, and disease

Expanded tandem repeat DNAs are associated with various unusual chromosomal lesions, despiralizations, multi-branched inter-chromosomal associations, and fragile sites. Fragile sites cytogenetically manifest as localized gaps or discontinuities in chromosome structure and are an important genetic, biological, and health-related phenomena. Common fragile sites (∼230), present in most individuals, are induced by aphidicolin and can be associated with cancer; of the 27 molecularly-mapped common sites, none are associated with a particular DNA sequence motif. Rare fragile sites ( ≳ 40 known), ≤ 5% of the population (may be as few as a single individual), can be associated with neurodevelopmental disease. All 10 molecularly-mapped folate-sensitive fragile sites, the largest category of rare fragile sites, are caused by gene-specific CGG/CCG tandem repeat expansions that are aberrantly CpG methylated and include FRAXA, FRAXE, FRAXF, FRA2A, FRA7A, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A. The minisatellite-associated rare fragile sites, FRA10B, FRA16B, can be induced by AT-rich DNA-ligands or nucleotide analogs. Despiralized lesions and multi-branched inter-chromosomal associations at the heterochromatic satellite repeats of chromosomes 1, 9, 16 are inducible by de-methylating agents like 5-azadeoxycytidine and can spontaneously arise in patients with ICF syndrome (Immunodeficiency Centromeric instability and Facial anomalies) with mutations in genes regulating DNA methylation. ICF individuals have hypomethylated satellites I-III, alpha-satellites, and subtelomeric repeats. Ribosomal repeats and subtelomeric D4Z4 megasatellites/macrosatellites, are associated with chromosome location, fragility, and disease. Telomere repeats can also assume fragile sites. Dietary deficiencies of folate or vitamin B12, or drug insults are associated with megaloblastic and/or pernicious anemia, that display chromosomes with fragile sites. The recent discovery of many new tandem repeat expansion loci, with varied repeat motifs, where motif lengths can range from mono-nucleotides to megabase units, could be the molecular cause of new fragile sites, or other chromosomal lesions. This review focuses on repeat-associated fragility, covering their induction, cytogenetics, epigenetics, cell type specificity, genetic instability (repeat instability, micronuclei, deletions/rearrangements, and sister chromatid exchange), unusual heritability, disease association, and penetrance. Understanding tandem repeat-associated chromosomal fragile sites provides insight to chromosome structure, genome packaging, genetic instability, and disease.

DHX15-independent roles for TFIP11 in U6 snRNA modification, U4/U6.U5 tri-snRNP assembly and pre-mRNA splicing fidelity

The U6 snRNA, the core catalytic component of the spliceosome, is extensively modified post-transcriptionally, with 2'-O-methylation being most common. However, how U6 2'-O-methylation is regulated remains largely unknown. Here we report that TFIP11, the human homolog of the yeast spliceosome disassembly factor Ntr1, localizes to nucleoli and Cajal Bodies and is essential for the 2'-O-methylation of U6. Mechanistically, we demonstrate that TFIP11 knockdown reduces the association of U6 snRNA with fibrillarin and associated snoRNAs, therefore altering U6 2'-O-methylation. We show U6 snRNA hypomethylation is associated with changes in assembly of the U4/U6.U5 tri-snRNP leading to defects in spliceosome assembly and alterations in splicing fidelity. Strikingly, this function of TFIP11 is independent of the RNA helicase DHX15, its known partner in yeast. In sum, our study demonstrates an unrecognized function for TFIP11 in U6 snRNP modification and U4/U6.U5 tri-snRNP assembly, identifying TFIP11 as a critical spliceosome assembly regulator.

Silencing non-SMC chromosome-associated polypeptide G inhibits proliferation and induces apoptosis in hepatocellular carcinoma cells

The present study was designed to investigate the significance of non-structural maintenance of chromosomes (non-SMC) chromosome-associated polypeptide G (NCAPG), a subunit of condensin complex I, in the development of hepatocellular carcinoma (HCC). NCAPG protein expression in human HCC and paracancerous hepatic tissues were examined using immunohistochemistry, and NCAPG mRNA expression in HCC cell lines were quantified using quantitative RT-PCR. Lentivirus-mediated RNA interference was used to silence NCAPG in HCC cells. Cell proliferation was monitored by MTT assay. Cell colony-forming capacity was measured by colony formation assay. Apoptosis was determined by flow cytometry. The results showed that increased protein expression of NCAPG was found in HCC tissues compared with the matched paracancerous hepatic tissues. At the mRNA level, increased expression of NCAPG was found in HCC cells as opposed to the normal hepatocytes. Silencing of NCAPG in BEL-7404 and SMMC-7721 cells led to decreased cell proliferation and increased apoptosis. These changes were associated with increased mRNA expressions of P53, P27, and Bad, but decreased mRNA expression of EGFR, Akt, survivin, and JNK. NCAPG might play an oncogenic role in the development of liver cancer. Further studies to clarify its role and underlying mechanisms in the development of liver cancer are warranted.

Smc1β is required for activation of SAC during mouse oocyte meiosis

Smc1β is a meiosis-specific cohesin subunit that is essential for sister chromatid cohesion and DNA recombination. Previous studies have shown that Smc1β-deficient mice in both sexes are sterile. Ablation of Smc1β during male meiosis leads to the blockage of spermatogenesis in pachytene stage, and ablation of Smc1β during female meiosis generates a highly error-prone oocyte although it could develop to metaphase II stage. However, the underlying mechanisms regarding how Smc1β maintains the correct meiotic progression in mouse oocytes have not been clearly defined. Here, we find that GFP-fused Smc1β is expressed and localized to the chromosomes from GV to MII stages during mouse oocyte meiotic maturation. Knockdown of Smc1β by microinjection of gene-specific morpholino causes the impaired spindle apparatus and chromosome alignment which are highly correlated with the defective kinetochore-microtubule attachments, consequently resulting in a prominently higher incidence of aneuploid eggs. In addition, the premature extrusion of polar bodies and escape of metaphase I arrest induced by low dose of nocodazole treatment in Smc1β-depleted oocytes indicates that Smc1β is essential for activation of spindle assembly checkpoint (SAC) activity. Collectively, we identify a novel function of Smc1β as a SAC participant beyond its role in chromosome cohesion during mouse oocyte meiosis.

Stag3 regulates microtubule stability to maintain euploidy during mouse oocyte meiotic maturation

Stag3, a meiosis-specific subunit of cohesin complex, has been demonstrated to function in both male and female reproductive systems in mammals. However, its roles during oocyte meiotic maturation have not been fully defined. In the present study, we report that Stag3 uniquely accumulates on the spindle apparatus and colocalizes with microtubule fibers during mouse oocyte meiotic maturation. Depletion of Stag3 by gene-targeting morpholino disrupts normal spindle assembly and chromosome alignment in oocytes. We also find that depletion of Stag3 reduces the acetylated level of tubulin and microtubule resistance to microtubule depolymerizing drug, suggesting that Stag3 is required for microtubule stability. Consistent with these observations, kinetochore-microtubule attachment, an important mechanism controlling chromosome alignment, is severely impaired in Stag3-depleted oocytes, resultantly causing the significantly increased incidence of aneuploid eggs. Collectively, our data reveal that Stag3 is a novel regulator of microtubule dynamics to ensure euploidy during moue oocyte meiotic maturation.