The presence of ribonucleotides in genomic DNA is undesirable given their increased susceptibility to hydrolysis. Ribonuclease (RNase) H enzymes that recognize and process such embedded ...ribonucleotides are present in all domains of life. However, in unicellular organisms such as budding yeast, they are not required for viability or even efficient cellular proliferation, while in humans, RNase H2 hypomorphic mutations cause the neuroinflammatory disorder Aicardi-Goutières syndrome. Here, we report that RNase H2 is an essential enzyme in mice, required for embryonic growth from gastrulation onward. RNase H2 null embryos accumulate large numbers of single (or di-) ribonucleotides embedded in their genomic DNA (>1,000,000 per cell), resulting in genome instability and a p53-dependent DNA-damage response. Our findings establish RNase H2 as a key mammalian genome surveillance enzyme required for ribonucleotide removal and demonstrate that ribonucleotides are the most commonly occurring endogenous nucleotide base lesion in replicating cells.
Display omitted
► Ribonucleotides are the most common nucleotide base lesion in the mouse genome ► RNase H2 is a key genome surveillance enzyme required for removal of nucleotides ► RNase H2 is essential for mammalian development ► Without RNase H2, cells exhibit genome instability and p53 pathway activation
DNA polymerases can incorporate more than a million ribonucleotides into replicating mouse DNA in each cell, making ribonucleotides the most abundant kind of DNA lesion. RNase H2, which removes this “damage,” has an essential role in genome surveillance and is required for embryonic development.
Compaction of chromosomes is essential for accurate segregation of the genome during mitosis. In vertebrates, two condensin complexes ensure timely chromosome condensation, sister chromatid ...disentanglement, and maintenance of mitotic chromosome structure. Here, we report that biallelic mutations in NCAPD2, NCAPH, or NCAPD3, encoding subunits of these complexes, cause microcephaly. In addition, hypomorphic Ncaph2 mice have significantly reduced brain size, with frequent anaphase chromatin bridge formation observed in apical neural progenitors during neurogenesis. Such DNA bridges also arise in condensin-deficient patient cells, where they are the consequence of failed sister chromatid disentanglement during chromosome compaction. This results in chromosome segregation errors, leading to micronucleus formation and increased aneuploidy in daughter cells. These findings establish "condensinopathies" as microcephalic disorders, with decatenation failure as an additional disease mechanism for microcephaly, implicating mitotic chromosome condensation as a key process ensuring mammalian cerebral cortex size.
Centrioles are essential for ciliogenesis. However, mutations in centriole biogenesis genes have been reported in primary microcephaly and Seckel syndrome, disorders without the hallmark clinical ...features of ciliopathies. Here we identify mutations in the genes encoding PLK4 kinase, a master regulator of centriole duplication, and its substrate TUBGCP6 in individuals with microcephalic primordial dwarfism and additional congenital anomalies, including retinopathy, thereby extending the human phenotypic spectrum associated with centriole dysfunction. Furthermore, we establish that different levels of impaired PLK4 activity result in growth and cilia phenotypes, providing a mechanism by which microcephaly disorders can occur with or without ciliopathic features.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Studies into disorders of extreme growth failure (for example, Seckel syndrome and Majewski osteodysplastic primordial dwarfism type II) have implicated fundamental cellular processes of DNA damage ...response signaling and centrosome function in the regulation of human growth. Here we report that mutations in ORC1, encoding a subunit of the origin recognition complex, cause microcephalic primordial dwarfism resembling Meier-Gorlin syndrome. We establish that these mutations disrupt known ORC1 functions including pre-replicative complex formation and origin activation. ORC1 deficiency perturbs S-phase entry and S-phase progression. Additionally, we show that Orc1 depletion in zebrafish is sufficient to markedly reduce body size during rapid embryonic growth. Our data suggest a model in which ORC1 mutations impair replication licensing, slowing cell cycle progression and consequently impeding growth during development, particularly at times of rapid proliferation. These findings establish a novel mechanism for the pathogenesis of microcephalic dwarfism and show a surprising but important developmental impact of impaired origin licensing.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Meier-Gorlin syndrome (ear, patella and short-stature syndrome) is an autosomal recessive primordial dwarfism syndrome characterized by absent or hypoplastic patellae and markedly small ears¹⁻³. Both ...pre- and post-natal growth are impaired in this disorder, and although microcephaly is often evident, intellect is usually normal in this syndrome. We report here that individuals with this disorder show marked locus heterogeneity, and we identify mutations in five separate genes: ORC1, ORC4, ORC6, CDT1 and CDC6. All of these genes encode components of the pre-replication complex, implicating defects in replication licensing as the cause of a genetic syndrome with distinct developmental abnormalities.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Aicardi-Goutières syndrome (AGS) presents as a severe neurological brain disease and is a genetic mimic of the sequelae of transplacentally acquired viral infection. Evidence exists for a ...perturbation of innate immunity as a primary pathogenic event in the disease phenotype. Here, we show that TREX1, encoding the major mammalian 3′ → 5′ DNA exonuclease, is the AGS1 gene, and AGS-causing mutations result in abrogation of TREX1 enzyme activity. Similar loss of function in the Trex1−/− mouse leads to an inflammatory phenotype. Our findings suggest an unanticipated role for TREX1 in processing or clearing anomalous DNA structures, failure of which results in the triggering of an abnormal innate immune response.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Non-homologous end joining (NHEJ) is a key cellular process ensuring genome integrity. Mutations in several components of the NHEJ pathway have been identified, often associated with severe combined ...immunodeficiency (SCID), consistent with the requirement for NHEJ during V(D)J recombination to ensure diversity of the adaptive immune system. In contrast, we have recently found that biallelic mutations in LIG4 are a common cause of microcephalic primordial dwarfism (MPD), a phenotype characterized by prenatal-onset extreme global growth failure. Here we provide definitive molecular genetic evidence supported by biochemical, cellular, and immunological data for mutations in XRCC4, encoding the obligate binding partner of LIG4, causing MPD. We report the identification of biallelic mutations in XRCC4 in five families. Biochemical and cellular studies demonstrate that these alterations substantially decrease XRCC4 protein levels leading to reduced cellular ligase IV activity. Consequently, NHEJ-dependent repair of ionizing-radiation-induced DNA double-strand breaks is compromised in XRCC4 cells. Similarly, immunoglobulin junctional diversification is impaired in cells. However, immunoglobulin levels are normal, and individuals lack overt signs of immunodeficiency. Additionally, in contrast to individuals with LIG4 mutations, pancytopenia leading to bone marrow failure has not been observed. Hence, alterations that alter different NHEJ proteins give rise to a phenotypic spectrum, from SCID to extreme growth failure, with deficiencies in certain key components of this repair pathway predominantly exhibiting growth deficits, reflecting differential developmental requirements for NHEJ proteins to support growth and immune maturation.
Persistently activated Stat3 is found in many different cancers, including ≈60% of breast tumors. Here, we demonstrate that a constitutively activated Stat3 transforms immortalized human mammary ...epithelial cells and that this oncogenic event requires the activity of matrix metalloproteinase-9 (MMP-9). By immunohistochemical analysis, we observe a positive correlation between strong MMP-9 expression and tyrosine phosphorylated Stat3 in primary breast cancer specimens. These results demonstrate a relationship between activated Stat3 and MMP-9 in breast oncogenesis.
Signal transducer and activator of transcription 3 (Stat3) is a transcription factor that is constitutively activated in a variety of human malignancies, including prostate, lung, brain, breast, and ...squamous cell carcinomas. Inhibition of activated Stat3 leads to decreased proliferation and apoptosis of many cancer-derived cell lines, while the introduction of a constitutively activated form of Stat3 into immortalized human breast epithelial cells and rodent fibroblasts results in cellular transformation. Collectively, these data suggest a role for Stat3 in oncogenesis. A new study from Chan et al. is the first to demonstrate a requirement for Stat3 in de novo epithelial carcinogenesis in vivo. Using the two-step model of chemically induced skin carcinogenesis, the authors demonstrated that mice deficient in Stat3 were completely resistant to skin tumor development.
DNA is strictly compartmentalized within the nucleus to prevent autoimmunity; despite this, cyclic GMP-AMP synthase (cGAS), a cytosolic sensor of double-stranded DNA, is activated in autoinflammatory ...disorders and by DNA damage. Precisely how cellular DNA gains access to the cytoplasm remains to be determined. Here, we report that cGAS localizes to micronuclei arising from genome instability in a mouse model of monogenic autoinflammation, after exogenous DNA damage and spontaneously in human cancer cells. Such micronuclei occur after mis-segregation of DNA during cell division and consist of chromatin surrounded by its own nuclear membrane. Breakdown of the micronuclear envelope, a process associated with chromothripsis, leads to rapid accumulation of cGAS, providing a mechanism by which self-DNA becomes exposed to the cytosol. cGAS is activated by chromatin, and consistent with a mitotic origin, micronuclei formation and the proinflammatory response following DNA damage are cell-cycle dependent. By combining live-cell laser microdissection with single cell transcriptomics, we establish that interferon-stimulated gene expression is induced in micronucleated cells. We therefore conclude that micronuclei represent an important source of immunostimulatory DNA. As micronuclei formed from lagging chromosomes also activate this pathway, recognition of micronuclei by cGAS may act as a cell-intrinsic immune surveillance mechanism that detects a range of neoplasia-inducing processes.
PDF
