Mouse Hus1, a homolog of the Schizosaccharomyces pombe hus1+ cell cycle checkpoint gene

Clamping down on mammalian meiosis

The RAD9A-RAD1-HUS1 (9-1-1) complex is a PCNA-like heterotrimeric clamp that binds damaged DNA to promote cell cycle checkpoint signaling and DNA repair. While various 9-1-1 functions in mammalian somatic cells have been established, mounting evidence from lower eukaryotes predicts critical roles in meiotic germ cells as well. This was investigated in 2 recent studies in which the 9-1-1 complex was disrupted specifically in the mouse male germline through conditional deletion of Rad9a or Hus1. Loss of these clamp subunits led to severely impaired fertility and meiotic defects, including faulty DNA double-strand break repair. While 9-1-1 is critical for ATR kinase activation in somatic cells, these studies did not reveal major defects in ATR checkpoint pathway signaling in meiotic cells. Intriguingly, this new work identified separable roles for 9-1-1 subunits, namely RAD9A- and HUS1-independent roles for RAD1. Based on these studies and the high-level expression of the paralogous proteins RAD9B and HUS1B in testis, we propose a model in which multiple alternative 9-1-1 clamps function during mammalian meiosis to ensure genome maintenance in the germline.

Dual inactivation of Hus1 and p53 in the mouse mammary gland results in accumulation of damaged cells and impaired tissue regeneration

In response to DNA damage, checkpoint proteins halt cell cycle progression and promote repair or apoptosis, thereby preventing mutation accumulation and suppressing tumor development. The DNA damage checkpoint protein Hus1 associates with Rad9 and Rad1 to form the 9-1-1 complex, which localizes to DNA lesions and promotes DNA damage signaling and repair. Because complete inactivation of mouse Hus1 results in embryonic lethality, we developed a system for regulated Hus1 inactivation in the mammary gland to examine roles for Hus1 in tissue homeostasis and tumor suppression. Hus1 inactivation in the mammary epithelium resulted in genome damage that induced apoptosis and led to depletion of Hus1-null cells from the mammary gland. Conditional Hus1 knockout females retained grossly normal mammary gland morphology, suggesting compensation by cells that failed to undergo Cre-mediated Hus1 deletion. p53-deficiency delayed the clearance of Hus1-null cells from conditional Hus1 knockout mice and caused the accumulation of damaged, dying cells in the mammary gland. Notably, compensatory responses were impaired following combined Hus1 and p53 loss, resulting in aberrant mammary gland morphology and lactation defects. Overall, these results establish a requirement for Hus1 in the survival and proliferation of mammary epithelium and identify a role for p53 in mammary gland tissue regeneration and homeostasis.

Increased common fragile site expression, cell proliferation defects, and apoptosis following conditional inactivation of mouse Hus1 in primary cultured cells

Targeted disruption of the mouse Hus1 cell cycle checkpoint gene results in embryonic lethality and proliferative arrest in cultured cells. To investigate the essential functions of Hus1, we developed a system for the regulated inactivation of mouse Hus1 in primary fibroblasts. Inactivation of a loxP site-flanked conditional Hus1 allele by using a cre-expressing adenovirus resulted in reduced cell doubling, cell cycle alterations, and increased apoptosis. These phenotypes were associated with a significantly increased frequency of gross chromosomal abnormalities and an S-phase-specific accumulation of phosphorylated histone H2AX, an indicator of double-stranded DNA breaks. To determine whether these chromosomal abnormalities occurred randomly or at specific genomic regions, we assessed the stability of common fragile sites, chromosomal loci that are prone to breakage in cells undergoing replication stress. Hus1 was found to be essential for fragile site stability, because spontaneous chromosomal abnormalities occurred preferentially at common fragile sites upon conditional Hus1 inactivation. Although p53 levels increased after Hus1 loss, deletion of p53 failed to rescue the cell-doubling defect or increased apoptosis in conditional Hus1 knockout cells. In summary, we propose that Hus1 loss leads to chromosomal instability during DNA replication, triggering increased apoptosis and impaired proliferation through p53-independent mechanisms.

DNA repair in mammalian embryos

Mammalian cells have developed complex mechanisms to identify DNA damage and activate the required response to maintain genome integrity. Those mechanisms include DNA damage detection, DNA repair, cell cycle arrest and apoptosis which operate together to protect the conceptus from DNA damage originating either in parental gametes or in the embryo's somatic cells. DNA repair in the newly fertilized preimplantation embryo is believed to rely entirely on the oocyte's machinery (mRNAs and proteins deposited and stored prior to ovulation). DNA repair genes have been shown to be expressed in the early stages of mammalian development. The survival of the embryo necessitates that the oocyte be sufficiently equipped with maternal stored products and that embryonic gene expression commences at the correct time. A Medline based literature search was performed using the keywords 'DNA repair' and 'embryo development' or 'gametogenesis' (publication dates between 1995 and 2006). Mammalian studies which investigated gene expression were selected. Further articles were acquired from the citations in the articles obtained from the preliminary Medline search. This paper reviews mammalian DNA repair from gametogenesis to preimplantation embryos to late gestational stages.

Disruption of the Rad9/Rad1/Hus1 (9–1–1) complex leads to checkpoint signaling and replication defects

The checkpoint sliding-clamp complex, Rad9/Rad1/Hus1, plays a critical role during initiation of checkpoint signals in response to DNA damage and replication disruption. We investigated the impact of loss of Rad1 on checkpoint function and on DNA replication in mammalian cells. We show that RAD1 is an essential gene for sustained cell proliferation and that loss of Rad1 causes destabilization of Rad9 and Hus1 and consequently disintegration of the sliding-clamp complex. In Rad1-depleted cells, Atr-dependent Chk1 activation was impaired whereas Atm-mediated Chk2 activation was unaffected, suggesting that the sliding clamp is required primarily in Atr-dependent signal activation. Disruption of sliding-clamp function also caused a major defect in S-phase control. Rad1-depleted cells exhibited an RDS phenotype, indicating that damage-induced S-phase arrest was compromised by Rad1 loss. Furthermore, lack of Rad1 also affected the efficiency of replication recovery from DNA synthesis blockage, resulting in a prolonged S phase. These deficiencies may perpetually generate DNA strand breakage as we have found chromosomal abnormalities in Rad1-depleted cells. We conclude that the Rad9/Rad1/Hus1 complex is essential for Atr-dependent checkpoint signaling, which may play critical roles in the facilitation of DNA replication and in the maintenance of genomic integrity.

Structure-function analysis of fission yeast Hus1-Rad1-Rad9 checkpoint complex

Hus1, Rad1, and Rad9 are three evolutionarily conserved proteins required for checkpoint control in fission yeast. These proteins are known to form a stable complex in vivo. Recently, computational studies have predicted structural similarity between the individual proteins of Hus1-Rad1-Rad9 complex and the replication processivity factor proliferating cell nuclear antigen (PCNA). This has led to the proposal that the Hus1-Rad1-Rad9 complex may form a PCNA-like ring structure, and could function as a sliding clamp during checkpoint control. In the present study, we have attempted to test the predictions of this model by asking whether the PCNA alignment identifies functionally important residues or explains mutant phenotypes of hus1, rad1, or rad9 alleles. Although some of our results are consistent with the PCNA alignment, others indicate that the Hus1-Rad1-Rad9 complex possesses unique structural and functional features.

HDAC1, a histone deacetylase, forms a complex with Hus1 and Rad9, two G2/M checkpoint Rad proteins

HDAC1 is a member of the histone deacetylase family, which plays an important role in modulating the eukaryotic chromatin structure. Numerous studies have demonstrated its involvement in transcription and in tumorigenesis. To better understand the functions and regulation of HDAC1, a yeast two-hybrid screening approach was chosen to identify novel interactions involving HDAC1. Human HDAC1 was found to interact specifically in yeast, mammalian cells, and in vitro with the human Hus1 gene product, whose Schizosaccharomyces pombe homolog has been implicated in G(2)/M checkpoint control. Both HDAC1 and Hus1 proteins localize to the nuclei. Furthermore, HDAC1 and Hus1 were found to exist in a complex with Rad9, a known Hus1-interacting factor. In addition, bioinformatics analysis of the protein sequences of Hus1, Rad1, and Rad9, three checkpoint Rad proteins that form a complex, revealed that they all contain a putative proliferating cell nuclear antigen (PCNA) fold, raising the possibility that these factors may bind to DNA in a PCNA-like ring structure. The results reported in this study strongly suggest a novel pathway involving HDAC1 in G(2)/M checkpoint control through the interaction with a functional Rad complex that may utilize a PCNA-like structure. Therefore, physically and functionally similar apparatus may function during G(2)/M checkpoint and DNA replication.

Isolation, characterization and targeted disruption of mouse ppia: cyclophilin A is not essential for mammalian cell viability

Cyclophilins (CyPs) are a family of proteins found in organisms ranging from prokaryotes to humans. These molecules exhibit peptidyl-prolyl isomerase activity in vitro, suggesting that they influence the conformation of proteins in cells. CyPs also bind with varying affinities to the immunosuppressive drug cyclosporin A (CsA), a compound used clinically to prevent allograft rejection. The founding member of the family, cyclophilin A (CyPA), is an abundant, ubiquitously expressed protein of unknown function that binds with nanomolar affinity to CsA. Here, we describe the isolation and characterization of mouse Ppia (mPpia), the gene encoding CyPA. Ppia was isolated using a PCR screen that distinguishes the expressed gene from multiple pseudogenes present in the mouse genome. mPpia consists of 5 exons and 4 introns spanning roughly 4.5 kb and maps to chromosome 11 near the centromere. Sequence analysis of a 369-bp fragment from the proximal promoter region of mPpia revealed the presence of a TATA box and sites recognized by several transcriptional regulators, including Sp1, AP-2, GATA factors, c-Myb, and NF-IL-6. This region is sufficient to drive high-level reporter gene expression in transfected cells. Both copies of Ppia were disrupted in murine embryonic stem (ES) cells via gene targeting. Ppia(-/-) ES cells grow normally and differentiate into hematopoeitic precursor cells in vitro, indicating that CyPA is not essential for mammalian cell viability.

Inactivation of mouse Hus1 results in genomic instability and impaired responses to genotoxic stress

The eukaryotic cell cycle is overseen by regulatory mechanisms, termed checkpoints, that respond to DNA damage, mitotic spindle defects, and errors in the ordering of cell cycle events. The DNA replication and DNA damage cell cycle checkpoints of the fission yeastSchizosaccharomyces pombe require the hus1+(hydroxyurea sensitive) gene. To determine the role of the mouse homolog of hus1+ in murine development and cell cycle checkpoint function, we produced a targeted disruption of mouse Hus1. Inactivation of Hus1results in mid-gestational embryonic lethality due to widespread apoptosis and defective development of essential extra-embryonic tissues. DNA damage-inducible genes are up-regulated inHus1-deficient embryos, and primary cells fromHus1-null embryos contain increased spontaneous chromosomal abnormalities, suggesting that loss of Hus1 leads to an accumulation of genome damage. Embryonic fibroblasts lackingHus1 fail to proliferate in vitro, but inactivation ofp21 allows for the continued growth of Hus1-deficient cells.Hus1−/−p21−/−cells display a unique profile of significantly heightened sensitivity to hydroxyurea, a DNA replication inhibitor, and ultraviolet light, but only slightly increased sensitivity to ionizing radiation. Taken together, these results indicate that mouse Hus1 functions in the maintenance of genomic stability and additionally identify an evolutionarily-conserved role for Hus1 in mediating cellular responses to genotoxins.