Genomic structure and chromosomal assignment of the mouse Ku70 gene

Characterization of the human artemis promoter by heterologous gene expression in vitro and in vivo

Artemis is an endonucleolytic enzyme involved in nonhomologous double-strand break repair and V(D)J recombination. Deficiency of Artemis results in a B- T- radiosensitive severe combined immunodeficiency, which may potentially be treatable by Artemis gene transfer into hematopoietic stem cells. However, we recently found that overexpression of Artemis after lentiviral transduction resulted in global DNA damage and increased apoptosis. These results imply the necessity of effecting natural levels of Artemis expression, so we isolated a 1 kilobase DNA sequence upstream of the human Artemis gene to recover and characterize the Artemis promoter (APro). The sequence includes numerous potential transcription factor-binding sites, and several transcriptional start sites were mapped by 5' rapid amplification of cDNA ends. APro and deletion constructs conferred significant reporter gene expression in vitro that was markedly reduced in comparison to expression regulated by the human elongation factor 1-α promoter. Ex vivo lentiviral transduction of an APro-regulated green fluorescent protein (GFP) construct in mouse marrow supported GFP expression throughout hematopoeitic lineages in primary transplant recipients and was sustained in secondary recipients. The human Artemis promoter thus provides sustained and moderate levels of gene expression that will be of significant utility for therapeutic gene transfer into hematopoeitic stem cells.

Suppression of a DNA double-strand break repair gene, Ku70, increases radio- and chemosensitivity in a human lung carcinoma cell line

Ku70 protein, cooperating with Ku80 and DNA-dependent protein kinase (DNA-PK) catalytic subunit (DNA-PKcs), is involved in DNA double-strand break (DNA DSB) repair and V(D)J recombination. Recent studies have revealed increased ionizing radiosensitivity in Ku70-deficient cells. The presented study, using a human squamous cell lung carcinoma cell line, demonstrated that introduction of an antisense Ku70 nucleic acid made the cells more radio- and chemosensitive than the parental cells. Ku70 protein expression was suppressed in the cells with antisense Ku70 construct when compared to the wild-type cells. A relatively small but statistically significant increase in radiosensitivity of the cells was achieved by the introduction of the antisense Ku70. The increased radiosensitivity in vitro was accompanied by an approximately two-fold increase in alpha and alpha/beta values in a linear-quadratic model. The antisense Ku70 increased the chemosensitivity of the cells to some DNA-damaging agents such as bleomycin and methyl methanesulfonate, but not to cisplatin, mitomycin C, and paclitaxel. This system provides us with partial suppression of Ku70, and will be a useful experimental model for investigating the physiological roles of the DNA DSB repair gene.

The nuclear localization signal of the human Ku70 is a variant bipartite type recognized by the two components of nuclear pore-targeting complex

Ku protein is a complex of two subunits, Ku70 and Ku80. Ku is suspected to participate in both DNA double-strand break repair and transcription. Since both of these processes take place in the cell nucleus, we have been investigating the subcellular localization and nuclear transport of Ku proteins. In the present study, we analyzed the subcellular localization and nuclear localization signal (NLS) of Ku70. Fusion proteins of Ku70 and green fluorescent protein (GFP) transiently expressed in cells were clearly localized in the nuclei of interphase cells. Ku70 staining was distributed throughout both the nucleus and the cytoplasm in late telophase to early G1 phase cells. The NLS of Ku70 was located at the region composed of 18 amino acid residues (positions 539 to 556). This region overlapped with the Ku80-independent DNA-binding domain reported previously. The Ku70 NLS consisted of two basic subregions and a nonbasic intervening region. All the subregions were necessary for complete NLS activity. The amino acids in the nonbasic intervening region of Ku70 might be important for full NLS activity not only to provide sufficient length between the two separated clusters of basic amino acids but also to have an adequate amino acid sequence. All of the basic amino acid residues in the basic subregions were conserved among mammalian and avian homologues, confirming their importance in the nuclear translocation of Ku70. The structure of the Ku70 NLS resembled the consensus of a bipartite-type NLS. The Ku70 NLS was mediated to target to the nuclear rim by two components of the nuclear pore-targeting complex, PTAC58 and PTAC97.

MEC1-dependent redistribution of the Sir3 silencing protein from telomeres to DNA double-strand breaks

The yeast Sir2/3/4p complex is found in abundance at telomeres, where it participates in the formation of silent heterochromatin and telomere maintenance. Here, we show that Sir3p is released from telomeres in response to DNA double-strand breaks (DSBs), binds to DSBs, and mediates their repair, independent of cell mating type. Sir3p relocalization is S phase specific and, importantly, requires the DNA damage checkpoint genes MEC1 and RAD9. MEC1 is a homolog of ATM, mutations in which cause ataxia telangiectasia (A-T), a disease characterized by various neurologic and immunologic abnormalities, a predisposition for cancer, and a cellular defect in repair of DSBs. This novel mode by which preformed DNA repair machinery is mobilized by DNA damage sensors may have implications for human diseases resulting from defective DSB repair.

Subcellular localization and protein-protein interaction regions of Ku proteins

The Ku protein is a complex of Ku70 and Ku80 subunits and is capable of binding promoters in a sequence-specific manner, although it remains unclear whether Ku is involved in transcriptional regulation. We examined the subcellular localization and determined the interaction regions of Ku. Our results indicate that heterodimers of Ku70 and Ku80 are localized in the nucleus, and that the stretches from amino acid (aa) 378 to 482 of Ku70 and from aa 374 to 502 of Ku80 are necessary for heterodimerization. These interaction regions do not contain any previously recognized protein-protein interaction motifs. To determine whether Ku contains a potential transcriptional activation domain, we examined N- and C-terminal deletion mutants of Ku70 and Ku80 for their ability to activate transcription in the GAL4-based one-hybrid system. We found that the whole Ku protein had no transcriptional activity, although the N-terminal peptide fragment of Ku70 was capable of activating transcription of the HIS3 and lacZ reporter genes in yeast cells.

Ku70: a candidate tumor suppressor gene for murine T cell lymphoma

We present evidence that inactivation of the Ku70 gene leads to a propensity for malignant transformation both in vitro and in vivo. In vitro, Ku70-/- mouse fibroblasts displayed an increased rate of sister chromatid exchange and a high frequency of spontaneous neoplastic transformation. In vivo, Ku70-/- mice, known to be defective in B but not T lymphocyte maturation, developed thymic and disseminated T cell lymphomas at a mean age of 6 months with CD4+CD8+ tumor cells. These findings directly demonstrate that Ku70 deficiency facilitates neoplastic growth and suggest a novel role of the Ku70 locus in tumor suppression.

The promoters for human DNA-PKcs (PRKDC) and MCM4: divergently transcribed genes located at chromosome 8 band q11

A 30-kb genomic segment containing the promoter and first 9 exons of PRKDC, the gene encoding the catalytic subunit (DNA-PKcs) of the human DNA-activated protein kinase, DNA-PK, was isolated and partially sequenced. Sequence comparison with the NCBI nonredundant database revealed the locations of the first 13 exons of the upstream gene, MCM4. MCM4 is an essential component of a protein complex that prevents DNA from being replicated more than once per cell cycle. The MCM4 and DNA-PKcs promoters are in CpG islands separated by approximately 700 bp, and transcription from each initiates at multiple, closely spaced sites. Both promoters lack TATA boxes, and the MCM4 promoter also lacks an initiator (Inr) element but has an inverted CCAAT box. The DNA-PKcs promoter has an Inr-like sequence as well as a downstream MED-1 element. The two promoters appear to function independently, as sequences required for core promoter activity do not overlap, and sequences extending into the 5' region of each gene had little or no effect on transcription of the other gene, as shown in transient transfection assays. The arrangement of the PRKDC/MCM4 gene pair is similar to that of the ATM/E14(NPAT) gene pair. ATM, the product of the gene mutated in ataxia telangiectasia, and DNA-PKcs function in pathways that detect or repair DNA damage and are members of a family of large, serine/threonine kinases that are closely related to phosphatidylinositol 3 kinases.

Comparative mapping of the human 22q11 chromosomal region and the orthologous region in mice reveals complex changes in gene organization

The region of human chromosome 22q11 is prone to rearrangements. The resulting chromosomal abnormalities are involved in Velo-cardio-facial and DiGeorge syndromes (VCFS and DGS) (deletions), "cat eye" syndrome (duplications), and certain types of tumors (translocations). As a prelude to the development of mouse models for VCFS/DGS by generating targeted deletions in the mouse genome, we examined the organization of genes from human chromosome 22q11 in the mouse. Using genetic linkage analysis and detailed physical mapping, we show that genes from a relatively small region of human 22q11 are distributed on three mouse chromosomes (MMU6, MMU10, and MMU16). Furthermore, although the region corresponding to about 2.5 megabases of the VCFS/DGS critical region is located on mouse chromosome 16, the relative organization of the region is quite different from that in humans. Our results show that the instability of the 22q11 region is not restricted to humans but may have been present throughout evolution. The results also underscore the importance of detailed comparative mapping of genes in mice and humans as a prerequisite for the development of mouse models of human diseases involving chromosomal rearrangements.

Ku70 is required for DNA repair but not for T cell antigen receptor gene recombination In vivo

Ku is a complex of two proteins, Ku70 and Ku80, and functions as a heterodimer to bind DNA double-strand breaks (DSB) and activate DNA-dependent protein kinase. The role of the Ku70 subunit in DNA DSB repair, hypersensitivity to ionizing radiation, and V(D)J recombination was examined in mice that lack Ku70 (Ku70(-/-)). Like Ku80(-/-) mice, Ku70(-/-) mice showed a profound deficiency in DNA DSB repair and were proportional dwarfs. Surprisingly, in contrast to Ku80(-/-) mice in which both T and B lymphocyte development were arrested at an early stage, lack of Ku70 was compatible with T cell receptor gene recombination and the development of mature CD4+CD8- and CD4-CD8+ T cells. Our data shows, for the first time, that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR V(D)J recombination. These results suggest that distinct but overlapping repair pathways may mediate DNA DSB repair and V(D)J recombination.

Ku70-deficient embryonic stem cells have increased ionizing radiosensitivity, defective DNA end-binding activity, and inability to support V(D)J recombination

V(D)J recombination requires both lymphoid-specific and generally expressed enzymatic activities. All three known generally expressed activities involved in V(D)J recombination are also involved in DNA double-strand break repair (DSBR). Two of these are components of the DNA-dependent protein kinase (DNA-PK) and include Ku80 and DNA-PK catalytic subunit (DNA-PKcs); the third, XRCC4, is a protein of unknown function. The Ku70 protein is an additional component of DNA-PK; Ku70 forms a heterodimer with Ku80 to generate the DNA end-binding component of the enzyme. To test putative functions for Ku70, we have used gene-targeted mutation to generate a murine embryonic stem cell line which lacks Ku70 expression. We find that the Ku70(-/-) cells produce no detectable Ku70 and very little Ku80, suggesting a direct interrelationship between their levels. Correspondingly, these cells lack the nonspecific DNA end-binding activity associated with Ku. Significantly, the Ku70(-/-) embryonic stem cells have markedly increased sensitivity to gamma-irradiation relative to Ku70(+/-) or wild-type embryonic stem cells. Furthermore, the Ku70(-/-) cells lack the ability to effectively rejoin signal and coding ends liberated in transiently introduced V(D)J recombination substrates by enforced RAG-1 and RAG-2 expression. We conclude that the Ku70 gene product is involved in DSBR and V(D)J recombination and confirm that the Ku70 gene can be classified as a member of the x-ray cross-complementation group 6 (XRCC6). Potential differences between the Ku70(-/-) and Ku80(-/-) V(D)J recombination defects are discussed.