ATM protein purified from vaccinia virus expression system: DNA binding requirements for kinase activation

Arginine-rich cell-penetrating peptide dramatically enhances AMO-mediated ATM aberrant splicing correction and enables delivery to brain and cerebellum

Antisense morpholino oligonucleotides (AMOs) can reprogram pre-mRNA splicing by complementary binding to a target site and regulating splice site selection, thereby offering a potential therapeutic tool for genetic disorders. However, the application of this technology into a clinical scenario has been limited by the low correction efficiency in vivo and inability of AMOs to efficiently cross the blood brain barrier and target brain cells when applied to neurogenetic disorders such as ataxia-telangiecatasia (A-T). We previously used AMOs to correct subtypes of ATM splicing mutations in A-T cells; AMOs restored up to 20% of the ATM protein and corrected the A-T cellular phenotype. In this study, we demonstrate that an arginine-rich cell-penetrating peptide, (RXRRBR)(2)XB, dramatically improved ATM splicing correction efficiency when conjugated with AMOs, and almost fully corrected aberrant splicing. The restored ATM protein was close to normal levels in cells with homozygous splicing mutations, and a gene dose effect was observed in cells with heterozygous mutations. A significant amount of the ATM protein was still detected 21 days after a single 5 µm treatment. Systemic administration of an fluorescein isothiocyanate-labeled (RXRRBR)(2)XB-AMO in mice showed efficient uptake in the brain. Fluorescence was evident in Purkinje cells after a single intravenous injection of 60 mg/kg. Furthermore, multiple injections significantly increased uptake in all areas of the brain, notably in cerebellum and Purkinje cells, and showed no apparent signs of toxicity. Taken together, these results highlight the therapeutic potential of (RXRRBR)(2)XB-AMOs in A-T and other neurogenetic disorders.

Nonaminoglycoside compounds induce readthrough of nonsense mutations

Large numbers of genetic disorders are caused by nonsense mutations for which compound-induced readthrough of premature termination codons (PTCs) might be exploited as a potential treatment strategy. We have successfully developed a sensitive and quantitative high-throughput screening (HTS) assay, protein transcription/translation (PTT)–enzyme-linked immunosorbent assay (ELISA), for identifying novel PTC-readthrough compounds using ataxia-telangiectasia (A-T) as a genetic disease model. This HTS PTT-ELISA assay is based on a coupled PTT that uses plasmid templates containing prototypic A-T mutated (ATM) mutations for HTS. The assay is luciferase independent. We screened ∼34,000 compounds and identified 12 low-molecular-mass nonaminoglycosides with potential PTC-readthrough activity. From these, two leading compounds consistently induced functional ATM protein in ATM-deficient cells containing disease-causing nonsense mutations, as demonstrated by direct measurement of ATM protein, restored ATM kinase activity, and colony survival assays for cellular radiosensitivity. The two compounds also demonstrated readthrough activity in mdx mouse myotube cells carrying a nonsense mutation and induced significant amounts of dystrophin protein.

Rapid flow cytometry-based structural maintenance of chromosomes 1 (SMC1) phosphorylation assay for identification of ataxia-telangiectasia homozygotes and heterozygotes

BACKGROUND

No rapid reliable method exists for identifying ataxia-telangiectasia (A-T) homozygotes or heterozygotes. Heterozygotes are at an increased risk of cancer and are more sensitive to the effects of ionizing radiation (IR) than the general population. We report a rapid flow cytometry (FC)-based ataxia-telangiectasia mutated (ATM) kinase assay that measures ATM- dependent phosphorylation of structural maintenance of chromosomes 1 (SMC1) following DNA damage (FC-pSMC1 assay).

METHODS

After optimizing conditions with lymphoblastoid cell lines (LCLs), we studied peripheral blood mononuclear cells (PBMCs) isolated from 16 healthy donors (unknowns), 10 obligate A-T heterozygotes, and 6 unrelated A-T patients. One hour after DNA damage (by either IR or bleomycin), the cells were fixed and incubated with a primary antibody to SMC1pSer966. We analyzed the stained cells by FC to determine the difference in geometric mean fluorescence intensity (DeltaGMFI) of untreated and treated cells; this difference was expressed as a percentage of daily experimental controls.

RESULTS

The FC-pSMC1 assay reliably distinguished ATM heterozygotes and homozygotes from controls. Average DeltaGMFI percentages (SD) of daily controls were, for unknowns, 106.1 (37.6); for A-T heterozygotes, 37.0 (18.7); and for A-T homozygotes; -8.73 (16.2). Values for heterozygotes and homozygotes were significantly different from those of controls (P < 0.0001).

CONCLUSIONS

The FC-pSMC1 assay shortens the turnaround time for diagnosing A-T homozygotes from approximately 3 months to approximately 3 h. It also identifies A-T heterozygotes and can be used for prenatal counseling or for screening individuals in large study cohorts for potential ATM heterozygosity, which can then be confirmed by sequencing.

Enhanced phosphorylation of transcription factor sp1 in response to herpes simplex virus type 1 infection is dependent on the ataxia telangiectasia-mutated protein

The ataxia telangiectasia-mutated (ATM) protein, a member of the related phosphatidylinositol 3-like kinase family encoded by a gene responsible for the human genetic disorder ataxia telangiectasia, regulates cellular responses to DNA damage and viral infection. It has been previously reported that herpes simplex virus type 1 (HSV-1) infection induces activation of protein kinase activity of ATM and hyperphosphorylation of transcription factor, Sp1. We show that ATM is intimately involved in Sp1 hyperphosphorylation during HSV-1 infection rather than individual HSV-1-encoded protein kinases. In ATM-deficient cells or cells silenced for ATM expression by short hairpin RNA targeting, hyperphosphorylation of Sp1 was prevented even as HSV-1 infection progressed. Mutational analysis of putative ATM phosphorylation sites on Sp1 and immunoblot analysis with phosphopeptide-specific Sp1 antibodies clarified that at least Ser-56 and Ser-101 residues on Sp1 became phosphorylated upon HSV-1 infection. Serine-to-alanine mutations at both sites on Sp1 considerably abolished hyperphosphorylation of Sp1 upon infection. Although ATM phosphorylated Ser-101 but not Ser-56 on Sp1 in vitro, phosphorylation of Sp1 at both sites was not detected at all upon infection in ATM-deficient cells, suggesting that cellular kinase(s) activated by ATM could be involved in phosphorylation at Ser-56. Upon viral infection, Sp1-dependent transcription in ATM expression-silenced cells was almost the same as that in ATM-intact cells, suggesting that ATM-dependent phosphorylation of Sp1 might hardly affect its transcriptional activity during the HSV-1 infection. ATM-dependent Sp1 phosphorylation appears to be a global response to various DNA damage stress including viral DNA replication.

ATM Activation by Ionizing Radiation Requires BRCA1-associated BAAT1

ATM (ataxia telangiectasia mutated) is required for the early response to DNA-damaging agents such as ionizing radiation (IR) that induce DNA double-strand breaks. Cells deficient in ATM are extremely sensitive to IR. It has been shown that IR induces immediate phosphorylation of ATM at Ser(1981), leading to catalytic activation of the protein. We recently isolated a novel BRCA1-associated protein, BAAT1 (BRCA1-associated protein required for ATM activation-1), by yeast two-hybrid screening and found that BAAT1 also binds to ATM, localizes to double-strand breaks, and is required for Ser(1981) phosphorylation of ATM. Small interfering RNA-mediated stable or transient reduction of BAAT1 resulted in decreased phosphorylation of both ATM at Ser(1981) and CHK2 at Thr(68). Treatment of BAAT1-depleted cells with okadaic acid greatly restored phosphorylation of ATM at Ser(1981), suggesting that BAAT1 is involved in the regulation of ATM phosphatase. Protein phosphatase 2A-mediated dephosphorylation of ATM was partially blocked by purified BAAT1 in vitro. Significantly, acute loss of BAAT1 resulted in increased p53, leading to apoptosis. These results demonstrate that DNA damage-induced ATM activation requires a coordinated assembly of BRCA1, BAAT1, and ATM.

Immunoassay to measure ataxia-telangiectasia mutated protein in cellular lysates

BACKGROUND

Ataxia-telangiectasia (A-T) is a neurologic disorder caused by mutations in the ataxia-telangiectasia mutated (ATM) gene. A clinical diagnosis of A-T is confirmed by radiosensitivity testing and immunoblotting for ATM protein. Because both of these tests have long turnaround times (> or =3 months), we developed a rapid immunoassay to measure ATM protein and determined its sensitivity and specificity for diagnosing A-T.

METHODS

Recombinant ATM protein was used for standardization. Lysates of lymphoblastoid cell lines (LCLs) and peripheral blood mononuclear cells (PBMCs) from A-T patients, controls, and A-T heterozygotes were tested for ATM protein by immunoassay.

RESULTS

Between-run imprecision (CV) was < or =13%. Nuclear lysates from control LCLs and PBMCs had ATM protein concentrations of 49-610 microg/L and 48-943 microg/L, respectively. ATM protein was not detectable in LCL nuclear lysates from 18 of 21 A-T patients. The three remaining A-T patients had trace amounts of ATM protein, which was confirmed on immuoblots. ATM protein was also detectable in whole-cell lysates from 4 x 10(6) cells at concentrations of 64-463 microg/L and 42-444 microg/L for control LCLs and PBMCs, respectively. A-T heterozygotes had ATM protein concentrations of 52-98 microg/L. ATM protein was stable in PBMCs stored for 1 month at -70 degrees C, but rapidly decreased after 1 day in unprocessed blood.

CONCLUSIONS

This ATM protein immunoassay can be used to confirm a diagnosis of A-T in 2 days on small numbers of PBMCs and can potentially identify A-T carriers and individuals at increased risk for cancer.