Detection of OG:A Lesion Mispairs by MutY Relies on a Single His Residue and the 2-Amino Group of 8-Oxoguanine.

MutY glycosylase excises adenines misincorporated opposite the oxidatively damaged lesion, 8-oxo-7,8-dihydroguanine (OG), to initiate base excision repair and prevent G to T transversion mutations. Successful repair requires MutY recognition of the OG:A mispair amidst highly abundant and structurally similar undamaged DNA base pairs. Herein we use a combination of in vitro and bacterial cell repair assays with single-molecule fluorescence microscopy to demonstrate that both a C-terminal domain histidine residue and the 2-amino group of OG base are critical for MutY detection of OG:A sites. These studies are the first to directly link deficiencies in MutY lesion detection with incomplete cellular repair. These results suggest that defects in lesion detection of human MutY (MUTYH) variants may prove predictive of early-onset colorectal cancer known an MUTYH-associated polyposis. Furthermore, unveiling these specific molecular determinants for repair makes it possible to envision new MUTYH-specific cancer therapies.

Intensity of chemotherapy for the initial management of newly diagnosed acute myeloid leukemia in older patients.

Aim: This study evaluated the overall survival (OS) of older patients (≥60 years) with acute myeloid leukemia based on the intensity of treatment. Methods: This single center, retrospective study included 211 patients diagnosed between 2000 and 2016, who received 10-day decitabine, low-intensity therapy or high-intensity therapy. Cox regression examined the impact of therapy on OS. Results: Younger patients were more likely to receive high-intensity therapy. Patients who received low-intensity therapy had worse OS compared with high-intensity therapy (median OS: 1.2 vs 8.5 months; p < 0.01). OS was similar with 10-day decitabine (median OS of 6.3 months) compared with either low-intensity therapy or high-intensity therapy. Conclusion: Ten-day decitabine is an effective alternative in older patients with newly diagnosed acute myeloid leukemia.

Single molecule glycosylase studies with engineered 8-oxoguanine DNA damage sites show functional defects of a MUTYH polyposis variant.

Proper repair of oxidatively damaged DNA bases is essential to maintain genome stability. 8-Oxoguanine (7,8-dihydro-8-oxoguanine, 8-oxoG) is a dangerous DNA lesion because it can mispair with adenine (A) during replication resulting in guanine to thymine transversion mutations. MUTYH DNA glycosylase is responsible for recognizing and removing the adenine from 8-oxoG:adenine (8-oxoG:A) sites. Biallelic mutations in the MUTYH gene predispose individuals to MUTYH-associated polyposis (MAP), and the most commonly observed mutation in some MAP populations is Y165C. Tyr165 is a 'wedge' residue that intercalates into the DNA duplex in the lesion bound state. Here, we utilize single molecule fluorescence microscopy to visualize the real-time search behavior of Escherichia coli and Mus musculus MUTYH WT and wedge variant orthologs on DNA tightropes that contain 8-oxoG:A, 8-oxoG:cytosine, or apurinic product analog sites. We observe that MUTYH WT is able to efficiently find 8-oxoG:A damage and form highly stable bound complexes. In contrast, MUTYH Y150C shows decreased binding lifetimes on undamaged DNA and fails to form a stable lesion recognition complex at damage sites. These findings suggest that MUTYH does not rely upon the wedge residue for damage site recognition, but this residue stabilizes the lesion recognition complex.

Hide and seek: How do DNA glycosylases locate oxidatively damaged DNA bases amidst a sea of undamaged bases?

The first step of the base excision repair (BER) pathway responsible for removing oxidative DNA damage utilizes DNA glycosylases to find and remove the damaged DNA base. How glycosylases find the damaged base amidst a sea of undamaged bases has long been a question in the BER field. Single molecule total internal reflection fluorescence microscopy (SM TIRFM) experiments have allowed for an exciting look into this search mechanism and have found that DNA glycosylases scan along the DNA backbone in a bidirectional and random fashion. By comparing the search behavior of bacterial glycosylases from different structural families and with varying substrate specificities, it was found that glycosylases search for damage by periodically inserting a wedge residue into the DNA stack as they redundantly search tracks of DNA that are 450-600bp in length. These studies open up a wealth of possibilities for further study in real time of the interactions of DNA glycosylases and other BER enzymes with various DNA substrates.

Visualizing the Search for Radiation-damaged DNA Bases in Real Time.

The Base Excision Repair (BER) pathway removes the vast majority of damages produced by ionizing radiation, including the plethora of radiation-damaged purines and pyrimidines. The first enzymes in the BER pathway are DNA glycosylases, which are responsible for finding and removing the damaged base. Although much is known about the biochemistry of DNA glycosylases, how these enzymes locate their specific damage substrates among an excess of undamaged bases has long remained a mystery. Here we describe the use of single molecule fluorescence to observe the bacterial DNA glycosylases, Nth, Fpg and Nei, scanning along undamaged and damaged DNA. We show that all three enzymes randomly diffuse on the DNA molecule and employ a wedge residue to search for and locate damage. The search behavior of the Escherichia coli DNA glycosylases likely provides a paradigm for their homologous mammalian counterparts.

Insights into the glycosylase search for damage from single-molecule fluorescence microscopy.

The first step of base excision repair utilizes glycosylase enzymes to find damage within a genome. A persistent question in the field of DNA repair is how glycosylases interact with DNA to specifically find and excise target damaged bases with high efficiency and specificity. Ensemble studies have indicated that glycosylase enzymes rely upon both sliding and distributive modes of search, but ensemble methods are limited in their ability to directly observe these modes. Here we review insights into glycosylase scanning behavior gathered through single-molecule fluorescence studies of enzyme interactions with DNA and provide a context for these results in relation to ensemble experiments.

Single-Molecule Analysis of Cytochrome c Folding by Monitoring the Lifetime of an Attached Fluorescent Probe.

Conformational dynamics of proteins are important for function. However, obtaining information about specific conformations is difficult for samples displaying heterogeneity. Here, time-resolved fluorescence resonance energy transfer is used to characterize the folding of single cytochrome c molecules. In particular, measurements of the fluorescence lifetimes of individual cytochrome c molecules labeled with a single dye that is quenched by energy transfer to the heme were used to monitor conformational transitions of the protein under partially denaturing conditions. These studies indicate significantly more conformational heterogeneity than has been described previously. Importantly, the use of a purified singly-labeled sample made a direct comparison to ensemble data possible. The distribution of lifetimes of single-proteins was compared to the distribution of lifetimes determined from analysis of ensemble lifetime fluorescence data. The results show broad agreement between single-molecule and ensemble data, with a similar range of lifetimes. However, the single-molecule data reveal greater conformational heterogeneity.

Triple negative status is a poor prognostic indicator in Chinese women with breast cancer: a ten year review.

BACKGROUND: Ethnic variation in tumor characteristics and clinical presentation of breast cancer is increasingly being emphasized. We studied the tumor characteristics and factors which may influence the presentation and prognosis of triple negative breast cancers (TNC) in a cohort of Chinese women. METHODS: A prospective cohort of 1800 Chinese women with breast cancer was recruited in a tertiary referral unit in Hong Kong between 1995 and 2006 and was followed up with a median duration of 7.2 years. Of the total, 216 (12.0%) had TNC and 1584 (88.0%) had non-TNC. Their clinicopathological variables, epidemiological variables and clinical outcomes were evaluated. RESULTS: Patients with TNC had similar age of presentation as those with non-TNC, while presenting at earlier stages (82.4% were stage 1-2, compared to 78.4% in non-TNC, p=0.035). They were likely to be associated with grade 3 cancer (Hazard Ratio(HR)=5.8, p<0.001). TNC showed higher chance of visceral relapse (HR=2.69, p<0.001), liver metastasis (HR=1.7, p=0.003) and brain metastasis (HR=1.8, p=0.003). Compared with non-TNC group, TNC had similar 10-year disease-free survival (82% vs 84%, p=0.148), overall survival (78% vs 79%, p=0.238) and breast cancer-specific mortality (18% vs 16%, p=0.095). However, TNC showed poorer 10-year stage 3 and 4 specific survival (stage 3: 53% vs. 67%, p=0.010; stage 4: 0% vs. 40%, p=0.035). CONCLUSIONS: Chinese women with triple negative breast cancer do not have less aggressive biological behavior compared to the West and presentation at a later stage results in worse prognosis compared with those with non triple negative breast cancer.

Bright fluorescence from individual single-walled carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and ß-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.

Fetal responses to lipopolysaccharide-induced chorioamnionitis alter immune and airway responses in 7-week-old sheep.

OBJECTIVE: We hypothesized that fetal innate immune responses to lipopolysaccharide-induced chorioamnionitis would alter postnatal systemic immune and airway responsiveness. STUDY DESIGN: Ewes received intraamniotic injections with saline or lipopolysaccharide at 90, 100, and 110 days of gestation. Immune status and airway responsiveness were evaluated at term and at 7 weeks of age. RESULTS: At term, lymphocytes, monocytes, and neutrophils were significantly increased (respectively, 24-fold, 127-fold, and 31,000-fold) in lungs and blood monocytes became Toll-like receptor 2 responsive after lipopolysaccharide exposures. Furthermore, CD4 and CD4/CD25 lymphocytes were increased in thymus and lymph nodes. At 7 weeks, airway reactivity decreased and concentrations of CD8 cytotoxic T lymphocytes changed in the lungs and thymus relative to controls. CONCLUSION: Early gestational lipopolysaccharide exposure increased leukocyte responsiveness at term. Decreased airway reactivity and changes in lymphocytes at 7 weeks postnatal demonstrate persistent effects of fetal exposure to LPS.

Zinc porphyrin as a donor for FRET in Zn(II)cytochrome c.

We demonstrate that Zn(II) porphyrin in Zn(II)cytochrome c (Zn cyt c) is a fluorescence resonance energy transfer (FRET) donor to an Alexa660 dye acceptor. The energy transfer efficiency is dependent on the distance between the two fluorophores as shown through protein denaturation studies of five Zn cyt c variants labeled with Alexa660 in different positions. The relative quantum yield, excitation and emission energies, and labeling efficiencies of this donor-acceptor pair allow for a method of analysis based on sensitized emission of the acceptor. These studies show that Zn(II) porphyrin is an effective energy donor for measurement of molecular-scale distances by FRET.

Nitric oxide photogeneration from trans-Cr(cyclam)(ONO)(2)(+) in a reducing environment. activation of soluble guanylyl cyclase and arterial vasorelaxation.

The chromium(III) nitrito complex trans-Cr(cyclam)(ONO)(2)(+) (1) is a very promising photochemical precursor for nitric oxide delivery to physiological targets. Here, we demonstrate that visible wavelength excitation of 1 in solutions containing thiol reductants such as the biological antioxidant glutathione (GSH) leads to permanent reaction even under anaerobic conditions, resulting in high quantum yield NO release. The nitric oxide formed under such conditions is sufficient, even at muM concentrations of 1 and using a low-intensity light source, to activate the enzyme soluble guanylyl cyclase (sGC). We also demonstrate that photolysis of 1 in the nM concentration range with a portable blue LED leads to vasorelaxation of porcine coronary arterial rings, a process also attributed to the NO activation of sGC.

Guanidine hydrochloride-induced unfolding of the three heme coordination states of the CO-sensing transcription factor, CooA.

CooA is a heme-dependent CO-sensing transcription factor that has three observable heme coordination states. There is some evidence that each CooA heme state has a distinct protein conformation; the goal of this study was to characterize these conformations by measuring their structural stabilities through guanidine hydrochloride (GuHCl) denaturation. By studying the denaturation processes of the Fe(III) state of WT CooA and several variants, we were able to characterize independent unfolding processes for each domain of CooA. This information was used to compare the unfolding profiles of various CooA heme activation states [Fe(III), Fe(II), and Fe(II)-CO] to show that the heme coordination state changes the stability of the effector binding domain. A mechanism consistent with the data predicts that all CooA coordination states and variants undergo unfolding of the DNA-binding domain between 2 and 3 M GuHCl with a free energy of unfolding of approximately 17 kJ/mol, while unfolding of the heme domain is variable and dependent on the heme coordination state. The findings support a model in which changes in heme ligation alter the structural stability of the heme domain and dimer interface but do not alter the stability of the DNA-binding domain. These studies provide evidence that the domains of transcription factors are modular and that allosteric signaling occurs through changes in the relative positions of the protein domains without affecting the structure of the DNA-binding region.

The effects of nitroxyl (HNO) on soluble guanylate cyclase activity: interactions at ferrous heme and cysteine thiols.

It has been previously proposed that nitric oxide (NO) is the only biologically relevant nitrogen oxide capable of activating the enzyme soluble guanylate cyclase (sGC). However, recent reports implicate HNO as another possible activator of sGC. Herein, we examine the affect of HNO donors on the activity of purified bovine lung sGC and find that, indeed, HNO is capable of activating this enzyme. Like NO, HNO activation appears to occur via interaction with the regulatory ferrous heme on sGC. Somewhat unexpectedly, HNO does not activate the ferric form of the enzyme. Finally, HNO-mediated cysteine thiol modification appears to also affect enzyme activity leading to inhibition. Thus, sGC activity can be regulated by HNO via interactions at both the regulatory heme and cysteine thiols.

Nuclear receptors homo sapiens Rev-erbbeta and Drosophila melanogaster E75 are thiolate-ligated heme proteins which undergo redox-mediated ligand switching and bind CO and NO.

Nuclear receptors E75, which regulates development in Drosophila melanogaster, and Rev-erbbeta, which regulates circadian rhythm in humans, bind heme within their ligand binding domains (LBD). The heme-bound ligand binding domains of E75 and Rev-erbbeta were studied using electronic absorption, MCD, resonance Raman, and EPR spectroscopies. Both proteins undergo redox-dependent ligand switching and CO- and NO-induced ligand displacement. In the Fe(III) oxidation state, the nuclear receptor hemes are low spin and 6-coordinate with cysteine(thiolate) as one of the two axial heme ligands. The sixth ligand is a neutral donor, presumably histidine. When the heme is reduced to the Fe(II) oxidation state, the cysteine(thiolate) is replaced by a different neutral donor ligand, whose identity is not known. CO binds to the Fe(II) heme in both E75(LBD) and Rev-erbbeta(LBD) opposite a sixth neutral ligand, plausibly the same histidine that served as the sixth ligand in the Fe(III) state. NO binds to the heme of both proteins; however, the NO-heme is 5-coordinate in E75 and 6-coordinate in Rev-erbbeta. These nuclear receptors exhibit coordination characteristics that are similar to other known redox and gas sensors, suggesting that E75 and Rev-erbbeta may function in heme-, redox-, or gas-regulated control of cellular function.

DNA binding by an imidazole-sensing CooA variant is dependent on the heme redox state.

CooA is a transcription factor from Rhodospirillum rubrum that is regulated by the binding of the small molecule effector, CO, to a heme moiety in the protein. The heme in CooA is axially ligated by two endogenous donors in the Fe(III) and Fe(II) states of the protein, and CO binding to the Fe(II) state results in replacement of the distal ligand. Reduction of the heme in the absence of CO results in a ligand switch on the proximal side, in which a cysteine thiolate in the Fe(III) state is replaced by a histidine in the Fe(II) state. Recently, a variant, termed RW CooA, was designed to respond to a new effector; Fe(II) RW CooA shows high specificity and induced DNA-binding activity in the presence of imidazole. Spectroscopic characterization of the imidazole adducts of RW CooA revealed that, unlike CO, imidazole binds to both Fe(III) RW CooA and Fe(II) RW CooA. The spectral characteristics are consistent with normal function of the redox-mediated ligand switch; Fe(III)-imidazole RW CooA bears a thiolate ligand and Fe(II)-imidazole RW CooA bears a neutral donor ligand. Since the effector binds to both redox states, RW CooA was used to probe the role of the redox-mediated ligand switch in the CooA activation mechanism. Functional studies of Fe(III)-imidazole and Fe(II)-imidazole ligated RW CooA demonstrate that only the Fe(II)-imidazole form is active for DNA binding. Thus, the ligand switch is essential for the activating conformational change and may prevent aberrant activation of CooA by other neutral diatomic molecules.

Unexpected NO-dependent DNA binding by the CooA homolog from Carboxydothermus hydrogenoformans.

CooA, the CO-sensing heme protein from Rhodospirillum rubrum, regulates the expression of genes that encode a CO-oxidation system, allowing R. rubrum to use CO as a sole energy source. To better understand the gas-sensing regulation mechanism used by R. rubrum CooA and its homologs in other organisms, we characterized spectroscopically and functionally the Fe(II), Fe(II)-NO, and Fe(II)-CO forms of CooA from Carboxydothermus hydrogenoformans. Surprisingly, and unlike R. rubrum CooA, C. hydrogenoformans CooA binds NO to form a six-coordinate Fe(II)-NO heme that is active for DNA binding in vitro and in vivo. In contrast, R. rubrum CooA, which is exquisitely specific for CO, forms a five-coordinate Fe(II)-NO adduct that is inactive for DNA binding. Based on analyses of protein variants and temperature studies, NO-dependent DNA binding by C. hydrogenoformans CooA is proposed to result from a greater apparent stability of the six-coordinate Fe(II)-NO adduct at room temperature. Results from the present study strengthen the proposal that CO specificity in the CooA activation mechanism is based on the requirement for a small, neutral distal ligand, which in turn affects the relative positioning of the ligand-bound heme.