Homology-directed repair protects the replicating genome from metabolic assaults

Homology-directed repair (HDR) safeguards DNA integrity under various forms of stress, but how HDR protects replicating genomes under extensive metabolic alterations remains unclear. Here, we report that besides stalling replication forks, inhibition of ribonucleotide reductase (RNR) triggers metabolic imbalance manifested by the accumulation of increased reactive oxygen species (ROS) in cell nuclei. This leads to a redox-sensitive activation of the ATM kinase followed by phosphorylation of the MRE11 nuclease, which in HDR-deficient settings degrades stalled replication forks. Intriguingly, nascent DNA degradation by the ROS-ATM-MRE11 cascade is also triggered by hypoxia, which elevates signaling-competent ROS and attenuates functional HDR without arresting replication forks. Under these conditions, MRE11 degrades daughter-strand DNA gaps, which accumulate behind active replisomes and attract error-prone DNA polymerases to escalate mutation rates. Thus, HDR safeguards replicating genomes against metabolic assaults by restraining mutagenic repair at aberrantly processed nascent DNA. These findings have implications for cancer evolution and tumor therapy.

Conduction block in immune-mediated neuropathy: paranodopathy versus axonopathy

BACKGROUND AND PURPOSE

Conduction block is a pathognomonic feature of immune-mediated neuropathies. The aim of this study was to advance understanding of pathophysiology and conduction block in chronic inflammatory demyelinating polyneuropathy (CIDP) and multifocal motor neuropathy (MMN).

METHODS

A multimodal approach was used, incorporating clinical phenotyping, neurophysiology, immunohistochemistry and structural assessments.

RESULTS

Of 49 CIDP and 14 MMN patients, 25% and 79% had median nerve forearm block, respectively. Clinical scores were similar in CIDP patients with and without block. CIDP patients with median nerve block demonstrated markedly elevated thresholds and greater threshold changes in threshold electrotonus, whilst those without did not differ from healthy controls in electrotonus parameters. In contrast, MMN patients exhibited marked increases in superexcitability. Nerve size was similar in both CIDP groups at the site of axonal excitability. However, CIDP patients with block demonstrated more frequent paranodal serum binding to teased rat nerve fibres. In keeping with these findings, mathematical modelling of nerve excitability recordings in CIDP patients with block support the role of paranodal dysfunction and enhanced leakage of current between the node and internode. In contrast, changes in MMN probably resulted from a reduction in ion channel density along axons.

CONCLUSIONS

The underlying pathologies in CIDP and MMN are distinct. Conduction block in CIDP is associated with paranodal dysfunction which may be antibody-mediated in a subset of patients. In contrast, MMN is characterized by channel dysfunction downstream from the site of block.

Rad54 Phosphorylation Promotes Homologous Recombination by Balancing Rad54 Mobility and DNA Binding

The repair of DNA double-strand breaks by homologous recombination is of crucial importance for maintaining genomic stability. Two major players during this repair pathway are Rad51 and Rad54. Previously, it was shown that Rad54 exists as a monomer or oligomer when bound to DNA and drives the displacement of Rad51 by translocating along the DNA. Moreover, phosphorylation of Rad54 was reported to stimulate this clearance of Rad51 from DNA. However, it is currently unclear how phosphorylation of Rad54 modulates its molecular-structural function and how it affects the activity of monomeric or oligomeric Rad54 during the removal of Rad51. To examine the impact of Rad54 phosphorylation on a molecular-structural level, we applied molecular dynamics simulations of Rad54 monomers and hexamers in the absence or presence of DNA. Our results suggest that 1) phosphorylation of Rad54 stabilizes the monomeric form by reducing the interlobe movement of Rad54 monomers and might therefore facilitate multimer formation around DNA and 2) phosphorylation of Rad54 in a higher-order hexamer reduces its binding strength to DNA, which is a requirement for efficient mobility on DNA. To further address the relationship between the mobility of Rad54 and its phosphorylation state, we performed fluorescence recovery after photobleaching experiments in living cells, which expressed different versions of the Rad54 protein. Here, we could measure that the phosphomimetic version of Rad54 was highly mobile on DNA, whereas a nonphosphorylatable mutant displayed a mobility defect. Taken together, these data show that the phosphorylation of Rad54 is a critical event in balancing the DNA binding strength and mobility of Rad54 and might therefore provide optimal conditions for DNA translocation and subsequent removal of Rad51 during homologous recombination repair.

53BP1 nuclear bodies enforce replication timing at under-replicated DNA to limit heritable DNA damage

Failure to complete DNA replication is a stochastic by-product of genome doubling in almost every cell cycle. During mitosis, under-replicated DNA (UR-DNA) is converted into DNA lesions, which are inherited by daughter cells and sequestered in 53BP1 nuclear bodies (53BP1-NBs). The fate of such cells remains unknown. Here, we show that the formation of 53BP1-NBs interrupts the chain of iterative damage intrinsically embedded in UR-DNA. Unlike clastogen-induced 53BP1 foci that are repaired throughout interphase, 53BP1-NBs restrain replication of the embedded genomic loci until late S phase, thus enabling the dedicated RAD52-mediated repair of UR-DNA lesions. The absence or malfunction of 53BP1-NBs causes premature replication of the affected loci, accompanied by genotoxic RAD51-mediated recombination. Thus, through adjusting replication timing and repair pathway choice at under-replicated loci, 53BP1-NBs enable the completion of genome duplication of inherited UR-DNA and prevent the conversion of stochastic under-replications into genome instability.

DNA Double-Strand Break Resection Occurs during Non-homologous End Joining in G1 but Is Distinct from Resection during Homologous Recombination

Canonical non-homologous end joining (c-NHEJ) repairs DNA double-strand breaks (DSBs) in G1 cells with biphasic kinetics. We show that DSBs repaired with slow kinetics, including those localizing to heterochromatic regions or harboring additional lesions at the DSB site, undergo resection prior to repair by c-NHEJ and not alt-NHEJ. Resection-dependent c-NHEJ represents an inducible process during which Plk3 phosphorylates CtIP, mediating its interaction with Brca1 and promoting the initiation of resection. Mre11 exonuclease, EXD2, and Exo1 execute resection, and Artemis endonuclease functions to complete the process. If resection does not commence, then repair can ensue by c-NHEJ, but when executed, Artemis is essential to complete resection-dependent c-NHEJ. Additionally, Mre11 endonuclease activity is dispensable for resection in G1. Thus, resection in G1 differs from the process in G2 that leads to homologous recombination. Resection-dependent c-NHEJ significantly contributes to the formation of deletions and translocations in G1, which represent important initiating events in carcinogenesis.

Nek1 Regulates Rad54 to Orchestrate Homologous Recombination and Replication Fork Stability

Never-in-mitosis A-related kinase 1 (Nek1) has established roles in apoptosis and cell cycle regulation. We show that human Nek1 regulates homologous recombination (HR) by phosphorylating Rad54 at Ser572 in late G2 phase. Nek1 deficiency as well as expression of unphosphorylatable Rad54 (Rad54-S572A) cause unresolved Rad51 foci and confer a defect in HR. Phospho-mimic Rad54 (Rad54-S572E), in contrast, promotes HR and rescues the HR defect associated with Nek1 loss. Although expression of phospho-mimic Rad54 is beneficial for HR, it causes Rad51 removal from chromatin and degradation of stalled replication forks in S phase. Thus, G2-specific phosphorylation of Rad54 by Nek1 promotes Rad51 chromatin removal during HR in G2 phase, and its absence in S phase is required for replication fork stability. In summary, Nek1 regulates Rad51 removal to orchestrate HR and replication fork stability.

Effect of leflunomide on the peripheral nerves in rheumatoid arthritis

BACKGROUND

The objective of this study was to determine the neurophysiological effects of leflunomide on peripheral nerves in rheumatoid arthritis.

METHODS

We conducted a prospective cohort trial of 32 patients with rheumatoid arthritis with 16 patients receiving leflunomide treatment and 16 receiving other disease-modifying anti-rheumatic drug therapies. Clinical, laboratory and neurophysiological measurements were used to determine the presence of a peripheral neuropathy in these patients at study entry and then after a further 3 and 6 months.

RESULTS

Fifty-four per cent of the leflunomide group and 8% of the control group had an increase in their neuropathy symptom score 6 months into the study (P = 0.01). No correlation was found between the electrophysiological findings and the clinical symptoms. There was no significant difference between the two groups in upper and lower limb sensory and motor amplitudes and conduction velocities recorded at 3 and 6 months. One patient developed both clinical and neurophysiological evidence of a peripheral neuropathy 5 months into the study that improved after cessation of leflunomide therapy and cholestyramine washout.

CONCLUSION

After 6 months of exposure we found that leflunomide was associated with an apparent increase in the clinical symptoms of peripheral neuropathy in patients with rheumatoid arthritis. These symptoms did not correlate with neurophysiological studies.

Effect of magnesium supplementation on the fractional intestinal absorption of 45CaCl2 in women with a low erythrocyte magnesium concentration

The cosupplementation of magnesium with calcium has been suggested to be beneficial in the prevention of osteoporosis. We investigated the effect of magnesium supplementation on parameters of bone resorption and fractional 45Ca absorption. Twenty apparently healthy women with a mean age of 39.2 +/- 9.2 years and an erythrocyte magnesium concentration less than 1.97 mmol/L were recruited into a controlled magnesium supplementation trial. During weeks 1 to 4, they received a daily control preparation, potassium/sodium citrate malate (PSCM). During weeks 5 to 8, the subjects received magnesium citrate malate (MCM) equivalent to 250 mg magnesium per day. During the fourth and eighth weeks, blood was collected for measurement of the serum intact parathyroid hormone (PTH) concentration and serum and erythrocyte magnesium concentration. Urine was collected for measurement of calcium, magnesium, creatinine, and deoxypyridinoline excretion. On the final day of each treatment period, 5 microCi45CaCl2 was administered orally, and the isotope was traced in the blood and urine over 7 hours. Urinary calcium, 45Ca, and deoxypyridinoline excretion, as well as serum intact PTH levels, showed no statistically significant changes as a result of magnesium supplementation. However, urinary magnesium excretion increased by 31.1% (P < .005) while fractional 45Ca absorption decreased by 23.5% (P < .001) as a result of magnesium supplementation. It is concluded that magnesium supplementation does not result in changes in bone resorption, while the fractional intestinal absorption of 45Ca appears to decrease.

The histopathology of cutaneous lesions of Kikuchi's disease (necrotizing lymphadenitis): a report of five cases

Kikuchi's disease (KD) is an idiopathic, self-limited necrotizing lymphadenitis that can clinically and histologically mimic high-grade lymphoma, including Hodgkin's disease, or can be mistaken for the lymphadenitis of systemic lupus erythematosus (SLE). Involvement of extranodal sites is unusual but well documented, especially in Asia, where KD is more common than in North America or Europe. The successful distinction of KD from malignant lymphoma and SLE is imperative for the appropriate treatment of affected patients. We describe five patients with cutaneous involvement by KD, all of whom presented with fever, lymphadenopathy, and an eruption on the skin of the upper body, which in one case was clinically suspected to be due to SLE and in another, polymorphous light eruption. The patients ranged in age from 10 months to 42 years (median, 33 years) and included three females and two males. All five patients had negative serologic studies for collagen vascular disease. Each patient had a lymph node biopsy showing the typical necrotizing lymphadenitis of KD. Skin biopsies from all five patients shared a specific constellation of histologic features: vacuolar interface change with necrotic keratinocytes, a dense lymphohistiocytic superficial and deep perivascular and interstitial infiltrate, varying amounts of papillary dermal edema, and abundant karyorrhectic debris with a conspicuous absence of neutrophils and a paucity of plasma cells, paralleling the nodal histology in KD. CD68 immunohistochemistry on paraffin-embedded sections showed many histiocytes and plasmacytoid monocytes in all cases, whereas CD3, CD4, and CD8 showed highly variable staining among the cases. There was only rare staining with TIA-1 and CD30. We believe that the papular eruption of KD has recognizable histopathologic features and that a CD68 stain that marks many cells that initially seem to be lymphocytes can be performed to confirm the diagnosis.

A rapid novel derivatization of amphetamine and methamphetamine using 2,2,2-trichloroethyl chloroformate for gas chromatography electron ionization and chemical ionization mass spectrometric analysis

Amphetamine and methamphetamine are commonly abused central nervous system stimulants. We describe a rapid new derivatization of amphetamine and methamphetamine using 2,2,2-trichloroethyl chloroformate for gas chromatography-mass spectrometric analysis. Amphetamine and methamphetamine, along with N-propyl amphetamine (internal standard), were extracted from urine using 1-chlorobutane. The derivatization with 2,2,2-trichloroethyl chloroformate can be achieved at room temperature in 10 minutes. The electron ionization mass spectrum of amphetamine 2,2,2-trichloroethyl carbamate showed two weak molecular ions at m/z 309 and 311, but showed diagnostic strong peaks at m/z 218, 220, and 222. In contrast, chemical ionization of the mass spectrum of amphetamine 2,2,2-trichloroethyl carbamate showed strong (M + 1) ions at m/z 310 and 312 and other strong diagnostic peaks at m/z 274 and 276. The major advantages of this derivative are the presence of a diagnostic cluster of peaks due to the isotopic effect of three chlorine atoms (isotopes 35 and 37) in the derivatized molecule and the relative ease of its preparation. We also observed strong molecular ions for derivatized methamphetamine in the chemical ionization mass spectrum, but the molecular ions were very weak in the electron ionization mass spectrum. We used the scan mode of mass spectrometry in all analyses. When using a urine standard containing 1,000 ng/mL of amphetamine (a 7.4-micromol/L concentration) and methamphetamine (a 6.7-micromol/L concentration), the within-run precisions were 4.8% for amphetamine and 3.6% for methamphetamine. The corresponding between-run precisions were 5.3% for amphetamine and 6.7% for methamphetamine. The assay was linear for amphetamine and methamphetamine concentrations of 250 to 5,000 ng/mL (amphetamine, 1.9-37.0 micromol/L; methamphetamine, 1.7-33.6 micromol/L). The detection limit was 100 ng/mL (amphetamine, 0.74 micromol/L; methamphetamine, 0.67 micromol/L) using the scan mode of electron ionization mass spectrometry. We observed good a correlation between the concentrations of amphetamine and methamphetamine in five urine specimens positive for amphetamines using the more conventional pentafluoropropionyl derivative and our new derivative using 2,2,2-trichloroethyl chloroformate.

Chromosomal translocation involving the beta T cell receptor gene in acute leukemia

DNA spanning a t(7;19) chromosomal translocation breakpoint was isolated from the human T cell line SUP-T7 established from an acute lymphoblastic leukemia. Nucleotide sequence analysis showed that the point of crossover on chromosome 7 occurred immediately adjacent to joining segment J beta 1.1 within the TCR-beta gene, suggesting that this translocation resulted from an error in TCR gene rearrangement. On chromosome 19, the translocation occurred within a previously uncharacterized transcriptional unit for which we propose the name lyl-1. An approximately 1.5-kb RNA is transcribed from this gene in a wide variety of hematolymphoid cell lines. The t(7;19) results in truncation of the lyl-1 gene and production of abnormal-sized RNAs, suggesting a role for lyl-1 in the pathogenesis of this leukemia.

Representativity of human mammary tumor cell cultures: DNA-cytophotometry as a method for checking tumour cell characteristics

Short term cultures (3-6 days) of 40 primary human mammary carcinomas were prepared and compared with the original tumours from which they were derived. As a criterion the nuclear DNA Frequency Distribution Pattern (FDP), cytophotometrically measured, was used. Comparisons were made between the FDPs of smears of freshly-cut tumour surfaces and their cultures. Twenty-nine (73%) cultures showed FDPs identical with the smears. Eleven cultures (27%) showed gross shifts in ratios between different peaks or showed a complete loss of one or more peaks in the FDP and were classified as not representative. Our results show that it is necessary to check primary mammary tumour cultures to determine whether or not they are representative of the original tumour. This is especially so if conclusions are to be drawn from the cultures about the original tumour. Analysis of FDPs in the cell islands of the cultures (migration of cells from attached clumps) resulted in a better understanding of the FDPs found in the smears. We showed that cultures of human mammary tumours either can be composed of cell islands with identical FDPs (diploid or aneuploid) or may show heterogeneity between different cell islands within one tumour.

Feline mammary carcinomas as a model for human breast cancer. I. Sensitivity of mammary tumor cells in culture to cytostatic drugs. A preliminary investigation of a predictive test

Feline mammary carcinomas were found to maintain well in short-term cultures. Principally the same types of nuclear DNA frequency distribution histograms were recognized in feline mammary carcinomas as in human mammary carcinomas. However, the more abnormal histograms are less frequent in feline than in human mammary carcinomas. Feline mammary carcinomas appeared, at least in vitro, most sensitive to Doxorubicin and 5-fluorouracil. Preliminary, thymidine incorporation studies indicate both cytotoxic and cytostatic effects of Doxorubicin and 5-FU. Methotrexate was found to stimulate thymidine incorporation.

[Relation between atmospheric events and internal medicine emergency situations]

Over a period of 2,541 days (7 years) patients with the diagnosis groups acute oesophageal and gastrointestinal haemorrhage, respectively (n = 395), cerebrovascular insufficiency (n = 689) and acute myocardial infarction (n = 701) were registered. From the individual meteorological data registered in the same period the temperature the evaporation warmth of the water, the specific warmth capacity of the air, the air pressure and a constant calculation factor were used for the calculation of the equivalent temperature. Thus by its means a measure for the warmth content of the air was at our disposal which is determined by a series of factors. Between the emergency situations mentioned and the equivalent temperature established a correlation test for significance was performed. In these cases was the result that there are significant relations between the cerebrovascular insufficiency. Though already a larger number of meteorological parameters entered into our investigations, the result supposes that further important meteorological data should not be neglected, such as phases of weather and front transits. This shall be taken into consideration in future investigations.