POT1 recruits and regulates CST-Polα/Primase at human telomeres

Telomere maintenance requires extension of the G-rich telomeric repeat strand by telomerase and fill-in synthesis of the C-rich strand by Polα/Primase. Telomeric Polα/Primase is bound to Ctc1-Stn1-Ten1 (CST), a single-stranded DNA-binding complex. Like mutations in telomerase, mutations affecting CST-Polα/Primase result in pathological telomere shortening and cause a telomere biology disorder, Coats plus (CP). We determined cryogenic electron microscopy structures of human CST bound to the shelterin heterodimer POT1/TPP1 that reveal how CST is recruited to telomeres by POT1. Phosphorylation of POT1 is required for CST recruitment, and the complex is formed through conserved interactions involving several residues mutated in CP. Our structural and biochemical data suggest that phosphorylated POT1 holds CST-Polα/Primase in an inactive auto-inhibited state until telomerase has extended the telomere ends. We propose that dephosphorylation of POT1 releases CST-Polα/Primase into an active state that completes telomere replication through fill-in synthesis.

CST-Polα/Primase: the second telomere maintenance machine

It has been known for decades that telomerase extends the 3' end of linear eukaryotic chromosomes and dictates the telomeric repeat sequence based on the template in its RNA. However, telomerase does not mitigate sequence loss at the 5' ends of chromosomes, which results from lagging strand DNA synthesis and nucleolytic processing. Therefore, a second enzyme is needed to keep telomeres intact: DNA polymerase α/Primase bound to Ctc1-Stn1-Ten1 (CST). CST-Polα/Primase maintains telomeres through a fill-in reaction that replenishes the lost sequences at the 5' ends. CST not only serves to maintain telomeres but also determines their length by keeping telomerase from overelongating telomeres. Here we discuss recent data on the evolution, structure, function, and recruitment of mammalian CST-Polα/Primase, highlighting the role of this complex and telomere length control in human disease.

Cryo-EM structure of the human CST-Polα/primase complex in a recruitment state

The CST–Polα/primase complex is essential for telomere maintenance and functions to counteract resection at double-strand breaks. We report a 4.6-Å resolution cryo-EM structure of human CST–Polα/primase, captured prior to catalysis in a recruitment state stabilized by chemical cross-linking. Our structure reveals an evolutionarily conserved interaction between the C-terminal domain of the catalytic POLA1 subunit and an N-terminal expansion in metazoan CTC1. Cross-linking mass spectrometry and negative-stain EM analysis provide insight into CST binding by the flexible POLA1 N-terminus. Finally, Coats plus syndrome disease mutations previously characterized to disrupt formation of the CST–Polα/primase complex map to protein–protein interfaces observed in the recruitment state. Together, our results shed light on the architecture and stoichiometry of the metazoan fill-in machinery.

Cryo-EM analysis of the human CST–Polα/primase complex reveals a metazoan-specific mode of interaction between CST and DNA polymerase α that is proposed to function in telomeric recruitment of Polα/primase for C-strand maintenance.

The evolution of metazoan shelterin

In this study, Myler et al. investigated the evolutionary origins of shelterin complex, which is comprised of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1; blocks the DNA damage response at chromosome ends; and interacts with telomerase and the CST complex to regulate telomere length. They describe the evolution of metazoan shelterin, showing that TRF1 emerged in vertebrates upon duplication of a TRF2-like ancestor, and providing insights into the biology of shelterin and its evolution from ancestral telomeric DNA-binding proteins.

The mammalian telomeric shelterin complex—comprised of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1—blocks the DNA damage response at chromosome ends and interacts with telomerase and the CST complex to regulate telomere length. The evolutionary origins of shelterin are unclear, partly because unicellular organisms have distinct telomeric proteins. Here, we describe the evolution of metazoan shelterin, showing that TRF1 emerged in vertebrates upon duplication of a TRF2-like ancestor. TRF1 and TRF2 diverged rapidly during vertebrate evolution through the acquisition of new domains and interacting factors. Vertebrate shelterin is also distinguished by the presence of an HJRL domain in the split C-terminal OB fold of POT1, whereas invertebrate POT1s carry inserts of variable nature. Importantly, the data reveal that, apart from the primate and rodent POT1 orthologs, all metazoan POT1s are predicted to have a fourth OB fold at their N termini. Therefore, we propose that POT1 arose from a four-OB-fold ancestor, most likely an RPA70-like protein. This analysis provides insights into the biology of shelterin and its evolution from ancestral telomeric DNA-binding proteins.

[Non-surgical treatment of acute pancreatitis with colonic fistula]

Objective: To examine the effectiveness of non-operative of colonic fistula following acute pancreatitis. Methods: Retrospective analysis of 354 patients with acute pancreatitis who were admitted to Department of Hepatobiliary Surgery of Chinese People's Liberation Army General Hospital from January 2013 to December 2018. Age of the patients was (46±14) years (range: 14-85 years); 249 cases (70.3%) were males. There were 41 cases of acute edematous pancreatitis and 313 cases of acute necrotising pancreatitis. Two hundred and fifteen cases were diagnosed as moderate severe acute pancreatitis and 139 were diagnosed as severe acute pancreatitis. Among 313 cases of acute necrotising pancreatitis, 62 cases underwent non-surgical treatment, 251 cases underwent surgical treatment in which 218 of minimal access retroperitoneal pancreatic necrosectomy underwent percutaneous nephroliguectomy with peripancreatic necrotic tissue removal technique. Results: There were 15 cases of colon fistula following acute necrotising pancreatitis, and the incidence rate was 4.2%(15/354). There were 7 males and 8 females, with age of (39±8) years (range: 27 to 50 years). The median interval between acute pancreatitis onset and diagnosis of colonic fistula was 71 days(27-134) days. Two cases occurred at the hepatic flexure of the colon, 4 cases at transverse colon, and 9 cases at splenic flexure of colon. Of the 354 patients, 39 cases died and the mortality was 11.0%. Two patients underwent laparotomy, and one of them died. The remaining 13 patients underwent non-surgical treatment and were discharged. Conclusion: Acute pancreatitis with colonic fistula can be treated with non-surgical treatment and can achieve good prognosis.

Structural and functional analysis of mRNA export regulation by the nuclear pore complex

The nuclear pore complex (NPC) controls the passage of macromolecules between the nucleus and cytoplasm, but how the NPC directly participates in macromolecular transport remains poorly understood. In the final step of mRNA export, the DEAD-box helicase DDX19 is activated by the nucleoporins Gle1, Nup214, and Nup42 to remove Nxf1•Nxt1 from mRNAs. Here, we report crystal structures of Gle1•Nup42 from three organisms that reveal an evolutionarily conserved binding mode. Biochemical reconstitution of the DDX19 ATPase cycle establishes that human DDX19 activation does not require IP₆, unlike its fungal homologs, and that Gle1 stability affects DDX19 activation. Mutations linked to motor neuron diseases cause decreased Gle1 thermostability, implicating nucleoporin misfolding as a disease determinant. Crystal structures of human Gle1•Nup42•DDX19 reveal the structural rearrangements in DDX19 from an auto-inhibited to an RNA-binding competent state. Together, our results provide the foundation for further mechanistic analyses of mRNA export in humans.

The export of mRNA to the cytosol depends on the nuclear pore complex (NPC) and the activation of the helicase DDX19, but their interplay in humans remains poorly understood. Here, the authors present a structural and functional analysis of DDX19 activation, revealing how the human NPC regulates mRNA export.

[The research advances and applications of genome editing in hereditary eye diseases]

Genome editing is a cutting-edge technology that generates DNA double strand breaks at the specific genomic DNA sequence through nuclease recognition and cleavage, and then achieves insertion, replacement, or deletion of the target gene via endogenous DNA repair mechanisms, such as non-homologous end joining, homology directed repair, and homologous recombination. So far, more than 600 human hereditary eye diseases and systemic hereditary diseases with ocular phenotypes have been found. However, most of these diseases are of incompletely elucidated pathogenesis and without effective therapies. Genome editing technology can precisely target and alter the genomes of animals, establish animal models of the hereditary diseases, and elucidate the relationship between the target gene and the disease phenotype, thereby providing a powerful approach to studying the pathogenic mechanisms underlying the hereditary eye diseases. In addition, correction of gene mutations by the genome editing brings a new hope to gene therapy for the hereditary eye diseases. This review introduces the molecular characteristics of 4 major enzymes used in the genome editing, including homing endonucleases, zinc finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR-associated protein 9 (Cas9), and summarizes the current applications of this technology in investigating the pathogenic mechanisms underlying the hereditary eye diseases. (Chin J Ophthalmol, 2017, 53: 386-371).

[Analysis of the spectrum and resistance of pathogen causing sepsis in patients with severe acute pancreatitis]

Objective: To investigate the characteristics of spectrum and drug resistance of pathogens causing sepsis in patients with severe acute pancreatitis(SAP). Methods: The clinical data of 63 SAP patients with sepsis admitted in Department of Hepatobiliary, People's Liberation Army General Hospital from January 2014 to December 2015 were retrospectively studied. There were 47 males and 16 females, aged from 22 to 73 years, with an average age of (52±11)years. Samples were collected mainly from: (1)pancreatic and peripancreatic necrosis and abdominal drainage; (2)bile; (3) blood or deep venous catheter; (4) sputum and tracheal catheter and thoracic drainage; (5) urine. Strain identification and drug-resistance test were preformed on positive specimens. Results: Of 244 pathogenic isolates, mainly derived from abdominal cavity(36.0%), blood stream (14.0%), central venous catheter(11.8%), necrotic tissue(9.1%) and sputum(8.1%); 154(63.1%) were gram-negative bacteria, 68 cases(27.9%) were gram-positive bacteria and 22 cases(9.0%) were fungi respectively. The top six common pathogens isolated were E. coli(16.0%), E.faecium and faecalis(15.2%), P.aeruginosa(10.7%), K.pneumonia(9.8%), Acinetobacter baumanni(8.2%), Stenotrophomonas maltophilia(5.3%)respectively. The detection rate of E. coli and K. pneumonia extended-spectrum β-lactamases(ESBL) was 84.6%(33/39) and 70.8%(17/24), the resistance rate to imipeniem was 12.8% and 25.0%, to cefperazone-sulbactam was 28.2% and 29.2%. As to P. aeruginosa and Acinetobacter bacillus, the resistance rate to imipeniem was 50.0% and 75.0%, to cefperazone-sulbactam was 42.3% and 70.0%; Stenotrophomonas maltophilia was completely resistant to cefperazone-sulbactam, but sensitive to minocycline, SMZ-TMP with the resistance rate less than 40.0%. Gram-positive bacterium strains mainly included E. faecium(38.2%, 26/68), E.faecalis(16.2%, 11/68) and Staphylococcus(35.3%, 24/68) which maintained high sensitivity to vancomycin, teicoplanin and linezolid, there was only one isolate resistant to vancomycin. Candida were the sole pathogens of fungal infections, sensitive to common antifungal drugs overall. Conclusions: The gram-negative bacteria are the predominant pathogens mainly including ESBL-producing isolates(E.coli and K. pneumonia) and non-fermentation bacteria(P.aeruginosa and Acinetobacter bacillus) causing sepsis in SAP. The infection rate and drug-resistance rate of these two kinds of pathogens are relatively higher.

[Experience of minimal-access video-assisted retroperitoneal debridement in treatment of infected pancreatic necrosis]

Objective: To explore the experience of minimal-access video-assisted retroperitoneal debridement in treatment of infected pancreatic necrosis(IPN). Methods: A retrospective review was performed on 12 patients with IPN who underwent minimal-access video-assisted retroperitoneal debridement between June 2008 and June 2013 in People's Liberation Army General Hospital and First Affiliated Hospital of People's Liberation Army General Hospital, respectively.There were 10 male patients and 2 female patients aging from 33 to 55 years with mean age of(43±8)years.Pancreas infective necrosis, the serious complications of severe acute pancreatitis occurred in all of the patients among which there were 2 patients with infection after percutaneous catheter drainage(PCD)in early phase of disease, and 12 patients with spontaneous during the late phase. The technical strategies of the minimally invasive treatment mainly included PCD, minimal-access video-assisted retroperitoneal debridement, and irrigation. Results: Ten patients received PCD and the median time from onset of acute necrotizing pancreatitis to PCD was mean of 24 days(range 8-86 days). Minimal-access video-assisted retroperitoneal debridement was performed after 18 days(range 3-29 days) after PCD.Three patients died after surgery.Five patients had hemorrhage complication and 3 had colonic fistula.Pancreatic fistula occurred in 2 patients. Conclusions: The technique of minimal-access video-assisted retroperitoneal debridement has advantage and some minor disadvantage.Delayed minimal-access video-assisted retroperitoneal debridement is recommended.

Cadmium exposure and health effects among residents in an irrigation area with ore dressing wastewater

About 0.05 mg Cd l-1 was found in the irrigation water contaminated by the wastewater discharged from tungsten ore dressing plants, and about 1 mg Cd kg-1 was found in irrigated acid soils. The main sources of cadmium intake by residents of the polluted areas are agricultural products. The average intake of cadmium was 367-382 micrograms day-1, and for smokers 417 micrograms day-1. Among the residents who have been exposed to cadmium for more than 25 years, cadmium absorption (urinary cadmium greater than or equal to 10 micrograms g-1 creatine) was 60% and an early effect on the target organ (urinary cadmium greater than or equal to 15 micrograms g-1 creatinine and urinary beta 2-microglobulin greater than 500 micrograms g-1 creatinine) was detected in 17% of the sample of 433 persons. Levels of urinary cadmium and blood cadmium exceeded the critical value in the exposed group. The concentrations of urinary calcium, beta 2-microglobulin and N-acetyl-beta-D-glucosaminidase were significantly higher in residents of contaminated areas than in those of control areas. Case-control study of the residents shows that many have suffered from a nephropathy with tubulo-interstitial abnormalities. Owing to the combined cadmium effect, the cases with fractional beta 2-microglobulin excretion greater than 0.10 were 46% in the exposed population, and were much higher than that in the control group (20.3%).

[Cadmium exposure and health effects among residents in irrigation area with ore dressing wastewater]

About 0.05 mg Cd/L was found in the irrigation water contaminated by the wastewater discharged from the wolfram ore dressing plants, and about 1 mg Cd/kg was found in irrigated acid soil. The main sources of the cadmium intake by residents, living in pollution areas, were agricultural products. The average intake of cadmium was 366.7-381.9 micrograms/d, smokers 416.6 micrograms/d. Among residents, who have been exposed to cadmium for more than 25 years, cadmium absorption (CdU greater than or equal to 10 micrograms/g Cre) was 60% and early effect of target organ (CdU greater than or equal to 15 micrograms/g Cre and beta 2 -mU greater than or equal to 500 micrograms/g Cre) was 17%. Levels of CdU and CdB were beyond the critical value in the observation areas. The concentrations of CaU, beta 2-mU and NAGU were significantly higher among residents in observation areas compared with control areas. Owing to the combined cadmium effect the cases with fractional beta 2-m excretion greater than 0.10 were 46% in the observation areas, and it was much higher than that in control areas (20.3%) in a case-control study among residents who have been suffering from an original nephropathy with tubulo-interstitial abnormalities.