Effects of solution conditions on virus retention by the Viresolve® NFP filter.

Virus filtration can provide a robust method for removal of adventitious parvoviruses in the production of biotherapeutics. Although virus filtration is typically thought to function by a purely size-based removal mechanism, there is limited data in the literature indicating that virus retention is a function of solution conditions. The objective of this work was to examine the effect of solution pH and ionic strength on virus retention by the Viresolve(®) NFP membrane. Data were obtained using the bacteriophage ϕX174 as a model virus, with retention data complemented by the use of confocal microscopy to directly visualize capture of fluorescently labeled ϕX174 within the filter. Virus retention was greatest at low pH and low ionic strength, conditions under which there was an attractive electrostatic interaction between the negatively charged membrane and the positively charged phage. In addition, the transient increase in virus transmission seen in response to a pressure disruption at pH 7.8 and 10 was completely absent at pH 4.9, suggesting that the trapped virus are unable to overcome the electrostatic attraction and diffuse out of the pores when the pressure is released. Further confirmation of this physical picture was provided by confocal microscopy. Images obtained at pH 10 showed the migration of previously captured phage; this phenomenon was absent at pH 4.9. These results provide important new insights into the factors governing virus retention using virus filtration membranes.

The tenacious recognition of yeast telomere sequence by Cdc13 is fully exerted by a single OB-fold domain.

Cdc13, the telomere end-binding protein from Saccharomyces cerevisiae, is a multidomain protein that specifically binds telomeric single-stranded DNA (ssDNA) with exquisitely high affinity to coordinate telomere maintenance. Recent structural and genetic data have led to the proposal that Cdc13 is the paralog of RPA70 within a telomere-specific RPA complex. Our understanding of Cdc13 structure and biochemistry has been largely restricted to studies of individual domains, precluding analysis of how each domain influences the activity of the others. To better facilitate a comparison to RPA70, we evaluated the ssDNA binding of full-length S. cerevisiae Cdc13 to its minimal substrate, Tel11. We found that, unlike RPA70 and the other known telomere end-binding proteins, the core Cdc13 ssDNA-binding activity is wholly contained within a single tight-binding oligosaccharide/oligonucleotide/oligopeptide binding (OB)-fold. Because two OB-folds are implicated in dimerization, we also evaluated the relationship between dimerization and ssDNA-binding activity and found that the two activities are independent. We also find that Cdc13 binding exhibits positive cooperativity that is independent of dimerization. This study reveals that, while Cdc13 and RPA70 share similar domain topologies, the corresponding domains have evolved different and specialized functions.

DNA repair: from genome maintenance to biomarker and therapeutic target.

A critical link exists between an individual's ability to repair cellular DNA damage and cancer development, progression, and response to therapy. Knowledge gained about the proteins involved and types of damage repaired by the individual DNA repair pathways has led to the development of a variety of assays aimed at determining an individual's DNA repair capacity. These assays and their use in the analysis of clinical samples have yielded useful though somewhat conflicting data. In this review article, we discuss the major DNA repair pathways, the proteins and genes required for each, assays used to analyze activity, and the relevant clinical studies to date. With the recent results from clinical trials targeting specific DNA repair proteins for the treatment of cancer, accurate, reproducible, and relevant analysis of DNA repair takes on an even greater significance. We highlight the strengths and limitations of these DNA repair studies and assays, with respect to the clinical assessment of DNA repair capacity to determine cancer development and response to therapy.

Interrogation of nucleotide excision repair capacity: impact on platinum-based cancer therapy.

DNA repair is essential for routine monitoring and repair of damage imparted to our genetic material by exposure to endogenous and exogenous carcinogens, including reactive oxygen species, UV light, and chemicals such as those found in cigarette smoke. Without DNA repair pathways, the continual assault on our DNA would be highly mutagenic and the risk of cancer increased. Paradoxically, the same pathways that help prevent cancer development are detrimental to the efficacy of DNA-damaging cancer therapeutics such as cisplatin. Recent studies demonstrate the inverse relationship between DNA repair capacity and efficacy of platinum-based chemotherapeutics: increased DNA repair capacity leads to resistance, while decreased capacity leads to increased sensitivities. Cisplatin's cytotoxic effects are mediated by formation of intrastrand DNA crosslinks, which are predominantly repaired via the nucleotide excision repair (NER) pathway. In an effort to personalize the treatment of cancers based on DNA repair capacity, we developed an ELISA-based assay to measure NER activity accurately and reproducibly as a prognostic for platinum-based treatments. Here we present an overview of DNA repair and its link to cancer and therapeutics. We also present data demonstrating the ability to detect the proteins of the pre-incision complex within the NER pathway from cell and tissue extracts.

Targeting the OB-Folds of Replication Protein A with Small Molecules.

Replication protein A (RPA) is the main eukaryotic single-strand (ss) DNA-binding protein involved in DNA replication and repair. We have identified and developed two classes of small molecule inhibitors (SMIs) that show in vitro inhibition of the RPA-DNA interaction. We present further characterization of these SMIs with respect to their target binding, mechanism of action, and specificity. Both reversible and irreversible modes of inhibition are observed for the different classes of SMIs with one class found to specifically interact with DNA-binding domains A and B (DBD-A/B) of RPA. In comparison with other oligonucleotide/oligosaccharide binding-fold (OB-fold) containing ssDNA-binding proteins, one class of SMIs displayed specificity for the RPA protein. Together these data demonstrate that the specific targeting of a protein-DNA interaction can be exploited towards interrogating the cellular activity of RPA as well as increasing the efficacy of DNA-damaging chemotherapeutics used in cancer treatment.