ADAR (adenosine deaminase that acts on RNA) editing enzymes target coding and noncoding double-stranded RNA (dsRNA) and are essential for neuronal function. Early studies showed that ADARs preferentially target adenosines with certain 5′ and 3′ neighbours. Here we use current Sanger sequencing protocols to perform a more accurate and quantitative analysis. We quantified editing sites in an ∼800-bp dsRNA after reaction with human ADAR1 or ADAR2, or their catalytic domains alone. These large data sets revealed that neighbour preferences are mostly dictated by the catalytic domain, but ADAR2's dsRNA-binding motifs contribute to 3′ neighbour preferences. For all proteins, the 5′ nearest neighbour was most influential, but adjacent bases also affected editing site choice. We developed algorithms to predict editing sites in dsRNA of any sequence, and provide a web-based application. The predictive power of the algorithm on fully base-paired dsRNA, compared with biological substrates containing mismatches, bulges and loops, elucidates structural contributions to editing specificity.
We have generated extreme ionizing radiation resistance in a relatively sensitive bacterial species, Escherichia coli, by directed evolution. Four populations of Escherichia coli K-12 were derived independently from strain MG1655, with each specifically adapted to survive exposure to high doses of ionizing radiation. D 37 values for strains isolated from two of the populations approached that exhibited by Deinococcus radiodurans. Complete genomic sequencing was carried out on nine purified strains derived from these populations. Clear mutational patterns were observed that both pointed to key underlying mechanisms and guided further characterization of the strains. In these evolved populations, passive genomic protection is not in evidence. Instead, enhanced recombinational DNA repair makes a prominent but probably not exclusive contribution to genome reconstitution. Multiple genes, multiple alleles of some genes, multiple mechanisms, and multiple evolutionary pathways all play a role in the evolutionary acquisition of extreme radiation resistance. Several mutations in the recA gene and a deletion of the e14 prophage both demonstrably contribute to and partially explain the new phenotype. Mutations in additional components of the bacterial recombinational repair system and the replication restart primosome are also prominent, as are mutations in genes involved in cell division, protein turnover, and glutamate transport. At least some evolutionary pathways to extreme radiation resistance are constrained by the temporally ordered appearance of specific alleles.
Genetic testing has the potential to guide the prevention and treatment of disease in a variety of settings, and recent technical advances have greatly increased our ability to acquire large amounts of genetic data. The interpretation of this data remains challenging, as the clinical significance of genetic variation detected in the laboratory is not always clear. Although regulatory agencies and professional societies provide some guidance regarding the classification, reporting, and long-term follow-up of variants, few protocols for the implementation of these guidelines have been described. Because the primary aim of clinical testing is to provide results to inform medical management, a variant classification program that offers timely, accurate, confident and cost-effective interpretation of variants should be an integral component of the laboratory process. Here we describe the components of our laboratory's current variant classification program (VCP), based on 20 years of experience and over one million samples tested, using the BRCA1/2 genes as a model. Our VCP has lowered the percentage of tests in which one or more BRCA1/2 variants of uncertain significance (VUSs) are detected to 2.1% in the absence of a pathogenic mutation, demonstrating how the coordinated application of resources toward classification and reclassification significantly impacts the clinical utility of testing.
Cells rely on a network of conserved pathways to govern DNA replication fidelity. Loss of polymerase proofreading or mismatch repair elevates spontaneous mutation and facilitates cellular adaptation. However, double mutants are inviable, suggesting that extreme mutation rates exceed an error threshold. Here we combine alleles that affect DNA polymerase δ (Pol δ) proofreading and mismatch repair to define the maximal error rate in haploid yeast and to characterize genetic suppressors of mutator phenotypes. We show that populations tolerate mutation rates 1,000-fold above wild-type levels but collapse when the rate exceeds 10−3 inactivating mutations per gene per cell division. Variants that escape this error-induced extinction (eex) rapidly emerge from mutator clones. One-third of the escape mutants result from second-site changes in Pol δ that suppress the proofreading-deficient phenotype, while two-thirds are extragenic. The structural locations of the Pol δ changes suggest multiple antimutator mechanisms. Our studies reveal the transient nature of eukaryotic mutators and show that mutator phenotypes are readily suppressed by genetic adaptation. This has implications for the role of mutator phenotypes in cancer.
Single-stranded DNA (ssDNA)-binding (SSB) proteins are uniformly required to bind and protect single-stranded intermediates in DNA metabolic pathways. All bacterial and eukaryotic SSB proteins studied to date oligomerize to assemble four copies of a conserved domain, called an oligonucleotide͞oligosaccharide-binding (OB) fold, that cooperate in nonspecific ssDNA binding. The vast majority of bacterial SSB family members function as homotetramers, with each monomer contributing a single OB fold. However, SSB proteins from the Deinococcus-Thermus genera are exceptions to this rule, because they contain two OB folds per monomer. To investigate the structural consequences of this unusual arrangement, we have determined a 1.8-Å-resolution x-ray structure of Deinococcus radiodurans SSB. The structure shows that D. radiodurans SSB comprises two OB domains linked by a -hairpin motif. The protein assembles a four-OB-fold arrangement by means of symmetric dimerization. In contrast to homotetrameric SSB proteins, asymmetry exists between the two OB folds of D. radiodurans SSB because of sequence differences between the domains. These differences appear to reflect specialized roles that have evolved for each domain. Extensive crystallographic contacts link D. radiodurans SSB dimers in an arrangement that has important implications for higher-order structures of the protein bound to ssDNA. This assembly utilizes the N-terminal OB domain and the -hairpin structure that is unique to Deinococcus and Thermus species SSB proteins. We hypothesize that differences between D. radiodurans SSB and homotetrameric bacterial SSB proteins may confer a selective advantage to D. radiodurans cells that aids viability in environments that challenge genomic stability.I n all organisms, single-stranded DNA (ssDNA)-binding (SSB) proteins sequester and protect ssDNA intermediates that arise during DNA replication, recombination, and repair (1). Their prominent roles in genome maintenance reactions make SSB proteins a requirement for cellular life (2). Although the sequences of SSB family members are highly variable, two common functional themes have emerged that link this class of proteins across evolution. The first is that SSB proteins use a conserved domain called an oligonucleotide͞oligosaccharide-binding (OB) fold to bind ssDNA (3, 4). OB domains bind ssDNA in a cleft formed primarily by -strands, by using aromatic residues that stack against nucleotide bases and positively charged residues that form ionic interactions with the DNA backbone (5-8). The second common feature of cellular SSB proteins is obligate oligomerization that brings together four DNA-binding OB folds. For example, Escherichia coli SSB contains a single OB fold per monomer, but the active form of the protein is a homotetramer with four OB folds (1). This general arrangement appears to define a structural paradigm for bacterial SSB family proteins because all but three of the Ͼ250 currently identifiable bacterial ssb genes encode proteins with a single OB fold.Different...
Single-nucleotide substitutions and small in-frame insertions or deletions identified in human breast cancer susceptibility genes BRCA1 and BRCA2 are frequently classified as variants of unknown clinical significance (VUS) due to the availability of very limited information about their functional consequences. Such variants can most reliably be classified as pathogenic or non-pathogenic based on the data of their co-segregation with breast cancer in affected families and/or their co-occurrence with a pathogenic mutation. Biological assays that examine the effect of variants on protein function can provide important information that can be used in conjunction with available familial data to determine the pathogenicity of VUS. In this report, we have used a previously described mouse embryonic stem (mES) cell-based functional assay to characterize eight BRCA2 VUS that affect highly conserved amino acid residues and map to the N-terminal PALB2-binding or the C-terminal DNA-binding domains. For several of these variants, very limited co-segregation information is available, making it difficult to determine their pathogenicity. Based on their ability to rescue the lethality of Brca2-deficient mES cells and their effect on sensitivity to DNA-damaging agents, homologous recombination and genomic integrity, we have classified these variants as pathogenic or non-pathogenic. In addition, we have used homology-based modeling as a predictive tool to assess the effect of some of these variants on the structural integrity of the C-terminal DNA-binding domain and also generated a knock-in mouse model to analyze the physiological significance of a residue reported to be essential for the interaction of BRCA2 with meiosis-specific recombinase, DMC1.
D. radiodurans single stranded (ss) DNA binding protein (DrSSB) originates from a radiation-resistant bacterium and participates in DNA recombination, replication and repair. Although it functions as a homodimer, it contains four DNA binding domains (OB folds) and thus is structurally similar to the E. coli SSB (EcoSSB) homotetramer. We examined the equilibrium binding of DrSSB to ssDNA to compare with EcoSSB. We find that the occluded site size of DrSSB on poly(dT) is ~45 nucleotides in low salt (<0.02M NaCl) but increases to 50–55 nucleotides at [NaCl] ≥ 0.2M. This suggests that DrSSB undergoes a transition between ssDNA binding modes as is observed for EcoSSB, although the site size difference between modes is not as large as for EcoSSB, suggesting that the pathways of ssDNA wrapping differ for these two proteins. The occluded site size corresponds well to the contact site size (52 nucleotides) determined by Isothermal Titration Calorimetry (ITC). Electrophoretic studies of complexes of DrSSB with phage M13ssDNA indicate the formation of stable, highly cooperative complexes at low salt conditions. Using ITC we find that DrSSB binding to oligo(dT)s with lengths close to the determined site size (50–55 nts) is stoichiometric with ΔHobs ≈-94±4 kcal/mole, somewhat smaller than for EcoSSB (≈-130 kcal/mole) under the same conditions. The observed binding enthalpy shows a large sensitivity to NaCl concentration, similar to that observed for EcoSSB. With the exception of the less dramatic change in occluded site size, the behavior of DrSSB is similar to that of EcoSSB protein (although, clear quantitative differences exist). These common features for SSB proteins having multiple DNA binding domains enable versatility of SSB function in vivo.
Genetic variants of uncertain clinical significance (VUSs) are a common outcome of clinical genetic testing. Locus-specific variant databases (LSDBs) have been established for numerous disease-associated genes as a research tool for the interpretation of genetic sequence variants to facilitate variant interpretation via aggregated data. If LSDBs are to be used for clinical practice, consistent and transparent criteria regarding the deposition and interpretation of variants are vital, as variant classifications are often used to make important and irreversible clinical decisions. In this study, we performed a retrospective analysis of 2017 consecutive BRCA1 and BRCA2 genetic variants identified from 24,650 consecutive patient samples referred to our laboratory to establish an unbiased dataset representative of the types of variants seen in the US patient population, submitted by clinicians and researchers for BRCA1 and BRCA2 testing. We compared the clinical classifications of these variants among five publicly accessible BRCA1 and BRCA2 variant databases: BIC, ClinVar, HGMD (paid version), LOVD, and the UMD databases. Our results show substantial disparity of variant classifications among publicly accessible databases. Furthermore, it appears that discrepant classifications are not the result of a single outlier but widespread disagreement among databases. This study also shows that databases sometimes favor a clinical classification when current best practice guidelines (ACMG/AMP/CAP) would suggest an uncertain classification. Although LSDBs have been well established for research applications, our results suggest several challenges preclude their wider use in clinical practice.Electronic supplementary materialThe online version of this article (doi:10.1007/s12687-015-0220-x) contains supplementary material, which is available to authorized users.
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