Unpredictable repeatability in molecular evolution.

The extent of parallel evolution at the genotypic level is quantitatively linked to the distribution of beneficial fitness effects (DBFE) of mutations. The standard view, based on light-tailed distributions (i.e., distributions with finite moments), is that the probability of parallel evolution in duplicate populations is inversely proportional to the number of available mutations and, moreover, that the DBFE is sufficient to determine the probability when the number of available mutations is large. Here, we show that when the DBFE is heavy-tailed, as found in several recent experiments, these expectations are defied. The probability of parallel evolution decays anomalously slowly in the number of mutations or even becomes independent of it, implying higher repeatability of evolution. At the same time, the probability of parallel evolution is non-self-averaging-that is, it does not converge to its mean value, even when a large number of mutations are involved. This behavior arises because the evolutionary process is dominated by only a few mutations of high weight. Consequently, the probability varies widely across systems with the same DBFE. Contrary to the standard view, the DBFE is no longer sufficient to determine the extent of parallel evolution, making it much less predictable. We illustrate these ideas theoretically and through analysis of empirical data on antibiotic-resistance evolution.

Conflicting effects of recombination on the evolvability and robustness in neutrally evolving populations.

Understanding the benefits and costs of recombination under different scenarios of evolutionary adaptation remains an open problem for theoretical and experimental research. In this study, we focus on finite populations evolving on neutral networks comprising viable and unfit genotypes. We provide a comprehensive overview of the effects of recombination by jointly considering different measures of evolvability and mutational robustness over a broad parameter range, such that many evolutionary regimes are covered. We find that several of these measures vary non-monotonically with the rates of mutation and recombination. Moreover, the presence of unfit genotypes that introduce inhomogeneities in the network of viable states qualitatively alters the effects of recombination. We conclude that conflicting trends induced by recombination can be explained by an emerging trade-off between evolvability on the one hand, and mutational robustness on the other. Finally, we discuss how different implementations of the recombination scheme in theoretical models can affect the observed dependence on recombination rate through a coupling between recombination and genetic drift.

Accessibility percolation on Cartesian power graphs.

A fitness landscape is a mapping from a space of discrete genotypes to the real numbers. A path in a fitness landscape is a sequence of genotypes connected by single mutational steps. Such a path is said to be accessible if the fitness values of the genotypes encountered along the path increase monotonically. We study accessible paths on random fitness landscapes of the House-of-Cards type, on which fitness values are independent, identically and continuously distributed random variables. The genotype space is taken to be a Cartesian power graph [Formula: see text], where [Formula: see text] is the number of genetic loci and the allele graph [Formula: see text] encodes the possible allelic states and mutational transitions on one locus. The probability of existence of accessible paths between two genotypes at a distance linear in [Formula: see text] displays a transition from 0 to a positive value at a threshold [Formula: see text] for the fitness difference between the initial and final genotype. We derive a lower bound on [Formula: see text] for general [Formula: see text] and show that this bound is tight for a large class of allele graphs. Our results generalize previous results for accessibility percolation on the biallelic hypercube, and compare favorably to published numerical results for multiallelic Hamming graphs.

Geometry of fitness landscapes: peaks, shapes and universal positive epistasis.

Darwinian evolution is driven by random mutations, genetic recombination (gene shuffling) and selection that favors genotypes with high fitness. For systems where each genotype can be represented as a bitstring of length L, an overview of possible evolutionary trajectories is provided by the L-cube graph with nodes labeled by genotypes and edges directed toward the genotype with higher fitness. Peaks (sinks in the graphs) are important since a population can get stranded at a suboptimal peak. The fitness landscape is defined by the fitness values of all genotypes in the system. Some notion of curvature is necessary for a more complete analysis of the landscapes, including the effect of recombination. The shape approach uses triangulations (shapes) induced by fitness landscapes. The main topic for this work is the interplay between peak patterns and shapes. Because of constraints on the shapes for [Formula: see text] imposed by peaks, there are in total 25 possible combinations of peak patterns and shapes. Similar constraints exist for higher L. Specifically, we show that the constraints induced by the staircase triangulation can be formulated as a condition of universal positive epistasis, an order relation on the fitness effects of arbitrary sets of mutations that respects the inclusion relation between the corresponding genetic backgrounds. We apply the concept to a large protein fitness landscape for an immunoglobulin-binding protein expressed in Streptococcal bacteria.

Stochastic establishment of ß-lactam-resistant Escherichia coli mutants reveals conditions for collective resistance.

For antibiotic resistance to arise, new resistant mutants must establish in a bacterial population before they can spread via natural selection. Comprehending the stochastic factors that influence mutant establishment is crucial for a quantitative understanding of antibiotic resistance emergence. Here, we quantify the single-cell establishment probability of four Escherichia coli strains expressing ß-lactamase alleles with different activity against the antibiotic cefotaxime, as a function of antibiotic concentration in both unstructured (liquid) and structured (agar) environments. We show that concentrations well below the minimum inhibitory concentration (MIC) can substantially hamper establishment, particularly for highly resistant mutants. While the pattern of establishment suppression is comparable in both tested environments, we find greater variability in establishment probability on agar. Using a simple branching model, we investigate possible sources of this stochasticity, including environment-dependent lineage variability, but cannot reject other possible causes. Lastly, we use the single-cell establishment probability to predict each strain's MIC in the absence of social interactions. We observe substantially higher measured than predicted MIC values, particularly for highly resistant strains, which indicates cooperative effects among resistant cells at large cell numbers, such as in standard MIC assays.

Safety and acute efficacy of cryoballoon ablation for atrial fibrillation at community hospitals.

AIMS: Pulmonary vein isolation (PVI) using cryoballoon ablation (CBA) is an established procedure for treating symptomatic paroxysmal and persistent atrial fibrillation (AF). The safety and efficacy of PVI performed at community hospitals are unknown. We aimed to determine the safety and acute efficacy of PVI using CBA performed at community hospitals with limited annual case numbers. METHODS AND RESULTS: This registry study included 1004 consecutive patients who had PVI performed for symptomatic paroxysmal (n = 563) or persistent AF (n = 441) from January 2019 to September 2020 at 20 hospitals. Each hospital performed fewer than 100 CBA-PVI procedures/year according to local standards. Procedural data, efficacy, and complication rates were determined. The mean number of CBA procedures performed/year at each centre was 59 ± 25. The average procedure time was 90.1 ± 31.6 min and the average fluoroscopy time was 19.2 ± 11.4 min. Isolation of all pulmonary veins was documented in 97.9% of patients. The most frequent reason for not achieving complete isolation was development of phrenic nerve palsy. No hospital deaths were observed. Two patients (0.2%) suffered a clinical stroke. Pericardial effusion occurred in six patients (0.6%), two of whom (0.2%) required pericardial drainage. Vascular complications occurred in 24 patients (2.4%), two of whom (0.2%) required vascular surgery. Phrenic nerve palsy occurred in 48 patients (4.8%) and persisted up to hospital discharge in six patients (0.6%). CONCLUSION: Pulmonary vein isolation procedures for paroxysmal or persistent AF using CBA can be performed at community hospitals with high acute efficacy and low complication rates.

Recombination and mutational robustness in neutral fitness landscapes.

Mutational robustness quantifies the effect of random mutations on fitness. When mutational robustness is high, most mutations do not change fitness or have only a minor effect on it. From the point of view of fitness landscapes, robust genotypes form neutral networks of almost equal fitness. Using deterministic population models it has been shown that selection favors genotypes inside such networks, which results in increased mutational robustness. Here we demonstrate that this effect is massively enhanced by recombination. Our results are based on a detailed analysis of mesa-shaped fitness landscapes, where we derive precise expressions for the dependence of the robustness on the landscape parameters for recombining and non-recombining populations. In addition, we carry out numerical simulations on different types of random holey landscapes as well as on an empirical fitness landscape. We show that the mutational robustness of a genotype generally correlates with its recombination weight, a new measure that quantifies the likelihood for the genotype to arise from recombination. We argue that the favorable effect of recombination on mutational robustness is a highly universal feature that may have played an important role in the emergence and maintenance of mechanisms of genetic exchange.

Empirical fitness landscapes and the predictability of evolution.

The genotype-fitness map (that is, the fitness landscape) is a key determinant of evolution, yet it has mostly been used as a superficial metaphor because we know little about its structure. This is now changing, as real fitness landscapes are being analysed by constructing genotypes with all possible combinations of small sets of mutations observed in phylogenies or in evolution experiments. In turn, these first glimpses of empirical fitness landscapes inspire theoretical analyses of the predictability of evolution. Here, we review these recent empirical and theoretical developments, identify methodological issues and organizing principles, and discuss possibilities to develop more realistic fitness landscape models.

Exactly Solvable Record Model for Rainfall.

Daily precipitation time series are composed of null entries corresponding to dry days and nonzero entries that describe the rainfall amounts on wet days. Assuming that wet days follow a Bernoulli process with success probability p, we show that the presence of dry days induces negative correlations between record-breaking precipitation events. The resulting nonmonotonic behavior of the Fano factor of the record counting process is recovered in empirical data. We derive the full probability distribution P(R,n) of the number of records R_{n} up to time n, and show that for large n, it converges to a Poisson distribution with parameter ln(pn). We also study in detail the joint limit p→0, n→∞, which yields a random record model in continuous time t=pn.

Competing Universalities in Kardar-Parisi-Zhang Growth Models.

We report on the universality of height fluctuations at the crossing point of two interacting (1+1)-dimensional Kardar-Parisi-Zhang interfaces with curved and flat initial conditions. We introduce a control parameter p as the probability for the initially flat geometry to be chosen and compute the phase diagram as a function of p. We find that the distribution of the fluctuations converges to the Gaussian orthogonal ensemble Tracy-Widom distribution for p<0.5, and to the Gaussian unitary ensemble Tracy-Widom distribution for p>0.5. For p=0.5 where the two geometries are equally weighted, the behavior is governed by an emergent Gaussian statistics in the universality class of Brownian motion. We propose a phenomenological theory to explain our findings and discuss possible applications in nonequilibrium transport and traffic flow.

Unraveling the causes of adaptive benefits of synonymous mutations in TEM-1 ß-lactamase.

While synonymous mutations were long thought to be without phenotypic consequences, there is growing evidence they can affect gene expression, protein folding, and ultimately the fitness of an organism. In only a few cases have the mechanisms by which synonymous mutations affect the phenotype been elucidated. We previously identified 48 mutations in TEM-1 ß-lactamase that increased resistance of Escherichia coli to cefotaxime, 10 of which were synonymous. To better understand the molecular mechanisms underlying the beneficial effect of these synonymous mutations, we made a series of measurements for a panel containing the 10 synonymous together with 10 non-synonymous mutations as a reference. Whereas messenger levels were unaffected, we found that total and functional TEM protein levels were higher for 5 out of 10 synonymous mutations. These observations suggest that some of these mutations act on translation or a downstream process. Similar effects were observed for some small-benefit non-synonymous mutations, suggesting a similar causal mechanism. For the synonymous mutations, we found that the cost of resistance scales with TEM protein levels. A resistance landscape for four synonymous mutations revealed strong epistasis: none of the combinations of mutations exceeded the resistance of the largest-effect mutation and there were synthetically neutral combinations. By considering combined effects of these mutations, we could infer that functional TEM protein level is a multi-dimensional phenotype. These results suggest that synonymous mutations may have beneficial effects by increasing the expression of an enzyme with low substrate activity, which may be realized via multiple, yet unknown, post-transcriptional mechanisms.

Mutation supply and the repeatability of selection for antibiotic resistance.

Whether evolution can be predicted is a key question in evolutionary biology. Here we set out to better understand the repeatability of evolution, which is a necessary condition for predictability. We explored experimentally the effect of mutation supply and the strength of selective pressure on the repeatability of selection from standing genetic variation. Different sizes of mutant libraries of antibiotic resistance gene TEM-1 ß-lactamase in Escherichia coli, generated by error-prone PCR, were subjected to different antibiotic concentrations. We determined whether populations went extinct or survived, and sequenced the TEM gene of the surviving populations. The distribution of mutations per allele in our mutant libraries followed a Poisson distribution. Extinction patterns could be explained by a simple stochastic model that assumed the sampling of beneficial mutations was key for survival. In most surviving populations, alleles containing at least one known large-effect beneficial mutation were present. These genotype data also support a model which only invokes sampling effects to describe the occurrence of alleles containing large-effect driver mutations. Hence, evolution is largely predictable given cursory knowledge of mutational fitness effects, the mutation rate and population size. There were no clear trends in the repeatability of selected mutants when we considered all mutations present. However, when only known large-effect mutations were considered, the outcome of selection is less repeatable for large libraries, in contrast to expectations. We show experimentally that alleles carrying multiple mutations selected from large libraries confer higher resistance levels relative to alleles with only a known large-effect mutation, suggesting that the scarcity of high-resistance alleles carrying multiple mutations may contribute to the decrease in repeatability at large library sizes.

Impact of surround flow catheter tip irrigation in contact force ablation on the incidence of asymptomatic oesophageal lesions after atrial fibrillation ablation: a prospective comparative study.

AIMS: Aim of this study was to evaluate the impact of a recently introduced contact force ablation catheter with modified irrigation technology compared with a conventionally irrigated ablation catheter on the incidence of endoscopically detected oesophageal lesions (EDEL). METHODS AND RESULTS: Patients with symptomatic, drug-refractory paroxysmal or persistent atrial fibrillation (AF) who underwent left atrial radiofrequency (RF) catheter ablation were prospectively enrolled. Patients were ablated using a single-tip RF contact force ablation catheter with conventional irrigation (Group 1; n = 50) or with a recently introduced intensified 'surround flow' irrigation technology (Group 2; n = 50). Assessment of EDEL was performed by oesophagogastroduodenoscopy in all patients after ablation. A total of 100 patients (mean age 63.6 ± 12.1 years; men 58%) with paroxysmal (n = 41; 41%) or persistent AF were included. Groups 1 and 2 patients were comparable in regard to baseline characteristics and procedural parameters, especially ablation time at posterior left atrial wall. Overall, 13 patients (13%) developed EDEL after AF ablation (8 oesophageal ulcerations, 5 erythema). The incidence of EDEL including oesophageal ulcerations was higher in Group 2 compared with Group 1 patients without statistical significance (18 vs. 8%, P = 0.23). One pericardial tamponade and one access site bleeding occurred in Group 2. No further adverse events were reported in both groups. CONCLUSION: According to these preliminary results, the use of an improved ablation catheter irrigation technology (surround flow) in conjunction with contact force measurement was associated with a higher but not statistically significant probability of oesophageal thermal lesions. Further studies including larger patient cohorts are needed.

Incidence of asymptomatic oesophageal lesions after atrial fibrillation ablation using an oesophageal temperature probe with insulated thermocouples: a comparative controlled study.

AIMS: Oesophageal probes to monitor luminal oesophageal temperature (LET) during atrial fibrillation (AF) catheter ablation have been proposed, but their effects remain unclear. Aim of this study is to evaluate the effects of an oesophageal temperature probe with insulated thermocouples. METHODS AND RESULTS: Patients with symptomatic, drug-refractory paroxysmal or persistent AF who underwent left atrial radiofrequency (RF) catheter ablation were prospectively enrolled. Patients were ablated using a single-tip RF contact force ablation catheter. An intraluminal oesophageal temperature probe was used in Group 1. In Group 2, patients were ablated without LET monitoring. Assessment of asymptomatic endoscopically detected oesophageal lesions (EDEL) was performed by oesophagogastroduodenoscopy (EGD) in all patients. Eighty patients (mean age 63.7 ± 10.7 years; men 56%) with symptomatic, drug-refractory paroxysmal (n = 28; 35%) or persistent AF were included. Group 1 and Group 2 patients (n = 40 in each group) were comparable in regard to baseline characteristics, but RF duration on the posterior wall was significantly shorter in Group 1 patients. Overall, seven patients (8.8%) developed EDEL (four ulcerations, three erythema). The incidence of EDEL in Group 1 and Group 2 patients was comparable (7.5 vs. 10%, P = 1.0). No major adverse events were reported in both groups. CONCLUSION: According to these preliminary results, the use of oesophageal temperature probes with insulated thermocouples seems to be feasible in patients undergoing AF RF catheter ablation. The incidence of post-procedural EDEL when using a cut-off of 39°C is comparable to the incidence of EDEL without using a temperature probe.

Diminishing-returns epistasis among random beneficial mutations in a multicellular fungus.

Adaptive evolution ultimately is fuelled by mutations generating novel genetic variation. Non-additivity of fitness effects of mutations (called epistasis) may affect the dynamics and repeatability of adaptation. However, understanding the importance and implications of epistasis is hampered by the observation of substantial variation in patterns of epistasis across empirical studies. Interestingly, some recent studies report increasingly smaller benefits of beneficial mutations once genotypes become better adapted (called diminishing-returns epistasis) in unicellular microbes and single genes. Here, we use Fisher's geometric model (FGM) to generate analytical predictions about the relationship between the effect size of mutations and the extent of epistasis. We then test these predictions using the multicellular fungus Aspergillus nidulans by generating a collection of 108 strains in either a poor or a rich nutrient environment that each carry a beneficial mutation and constructing pairwise combinations using sexual crosses. Our results support the predictions from FGM and indicate negative epistasis among beneficial mutations in both environments, which scale with mutational effect size. Hence, our findings show the importance of diminishing-returns epistasis among beneficial mutations also for a multicellular organism, and suggest that this pattern reflects a generic constraint operating at diverse levels of biological organization.

Spatial Relationship of Focal Impulses, Rotors and Low Voltage Zones in Patients With Persistent Atrial Fibrillation.

INTRODUCTION: Focal impulses (FI) and rotors are sources associated with the initiation and maintenance of atrial fibrillation (AF). Their ablation results in a lower recurrence rate. The aim of this study was to characterize for the first time the spatial relationship between such sources and atrial low voltage zones (LVZ) representing fibrosis. METHODS: Twenty-five consecutive patients undergoing their first ablation for persistent AF were included. Voltage mapping of both atria was done during AF. Endocardial mapping of FI and rotors (sources) was performed using a basket catheter and displayed using RhythmView(TM) (Topera Inc.) before ablation. Spatial relationship of LVZ and sources was analyzed. RESULTS: LVZs covered 13 ± 12% of right atrial (RA) endocardial surface and 33 ± 25% of left atrial (LA) endocardial surface. The median number of sources was 1 [1-3] in RA and 3 [1-4] in LA. Of LA sources, 18 (30%) were definitely not associated with LVZs or pulmonary vein (PV) antra. Of RA sources, 32 (84%) were remote from LVZ. During ablation of such sources substantial cycle length (CL) prolongation or AF conversion occurred in 11/23 patients (48%). Altogether, 8/11 (73%) of these pertinent sources were located remotely from LVZ and PV antra. CONCLUSIONS: There is a wide discrepancy in distribution of LVZ areas and sites of identified rotors. Site and incidence of FIRM sources appear to be unpredictable with atrial substrate mapping. Further prospective, randomized studies are necessary to elucidate the impact of additional ablation of such sources in patients with persistent or longstanding persistent AF.

Greedy adaptive walks on a correlated fitness landscape.

We study adaptation of a haploid asexual population on a fitness landscape defined over binary genotype sequences of length L. We consider greedy adaptive walks in which the population moves to the fittest among all single mutant neighbors of the current genotype until a local fitness maximum is reached. The landscape is of the rough mount Fuji type, which means that the fitness value assigned to a sequence is the sum of a random and a deterministic component. The random components are independent and identically distributed random variables, and the deterministic component varies linearly with the distance to a reference sequence. The deterministic fitness gradient c is a parameter that interpolates between the limits of an uncorrelated random landscape (c=0) and an effectively additive landscape (c→∞). When the random fitness component is chosen from the Gumbel distribution, explicit expressions for the distribution of the number of steps taken by the greedy walk are obtained, and it is shown that the walk length varies non-monotonically with the strength of the fitness gradient when the starting point is sufficiently close to the reference sequence. Asymptotic results for general distributions of the random fitness component are obtained using extreme value theory, and it is found that the walk length attains a non-trivial limit for L→∞, different from its values for c=0 and c=∞, if c is scaled with L in an appropriate combination.

Predictability of evolution depends nonmonotonically on population size.

To gauge the relative importance of contingency and determinism in evolution is a fundamental problem that continues to motivate much theoretical and empirical research. In recent evolution experiments with microbes, this question has been explored by monitoring the repeatability of adaptive changes in replicate populations. Here, we present the results of an extensive computational study of evolutionary predictability based on an experimentally measured eight-locus fitness landscape for the filamentous fungus Aspergillus niger. To quantify predictability, we define entropy measures on observed mutational trajectories and endpoints. In contrast to the common expectation of increasingly deterministic evolution in large populations, we find that these entropies display an initial decrease and a subsequent increase with population size N, governed, respectively, by the scales Nµ and Nµ(2), corresponding to the supply rates of single and double mutations, where µ denotes the mutation rate. The amplitude of this pattern is determined by µ. We show that these observations are generic by comparing our findings for the experimental fitness landscape to simulations on simple model landscapes.

Effect of Different Ablation Settings on Acute Complications Using the Novel Irrigated Multipolar Radiofrequency Ablation Catheter (nMARQ).

BACKGROUND: Single-shot ablation devices for pulmonary vein isolation (PVI) in patients with symptomatic atrial fibrillation (AF) have been increasingly used in clinical practice. OBJECTIVE: A novel mapping-system integrated irrigated multipolar circular ablation catheter (nMARQ) has been introduced for PVI but data on larger patient cohorts on acute safety and efficacy are lacking. METHODS: A total of 145 consecutive patients undergoing AF ablation treated with the nMARQ underwent endoscopic evaluation of esophageal thermal damage (EDEL) and brain MRI for detection of silent cerebral events (SCE). During the course of our experience different modifications of the ablation strategy, including energy delivery at the left atrial posterior wall, were evaluated. RESULTS: Effective PVI was achieved in 99% of all PVs during a mean procedure-duration of 115 (±36) minutes and ablation-duration of 18 (±8) minutes. Acute major complications occurred in 3 patients (2.1%) and asymptomatic complications like SCE in 26% and EDEL in 21%. There was a significant reduction in EDEL when not using a thermal esophageal probe (0% vs. 28%, P < 0.0001). Ablation under oral anticoagulation led to lower SCE incidences compared to interrupted anticoagulation regimen (15% vs. 31%, P = 0.7). Out of 65 patients with completed 12-month follow-up, 43 (66%) were in stable sinus rhythm. CONCLUSIONS: PVI using the nMARQ is safe and effective in patients with symptomatic AF. Not using an esophageal temperature probe during ablation has relevantly reduced the incidence of EDEL. Ablations under continued oral anticoagulation have reduced incidence of SCE. Further studies on long-term efficacy are needed.

Silent cerebral events/lesions related to atrial fibrillation ablation: a clinical review.

Brain magnetic resonance imaging (MRI) has identified a high incidence of cerebral ischemia in asymptomatic patients after atrial fibrillation (AF) ablation (silent). Detection of cerebral ischemic events on MRI is based on acute hyperintense lesions on diffusion-weighted imaging. In the literature, the incidence is related to specifications of MRI and depends on the definition applied. In comparative studies, silent cerebral events (SCE, diffusion-weighted MRI [DWI] positive only) appear to be approximately 3 times more common compared to using a definition of silent cerebral lesions (SCL; without fluid attenuated inverse recovery sequence [FLAIR] positivity). Whereas the FLAIR sequence may turn positive within days after the ischemic event, SCE definition is highly sensitive for early phases of ischemic brain damage. SCE/SCL appear to represent cerebral ischemic infarcts and determine the "embolic fingerprint" of a specific ablation technology and strategy used. The optimum time point for detecting SCE is early after AF ablation (24-72 hours), whereas detection of SCL can only be performed within the first 2-7 days (due to delay of FLAIR positivity). Different technology-, procedure-, and patient-related parameters have been identified to play a role in the multifactorial genesis of SCE/SCL. In recent years, evidence has been gathered that there may be differences of SCE/SCL rates depending upon the ablation technology used, but small patient numbers and a large number of potential confounders hamper all studies. As major findings of recent studies, mode of periprocedural and intraprocedural anticoagulation has been identified as a major predictor for incidences of SCE/SCL. Whereas procedural characteristics related to higher SCE/SCL-rates may be modified, unchangeable patient-related factors should be taken into account for future individualized risk assessment. Novel ablation devices introduced into the market should be tested for their potential embolic fingerprint and refinements of ablation procedures to reduce their embolic potential should be prompted. The knowledge of "best practice" in terms of low SCE/SCL rates has prompted changes in work-flow, which have been implemented into ablation procedures using novel ablation devices. So far, no study has linked SCE/SCL to neuropsychological decline and the low number of AF-ablation-associated events needs to be weighted against the multitude of preexisting asymptomatic MRI-detected brain lesions related to the course of AF itself. Future studies are needed to evaluate if more white matter hyperintensities due to AF may be prevented by AF ablation (producing only a small number of SCE/SCL).