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A H Andersen

Publications and source records attributed to A H Andersen.

At least 19 recordsLinked to original sources
PubMed2006

The effect of ignoring individual heterogeneity in Weibull log-normal sire frailty models.

The objective of this study was, by means of simulation, to quantify the effect of ignoring individual heterogeneity in Weibull sire frailty models on parameter estimates and to address the consequences for genetic inferences. Three simulation studies were evaluated, which included 3 levels of individual heterogeneity combined with 4 levels of censoring (0, 25, 50, or 75%). Data were simulated according to balanced half-sib designs using Weibull log-normal animal frailty models with a normally distributed residual effect on the log-frailty scale. The 12 data sets were analyzed with 2 models: the sire model, equivalent to the animal model used to generate the data (complete sire model), and a corresponding model in which individual heterogeneity in log-frailty was neglected (incomplete sire model). Parameter estimates were obtained from a Bayesian analysis using Gibbs sampling, and also from the software Survival Kit for the incomplete sire model. For the incomplete sire model, the Monte Carlo and Survival Kit parameter estimates were similar. This study established that when unobserved individual heterogeneity was ignored, the parameter estimates that included sire effects were biased toward zero by an amount that depended in magnitude on the level of censoring and the size of the ignored individual heterogeneity. Despite the biased parameter estimates, the ranking of sires, measured by the rank correlations between true and estimated sire effects, was unaffected. In comparison, parameter estimates obtained using complete sire models were consistent with the true values used to simulate the data. Thus, in this study, several issues of concern were demonstrated for the incomplete sire model.

Animals↗
PubMed2005

Longitudinal functional alterations in asymptomatic women at risk for Alzheimer's disease.

PURPOSE: The authors sought to determine whether known alterations of brain function in normal individuals who are at high risk for Alzheimer's disease (AD) worsen or stay the same after a significant interval of time. METHODS: The authors used functional magnetic resonance imaging (fMRI) to observe cortical activation during confrontation naming in 14 women with high AD risk and 10 with low risk, based on family history and apolipoprotein-E4 allele status. They repeated the identical scan protocol in the same patients after 4 years. RESULTS: fMRI activation in high-AD-risk participants was found to be further diverged from that of their low-AD-risk counterparts over this period. CONCLUSION: fMRI may report on the presence and progression of neuropathology in the ventral temporal cortex or in functionally connected regions in presymptomatic AD.

Alzheimer Disease↗
PubMed2002

Women at risk for AD show increased parietal activation during a fluency task.

BACKGROUND: Imaging studies have shown disparities in resting metabolism and in functional activation between cognitively normal individuals at high and low risk for AD. A recent study has shown increased parietal activation in high-risk subjects during a paired associates recall task, which the authors postulated might overlap activation typically observed in verbal fluency. OBJECTIVE: To determine whether parietal activation is altered in a letter fluency task in cognitively normal individuals at high risk for AD. METHODS: fMRI was used to compare cortical activation between two groups of cognitively normal women differing in their risk for developing AD. A letter fluency task was used, which activates left frontal and parietal regions. The risk groups differed in family history of AD and APOE allele status but were matched in age, education, and measures of cognitive performance. Average age of the study participants was 53 years. RESULTS: The regional patterns of brain activation were similar between groups and similar to patterns observed by other investigators. However, the high-risk group showed significantly increased activation in the left parietal region despite identical letter fluency performance between risk groups. CONCLUSIONS: Cognitively normal individuals at high risk for AD show increased brain activation in the left parietal region with letter fluency, a region adjacent to that observed by others using a recall task. This convergence of results indicates disruption of functional circuits involving the left parietal lobe in asymptomatic individuals at increased risk for AD.

Alzheimer Disease↗
PubMed2001

Residues within the N-terminal domain of human topoisomerase I play a direct role in relaxation.

All eukaryotic forms of DNA topoisomerase I contain an extensive and highly charged N-terminal domain. This domain contains several nuclear localization sequences and is essential for in vivo function of the enzyme. However, so far no direct function of the N-terminal domain in the in vitro topoisomerase I reaction has been reported. In this study we have compared the in vitro activities of a truncated form of human topoisomerase I lacking amino acids 1-206 (p67) with the full-length enzyme (p91). Using these enzyme forms, we have identified for the first time a direct role of residues within the N-terminal domain in modulating topoisomerase I catalysis, as revealed by significant differences between p67 and p91 in DNA binding, cleavage, strand rotation, and ligation. A comparison with previously published studies showing no effect of deleting the first 174 or 190 amino acids of topoisomerase I (Stewart, L., Ireton, G. C., and Champoux, J. J. (1999) J. Biol. Chem. 274, 32950-32960; Bronstein, I. B., Wynne-Jones, A., Sukhanova, A., Fleury, F., Ianoul, A., Holden, J. A., Alix, A. J., Dodson, G. G., Jardillier, J. C., Nabiev, I., and Wilkinson, A. J. (1999) Anticancer Res. 19, 317-327) suggests a pivotal role of amino acids 191-206 in catalysis. Taken together the presented data indicate that at least part(s) of the N-terminal domain regulate(s) enzyme/DNA dynamics during relaxation most probably by controlling non-covalent DNA binding downstream of the cleavage site either directly or by coordinating DNA contacts by other parts of the enzyme.

Catalysis↗
PubMed2001

Neural substrates of facial emotion processing using fMRI.

We identified human brain regions involved in the perception of sad, frightened, happy, angry, and neutral facial expressions using functional magnetic resonance imaging (fMRI). Twenty-one healthy right-handed adult volunteers (11 men, 10 women; aged 18-45; mean age 21.6 years) participated in four separate runs, one for each of the four emotions. Participants viewed blocks of emotionally expressive faces alternating with blocks of neutral faces and scrambled images. In comparison with scrambled images, neutral faces activated the fusiform gyri, the right lateral occipital gyrus, the right superior temporal sulcus, the inferior frontal gyri, and the amygdala/entorhinal cortex. In comparisons of emotional and neutral faces, we found that (1) emotional faces elicit increased activation in a subset of cortical regions involved in neutral face processing and in areas not activated by neutral faces; (2) differences in activation as a function of emotion category were most evident in the frontal lobes; (3) men showed a differential neural response depending upon the emotion expressed but women did not.

Adult↗
PubMed2001

Differences in functional magnetic resonance imaging activation by category in a visual confrontation naming task.

OBJECTIVE: Cortical processing involved in seemingly similar tasks may differ in important ways. The authors mapped cortical regions engaged in a commonly performed picture naming task, seeking differences by semantic category. Functional magnetic resonance imaging was used during presentation of standardized line drawings in 18 healthy right-handed female participants, comparing living versus nonliving entities. During visual naming, across categories there was strong activation of left frontal (BA45/47), bilateral temporo-occipital junction (BA19), and inferior temporal regions (BA36/37). Activation of right inferior temporal cortex (BA19 and BA37) was greater during naming of living versus nonliving category items. No category differences in activation strength in the left temporal lobe were observed. The authors conclude that visual semantic operations may involve visual association cortex in the right temporal lobe in women.

Brain Mapping↗
PubMed2000

Position-specific effect of ribonucleotides on the cleavage activity of human topoisomerase II.

Beyond the normal DNA transactions mediated by topoisomerase II, we have recently demonstrated that the cleavage activity of the two human topoisomerase II isoforms is several-fold stimulated if a ribonucleotide rather than a deoxyribonucleotide is present at the scissile phosphodiester in one strand of the substrate. Here we show that ribonucleotides exert a position-specific effect on topoisomerase II-mediated cleavage without altering the sequence specificity of the enzyme. Ribonucleotides located within the 4 bp cleavage stagger stimulate topoisomerase II-mediated cleavage, whereas ribonucleotides located outside the stagger in general have an inhibitory effect. Results obtained from competition experiments indicate that the position-specific effect of ribonucleotides on topoisomerase II activity is caused by altered substrate interaction. When cleavage is performed with substrates containing one ribonucleotide in both strands or several ribonucleotides in one strand the effect of the individual ribonucleotides on cleavage is not additive. Finally, although topoisomerase II recognizes substrates with longer stretches of ribonucleotides, an RNA/DNA hybrid where one strand is composed entirely of RNA is not cleaved by the enzyme. The positional effect of ribonucleotides on topoisomerase II-mediated cleavage shares many similarities to the positional effect exerted by either abasic sites or base mismatches, demonstrating a general influence of DNA imperfections on topoisomerase II activity.

Base Pair Mismatch↗
PubMed2000

Communication between the ATPase and cleavage/religation domains of human topoisomerase IIalpha.

The DNA strand passage activity of eukaryotic topoisomerase II relies on a cascade of conformational changes triggered by ATP binding to the N-terminal domain of the enzyme. To investigate the interdomain communication between the ATPase and cleavage/religation domains of human topoisomerase IIalpha, we characterized a mutant enzyme that contains a deletion at the interface between the two domains, covering amino acids 350-407. The ATPase domain retained full activity with a rate of ATP hydrolysis that was severalfold higher than normal, but the ATPase activity was unaffected by DNA. The cleavage and religation activities of the enzyme were comparable with those of the wild-type enzyme both in the absence and presence of cancer chemotherapeutic agents. However, neither ATP nor a nonhydrolyzable ATP analog stimulated cleavage complex formation. Although both conserved domains retained full activity, the mutant enzyme was unable to coordinate these activities into strand passage. Our findings suggest that the normal conformational transitions occurring in the enzyme upon ATP binding are hampered or lacking in the mutant enzyme. Consistent with this hypothesis, the enzyme displayed an abnormal clamp closing activity. In summary, the region covering amino acids 350-407 in human topoisomerase IIalpha seems to be essential for correct interdomain communication and probably is involved in signaling ATP binding to the rest of the enzyme.

Adenosine Triphosphatases↗
PubMed2000

Cerebrovascular changes in the basal ganglia with HIV dementia.

BACKGROUND: HIV dementia is a form of subcortical dementia. Clinical, radiologic, pathologic, and biochemical studies suggest a major contribution of basal ganglia dysfunction to the pathogenesis of this disorder. Many investigators have proposed a contribution of a disrupted blood-brain barrier (BBB) to the pathogenesis of HIV dementia. OBJECTIVE: To identify microvascular abnormalities in vivo in basal ganglia or white matter of persons with HIV dementia. METHODS: Time course of MRI postcontrast enhancement was determined in basal ganglia and white matter of HIV-infected persons without dementia (Memorial Sloan Kettering [MSK] score of 0; n = 4); HIV-infected persons with mild dementia (MSK score of 0.5; n = 2); and HIV-infected persons with moderate-to-severe dementia (MSK > or = 1.0; n = 6). RESULTS: Increased basal ganglia enhancement was observed in individuals with moderate-to-severe dementia relative to nondemented individuals, both immediately and 30 minutes after contrast administration. Decline of basal ganglia enhancement was slower in the moderately to severely demented patients and, when normalized to intravascular enhancement of sagittal sinus, suggested leakage of contrast agent, consistent with increased permeability of BBB. A significant correlation between the postcontrast fractional enhancement at 30 minutes (FE30) and the MSK score was noted. White matter showed no significant differences in postcontrast enhancement among the three groups. CONCLUSION: Increased early enhancement in basal ganglia of the HIV dementia group is consistent with increased regional cerebral blood volume (rCBV). Increased late enhancement is strongly suggestive of BBB disruption. Similar abnormalities were absent in the white matter adjacent to the caudate nucleus.

AIDS Dementia Complex↗
PubMed2000

Functional MRI of apomorphine activation of the basal ganglia in awake rhesus monkeys.

Functional magnetic resonance imaging (fMRI) was used to analyze blood oxygen level-dependent (BOLD) responses in the nigrostriatal system (caudate nucleus, putamen and substantia nigra) of awake rhesus monkeys to systemic apomorphine administration. The study (1) measured BOLD responses as an index of neuronal activity in the three structures following injections of the mixed D1/D2 agonist, and (2) assessed the effects of isoflurane anesthesia on the fMRI responses. Compared to control saline injections, 0.1 mg/kg apomorphine significantly activated the caudate nucleus (P < or = 0.005), putamen (P < or = 0.001) and substantia nigra (P < or = 0.005). The responses were consistent with activation of GABAergic neurons in these three structures seen in other animal models. Isoflurane gas measurably blunted the response to apomorphine, so that a significant apomorphine activation was only seen in the substantia nigra of anesthetized animals. Even there, the mean MR signal change was reduced from 9.8% in awake monkeys to 2.3% in anesthetized animals. The data support the hypothesis that fMRI can be used to study the effects of drugs that alter basal ganglia activity in awake rhesus monkeys.

Animals↗
PubMed1999

Altered brain activation in cognitively intact individuals at high risk for Alzheimer's disease.

OBJECTIVE: To determine whether brain function is altered in cognitively normal individuals at high risk for AD several years before the typical age at onset for this illness. BACKGROUND: Neuropathologic alterations in AD precede cognitive impairment by several years. It is unknown whether functional alterations in neural circuitry accompany these neuropathologic changes, and if so, whether they may be detectable before onset of symptoms. METHODS: We used functional MRI to compare cortical activation between two groups of cognitively normal women differing only in their risk for developing AD. Visual naming and letter fluency tasks were used to activate brain areas subserving object and face recognition, previously described sites of hypometabolism and neuropathologic alteration in AD. The risk groups differed in family history of AD and apolipoprotein E allele status, but were matched in age, education, and measures of cognitive performance. Average age of the study participants was 52 years. RESULTS: The regional patterns of brain activation were similar between groups. However, the high risk group showed areas of significantly reduced activation in the mid- and posterior inferotemporal regions bilaterally during both tasks despite identical naming and letter fluency performance. CONCLUSIONS: Cognitively normal individuals at high risk for AD demonstrate decreased brain activation in key areas engaged during naming and fluency tasks. Decreased activation in the high risk group may be a consequence of the presence of subclinical neuropathology in the inferotemporal region or in the inputs to that region. If so, these findings provide evidence of a window of opportunity for disease-modifying treatment before the onset of symptomatic AD.

Adult↗
PubMed1999

Using a biochemical approach to identify the primary dimerization regions in human DNA topoisomerase IIalpha.

Eukaryotic topoisomerase II is a nuclear enzyme essential for DNA metabolism and chromosome dynamics. The enzyme has a dimeric structure, and subunit dimerization is vital to the cellular functions and activities of the enzyme. Two biochemical approaches based on metal ion affinity chromatography and immunoprecipitation have been carried out to map the dimerization region(s) in human topoisomerase IIalpha. The results demonstrate that two regions spanning amino acids 1053-1069 and 1124-1143 are both essential for dimerization. The regions correspond to the interaction domains revealed in yeast topoisomerase II after crystallization of a central fragment of this enzyme, indicating that the overall C-terminal dimerization structure of eukaryotic topoisomerase II is conserved from yeast to human. Furthermore, linker insertion analysis has demonstrated that the two dimerization regions are located in a highly flexible part of the enzyme. Topoisomerase IIalpha mutant enzymes unable to dimerize via the C-terminal primary dimerization regions due to lack of one of the defined dimerization regions can still be forced to dimerize if DNA and an ATP analog are added to the reaction mixture. The result indicates that secondary interactions occur by ATP analog-mediated clamp closing when the subunits are brought together on DNA.

Amino Acid Sequence↗
PubMed1999

Stimulated activity of human topoisomerases IIalpha and IIbeta on RNA-containing substrates.

Eukaryotic topoisomerase II is a dimeric nuclear enzyme essential for DNA metabolism and chromosome dynamics. Central to the activities of the enzyme is its ability to introduce transient double-stranded breaks in the DNA helix, where the two subunits of the enzyme become covalently attached to the generated 5'-ends through phosphotyrosine linkages. Here, we demonstrate that human topoisomerases IIalpha and IIbeta are able to cleave ribonucleotide-containing substrates. With suicide substrates, which are partially double-stranded molecules containing a 5'-recessed strand, cleavage of both strands was stimulated approximately 8-fold when a ribonucleotide rather than a deoxyribonucleotide was present at the scissile phosphodiester of the recessed strand. The existence of a ribonucleotide at the same position in a normal duplex substrate also enhanced topoisomerase II-mediated cleavage, although to a lesser extent. The enzyme covalently linked to the 5'-ribonucleotide in the cleavage complex efficiently performed ligation, and ligation occurred equally well to acceptor molecules terminated by either a 3'-ribo- or deoxyribonucleotide. Besides the enhanced topoisomerase II-mediated cleavage of ribonucleotide-containing substrates, cleavage of such substrates could be further stimulated by ATP or antitumor drugs. In conclusion, the observed in vitro activities of the human topoisomerase II isoforms indicate that the enzymes can operate on RNA or RNA-containing substrates and thus might possess an intrinsic RNA topoisomerase activity, as has previously been demonstrated for Escherichia coli topoisomerase III.

Adenosine Triphosphate↗
PubMed1999

Age-associated changes in rhesus CNS composition identified by MRI.

Multispectral automated segmentation of MR images of the brains of 10 young (5-8 years), 10 middle-aged (12-17 years), and 11 old (21-27 years) female rhesus monkeys revealed age-associated changes in brain volume and composition. Total brain parenchymal volume (expressed as fraction of intracranial volume-%ICV) decreased at a linear rate of 0.3+/-0.04% ICV/year. Up to age approximately 15 years, this loss was almost entirely due to gray matter loss, with a compensatory increase in cerebrospinal fluid (CSF), and possibly some white matter. Brain tissue composition, expressed as the gray matter/white matter volume ratio confirmed that gray matter loss exceeded white matter loss, but the rate of decline in the gray/white ratio began to slow after approximately 15 years. Comparison of these age-associated changes in rhesus brain with those in humans suggest that the brain aging in rhesus is a good model of human brain aging, but occurs approximately 3-fold faster.

Aging↗
PubMed1999

Functional magnetic resonance imaging evidence for task-specific activation of developmentally abnormal visual association cortex.

Functional magnetic resonance imaging was performed on a 36-year-old woman with muscular dystrophy, intractable epilepsy, and bilateral temporo-occipital lissencephaly. We observed islands of task-specific activation in lissencephalic cortex homologous to visual association regions activated in normal subjects on the same visual confrontation naming task. This result suggests lissencephalic cortex may develop specific functional connections with other brain regions.

Adult↗
PubMed1999

Functional MRI of basal ganglia responsiveness to levodopa in parkinsonian rhesus monkeys.

Functional MRI (fMRI) was used to study striatal sensitivity to levodopa in hemiparkinsonian rhesus monkeys. Responses consistent with increased neuronal activity were seen in areas whose normal dopaminergic input from the substantia nigra pars compacta had been ablated by MPTP. Sites of increased activity following levodopa included the lateral putamen, the ventral region of the caudate head, septal areas, and midlateral amygdala in the MPTP-lesioned hemisphere. Increased activity was also observed in the same areas in the nonlesioned hemisphere, but was less pronounced in spatial extent and magnitude, suggesting either subclinical contralateral damage and/or functional adaptations in the contralateral dopamine systems. The increases in neuronal activity following levodopa treatment were temporally correlated with increases in striatal dopamine levels. Chronic levodopa treatment reduced behavioral responsiveness to levodopa and abolished the fMRI response. These results suggest that fMRI can detect changes in dopamine receptor-mediated neuronal sensitivity to dopaminergic agents.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1999

Principal component analysis of the dynamic response measured by fMRI: a generalized linear systems framework.

Principal component analysis (PCA) is one of several structure-seeking multivariate statistical techniques, exploratory as well as inferential, that have been proposed recently for the characterization and detection of activation in both PET and fMRI time series data. In particular, PCA is data driven and does not assume that the neural or hemodynamic response reaches some steady state, nor does it involve correlation with any pre-defined or exogenous experimental design template. In this paper, we present a generalized linear systems framework for PCA based on the singular value decomposition (SVD) model for representation of spatio-temporal fMRI data sets. Statistical inference procedures for PCA, including point and interval estimation will be introduced without the constraint of explicit hypotheses about specific task-dependent effects. The principal eigenvectors capture both the spatial and temporal aspects of fMRI data in a progressive fashion; they are inherently matched to unique and uncorrelated features and are ranked in order of the amount of variance explained. PCA also acts as a variation reduction technique, relegating most of the random noise to the trailing components while collecting systematic structure into the leading ones. Features summarizing variability may not directly be those that are the most useful. Further analysis is facilitated through linear subspace methods involving PC rotation and strategies of projection pursuit utilizing a reduced, lower-dimensional natural basis representation that retains most of the information. These properties will be illustrated in the setting of dynamic time-series response data from fMRI experiments involving pharmacological stimulation of the dopaminergic nigro-striatal system in primates.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗