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T Sigmundsson

Publications and source records attributed to T Sigmundsson.

17 recordsLinked to original sources
PubMed2001

Genomewide genetic linkage analysis confirms the presence of susceptibility loci for schizophrenia, on chromosomes 1q32.2, 5q33.2, and 8p21-22 and provides support for linkage to schizophrenia, on chromosomes 11q23.3-24 and 20q12.1-11.23.

We have performed genetic linkage analysis in 13 large multiply affected families, to test the hypothesis that there is extensive heterogeneity of linkage for genetic subtypes of schizophrenia. Our strategy consisted of selecting 13 kindreds containing multiple affected cases in three or more generations, an absence of bipolar affective disorder, and a single progenitor source of schizophrenia with unilineal transmission into the branch of the kindred sampled. DNA samples from these families were genotyped with 365 microsatellite markers spaced at approximately 10-cM intervals across the whole genome. We observed LOD scores >3.0 at five distinct loci, either in the sample as a whole or within single families, strongly suggesting etiological heterogeneity. Heterogeneity LOD scores >3.0 in the sample as a whole were found at 1q33.2 (LOD score 3.2; P=.0003), 5q33.2 (LOD score 3.6; P=.0001), 8p22.1-22 (LOD score 3.6; P=.0001), and 11q21 (LOD score 3.1; P=.0004). LOD scores >3.0 within single pedigrees were found at 4q13-31 (LOD score 3.2; P=.0003) and at 11q23.3-24 (LOD score 3.2; P=.0003). A LOD score of 2.9 was also found at 20q12.1-11.23 within in a single family. The fact that other studies have also detected LOD scores >3.0 at 1q33.2, 5q33.2, 8p21-22 and 11q21 suggests that these regions do indeed harbor schizophrenia-susceptibility loci. We believe that the weight of evidence for linkage to the chromosome 1q22, 5q33.2, and 8p21-22 loci is now sufficient to justify intensive investigation of these regions by methods based on linkage disequilibrium. Such studies will soon allow the identification of mutations having a direct effect on susceptibility to schizophrenia.

Chromosome Aberrations↗
PubMed2001

Structural abnormalities in frontal, temporal, and limbic regions and interconnecting white matter tracts in schizophrenic patients with prominent negative symptoms.

OBJECTIVE: Imaging studies of schizophrenia have repeatedly demonstrated global abnormalities of cerebral and ventricular volumes. However, pathological changes at more local levels of brain organization have not yet been so clearly characterized because of the few brain regions of interest heretofore included in morphometric analyses as well as heterogeneity of patient samples. METHOD: Dual echo magnetic resonance imaging (MRI) data were acquired at 1.5 T from 27 right-handed patients who met DSM-IV criteria for schizophrenia with enduring negative symptoms and from 27 healthy comparison subjects. Between-group differences in gray and white matter volume were estimated at each intracerebral voxel after registration of the images in standard space. The relationship between clinical symptom scores and brain structure was also examined within the patient group. Spatial statistics and permutation tests were used for inference. RESULTS: Significant deficits of gray matter volume in the patient group were found at three main locations: 1) the left superior temporal gyrus and insular cortex, 2) the left medial temporal lobe (including the parahippocampal gyrus and hippocampus), and 3) the anterior cingulate and medial frontal gyri. The volume of these three regions combined was 14% lower in the patients relative to the comparison subjects. White matter deficits were found in similar locations in the left temporal lobe and extended into the left frontal lobe. The patient group showed a relative excess of gray matter volume in the basal ganglia. Within the patient group, basal ganglia gray matter volume was positively correlated with positive symptom scores. CONCLUSIONS: Anatomical abnormalities in these schizophrenic patients with marked negative symptoms were most evident in left hemispheric neocortical and limbic regions and related white matter tracts. These data are compatible with models that depict schizophrenia as a supraregional disorder of multiple, distributed brain regions and the axonal connections between them.

Adult↗
PubMed1999

Lack of normal pattern of cerebral asymmetry in familial schizophrenic patients and their relatives--The Maudsley Family Study.

Lack of the normal cerebral asymmetry has been reported in schizophrenia. We wished to test the hypothesis that this lack of the normal pattern of asymmetry is familial and that it can be found in both schizophrenic and non-schizophrenic family members. In particular, we wanted to know whether those relatives who appear to be transmitting liability to the illness also demonstrate the loss of normal asymmetry. We studied families with several members affected with schizophrenia. We carried out volumetric measurements of prefrontal, premotor, sensorimotor and occipitoparietal regions in each hemisphere using 3D reconstructed MRI images in 29 schizophrenic patients, 55 of their first degree relatives, and 39 unrelated control subjects on contiguous thin slices of the brain. Nine of the unaffected relatives appeared to be transmitting the liability for schizophrenia (e.g. the mother of a schizophrenic patient who, although not psychotic herself, had a schizophrenic parent or sibling). We termed them presumed obligate carriers and the remaining 46 relatives presumed non-obligate carriers. The healthy control subjects showed larger right than left prefrontal regions and larger left than right sensorimotor and occipitoparietal regions. The schizophrenic patients showed lack of this normal brain asymmetry in the prefrontal, sensorimotor and occipitoparietal cortical regions. The presumed obligate carriers were similar to the schizophrenic patients in exhibiting lack of asymmetries in these cortical regions, while the presumed non-obligate relatives showed lack of asymmetry only in the occipitoparietal region. There was no overall reduction in total or regional brain volumes among the groups. Our findings indicate that lack of the normal pattern of frontal and occipital asymmetry is a marker for genetic liability to schizophrenia in families multiply affected with schizophrenia.

Adult↗
PubMed1999

A modified fuzzy clustering algorithm for operator independent brain tissue classification of dual echo MR images.

Methods for brain tissue classification or segmentation of structural magnetic resonance imaging (MRI) data should ideally be independent of human operators for reasons of reliability and tractability. An algorithm is described for fully automated segmentation of dual echo, fast spin-echo MRI data. The method is used to assign fuzzy-membership values for each of four tissue classes (gray matter, white matter, cerebrospinal fluid and dura) to each voxel based on partition of a two dimensional feature space. Fuzzy clustering is modified for this application in two ways. First, a two component normal mixture model is initially fitted to the thresholded feature space to identify exemplary gray and white matter voxels. These exemplary data protect subsequently estimated cluster means against the tendency of unmodified fuzzy clustering to equalize the number of voxels in each class. Second, fuzzy clustering is implemented in a moving window scheme that accommodates reduced image contrast at the axial extremes of the transmitting/receiving coil. MRI data acquired from 5 normal volunteers were used to identify stable values for three arbitrary parameters of the algorithm: feature space threshold, relative weight of exemplary gray and white matter voxels, and moving window size. The modified algorithm incorporating these parameter values was then used to classify data from simulated images of the brain, validating the use of fuzzy-membership values as estimates of partial volume. Gray:white matter ratios were estimated from 20 twenty normal volunteers (mean age 32.8 years). Processing time for each three-dimensional image was approximately 30 min on a 170 MHz workstation. Mean cerebral gray and white matter volumes estimated from these automatically segmented images were very similar to comparable results previously obtained by operator dependent methods, but without their inherent unreliability.

Adult↗
PubMed1999

New DNA markers with increased informativeness show diminished support for a chromosome 5q11-13 schizophrenia susceptibility locus and exclude linkage in two new cohorts of British and Icelandic families.

Genetic linkage of schizophrenia to markers at 5q11.2-13.3 had been reported previously in five Icelandic and two British families, but attempts at replication in independent samples have been unsuccessful. We report here an update on the diagnoses and results of linkage analyses using newer highly polymorphic microsatellite markers at or near the loci D5S76 and D5S39 in the original sample of pedigrees and in two new family samples from Iceland and from Britain. The new results show a reduction in evidence for linkage in the original sample and evidence against linkage in the two new family samples. Although it is possible that a rare locus is present, perhaps in the region 5p14.1-13.1 rather than 5q11.2-13.3, it appears most likely that the original positive lod scores represent an exaggeration of the 'true' lod scores due to random effects and that the small lod scores we now obtain could have arisen by chance.

Chromosomes, Human, Pair 5↗
PubMed1999

No evidence for linkage of schizophrenia to DXS7 at chromosome Xp11.

There have been claims that a gene on the X chromosome may contribute to susceptibility to schizophrenia. Crow (1988) initially proposed that such a gene might lie in the pseudoautosomal region, but when evidence that weakened this hypothesis accumulated, he proposed that a susceptibility locus might be present elsewhere on the sex chromosomes instead. DeLisi et al. (1994) found a small nonsignificant positive lod score between the marker DXS7 and schizophrenia, but other failed to replicate this finding. Another study reported by Crow and DeLisi's group was also weakly positive for this marker (Dann et al., 1997). This locus was then investigated in a collaborative study by Laval et al. (1997), which produced a nonparametric lod score of 2.44. Using a sample of 17 pedigrees from Britain and Iceland, we have also tested the hypothesis of linkage between DXS7 and schizophrenia. The 17 families were selected from a larger sample on the basis of an absence of male-to-male transmission for schizophrenia. These families were originally selected for having multiple cases of schizophrenia within them and for having no cases of bipolar affective disorder. We genotyped subjects for a marker at DXS7 and performed classical lod score and model-free linkage analysis using broad and narrow definitions of affection with schizophrenia. We found strongly negative lod scores and no evidence for linkage using model-free analysis. Therefore, this study does not support the hypothesis of linkage of schizophrenia to DXS7, and the evidence for a susceptibility locus on this part of the X chromosome is weakened.

Chromosome Mapping↗
PubMed1998

Neurological abnormalities in familial and sporadic schizophrenia.

Neurological assessment was carried out on patients with schizophrenia from multiply and singly affected families, their relatives, and a normal control group (214 subjects). A systematic examination was used in which abnormal signs were divided into 'primary' and 'integrative' signs. Primary signs were elicited by a standard clinical neurological examination and included signs of focal damage to nuclei and tracts, whilst integrative signs were selected as reflecting distributed brain function. The assessments were carried out to test the hypotheses that (i) neurological abnormalities are present in schizophrenia, (ii) the pattern of abnormality is different in familial and sporadic schizophrenic subjects, and (iii) the well relatives of familial (but not sporadic) schizophrenic subjects will show neurological abnormalities. An excess of primary signs compared with the controls was demonstrated in the sporadic schizophrenic group only. Both the familial schizophrenics and their first-degree relatives (but not their sporadic counterparts) showed an increase in integrative signs. The results support the existence of different mechanisms of underlying brain dysfunction in familial and sporadic schizophrenia.

Adult↗
PubMed1998

Minor physical anomalies in familial and sporadic schizophrenia: the Maudsley family study.

OBJECTIVES: (1) To test the hypothesis that minor physical anomalies are increased in patients with schizophrenia and (2) to investigate differences in the prevalence of minor physical anomalies in patients with familial and sporadic schizophrenia and their first degree relatives. METHODS: A weighted Waldrop assessment was carried out on 214 subjects in five groups: schizophrenic patients from multiply affected families; first degree relatives of these familial schizophrenic patients; sporadic schizophrenic patients; first degree relatives of these sporadic schizophrenic patients, and normal controls. Broad and narrow criteria for abnormality were defined based on the distribution of minor physical anomalies in the control group. RESULTS: (1) The total schizophrenic group did not have a significant increase in minor physical anomalies using a narrow criterion of abnormality, but did when a broader criterion was used. (2) A significant increase in the proportion of subjects with an abnormally high number of minor physical abnormalities was shown in the group of sporadic schizophrenic patients (uncorrected p<0.01). Separate analyses for males and females showed a significant increase in the male sporadic group (uncorrected p<0.05), and a smaller non-significant increase in the female sporadic group. Neither the familial schizophrenic group nor either group of first degree relatives showed any significant increases in the proportion of patients with high abnormality scores. CONCLUSION: This work supports prenatal developmental abnormality as a mechanism for sporadic, but not familial, schizophrenia.

Adult↗
PubMed1998

Brain changes in schizophrenia. Volumetric MRI study of families multiply affected with schizophrenia--the Maudsley Family Study 5.

BACKGROUND: Structural brain abnormalities have been reported in schizophrenia. We tested the hypothesis that these abnormalities represented a marker for the genetic liability to schizophrenia in a sample of people with schizophrenia and their relatives from families multiply affected with the disorder. METHOD: We compared 31 people with schizophrenia, 57 relatives and 39 unrelated control subjects. Volumetric measurement of brain structures was carried out using stereological principles from three-dimensional reconstructed magnetic resonance images. RESULTS: Subjects with schizophrenia had larger lateral ventricles than their relatives and the normal control subjects. Relatives who were 'presumed obligate carriers' had larger left lateral ventricles than other relatives and the control subjects. Subjects with schizophrenia showed smaller whole brain and cerebellar volumes and larger lateral ventricles than their age- and gender-matched unaffected siblings. CONCLUSIONS: In families multiply affected with schizophrenia lateral ventricular enlargement distinguishes people with schizophrenia and presumed obligate carriers from other relatives and unrelated control subjects. These changes may be a marker for a genetic liability to schizophrenia.

Adult↗
PubMed1997

Exclusion of linkage between schizophrenia and the gene encoding a neutral amino acid glutamate/aspartate transporter, SLC1A5.

An abnormality in glutamatergic function has been hypothesized as being of etiological importance in schizophrenia. Twenty-three multiplex English and Icelandic schizophrenia families were genotyped with a polymorphic dinucleotide repeat sequence in the 3'-untranslated region of the glutamate/aspartate transporter gene called SLC1A5. Using the lod and a model-free method of linkage analysis (MFLINK), no evidence of linkage between SLC1A5 and schizophrenia was found. Our results do not support the hypothesis that SLC1A5 gene mutations or allelic variants provide a major gene contribution to the etiology of schizophrenia. However, because of the likelihood of heterogeneity of linkage in schizophrenia, there is a case for testing other pedigrees for linkage to the SLC1A5 locus. The SLC1A5 locus is one of a complex family of genes encoding neutral amino acid transporter proteins and the genetic relation between these other loci and schizophrenia has not yet been established.

Alleles↗
PubMed1997

The Maudsley Family Study. I: Structural brain changes on magnetic resonance imaging in familial schizophrenia.

1. The authors investigated the prevalence of qualitatively rated structural brain abnormalities in schizophrenic probands and their first-degree relatives from families multiply affected with schizophrenia. 2. Magnetic resonance imaging was used to evaluate brain morphology in 33 schizophrenic probands, 54 of their non-schizophrenic first-degree relatives (including 11 presumed obligate carriers) and 37 unrelated control subjects. Structural images were examined by a neuroradiologist who was blind to diagnostic and family status. 3. 52% of the schizophrenic subjects were rated as showing abnormalities compared with 27% of presumed obligate carriers, 16% of their non-schizophrenic relatives and 11% of unrelated controls. 4. Brain abnormalities were more frequent in schizophrenic subjects from multiplex families than in their first-degree relatives and controls. Abnormalities were also found in unaffected relatives particularly those who appear to be transmitting the disorder.

Adult↗
PubMed1996

Dopamine D2 receptor occupancy in vivo by the novel atypical antipsychotic olanzapine--a 123I IBZM single photon emission tomography (SPET) study.

We have studied striatal D2 dopamine binding in schizophrenic patients treated with the novel atypical antipsychotic drug, olanzapine. 123I iodobenzamide (IBZM) single photon emission tomography (SPET) was used to estimate striatal dopamine D2 receptor binding in vivo. Patients were recruited from a prospective, double blind controlled trial of olanzapine versus haloperidol treatment. In vivo striatal D2 binding data from olanzapine treated patients (n = 6) were compared with previously reported data from typical antipsychotic responsive (n = 10); clozapine (n = 10); and risperidone (n = 6) treated patient groups. Mean % Brief Psychiatric Rating Scale score (BPRS) improvement following olanzapine treatment was 49% (SD 44). The hypothesis that clinical improvement in olanzapine treated patients would be associated with higher mean striatal D2 binding of 123I IBZM (reflecting lower levels of D2 occupancy) than typical antipsychotic (1.25 +/- 0.05) or risperidone (1.24 +/- 0.04) treatment was confirmed. Olanzapine treated patients had similar levels of striatal D2 binding in vivo (1.41 +/- 0.06) as those treated with clozapine (1.49 +/- 0.04). This preliminary evidence suggests olanzapine is another atypical antipsychotic drug in which therapeutic response is not associated with a high degree of striatal D2 receptor occupancy in vivo.

Adult↗
PubMed1996

Linkage study of the D5 dopamine receptor gene (DRD5) in multiplex Icelandic and English schizophrenia pedigrees.

OBJECTIVE: The authors investigated the possibility that genetic variation or mutation of the dopamine D5 receptor gene might modify susceptibility to schizophrenia. METHOD: Twenty-three Icelandic and English pedigrees containing multiple cases of schizophrenia were genotyped by using a highly informative microsatellite for the D5 dopamine receptor gene DRD5. RESULTS: By means of three different affection models, negative lod scores were obtained under assumptions of autosomal dominant and recessive inheritance. There was no evidence for locus heterogeneity. Nonparametric extended relative pair analysis also produced negative results. CONCLUSIONS: These data indicate that mutations of the D5 dopamine receptor gene are not a major cause of schizophrenia in these pedigrees. Because of the probable existence of locus heterogeneity, the D5 receptor gene may be of etiologic importance in other families with schizophrenia.

Base Sequence↗
PubMed1996

Lack of evidence for close linkage of the glutamate GluR6 receptor gene with schizophrenia.

OBJECTIVE: Previous research has consistently implicated genetic factors in the pathogenesis of schizophrenia. It has been hypothesized that an abnormality in glutamatergic function is of etiologic importance in schizophrenia, and therefore the glutamate receptor family of genes are potential susceptibility loci for schizophrenia. To test this hypothesis the authors sought to detect linkage between the GluR6 glutamate receptor gene and schizophrenia. METHOD: Twenty-three English and Icelandic families containing multiple cases of schizophrenia were genotyped with a microsatellite trinucleotide repeat polymorphism localized at the GluR6 glutamate receptor locus. Lod scores, model-free linkage analysis, and extended relative pair analysis were used to test for linkage. RESULTS: No statistically significant evidence of linkage between GluR6 and schizophrenia was found. CONCLUSIONS: The results do not support the hypothesis that GluR6 allelic variants provide a major gene contribution to the etiology of schizophrenia in a large proportion of these pedigrees.

Alleles↗
PubMed1995

Linkage analysis of chromosome 22q12-13 in a United Kingdom/Icelandic sample of 23 multiplex schizophrenia families.

A possible linkage to a genetic subtype of schizophrenia and related disorders has been reported on the long arm of chromosome 22 at q12-13. (Pulver et al., 1994: Am J Med Genet 54:36-43; Coon et al., 1994: Am J Med Genet 54:72-79; Pulver et al., 1994: Am J Med Genet 54:44-50). However formal statistical tests in a combined sample could not reject homogeneity and prove that there was a linked subgroup of families. We have studied 23 schizophrenia pedigrees to test whether some multiplex schizophrenia families may be linked to the microsatellite markers D22S274 and D22S283 which span the 22q12-13 region. Two point followed by multipoint lod and non-parametric linkage analyses under the assumption of heterogeneity provided no evidence for linkage over the relevant region.

Chromosomes, Human, Pair 22↗