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Biomedical subjects

G Thomson

Publications and source records attributed to G Thomson.

At least 109 records · Page 6Linked to original sources
PubMed1989

Autoimmune thyroid disease in type I diabetic families.

The prevalence rate for autoimmune thyroid disease (ATD) is about 30 times higher in the type I diabetic (IDDM) families that were ascertained for Genetic Analysis Workshop 5 (GAW5) than in the general population. Two approaches were used to study the clustering of ATD and IDDM in these families: 1) HLA haplotype sharing in sib pairs in which one has IDDM and the other has ATD was analyzed with the genetic interrelationship method. The hypotheses of different alleles (at the same locus or at different loci) were rejected. Thus there must exist at least one common allele that predisposes to both IDDM and ATD. 2) The DR genotype frequencies suggest that in the GAW5 families two alleles may be predisposing to ATD; a recessive DR3-associated allele and a dominant DR4-associated allele.

Cross-Sectional Studies↗
PubMed1989

HLA and insulin gene associations with IDDM.

The HLA DR genotype frequencies in insulin-dependent diabetes mellitus (IDDM) patients and the frequencies of DR alleles transmitted from affected parent to affected child both indicate that the DR3-associated predisposition is more "recessive" and the DR4-associated predisposition more "dominant" in inheritance after allowing for the DR3/DR4 synergistic effect. B locus distributions on patient haplotypes indicate that only subsets of both DR3 and DR4 are predisposing. Heterogeneity is detected for both the DR3 and DR4 predisposing haplotypes based on DR genotypic class. With appropriate use of the family structure of the data a control population of "unaffected" alleles can be defined. Application of this method confirms the predisposing effect associated with the class 1 allele of the polymorphic region 5' to the insulin gene.

Child↗
PubMed1989

Relative predispositional effects (RPEs) of marker alleles with disease: HLA-DR alleles and Graves disease.

A method is described to reveal the relative predispositional effects (RPEs) (predisposing, protective, or neutral) of the HLA alleles or of any other marker system that is associated with a disease. When the disease is associated with two or more alleles of a locus, the RPE method identifies the associations sequentially according to their strength; thus the problem that a strong association with one allele can create misleading deviations in the frequencies of other alleles is alleviated. Using this method, we have examined the relative effects of HLA-DR alleles in susceptibility to Graves disease in the Caucasian population. The well-established positive association with DR3 was confirmed as the strongest effect. In addition, a negative association was found between DR5 and Graves disease. The reduced frequency of DR5 among patients is statistically significant and is not a result of the increase in DR3. Finally, when patients were divided according to the presence or absence of eye disease, the latter showed a significant increase in the frequency of DR4. With family data, linkage to HLA of Graves disease was established in both Caucasian and Chinese families by the sib-pair method.

Alleles↗
PubMed1988

Maternal-fetal interactions and the maintenance of HLA polymorphism.

There is some empirical evidence that a fetus with an HLA antigen not present in its mother has a higher survival than a fetus sharing antigens with its mother. We have developed both single locus and two-locus theoretical models to examine this mode of selection. First, this immunologically based model appears to have the potential to maintain many alleles at a single locus and to result in an excess of heterozygotes when selection is strong. Second, substantial gametic disequilibrium is maintained between alleles at two loci for this selection mode when recombination is that observed between HLA loci A, B, and DR. Overall, it appears that this mode of selection has the potential to strongly affect genetic variation in the HLA region.

Alleles↗
PubMed1988

Genetic heterogeneity, modes of inheritance, and risk estimates for a joint study of Caucasians with insulin-dependent diabetes mellitus.

From 11 studies, a total of 1,792 Caucasian probands with insulin-dependent diabetes mellitus (IDDM) are analyzed. Antigen genotype frequencies in patients, transmission from affected parents to affected children, and the relative frequencies of HLA-DR3 and -DR4 homozygous patients all indicate that DR3 predisposes in a "recessive"-like and DR4 in a "dominant"-like or "intermediate" fashion, after allowing for the DR3/DR4 synergistic effect. Removal of DR3 and DR4 reveals an overall protective effect of DR2, predisposing effects of DR1 and DRw8, and a slight protective effect of DR5 and a predisposing effect of DRw6. Analysis of affected-parent-to-affected-child data indicates that a subset of DR2 may predispose. The non-DR3, non-DR4 antigens are not independently associated with DR3 and DR4; the largest effect is a deficiency of DR2, followed by excesses of DR1, DRw8, and DRw6, in DR4 individuals, as compared with DR3 individuals. HLA-B locus distributions on patient haplotypes indicate that only subsets of both DR3 and DR4 are predisposing. The presence or absence of Asp at position 57 of the DQ beta gene, recently implicated in IDDM predisposition, is not by itself sufficient to explain the inheritance of IDDM. At a minimum, the distinguishing features of the DR3-associated and DR4-associated predisposition remain to be identified at the molecular level. Risk estimates for sibs of probands are calculated based on an overall sibling risk of 6%; estimates for those sharing two, one, or zero haplotypes are 12.9%, 4.5%, and 1.8%, respectively. Risk estimates subdivided by the DR type of the proband are also calculated, the highest being 19.2% for sibs sharing two haplotypes with a DR3/DR4 proband.

Adolescent↗
PubMed1987

Influence of blood lead on the ability and attainment of children in Edinburgh.

The effect of blood-lead on children's ability and attainment was investigated in a sample of 855 boys and girls aged 6-9 years from eighteen primary schools within a defined area of central Edinburgh. The geometric mean blood-lead value was 10.4 micrograms/dl. In a stratified subsample, 501 children completed individual tests of cognitive ability and educational attainment from the British Ability Scales (BAS). An extensive home interview with a parent was also done. Multiple regression analyses showed a significant negative relation between log blood-lead and BAS combined score, number skills, and word reading when thirty-three possible confounding variables were taken into account. There was a dose-response relation between blood-lead and test scores, with no evidence of a threshold. The size of the effect was small compared with that of other factors. Lead at low levels of exposure probably has a small harmful effect on the performance of children in ability and attainment tests.

Achievement↗
PubMed1987

A comprehensive search for segregation distortion in HLA.

Segregation distortion, the non-Mendelian segregation of gametes, has been well documented among diverse groups of organisms. These cases are characterized by extreme segregation ratios found only in males. Previous reports have suggested the existence of segregation distortion operating in the HLA system of humans, a tightly linked complex of genes which regulates the immune system. In mice, some alleles of the T/t complex, which is linked to H-2 (the HLA homologue of mice), cause extreme segregation distortion in wild mice populations. Here we report on the examination of a large body of pedigree data on non-diseased families, scored for the alleles of five HLA region loci. We searched for segregation distortion on the basis of five different models of inheritance: allelic, haplotypic, genotypic, diffuse occurrence in families, and autosomal effects on the sex ratio. There was no clear evidence for segregation distortion. In particular, the possibility of extreme levels of segregation distortion was firmly rejected in the populations examined, thus reducing the likelihood of common distortion-causing HLA associated haplotypes in our species.

Alleles↗
PubMed1987

Disequilibrium pattern analysis. I. Theory.

We have developed a method, disequilibrium pattern analysis, for examining the disequilibrium distribution of the entire array of two locus multiallelic haplotypes in a population. It is shown that a selected haplotype will produce a distinct pattern of linkage disequilibrium values for all generations while the selection is acting. This pattern will also presumably be maintained for many generations after the selection event, until the disequilibrium pattern is eventually broken down by genetic drift and recombination. Related haplotypes, sharing an allele with a selected haplotype, assume a value of linkage disequilibrium proportional to the frequency of the unshared allele and have a single negative value of the normalized linkage disequilibrium. The analysis assumes zero linkage disequilibrium for all allelic combinations initially. The same basic results continue to apply if the selection involves a new mutant, the occurrence of which creates linkage disequilibrium for some haplotypes. The disequilibrium pattern predicted under selection is robust with respect to the influence of migration and random genetic drift. This method is applicable to population data having linked polymorphic loci including that determined from protein or DNA sequencing.

Alleles↗
PubMed1987

Disequilibrium pattern analysis. II. Application to Danish HLA A and B locus data.

Disequilibrium pattern analysis, a general method for analyzing evolutionary events acting on pairs of tightly linked polymorphic loci, is applied to a large sample of Danish individuals typed for A and B loci of the HLA (human leukocyte antigen) system. Cases of selection on particular haplotypes are revealed from patterns of linkage disequilibrium among the HLA haplotypes. These patterns cannot be explained by either population admixture or random genetic drift. Six haplotypes out of the total array of 273 haplotypes have been identified which show in varying extents the patterns indicating selection.

Alleles↗
PubMed1987

The affected sib method. V. Testing the assumptions.

The affected sib methods, which are used to make inferences about the genetic components of HLA associated diseases, have many underlying assumptions which may not always be realistic. These include no selective disadvantage of affected individuals, little or no recombination between the marker loci and the 'disease' locus, a single panmictic population, Mendelian segregation of the disease locus alleles and random distribution of individuals over environments. The effects of breaking these assumptions have been investigated. We have explicitly derived the haplotype sharing identity by descent (IBD) expectations for the cases of selection against affected individuals and recombination between the HLA marker loci and the 'disease' predisposing locus for affected sib trios (as was previously done for affected sib pairs). We have also derived, for both affected sib pairs and trios, the haplotype sharing expectations for non-random mating (positive assortative), admixture, meiotic drive (of disease allele carrying haplotypes), and a random versus shared environmental component for sibs. In order to assess the sensitivity of the affected sib methods to perturbations in the assumptions, the expectation spaces of haplotype sharing in affected sib pairs and sib trios under the single diallelic locus model with varying penetrances and allele frequencies are fully described. The effects on haplotype sharing and subsequent disease parameter estimation are different for each of the factors we have considered. The affected sib methods are found to be robust in many situations.

Alleles↗
PubMed1987

Analysis of genetic interrelationship among HLA-associated diseases.

We have developed a method to study the genetic relationship between any two HLA-associated diseases. We have considered the following hypotheses: (1) both diseases are caused by a common allele; (2) different alleles at the same locus predispose to the two diseases; (3) one disease is predisposed by two alleles, one of which can also lead to the second disease; and (4) different HLA-linked loci are involved in the etiology of each disease. For each hypothesis, we have derived the expected HLA haplotype-sharing distribution in sib pairs who are affected with two diseases. The comparison of the expectations indicate that, in many cases, the alternate hypotheses can be distinguished, if the sample size is appropriately large. The knowledge of the mode of inheritance of each disease is not usually necessary; however, it can greatly increase the power of the test. Analyses of data on pairwise combinations of rheumatoid arthritis (RA), autoimmune thyroid disease (ATD), and insulin-dependent (type I) diabetes mellitus (IDDM) suggest that (a) IDDM is predisposed by two HLA-linked alleles, one of which also predisposes to ATD, (b) one of the IDDM alleles also confers susceptibility to RA, and (c) although the HLA-linked susceptibilities to RA and ATD appear to be primarily due to distinct alleles, the ATD allele may also have a minor role in predisposition to RA.

Alleles↗