Growth patterns in young adult monozygotic twin pairs discordant and concordant for obesity.

Weight discordance is very rare in monozygotic (MZ) twin pairs; when found, however, such pairs are advantageous in the search for either environmental or epigenetic causes and consequences of obesity. We analyzed the growth patterns of young adult MZ pairs discordant and concordant for obesity. Screening 5 consecutive birth cohorts (1975-1979) of 22- to 27-year-old Finnish twins (the FinnTwin16 study), we found 14 obesity discordant (Body Mass Index [BMI] difference > or = 4 kg/m2) MZ pairs out of 658. Ten pairs participated in clinical studies. Nine concordant pairs (BMI difference < or = 2 kg/m2) were examined as controls. Lifetime measured heights and weights recorded in hospitals and health centers were traced manually. Height development was similar in all the co-twins of both groups. The weight differences between the co-twins of the discordant pairs began to emerge at 18 years leading to an average discordance of 16.4 kg, 5.6 kg/m2 (p for both = .005) at 25.7 years. The heavier co-twin weighed 221 g (p = .066), 1.0 kg/m2 (p = .01) more already at birth than the leaner, but the differences waned by 6 months of age and reappeared only after adolescence. Both the leaner and the heavier co-twins of the discordant pairs weighed more than expected by the singleton reference values (Cole et al., 1998) after 8 years. The concordant co-twins, on the other hand, grew similarly and after 6 months, their mean growth was not distinguishable from the singleton patterns. Young adulthood represents a critical period of gaining weight irrespective of genetic background in this twin sample.     

Linkage analysis of extremely discordant and concordant sibling pairs identifies quantitative-trait loci that influence variation in the human personality trait neuroticism

Several theoretical studies have suggested that large samples of randomly ascertained siblings can be used to ascertain phenotypically extreme individuals and thereby increase power to detect genetic linkage in complex traits. Here, we report a genetic linkage scan using extremely discordant and concordant sibling pairs, selected from 34,580 sibling pairs in the southwest of England who completed a personality questionnaire. We performed a genomewide scan for quantitative-trait loci (QTLs) that influence variation in the personality trait of neuroticism, or emotional stability, and we established genomewide empirical significance thresholds by simulation. The maximum pointwise P values, expressed as the negative logarithm (base 10), were found on 1q (3.95), 4q (3.84), 7p (3.90), 12q (4.74), and 13q (3.81). These five loci met or exceeded the 5% genomewide significance threshold of 3.8 (negative logarithm of the P value). QTLs on chromosomes 1, 12, and 13 are likely to be female specific. One locus, on chromosome 1, is syntenic with that reported from QTL mapping of rodent emotionality, an animal model of neuroticism, suggesting that some animal and human QTLs influencing emotional stability may be homologous.     

Efficiency robust tests for mapping quantitative trait loci using extremely discordant sib pairs

In 1972, Haseman and Elston proposed a pioneering regression method for mapping quantitative trait loci using randomly selected sib pairs. Recently, the statistical power of their method was shown to be increased when extremely discordant sib pairs are ascertained. While the precise genetic model may not be known, prior information that constrains IBD probabilities is often available. We investigate properties of tests that are robust against model uncertainty and show that the power gain from further constraining IBD probabilities is marginal. The additional linkage information contained in the trait values can be incorporated by combining the Haseman-Elston regression method and a robust allele sharing test.     

A systematic genomewide linkage study in 353 sib pairs with schizophrenia

We undertook a genomewide linkage study in a total of 353 affected sib pairs (ASPs) with schizophrenia. Our sample consisted of 179 ASPs from the United Kingdom, 134 from Sweden, and 40 from the United States. We typed 372 microsatellite markers at approximately 10-cM intervals. Our strongest finding was a LOD score of 3.87 on chromosome 10q25.3-q26.3, with positive results being contributed by all three samples and a LOD-1 interval of 15 cM. This finding achieved genomewide significance (P<.05), on the basis of simulation studies. We also found two regions, 17p11.2-q25.1 (maximum LOD score [MLS] = 3.35) and 22q11 (MLS = 2.29), in which the evidence for linkage was highly suggestive. Linkage to all of these regions has been supported by other studies. Moreover, we found strong evidence for linkage (genomewide P<.02) to 17p11.2-q25.1 in a single pedigree with schizophrenia. In our view, the evidence is now sufficiently compelling to undertake detailed mapping studies of these three regions.     

Genetic association mapping based on discordant sib pairs: the discordant-alleles test.

Family-based tests of association provide the opportunity to test for an association between a disease and a genetic marker. Such tests avoid false-positive results produced by population stratification, so that evidence for association may be interpreted as evidence for linkage or causation. Several methods that use family-based controls have been proposed, including the haplotype relative risk, the transmission-disequilibrium test, and affected family-based controls. However, because these methods require genotypes on affected individuals and their parents, they are not ideally suited to the study of late-onset diseases. In this paper, we develop several family-based tests of association that use discordant sib pairs (DSPs) in which one sib is affected with a disease and the other sib is not. These tests are based on statistics that compare counts of alleles or genotypes or that test for symmetry in tables of alleles or genotypes. We describe the use of a permutation framework to assess the significance of these statistics. These DSP-based tests provide the same general advantages as parent-offspring trio-based tests, while being applicable to essentially any disease; they may also be tailored to particular hypotheses regarding the genetic model. We compare the statistical properties of our DSP-based tests by computer simulation and illustrate their use with an application to Alzheimer disease and the apolipoprotein E polymorphism. Our results suggest that the discordant-alleles test, which compares the numbers of nonmatching alleles in DSPs, is the most powerful of the tests we considered, for a wide class of disease models and marker types. Finally, we discuss advantages and disadvantages of the DSP design for genetic association mapping.     

Equivalence between Haseman-Elston and variance-components linkage analyses for sib pairs

The Haseman-Elston regression method offers a simpler alternative to variance-components (VC) models, for the linkage analysis of quantitative traits. However, even the "revisited" method, which uses the cross-product--rather than the squared difference--in sib trait values, is, in general, less powerful than VC models. In this report, we clarify the relative efficiencies of existing Haseman-Elston methods and show how a new Haseman-Elston method can be constructed to have power equivalent to that of VC models. This method uses as the dependent variable a linear combination of squared sums and squared differences, in which the weights are determined by the overall trait correlation between sibs in a population. We show how this method can be used for both the selection of maximally informative sib pairs for genotyping and the subsequent analysis of such selected samples.     

Linkage analysis of extremely discordant and concordant sibling pairs identifies quantitative trait loci influencing variation in human menopausal age

Age at natural menopause may be used as parameter for evaluating the rate of ovarian aging. Environmental factors determine only a small part of the large variation in menopausal age. Studies have shown that genetic factors are likely to be involved in variation in menopausal age. To identify quantitative-trait loci for this trait, we performed a genomewide linkage study with age at natural menopause as a continuous quantitative phenotype in Dutch sister pairs, through use of a selective sampling scheme. A total of 165 families were ascertained using extreme selected sampling and were genotyped for 417 markers. Data were analyzed by Haseman-Elston regression and by an adjusted variance-components analysis. Subgroup analyses for early and late menopausal age were conducted by Haseman-Elston regression. In the adjusted variance-components analysis, 12 chromosomes had a LOD score of > or =1.0. Two chromosomal regions showed suggestive linkage: 9q21.3 (LOD score 2.6) and Xp21.3 (LOD score 3.1). Haseman-Elston regression showed rather similar locations of the peaks but yielded lower LOD scores. A permutation test to obtain empirical P values resulted in a significant peak on the X chromosome. To our knowledge, this is the first study to attempt to identify loci responsible for variability in menopausal age and in which several chromosomal regions were identified with suggestive and significant linkage. Although the finding of the region on the X chromosome comes as no surprise, because of its widespread involvement in premature ovarian failure, the definition of which particular gene is involved is of great interest. The region on chromosome 9 deserves further consideration. Both findings require independent confirmation.     

Using discordant sib pairs to map loci for qualitative traits with high sibling recurrence risk.

A common approach for detecting genetic linkage using siblings is to collect affected sib pairs (ASPs) and to identify markers where allele sharing exceeds expectation. Alternatively, markers can be analyzed in discordant sib pairs (DSPs) for allele sharing below expectation. Relative to the ASP approach, according to Risch, the power of the DSP approach increases with sibling recurrence risk, the two approaches being equally effective at 50% recurrence risk. However, with many diseases associated with more moderate sibling recurrence risk, less emphasis has been placed on the use of DSPs and the development of the underlying theory. In this paper, we expand the work of Risch to provide a more general foundation for DSP studies, since power can be quite high under the appropriate conditions. For example, in some highly affected populations, such as the diabetes-prone Pima Indians, sibling recurrence risk can be very large and, thus, DSPs ideal. Similarly, as we show through simulation, DSPs are preferable for diabetic nephropathy due to a 70% recurrence rate among siblings with insulin-dependent diabetes mellitus. Following the diabetic nephropathy example, we consider more systematically the situations in which DSPs can provide an efficient alternative to ASPs.     

Cardiac structure and function in monozygotic twin pairs discordant for physical fitness.

Cross-sectional studies in athletes and untrained subjects suggest that exercise training induces adaptations in cardiac structure and function. However, the role of genetic variation on the results has largely been ignored in these studies. The purpose of this study was to investigate the effects of long-term volitionally increased physical activity on electrocardiographic and echocardiographic parameters in male monozygotic twin pairs discordant for physical activity and fitness. On the basis of the mailed questionnaires, a telephone interview, and the inclusion criteria, 12 pairs of young adult male monozygotic twins were recruited from a Finnish twin cohort. All subjects completed a maximal oxygen uptake (.VO2 (max)) test and electrocardiography and echocardiography studies. Nine pairs had at least 9% difference in .VO2(max) and were selected for further analysis and for a second echocardiography study. Twins were divided into the more (MAG) and less active group (LAG), according to their VO2(max). On average, MAG had 18% higher VO2(max) compared with LAG. In electrocardiography, MAG had 29% (P = 0.02) higher Cornell voltage and 37% (P = 0.01) higher right-side hypertrophy index. In echocardiography, no significant differences were observed between the groups, and left ventricular mass index was only 7% (P = 0.16) higher in MAG. These results show that the volitionally increased physical activity that has led to an 18% increase in cardiorespiratory fitness induces greater changes in electro- than echocardiographic parameters. Electrocardiographic changes were suggestive of left ventricular hypertrophy, and echocardiography showed a similar but statistically nonsignificant trend.     

HLA-DR typing in identical twins with insulin-dependent diabetes: difference between concordant and discordant pairs.

A total of 106 pairs of identical twins, of whom 56 were concordant and 50 discordant for insulin-dependent diabetes, were typed for HLA-DR. In both the concordant and discordant groups there was a high prevalence of the antigens DR3 and DR4, a low prevalence of DR5 and DR7, and a virtual absence of DR2. The heterozygous phenotype DR3,DR4 was more prevalent in concordant than discordant pairs. This was therefore the first demonstration of a genetic difference between concordant and discordant identical twin pairs. These findings suggest that possession of both DR3 and DR4 antigens confers a greater genetic predisposition to insulin-dependent diabetes than does the possession of either antigen alone.     

Lifetime prevalence of mood and anxiety disorders in twin pairs discordant for schizophrenia.

There have been long questions about the relationship of schizophrenia to other mental disorders. Lifetime DSM-III-R diagnoses of mood and anxiety disorders in twins with clinically diagnosed schizophrenia (n = 24) and their non-affected co-twins (n = 24) were compared with twins from pairs without schizophrenia (n = 3327) using a sample from the Vietnam Era Twin Registry. Schizophrenic probands had significantly elevated rates of all included disorders (bipolar disorder, major depression, dysthymia, generalized anxiety disorder, panic disorder, and PTSD) compared with controls (P<0.01). The odd ratios comparing co-twins of schizophrenic probands with controls was greater than three for every disorder, but did not attain statistical significance. A similar pattern was observed when analyses were restricted to only monozygotic twins (n = 12). Consistent with other studies, schizophrenics appeared to have higher rates of a range of mental disorders. Our results suggest that schizophrenia per se represents a risk factor for other psychiatric disorders, but the absence of significantly elevated risk among non-schizophrenic co-twins suggested that family environmental and/or genetic factors that contribute to risk of schizophrenia do not increase the risk of mood and anxiety disorders to the same extent that the risk of these other disorders is increased by the presence of schizophrenia.     

Multipoint linkage analysis using sib pairs: an interval mapping approach for dichotomous outcomes.

I propose an interval mapping approach suitable for a dichotomous outcome, with emphasis on samples of affected sib pairs. The method computes a lod score for each of a set of locations in the interval between two flanking markers and takes as its estimate of trait-locus location the maximum lod score in the interval, provided it exceeds the prespecified critical value. Use of the method depends on prior knowledge of the genetic model for the disease only through available estimates of recurrence risk to relatives of affected individuals. The method gives an unbiased estimate of location, provided the recurrence risk are correctly specified and provided the marker identity-by-descent probabilities are jointly, rather than individually, estimated. I also discuss use of the method for traits determined by two loci and give an approximation that has good power for a wide range of two-locus models.     

Insulin-gene sharing in sib pairs with insulin-dependent diabetes mellitus: no evidence for linkage

An association between insulin-dependent diabetes mellitus (IDDM) and an RFLP adjacent to the insulin gene has been consistently observed, but its etiological significance is unclear. We studied unrelated IDDM patients (N = 45) and controls (N = 65) to confirm the association--and assessed evidence for linkage in 22 families with at least two affected (IDDM) sibs--to determine whether the insulin-gene region actually contributes to susceptibility to IDDM. All individuals were typed for the RFLP in the 5'-flanking region of the insulin gene (5'FP) used in the previous studies, and the 12 families not fully informative for linkage with the 5'FP were typed for additional closely linked RFLPs. We found a higher frequency of class 1 alleles of the 5'FP in IDDM patients (.83) than in controls (.75), which is consistent with the reported association, but the difference was not statistically significant in our sample. Among the 33 affected sib pairs (ASPs) in 22 families, if maximum possible sharing is assumed when sharing is ambiguous, 10 pairs share both parental insulin genes, 17 pairs share one, and six share neither. This distribution is incompatible with close linkage. In contrast, for the HLA region, for which all 22 families are fully informative, 19 of the 33 ASPs share two haplotypes and the remaining 14 share one. There are no pairs that share neither HLA haplotype. Thus, although these data clearly illustrate the contribution of HLA-linked susceptibility to IDDM, they argue strongly against a contribution of similar magnitude by the insulin-gene region.     

A powerful and robust new linkage statistic for discordant sibling pairs

Previously, Szatkiewicz and colleagues evaluated the performance of a wide variety of statistics for quantitative-trait-locus linkage, using discordant sibling pairs. They found that the most powerful statistics, in general, were a score statistic and a "composite statistic." However, whereas these two statistics have equal power under ideal conditions, each has limitations that reduce its power in certain circumstances. The score statistic depends on estimates of trait parameters and can lose a lot of power if those estimates are incorrect. The composite statistic is not sensitive to trait-parameter estimates but does depend on arbitrary weights that must be chosen on the basis of the ascertainment scheme. In this report, we elucidate the algebraic relationship between the score and composite statistics and then use that relationship to suggest a new statistic that combines the best properties of both. We call our new statistic the "robust discordant pair" (RDP) statistic. We report simulation studies to show that the RDP statistic does, indeed, have all of the strengths and none of the weaknesses of the score and composite statistics.     

Mapping quantitative-trait loci in humans by use of extreme concordant sib pairs: selected sampling by parental phenotypes.

In two previous articles, we have considered sample sizes required to detect linkage for mapping quantitative-trait loci in humans, using extreme discordant sib pairs. Here, we examine further the use of extreme concordant sib pairs but consider the effect of parents' phenotypes. Sample sizes necessary to obtain a power of 80% with concordant sib pairs at a significance level of .0001 are given, stratified by parental phenotypes. When there is no residual correlation between sibs, the parental phenotypes have little impact on the sample sizes. When residual correlations between sibs exist, we show, however, that power can be considerably reduced by including extreme sib pairs when the parents also have similarly extreme values. Thus, we recommend the exclusion of such pairs from linkage studies. This recommendation reduces the required sample sizes by 3- to 28-fold. The degree of saving in the required sample sizes varies among different models and allele frequencies. The reduction is most dramatic (a 28-fold reduction) for a rare recessive gene.     

Erythrocyte sodium-lithium countertransport and blood pressure in identical twin pairs discordant for insulin dependent diabetes.

OBJECTIVE--To investigate whether insulin dependent diabetes is responsible for the abnormal behaviour of the carrier in sodium-lithium countertransport and whether the diabetic state is associated with rise in blood pressure. DESIGN--Case-control study. SETTING--London teaching hospital. SUBJECTS--44 twin pairs discordant for insulin dependent diabetes living in United Kingdom and 44 healthy control subjects matched for age, sex, and body mass index. None of the twin pairs or the controls had evidence of microalbuminuria. MAIN OUTCOME MEASURES--Sodium-lithium countertransport activity in erythrocytes and arterial blood pressure. RESULTS--The mean (95% confidence interval) sodium-lithium countertransport activity (mmol Li per litre of red blood cells per h) of the diabetic twins (0.291 (0.244 to 0.338)) was similar to that of their non-diabetic cotwins (0.247 (0.204 to 0.290)); both values were significantly higher than that of the controls (0.187 (0.157 to 0.216); p < 0.05). In addition, systolic blood pressure was higher in those twins with diabetes (127 (122 to 133) mm Hg) than in the non-diabetic cotwins (122 (117 to 127) mm Hg; p < 0.01). There were no significant differences in mean diastolic blood pressure between any of the groups studied. CONCLUSIONS--The raised erythrocyte sodium-lithium countertransport activity in the diabetic twins compared with the controls seems to be inherited rather than a consequence of overt diabetes. The higher systolic blood pressure in diabetic twins than non-diabetic cotwins indicates that insulin dependent diabetes does exert a small influence on systolic blood pressure.     

X-linked extension of the revised Haseman-Elston algorithm for linkage analysis in sib pairs

Haseman and Elston (H-E) proposed a regression-based robust test of linkage between a marker and an autosomal quantitative trait locus, using the squared sib pair trait difference as a dependent variable and the proportion of alleles shared identical by descent by the sib pair as an independent variable. Several authors have proposed improvement of the original H-E's seminal work by using an optimal linear combination of squared sum and squared difference as the dependent variable. In this paper, we extend Haseman and Elston's sib pair method to an X-linked locus. We give a general formulation of the complete regression model and details of the regression coefficients in terms of variance components. Simulation results are presented to describe the power of this technique for a theoretical best case scenario.     

Molecular characterization of a MSRV-like sequence identified by RDA from monozygotic twin pairs discordant for schizophrenia.

Retroviral-related amplicons were used in modified RDA to identify four sequences from affected members of three pairs of monozygotic twins discordant for schizophrenia. One sequence (schizophrenia associated retrovirus, SZRV-1, GenBank Accession No. AF135487) is characterized here. It is similar to two known sequences of retroviral origin: multiple sclerosis-associated retrovirus, MSRV (GenBank Accession No. AF009668), and ERV-9 (GenBank Accession No. S77575). It is present in multiple copies in the human genome and has been localized to six different chromosomal sites. A zooblot shows that this multicopy sequence is predominant in the primate lineage and present in rhesus monkeys and humans. SZRV-1 is expressed as a 9-kb RNA band in the placenta. This could offer support to the hypothesis that retroviral sequences transposing during fetal growth may alter neurodevelopmental genes and cause diseases, although its direct involvement in the causation of schizophrenia remains to be established.