Diabetes 53:855-860, 2004 © 2004 by the American Diabetes Association, Inc.
Evidence From a Large U.K. Family Collection That Genes Influencing Age of Onset of Type 2 Diabetes Map to Chromosome 12p and to the MODY3/NIDDM2 Locus on 12q24
1 Wellcome Trust Centre for Human Genetics, Oxford, U.K
Additional information on genetic susceptibility effects relevant to type 2 diabetes pathogenesis can be extracted from existing genome scans by extending examination to related phenotypes such as age at disease onset. In this study, we report the reanalysis of data from 573 U.K. sibships ascertained for multiplex type 2 diabetes, using age at onset (assessed by the proxy measure of age at diagnosis) as the phenotype of interest. Genome-wide evidence for linkage to age at diagnosis was evaluated using both variance components and Haseman-Elston (HECOM) regression approaches, with extensive simulations to derive empirical significance values. There was broad agreement across analyses with six regions of interest (logarithm of odds [LOD] 1.18) identified on chromosomes 1qter, 4p154q12, 5p15, 12p1312q13, 12q24, and 14q1214q21. The strongest empirically "suggestive" evidence for linkage comes from regions on chromosome 12. The first region (12p1312q13), peaking at D12S310 (variance components LOD [LODVC] = 2.08, empirical pointwise P = 0.0007; HECOM LOD [LODHECOM] = 2.58, P = 0.0010) seems to be novel. The second (12q24) peaking between D12S324 and D12S1659 (LODVC = 1.87, P = 0.0016; LODHECOM = 1.93, P = 0.0027) overlaps a region showing substantial prior evidence for diabetes linkage. These data provide additional evidence that genes mapping to these chromosomal regions are involved in the susceptibility to, and/or development of, type 2 diabetes.
Type 2 diabetes is a multifactorial disease of rising prevalence and increasing medical importance (1). The development of novel preventative and therapeutic strategies is largely contingent on an improved understanding of the molecular events involved in the pathogenesis of diabetes and related phenotypes such as obesity and insulin resistance (2). Identification and characterization of the susceptibility variants underlying predisposition to these conditions remains one of the most powerful routes to such understanding. Most genome-wide linkage scans to date have concentrated on the analysis of type 2 diabetes as a discrete dichotomous trait (3). However, it seems likely that additional useful information on genetic susceptibility effects can be extracted from existing scans by extending examination to pertinent continuous phenotypes, such as those reflecting the action of modifier genes influencing age at disease onset and presentation (4,5). We have previously reported (6) our analysis of the genome-wide evidence for linkage to type 2 diabetes in 573 U.K. affected sibships and recently demonstrated that the evidence for linkage to type 2 diabetes in this cohort comes disproportionately from patients with younger ages of onset (7). Here, we present an analysis of the same dataset designed to detect genes influencing age at onset (as assessed by the proxy measure of age at diagnosis) using two powerful complementary statistical approaches.
The subjects for study comprised the 573 full-sib pedigrees previously analyzed for evidence for linkage to type 2 diabetes (6). These pedigrees had been genotyped with 418 autosomal microsatellite markers (mean spacing 9.26 cM [Haldane units]), as previously described (6). Age at diagnosis was available for 1,233 affected individuals (1,223 offspring and 10 parents); mean age at diagnosis was 55.2 (SD 8.6) years in men (n = 661) and 56.2 (SD 8.8) years in women (n = 572). Genotypes were available for these 1,233 individuals and a further 173 individuals who were nondiabetic at the time of ascertainment. The phenotype for analysis in this study was age at diagnosis in affected individuals, distributed as a continuous quantitative variable. These measures were self-reported by subjects at the time of study recruitment. Though, in principle, it would have been desirable to include information derived from the age at study among unaffected relatives (treated as a censored trait, for example using the hazard function based approach of Hanson and Knowler [8]), attempts to do so generated a markedly bimodal distribution of data points, a consequence of the incomplete ascertainment of unaffected relatives. This distribution was not amenable to meaningful analysis with any available method; consequently, unaffected relatives were treated as unknown for the age at diagnosis phenotype. There was a marginally significant (P = 0.049) effect of sex on age at diagnosis. Heritability estimates for age at diagnosis, both before and after adjustment for sex, were obtained using MERLIN (9). BMI and other anthropometric measures around the time diabetes onset were not available, so adjustment was not possible for these measures. Sex-adjusted age at diagnosis was only slightly skewed (coefficient = -0.22) and platykurtic (coefficient = -0.42), and therefore untransformed sex-adjusted values were standardized and used in the linkage analyses.
Linkage analysis. To detect colocalization of regions of linkage to age at diagnosis and obesity, we also undertook linkage analysis for BMI in diabetic members of the same families. BMI values (measured at the time of study) were logarithmically transformed and adjusted for the effects of age and sex using normative data from the adult U.K. population (11). These data were then standardized against the population data and analyzed using HECOM regression.
Power calculations.
Empirical significance simulations. In addition, for both analytical methods, we estimated empirically the genome-wide null distribution of both LOD scores and the number of independent regions of linkage ("locus-counting") (13). This involved 1,000 replicates of the entire genome, generated given the observed phenotype, pedigree, and marker characteristics of the data. These complementary approaches provide empirically derived assessments of the genome-wide evidence for linkage to age at diagnosis in our dataset. By taking account of the incomplete extraction of inheritance information and the effects of pedigree structure on the linkage statistic, a more appropriate, less conservative measure of statistical significance can be obtained (13). Previous studies have demonstrated the profound effect these factors can have on genome-wide significance levels in quantitative trait linkage analysis (14) and the consequent necessity of empirical estimates of significance.
The heritability of age at diagnosis in these pedigrees was found to be 63.6% before adjustment, confirming the familial clustering of this trait. The effect of sex was marginal (P = 0.049), with age at diagnosis heritability increasing to 64.9% after adjustment for sex. All subsequent analyses were conducted on data adjusted for the effects of sex.
The results of the genome wide quantitative linkage analysis of age at diagnosis with both variance components and HECOM regression approaches are summarized in Fig. 1 and Table 1. Regions showing evidence for linkage to age at diagnosis (taking a threshold of LOD
The empirical genome-wide null distributions of LOD scores and independent regions of linkage, estimated using variance components and HECOM regression methods are shown in Fig. 2. The locus-counting simulations indicate more independent regions showing evidence for linkage than expected by chance for LOD scores from 1.18 to 2.00, with this excess oscillating around the 5% significance level. These simulations also indicate that when our data are analyzed by the variance components method evidence for linkage with LODVC = 1.56 occurs once per genome under the null hypothesis (defining the threshold for "suggestive" linkage), whereas a LODVC = 2.75 is associated with a genome-wide significance of 0.05 ("significant" linkage). With the HECOM regression analysis, equivalent empirical thresholds are LODHECOM = 1.69 and LODHECOM = 3.25, respectively.
The most interesting results were obtained for chromosome 12 and are shown in detail in Fig. 3. We observed a broad region showing evidence for linkage to age at diagnosis on 12p1312q13, peaking at D12S310, (LODVC = 2.08, empirical pointwise P = 0.0007; LODHECOM = 2.58, empirical pointwise P = 0.0010). Evidence for linkage was also observed on 12q24 between D12S324 and D12S1659 (LODVC = 1.87, empirical pointwise P = 0.0016; LODHECOM = 1.93, empirical pointwise P = 0.0027). Both results are robust to the removal of phenotypic outliers with values >3 SDs from the mean age at diagnosis. Both regions exceed the thresholds for "suggestive" linkage with both analytical approaches.
To detect colocalization between loci influencing age at diagnosis and obesity, we sought evidence of linkage to BMI measures taken at the time of study recruitment. Modest, nominally significant (P < 0.05) peaks for BMI were indeed found on chromosome 5pter (D5S1981, LODHECOM = 0.61) and on chromosome 12 (D12S1725, LODHECOM = 0.80, D12S346-D12S78, LODHECOM = 0.69). Neither of these lie within 20 cM of the maximum age at diagnosis LOD scores on chromosome 12p12 and 12q24, and theLODHECOM scores for linkage to BMI at these age-at-diagnosis peaks were 0.265 and 0.090, respectively. There were no instances of overlap between BMI and age-at-diagnosis loci on other chromosomes (data not shown). Simulations suggested that our study had good power to detect a suggestive linkage (6678%) but low power to detect a genome-wide significant linkage (3041%) to a QTL accounting for 30% of the trait variance with a residual 34.9% polygenic variance component using variance components and HECOM regression methods, respectively.
We have conducted a genome-wide analysis of a large set of families of U.K. origin seeking evidence for loci influencing age of diagnosis of type 2 diabetes using two complementary statistical approaches. We found evidence for linkage with a LOD 1.18 between age at diagnosis and markers on 1qter, 4p154q12, 5p15, 12p1312q13, 12q24, and 14q1221. These results are consistent across both analyses. Although none of these regions achieves genome-wide significance individually, locus counting simulations indicate that we observed significantly more evidence for linkage to age at diagnosis, genome wide, than expected by chance. These data imply that several of the regions detected in this scan may harbor susceptibility loci influencing age at diagnosis. Only replication by, or concordance with, other genome scans can indicate which regions are likely to represent the true positives. We have found no regions of concordance between the present age at diagnosis analysis and the previous genome-wide scans for type 2 diabetes in this dataset, either in the complete (6) or age-stratified (7) analyses, even in regions such as chromosome 1q that have been widely replicated (3) and therefore have the strongest likelihood of containing type 2 diabetes susceptibility genes. This lack of concordance may reflect intrinsic methodological differences between analyses that use age at diagnosis as a quantitative variable (where the aim, as here, is to detect genes influencing the age at onset of type 2 diabetes in susceptible individuals) and those that consider type 2 diabetes itself as the trait of interest, as in an affected subjectsonly analysis. An alternative conclusion is that these findings indicate that the genetic determinants of type 2 diabetes susceptibility and of disease progression are, at least in the U.K. population, distinct. There are few populations for which linkage data are available for both type 2 diabetes and age at diagnosis (5,15). Consequently, it is not yet possible to obtain a comprehensive view of the relationship between the genetic basis of susceptibility and that of diabetes onset or progression. Indeed, the influence of a type 2 diabetes disease gene on the spectrum of clinical phenotypes, from a strict susceptibility effect to a strict progression effect, may depend on genetic and environmental background; a gene principally influencing susceptibility in one population may influence progression in another.
The strongest evidence for linkage to age at diagnosis in the present study was seen in the two regions on chromosome 12. The first of these covered the pericentromeric region (12p1312q13), with evidence peaking on 12p12. There have been no previous reports of evidence for linkage to type 2 diabetes (or related traits) to 12p. However, a locus in the pericentromeric region of 12q has been implicated in two previous studies. In European-American pedigrees segregating early-onset autosomal-dominant type 2 diabetes, a heterogeneity LOD score of 2.5 (
The second region of interest on chromosome 12 lies on 12q24. This region represents one of the best replicated intervals for type 2 diabetes susceptibility with evidence for linkage generated in an appreciable number of previous studies (17,1922). The present study provides further support for functional variants on chromosome 12q influencing type 2 diabetes susceptibility and/or progression. This region includes the gene TCF1 (encoding hepatocyte nuclear factor-1 The other regions highlighted in our scan have limited support from previous scans of type 2 diabetes susceptibility. The San Antonio Family Heart Study (15) observed evidence for linkage to type 2 diabetes and age at diagnosis on 1q4344, some 15 cM(K) from our present evidence on 1q. The Amish Family Heart Study (24) observed evidence for linkage of HbA1c levels to markers on 4p, 20 cM telomeric from the peak LOD score on 4p in the present study, and evidence for linkage of glucose levels to markers overlapping our region of interest on 14q. The latter region was also implicated in linkage to type 2 diabetes in the GENNID study (17). A modifier locus influencing age at diagnosis in MODY3 families has been observed on 5p15 (25), some 15 cM from our evidence on 5p15. Obesity is a known risk factor for type 2 diabetes and likely to influence the age of disease onset. This raises the possibility of overlap between genetic variation influencing obesity susceptibility and that involved in age at diabetes onset. As we did not have access to measures of obesity taken at the pertinent time point (i.e., around the time of disease onset), we were unable to test this possibility directly (for example, by adjusting for BMI at disease onset in the analyses). However, comparison of our linkage findings for age at diagnosis with those for BMI measures taken at the time of study recruitment fail to provide convincing evidence for colocalization of the susceptibility regions for these two phenotypes. Though there were some modest signals for BMI on chromosomes 5q and 12, even allowing for the imprecision in linkage location estimates, the overlap with those for age at diagnosis is limited. Nevertheless, without access to BMI measurements earlier in life, we cannot entirely discount the possibility that some of the linkage to age at diagnosis is mediated through the effects of variation in BMI. In summary, this study provides evidence for two loci on chromosome 12 that influence age at diagnosis of type 2 diabetes in a northern European population. For both of these, the current data add to the growing weight of evidence indicating that these regions harbor genes influencing susceptibility to, and/or development of, type 2 diabetes.
The Warren 2 collection and genome scan were funded by Diabetes U.K. S.W. is a Wellcome Trust Career Development Fellow and L.R.C. is a Wellcome Trust Principal Fellow. This work was funded in part through the NIDDK award to the International Type 2 Diabetes Linkage Analysis Consortium (U01 DK058026). We thank the physicians, nurses, and subjects who participated in the ascertainment and the researchers who contributed to the genotyping. Address correspondence and reprint requests to Prof. Mark McCarthy, Oxford Centre for Diabetes, Endocrinology and Metabolism, Churchill Site, Old Road, Headington, Oxford OX3 7LJ, U.K. E-mail: mark.mccarthy{at}drl.ox.ac.uk Received for publication September 8, 2003 and accepted in revised form November 18, 2003
Abbreviations: HECOM, Haseman-Elston; IBD, identity-by-descent; LOD, logarithm of odds; LODHECOM, HECOM LOD; LODVC, variance components LOD; QTL, quantitative trait locus
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