BMI loci and longitudinal BMI from adolescence to young adulthood in an ethnically diverse cohort.

OBJECTIVE: The association of obesity susceptibility variants with change in body mass index (BMI) across the life course is not well understood. SUBJECTS: In ancestry-stratified models of 5962 European American (EA), 2080 African American (AA) and 1582 Hispanic American (HA) individuals from the National Longitudinal Study of Adolescent to Adult Health (Add Health), we examined associations between 34 obesity single-nucleotide polymorphisms (SNPs) with per year change in BMI, measured by the slope from a growth-curve analysis of two or more BMI measurements between adolescence and young adulthood. For SNPs nominally associated with BMI change (P<0.05), we interrogated age differences within data collection Wave and time differences between age categories that overlapped between Waves. RESULTS: We found SNPs in/near FTO, MC4R, MTCH2, TFAP2B, SEC16B and TMEM18 were significantly associated (P<0.0015≈0.05/34) with BMI change in EA and the ancestry-combined meta-analysis. rs9939609 in FTO met genome-wide significance at P<5e-08 in the EA and ancestry-combined analysis, respectively [Beta(se)=0.025(0.004);Beta(se)=0.021(0.003)]. No SNPs were significant after Bonferroni correction in AA or HA, although five SNPs in AA and four SNPs in HA were nominally significant (P<0.05). In EA and the ancestry-combined meta-analysis, rs3817334 near MTCH2 showed larger effects in younger respondents, whereas rs987237 near TFAP2B, showed larger effects in older respondents across all Waves. Differences in effect estimates across time for MTCH2 and TFAP2B are suggestive of either era or cohort effects. CONCLUSION: The observed association between variants in/near FTO, MC4R, MTCH2, TFAP2B, SEC16B and TMEM18 with change in BMI from adolescence to young adulthood suggest that the genetic effect of BMI loci varies over time in a complex manner, highlighting the importance of investigating loci influencing obesity risk across the life course.

The interaction between physical activity and obesity gene variants in association with BMI: Does the obesogenic environment matter?

Little is known about how obesity susceptibility single nucleotide polymorphisms (SNPs) interact with moderate to vigorous physical activity (MVPA) in relation to BMI during adolescence, once obesogenic neighborhood factors are accounted for. In race stratified models, including European (EA; N=4977), African (AA; N=1726), and Hispanic Americans (HA; N=1270) from the National Longitudinal Study of Adolescent to Adult Health (1996; ages 12-21), we assessed the evidence for a SNPxMVPA interaction with BMI-for-age Z score, once accounting for obesogenic neighborhood factors including physical activity amenities, transportation and recreation infrastructure, poverty and crime. Eight SNPxMVPA interactions with suggestive significance (p<0.10; three in each EA, and AA, two in HA) were observed showing attenuation on BMI-for-age Z score in adolescents with ≥5 versus <5 bouts/week MVPA, except for rs10146997 (near NRXN3). Findings were robust to the inclusion of neighborhood-level variables as covariates. These findings suggest that any attenuation from MVPA on a genetic susceptibility to obesity during adolescence is likely not operating through obesogenic neighborhood factors.

Genetic risk score and adiposity interact to influence triglyceride levels in a cohort of Filipino women.

BACKGROUND/OBJECTIVES: Individually, genetic variants only moderately influence cardiometabolic (CM) traits, such as lipid and inflammatory markers. In this study we generated genetic risk scores from a combination of previously reported variants influencing CM traits, and used these scores to explore how adiposity levels could mediate genetic contributions to CM traits. SUBJECTS/METHODS: Participants included 1649 women from the 2005 Cebu Longitudinal Health and Nutrition Survey. Three genetic risk scores were constructed for C-reactive protein (CRP), high-density lipoprotein cholesterol (HDL-C) and triglycerides (TGs). We used linear regression models to assess the association between each genetic risk score and its related trait. We also tested for interactions between each score and measures of adiposity. RESULTS: Each genetic risk score explained a greater proportion of variance in trait levels than any individual genetic variant. We found an interaction between the TG genetic risk score (2.29-14.34 risk alleles) and waist circumference (WC) (Pinteraction=1.66 × 10(-2)). Based on model predictions, for individuals with a higher TG genetic risk score (75th percentile=12), having an elevated WC (⩾80 cm) increased TG levels from 1.32 to 1.71 mmol l(-1). However, for individuals with a lower score (25th percentile=7), having an elevated WC did not significantly change TG levels. CONCLUSIONS: The TG genetic risk score interacted with adiposity to synergistically influence TG levels. For individuals with a genetic predisposition to elevated TG levels, our results suggest that reducing adiposity could possibly prevent further increases in TG levels and thereby lessen the likelihood of adverse health outcomes such as cardiovascular disease.

Moderate to vigorous physical activity interactions with genetic variants and body mass index in a large US ethnically diverse cohort.

BACKGROUND: Little is known about the interaction between genetic and behavioural factors during lifecycle risk periods for obesity and how associations vary across race/ethnicity. OBJECTIVE: The objective of this study was to examine joint associations of adiposity-related single-nucleotide polymorphisms (SNPs) and moderate to vigorous physical activity (MVPA) with body mass index (BMI) in a diverse adolescent cohort. METHODS: Using data from the National Longitudinal Study of Adolescent Health (n = 8113: Wave II 1996; ages 12-21, Wave III; ages 18-27), we assessed interactions of 41 well-established SNPs and MVPA with BMI-for-age Z-scores in European Americans (EA; n = 5077), African-Americans (AA; n = 1736) and Hispanic Americans (HA; n = 1300). RESULTS: Of 97 assessed, we found nominally significant SNP-MVPA interactions on BMI-for-age Z-score in EA at GNPDA2 and FTO and in HA at LZTR2/SEC16B. In EA, the estimated effect of the FTO risk allele on BMI-for-age Z-score was lower (ß = -0.13; 95% confidence interval [CI]: 0.08, 0.18) in individuals with ≥5 vs. <5 (ß = 0.24; CI: 0.16, 0.32) bouts of MVPA per week (P for interaction 0.02). Race/ethnicity-pooled meta-analysis showed nominally significant interactions for SNPs at TFAP2B, POC5 and LYPLAL1. CONCLUSIONS: High MVPA may attenuate underlying genetic risk for obesity during adolescence, a high-risk period for adult obesity.

Screen time behaviours may interact with obesity genes, independent of physical activity, to influence adolescent BMI in an ethnically diverse cohort.

BACKGROUND: There has been little investigation of gene-by-environment interactions related to sedentary behaviour, a risk factor for obesity defined as leisure screen time (ST; i.e. television, video and computer games). OBJECTIVE: To test the hypothesis that limiting ST use attenuates the genetic predisposition to increased body mass index (BMI), independent of physical activity. DESIGN: Using 7642 wave II participants of the National Longitudinal Study of Adolescent Health, (Add Health; mean = 16.4 years, 52.6% female), we assessed the interaction of ST (h week(-1) ) and 41 established obesity single nucleotide polymorphisms (SNPs) with age- and sex-specific BMI Z-scores in 4788 European-American (EA), 1612 African-American (AA) and 1242 Hispanic American (HA) adolescents. RESULTS: Nominally significant SNP*ST interaction were found for FLJ35779 in EA, GNPDA2 in AA and none in HA (EA: beta [SE] = 0.016[0.007]), AA: beta [SE] = 0.016[0.011]) per 7 h week(-1) ST and one risk allele in relation to BMI Z-score. CONCLUSIONS: While for two established BMI loci, we find evidence that high levels of ST exacerbate the influence of obesity susceptibility variants on body mass; overall, we do not find strong evidence for interactions between the majority of established obesity loci. However, future studies with larger sample sizes, or that may build on our current study and the growing published literature, are clearly warranted.

Replication of LIN28B SNP association with age of menarche in young Filipino women.

BACKGROUND: Age of menarche, or the timing of first menses in girls, is a physiological trait that shows substantial genetic heritability. Earlier age of menarche is associated with increased childhood adiposity and with adult risk of obesity and cardiovascular disease. OBJECTIVES: We sought to further characterize the single nucleotide polymorphism (SNP) rs7759938 from the menarche locus LIN28B in 827 young Filipino women from the Cebu Longitudinal Health and Nutrition Survey (CLHNS). METHODS: We tested rs7759938 for additive association with age of menarche and also tested whether childhood adiposity, as measured by body mass index (BMI) at age 8, mediated this relationship. RESULTS: We observed nominal association of rs7759938 with age of menarche (ß = -0.118 years, 95% confidence interval = (-0.216, -0.020), P = 0.019) with an effect direction consistent with the previous report. We also observed suggestive evidence that the effect of the SNP on age of menarche was independent of childhood BMI. CONCLUSIONS: These data confirm the strongest gene reported in Europeans (LIN28B) as a contributor to age of menarche in an Asian population.

Estimation of genetic effects on BMI during adolescence in an ethnically diverse cohort: The National Longitudinal Study of Adolescent Health.

OBJECTIVE: The contribution of genetic variants to body mass index (BMI) during adolescence across multiethnic samples is largely unknown. We selected genetic loci associated with BMI or obesity in European-descent samples and examined them in a multiethnic adolescent sample. DESIGN AND SAMPLE: In 5103 European American (EA), 1748 African American (AfA), 1304 Hispanic American (HA) and 439 Asian American (AsA) participants of the National Longitudinal Study of Adolescent Health (Add Health; ages 12-21 years, 47.5% male), we assessed the association between 41 established obesity-related single-nucleotide polymorphisms (SNPs) with BMI using additive genetic models, stratified by race/ethnicity, and in a pooled meta-analysis sample. We also compared the magnitude of effect for BMI-SNP associations in EA and AfA adolescents to comparable effect estimates from 11 861 EA and AfA adults in the Atherosclerosis Risk in Communities study (ages 45-64 years, 43.2% male). RESULTS: Thirty-five of 41 BMI-SNP associations were directionally consistent with published studies in European populations, 18 achieved nominal significance (P<0.05; effect sizes from 0.19 to 0.71 kg m(-2) increase in BMI per effect allele), while 4 (FTO, TMEM18, TFAP2B, MC4R) remained significant after Bonferroni correction (P<0.0015). Of 41 BMI-SNP associations in AfA, HA and AsA adolescents, nine, three and five, respectively, were directionally consistent and nominally significant. In the pooled meta-analysis, 36 of 41 effect estimates were directionally consistent and 21 of 36 were nominally significant. In EA adolescents, BMI effect estimates were larger (P<0.05) for variants near TMEM18, PTER and MC4R and smaller for variants near MTIF3 and NRXN3 compared with EA adults. CONCLUSION: Our findings suggest that obesity susceptibility loci may have a comparatively stronger role during adolescence than during adulthood, with variation across race/ethnic subpopulation.

A mechanism for low penetrance in an ALS family with a novel SOD1 deletion.

BACKGROUND: About 20% of familial amyotrophic lateral sclerosis (ALS) is caused by mutations in SOD1 and is typically transmitted as an autosomal dominant trait. However, due to reduced mutation penetrance, the disease may present in a recessive or sporadic manner. OBJECTIVE: To determine the factors responsible for the low penetrance of the SOD1 mutation. METHODS: Twelve members of a Canadian ALS family of Filipino origin were recruited for the study. SOD1 was sequenced in the proband. SOD1 expression was assessed by real-time-PCR and immunoblotting. RESULTS: The proband was a homozygous carrier of a novel 6 bp deletion in exon 2 (DeltaG27/P28), the pathologic significance of which was confirmed by immunohistochemistry. Eight living family members are heterozygotes and remain unaffected at ages ranging between 48 and 85 years. Haplotype analysis showed that the deletion is a single founder mutation likely common in the Cagayan province (Philippines). The low penetrance of the mutation is explained by the fact that it enhances the naturally occurring alternative splicing of exon 2 of the SOD1 mRNA, leading to reduced transcription of the mutant allele. Indeed, Western blot analysis demonstrated the low level of SOD1 protein in carriers of the DeltaG27/P28 compared to wild-type individuals or a carrier of the A4V SOD1 mutation. CONCLUSION: The enhanced splicing of exon 2 acts as a natural knock-down of the mutant SOD1 allele in the Filipino amyotrophic lateral sclerosis (ALS) family. There is a need for careful investigation of splicing isoforms of SOD1 and other ALS genes as factors influencing the severity of disease.

Replication of the association between variants in WFS1 and risk of type 2 diabetes in European populations.

AIMS/HYPOTHESIS: Mutations at the gene encoding wolframin (WFS1) cause Wolfram syndrome, a rare neurological condition. Associations between single nucleotide polymorphisms (SNPs) at WFS1 and type 2 diabetes have recently been reported. Thus, our aim was to replicate those associations in a northern Swedish case-control study of type 2 diabetes. We also performed a meta-analysis of published and previously unpublished data from Sweden, Finland and France, to obtain updated summary effect estimates. METHODS: Four WFS1 SNPs (rs10010131, rs6446482, rs752854 and rs734312 [H611R]) were genotyped in a type 2 diabetes case-control study (n = 1,296/1,412) of Swedish adults. Logistic regression was used to assess the association between each WFS1 SNP and type 2 diabetes, following adjustment for age, sex and BMI. We then performed a meta-analysis of 11 studies of type 2 diabetes, comprising up to 14,139 patients and 16,109 controls, to obtain a summary effect estimate for the WFS1 variants. RESULTS: In the northern Swedish study, the minor allele at rs752854 was associated with reduced type 2 diabetes risk [odds ratio (OR) 0.85, 95% CI 0.75-0.96, p=0.010]. Borderline statistical associations were observed for the remaining SNPs. The meta-analysis of the four independent replication studies for SNP rs10010131 and correlated variants showed evidence for statistical association (OR 0.87, 95% CI 0.82-0.93, p=4.5 x 10(-5)). In an updated meta-analysis of all 11 studies, strong evidence of statistical association was also observed (OR 0.89, 95% CI 0.86-0.92; p=4.9 x 10(-11)). CONCLUSIONS/INTERPRETATION: In this study of WFS1 variants and type 2 diabetes risk, we have replicated the previously reported associations between SNPs at this locus and the risk of type 2 diabetes.

Variation in the resistin gene is associated with obesity and insulin-related phenotypes in Finnish subjects.

AIMS/HYPOTHESIS: Resistin is a peptide hormone produced by adipocytes that is present at high levels in sera of obese mice and may be involved in glucose homeostasis through regulation of insulin sensitivity. Several studies in humans have found associations between polymorphisms in the resistin gene and obesity, insulin sensitivity and blood pressure. An association between variation in the resistin gene and type 2 diabetes has been reported in some, but not all studies. The aim of this study was to analyse variants of the resistin gene for association with type 2 diabetes and related traits in a Finnish sample. METHODS: In 781 cases with type 2 diabetes, 187 spouse controls and 222 elderly controls of Finnish origin, we genotyped four previously identified non-coding single-nucleotide polymorphisms (SNPs): -420C>G from the promoter region, +156C>T and +298G>A from intron 2, and +1084G>A from the 3' untranslated region. We then tested whether these SNPs were associated with type 2 diabetes and related traits. RESULTS: The SNPs were not significantly associated with type 2 diabetes. However, SNPs -420C>G, +156C>T and +298G>A and the common haplotype for these three markers were associated with increased values of weight-related traits and diastolic blood pressure in cases, lower weight in elderly control subjects, and lower insulin sensitivity and greater acute insulin response in spouses. Furthermore, the +1084G allele was associated with lower HDL cholesterol in both cases and controls, higher systolic blood pressure and waist circumference in cases, and greater acute insulin response in spouse controls. CONCLUSIONS/INTERPRETATION: Our results add to growing evidence that resistin is associated with variation in weight, fat distribution and insulin resistance.

Linkage disequilibrium between microsatellite markers extends beyond 1 cM on chromosome 20 in Finns.

Linkage disequilibrium (LD) is a proven tool for evaluating population structure and localizing genes for monogenic disorders. LD-based methods may also help localize genes for complex traits. We evaluated marker-marker LD using 43 microsatellite markers spanning chromosome 20 with an average density of 2.3 cM. We studied 837 individuals affected with type 2 diabetes and 386 mostly unaffected spouse controls. A test of homogeneity between the affected individuals and their spouses showed no difference, allowing the 1223 individuals to be analyzed together. Significant (P < 0.01) LD was observed using a likelihood ratio test in all (11/11) marker pairs within 1 cM, 78% (25/32) of pairs 1-3 cM apart, and 39% (7/18) of pairs 3-4 cM apart, but for only 12 of 842 pairs more than 4 cM apart. We used the human genome project working draft sequence to estimate kilobase (kb) intermarker distances, and observed highly significant LD (P < 10(-10)) for all six marker pairs up to 350 kb apart, although the correlation of LD with cM is slightly better than the correlation with megabases. These data suggest that microsatellites present at 1-cM density are sufficient to observe marker-marker LD in the Finnish population.

The peroxisome proliferator-activated receptor-gamma2 Pro12A1a variant: association with type 2 diabetes and trait differences.

Recent studies have identified a common proline-to-alanine substitution (Pro12Ala) in the peroxisome proliferator-activated receptor-gamma2 (PPAR-gamma2), a nuclear receptor that regulates adipocyte differentiation and possibly insulin sensitivity. The Pro12Ala variant has been associated in some studies with diabetes-related traits and/or protection against type 2 diabetes. We examined this variant in 935 Finnish subjects, including 522 subjects with type 2 diabetes, 193 nondiabetic spouses, and 220 elderly nondiabetic control subjects. The frequency of the Pro12Ala variant was significantly lower in diabetic subjects than in nondiabetic subjects (0.15 vs. 0.21; P = 0.001). We also compared diabetes-related traits between subjects with and without the Pro12Ala variant within subgroups. Among diabetic subjects, the variant was associated with greater weight gain after age 20 years (P = 0.023) and lower triglyceride levels (P = 0.033). Diastolic blood pressure was higher in grossly obese (BMI >40 kg/m2) diabetic subjects with the variant. In nondiabetic spouses, the variant was associated with higher fasting insulin (P = 0.033), systolic blood pressure (P = 0.021), and diastolic blood pressure (P = 0.045). These findings support a role for the PPAR-gamma2 Pro12Ala variant in the etiology of type 2 diabetes and the insulin resistance syndrome.

The Finland-United States investigation of non-insulin-dependent diabetes mellitus genetics (FUSION) study. I. An autosomal genome scan for genes that predispose to type 2 diabetes.

We performed a genome scan at an average resolution of 8 cM in 719 Finnish sib pairs with type 2 diabetes. Our strongest results are for chromosome 20, where we observe a weighted maximum LOD score (MLS) of 2.15 at map position 69.5 cM from pter and secondary weighted LOD-score peaks of 2.04 at 56.5 cM and 1.99 at 17.5 cM. Our next largest MLS is for chromosome 11 (MLS = 1.75 at 84.0 cM), followed by chromosomes 2 (MLS = 0.87 at 5.5 cM), 10 (MLS = 0.77 at 75.0 cM), and 6 (MLS = 0.61 at 112.5 cM), all under an additive model. When we condition on chromosome 2 at 8.5 cM, the MLS for chromosome 20 increases to 5.50 at 69.0 cM (P=.0014). An ordered-subsets analysis based on families with high or low diabetes-related quantitative traits yielded results that support the possible existence of disease-predisposing genes on chromosomes 6 and 10. Genomewide linkage-disequilibrium analysis using microsatellite marker data revealed strong evidence of association for D22S423 (P=.00007). Further analyses are being carried out to confirm and to refine the location of these putative diabetes-predisposing genes.

The Finland-United States investigation of non-insulin-dependent diabetes mellitus genetics (FUSION) study. II. An autosomal genome scan for diabetes-related quantitative-trait loci.

Type 2 diabetes mellitus is a complex disorder encompassing multiple metabolic defects. We report results from an autosomal genome scan for type 2 diabetes-related quantitative traits in 580 Finnish families ascertained for an affected sibling pair and analyzed by the variance components-based quantitative-trait locus (QTL) linkage approach. We analyzed diabetic and nondiabetic subjects separately, because of the possible impact of disease on the traits of interest. In diabetic individuals, our strongest results were observed on chromosomes 3 (fasting C-peptide/glucose: maximum LOD score [MLS] = 3.13 at 53.0 cM) and 13 (body-mass index: MLS = 3.28 at 5.0 cM). In nondiabetic individuals, the strongest results were observed on chromosomes 10 (acute insulin response: MLS = 3.11 at 21.0 cM), 13 (2-h insulin: MLS = 2.86 at 65.5 cM), and 17 (fasting insulin/glucose ratio: MLS = 3.20 at 9.0 cM). In several cases, there was evidence for overlapping signals between diabetic and nondiabetic individuals; therefore we performed joint analyses. In these joint analyses, we observed strong signals for chromosomes 3 (body-mass index: MLS = 3.43 at 59.5 cM), 17 (empirical insulin-resistance index: MLS = 3.61 at 0.0 cM), and 19 (empirical insulin-resistance index: MLS = 2.80 at 74.5 cM). Integrating genome-scan results from the companion article by Ghosh et al., we identify several regions that may harbor susceptibility genes for type 2 diabetes in the Finnish population.

The molecular basis of von Willebrand disease.

von Willebrand disease (VWD) is a clinically heterogeneous bleeding disorder that reflects a wide array of defects. Quantitative subtypes of the disorder, including types 1 and 3 VWD, result in bleeding due to reduced levels of circulating von Willebrand factor (VWF) protein. Qualitative subtypes, defined as type 2 VWD, act through altered VWF function. A range of molecular defects are responsible for many of these subtypes, including missense, nonsense, splicing, insertion, and deletion mutations, resulting in either dominant or recessive inheritance. While many mutations correspond to selected variants, the basis for variation in expression and the imperfect correlations between genotype and phenotype remain to be understood.

Mvwf, a dominant modifier of murine von Willebrand factor, results from altered lineage-specific expression of a glycosyltransferase.

We have identified altered lineage-specific expression of an N-acetylgalactosaminyltransferase gene, Galgt2, as the gain-of-function mechanism responsible for the action of the Mvwf locus, a major modifier of plasma von Willebrand factor (VWF) level in RIIIS/J mice. A switch of Galgt2 gene expression from intestinal epithelial cell-specific to a pattern restricted to the vascular endothelial cell bed leads to aberrant posttranslational modification and rapid clearance of VWF from plasma. Transgenic expression of Galgt2 directed to vascular endothelial cells reproduces the low VWF phenotype, confirming this switch in lineage-specific gene expression as the likely molecular mechanism for Mvwf. These findings identify alterations in glycosyltransferase function as a potential general mechanism for the genetic modification of plasma protein levels.

Comparative mapping of distal murine chromosome 11 and human 17q21.3 in a region containing a modifying locus for murine plasma von Willebrand factor level.

Type 1 von Willebrand disease (VWD) is a common inherited disorder characterized by mild to moderate bleeding and reduced levels of von Willebrand factor (VWF). An animal model for human type 1 VWD, the RIIIS/J mouse strain, exhibits a prolonged bleeding time and reduced plasma VWF levels. We have previously mapped the defect in RIIIS/J to distal mouse Chr 11, distinct from the Vwf locus on Chr 6. This locus, Mvwf, was localized to an approximately 0.5-cM interval, tightly linked to Gip, distal to Ngfr, and proximal to Hoxb. We have now used these genetic markers to construct a contig of yeast and bacterial artificial chromosomes and bacteriophage P1 clones spanning the approximately 300-kb Mvwf nonrecombinant interval. In a comparative mapping approach, mouse homologues of mapped human expressed sequence tags (ESTs) were localized relative to the candidate interval. Twenty-one sequence-tagged sites and ESTs from the corresponding human syntenic region 17q21.3 were ordered using the high-resolution Stanford TNG3 radiation hybrid panel. Based on the resulting radiation hybrid map and our mouse genetic and physical maps, the order of human and mouse genes in a >0.7-cM region appears to be conserved. Six genes localized to the Mvwf nonrecombinant interval by comparative mapping included orthologs of GNGT2, ATP6N1, and a nuclear domain protein. Seven other genes or ESTs were excluded from the candidate interval, including orthologs of PHB, PDK2, a speckle-type protein, and a UDP-galactose transporter. Using exon trapping, 10 additional putative expressed sequences were identified within the Mvwf nonrecombinant interval, including a previously cloned murine glycosyltransferase as well as exons showing sequence similarity to genes for Caenorhabditis elegans and Saccharomyces cerevisiae predicted proteins, an Arabidopsis thaliana ubiquitin-conjugating enzyme, and a Gallus gallus mRNA zipcode-binding protein. Further characterization of these putative genes could identify the dominant mutation responsible for low plasma VWF levels in RIIIS/J mice. These data may also aid in the localization of other disease loci mapped to this region, including the gene for tricho-dento-osseous syndrome and a murine locus for susceptibility to ozone-induced acute lung injury.

A novel modifier gene for plasma von Willebrand factor level maps to distal mouse chromosome 11.

Type 1 von Willebrand disease (VWD), characterized by reduced levels of plasma von Willebrand factor (VWF), is the most common inherited bleeding disorder in humans. Penetrance of VWD is incomplete, and expression of the bleeding phenotype is highly variable. In addition, plasma VWF levels vary widely among normal individuals. To identify genes that influence VWF level, we analyzed a genetic cross between RIIIS/J and CASA/Rk, two strains of mice that exhibit a 20-fold difference in plasma VWF level. DNA samples from F2 progeny demonstrating either extremely high or extremely low plasma VWF levels were pooled and genotyped for 41 markers spanning the autosomal genome. A novel locus accounting for 63% of the total variance in VWF level was mapped to distal mouse chromosome 11, which is distinct from the murine Vwf locus on chromosome 6. We designated this locus Mvwf for "modifier of VWF." Additional genotyping of as many as 2407 meioses established a high resolution genetic map with gene order Cola1-Itg3a-Ngfr-Mvwf/Gip-Hoxb9-Hoxb1++ +-Cbx'rs2-Cox5a-Gfap. The Mvwf candidate interval between Ngfr and Hoxb9 is approximately 0.5 centimorgan (cM). These results demonstrate that a single dominant gene accounts for the low VWF phenotype of RIIIS/J mice in crosses with several other strains. The pattern of inheritance suggests a gain-of-function mutation in a unique component of VWF biosynthesis or processing. Characterization of the human homologue for Mvwf may have relevance for a subset of type 1 VWD cases and may define an important genetic factor modifying penetrance and expression of mutations at the VWF locus.