Familial combined hyperlipidemia (FCHL), characterized by elevated levels of serum total cholesterol, triglycerides or both 1,2 , is observed in about 20% of individuals with premature coronary heart disease 1 . We previously identified a locus linked to FCHL on 1q21-q23 in Finnish families with the disease 3 . This region has also been linked to FCHL in families from other populations 4-6 as well as to type 2 diabetes mellitus 7-12 . These clinical entities have several overlapping phenotypic features, raising the possibility that the same gene may underlie the obtained linkage results. Here, we show that the human gene encoding thioredoxin interacting protein (TXNIP) on 1q, which underlies combined hyperlipidemia in mice 13 , is not associated with FCHL. We show that FCHL is linked and associated with the gene encoding upstream transcription factor 1 (USF1) in 60 extended families with FCHL, including 721 genotyped individuals (P = 0.00002), especially in males with high triglycerides (P = 0.0000009). Expression profiles in fat biopsy samples from individuals with FCHL seemed to differ depending on their carrier status for the associated USF1 haplotype. USF1 encodes a transcription factor known to regulate several genes of glucose and lipid metabolism 14-17 .To identify the gene on 1q21 associated with FCHL, we initially sequenced four functionally relevant regional candidates: TXNIP, USF1, retinoid X receptor gamma (RXRG) and apolipoprotein A-II (APOA2). In parallel, we carried out a functionally unbiased genetic analysis of 60 single-nucleotide polymorphisms (SNPs) in 26 genes in 42 families with FCHL, including the 31 families in the original linkage study 3 . We then genotyped the ten SNPs most likely to be relevant in the extended sample of 60 families of FCHL (Supplementary Table 1 online). Fifty SNPs were located in a 5.8-Mb region flanking the peak markers D1S104 and D1S1677 (Fig. 1). All the families that we studied included a proband with severe coronary heart disease and an abnormal lipid phenotype and an average of 5-6 members affected with FCHL.We sequenced the entire TXNIP gene and the 2,000-bp upstream DNA region in 60 FCHL probands. Of the 20 SNPs identified, none resulted in amino acid changes, and all were rare, with a maximal 7% allele frequency. We also did not observe the nonsense mutation causing hyperlipidemia in mice 13 . We genotyped the four most common SNPs in the 60 families with FCHL but found no evidence of association Table 2 for distances, SNP numbers and LD clusters of these SNPs). (c) The SNPs associated with triglyceride levels in men and (d) the SNPs associated with FCHL and triglycerides in all family members.

BackgroundThe acquired component of complex traits is difficult to dissect in humans. Obesity represents such a trait, in which the metabolic and molecular consequences emerge from complex interactions of genes and environment. With the substantial morbidity associated with obesity, a deeper understanding of the concurrent metabolic changes is of considerable importance. The goal of this study was to investigate this important acquired component and expose obesity-induced changes in biological pathways in an identical genetic background.Methods and FindingsWe used a special study design of “clonal controls,” rare monozygotic twins discordant for obesity identified through a national registry of 2,453 young, healthy twin pairs. A total of 14 pairs were studied (eight male, six female; white), with a mean ± standard deviation (SD) age 25.8 ± 1.4 y and a body mass index (BMI) difference 5.2 ± 1.8 kg/m2. Sequence analyses of mitochondrial DNA (mtDNA) in subcutaneous fat and peripheral leukocytes revealed no aberrant heteroplasmy between the co-twins. However, mtDNA copy number was reduced by 47% in the obese co-twin's fat. In addition, novel pathway analyses of the adipose tissue transcription profiles exposed significant down-regulation of mitochondrial branched-chain amino acid (BCAA) catabolism (p < 0.0001). In line with this finding, serum levels of insulin secretion-enhancing BCAAs were increased in obese male co-twins (9% increase, p = 0.025). Lending clinical relevance to the findings, in both sexes the observed aberrations in mitochondrial amino acid metabolism pathways in fat correlated closely with liver fat accumulation, insulin resistance, and hyperinsulinemia, early aberrations of acquired obesity in these healthy young adults.ConclusionsOur findings emphasize a substantial role of mitochondrial energy- and amino acid metabolism in obesity and development of insulin resistance.

The authors describe a new approach to studying cellular lipid profiles and propose a compensatory mechanism that may help maintain the normal membrane function of adipocytes in the context of obesity.