Metabolic syndrome is a cluster of clinical disturbances (abdominal obesity, hypertension, dyslipidemia, and impaired glucose tolerance/insulin resistance) associated with an increased risk for diabetes mellitus and cardiovascular disease. Mild hyperhomocysteinemia, a common finding in patients with arteriosclerosis, has been described as a possible component of metabolic syndrome.1,2 Because folates and B vitamins modulate metabolism of homocysteine and of other sulfur amino acids (eg, cysteine, methionine, and cystathionine), mild hyperhomocysteinemia may be mediated by nutritional or genetically determined deficiencies of these vitamins. There is a growing body of epidemiological and clinical evidence for an important role of folate deficiency in human metabolic disturbances and hypertension. [3][4][5] We have shown previously that nutritional folate deficiency leads to development of features of the metabolic syndrome in the spontaneously hypertensive rat (SHR), 6 a widely studied model of essential hypertension associated with increased risk for insulin resistance and dyslipidemia. In this study, in the BXH/HXB recombinant inbred (RI) strains derived from the SHR strain and the normotensive Brown Norway (BN) strain, 7 we used a combination of linkage and correlational analyses of physiological traits and gene expression levels to investigate genetic factors influencing folate and sulfur amino acid metabolism and risk for multiple features of the metabolic syndrome.
Methods
AnimalsSHR/OlaIpcv rats (referred to as the SHR strain), BN-Lx/Cub rats (referred to as the BN strain), BXH/HXB RI strains 7 derived from the SHR and BN progenitor strains, SHR.BN-D1Mit3/Igf2 congenic Abstract-Metabolism of homocysteine and other sulfur amino acids is closely associated with metabolism of folates. In this study, we analyzed the possible role of folates and sulfur amino acids in the development of features of the metabolic syndrome in the BXH/HXB recombinant inbred strains derived from the spontaneously hypertensive rat (SHR) and Brown Norway progenitors. We mapped a quantitative trait locus for cysteine concentrations to a region of chromosome 1 that contains a cis-acting expression quantitative trait locus regulating mRNA levels of folate receptor 1 (Folr1) in the kidney. Sequence analysis revealed a deletion variant in the Folr1 promoter region of the SHR. Transfection studies demonstrated that the SHR-promoter region of Folr1 is less effective in driving luciferase reporter gene expression than the Brown Norway promoter region of Folr1. Results in the SHR.BN-chr.1 congenic strain confirmed that the SHR variant in Folr1 cosegregates with markedly reduced renal expression of Folr1 and renal folate reabsorption, decreased serum levels of folate, increased serum levels of cysteine and homocysteine, increased adiposity, ectopic fat accumulation in liver and muscle, reduced muscle insulin sensitivity, and increased blood pressure. Transgenic rescue experiments performed by expressing a Folr1 transgene in the SHR ameliorated ...