Genetic diagnosis of familial hypercholesterolaemia: a mutation and a rare non-pathogenic amino acid variant in the same family.

Familial hypercholesterolaemia (FH), a relatively common inherited disorder, is caused by mutations in the gene for the low density lipoprotein (LDL) receptor (LDLR) that result in impaired clearance of LDL. Identification of mutations in patients with the clinical phenotype of FH allows unequivocal diagnosis in potentially affected relatives, but depends critically on distinguishing mutations that affect protein function from variants with no significant effect. A presumed functional mutation in LDLR (G198D in exon 4) was identified in two hypercholesterolaemic English brothers by high throughput screening and was not found in 550 controls. However, a second variant (L458P) was identified separately in their mother that co-segregated with hypercholesterolaemia in the entire pedigree. L458, but not G198, is strongly conserved between species and lies in a region important for beta-propeller stability. G198D was inherited from their normolipidaemic father by two of three siblings heterozygous for L458P; they appeared less severely hypercholesterolaemic and more responsive to statins than the third affected brother and their mother. This study emphasises that apparent co-segregation of an amino acid substitution in a critical region of the protein with hypercholesterolaemia and its absence from a large control population is insufficient evidence that a variant of the LDL receptor is necessarily deleterious to its function.

Autosomal recessive hypercholesterolaemia: long-term follow up and response to treatment.

Autosomal recessive hypercholesterolaemia (ARH) is caused by mutations in ARH on chromosome 1p35-36, encoding a putative adaptor protein. Mutations in the gene prevent normal internalisation of the low density lipoprotein (LDL) receptor by cultured lymphocytes and monocyte-derived macrophages, but not skin fibroblasts. This newly identified disorder is characterised by severe hypercholesterolaemia, large tendon, tuberous and planar xanthomas and premature atherosclerosis. We describe long-term (9-23 years) follow up and response to treatment of eight subjects with ARH from four families (Turkish/Lebanese, Indian-Asian, English and Italian). The clinical phenotype of ARH is similar to that of classical homozygous familial hypercholesterolaemia (FH) caused by mutations in the LDL-receptor gene but is more variable, less severe and is more responsive to lipid-lowering therapy with bile acid sequestrants and/or HMG-CoA reductase inhibitors. The latter reduced total serum cholesterol by up to 60% and the former by 20-35%. The cardiovascular complications of premature atherosclerosis seem to be delayed in some individuals and the involvement of the aortic root and valve are rarer in comparison with homozygous FH.

Linkage and association between distinct variants of the APOA1/C3/A4/A5 gene cluster and familial combined hyperlipidemia.

OBJECTIVE: Combined hyperlipidemia is a common disorder, characterized by a highly atherogenic lipoprotein profile and a substantially increased risk of coronary heart disease. The purpose of this study was to establish whether variations of apolipoprotein A5 (APOA5), a newly discovered gene of lipid metabolism located 30 kbp downstream of the APOA1/C3/A4 gene cluster, contributes to the transmission of familial combined hyperlipidemia (FCHL). METHODS AND RESULTS: We performed linkage and association tests on 128 families. Two independent alleles, APOA5c.56G and APOC3c.386G, of the APOA1/C3/A4/A5 gene cluster were overtransmitted in FCHL (P=0.004 and 0.007, respectively). This was paired with reduced transmission of the common APOA1/C3/A4/A5 haplotype (frequency 0.4461) to affected subjects (P=0.012). The APOA5c.56G genotype accounted for 7.3% to 13.8% of the variance in plasma triglyceride levels in probands (P<0.004). The APOC3c.386G genotypes accounted for 4.4% to 5.1% of the variance in triglyceride levels in FCHL spouses (P<0.007), suggesting that this allele marks a FCHL quantitative trait as well as representing a susceptibility locus for the condition. CONCLUSIONS: A combined linkage and association analysis establishes that variation at the APOA1/C3/A4/A5 gene cluster contributes to FCHL transmission in a substantial proportion of northern European families.

Autosomal recessive hypercholesterolemia.

Autosomal recessive hypercholesterolemia (ARH) presents with a clinical phenotype similar to that of classical homozygous familial hypercholesterolemia (FH) caused by defects in the low-density lipoprotein (LDL) receptor gene but is more variable, generally less severe, and more responsive to lipid-lowering therapy than homozygous FH; furthermore, FH is inherited with a dominant pattern. The approximately 50 known affected ARH individuals are mostly of Sardinian or Middle Eastern origin, but rare cases of ARH have occurred worldwide. The physiological defect in ARH is a failure of some, but not all, cell types to mediate LDL receptor-dependent internalization of LDL and is caused by mutations in the gene for a putative adaptor protein called ARH. In affected cells, the LDL receptor gene is normal but LDL receptor protein accumulates at the cell surface; this also occurs in livers of recombinant mice lacking ARH, providing an explanation for the failure of clearance of LDL from plasma in ARH patients. The structural features of the ARH protein and its capacity to interact with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest that it plays a key role in the LDL receptor pathway.

Confirmed locus on chromosome 11p and candidate loci on 6q and 8p for the triglyceride and cholesterol traits of combined hyperlipidemia.

UNLABELLED: Background- Combined hyperlipidemia is a common disorder characterized by a highly atherogenic lipoprotein profile and increased risk of coronary heart disease. The etiology of the lipid abnormalities (increased serum cholesterol and triglyceride or either lipid alone) is unknown. METHODS AND RESULTS: We assembled 2 large cohorts of families with familial combined hyperlipidemia (FCHL) and performed disease and quantitative trait linkage analyses to evaluate the inheritance of the lipid abnormalities. Chromosomal regions 6q16.1-q16.3, 8p23.3-p22, and 11p14.1-q12.1 produced evidence for linkage to FCHL. Chromosomes 6 and 8 are newly identified candidate loci that may respectively contribute to the triglyceride (logarithm of odds [LOD], 1.43; P=0.005) and cholesterol (LOD, 2.2; P=0.0007) components of this condition. The data for chromosome 11 readily fulfil the guidelines required for a confirmed linkage. The causative alleles may contribute to the inheritance of the cholesterol (LOD, 2.04 at 35.2 cM; P=0.0011) component of FCHL as well as the triglyceride trait (LOD, 2.7 at 48.7 cM; P=0.0002). CONCLUSIONS: Genetic analyses identify 2 potentially new loci for FCHL and provide important positional information for cloning the genes within the chromosome 11p14.1-q12.1 interval that contributes to the lipid abnormalities of this highly atherogenic disorder.

Genetics, clinical phenotype, and molecular cell biology of autosomal recessive hypercholesterolemia.

The recent characterization of a rare genetic defect causing autosomal recessive hypercholesterolemia (ARH) has provided new insights into the underlying mechanism of clathrin-mediated internalization of the LDL receptor. Mutations in ARH on chromosome 1p35-36.1 prevent normal internalization of the LDL receptor by cultured lymphocytes and monocyte-derived macrophages but not by skin fibroblasts. In affected cells, LDL receptor protein accumulates at the cell surface; this also occurs in the livers of recombinant mice lacking ARH, thereby providing an explanation for the failure of clearance of LDL from the plasma in subjects lacking ARH. The approximately 50 known affected individuals are mostly of Sardinian or Middle Eastern origin. The clinical phenotype of ARH is similar to that of classic homozygous familial hypercholesterolemia caused by defects in the LDL receptor gene, but it is more variable, generally less severe, and more responsive to lipid-lowering therapy. Structural features of the ARH protein and its capacity to interact simultaneously with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest how ARH can play a pivotal role in gathering the LDL receptor into forming endocytic carrier vesicles.

Mutations in a Sar1 GTPase of COPII vesicles are associated with lipid absorption disorders.

Dietary fat is an important source of nutrition. Here we identify eight mutations in SARA2 that are associated with three severe disorders of fat malabsorption. The Sar1 family of proteins initiates the intracellular transport of proteins in COPII (coat protein)-coated vesicles. Our data suggest that chylomicrons, which vastly exceed the size of typical COPII vesicles, are selectively recruited by the COPII machinery for transport through the secretory pathways of the cell.

Restoration of LDL receptor function in cells from patients with autosomal recessive hypercholesterolemia by retroviral expression of ARH1.

Familial hypercholesterolemia is an autosomal dominant disorder with a gene-dosage effect that is usually caused by mutations in the LDL receptor gene that disrupt normal clearance of LDL. In the homozygous form, it results in a distinctive clinical phenotype, characterized by inherited hypercholesterolemia, cholesterol deposition in tendons, and severe premature coronary disease. We described previously two families with autosomal recessive hypercholesterolemia that is not due to mutations in the LDL receptor gene but is characterized by defective LDL receptor-dependent internalization and degradation of LDL by transformed lymphocytes from the patients. We mapped the defective gene to chromosome 1p36 and now show that the disorder in these and a third English family is due to novel mutations in ARH1, a newly identified gene encoding an adaptor-like protein. Cultured skin fibroblasts from affected individuals exhibit normal LDL receptor activity, but their monocyte-derived macrophages are similar to transformed lymphocytes, being unable to internalize and degrade LDL. Retroviral expression of normal human ARH1 restores LDL receptor internalization in transformed lymphocytes from an affected individual, as demonstrated by uptake and degradation of (125)I-labeled LDL and confocal microscopy of cells labeled with anti-LDL-receptor Ab.

A 6-month randomized trial of thyroxine treatment in women with mild subclinical hypothyroidism.

PURPOSE: The role of thyroxine replacement in subclinical hypothyroidism remains unclear. We performed a 6-month randomized, double-blind, placebo-controlled trial to evaluate the effects of thyroxine treatment for mild subclinical hypothyroidism, defined as a serum thyroid-stimulating hormone level between 5 to 10 microU/mL with a normal serum free thyroxine level (0.8-16 ng/dL). SUBJECTS AND METHODS: We randomly assigned 40 women with mild subclinical hypothyroidism who had presented to their family practitioners to either thyroxine treatment (n = 23; 50 to 100 microg daily) or placebo (n = 17). Health-related quality of life (Hospital Anxiety and Depression scale, 30-item General Health Questionnaire), fasting lipid profiles, body weight, and resting energy expenditure were measured at baseline and 6 months. RESULTS: The most common presenting symptoms were fatigue (n = 33 [83%]) and weight gain (n = 32 [80%]). At presentation, 20 women (50%) had elevated anxiety scores and 22 (56%) had elevated scores on the General Health Questionnaire. Thirty-five women completed the study. There were no significant differences in the changes from baseline to 6 months between women in the thyroxine group and the placebo group for any of the metabolic, lipid, or anthropometric variables measured, expressed as the mean change in the thyroxine group minus the mean change in the placebo group: body mass index, -0.3 kg/m(2) (95% confidence interval [CI]: -0.9 to 0.4 kg/m(2)); resting energy expenditure, -0.2 kcal/kg/24 h (95% CI: -1.3 to 1.0 kcal/kg/24 h); and low-density lipoprotein cholesterol, -4 mg/dL (95% CI: -23 to 15 mg/dL). There was a significant worsening in anxiety scores in the thyroxine group (scores increased in 8 of 20 women and were unchanged in 2 of 20) compared with the placebo group (scores increased in 1 of 14 women and were unchanged in 6 of 14; P = 0.03). CONCLUSIONS; We observed no clinically relevant benefits from 6 months of thyroxine treatment in women with mild subclinical hypothyroidism.