Down regulation of CD11b and CD18 expression in children with hypercholesterolemia: a preliminary report

BACKGROUND AND AIM

Cell adhesion molecules play an important role in the development of atherosclerosis mediating the attachment of monocytes to the endothelium. The aim of our study was to assess the cell surface expression of CD11b/CD18 integrin on the phagocytes of children affected by hypercholesterolemia.

METHODS AND RESULTS

Twenty-six children with hypercholesterolemia (15 males, mean age 8.3, range 2-18) with a family history of early cardiovascular disease, as well as 26 children with normocholesterolemia matched for gender and age (15 males, mean age 8.3) were studied. Cell surface expression of CD11b/CD18 on peripheral blood mononuclear cells (PBMC) were analyzed by flow cytometry. The geometric mean percentages of CD11b and CD18 expression were significantly lower in the hypercholesterolemic group [52 (95% confidence intervals, 40-68) and 88 (84-93)] than in the control group [87 (83-91), P<0.0001 and 93 (89-96), P<0.05], respectively. After correction for age, gender, and pubertal status, CD11b cell surface expression on PBMC was inversely and independently correlated with total cholesterol concentrations (r=-0.395; P<0.01) and LDL (r=-0.307; P<0.05), as well as with triglycerides (r=-0.406; P<0.01).

CONCLUSIONS

In children with hypercholesterolemia, cell surface expression of CD11b and CD18 on PBMC was significantly decreased. Follow-up studies are necessary to determine the clinical implications of these findings in the context of the natural course and progression of atherosclerosis in high risk children.

A novel mutation of the extracellular matrix protein 1 gene (ECM1) in a patient with lipoid proteinosis (Urbach-Wiethe disease) from Sicily

BACKGROUND

Lipoid proteinosis (LP), also known as Urbach-Wiethe disease, is a rare autosomal recessive disorder characterized by a hoarse voice, warty skin infiltration and scarring. Mutations within the extracellular matrix protein 1 (ECM1) gene cause LP.

OBJECTIVES

We report the molecular analysis of the ECM1 gene in a Sicilian patient with LP in order to extend the mutation spectrum of this genodermatosis.

METHODS

We studied a 32-year-old female born from consanguineous parents who was diagnosed at the age of 11 years as having LP. She has a clinical phenotype corresponding to Urbach-Wiethe disease characterized by papules/nodules, indurated plaques and sometimes ulcerated lesions primarily involving the skin and mucous membranes, and extracutaneous features such as epilepsy, hoarseness of the voice and neuropsychiatric abnormalities. Samples of clinically affected skin obtained by biopsies were analysed after staining with haematoxylin and eosin, periodic acid-Schiff (PAS), and PAS-diastase. The whole ECM1 gene was analysed by direct sequencing.

RESULTS

We identified a homozygous nonsense mutation in exon 6 of the ECM1 gene, C589T (Q197Ter).

CONCLUSIONS

Over 60% of mutations occur in exons 6 and 7. Exon 7 is alternatively spliced and frameshift mutations in exon 7 lead to ablation of the ECM1a transcript, but not the shorter ECM1b transcript that normally lacks this exon. Homozygous nonsense or frameshift mutations in exon 6 are predicted to affect both full-length ECM1a and ECM1b transcripts, whereas ECM1b should be unaffected for similar types of mutation in exon 7. It has been suggested that individuals with mutations in exon 7 have a slightly milder phenotype than those with exon 6 mutations. This is the first report with respect to a novel mutation of the ECM1 gene responsible for recessive LP in Sicily.

Regulation of the apolipoprotein B in heterozygous hypobetalipoproteinemic knock-out mice expressing truncated apoB, B81. Low production and enhanced clearance of apoB cause low levels of apoB

Low levels of cholesterol are protective against development of coronary artery disease. Heterozygous hypobetalipoproteinemic individuals expressing truncated apolipoprotein (apo)B as a result of mutation in the apob gene have low levels of cholesterol and apoB in their plasma. To study the molecular mechanism of low levels of apoB in these individuals, we employed a previously reported knock out mouse model generated by targeted modification of the apob gene. The heterozygous, apoB-100/B-81, mice express full length and truncated apoB, B-81, and have 20 and 35% lower levels of total cholesterol and apoB, respectively, when compared to WT (apoB-100/B-100) mice. The majority of the truncated apoB, B-81, fractionated in the VLDL- density range. The mechanism of low levels of apoB in B-100/B-81 mice was examined. Total hepatic apoB mRNA levels decreased by 15%, primarily due to lower levels of apoB-81 mRNA. Since apoB mRNA transcription rates were similar in B-100/B-100 and B-100/B-81 mice, low levels of mutant apoB-81 mRNA occurred by enhanced degradation of apoB mRNA transcript containing premature translational stop codon. ApoB synthesis measured on isolated hepatocytes decreased in B-100/B-81 mice by 35%, while apoB-48, apoE, and apoAI syntheses remained unchanged. Metabolic studies using whole animal showed a 32% decrease in triglyceride secretion rates, consistent with the apoB secretion rates. Inhibition of receptor-mediated clearance of apoB-81-containing particles resulted in greater relative accumulation of apoB-81 in plasma than apoB-100, suggesting enhanced clearance of apoB-81-containing particles. These results demonstrate that low levels of apoB in heterozygous hypobetalipoproteinemic mice occurs by low rates of apoB secretion, and increased clearance of truncated apoB. Similar mechanisms appear to contribute to low levels of apoB in hypobetalipoproteinemic humans.

Molecular bases of low production rates of apolipoprotein B-100 and truncated apoB-82 in a mutant HepG2 cell line generated by targeted modification of the apolipoprotein B gene

In subjects with familial hypobetalipoproteinemia heterozygous for truncated forms of apolipoprotein B, both apoB-100 and the truncated forms are produced at lower than expected rates. We studied the mechanism of low levels of apoB in a cell model produced by targeted modification of the apob gene of HepG2 cells. One of the three alleles of apob was found to be targeted. The targeted cells expressed apoB-100 and B-82. The media of mutant cells contained 56% of the levels of apoB-100 present in the media of wild-type (WT) HepG2 cells. ApoB-82 was present at 11% of the apoB-100 levels in mutant cell media. An 85-kD protein (apoB-15) representing the N-terminal fragment of apoB was also secreted, but only in the mutant cell media. We examined the mechanism of low levels of apoB-82. Cellular apoB-82 mRNA was 11% of apoB-100 mRNA, lower than the 33% expected, but consistent with relative levels of apoB-82 in the media. ApoB mRNA transcription in WT and the mutant cells did not differ, while the levels of apoB-82 mRNA in nuclei and polysomes were 46% and 12% of the levels of apoB-100 mRNA, respectively, suggesting that the lower levels of apoB-82 mRNA were due to altered message stability. In a pulse/chase experiment with [35S] methionine, at zero time of chase, the amounts of apoB-100 in mutant cells was 66% that of WT levels, consistent with the modification of one allele. The fractions of newly synthesized apoB-100 secreted into the media at 2 h were 10% in the mutant cells and 19% in the WT cells, suggesting greater presecretory degradation of apoB-100 in the mutant cells. Thus, low levels of mutant apoB-82 mRNA gave rise to the low levels of apoB-82, while low levels of apoB-100 were due to low rates of secretion.