In vitro normalization of cholesteryl ester content and particle size of fish eye disease high density lipoproteins

A kindred with fish eye disease, corneal opacities, marked high-density lipoprotein deficiency, and statin therapy

A kindred affected with fish eye disease (FED) from Oklahoma is reported. Two probands with corneal opacification had mean levels of high-density lipoprotein (HDL) cholesterol (C), apolipoprotein (apo) A-I, and apoA-I in very large alpha-1 HDL particles that were 9%, 17%, and 5% of normal, whereas their parents and 1 sibling had values that were 61%, 77%, and 72% of normal. The probands had no detectable lipoprotein-X, and had mean low-density lipoprotein cholesterol (LDL-C) and triglyceride levels that were elevated. Their mean lecithin cholesterol acyltransferase (LCAT) activities, cholesterol esterification rates, and free cholesterol levels were 8%, 42%, and 258% of normal, whereas their parents and 1 sibling had values that were 55%, 49%, and 114% of normal. The defect was due to 1 common variant in the LCAT gene in exon 1: c101t causing a proline34leucine substitution and a novel mutation c1177t causing a threonine37methionine substitution, with the former variant being found in the father and 1 sibling, and the latter mutation being found in the mother, and both mutations being present in the 2 probands. FED is distinguished from familial LCAT deficiency (FLD) by the lack of anemia, splenomegaly, and renal insufficiency as well as normal or increased LDL-C. Both FLD and FED cases have marked HDL deficiency and corneal opacification, and FED cases may have premature coronary heart disease in contrast to FLD cases. Therapy, using presently available agents, in FED should be to optimize LDL-C levels, and 1 proband responded well to statin therapy. The investigational use of human recombinant LCAT as an enzyme source is ongoing.

Different substrate specificities of plasma lecithin: cholesterol acyl transferase in fish eye disease and Tangier disease

Esterification of plasma free cholesterol is mediated by lecithin:cholesterol acyl transferase (LCAT). The free cholesterol of plasma high density lipoproteins (HDL) is considered to be the preferred substrate for LCAT. It therefore appeared as a paradox that plasma cholesterol esterification, both in vivo and in vitro, is normal in fish eye disease and Tangier disease, two familial conditions with extremely low plasma HDL levels. Fish eye disease plasma, however, was shown to have LCAT activity primarily acting on combined very low (VLDL) and low (LDL) density lipoproteins, denominated beta-LCAT, while it lacked LCAT activity esterifying HDL cholesterol (alpha-LCAT). Here we show that Tangier plasma, in contrast, has both alpha- and beta-LCAT. Thus, in both fish eye and Tangier diseases it is beta-LCAT that explains the apparent normal plasma cholesterol esterification. We also show that Tangier plasma, having alpha-LCAT activity, normalizes the low cholesteryl ester content as well as the abnormally small size of fish eye disease HDL particles during incubation.

Substrate specificity of plasma lecithin: cholesterol acyltransferase in abetalipoproteinemia

The lecithin: cholesterol acyltransferase (LCAT) activity of lipoprotein-depleted plasma from a patient with abetalipoproteinemia has been assayed in a modified Glomset-Wright incubation system with three different normal lipoprotein substrates consisting of an authentic mixture of very low (VLDL), low (LDL) and high (HDL) density lipoproteins for the assay of total LCAT activity, HDL to assay alpha-LCAT activity and combined VLDL and LDL to assay beta-LCAT activity, respectively. Although reduced to about half the normal control values, both alpha- and beta-LCAT activities were present in the patient's plasma. It has been shown earlier that secretion of LCAT is linked to that of VLDL, but since patients with abetalipoproteinemia cannot form either chylomicrons or VLDL, our results suggest that a secretion of these triglyceride-rich lipoproteins do not seem to be a prerequisite for a basal secretion of beta-LCAT.

Two different allelic mutations in the lecithin-cholesterol acyltransferase gene associated with the fish eye syndrome. Lecithin-cholesterol acyltransferase (Thr123----Ile) and lecithin-cholesterol acyltransferase (Thr347----Met)

We have elucidated the genetic defect in a 66-yr-old patient with fish eye syndrome (FES) presenting with severe corneal opacities and hypoalphalipoproteinemia. The patient's plasma concentration of high density lipoprotein (HDL) cholesterol was reduced at 7.7 mg/dl (35.1-65.3 mg/dl in controls) and the HDL cholesteryl ester content was 31% (60-80% in controls); however, total plasma cholesteryl esters were similar to normal (60% of total cholesterol vs. a mean of 66% in controls). The patient's plasma cholesterol esterification rate was slightly reduced at 51 nmol/ml per h (control subjects: 61-106 nmol/ml per h), whereas lecithin-cholesterol acyltransferase (LCAT) activity, assayed using a HDL-like exogenous proteoliposome substrate, was virtually absent (0.9 nmol/ml per h vs. 25.1-27.9 nmol/ml per h in control subjects). DNA sequence analysis of the proband's LCAT gene revealed two separate C to T transitions resulting in the substitution of Thr123 with Ile and Thr347 with Met. The mutation at codon 347 created a new restriction site for the enzyme Nla III. Analysis of the patient's polymerase chain reaction-amplified DNA containing the region of the Thr347 mutation by digestion with Nla III confirmed that the proband is a compound heterozygote for both defects. The patient's daughter, who is asymptomatic despite a 50% reduction of LCAT activity, is heterozygous for the Thr123----Ile mutation. Our data indicate that the regions adjacent to Thr123 and Thr347 of LCAT may play an important role in HDL cholesterol esterification, suggesting that these regions may contain a portion of the LCAT binding domain(s) for HDL.

Normalization of high density lipoprotein in fish eye disease plasma by purified normal human lecithin: cholesterol acyltransferase

Plasma from a patient with fish eye disease has been enriched with autologous high density lipoproteins (HDL) and supplemented with highly purified normal human plasma lecithin:cholesterol acyltransferase (LCAT). Incubation of such plasma at 37 C in vitro resulted in normalization of its low HDL cholesteryl ester percentage, from 23% to 79%, associated with a two-fold increase in both the cholesteryl ester and triglyceride contents of the HDL fraction, as compared to incubation experiments with absent or heat-inactivated purified normal LCAT. The normalization of the HDL cholesteryl ester percentage induced by incubation with purified normal LCAT also was accompanied by an increase in the size of the original fish eye disease HDL particles, which had a mean mass of 115 kd, to HDL particle populations with mean particle masses ranging from 130-220 kd, depending on the concentration of purified LCAT in the incubate. Both HDL cholesterol esterification and particle enlargement were abolished completely by the LCAT inhibitor DTNB and by heat inactivation of the purified normal LCAT. The results give further evidence that fish eye disease is an alpha-LCAT deficiency.

Net lipid transfer between lipoproteins in fish-eye disease plasma supplemented with normal high density lipoproteins

Native fish-eye disease plasma, which is deficient of both high density lipoproteins (HDL) and lecithin-cholesterol acyltransferase activity (alpha-LCAT), processing the free cholesterol of these lipoproteins, has been supplemented with normal isolated HDL2 or HDL3 and incubated in vitro at 37 C. After incubation for 0, 7.5 and 24 hr the very low density (VLDL) and low density (LDL) lipoproteins as well as HDL were isolated, and their contents of triglycerides, phospholipids and free, esterified and total cholesterol were quantified. The resulting net mass transfer of the different lipids revealed a functioning transfer of cholesteryl esters and all other analyzed lipids between the lipoproteins, although no de novo esterification of the HDL cholesterol by LCAT in this plasma occurred. In accordance with previous findings there was a functioning esterification process of the free cholesterol of the combined VLDL and LDL of fish-eye disease plasma. The present reports make it reasonable to conclude that the lack of HDL cholesterol esterification in this disease is not a result of a deficiency of cholesteryl ester transfer or lipid transfer activities.