Plasma lipid transfer in fish-eye disease

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.

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

Isolated high density lipoprotein (HDL) from the two living fish eye disease patients have been incubated in vitro with autologous lipoprotein depleted plasma or with lipoprotein depleted plasma from domestic pig (Sus domesticus), with and without the presence of LCAT inhibitor for 24 hours at 0 and 37 degrees C. The lecithin:cholesterol acyltransferase (LCAT) activity in lipoprotein depleted pig plasma increased the abnormally low cholesteryl ester content of the fish eye disease HDL particles from about 20 to 100% and increased their exceptionally small mean particle size, probably by particle fusion, to a range which is representative of normal HDL3. Both esterification and particle enlargement were totally blocked by the LCAT inhibitor. Incubation of concentrated fish eye disease HDL with autologous lipoprotein depleted plasma for 24 hours at 37 degrees C resulted in a small increase in its cholesteryl ester percentage to 37%, without affecting the apparent HDL particle size. This finding confirms a deficiency of HDL lecithin:cholesterol acyltransferase activity (alpha-LCAT) in fish eye disease. The observed normalization of both HDL cholesteryl ester percentage and particle size by lipoprotein depleted pig plasma which contains virtually no cholesteryl ester transfer activity indicates that the latter is not a requisite for esterification of the free cholesterol of fish eye disease HDL.

Paradoxical esterification of plasma cholesterol in fish eye disease

The activity of lecithin: cholesterol acyl transferase (LCAT), the enzyme which catalyses the esterification of human plasma cholesterol, has been measured by two independent methods in plasma from the two known living Swedish patients with fish eye disease. The enzyme activity was in both cases about 15% of that of normal plasma. Paradoxically, however, the percentage of plasma cholesterol which was esterified was almost normal in both patients. In addition, a normal spectrum of the fatty acids of the cholesteryl esters was present indicating a normal cholesterol esterification pathway in vivo. Incubation experiments in vitro of plasma from the two patients also yielded normal cholesterol esterification rates when measured by two different methods. These paradoxical results for cholesterol esterification are discussed on the basis of the present biochemical knowledge of fish eye disease and LCAT deficiency.

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.

Familial LCAT deficiency and fish-eye disease

Familial LCAT deficiency is due to deficiency of plasma lecithin-cholesterol acyltransferase. The plasma is rich in free cholesterol and lecithin while cholesterol ester and lysolecithin levels are reduced. Analysis of the abnormal lipoproteins has helped our understanding of plasma lipid and lipoprotein metabolism in normals and in patients with liver disease. Proteinuria and anaemia are common and there is marked corneal lipid deposition. Eventually renal function deteriorates and dialysis and/or renal transplantation may be necessary. The human LCAT gene has been sequenced and been shown to be present on chromosomal segment 16q22-the region predicted on the basis of recombination studies as the site of the LCAT deficiency gene. The gene defect has been identified in some cases, but the mechanism remains unclear as the mutations were not in the region presumed to be the enzyme's active site. Only three cases of fish-eye disease have been described; all were elderly and had obvious corneal opacities. They had fasting hypertriglyceridaemia and increased VLDL. IDL and LDL were increased and were triglyceride rich. HDL, reduced by 90%, was mainly HDL3--with a high free and low ester cholesterol. LCAT activity in fish-eye plasma was normal but when measured in an exogenous substrate it was only 10-15% of normal. Fish-eye HDL is a substrate for purified LCAT, but fish-eye LCAT does not esterify free cholesterol of HDL (normal or fish-eye), although it esterifies free cholesterol of VLDL and LDL. It has been suggested that one type of LCAT activity acts on HDL (alpha-LCAT) and another on VLDL and LDL (beta-LCAT)--and that fish-eye disease is due to alpha-LCAT deficiency, and classical familial LCAT deficiency due to lack of both components.

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.

Electron microscopic structure of serum lipoproteins from patients with fish eye disease

The structure and composition of lipoprotein fractions from two patients with fish eye disease were examined. The composition of very low density lipoproteins (VLDL) was normal, although total mass was greatly elevated. The mean particle sizes of the VLDL were 44.6 +/- 22.2 and 42.8 +/- 19.8 nm for Patients 1 and 2, respectively. Intermediate density lipoprotein (IDL) concentrations in patients were elevated and contained increased triglyceride content; particle sizes for Patients 1 and 2 were 29.4 +/- 3.5 nm and 28.0 +/- 4.1 nm, respectively. In both patients, the triglyceride/cholesteryl ester ratio in LDL was approximately tenfold higher than in normal individuals; however, the LDL particles were somewhat smaller in diameter (23.5 +/- 3.0 nm for Patient 1 and 23.3 +/- 3.8 nm for Patient 2) than those of controls (25.8 +/- 3.0 nm and 24.9 +/- 3.4 nm). In both patients, large vesicular structures were occasionally encountered in the LDL region. The high density lipoprotein (HDL) fraction of fish eye disease patients showed the greatest abnormalities. Not only was the total HDL concentration extremely low (approximately 10% of control levels), but unesterified cholesterol was increased relative to cholesteryl ester. Particle morphology was heterogeneous; the major HDL species was a small spherical particle with a diameter of 7.6 nm. Discoidal particles with a thickness of 4.4 nm and diameters between 17.4 and 20.8 nm were also present, together with large (40-90 nm) vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)