Fish eye syndrome: A molecular defect in the lecithin-cholesterol acyltransferase (LCAT) gene associated with normal α-LCAT-specific activity: Implications for classification and prognosis

Abstract

We have identified the molecular defect in two siblings presenting with classical clinical and biochemical features of Fish Eye disease (FED), including corneal opacities, HDL cholesterol < 10 mg/dl, normal plasma cholesteryl esters, and elevated triglycerides. In contrast to previously reported patients with FED who are unable to esterify HDL-associated cholesterol, our patients' plasma lecithin-cholesterol acetyltransferase (α-LCAT)-specific activities assayed using an HDL-like proteoliposome substrate were 12.7-25.7 nmol/μg (19.5±1.8 in controts). In addition, significant residual cholesterol esterification was present in VLDL/LDL-depleted plasma, confirming the presence of HDL-associated α-LCAT activity. DNA sequence analysis of the proband's LCAT gene identified deletion of the triplet coding for leu300, which resulted in the loss of a restriction site for Mini. Digestion of PCR-amplified DNA using Mini established that both siblings are homozygous for this defect. Expression of LCAT300-del. in human embryonic kidney-293 cells revealed normal mRNA and intracellular LCAT concentrations. However, reduced amounts of LCAT300-del., which had a normal specific α-LCAT activity, were present in the media. In summary, we report the first case of FED associated with a mutant enzyme that has a normal α-LCAT-specific activity. The functional significance of this LCAT gene defect has been established in an in vitro expression system, which demonstrates that very small amounts of this functional LCAT mutant enzyme accumulate in the media. Characterization of LCAT300-del. established tnat selective α-LCAT deficiency is not a prerequisite for the development of FED. On the basis of our combined results, we propose that the residual amounts of total plasma LCAT activity and not its distribution on lipoproteins primarily determines the heterogeneity in phenotypic expression observed in familial LCAT deficiency syndromes.