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.