Broad-β bandを示す高脂血症患者にみられた新しいアポリポ蛋白の遺伝的変異種

Abstract

A new variant of apolipoprotein E (apo E-Kochi) was identified in a serum of a 29-year-old man with hyperlipidemia.This patient was found to have hyperlipidemia in September, 1984, but had no symptom of cardiovascular disease. On examination, xanthomas, corneal arcus and hepatosplenomegaly were not evident. Electrocardiogram was no abnormal finding. Liver function tests, oral glucose tolerance test and thyroid function test were normal.His serum cholesterol and triglyceride values ranged from 214-308mg/dl and 546-1, 568mg/dl, respectively. An agarose electrophoretogram of the serum showed broad-beta band. Lipid concentrations of the VLDL and IDL were elevated, and these fractions accounted 67 % of the serum cholesterol. But the ratio of VLDL-cholesterol to VLDL-triglyceride was 0.26. The hyperlipoprotein phenotype of the patient was difficult to classify, but resembled mixed type of III and IV. Apo E level of the serum was elevated markedly, that was 22.4mg/dl.The isoelectric focusing pattern of the VLDL from the patient revealed major double bands in the apo E3 region and minor double bands in the sialated isoform regions (E3s1, Es2) of the gel. In the family study, the proband, his mother, maternal uncle and grandmother were found to have the same pattern of apo E. His father, brother and sister were found to possess ordinary E3/E3 phenotype.The cationic component of the double bands of apo E of the proband was focused exactly in the E3 region, and the more anionic component was equal in amount to cationic band and different in isoelectric point by a half charge unit (≅0.06 pH). After cysteamine modification, the double bands shifted one charge range toward cathode, indicating that each component of the double bands contain one residue of cysteine. Each component of the double bands was revealed to have equal molecular weight by SDS-polyacrylamide gel electrophoresis, and same affinity to heparin-Sepharose gel column. Purified protein from the anionic band, as well as protein from the cationic band and ordinary apo E3 showed immunochemical precipitation reaction with anti-apo E goat serum.The results from the above have indicated that the cationic band from the patient was apo E3 and the more anionic band was a new variant isoform of apo E, that was designated as apo E-Kochi. We presume that the apo E-Kochi may has histidin/neutral amino acid interchange at residue 140 of apo E3 that are sufficient to account for the half-charge difference between apo E-Kochi and E3 isoform.