title: Structure-function analyses of microsomal triglyceride transfer protein missense mutations in abetalipoproteinemia and hypobetalipoproteinemia subjects.
abstract: We describe two new hypolipidemic patients with very low plasma triglyceride and apolipoprotein B (apoB) levels with plasma lipid profiles similar to abetalipoproteinemia (ABL) patients. In these patients, we identified two previously uncharacterized missense mutations in the microsomal triglyceride transfer protein (MTP) gene, R46G and D361Y, and studied their functional effects. We also characterized three missense mutations (H297Q, D384A, and G661A) reported earlier in a familial hypobetalipoproteinemia patient. R46G had no effect on MTP expression or function and supported apoB secretion. H297Q, D384A, and G661A mutants also supported apoB secretion similarly to WT MTP. Contrary to these four missense mutations, D361Y was unable to support apoB secretion. Functional analysis revealed that this mutant was unable to bind protein disulfide isomerase (PDI) or transfer lipids. The negative charge at residue 361 was critical for MTP function as D361E was able to support apoB secretion and transfer lipids. D361Y most likely disrupts the tightly packed middle α-helical region of MTP, mitigates PDI binding, abolishes lipid transfer activity, and causes ABL. On the other hand, the hypolipidemia in the other two patients was not due to MTP dysfunction. Thus, in this study of five missense mutations spread throughout MTP's three structural domains found in three hypolipidemic patients, we found that four of the mutations did not affect MTP function. Thus, novel mutations that cause severe hypolipidemia probably exist in other genes in these patients, and their recognition may identify novel proteins involved in the synthesis and/or catabolism of plasma lipoproteins.
authors.affiliation: School of Graduate Studies, Molecular and Cell Biology Program, State University of New York Downstate Medical Center, Brooklyn, NY 11203, United States; Department of Cell Biology, State University of New York Downstate Medical Center, Brooklyn, NY 11203, United States.
authors.forename: Meghan T
authors.initials: MT
authors.lastname: Walsh
authors.affiliation: Department of Life Sciences, University of Modena and Reggio Emilia, Modena, Italy.
authors.forename: Enza
authors.initials: E
authors.lastname: Di Leo
authors.affiliation: Department of Pediatric Metabolism and Nutrition, Gazi University School of Medicine, Ankara, Turkey.
authors.forename: Ilyas
authors.initials: I
authors.lastname: Okur
authors.affiliation: Department of Life Sciences, University of Modena and Reggio Emilia, Modena, Italy.
authors.forename: Patrizia
authors.initials: P
authors.lastname: Tarugi
authors.affiliation: Department of Cell Biology, State University of New York Downstate Medical Center, Brooklyn, NY 11203, United States; Department of Pediatrics, SUNY Downstate Medical Center, Brooklyn, NY 11203, United States; VA New York Harbor Healthcare System, Brooklyn, NY 11209, United States; Winthrop University Hospital, Mineola, NY 11501, United States. Electronic address: MHussain@winthrop.org.
authors.forename: M Mahmood
authors.initials: MM
authors.lastname: Hussain
mesh_terms: D000012:Abetalipoproteinemia / Q000097:blood / Q000235:genetics*; D000595:Amino Acid Sequence; D000818:Animals; D001055:Apolipoproteins B / Q000378:metabolism; D019556:COS Cells; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics*; D002648:Child; D002522:Chlorocebus aethiops; D003198:Computer Simulation; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D006995:Hypobetalipoproteinemias / Q000097:blood / Q000235:genetics*; D007223:Infant; D050356:Lipid Metabolism / Q000235:genetics; D008297:Male; D020125:Mutation, Missense / Q000235:genetics*; D010641:Phenotype; D011485:Protein Binding; D019704:Protein Disulfide-Isomerases / Q000378:metabolism; D016415:Sequence Alignment; D013329:Structure-Activity Relationship; D014280:Triglycerides / Q000378:metabolism; D014815:Vitamins / Q000097:blood; D055815:Young Adult
chemical_list: D001055:Apolipoproteins B; D002352:Carrier Proteins; D014280:Triglycerides; D014815:Vitamins; C106092:microsomal triglyceride transfer protein; D019704:Protein Disulfide-Isomerases
keywords: Abetalipoproteinemia; Apolipoprotein B; Hypobetalipoproteinemia; Lipoprotein; Low density; Microsomal triglyceride transfer protein