Structure-function analyses of microsomal triglyceride transfer protein missense mutations in abetalipoproteinemia and hypobetalipoproteinemia subjects

• Published in: Biochimica et biophysica acta Vol. 1861; no. 11; pp. 1623 - 1633
• Main Authors: Walsh, Meghan T., Di Leo, Enza, Okur, Ilyas, Tarugi, Patrizia, Hussain, M. Mahmood
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.11.2016
• Subjects: Abetalipoproteinemia; Abetalipoproteinemia - blood; Abetalipoproteinemia - genetics; Amino Acid Sequence; Animals; Apolipoprotein B; Apolipoproteins B - metabolism; blood lipids; Carrier Proteins - chemistry; Carrier Proteins - genetics; Cercopithecus aethiops; Child; Computer Simulation; COS Cells; Endoplasmic Reticulum - metabolism; Gene Expression Regulation; genes; Humans; Hypobetalipoproteinemia; Hypobetalipoproteinemias - blood; Hypobetalipoproteinemias - genetics; hypolipoproteinemia; Infant; lipid composition; Lipid Metabolism - genetics; Lipoprotein; Low density; Male; metabolism; Microsomal triglyceride transfer protein; missense mutation; mutants; Mutation, Missense - genetics; patients; Phenotype; Protein Binding; protein disulfide-isomerase; Protein Disulfide-Isomerases - metabolism; secretion; Sequence Alignment; Structure-Activity Relationship; triacylglycerols; Triglycerides - metabolism; Vitamins - blood; Young Adult; Apolipoprotein B; Microsomal triglyceride transfer protein; Low density; Hypobetalipoproteinemia; Abetalipoproteinemia; Lipoprotein
• ISSN: 1388-1981; 0006-3002; 1879-2618
• DOI: 10.1016/j.bbalip.2016.07.015

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. •We analyzed MTP missense mutations in abeta- and hypobetalipoproteinemia patients.•A novel D361Y mutation abolished MTP activity.•D361Y interferes with M subunit-PDI binding.•Four out of five missense mutations had no effect on MTP function.•Other unidentified genes may contribute to low plasma apoB-containing lipoproteins.