Novel missense MTTP gene mutations causing abetalipoproteinemia

Area of Science:

• Biochemistry
• Genetics
• Molecular Biology

Background:

• Microsomal triglyceride transfer protein (MTTP) is crucial for hepatic very low-density lipoprotein assembly.
• Abetalipoproteinemia (ABL) is a rare genetic disorder characterized by severely reduced levels of apolipoprotein B-containing lipoproteins due to MTTP deficiency.

Purpose of the Study:

• To investigate the functional impact of four novel MTTP missense mutations on protein interactions, expression, and lipid-transfer activity.
• To identify the specific mutations responsible for the ABL phenotype in two affected patients.

Main Methods:

• Identification of novel MTTP mutations (frameshift and missense) in ABL patients.
• Transient expression of mutant MTTP proteins in COS-7 cells.
• Assessment of MTTP interactions with apolipoprotein B (apoB) and protein disulfide isomerase (PDI).
• Quantification of MTTP lipid-transfer activity.

Main Results:

• Four novel missense mutations (G264R, Y528H, R540C, N649S) and one frameshift mutation were identified in MTTP.
• All missense mutations maintained interaction with apoB and PDI.
• Mutations Y528H and R540C resulted in negligible MTTP activity; N649S showed partial reduction.
• Mutation G264R retained wild-type MTTP lipid-transfer activity.

Conclusions:

• The missense mutations Y528H, R540C, and N649S contribute to ABL by impairing MTTP lipid-transfer activity.
• The identified mutations do not affect MTTP binding to PDI or apoB.
• Amino acids 528 and 540 in MTTP are critical for its lipid-transfer function.