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Endoplasmic reticulum quality control of LDLR variants associated with familial hypercholesterolemia

Praseetha Kizhakkedath¹, Anne John¹, Buthaina K Al-Sawafi¹

¹Department of Pathology, College of Medicine and Health Sciences, United Arab Emirates University, Al-Ain, United Arab Emirates.

|October 7, 2019

Insights

Familial hypercholesterolemia (FH) can result from LDLR gene mutations. This study reveals ER stress and degradation mechanisms for specific LDLR variants, advancing FH understanding.

Area of Science:

Genetics
Molecular Biology
Cellular Biology

Background:

Loss-of-function mutations in the low-density lipoprotein receptor (LDLR) gene are a primary cause of familial hypercholesterolemia (FH).
Functional evidence for the pathogenicity of many LDLR mutations remains limited.
Understanding cellular mechanisms is crucial for FH diagnosis and treatment.

Purpose of the Study:

To investigate the cellular pathogenic mechanisms of three specific LDLR mutations causing FH.
To compare the behavior of these LDLR mutants with structurally identical VLDLR mutants.
To elucidate the role of endoplasmic reticulum (ER) stress and degradation pathways in FH pathogenesis.

Main Methods:

Investigated cellular localization and trafficking of mutant LDLR proteins using techniques like N-glycosylation profiling.
Assessed ER stress induction by measuring spliced X-box binding protein-1 (XBP-1) mRNA levels.
Examined mutant protein association with ER quality control components and stability using proteasome inhibitors.

Main Results:

Two LDLR mutants (D482H and C667F) were retained in the ER, similar to VLDLR mutants.
One mutant (D445E) showed normal trafficking to the plasma membrane.
ER-retained mutants induced ER stress and associated with ER quality control components, with enhanced stability via proteasome inhibition.

Conclusions:

These findings identify novel transport-deficient class II LDLR variants contributing to FH.
The study provides evidence for the involvement of endoplasmic reticulum-associated degradation (ERAD) in the stability of these pathogenic LDLR variants.
This research deepens the understanding of molecular mechanisms underlying FH and highlights potential therapeutic targets.

Read full analysis

Abstract:

Loss-of-function mutations in the low-density lipoprotein receptor (LDLR) gene can cause familial hypercholesterolemia (FH), but detailed functional evidence for pathogenicity is limited to a few reported mutations. Here, we investigated the cellular pathogenic mechanisms of three mutations in LDLR causing FH, which are structurally identical to pathogenic mutations in the very low-density lipoprotein receptor (VLDLR). Similar to the VLDLR mutants, LDLR mutants D482H and C667F were found to be localized to the ER, while D445E, which is a conserved amino acid change, did not affect the trafficking of the receptor to the plasma membrane, as confirmed by the N-glycosylation profile. Although the ER-retained mutant proteins were soluble, induction of ER stress was observed as indicated by spliced X-box binding protein-1 (XBP-1) mRNA levels. The mutants were found to associate with ER quality control components, and their stability was enhanced by inhibitors of proteasome. Our results contribute to the growing list of transport-deficient class II LDLR variants leading to FH and provide evidence for the involvement of endoplasmic reticulum-associated degradation in their stability.

Keywords:

ERAD FH LDLR VLDLR missense mutation

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