Abstract 4142287: 4-Phenylbutyric Acid Reduces Endoplasmic Reticulum Retention and Partially Restores Function of LDLR p.D622N Mutation In Vitro: A Potential Therapy for Hypercholesterolemia

Background: We identified a family with familial hypercholesterolemia (FH). The proband presented with corneal arcus, skin, and tendon xanthomas, significant stenosis in the coronary circumflex and right coronary arteries, and severely elevated plasma LDL levels. Treatment with statins, ezetimibe, PCSK9 monoclonal antibodies (Alirocumab and Evolocumab), and inclisiran, which target LDLR function, resulted in minimal LDL reduction.In China, evinacumab is not available, so the patient has to rely on long-term lipid apheresis (plasma exchange). Methods: Genetic testing revealed the proband harbored three mutations in the LDLR gene: c.1864G>A (p.D622N), c.1448G>A (p.W483*), and c.292G>A (p.G98S). D622N and G98S were on the same allele, while W483* was on the other allele. Mutant plasmids were constructed and transfected into HEK293T cells. Western blot and immunofluorescence co-localization assays were used to assess LDLR protein expression and localization. The LDLR function was evaluated using Dil-LDL uptake assays. Additionally, cells were treated with 4 mM 4-phenylbutyric acid for 48 hours to examine its impact on LDLR function. Results: The Western Blot test showed that the W483* mutation broke down the LDLR protein almost completely. The D622N mutation, on the other hand, only showed the precursor form and not the mature form. Finally, the G98S mutation showed that LDLR expression was normal. According to immunofluorescence co-localization, the D622N mutation kept LDLR in the endoplasmic reticulum (ER). Di-LDL uptake tests showed that the G98S mutation could still bind and transport LDL normally. On the other hand, the W483* mutation had almost no LDLR expression or function, and the D622N mutation had ER-retained LDLR that could not bind or transport LDL. After 48 hours of treatment with 4 mM 4-phenylbutyric acid, some of the endoplasmic reticulum retention of D622N-mutant LDLR was alleviated. This facilitated its movement to the cell membrane, partially restoring its ability to bind and transport LDL. Conclusions: 4-phenylbutyric acid may serve as a potential therapeutic approach for FH patients with LDLR mutations causing ER retention.