Does elevated LDL-C levels mean familial hypercholesterolemia?

Recently, an article titled "When LDL Cholesterol Is Not LDL Cholesterol" was published in the journal JACC: CASE REPORTS. This article emphasizes the importance of distinguishing between low-density lipoprotein cholesterol (LDL-C) and lipoprotein X (LpX) through the analysis and study of two cases of secondary LDL-C elevation.

Highlights

Preface

The distinction between LpX and LDL-C is important because they differ in their respective risks and treatments. LpX ​​is a lipoprotein with the same density as LDL but without ApoB (Figure 1), so measuring the ApoB content is a better method of differentiation.

Case

The first case is a patient with hepatitis, whose LDL-C level was 9.0 mmol/L and ApoB was 1.8 g/L. The ApoB level did not increase significantly with the increase in LDL-C. This patient had a secondary increase in LDL-C due to the formation of LpX caused by cholestasis during liver disease, and the main treatment was to resolve cholestasis. Therefore, after taking drugs to treat hepatitis, his LDL-C and ApoB levels dropped to normal ranges.

The second case is a patient with drug-induced liver injury, whose LDL-C level was 13.1mmol/L and ApoB was 2.2g/L. Because the patient's ApoB was elevated, he needed to receive both cholestasis and lipid-lowering treatment. After lipid-lowering drugs and liver injury drug treatment, his LDL-C and ApoB levels also dropped to normal ranges.

The two cases presented here are secondary LDL-C elevations due to LpX formation caused by cholestasis that occurs in liver disease. During standard LDL-lowering therapy, LpX cannot be absorbed through the LDL/ApoB receptor and remains circulating in the blood. Therefore, the treatment of such patients is primarily based on resolving the cause of the cholestasis. With the restoration of bile excretion, LpX formation is stopped.

Figure 1: The process by which LpX is produced and leads to elevated LDL-C levels

Summarize

LpX is a lipoprotein formed under cholestatic conditions that is often misreported as an increase in LDL-C levels. Lower-than-expected levels of ApoB may be the key to distinguishing LpX: if the ApoB level is lower than expected, it can be identified as LpX. Treatment of elevated LDL-C due to cholestasis should not automatically focus on lowering lipid levels but should focus primarily on addressing the cause of the cholestasis. Statins may be effective but should be used with caution because statin monotherapy may have side effects. Therefore, it is important to consider LpX as a possible cause of hypercholesterolemia in patients with hypercholesterolemia and cholestasis.

Hypercholesterolemia is caused by acquired bad living habits, such as excessive drinking, smoking, obesity, etc. Familial hypercholesterolemia (FH) is an autosomal dominant genetic disease, with genetic factors accounting for 60%-80%. Its main clinical manifestations are significantly increased serum LDL-C levels. Some patients may develop xanthomas and corneal arcus of the skin/tendons, and even premature coronary heart disease. However, an increase in LDL-C levels does not mean hypercholesterolemia, nor is it equivalent to FH. Therefore, when performing FH testing, it is necessary to exclude secondary FH factors first.

References

Huygen LPM, Westerink J, MolGC, Bemelmans RHH. When LDL Cholesterol Is Not LDLCholesterol: LpX, A ClinicalLesson. JACC Case Rep. 2022 Jun 1;4(11):690-693. doi:10.1016/j.jaccas.2022.03.009. PMID: 35677796; PMCID: PMC9168776.

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