Beyond Classic LPL Deficiency: The Genetic Complexity of FCS

The genetic architecture of familial chylomicronemia syndrome extends well beyond classic biallelic LPL mutations, and understanding that broader landscape changes how clinicians recognize and classify patients with severe chylomicronemia.

Stephan P. Babirak, MD, PhD, who was the first to characterize heterozygosity for lipoprotein lipase deficiency in the late 1980s, describes how variants in LPL, apolipoprotein A-V, apolipoprotein C-II, lipase maturation factor, and related loci can individually or in combination substantially reduce LPL activity, producing a phenotype that resembles classic FCS but is far more common in specialty practice. He has also published work suggesting that patients can carry multiple defects across LPL-related genes, compounding their functional LPL impairment.

Alan Brown, MD, MHA, agrees, citing a case of a patient with type III hyperlipidemia carrying a single APOA5 mutation whose chylomicronemia phenotype was indistinguishable from classic FCS. Across his own practice, patients have sorted into 3 broad groups: those with biallelic pathogenic mutations in one of the 5 canonical LPL-pathway genes; a larger group with clinical FCS and a nonclassic genotype; and a third category likely harboring clusters of smaller SNPs that together depress LPL function enough to produce full chylomicronemia without an identifiable large-gene mutation.

Brown notes that splitting hairs over whether a patient has 'true' FCS versus clinical FCS becomes less meaningful when the phenotype is the same: recurrent pancreatitis, no secondary causes, and no response to conventional therapy.

Babirak brings cohort data to the point. In the first 61 consecutive patients he evaluated with chylomicronemia syndrome, 56 had at least one identifiable genetic variant, and virtually all carried at least one additional metabolic or pharmacologic contributor. Each episode of pancreatitis carries roughly a 6% mortality rate, and the cumulative exocrine and endocrine consequences, including secondary diabetes, chronic pancreatic insufficiency, and hepatic steatosis, can permanently alter a patient's life. Both experts note that cascade genetic testing in families is among the most productive diagnostic moves available.

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Our Panelists:

Alan Brown, MD, MHA, is a professor of cardiovascular medicine at Wake Forest University and a past president of the National Lipid Association.

Stephan P. Babirak, MD, PhD, is the founder and medical director of Metabolic Leader, a metabolic and endocrine care center in Scarborough, Maine, where he specializes in lipoprotein disorders.

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Disclosures for Brown include New Amsterdam, Merck, Novartis, Arrowhead, Amgen, Regeneron, and Ionis. Babirak has no relevant disclosures to report.

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References

1. Javed F, et al. Familial chylomicronemia syndrome: an expert clinical review from the National Lipid Association. J Clin Lipidol. 2025;19(3):382-403. doi:10.1016/j.jacl.2025.03.013

2. US Food and Drug Administration. FDA approves drug to reduce triglycerides in adults with familial chylomicronemia syndrome. FDA. November 18, 2025. Accessed June 12, 2026. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-drug-reduce-triglycerides-adults-familial-chylomicronemia-syndrome

3. US Food and Drug Administration. FDA approves drug to reduce triglycerides in adult patients with familial chylomicronemia syndrome. FDA. December 19, 2024. Accessed June 12, 2026. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-drug-reduce-triglycerides-adult-patients-familial-chylomicronemia-syndrome