Familial Chylomicronemia Syndrome (FCS) Testing Program

In the US, FCS is estimated to affect between 1 – 13 individuals per 1,000,000.^1,2 FCS usually appears in newborns and young children, and at the latest, during adolescence.³ FCS is characterized by severely high plasma triglyceride levels resulting from the improper breakdown of chylomicron lipoproteins by LPL.^4,5 Triglyceride levels below 1.7 mmol/L (150 mg/dL) are considered normal, while patients with FCS typically have triglyceride levels over 10 mmol/L (885 mg/dL).^6,7 Patients with FCS frequently present with debilitating abdominal pain and severe and recurrent episodes of acute pancreatitis. Other clinical manifestations of FCS include hepatosplenomegaly, eruptive xanthomas, and lipemia retinalis. Patients may also experience sensations of numbness or tingling, along with fatigue and cognitive impairment.⁸ Laboratory findings include chylomicronemia, hyperlipoproteinemia, severe hypertriglyceridemia, decreased plasma apolipoprotein C-II, and cloudy or pinkish-colored blood.^4,9

It is difficult to differentiate between FCS and other forms of chylomicronemia because they have similar clinical symptoms and laboratory findings.³ Reduced LPL enzyme activity is a key feature of FCS, and functional assays of LPL activity are used for making a diagnosis. However, such assays are not always readily available, and the output of these assays often shows considerable variability.⁵

Genetic testing provides specific information regarding the altered gene and therefore allows for the differentiation of FCS from other much more common causes of chylomicronemia such as multifactorial chylomicronemia syndrome (MCS) and familial partial lipodystrophy (FPLD). Genetic testing is not widely available, however, and is expensive or not covered by insurance in the US.⁵ Nonetheless, it is important to determine the cause of the chylomicronemia because management approaches to lowering triglyceride levels vary depending upon the cause of chylomicronemia. The top global recommendations for treatment of FCS are to (1) limit fat to <15 to 20 g per day (<10%-15% of total daily energy intake); (2) meet recommendations for essential fatty acids: α-linolenic acid and linoleic acid; (3) choose complex carbohydrate foods while limiting simple and refined carbohydrate foods; (4) supplement with fat-soluble vitamins, minerals, and medium-chain triglyceride oil, as needed; and (5) adjust calories for weight management.¹⁰

FCS is a rare monogenic autosomal recessive disorder. FCS is caused by biallelic loss-of-function (LoF) homozygous, compound heterozygous, or double heterozygous pathogenic variants in LPL or other genes required for regulating LPL activity including APOC2, APOA5, LMF1, GPIHBP1. In addition to these, rare cases of variants of GPD1, and CREB3L3 have also been reported.¹¹ Conversely, MCS is caused by both heterozygous variants in LPL, APOA5, GCKR, APOB, LMF1, GPIHBP1, CBREB3L3, APOC2, APOE, or the cumulative burden of small-effect variants, together with secondary factors.¹²

The table below outlines the functions of the gene products of the genes included in the FCS gene panel as well as the molecular features of the aberrant form.

*Variants in GDP1 cause transient infantile hypertriglyceridemia.¹⁹

CREB3L3, CAMP - responsive element - binding protein 3 - like protein 3; FFA, free fatty acids; GPD1, glycerol - 3 - phosphate dehydrogenase 1; GPIHBP1, glycosylphosphatidylinositol anchored HDL - binding protein 1; LMF1, lipase maturation factor 1; LPL, lipoprotein lipase; TG, triglycerides; VLDL: very - low - density lipoprotein.

Mutations in the LPL gene account for most cases of FCS and more than 180 mutations have been identified.⁶ In one of the largest studied cohorts of patients with FCS, 52 individuals were confirmed to have a genetic diagnosis of FCS. Approximately 80% of these patients harbored rare biallelic variants in the LPL gene. Among the remaining patients, 45% had biallelic pathogenic variants in GPIHBP1, 22% in APOA5, 11% in LMF1 and 11% in APOC2. Additionally, others were double heterozygotes with mutations in LPL and either APOA5 (11%) or LMF1 (11%).^21,22 In the same cohort, pathogenic LoF variants in FCS comprised mainly missense mutations, although nonsense and splicing variants were also observed. Approximately 5% of the cases involved copy number variations (CNVs), with a large deletion spanning 3-5 exons in GPIHBP7 being the most frequent.^21,22 Very rare instances of de novo variants have been observed in the FCS genes, but the vast majority of pathogenic variants are inherited from parental carriers.²³

This test is performed via Sequencing and CNV Detection by NextGen Sequencing.

This panel provides 100% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non coding regions in which pathogenic variants have been identified at Prevention Genetics or reported elsewhere. We define coverage as 2:2.0X NGS reads or Sanger sequencing.

To qualify for the testing program, candidates must reside in the United States or Canada and have evidence of severe refractory hypertriglyceridemia, defined by a minimum of 2 consecutive fasting triglyceride levels > 880 mg/dL or 10 mmol/L in the absence of secondary causes or medical conditions known to cause sHTG. Informed consent must be provided by the patient.

Specimen Collection

WHOLE BLOOD

Collect 3 ml - 5 ml of whole blood in EDTA (purple top tube) or ACD (yellow top tube).

SALIVA

Oragene^TM or GeneFiX^TM Saliva Collection kit used according to manufacturer instructions.

OCD-100 BUCCAL SWAB

OCD-100 Buccal Swab used according to manufacturer instructions.

Shipping and Handling Instructions

Label all specimen containers with the patient's name, date of birth, and/or ID number. At least two identifiers should be listed on specimen containers. Specimen deliveries are accepted Monday-Saturday for all specimen types. Holiday schedules will be posted on our website at least one week prior to major holidays.

WHOLE BLOOD

DO NOT FREEZE. During hot weather, include a frozen ice pack in the shipping container. Place a paper towel or other thin material between the ice pack and the blood tube. In cold weather include an unfrozen ice pack in the shipping container as insulation. At room temperature, blood specimens are stable for up to 48 hours. If refrigerated, blood specimens are stable for up to one week.

SALIVA AND BUCCAL

Specimens may be shipped at room temperature.

Specimen collection kits: Blood, saliva, or buccal specimen collection kits, which contain the TRF and the shipping label, may be requested through the kit order form.

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