Olezarsen for Familial Chylomicronemia Syndrome in Kids

Key Inclusion Criteria: 1. Parental or legally authorized representative consent must be obtained, and the participants must provide age-appropriate or cognition-appropriate assent, as determined by the Investigator. The parent or legal guardian must be able to understand and comply with the study visit schedule and all other study procedures. 2. Must be able to comply with all study procedures. 3. Age 12 to less than 18 years at the time of informed consent/assent (Cohort 1); age 2 to less than 12 years at time of informed consent/assent (Cohort 2). 4. Willing to fast for at least 10 hours before visits requiring fasted blood sampling. 5. A diagnosis of Familial Chylomicronemia Syndrome (type 1 Hyperlipoproteinemia) by documentation of confirmed homozygote, compound heterozygote or double heterozygote for loss-of-function mutations in type 1-causing genes. 6. Fasting TGs ≥880 mg/dL at screening. If fasting TG is \< 880 mg/dL, up to two additional tests may be performed during the screening period with any single test used to qualify. Key Exclusion Criteria: 1. Diabetes mellitus with any of the following: 1. Newly diagnosed within 12 weeks prior to screening or during the screening period. 2. Hemoglobin A1c (HbA1c) ≥9.5% at screening. 3. Change in basal insulin regimen \>20% within 3 months prior to screening or during the screening period. 4. For participants with type 1 diabetes: episode of diabetic ketoacidosis, or ≥3 episodes of severe hypoglycemia within 6 months prior to screening or during the screening period. 2. History of bleeding, diathesis, or coagulopathy. 3. Major surgery within 3 months of screening. 4. Plasma apheresis within 4 weeks prior to screening or planned during the study. 5. Treatment with another investigational drug, biological agent, or device within one month of screening, or 5 half-lives of investigational agent, whichever is longer. 6. Active pancreatitis within 4 weeks prior to screening or during the screening period. 7. Malignancy diagnosed or treated within 5 years prior to screening or during the screening period. Note: Other protocol-specified inclusion/exclusion criteria may apply.