A severe type of hyperlipidemia, sometimes familial, that is characterized by the elevation of both plasma chylomicrons and triglycerides contained in very-low-density lipoproteins. Type V hyperlipoproteinemia is often associated with diabetes mellitus and is not caused by reduced lipoprotein lipase activity as in hyperlipoproteinemia type I.

Chylomicron retention disease (CMRD), characterized by the inability to secrete chylomicrons from the enterocytes following the ingestion of fat, typically presents in infancy with failure to thrive, diarrhea, vomiting, abdominal distention, and malabsorption of fat. This leads to steatorrhea – the severity of which relates to the fat content of the diet – and in some cases, hepatomegaly. Organ systems outside of the gastrointestinal tract may also be affected (often due to malnutrition and deficiencies of fat-soluble vitamins), including neuromuscular abnormalities (typically in the first or second decade of life) secondary to vitamin E deficiency, poor bone mineralization and delayed bone maturation due to vitamin D deficiency, prolonged international normalized ratio (INR) due to vitamin K deficiency, mild ophthalmologic issues (e.g., micronystagmus, delayed dark adaptation, abnormal visual evoked potentials, and abnormal scotopic electroretinograms), and (in a small proportion of adults) cardiomyopathy with decreased ejection fraction. Affected individuals typically have marked hypocholesterolemia, low plasma apolipoprotein B levels, normal-to-low plasma triglyceride levels, and low serum concentrations of fat-soluble vitamins (A, D, E, and K). Endoscopy typically demonstrates a gelée blanche ("white hoar frosting") appearance of the duodenal mucosa.

Pantothenate kinase-associated neurodegeneration (PKAN) is a type of neurodegeneration with brain iron accumulation (NBIA). The phenotypic spectrum of PKAN includes classic PKAN and atypical PKAN. Classic PKAN is characterized by early-childhood onset of progressive dystonia, dysarthria, rigidity, and choreoathetosis. Pigmentary retinal degeneration is common. Atypical PKAN is characterized by later onset (age >10 years), prominent speech defects, psychiatric disturbances, and more gradual progression of disease.

Hypobetalipoproteinemia (HBL) is defined as permanently low levels, below the 5th percentile of sex- and age-matched individuals in the population, of apolipoprotein B (apoB), total cholesterol, and low-density lipoprotein (LDL) cholesterol; the lipid profile in FHBL2 includes low HDL cholesterol as well. HBL can result from environmental factors such as a strict vegetarian diet, or can be secondary to certain diseases such as intestinal fat malabsorption, chronic pancreatitis, severe liver disease, malnutrition, or hyperthyroidism. Heritable primary causes of HBL include chylomicron retention disease (CMRD; 246700), abetalipoproteinemia (200100), and familial hypobetalipoproteinemia (FHBL) (summary by Martin-Campos et al., 2012). For a discussion of genetic heterogeneity of familial hypobetalipoproteinemia, see FHBL1 (615558).

Familial hypercholesterolemia (FH) is characterized by significantly elevated low-density lipoprotein cholesterol (LDL-C) that leads to atherosclerotic plaque deposition in the coronary arteries and proximal aorta at an early age and increases the risk of premature cardiovascular events such as angina and myocardial infarction; stroke occurs more rarely. Xanthomas (cholesterol deposits in tendons) may be visible in the Achilles tendons or tendons of the hands and worsen with age as a result of extremely high cholesterol levels. Xanthelasmas (yellowish, waxy deposits) can occur around the eyelids. Individuals with FH may develop corneal arcus (white, gray, or blue opaque ring in the corneal margin as a result of cholesterol deposition) at a younger age than those without FH. Individuals with a more severe phenotype, often as a result of biallelic variants, can present with very significant elevations in LDL-C (>500 mg/dL), early-onset coronary artery disease (CAD; presenting as early as childhood in some), and calcific aortic valve disease.

Hyperlipoproteinemia type ID is a rare autosomal recessive disorder characterized by impaired clearance of triglyceride (TG)-rich lipoproteins in plasma, leading to severe hypertriglyceridemia (chylomicronemia). Clinical features include eruptive xanthomas, lipemia retinalis, hepatosplenomegaly, episodes of abdominal pain, and pancreatitis. Onset usually occurs in adulthood (summary by Brahm and Hegele, 2013). For a discussion of genetic heterogeneity of familial chylomicronemia, see 238600.

Individuals with biallelic APOB-related familial hypobetalipoproteinemia (APOB-FHBL) may present from infancy through to adulthood with a range of clinical symptoms including deficiency of fat-soluble vitamins and gastrointestinal and neurologic dysfunction. Affected individuals typically have plasma total cholesterol, LDL cholesterol, and apo B levels below the fifth centile for age and sex. Acanthocytosis, elevated liver enzymes, and hyperbilirubinemia may also be found. The most common clinical findings are hepatomegaly, steatorrhea, and failure to thrive / growth deficiency. In the absence of treatment, affected individuals can develop atypical pigmentation of the retina; progressive loss of deep tendon reflexes, vibratory sense, and proprioception; muscle pain or weakness; dysarthria; ataxia; tremors; and steatohepatitis, fibrosis, and rarely, cirrhosis of the liver. Individuals with a heterozygous, typically truncating pathogenic variant in APOB are usually asymptomatic with mild liver dysfunction and hepatic steatosis. However, about 5%-10% of individuals with heterozygous APOB-FHBL develop relatively more severe nonalcoholic steatohepatitis requiring medical attention and occasionally progressing to cirrhosis, albeit very rarely.

Squalene synthase deficiency (SQSD) is a rare inborn error of cholesterol biosynthesis with multisystem clinical manifestations similar to Smith-Lemli-Optiz syndrome. Key clinical features include facial dysmorphism, a generalized seizure disorder presenting in the neonatal period, nonspecific structural brain malformations, cortical visual impairment, optic nerve hypoplasia, profound developmental delay / intellectual disability, dry skin with photosensitivity, and genital malformations in males.

Microcephaly, congenital cataract, and psoriasiform dermatitis (MCCPD) represents an inborn error of cholesterol metabolism that is characterized by accumulation of a large amount of methylsterols, particularly dimethylsterols, in affected patients. The associated features of immune dysregulation, skin disease, and growth delay can be at least partially corrected with cholesterol and statin supplements (He et al., 2014).