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.

This syndrome is characterized by the association of an axonal sensory and autonomic neuropathy with spastic paraplegia.

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).

Congenital disorder of glycosylation type IIt (CDG2t) is an autosomal recessive multisystemic metabolic disorder characterized by global developmental delay, poor overall growth, severely impaired intellectual development with absent language, and behavioral abnormalities. Most patients develop early-onset seizures; brain imaging tends to show white matter abnormalities. Variable dysmorphic features, including long face, almond-shaped eyes, protruding maxilla, and short philtrum, are also present. The disorder, which is associated with low levels of HDL cholesterol, results from defective posttranslational O-linked glycosylation of certain plasma lipids and proteins (summary by Zilmer et al., 2020). For an overview of congenital disorders of glycosylation, see CDG1A (212065) and CDG2A (212066).

Apolipoprotein (apo) A-I is the major protein of HDL cholesterol, whereas apoC-III and apoA-IV are minor components. The genes coding for apoA-I, apoC-III, and apoA-IV are adjacent to one another on the long arm of chromosome 11. Familial apolipoprotein gene cluster deletion syndrome has been described in 1 family and found to be a homozygous deletion of the entire APOA1/C3/A4 gene complex. This results in a lack of expression of these plasma lipoproteins, with marked HDL-C deficiency in the homozygote and approximately half-normal levels of these apolipoproteins and HDL-C in the heterozygotes.