OMIM ID:
Abetalipoproteinemia
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
The major ocular manifestations of abetalipoproteinemia are in the retina which develops diffuse and sometimes patchy pigmentary changes often called atypical retinitis pigmentosa. In other cases the picture resembles retinitis punctata albescens with perivascular white spots in the peripheral retina. Night blindness is an early and prominent symptom with abnormal dark adaptation thresholds evident before fundus pigment changes are seen. The ERG shows loss of rod function before that of cone function. The macula may or may not be affected while peripheral fields are often severely constricted. Loss of photoreceptors occurs throughout life and visual fields show progressive constriction, sometimes with central sparing. A single case of bilateral disc swelling in a 9 year-old girl has been reported.
Systemic Features
Celiac disease and steatorrhea due to a deficiency of circulating chylomicra underlie the malabsorption of vitamins A and E which is probably responsible for the majority of systemic manifestations. Red blood cells have a peculiar burr-like morphology that has led to the designation ‘acanthocytes’. Liver failure and cirrhosis sometimes occur. Plasma lipids are generally low including cholesterol, triglycerides, and beta lipoproteins. Central and peripheral nerve demyelination occurs leading to a progressive ataxia and other neurological symptoms.
Genetics
Inheritance
This autosomal recessive disease seems to result from an inability to synthesize the apoB peptide that is a part of the LDL and VLDL. A mutation in the MTP gene (4q22-q24) is responsible. The gene is sometimes called MTTP as it codes for micosomal triglyceride transfer protein.
Acanthocytosis is also a feature in the autosomal recessive condition known as chorea-acanthocytosis (200150), a progressive degenerative movement disorder primarily affecting the limbs resulting from mutations in the VPS13A gene.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.