Familial hypobetalipoproteinemia is a codominant disorder characterized by low plasma levels of
low-density lipoprotein cholesterol and
apolipoprotein B (
apoB), which in ∼50% of the cases is due to mutations in
APOB gene. In most cases, these mutations cause the formation of truncated apoBs of various sizes, which have a reduced capacity to bind
lipids and form
lipoprotein particles. Here, we describe 2 children with severe
hypobetalipoproteinemia found to be homozygous for novel
APOB gene mutations. The first case (HBL-201) was an asymptomatic 13-year-old boy incidentally found to have slightly elevated serum
transaminases associated with hepatic steatosis. He was homozygous for a truncated
apoB (2211
amino acids,
apoB-48.74) whose size is similar to that of wild-type
apoB-48 (2152
amino acids) produced by the intestine. ApoB-48.74 is expected to be incorporated into
chylomicrons in the intestine but might have a reduced capacity to form secretion-competent
very low-density lipoprotein in the liver. The second patient (HBL-96) was a 6-month-old girl suspected to have
abetalipoproteinemia, for the presence of chronic
diarrhea,
failure to thrive, extremely severe
hypobetalipoproteinemia, and low plasma levels of
vitamin E and
vitamin A. She was homozygous for a
nonsense mutation (Gln513*) resulting in a short truncated
apoB (apoB-11.30), which is not secreted into the plasma. In this patient, the impaired
chylomicron formation is responsible for the severe clinical manifestations and growth retardation. In homozygous
familial hypobetalipoproteinemia, the capacity of truncated apoBs to form
chylomicrons is the major factor, which affects the severity of the clinical manifestations.