Homozygous familial hypobetalipoproteinemia: two novel mutations in the splicing sites of apolipoprotein B gene and review of the literature

Objective: Familial hypobetalipoproteinemia (FHBL) is autosomal codominant disorder of lipoprotein metabolism characterized by low plasma levels of total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and apolipoprotein B (apoB) below the 5th percentile of the distribution in the population. Patients with the clinical diagnosis of homozygous FHBL (Ho-FHBL) are extremely rare and few patients have been characterized at the molecular level. Here we report the medical history and the molecular characterization of one paediatric patient with clinical features of Ho-FHBL. Methods: A one month old infant with failure to thrive, severe hypocholesterolemia and acanthocytosis was clinically and genetically characterized. Molecular characterization of the proband and her parents was performed by direct sequencing of the APOB gene and functional role of the identified mutations was assessed by the minigene methodology. Results: The proband was found carrying two novel splicing mutations of the APOB gene (c.3696+1G>C and c.3697-1G>A). CHOK1H8 cells expressing minigenes harbouring the mutations showed that these two mutations were associated with the retention of intron 23 and skipping of exon 24, resulting in two truncated apoB fragments of approximate size of 26-28 % of ApoB-100 and the total absence of apoB. Conclusion: We describe the first case of Ho-FHBL due to two splicing mutations affecting both the donor and the acceptor splice sites of the same intron of the APOB gene occurring in the same patient.The clinical management of the proband is discussed and a review of the clinical and genetic features of the published Ho-FHBL cases is reported.