Background: Familial Hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by fatty liver and reduced plasma levels of low density lipoprotein (LDL) and its protein constituent apolipoprotein B (apoB). FHBL is linked to APOB gene in some but not all known cases. In a group of 59 FHBL patients genotyped for APOB gene mutations we found three novel splice site mutations: c.904+4A>G in intron 8, c.3843-2A>G in intron 24 and c.4217-1G>T in intron 25.
Objective: To assess the effects of these mutations of apoB pre-mRNA splicing.
Methods: We analysed apoB mRNA in the liver of one proband and in cells expressing APOB minigenes harbouring the mutations found in the other probands.
Results: In the liver of the c.3843-2A>G carrier we identified an apoB mRNA devoid of exon 25, predicted to encode a truncated peptide of 1260 amino acids. The analysis of minigene transcripts in COS-1 cells showed that c.904+4A>G mutation caused the formation of an mRNA devoid of exon 8,predicted to encode a short apoB of 247 amino acids. The minigene harbouring the c.4217-1G>T mutation in intron 25 generated an mRNA in which exon 25 joined to a partially deleted exon 26, resulting from the activation of an acceptor site in exon 26; this mRNA is predicted to encode a truncated protein of 1380 amino acids. All these truncated apoBs were not secreted as constituents of plasma lipoproteins.
Conclusions: These findings demonstrate the pathogenic effect of rare splice site mutations of APOB gene found in FHBL.
- Apolipoprotein B gene
- Familial hypobetalipoproteinemia
- Intronic mutations
- Splicing defects
- Truncated apolipoprotein B