Molecular basis of an apolipoprotein[a] null allele: a splice sitemutation is associated with deletion of a single exon

Abstract

Apolipoprotein[a] (apo[a]), a unique component of atherogenic lipoprotein[a], is highly polymorphic in human and nonhuman primates. Null alleles, producing no detectable circulating Lp[a] or apo[a] isoforms, are found at high frequencies. The molecular basis of null alleles is not yet known. In baboons, approximately two-thirds of null alleles do not produce detectable hepatic transcripts (transcript negative nulls), and one-third of null alleles produce normal amounts of apo[a] transcripts (transcript positive nulls). We have cloned apo[a] cDNA from a baboon carrying a transcript positive null allele defective in secretion from primary hepatocytes. Compared with wild-type cDNA, the null allele contained an in-frame 47 amino acid deletion in the protease domain corresponding to one exon of the apo[a] gene. The null allele contains an A ¨ T substitution in the third nucleotide position of the intron downstream of the deleted exon which alters the donor splice site consensus sequence. Thus, this null is likely due to a mutation that prevents normal mRNA splicing, yielding a shortened protein that may be defective in intramolecular interactions required for normal processing and secretion of apo[a]. This is the first report of a molecular basis for apo[a] null alleles.-Cox, L. A., C. Jett, and J. E. Hixson. Molecular basis of an apolipoprotein[a] null allele: a splice site mutation is associated with deletion of a single exon.

Role of Calnexin, Calreticulin, and Endoplasmic Reticulum Mannosidase I in Apolipoprotein(a) Intracellular Targeting

“…Using a gel-based folding assay (21), however, we were unable to demonstrate any effect on apo(a) folding when apo(a) interaction with CNX and CRT was either enhanced or prevented by the co-or post-translational inhibition of glucosidase II (ref 21, and data not shown). This may reflect an inability of the folding assay to detect disulfide bond-independent changes in conformation, or differences restricted to one or a few domains in apo(a) (21,23,39). More sensitive assays will be required to determine the precise role of CNX and CRT in the apo(a) folding pathway.…”

“…The mechanism of apo(a) retrotranslocation and whether this involves the Sec61 protein channel or transport to a post-ER compartment (not indicated) are uncertain. Previous studies have demonstrated that a deletion in the protease domain of apo(a) is sufficient to cause complete ER retention and ERAD of apo(a) ( , ). …”

“…Large apo(a) proteins have longer ER residence times and tend to be subject to more ERAD than small apo(a) isoforms (15,16,22), accounting for the inverse correlation between apo(a) size and plasma Lp(a) levels (17,18). Other amino acid sequence variations can also influence the extent of apo(a) ER retention and degradation (15,21,23). These unique properties make apo(a) a valuable tool with which to analyze the mechanisms of protein processing and quality control in the ER.…”

Lipoprotein(a) beyond the kringle IV repeat polymorphism: The complexity of genetic variation in the LPA gene

“…On the one hand, large isoforms may fail to mature properly in the endoplasmatic reticulum and are degraded before being secreted [ 27 , 28 , 110 ]. On the other hand, LOF variants can suppress mRNA or protein production [ 24 , 63 , 111 – 113 ]. In contrast to several other examples described in this review, such variants may act independently from the background apo (a) isoform; however, depending on the apo(a) isoform with which they occur, the size of the Lp(a) lowering effect might be variable ( Fig.…”

Determinants of human apolipoprotein [a] secretion from mouse hepatocyte cultures

“…1 To whom correspondence should be addressed. (30), presumably because of the presence of destabilizing amino acid sequences in some of these variants (33,34). Although plasma Lp[a] levels vary enormously among individuals, levels within an individual remain remarkably constant (4) and, in comparison with other lipoproteins, are remarkably refractory to modulation by dietary and pharmacological intervention.…”