Apolipoprotein(a) (Apo(a)) Glycoprotein Isoforms Result from Size Differences in Apo(a) mRNA in Baboons

Plasma lipoprotein [a] (Lp[a]) concentrations are inversely associated with, and largely determined by, apolipoprotein [a] (apo[a]) gene size, a highly polymorphic trait. We studied if, within an individual, the smaller apo[a] isoform always dominated, whether there was interaction between the two alleles, and whether these features differed between Caucasians and African Americans. We determined apo[a] gene sizes, apo[a] protein sizes and relative amounts, and plasma Lp[a] levels in 430 individuals (263 Caucasians and 167 African Americans). Of the 397 heterozygotes with at least one detectable apo[a] isoform (238 Caucasians and 159 African Americans), the larger allele dominated in 28% of Caucasians and 23% of African Americans, while the smaller allele dominated in 56% of Caucasians and 45% of African Americans. In Caucasians, dominance of the smaller allele increased with Lp[a] levels, from 44% at Lp[a] р 30 nM to 81% at Lp[a] Ͼ 100 nM ( P Ͻ 0.0001). Dominance by the smaller allele increased with increasing size of the larger allele in both groups but with the smaller allele only in African Americans. There was no interaction between apo[a] alleles within genotypes; one apo[a] isoform level was not associated with the other isoform level, and isoform levels were not affected by the difference in size. More of the dominance pattern was explained by Lp[a] level and apo[a] genotype in African Americans than in Caucasians (29% vs. 13%). Thus, genotype influences isoform-specific Lp[a] levels and dominance patterns differently in African Americans and in Caucasians.

Section: Discussionmentioning

confidence: 99%

Apolipoprotein [a] genotype influences isoform dominance pattern differently in African Americans and Caucasians

Rubin

Paultre

Tuck

et al. 2002

“…It seems likely that mutations in other, as yet unknown, promoter elements must be re- sponsible for transcript negative nulls. Previous studies in baboons found that transcript positive null alleles produced different sized hepatic apo[a] transcripts, presumably resulting from different sized apo[a] alleles (10). Previous studies in humans found that null alleles were associated with different sizes of apo[a] alleles as measured by Southern blotting of genomic DNA (33).…”

Section: Discussionmentioning

confidence: 99%

“…Little is known about the molecular basis of apo [a] null alleles or the physiological consequences of carrying apo[a] null alleles. In baboons, we have shown that approximately two-thirds of null alleles do not produce hepatic apo [a] transcripts that are detectable by Northern blot analysis (called transcript negative nulls) (10). Approximately one-third of null alleles produce normal levels of hepatic apo[a] transcripts (transcript positive nulls).…”

Section: Lipoprotein[a] (Lp[a]mentioning

confidence: 99%

See 1 more Smart Citation

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

Cox

Jett

Hixson

1998

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.

“…The rate of apo[a] secretion is likely determined at multiple levels. Differences in hepatic apo[a] mRNA concentration imply a major role for apo [a] gene transcription rate and/ or mRNA stability (27)(28)(29). Posttranscriptional mechanisms are also important.…”

Section: Apolipoprotein [A] (Apo[a]) Is the Unique Glycoprotein Component Of Plasma Lipoprotein [A] (Lp[a]mentioning

confidence: 99%

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

Wang

Boedeker

Hobbs

et al. 2001

Efforts to develop an in vitro model system to analyze apolipoprotein [a] (apo[a]) gene transcription, mRNA translation, and protein secretion have been complicated by the limited tissue and species distribution of apo[a] and the presence of regulatory DNA sequences remote from the apo[a] transcription start site. In the current study we examined primary hepatocytes cultured from apo[a] transgenic mice as a model system for analyzing apo[a] biogenesis. Hepatocytes from mice transgenic for a yeast artificial chromosome (YAC) encoding the entire apo[a] gene in its own genomic context (YAC-apo[a] hepatocytes) were unable to maintain apo[a] expression beyond 48 h of culture. This suggests that the apo[a] promoter was not active in cultured YAC-apo[a] hepatocytes. In contrast, apo[a] expression was maintained for at least 7 days in hepatocytes cultured from mice transgenic for an apo[a] cDNA under control of the mouse transferrin promoter (transferrin-apo[a] hepatocytes). Pulse-chase experiments established that more than 80% of apo[a] synthesized by both transferrin-apo[a] and YAC-apo[a] hepatocytes was degraded prior to secretion, independently of the coexpression of human apoB. Thus, low secretion efficiency appears to be a general characteristic of human apo[a] proteins in mouse liver. Apo[a] secretion was increased somewhat (from 18% to 32%) in the presence of lipoprotein-containing serum. Transformed cell lines derived from transferrin apo[a] hepatocytes retained characteristics of apo[a] secretion similar to those observed in primary cells. Primary and transformed apo[a] transgenic hepatocytes may provide valuable additional models with which to study posttranslational mechanisms regulating apo[a] secretion. -Wang, J., J. Boedeker, H. H. Hobbs, and A. L. White. Determinants of human apolipoprotein [a] secretion from mouse hepatocyte cultures.