The Lp(a) lipoprotein represents a quantitative genetic trait. It contains two different polypeptide chains, the Lp(a) glycoprotein and apo B-100. We have demonstrated the Lp(a) glycoprotein directly in human sera by sodium dodecyl sulfate-gel electrophoresis under reducing conditions after immunoblotting using anti-Lp(a) serum and have observed inter-and intraindividual size heterogeneity of the glycoprotein with apparent molecular weights ranging from 400,000-700,000 D. According to their relative mobilities compared with apo B-100 Lp(a) patterns were categorized into phenotypes F (faster than apo B-100), B (similar to apo B-100), Si, S2, S3, and S4 (all slower than apo B-100), and into the respective double-band phenotypes. Results from neuraminidase treatment of isolated Lp(a) glycoprotein indicate that the phenotypic differences do not reside in the sialic acid moiety of the glycoprotein. Family studies are compatible with the concept that Lp(a) glycoprotein phenotypes are controlled by a series of autosomal alleles (LplaIF, LplajB, Lplar', Lplar2, Lplae, Lpla'4, and Lplar) at a single locus. Comparison of Lp(a) plasma concentrations in different phenotypes revealed a highly significant association of phenotype with concentration.Phenotypes B, Si, and S2 are associated with high and phenotypes S3 and S4 with low Lp(a) concentrations. This suggests that the same gene locus is involved in determining Lp(a) glycoprotein phenotypes and' Lpa) lipoprotein concentrations in plasma and is the first indication for structural differences underlying the quantitative genetic Lp(a)-trait.

Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive malformation syndrome, ranges in clinical severity from mild dysmorphism and moderate mental retardation to severe congenital malformation and intrauterine lethality. Mutations in the gene for Delta7-sterol reductase (DHCR7), which catalyzes the final step in cholesterol biosynthesis in the endoplasmic reticulum (ER), cause SLOS. We have determined, in 84 patients with clinically and biochemically characterized SLOS (detection rate 96%), the mutational spectrum in the DHCR7 gene. Forty different SLOS mutations, some frequent, were identified. On the basis of mutation type and expression studies in the HEK293-derived cell line tsA-201, we grouped mutations into four classes: nonsense and splice-site mutations resulting in putative null alleles, missense mutations in the transmembrane domains (TM), mutations in the 4th cytoplasmic loop (4L), and mutations in the C-terminal ER domain (CT). All but one of the tested missense mutations reduced protein stability. Concentrations of the cholesterol precursor 7-dehydrocholesterol and clinical severity scores correlated with mutation classes. The mildest clinical phenotypes were associated with TM and CT mutations, and the most severe types were associated with 0 and 4L mutations. Most homozygotes for null alleles had severe SLOS; one patient had a moderate phenotype. Homozygosity for 0 mutations in DHCR7 appears compatible with life, suggesting that cholesterol may be synthesized in the absence of this enzyme or that exogenous sources of cholesterol can be used.

Lipoprotein(a) [Lp(a)] is a quantitative trait in human plasma. Lp(a) consists of a low-density lipoprotein and the plasminogen-related apolipoprotein(a) [apo(a)]. The apo(a) gene determines a size polymorphism of the protein, which is related to Lp(a) levels in plasma. In an attempt to gain a deeper insight into the genetic architecture of this risk factor for coronary heart disease, we have investigated the basis of the apo(a) size polymorphism by pulsed field gel electrophoresis of genomic DNA employing various restriction enzymes (SwaI, KpnI, KspI, SfiI, NotI) and an apo(a) kringle-IV-specific probe. All enzymes detected the same size polymorphism in the kringle IV repeat domain of apo(a). With KpnI, 26 different alleles were identified among 156 unrelated subjects; these alleles ranged in size from 32 kb to 189 kb and differed by increments of 5.6 kb, corresponding to one kringle IV unit. There was a perfect match between the size of the apo(a) DNA phenotypes and the size of apo(a) isoforms in plasma. The apo(a) DNA polymorphism was further used to estimate the magnitude of the apo(a) gene effect on Lp(a) levels by a sib-pair comparison approach based on 253 sib-pairs from 64 families. Intra-class correlation of log-transformed Lp(a) levels was high in sib-pairs sharing both parental alleles (r = 0.91), significant in those with one common allele (r = 0.31), and absent in those with no parental allele in common (r = 0.12). The data show that the intra-individual variability in Lp(a) levels is almost entirely explained by variation at the apo(a) locus but that only a fraction (46%) is explained by the DNA size polymorphism. This suggests further heterogeneity relating to Lp(a) levels in the apo(a) gene.

Lipoprotein(a) contains one copy each of apolipoprotein B-100 and apolipoprotein(a). It has been hypothesized that a disulfide bond might exist between Cys4O" of apolipoprotein (a) Lp(a) represent an independent risk factor for the development of premature atherosclerosis (4, 5). Mice transgenic for human apo(a) develop atherosclerosis upon cholesterol feeding (6).Cloning and sequencing of the first complete apo(a) cDNA from human liver (7) revealed a striking homology between apo(a) and plasminogen. Both proteins belong to a protein family characterized by the presence of one or more kringle domains with six highly conserved cysteine residues, which are believed to be involved in three intradomain disulfide bridges. Plasminogen contains single copies of five different kringle domains and a serine protease domain. Referring to the numbering of plasminogen kringles, the first sequenced apo(a) cDNA encoded a glycoprotein with 37 tandemly repeated kringle IV-like domains, a single kringle V domain, and a serine protease domain. Ten distinct kringle IV domains were encoded by the sequenced apo(a) cDNA. One of them occurred in 28 identical copies; the others were unique.In human plasma, almost all apo(a) is present as a macromolecular Lp(a) complex. As the dissociation of the LDL moiety and apo(a) in Lp(a) requires reducing conditions, a

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