Linkage analysis in Dutch familial atypical multiple mole-melanoma (FAMMM) syndrome families. Effect of naevus count

Familial atypical multiple mole-melanoma (FAMMM) syndrome is characterized by the familial occurrence of malignant melanoma of the skin in combination with multiple atypical precursor naevi. In the present study we performed linkage analysis in seven Dutch FAMMM families to define the relationship between the ultimate phenotype melanoma and the postulated precursors, atypical (dysplastic) naevi. Various models were defined, varying from melanoma only to various combinations of melanoma and atypical naevi, reflecting the FAMMM phenotype. Using 124 microsatellite markers spread across all autosomes, hints for linkage were obtained between several chromosome 9p markers and a melanoma locus (D9S171; odds for linkage, 275:1). In a model including melanoma and a florid manifestation of atypical naevi a considerably higher lod score was obtained with D9S171 (odds for linkage, 4365:1); models including milder manifestations yielded less support. We conclude that, also in the Dutch FAMMM families, a melanoma gene is located on the short arm of chromosome 9 and that multiple atypical naevi, at least in certain cases, seems to be a component of the FAMMM phenotype.

Structure and expression of the mouse apolipoprotein C2 gene

Three cDNA clones containing the mouse apolipoprotein C2 (Apoc2) gene were isolated from a mouse liver cDNA library. The inserts from two cDNA clones were 500 bp in size while the insert from the third clone was unexpectedly large, 962 bp. All three clones contained a single open reading frame encoding apoC2. The exon-intron structure of the mouse Apoc2 gene was determined by sequence analysis. Northern blotting and primer extension analysis of mouse RNA showed that the major liver transcript is 500 bp in size and is encoded by four exons. Transcripts for Apoc2 were found in fetal liver, adult liver, intestine, and peritoneal macrophages. The largest cDNA clone, mAPOC2c4, contained an additional 440 bp at the 5' end that are evolutionary conserved between man and mouse. These additional sequences are encoded by two exons located 5' to the major liver start site. Although the larger transcript could not be detected by Northern blot analysis, products resulting from an upstream transcription initiation site were detected in the liver using RT-PCR analysis. The sizes of the RT-PCR products are consistent with alternative splicing.

Identification of a splice-site mutation in the low density lipoprotein receptor gene by denaturing gradient gel electrophoresis

We have applied the denaturing gradient gel electrophoresis (DGGE) technique to detect sequence variations in exon 9 of the low density lipoprotein receptor (LDLR) gene in individuals with heterozygous familial hypercholesterolemia (FH). A fragment containing exon 9 and 25 base pairs (bp) of the intron boundary sequence at either side was amplified. To this fragment a 40-bp GC-clamp was attached by the polymerase chain reaction (PCR). We have analyzed a total of 165 DNA samples of FH patients and have detected a mutation in three cases. Two patients were found to have the previously described "South African" G to A transition in codon 408. In a third patient, we observed a different banding pattern of the DNA fragments on DGGE indicating a different mutation. The mutant homoduplex band of this sample was purified from the gel, cloned in an AT-vector and sequenced. Sequence analysis demonstrated a G to A transition of the consensus G-nucleotide at the intron 9 splice donor site. Cosegregation between this mutation and elevated plasma cholesterol levels was observed in family members of this FH patient. This mutation probably prevents normal splicing of the mRNA and represents the first identified splice-site mutation in the LDLR gene. We conclude that the use of DGGE of GC-clamped PCR-amplified exon sequences offers a general strategy for the detection of disease-producing mutations in the LDLR gene.

Hereditary spastic paraparesis. Clinical and genetic data from a large Dutch family

A large Dutch family is presented with pure hereditary spastic paraparesis. Pedigree analysis indicates autosomal dominant inheritance with complete penetrance and possibly anticipation in successive generations. These observations may have implications for genetic studies. Linkage studies in this family have excluded more than 40% of the autosomal genome.

Transgenic mice carrying the apolipoprotein E3-Leiden gene exhibit hyperlipoproteinemia

Apolipoprotein (apo) E3-Leiden, described in a large Dutch family, is associated with a dominantly inherited form of familial dysbetalipoproteinemia. To study the effect of the APOE*3-Leiden mutation in vivo, transgenic mice were generated using a genomic 27-kilobase DNA construct isolated from the APOE*3-Leiden proband. This construct carried the APOE gene, the APOC1 gene, and all known regulatory elements including an element that mediates liver expression. Three strains were generated that showed human APOE and APOC1 expression. All strains had significantly elevated levels of total plasma cholesterol and triglycerides on a regular diet. When mice of one strain were fed a semisynthetic cholesterol-rich diet, total plasma cholesterol and triglyceride levels increased dramatically. This increase was observed mainly in the very low density lipoprotein (VLDL)- and low density lipoprotein (LDL)-sized fractions. In cholesterol-fed mice, the apoE3-Leiden protein became equally distributed between the VLDL/LDL and HDL-sized fractions, while in mice kept on a regular diet, apoE3-Leiden protein was mainly associated with HDL-sized fractions. The presence of hyperlipoproteinemia in the APOE*3-Leiden-expressing transgenic mice supports our finding that the apoE3-Leiden variant behaves like a dominant trait in the expression of familial dysbetalipoproteinemia. ApoE3-Leiden transgenic mice may serve as a model to elucidate additional factors involved in the metabolism of apoE containing remnant lipoproteins in general and the etiology of familial dysbetalipoproteinemia in particular.

Low density lipoprotein receptor internalizes low density and very low density lipoproteins that are bound to heparan sulfate proteoglycans via lipoprotein lipase

It has previously been shown that lipoprotein lipase (LPL) enhances the binding of low density lipoproteins (LDL) and very low density lipoproteins (VLDL) to HepG2 cells and fibroblasts, up to 80-fold. This increase in binding is LDL receptor-independent and is due to a bridging of LPL between extracellular heparan sulfate proteoglycans (HSPG) and the lipoproteins. In the present paper, we show that preincubation of the cells with LPL, followed by washing prior to the binding experiment, increased binding to the same extent as occurs when the binding is performed in the presence of LPL. This indicates that the formation of a complex of LPL with the lipoproteins is not a prerequisite of binding. Binding curves and Scatchard analyses reveal that both the number of binding sites and the affinity of the binding are increased 20-30-fold by the addition of 3.4 micrograms/ml LPL. The addition of LPL also resulted in an enhanced uptake and subsequent lysosomal degradation of both LDL and VLDL when compared with binding, although to a lesser extent (up to 25-fold when measured after 5 h at 37 degrees C). Strikingly, enhanced uptake did not occur in LDL receptor-negative fibroblasts. In addition, down-regulation of the LDL receptor activity by preincubation of the cells for 48 h with either LDL or beta-VLDL resulted in a parallel decrease in the uptake of LPL-mediated HSPG-bound LDL, whereas the LPL-mediated binding itself was not diminished. These observations indicate that the uptake of LPL-mediated HSPG-bound LDL and VLDL mainly proceeds via the LDL receptor. Binding of labeled LDL to the cells at 4 degrees C for 2 h followed by a chase period at 37 degrees C revealed that in absolute terms, the initial rate of internalization of HSPG-bound LDL is comparable with that of LDL receptor-bound LDL (0.58 and 0.44 ng/min/mg of cell protein, respectively). We conclude that in LDL receptor-positive cells, the LPL-mediated binding of LDL and VLDL to HSPG is followed by internalization of the lipoproteins mainly through the rapid process of the classical LDL receptor recycling system, whereas only a minor portion is internalized via the much slower process of HSPG uptake.

The effect of the apolipoprotein E phenotype on plasma lipids is not influenced by environmental variability: results of a Dutch twin study

We tested the influence of the apolipoprotein E (apoE) polymorphism on the intrapair differences in the levels of plasma cholesterol, plasma triglycerides, low density lipoprotein-cholesterol, apoB and apoE in monozygotic (MZ) twins, and estimated whether or not there was a interaction between the apoE polymorphism and environmental factors. In 65 MZ twin pairs, the intrapair differences in the measured lipoprotein parameters were similar in the different apoE phenotype classes. This indicates that the effect of the apoE polymorphism is not influenced by environmental variability between the MZ pair members and accordingly identifies the APOE gene as a "level" gene.

The mouse low density lipoprotein receptor gene: cDNA sequence and exon-intron structure

The low density lipoprotein (LDL) receptor plays a central role in the cholesterol metabolism. The cDNA sequence of the mouse low density lipoprotein receptor (Ldlr) gene has been determined and shows 76% homology with the human gene. The exon-intron structure has been determined for the 129/J mouse strain. The gene is composed of 18 exons and spans a region of 28 kb. In addition, the promoter regions of the mouse and human genes are homologous. Northern blot analysis revealed an mRNA of approximately 5 kb. The cloning of the Ldlr gene will enhance the usefulness of the mouse for the study of cholesterol metabolism and, in particular, for carrying out gene targeting experiments.

Low-density lipoproteins are degraded in HepG2 cells with low efficiency

In previous studies we have shown that in HepG2 cells, as compared with fibroblasts, the low-density lipoprotein (LDL) receptor is only weakly down-regulated upon incubation of the cells with LDL. To explain this difference in down-regulation of the LDL-receptor activity, we studied simultaneously the intracellular processing of 125I-labelled LDL in both cell lines. Upon incubation of HepG2 cells with 125I-LDL, the appearance of degradation products started at 90 min, whereas in fibroblasts this lag time was only 30 min. The degradation efficiency (representing the ratio degradation/cell association of LDL) in HepG2 was less than 50% of that in fibroblasts up to 5h of incubation at 37 degrees C. The longer lag time and low efficiency of the degradation of LDL in HepG2 cells were independent of the cell density. Pulse-chase experiments indicated that the internalization rate of surface-bound LDL in HepG2 cells is similar to that of fibroblasts. Endosomal loading of 125I-LDL by incubation at 18 degrees C for 4.5 h, followed by a shift to 37 degrees C, resulted in degradation of LDL within 30 min in fibroblasts, whereas in HepG2 cells the lag time of the degradation was 90 min. In parallel experiments using subcellular fractionation by Percoll-gradient centrifugation of homogenized cells and 125I-tyramine-cellobiose-labelled LDL, we observed that in both cell types LDL is equally rapidly shifted from a low- to a high-density compartment (within 15 min), representing the endosomal and the late-endosomal plus lysosomal compartment respectively. We conclude that in HepG2 cells the cell-bound LDL, upon internalization, goes through the intracellular itinerary at the same rate as in fibroblasts, but that either the fusion between late endosomes and lysosomes or the lysosomal degradation itself is proceeding at a lower efficiency. A low degradation rate of LDL may contribute to explain the relatively weak down-regulation of the LDL-receptor activity in HepG2 cells by LDL.

Facioscapulohumeral muscular dystrophy: the impact of genetic research

Recent developments in genetic research have led to the localization and identification of the causative gene defect in a large number of neurological diseases. This paper describes some of the basic principles of molecular genetics and the strategies that have been followed in the search for the gene for facioscapulohumeral muscular dystrophy (FSHD), beginning with the recent localization to chromosome 4q. Many questions remain concerning the pathogenesis and possible genetic heterogeneity of this autosomal dominant myopathy. Hitherto, most evidence favours a genetically homogeneous disorder, but only the isolation and detailed characterization of the FSHD gene will resolve these issues completely.

Treatment of E2E2 homozygous familial dysbetalipoproteinemic subjects with gemfibrozil does not enhance the binding of their d < 1.019 lipoprotein fraction to the low-density lipoprotein receptor

Six E2E2 homozygous familial dysbetalipoproteinemic (FD) patients were treated with gemfibrozil (2 x 600 mg/d) for a period of 4 weeks. For all subjects, normalization of serum cholesterol concentrations with treatment did not result in a significant change in the cholesterol/triglyceride ratio of the d < 1.019 lipoprotein fraction. In addition, the binding efficiency of this lipoprotein fraction to the low-density lipoprotein (LDL) receptor on HepG2 cells did not change consistently with treatment. We conclude that normalization of serum cholesterol concentrations in FD patients by treatment with gemfibrozil is the result of an effect of gemfibrozil on the synthesis of d < 1.019 lipoproteins rather than an effect on the receptor-mediated clearance of these particles.

Evolutionary conservation of the mouse apolipoprotein e-c1-c2 gene cluster: structure and genetic variability in inbred mice

The human apolipoprotein E (APOE), APOC1, pseudo APOC1 (APOC1'), and APOC2 genes are clustered within 48 kb on the long arm of chromosome 19. A mouse Apoe cDNA probe was used to isolate overlapping cosmid clones from a cosmid library of the C57BL/Rij inbred mouse strain. These clones were investigated for the presence of the Apoc1 and Apoc2 genes by heterologous hybridization. Our results show that the Apoe-c1-c2 gene cluster is conserved in the mouse. In line with evolutionary data, the mouse lacks the equivalent of APOC1'. These data were confirmed using a mouse Apoc2 cDNA clone, and surprisingly the cDNA clone isolated here was 965 bp in size, which is on average 450 bp longer than other APOC2 cDNAs described so far. Correspondingly, the Apoc2 gene occupies an unusually large genomic region, due to an extended 5' end. Interestingly, a variable number of tandem repeat (VNTR) in the third intron of the human APOC2 gene shows a high sequence homology and is located at the identical position in the mouse gene. Despite the high copy number of this VNTR (27 or 34 copies) only two variants were found among 11 different inbred strains. With the aid of six restriction fragment length variations in this gene cluster only two different haplotypes could be deduced, indicating that the Apoe-c1-c2 gene cluster is highly conserved in the inbred strains that were studied.

Lipoprotein profile of a Greenland Inuit population. Influence of anthropometric variables, Apo E and A4 polymorphism, and lifestyle

Previously it has been reported that Greenland Inuit (Eskimos) from the Uummannaq district display low levels of plasma cholesterol and triglycerides and relatively high levels of high density lipoprotein (HDL) when compared with healthy Danish control subjects (Lancet 1971;1:1143-1146). Here we present data obtained in 1989 that show the following. In a group of 133 healthy adult Greenland Inuit from Nanortalik, the levels of plasma cholesterol and low density lipoprotein (LDL) cholesterol (6.39 and 4.39 mmol/l, respectively) were slightly higher than "normal" values found in western societies, whereas the HDL cholesterol level was markedly higher (1.64 mmol/l). Compared with most Caucasian populations, the Inuit population we studied exhibits a high apolipoprotein (APO)E*4 allele frequency (0.229), whereas the APOE*2 allele frequency was extremely low (0.015). In contrast to Caucasian populations, in the Inuit population the apoE polymorphism showed only a minor influence on the plasma lipid and (apo)lipoprotein levels, as evaluated by multiple regression analysis, with the exception of apoE levels. This absence of an effect could be explained by the low very low density lipoprotein (VLDL) plus intermediate density lipoprotein (IDL) cholesterol levels. The contributions of eicosapentaenoic acid and linoleic acid to the total amount of fatty acids in plasma cholesterol esters differed markedly from those reported in 1971 for another Greenland Inuit population (3.2% versus 15.8% and 49.5% versus 20.4%, respectively), thereby resembling values now found in the average western population. Even in those Inuit who reported exclusive consumption of the traditional Inuit diet (13% of the population), the fatty acid composition of the plasma cholesterol esters closely resembled the values measured in western populations.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of mutations in the promoter region of the low density lipoprotein receptor gene in a large number of familial hypercholesterolaemia patients as revealed by denaturing gradient gel electrophoresis

Denaturing gradient gel electrophoresis (DGGE) was used in combination with the polymerase chain reaction (PCR) to detect sequence variations in the promoter region of the low density lipoprotein receptor (LDLR) gene. On the basis of calculated predictive melting properties we designed primers to amplify a 447-bp fragment of the promoter region from position -512 to -66, containing previously identified regulatory sequences. Using a primer with a GC-clamp in combination with restriction enzyme digestion, two melting domains could be analysed simultaneously. By oligonucleotide-directed mutagenesis artificial mutants were generated to optimize the conditions and to test the sensitivity of the method. All mutants were readily detected by electrophoresis in a 9% polyacrylamide gel containing a 10%-60% linear denaturing gradient. Using this method, we analysed DNA samples of 350 heterozygous familial hypercholesterolaemia (FH) patients. No mutations were detected, suggesting that mutations in the regulatory elements of the promoter sequence do not play a significant role in the etiology of FH.

The familial hypercholesterolemia (FH)-North Karelia mutation of the low density lipoprotein receptor gene deletes seven nucleotides of exon 6 and is a common cause of FH in Finland

A mutation of the LDL receptor gene very common among Finnish patients with heterozygous familial hypercholesterolemia (FH) was identified. This mutation, designated as FH-North Karelia, deletes seven nucleotides from exon 6 of the LDL receptor gene, causes a translational frameshift, and is predicted to result in a truncated receptor protein. Only minute quantities of mRNA corresponding to the deleted gene were detected. Functional studies using cultured fibroblasts from the patients revealed that the FH-North Karelia gene is associated with a receptor-negative (or binding-defective) phenotype of FH. Carriers of the FH-North Karelia gene showed a typical xanthomatous form of FH, with mean serum total and LDL cholesterol levels of 12 and 10 mmol/liter, respectively. This mutation was found in 69 (34%) out of 201 nonrelated Finnish FH patients and was especially abundant (prevalence 79%) in patients from the eastern Finland. These results, combined with our earlier data on another LDL receptor gene deletion (FH-Helsinki), demonstrate that two "Finnish-type" mutant LDL receptor genes make up about two thirds of FH mutations in this country, reflecting a founder gene effect. This background provides good possibilities to examine whether genetic heterogeneity affects the clinical presentation or responsiveness to therapeutic interventions in FH.

Heparan sulphate proteoglycans are involved in the lipoprotein lipase-mediated enhancement of the cellular binding of very low density and low density lipoproteins

We found that LPL enhances the binding to HepG2 cells and fibroblasts of both VLDL and apoE free LDL. In the presence of 1.7 micrograms/ml of purified bovine LPL, the binding of LDL and VLDL was up to 60 fold increased as compared to the control binding. In addition, LPL enhances the binding in LDL-receptor negative fibroblasts to the same extent as it does in normal fibroblasts. The presence of 10 mM of EGTA could not prevent the LPL-mediated enhancement of the binding of both LDL and VLDL to fibroblasts, indicating that the binding is calcium independent. Furthermore, up- and down regulation of the LDL receptor did not influence the binding of these lipoproteins in the presence of LPL. Strikingly, we found that the enhancing effect of LPL on the binding of LDL and VLDL to HepG2 cells could be abolished by preincubation of the cells with heparinase, suggesting that heparan sulphate proteoglycans are involved in the LPL-mediated stimulation. We hypothesize that the enhancement of the cellular binding of LDL and VLDL in the presence of LPL is caused by an LPL-bridging between proteoglycans present on the plasma membrane and the lipoproteins, and that the LDL receptor and LRP are not involved.

Familial dysbetalipoproteinemia: a genetically heterogenous disease caused by mutations of the ligand apolipoprotein E

Apolipoprotein E is present on the surface of very-low-density lipoprotein (VLDL) and chylomicron-remnants and is essential for the receptor mediated endocytosis of these particles via hepatic receptors. Several types of mutations of the apoE can cause a deficiency in the clearance of these remnant particles. An accumulation of lipoprotein-remnant particles may occur and familial dysbetalipoproteinemia (FD) develops. Genotyping of the various apoE variants and relation of these mutations with their effect on the lipoprotein-remnant removal have provided more insight in the structure-relationship of apoE ligand-receptor interactions. It is postulated that the apoE2 (Arg158----Cys) mutation is just outside the binding domain and that its deficient binding can be stimulated by exogenous factors. This hypothesis can explain why apoE2/E2 homozygosity can only induce FD under certain circumstances. ApoE mutations that occur in the binding domain, e.g., apoE2 (Lys146----Gln) have a direct effect on the ligand-receptor binding and, in these individuals, FD is inherited in an autosomal dominant way. Finally, apoE3-Leiden has an arginine residue at 112 and has a repeat of seven extra amino acid residues just outside the binding domain. Because of this repeat, conformational changes of the binding domain can ensue. Due to the fact that in apo E3-Leiden the arginine residue is present at 112, apoE3 Leiden is predominantly present on chylomicron and VLDL remnants. In these persons FD is also inherited in an autosomal dominant way.

Protease inhibitor (Pi) locus, fertility and twinning

In a sample of 160 Dutch twin pairs and their parents, we found that mothers of dizygotic twins had frequencies of the S and Z alleles at the protease inhibitor (Pi) locus that were 3 times higher than a control sample. Mothers of identical twins also had a higher frequency of S than controls. The S allele may thus both increase ovulation rate and enhance the success of multiple pregnancies. There was also an increased frequency of the S allele in fathers of dizygotic twins; however, this may be a secondary effect of assortative mating for family size (indicating by the number of siblings of the parents), for which a correlation of 0.2 was observed. Parents of dizygotic twins came from larger families than parents of monozygotic twins, but no effect of Pi type on family size was seen.

Immunological discrimination between the human apolipoprotein E2(Arg158----Cys) and E3 isoforms

A specific anti-apoE2(Arg158----Cys) monoclonal antibody was raised by means of immunization of mice with a variant specific synthetic peptide. The peptide sequences used were homologous to apolipoprotein E of human and mouse. Consequently, the mouse immune system was tolerant to most of the selected sequences. Immunization with only one of selected peptides (amino acids 154-172) evoked an anti-peptide and anti-native protein response. Surprisingly, this peptide was predicted to have a low antigenicity index, in contrast to the other used peptides. The variant specific anti-peptide MAb that was generated with this sequence, recognizes apoE2(Arg158----Cys) and not apoE3. We here describe a sensitive, time saving, and simple immunoblot assay to detect apoE2(Arg158----Cys) in human sera without prior isoelectric focusing of serum proteins.