A deletion of one nucleotide results in functional deficiency of apolipoprotein CII (apo CII Toronto)

Understanding Hypertriglyceridemia: Integrating Genetic Insights

Hypertriglyceridemia is an exceptionally complex metabolic disorder characterized by elevated plasma triglycerides associated with an increased risk of acute pancreatitis and cardiovascular diseases such as coronary artery disease. Its phenotype expression is widely heterogeneous and heavily influenced by conditions as obesity, alcohol consumption, or metabolic syndromes. Looking into the genetic underpinnings of hypertriglyceridemia, this review focuses on the genetic variants in LPL, APOA5, APOC2, GPIHBP1 and LMF1 triglyceride-regulating genes reportedly associated with abnormal genetic transcription and the translation of proteins participating in triglyceride-rich lipoprotein metabolism. Hypertriglyceridemia resulting from such genetic abnormalities can be categorized as monogenic or polygenic. Monogenic hypertriglyceridemia, also known as familial chylomicronemia syndrome, is caused by homozygous or compound heterozygous pathogenic variants in the five canonical genes. Polygenic hypertriglyceridemia, also known as multifactorial chylomicronemia syndrome in extreme cases of hypertriglyceridemia, is caused by heterozygous pathogenic genetic variants with variable penetrance affecting the canonical genes, and a set of common non-pathogenic genetic variants (polymorphisms, using the former nomenclature) with well-established association with elevated triglyceride levels. We further address recent progress in triglyceride-lowering treatments. Understanding the genetic basis of hypertriglyceridemia opens new translational opportunities in the scope of genetic screening and the development of novel therapies.

Low circulating PCSK9 levels in LPL homozygous children with chylomicronemia syndrome in a syrian refugee family in Lebanon

Familial chylomicronemia syndrome is a rare autosomal recessive disorder of lipoprotein metabolism characterized by the presence of chylomicrons in fasting plasma and an important increase in plasma triglycerides (TG) levels that can exceed 22.58 mmol/l. The disease is associated with recurrent episodes of abdominal pain and pancreatitis, eruptive cutaneous xanthomatosis, lipemia retinalis, and hepatosplenomegaly. A consanguineous Syrian family who migrated to Lebanon was referred to our laboratory after perceiving familial chylomicronemia syndrome in two children. The LPL and PCSK9 genes were sequenced and plasma PCSK9 levels were measured. Sanger sequencing of the LPL gene revealed the presence of the p.(Val227Phe) pathogenic variant in exon 5 at the homozygous state in the two affected children, and at the heterozygous state in the other recruited family members. Interestingly, PCSK9 levels in homozygous carriers of the p.(Val227Phe) were ≈50% lower than those in heterozygous carriers of the variant (p-value = 0.13) and ranged between the 5th and the 7.5th percentile of PCSK9 levels in a sample of Lebanese children of approximately the same age group. Moreover, this is the first reported case of individuals carrying simultaneously an LPL pathogenic variant and PCSK9 variants, the L10 and L11 leucine insertion, which can lower and raise low-density lipoprotein cholesterol (LDL-C) levels respectively. TG levels fluctuated concomitantly between the two children, were especially high following the migration from a country to another, and were reduced under a low-fat diet. This case is crucial to raise public awareness on the risks of consanguineous marriages to decrease the emergence of inherited autosomal recessive diseases. It also highlights the importance of the early diagnosis and management of these diseases to prevent serious complications, such as recurrent pancreatitis in the case of familial hyperchylomicronemia.

A novel APOC2 gene mutation identified in a Chinese patient with severe hypertriglyceridemia and recurrent pancreatitis

BACKGROUND

The severe forms of hypertriglyceridemia are usually caused by genetic defects. In this study, we described a Chinese female with severe hypertriglyceridemia caused by a novel homozygous mutation in the APOC2 gene.

METHODS

Lipid profiles of the pedigree were studied in detail. LPL and HL activity were also measured. The coding regions of 5 candidate genes (namely LPL, APOC2, APOA5, LMF1, and GPIHBP1) were sequenced using genomic DNA from peripheral leucocytes. The ApoE gene was also genotyped.

RESULTS

Serum triglyceride level was extremely high in the proband, compared with other family members. Plasma LPL activity was also significantly reduced in the proband. Serum ApoCII was very low in the proband as well as in the heterozygous mutation carriers. A novel mutation (c.86A > CC) was identified on exon 3 [corrected] of the APOC2 gene, which converted the Asp [corrected] codon at position 29 into Ala, followed by a termination codon (TGA).

CONCLUSIONS

This study presented the first case of ApoCII deficiency in the Chinese population, with a novel mutation c.86A > CC in the APOC2 gene identified. Serum ApoCII protein might be a useful screening test for identifying mutation carriers.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Missense mutation Leu72Pro located on the carboxyl terminal amphipathic helix of apolipoprotein C-II causes familial chylomicronemia syndrome

BACKGROUND

Chylomicronemia syndrome can be caused by 2 autosomal recessive disorders - lipoprotein lipase (LPL) deficiency and apolipoprotein C-II (apo C-II) deficiency.

METHODS

We described 2 siblings with chylomicronemia syndrome of a consanguineous family. To determine the molecular basis of chylomicronemia syndrome in this family, we performed direct DNA sequencing of the LPL and APOC2 genes of the proband.

RESULTS

A novel homozygous mutation, Leu72Pro, in the APOC2 gene was found in both siblings whereas their parents were carriers. No LPL mutations were detected in the siblings. Apo C-II contains 3 amphipathic alpha helices; the C-terminal alpha helix is composed of residues 64 to 74. Substitution of residue 72 from a helix former leucine to a helix breaker, proline, is predicted to change the secondary structure of the C-terminal helix and subsequently alter the interaction between apo C-II and LPL.

CONCLUSIONS

To our knowledge, Leu72Pro is the first missense mutation identified in the C-terminal of apo C-II. The result is consistent with the current biochemical and structural findings that the C-terminal helix of apo C-II is important for activation of LPL.

A new case of apo C-II deficiency with a nonsense mutation in the apo C-II gene

The apo C-II gene from a patient with apo C-II deficiency has been sequenced after amplification by the polymerase chain reaction (PCR). The sequence analysis revealed a substitution of adenosine for cytosine at position 3,002 in exon 3, leading to the introduction of a premature stop codon (TAA) at a position corresponding to aminoacid 37 of mature apo C-II. This mutation creates a new Rsa I restriction enzyme site in the apo C-II gene. Amplification of DNA from family members by PCR and digestion with Rsa I established that the patient is a true homozygote for this mutation. The same nucleotide has been substituted for the mutation apo C-IIPadova and apo C-IIBari previously described in two kindreds from Italy. From these data we speculate that base pair 3,002 in the apo C-II gene may represent a hot spot for mutation.

Heterozygous apolipoprotein C-II deficiency: lipoprotein and apoprotein phenotype and RsaI restriction enzyme polymorphism in the Apo C-IIPadova kindred

Deficiency of apolipoprotein C-II (apo C-II), the cofactor for lipoprotein lipase, results in the familial chylomicronaemia syndrome characterized by severe hypertriglyceridaemia and fasting chylomicronaemia. To investigate the biochemical features of the heterozygous state for apo C-II deficiency, we characterized the lipid, lipoprotein and apolipoprotein profiles in 18 relatives of two affected individuals (brother and sister) homozygous for the apo C-IIPadova gene defect which results in the synthesis of a truncated 36 amino acid apolipoprotein. Carrier status was established in first degree relatives as well as in seven non-obligate heterozygotes by restriction enzyme analysis of amplified apo C-II genomic DNA using RsaI. No significant differences in lipid, lipoprotein and apo C-II levels were observed in heterozygotes when compared to unaffected family members. Thus, in this study, the carrier state was not associated with hypertriglyceridaemia or reduced plasma levels of apo C-II. However, analysis of amplified DNA from members of the apo C-IIPadova kindred by digestion with the enzyme RsaI, which identifies the mutant apo C-II, permitted the identification of heterozygous family members which could not be recognized by measuring either fasting triglycerides or plasma apo C-II levels. This study provides further evidence that apo C-II deficiency syndrome is a heterogeneous disease not only at the molecular level but also on the clinical ground with variable phenotypic expression in heterozygous individuals from different kindreds.

Apo C-II deficiency type bari

We formerly studied an Italian family with apo C-II deficiency. Two probands were homozygous for the defect (unmeasurable circulating apolipoprotein C-II and absence of C-II bands on immunoelectrophoresis). We documented the synthesis of the protein at the intestinal level in the probands with immunohistological techniques. With the purpose of investigating the molecular basis of the defect, Southern analysis, polymerase chain reaction (PCR) amplification and sequence analysis were carried out on one of the two cases. We identified a point mutation C to G transversion in the third exon of the gene causing a premature stop codon. Our hypothesis is that the truncated protein of 36 aa., instead of 79 aa., lacks its functional domain. This causes inefficiency in the activation of lipoprotein lipase (LPL) and the instability of the circulating molecule, which could have an higher catabolic rate compared to a normal protein. The faster disappearance from the circulating compartment make it unmeasurable. The mutation destroys a Rsa I site, present in the normal gene sequence. We suggest the use of this site for a rapid Restriction Fragment Length Polymorphism (RFLP) on PCR amplification products to screen this defect in the Italian population.

Studies on an apolipoprotein C-II variant occurring in Caucasians

Apolipoproteins C (apo C-II, apo C-III0, apo C-III1 and apo C-III2) from delipidated very low density lipoproteins (VLDL) of 522 normo- and hyperlipoproteinemic Caucasians were screened by analytical isoelectric focusing. The immobilized pH gradient used was pH 4.0-5.0 with 7 M urea, which raised the apparent pH range to 4.8-5.7. As identified by immunoblotting, six unrelated persons had two major isoforms of apo C-II, the normal apo C-II-1 (which focuses between apo C-III0 and apo C-III1) and a variant, designated apo C-II-v according to Huff et al., focusing between apo C-III1 and apo C-III2 due to a more acidic pI. In narrow pH gradients, apo C-II-v can readily be discriminated from the minor isoform, apo C-II-2, due to its slightly more basic pI, corresponding to a difference of 0.01 pH units. Neuraminidase treatment did not alter the pI of apo C-II-v and on two-dimensional electrophoresis the molecular weights of apo C-II-1 and apo C-II-v were indistinguishable. The frequency of apo C-II-v was 1.2%. It was the same in males and females and was independent of hypertriglyceridemia. The autosomal codominant inheritance could be demonstrated in the pedigree of one family. Electroblotting of apo C-II-1 and apo C-II-v onto activated glass fiber sheets, followed by amino acid sequence analysis of the amino terminal ends, revealed an exchange of the amino acid lysine at position 19 by threonine in apo C-II-v.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between variant apolipoproteins C-II and E that affects plasma lipoprotein concentrations

The genes for apolipoprotein (apo) C-II, a cofactor for activation of lipoprotein lipase, and apo E, a ligand for receptor-mediated uptake of triglyceride-rich lipoproteins, are physically linked on chromosome 19q13.1. In a large Caribbean Caucasian family, several individuals had clinical features of the complete absence of lipoprotein lipase activity and were homozygous for a DNA frameshift mutation of apo C-II, imparting functional inactivity to the mutant protein. Plasma from heterozygous carriers of this mutation, when compared with plasma from relatives who were noncarriers, had significantly diminished capacity to activate lipoprotein lipase in vitro. We also observed in heterozygotes for this mutation a wide range of serum lipid and lipoprotein levels. When age and sex were taken into account, the presence of a single apo E allele encoding the E4 isoform occurring in individuals with a single mutant apo C-II allele was strongly associated with higher levels of cholesterol, triglycerides, very low density lipoprotein cholesterol, and non-high density lipoprotein cholesterol when compared with those of relatives who carried neither or only one variant allele. This suggests that a single genetic mutation that usually has a recessive effect on lipoprotein metabolism can have an interactive effect on lipid phenotype when it is coinherited with a single mutation at another gene whose product affects the same metabolic pathway.

Ionizing radiation and genetic risks. I. Epidemiological, population genetic, biochemical and molecular aspects of Mendelian diseases

This paper reviews the currently available information on naturally occurring Mendelian diseases in man; it is aimed at providing a background and framework for discussion of experimental data on radiation-induced mutations (papers II and III) and for the estimation of the risk of Mendelian disease in human populations exposed to ionizing radiation (paper IV). Current consensus estimates indicate that a total of about 125 per 10(4) livebirths are directly affected by one or another naturally occurring Mendelian disease (autosomal dominants, 95/10(4); X-linked ones, 5/10(4); and autosomal recessives, 25/10(4). These estimates are conservative and take into account conditions which are very rare and for which prevalence estimates are unavailable. Most, although not all, of the recognized "common" dominants have onset in adult ages while most sex-linked and autosomal recessives have onset at birth or in childhood. Autosomal dominant and X-linked diseases (i.e., the responsible mutant alleles) presumed to be maintained in the population due to a balance between mutation and selection are the ones which may be expected to increase in frequency as a result of radiation exposures. Viewed from this standpoint, the above assumption seems safe only for a small proportion of such diseases; for the remainder, there is no easy way to discriminate between different mechanisms that may be responsible or to rigorously exclude some in favor of some others. Mutations in genes that code for enzymic proteins are more often recessive in contrast to those that code for non-enzymic proteins, which are more often dominant. At the molecular level, with recessives, a wide variety of changes is possible and these include specific types of point mutations, small and large intragenic deletions, multilocus deletions and rearrangements. In the case of dominants, however, the kinds of recoverable point mutations and deletion-type changes are less extensive because of functional constraints. The mutational potential of genes varies, depending on the gene, its size, sequence content and arrangement, location and its normal functions, and can be grouped into three groups: those in which only point mutations have been found to occur, those in which only deletions or other gross changes have been recovered and those in which both kinds of changes are known. Molecular data are available for about 75 Mendelian conditions and these suggest that in approximately 50% of them, the changes categorized to date are point mutations and in the remainder, intragenic deletions or other gross changes; there does not seem to be any fundamental difference between dominants and recessives with respect to the underlying molecular defect.(ABSTRACT TRUNCATED AT 400 WORDS)

Gene deletions causing human genetic disease: mechanisms of mutagenesis and the role of the local DNA sequence environment

Reports describing short (less than 20 bp) gene deletions causing human genetic disease were collated in order to study underlying causative mechanisms. Deletion breakpoint junction regions were found to be non-random both at the nucleotide and dinucleotide sequence levels, an observation consistent with an endogenous sequence-directed mechanism of mutagenesis. Direct repeats of between 2bp and 8bp were found in the immediate vicinity of all but one of the 60 deletions analysed. Direct repeats are a feature of a number of recombination, replication or repair-based models of deletion mutagenesis and the possible contribution of each to the spectrum of mutations examined was assessed. The influence of parameters such as repeat length and length of DNA between repeats was studied in relation to the frequency, location and extent of these deletions. Findings were broadly consistent with a slipped mispairing model but the predicted deletion of one whole repeat copy was found only rarely. A modified version of the slipped mispairing hypothesis was therefore proposed and was shown to possess considerable explanatory value for approximately 25% of deletions examined. Whereas the frequency of inverted repeats in the vicinity of gene deletions was not significantly elevated, these elements may nevertheless promote instability by facilitating the formation of secondary structure intermediates. A significant excess of symmetrical sequence elements was however found at sites of single base deletions. A new model to explain the involvement of symmetric elements in frameshift mutagenesis was devised, which successfully accounted for a majority of the single base deletions examined. In general, the loss of one or a few base pairs of DNA was found to be more compatible with a replication-based model of mutagenesis than with a recombination or repair hypothesis. Seven hitherto unrecognized hotspots for deletion were noted in five genes (AT3, F8, HBA, HBB and HPRT). Considerable sequence homology was found between these different sites, and a consensus sequence (TGA/GA/GG/TA/C) was drawn up. Sequences fitting this consensus (i) were noted in the immediate vicinity of 41% of the other (sporadic) gene deletions, (ii) were found frequently at sites of spontaneous deletion in the hamster APRT gene, (iii) were found to be associated with many larger human gene deletions/translocations, (iv) act as arrest sites for human polymerase alpha during DNA replication and (v) have been shown by in vitro studies of human polymerase alpha to be especially prone to frameshift mutation.(ABSTRACT TRUNCATED AT 400 WORDS)

Diagnosis and management of familial dyslipoproteinemia in children and adolescents

CAD results from atherosclerosis, a chronic disease process that has its origin in childhood. Children and adolescents can be at higher risk for CAD by virtue of being from families with premature CAD or familial dyslipoproteinemias. The plasma lipid and lipoprotein levels result from a number of complex metabolic processes that are under the control of genetic and environmental (e.g., diet) influences. The normal ranges of plasma lipids and lipoproteins in children are known, and children and adolescents with dyslipoproteinemia are ordinarily defined as those having levels of plasma total, LDL, or triglyceride above the 95th percentile or with a low HDL cholesterol below the 5th percentile. Children of a parent with documented dyslipoproteinemia or with family history of premature CAD may be screened in the fasting state any time after 2 years of age. Following the exclusion of secondary causes of dyslipoproteinemia, the diagnosis of primary dyslipoproteinemia can be made. Lipoprotein patterns are not diagnostic for a given genotype. Efforts to determine further the biochemical defects responsible for a given phenotype have led to the investigation of gene coding for the apolipoproteins, the key enzymes in the lipoproteins pathways (LPL, HDL, and LCAT) and the receptors that process lipoproteins, such as the LDL receptor and the chylomicron remnant receptor. From a practical standpoint, the diagnosis of the kind of dyslipoproteinemia in a child will depend upon the nature and severity of the dyslipoproteinemia, both in the child (or adolescent) and in parents and siblings. Marked increases in plasma total and LDL cholesterol in the child and in at least one of the parents often reflect the presence of familial hypercholesterolemia, an inherited dominant condition due to a defect in the LDL receptor gene. The triglyceride levels are often normal. If the child has a different dyslipoproteinemia pattern from siblings and parents, then the diagnosis of familial combined hyperlipidemia or hyperapobetalipoproteinemia should be considered. Most children with mild or borderline elevations in total and LDL cholesterol will have polygenic hypercholesterolemia. Triglyceride problems in children and adolescents are relatively uncommon, particularly the more severe hypertriglyceridemia such as that found in lipoprotein lipase and apoC-II deficiency, dysbetalipoproteinemia, and type V hyperlipoproteinemia. High levels of Lp(a) lipoprotein, in isolation or in combination with other dyslipoproteinemia, accelerate risk for CAD. Low levels of HDL cholesterol in the absence of other abnormalities suggest the diagnosis of hypoalphalipoproteinemia.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the mutation responsible for a case of plasmatic apolipoprotein CII deficiency (Apo CII-Bari)

We studied a case of familial Apolipoprotein CII deficiency. By Southern hybridization, amplification and sequence analysis, the genetic defect was identified. It consists in a point mutation C- greater than G in the third exon of the gene causing a premature stop codon. Truncated at the aa. 36 of the mature form, the protein loses its functional domains, becomes inefficient and cannot be detected in the plasma, because of its high instability. The mutation destroys an RsaI site, present in the normal gene sequence. This point mutation is useful in the diagnosis of this Apolipoprotein CII deficiency.

The genetic dyslipoproteinemias--nosology update 1990

The number of discrete disorders of lipid transport is growing. Concomitantly, the classification of the disorders is changing, from one based on altered concentrations of lipoproteins, to one based on current understanding of the genetics of the disorders and of lipoprotein biochemistry and physiology. Many disorders are now traceable to deficiencies of essential proteins such as apolipoproteins, enzymes, lipid transfer proteins and cellular receptors.