Hypermutability of CpG dinucleotides in the propeptide-encoding sequence of the human albumin gene

Variations in the Human Serum Albumin Gene: Molecular and Functional Aspects

The human albumin gene, the most abundant serum protein, is located in the long arm of chromosome 4, near the centromere, position 4q11–3. It is divided by 14 intervening introns into 15 exons, the last of which is untranslated. To date, 74 nucleotide substitutions (mainly missense) have been reported, determining the circulating variants of albumin or pre-albumin. In a heterozygous state, this condition is known as alloalbuminaemia or bisalbuminaemia (OMIM # 103600). The genetic variants are not associated with disease, neither in the heterozygous nor in the homozygous form. Only the variants resulting in familial dysalbuminaemic hyperthyroxinaemia and hypertriiodothyroninaemia are of clinical relevance because affected individuals are at risk of inappropriate treatment or may have adverse drug effects. In 28 other cases, the pathogenic variants (mainly affecting splicing, nonsense, and deletions), mostly in the homozygous form, cause a premature stop in the synthesis of the protein and lead to the condition known as congenital analbuminaemia. In this review, we will summarize the current knowledge of genetic and molecular aspects, functional consequences and potential therapeutic uses of the variants. We will also discuss the molecular defects resulting in congenital analbuminaemia, as well as the biochemical and clinical features of this rare condition

Mutations and polymorphism in bottlenose dolphin (Tursiops truncatus, Montagu 1821) albumin gene: First identification of mutations responsible for inherited bisalbuminemia

Hereditary bisalbuminemia is an asymptomatic and heterozygous condition in a range of species characterized by the presence of two serum albumin fractions with different electrophoretic mobility resulting in a bicuspid pattern on serum electrophoresis. Bisalbuminemia has been diagnosed by electrophoresis in two bottlenose dolphin (Tursiops truncatus) families, but causative mutations and the inheritance pattern have not been identified. The aims of this work are: to investigate polymorphisms of the bottlenose dolphin albumin gene and to identify mutations causative of bisalbuminemia; to identify the inheritance pattern in two bottlenose dolphin families. Coding regions of the albumin gene were screened for mutations in 15 bottlenose dolphins kept under human care from two distinct families. Eighteen albumin mutations (three synonymous and 15 non-synonymous) were identified. Two non-synonymous variations co-segregated with bisalbuminemic phenotype: p.Phe146Leu in exon 4 and p.Tyr163His in exon 5. The amino acid change in exon 5 was associated with the secondary and/or tertiary structure variation of the protein and has been reported as causative of bisalbuminemia in humans. Pedigree analysis of the dolphin families showed an autosomal codominant inheritance pattern. In this work, the mutations potentially responsible for bisalbuminemia were identified and confirmed the autosomal codominant trait in bottlenose dolphins.

Clinical, Genetic, and Protein Structural Aspects of Familial Dysalbuminemic Hyperthyroxinemia and Hypertriiodothyroninemia

Familial dysalbuminemic hyperthyroxinemia (FDH-T4) and hypertriiodothyroninemia (FDH-T3) are dominantly inherited syndromes characterized by a high concentration of thyroid hormone in the blood stream. The syndromes do not cause disease, because the concentration of free hormone is normal, but affected individuals are at risk of erroneous treatment. FDH-T4 is the most common cause of euthyroid hyperthyroxinemia in Caucasian populations in which its prevalence is about 1 in 10,000 individuals, but the prevalence can be much higher in some ethnic groups. The condition is caused by a genetic variant of human serum albumin (HSA); Arg218 is mutated to histidine, proline, or serine or Arg222 is changed to isoleucine. The disorder is characterized by greater elevation in serum l-thyroxine (T4) than in serum triiodothyronine (T3); T4 can be increased by a factor 8-15. The high serum concentration of T4 is due to modification of a binding site located in the N-terminal half of HSA (in subdomain IIA). Thus, mutating Arg218 or Arg222 for a smaller amino acid reduces the steric restrictions in the site and creates a high-affinity binding site. The mutations can also affect binding of other ligands and can perhaps cause modified pharmacokinetics of albumin-binding drugs. In normal HSA, the high-affinity site has another location (in subdomain IIIB). Different locations of these sites imply that persons with and without FDH-T4 can have different types of interactions, and thereby complications, when given albumin-binding drugs. FDH-T3 is caused by a leucine to proline mutation in position 66 of HSA, which results in a large increment of the binding affinity for T3 but not for T4. For avoiding unwanted treatment of euthyroid persons with hyperthyroxinemia or hypertriiodothyroninemia, protein sequencing and/or sequencing of the albumin gene should be performed.

Fibrinogen storage disease in a Chinese boy with de novo fibrinogen Aguadilla mutation: Incomplete response to carbamazepine and ursodeoxycholic acid

Background

Fibrinogen storage disease (FSD) is a rare autosomal-dominant disorder caused by mutation in FGG, encoding the fibrinogen gamma chain. Here we report the first Han Chinese patient with FSD, caused by de novo fibrinogen Aguadilla mutation, and his response to pharmacologic management.

Case presentation

Epistaxis and persistent clinical-biochemistry test-result abnormalities prompted liver biopsy in a boy, with molecular study of FGG in him and his parents. He was treated with the autophagy enhancer carbamazepine, reportedly effective in FSD, and with ursodeoxycholic acid thereafter. Inclusion bodies in hepatocellular cytoplasm stained immune-histochemically for fibrinogen. Selective analysis of FGG found the heterozygous mutation c.1201C > T (p.Arg401Trp), absent in both parents. Over more than one year’s follow-up, transaminase and gamma-glutamyl transpeptidase activities have lessened but not normalized.

Conclusion

This report expands the epidemiology of FSD and demonstrates idiosyncrasy in response to oral carbamazepine and/or ursodeoxycholic acid in FSD.

Electronic supplementary material

The online version of this article (doi:10.1186/s12876-016-0507-3) contains supplementary material, which is available to authorized users.

Unique albumin with two silent substitutions (540Thr→Ala and 546Ala→Ser): Insights into how albumin is recycled

OBJECTIVES

To determine the cause of an albumin abnormality detected by chance on electrospray time-of-flight mass spectrometry (TOF MS) of whole plasma, and to assess its physiological consequences.

METHOD

Plasma was examined by TOF MS and tryptic mapping was used to locate mutation sites and determine the relative expression level of the variant and normal albumins. DNA sequencing was used to precisely define mutations.

RESULTS

Whole protein electrospray TOF MS indicated a decrease of 14Da in the mass of albumin. Peptide mass mapping and DNA sequencing established the presence of two novel heterozygous point mutations (540Thr→Ala and 546Ala→Ser) whose combined mass changes (-30 and +16Da) indicated both mutations occurred on the same allele. Peptide ratios showed the variant albumin was present at a lower level than normal with an expression ratio of approximately 1:2 (variant:normal). Phylogenetic sequence alignments show Thr540 is highly conserved while Ala546 has wide species variation, suggesting 540Thr→Ala might compromise the protein.

CONCLUSION

Both mutations occur close together in domain IIIB, a region involved in albumin scavenging and recycling. In particular, Thr540 is close to His535, a residue directly involved in pH-dependent binding and release of albumin from its recycling neonatal Fc receptor. Compromised receptor binding would explain the low albumin (34g/l) concentration and the diminished variant expression level.

Human serum albumin isoforms: genetic and molecular aspects and functional consequences

BACKGROUND

At present, 67 different genetic variants of human serum albumin and proalbumin have been molecularly characterized at the protein and/or gene level.

SCOPE OF REVIEW

This review summarizes present knowledge about genetic and molecular aspects, functional consequences and potential uses of the variants.

MAJOR CONCLUSIONS

The frequency of bisalbuminemia in the general population is probably about 1:1000, but it can be much higher in isolated populations. Mutations are often due to hypermutable CpG dinucleotides, and in addition to single-amino acid substitutions, glycosylated variants and C-terminally modified alloalbumins have been found. Some mutants show altered stability in vivo and/or in vitro. High-affinity binding of Ni(++) and Cu(++) is blocked, or almost so, by amino acid changes at the N-terminus. In contrast, substitution of Leu90 and Arg242 leads to strong binding of triiodothyronine and l-thyroxine, respectively, resulting in two clinically important syndromes. Variants often have modified plasma half-lives and organ uptakes when studied in mice.

GENERAL SIGNIFICANCE

Because alloalbumins do not seem to be associated with disease, they can be used as markers of migration and provide a model for study of neutral molecular evolution. They can also give valuable molecular information about albumins binding sites, antioxidant and enzymatic properties, as well as stability. Mutants with increased affinity for endogenous or exogenous ligands could be therapeutically relevant as antidotes, both for in vivo and extracorporeal treatment. Variants with modified biodistribution could be used for drug targeting. In most cases, the desired function can be further elaborated by producing site-directed, recombinant mutants. This article is part of a Special Issue entitled Serum Albumin.

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

Mutations and polymorphisms of the gene of the major human blood protein, serum albumin

We have tabulated the 77 currently known mutations of the familiar human blood protein, serum albumin (ALB). A total of 65 mutations result in bisalbuminemia. Physiological and structural effects of these mutations are included where observed. Most of the changes are benign. The majority of them were detected upon clinical electrophoretic studies, as a result of a point mutation of a charged amino acid residue. Three were discovered by their strong binding of thyroxine or triiodothyronine. A total of 12 of the tabulated mutations result in analbuminemia, defined as a serum albumin concentration of <1 g/L. These were generally detected upon finding a low albumin concentration in patients with mild edema, and involve either splicing errors negating translation or premature stop codons producing truncated albumin molecules. A total of nine mutations, five of those with analbuminemia and four resulting in variants modified near the C-terminal end, cause frameshifts. Allotypes from three of the point mutations become N-glycosylated and one C-terminal frameshift mutation shows O-glycosylation.

Analbuminemia in a Swiss family is caused by a C → T transition at nucleotide 4446 of the albumin gene

OBJECTIVE

To define the molecular defect that causes analbuminemia in an apparently healthy boy, son of non-consanguineous Swiss parents.

DESIGN AND METHODS

Total DNA, extracted from peripheral blood samples from the proband and from both parents, was PCR-amplified using oligonucleotide primers designed to amplify the 14 exons of the human albumin gene and the flanking intron regions. The products were screened for mutations by single-strand conformation polymorphism (SSCP) and heteroduplex analyses (HA) either directly or after digestion with restriction enzymes. The combination of these methods identified the abnormal fragment, which was then sequenced.

RESULTS

DNA sequence analysis identified in the homozygous proband a C --> T transition at nucleotide 4446. The mutation changes the codon CGA for Arg 114 to a stop codon TGA, resulting in premature termination and is therefore responsible for the analbuminemic trait. The same mutation has been previously reported to cause analbuminemia in an American female. The putative protein product would have a length of 113 residues. The parents were found to be heterozygous for the mutation.

CONCLUSIONS

Gel-based mutation detection and DNA sequencing confirmed the diagnosis of congenital analbuminemia in the proband. Our results show that the combination of SSCP and HA represents a powerful tool to study the molecular defects causing analbuminemia in humans.

Effect of genetic variation on the thermal stability of human serum albumin

Reversible thermal denaturation of 33 genetic variants of human serum albumin (HSA) appeared to be a two-state process when studied by circular dichroism (CD). Fourteen single-residue variants have Tm values (midpoint of denaturation) higher than, and nine have Tm values lower than, their endogenous, wild-type counterpart. Nine single-residue variants have DeltaHv values (van't Hoff enthalpy) higher than, and 14 have DeltaHv values lower than, normal albumin. All types of combinations of positive and negative DeltaTm values and Delta(DeltaHv) values were found. Good linear correlations between mutation-induced changes of alpha-helical content and Delta(DeltaHv) values, but not DeltaTm values, were found especially for the variants mutated in domains I and III. The effect of altered chain length and glycosylation on Tm and DeltaHv was also studied. For all variants, no clear relationship was found between the changes in the thermodynamic parameters and the type of substitution, changes in protein charge or hydrophobicity. However, the protein changes taking place in domain I have a rather uniform effect (almost all of the nine variants have positive DeltaTm values and negative Delta(DeltaHv) values, i.e., they denature more easily than normal albumin but they do so at a higher temperature). The present results can be of both protein chemical relevance and of clinical interest, because they could be useful when designing stable, recombinant HSAs for clinical applications.

Application of the S-pyridylethylation reaction to the elucidation of the structures and functions of proteins

Cysteine (Cys) and cystine residues in proteins are unstable under conditions used for acid hydrolysis of peptide bonds. To overcome this problem, we proposed the use of the S-pyridylethylation reaction to stabilize Cys residues as pyridylethyl-cysteine (PEC) protein derivatives. This suggestion was based on our observation that two synthetic derivatives formed by pyridylethylation of the SH group of Cys with either 2-vinylpyridine (2-VP) or 4-vinylpyridine (4-VP), designated as S-beta-(2-pyridylethyl)-L-cysteine (2-PEC) and S-beta-(4-pyridylethyl)-L-cysteine (4-PEC), were stable under acid conditions used to hydrolyze proteins. This was also the case for protein-bound PEC groups. Since their discovery over 30 years ago, pyridylethylation reactions have been widely modified and automated for the analysis of many structurally different proteins at levels as low as 20 picomoles, to determine the primary structures of proteins and to define the influence of SH groups and disulfide bonds on the structures and functional, enzymatic, medical, nutritional, pharmacological, and toxic properties of proteins isolated from plant, microbial, marine, animal, and human sources. Pyridylethylation has been accepted as the best method for the modification of Cys residues in proteins for subsequent analysis and sequence determination. The reaction has also been proposed to measure D-Cys, homocysteine, glutathione, tryptophan, dehydroalanine, and furanthiol food flavors. This integrated overview of the diverse literature on these reactions emphasizes general concepts. It is intended to serve as a resource and guide for further progress based on the reported application of pyridylethylation reactions to more than 150 proteins.

A nucleotide insertion and frameshift cause albumin Kénitra, an extended and O-glycosylated mutant of human serum albumin with two additional disulfide bridges

Albumin Kenitra is a new type of genetic variant of human serum albumin that has been found in two members of a family of Sephardic Jews from Kenitra (Morocco). The slow-migrating variant and the normal protein were isolated by anion-exchange chromatography and, after treatment with CNBr, the digests were analyzed by two-dimensional electrophoresis in a polyacrylamide gel. The CNBr peptides of the variant were purified by reverse-phase high performance liquid chromatography and submitted to sequence analysis. Albumin Kenitra is peculiar because it has an elongated polypeptide chain, 601 residues instead of 585, and its sequence is modified beginning from residue 575. DNA structural studies showed that the variant is caused by a single-base insertion, an adenine at nucleotide position 15 970 in the genomic sequence, which leads to a frameshift with the subsequent translation to the first termination codon of exon 15. Mass spectrometric analyses revealed that the four additional cysteine residues of the variant form two new S-S bridges and showed that albumin Kenitra is partially O-glycosylated by a monosialylated HexHexNAc structure. This oligosaccharide chain has been located to Thr596 by amino-acid sequence analysis of the tryptic fragment 592-597.

Implications of the distribution of Albumin Naskapi and Albumin Mexico for new world prehistory

The known distributions of two mutational variants of the albumin gene that are restricted to Mexico and/or North America, Albumin Mexico (AL*Mexico) and Albumin Naskapi (AL*Naskapi), were expanded by the electrophoretic analysis of sera collected from more than 3, 500 Native Americans representing several dozen tribal groups. With a few exceptions that could be due to recent, isolated cases of admixture, AL*Naskapi is limited to groups that speak Athapaskan and Algonquian, two widely distributed language families not thought to be related, and to several linguistically unrelated groups geographically proximate to its probable ancestral homeland. Similarly, AL*Mexico is limited to groups that speak Yuman or Uto-Aztecan, two language groups in the American Southwest and Baja California not thought to be closely related to each other, and to several linguistically unrelated groups throughout Mexico. The simultaneous consideration of genetic, historical, linguistic, and archaeological evidence suggests that AL*Naskapi probably originated on the northwestern coast of North America, perhaps in some group ancestral to both Athapaskans and Algonquians, and then spread by migration and admixture to contiguous unrelated, or distantly related, tribal groups. AL*Mexico probably originated in Mexico before 3,000 years BP then spread northward along the Tepiman corridor together with cultural influences to several unrelated groups that participated in the Hohokam culture.

Genetic variants showing apparent hot-spots in the human serum albumin gene

The molecular defects of three different slow-migrating genetic variants of human serum albumin, albumins Kamloops (formerly RIH), Stirling and Amsterdam, previously characterized only by electrophoretic and dye-binding studies, are now reported. Two of them are proalbumin variants: sequential analysis of the purified whole proteins has established the mutation responsible for albumin Kamloops as -1Arg-->Gln, and for albumin Stirling as -2Arg-->His. A Glu-->Lys substitution in position 570 of the mature albumin molecule was determined in albumin Amsterdam by sequential analysis of two abnormal tryptic fragments. The three alloalbumins are caused by single-base changes all of which seem to represent hot-spots in the albumin gene. The possible functional consequences of the presence of a circulating alloalbumin are discussed.

Structural characterization, stability and fatty acid-binding properties of two French genetic variants of human serum albumin

Four bisalbuminemic, unrelated persons were found in Bretagne, France, and their variant and normal albumins were isolated by DEAE ion exchange chromatography, reduced, carboxymethylated and treated with CNBr. Comparative two-dimensional electrophoresis of the CNBr digests showed that three of the variants were modified in fragment CB4, whereas the fourth had an abnormal fragment CB1. These fragments were isolated, digested with trypsin and mapped by reverse-phase HPLC. Sequencing of altered tryptic peptides showed that the three variants modified in CB4 were caused by the same, previously unreported, amino acid substitution: Asp314-->Val (albumin Brest). The fourth, however, was a proalbumin variant with the change Arg-2-->Cys (albumin Ildut). Both amino acid substitutions can be explained by point mutations in the structural gene: GAT-->GTT (albumin Brest) and CGT-->TGT (albumin Ildut). The proalbumin Ildut is very unstable and already in vivo it is to a large extent cleaved posttranslationally to Arg-Albumin and normal albumin. Furthermore, we observed that during a lengthy isolation procedure the remaining proalbumin was changed to Arg-Albumin or proalbumin lacking Arg-6. In addition, part of normal albumin had lost Asp1. Gas chromatographic investigations using isolated proteins indicated that albumin Brest has improved in vivo fatty acid-binding properties, whereas the structural modification(s) of albumin Ildut does not affect fatty acid binding.

Electrospray ionization mass analysis of normal and genetic variants of human serum albumin

Both normal albumin (Al A) and genetically modified forms were isolated from six heterozygous subjects. Albumins from each individual were analyzed by electrospray ionization mass spectrometry (ESI MS), and the mass was compared with that predicted from the protein sequence. In all cases, the Al A was heterogeneous, with components of mass (± SE) 66 463 ± 4, 66 586 ± 3, and 66 718 ± 5 Da. Each genetic variant showed similar heterogeneity. The mass increase in Al Casebrook (2214 Da) was very close to that predicted (2205 Da) from protein and carbohydrate sequence analysis, whereas the increase in Al Redhill (2378 Da) was close to that expected (2392 Da) for an Arg-albumin with a disialylated N-linked biantennary oligosaccharide and an Ala→Thr mutation. The circulating proalbumins, Christchurch and Blenheim, had mass increases of 748 and 756 Da, respectively, over Al A; in excellent agreement with theoretical values of 744 and 756. Clear shifts in mass were also detected for the point substitutions 177Cys→Phe (44 Da), 1Asp→Val (20 Da), and Arg-albumin (160 Da).

Effect of genetic variation on the fatty acid-binding properties of human serum albumin and proalbumin

In the circulation, non-esterified fatty acids are transported by albumin which also facilitates their removal from donor cells and uptake into receptor cells. We have studied whether genetic variations in the albumin molecule can affect its in vivo fatty acid-binding properties. The fatty acids bound to 25 structurally different variants and to their wildtype counterparts, isolated from heterozygous carriers, were determined gas chromatographically. The variants were proalbumins, albumins with single amino acid substitutions and glycosylated or truncated albumins. In eight cases the total amount bound to the variants was diminished (0.4-0.8-fold), and in seven cases the load was increased to 1.3 or more of normal. Twenty-one fatty acids were quantitated, and for 19 alloalbumins significant deviations from normal were found. Usually, changes in total and individual fatty acid binding were of the same type, but several exceptions to this rule was found. The glycosylated albumin Casebrook showed the largest changes, the total load and the amount of bound palmitate was 8.6 and 14 times, respectively, the normal. The most pronounced changes and the majority of cases of increased binding were caused by molecular changes in domain III. Mutations in domain I, II and the propeptide resulted in smaller effects, if any, and these were often reductions in binding.

Structural characterization of four genetic variants of human serum albumin associated with alloalbuminemia in Italy

A long-term electrophoretic survey on plasma proteins, which was carried out in several clinical laboratories in Italy, identified 28 different genetic variants of human serum albumin and four cases of analbuminemia. We have previously characterized 16 point mutations, 3 C-terminal mutants, and the genetic defects in two analbuminemic subjects. Here, we report the molecular defects of four alloalbumins that have been characterized by protein structural analysis. Of these, three represent new single-point mutations: albumins Tregasio, Val122-->Glu, Bergamo, Asp314-->Gly, and Maddaloni, Val533-->Met. The fourth, albumin Besana Brianza, has the same Asp494-->Asn mutation that introduces a glycosylation site which has been previously reported in a variant from New Zealand, albumin Casebrook. However, in contrast to albumin Casebrook, albumin Besana Brianza is only partially glycosylated and the oligosaccharide is heterogeneous, consisting of a biantennary complex type N-glycan with either two or one sialic acid residue(s) on the antennae. Both albumin Maddaloni and Besana Brianza represent mutations at hypermutable CpG dinucleotide sites; albumin Maddaloni is a mutant that does not involve a charged amino acid.

A genetic variant of albumin (albumin Asola; Tyr140-->Cys) with no free -SH group but with an additional disulfide bridge

A slow migrating variant of human serum albumin, present in lower amount than the normal protein, has been detected by routine clinical electrophoresis at pH 8.6 in two members of a family living in Asola (Lombardia, Italy). Ion-exchange chromatography of serum samples failed to separate the normal protein from the variant. Analysis of the albumin peak by SDS/PAGE revealed that the variant had a lower apparent molecular mass than its normal counterpart. However, the abnormal band was not detectable when the separation was performed under reducing conditions or when both albumins were carboxymethylated. Isoelectric-focusing analysis of CNBr fragments localized the mutation to fragment CNBr 3 (residues 124-298). This fragment was isolated on a preparative scale and subjected to tryptic digestion. Sequence determination of the abnormal tryptic peptide revealed that the variant arises from a Tyr140--> Cys substitution. This result was confirmed by DNA sequence analysis, which showed a single transition of TAT-->TGT at nucleotide position 5074. Despite the presence of an additional cysteine residue, several lines of evidence indicated that albumin Asola has no free -SH group; therefore, we propose the formation of a new S-S bond between Cys140 and Cys34, the only free sulphydryl group present in the normal protein. The relatively low level of the variant in serum and its abnormal mobility on cellulose acetate electrophoresis and SDS/PAGE are probably caused by a gross conformational change of the molecule induced by the new S-S bridge.

Analbuminemia: three cases resulting from different point mutations in the albumin gene

Analbuminemia is a very rare recessive disorder in which subjects have little or no circulating albumin, although albumin is normally the most abundant plasma protein and has many functions. Analbuminemia is caused by a variety of mutations in the albumin gene and is exhibited only by subjects homozygous for the defect. Previously the mutation had been identified at the molecular level in only two human cases; in one case it resulted from an exon-splicing defect, and in the other case it was caused by a nucleotide insertion that caused a frameshift and premature stop codon. In this investigation we identified the mutations in three unrelated subjects from different countries. In each instance a single-nucleotide mutation produced a stop codon, but the mutations occurred at three different sites: (i) in an Italian male a C-->T transition at nt 2368 in the genomic sequence of albumin, (ii) a C-->T transition at nt 4446 for an American female, and (iii) a G-->A transition at nt 7708 in a Canadian male. The size of the albumin fragment that might have been produced for the three cases varied from 31- to 213-amino acid residues, but no evidence for a circulating albumin fragment was obtained. The paradox is that analbuminemia is extremely rare (frequency < 1 x 10(6)); yet the virtual absence of albumin is tolerable despite its multiple functions.

Genetic variants of human serum albumin in Italy: point mutants and a carboxyl-terminal variant

Of the > 50 different genetic variants of human serum albumin (alloalbumins) that have been characterized by amino acid or DNA sequence analysis, almost half have been identified in Italy through a long-term electrophoretic survey of serum. Previously we have reported structural studies of 11 Italian alloalbumins with point mutations, 2 different carboxyl-terminal variants, and 1 case of analbuminemia in an Italian family. This article describes confirmation by DNA sequencing of mutations previously inferred from protein sequencing of 4 of the above alloalbumins; it also reports the mutations identified by protein and DNA sequence analysis of 4 other Italian alloalbumins not previously recorded: albumin Larino, His3-->Tyr; Tradate-2 (protein sequencing only), Lys225-->Gln; Caserta, Lys276-->Asn; and Bazzano, a carboxyl-terminal variant. The first 3 have point mutations that produce a single amino acid substitution, but a nucleotide deletion causes a frameshift and an altered and truncated carboxyl-terminal sequence in albumin Bazzano. In these 4 instances the expression of the alloalbumin is variable, ranging from 10% to 70% of the total albumiN, in contrast to the usual 50% each for the normal and mutant albumin. The distribution of point mutations in the albumin gene is nonrandom; most of the 47 reported point substitutions involve charged amino acid residues on the surface of the molecule that are not concerned with ligand-binding sites.

Modified high-affinity binding of Ni2+, Ca2+ and Zn2+ to natural mutants of human serum albumin and proalbumin

High-affinity binding of radioactive Ni2+, Ca2+ and Zn2+ to six genetic albumin variants and to normal albumin isolated from the same heterozygote carriers was studied by equilibrium dialysis at pH 7.4. The three cations bind differently to albumin. Ni2+ binds to a site in the N-terminal region of the protein which is partially blocked by the presence of a propeptide as in proalbumin (proAlb) Varese (Arg-2-->His), proAlb Christchurch (Arg-1-->Gln) and proAlb Blenheim (Asp1-->Val) and by the presence of only an extra Arg residue (Arg-1) as in Arg-Alb and albumin (Alb) Redhill. The association constants are decreased by more than one order of magnitude in these cases, suggesting biological consequences for the ligand. The additional structural changes in Alb Redhill have no effect on Ni2+ binding. Finally, the modification of Alb Blenheim (Asp1-->Val) reduces the binding constant to 50%. Ca2+ binding is decreased to about 60-80% by the presence of a propeptide and the mutation Asp1-->Val. Arg-1 alone does not affect binding, whereas Alb Redhill binds Ca2+ more strongly than the normal protein (125%). In contrast with binding of Ni2+ and Ca2+, albumin shows heterogeneity with regard to binding of Zn2+, i.e. the number of high-affinity sites was calculated to be, on average, 0.43. The binding constant for Zn2+ is increased to 125% in the case of proAlb Varese, decreased to 50-60% for proAlb Christchurch and Alb Redhill but is normal for proAlb Blenheim, Alb Blenheim and Arg-Alb. The effects of the mutations on binding of Ca2+ and Zn2+ indicate that primary binding, when operative, is to as yet unidentified sites in domain I of the albumin molecule.

A nucleotide insertion and frameshift cause analbuminemia in an Italian family

In analbuminemia, a very rare inherited syndrome, subjects produce little or no albumin (1/100th to 1/1000th normal), presumably because of a mutation in the albumin gene; yet, they have only moderate edema and few related symptoms owing to a compensatory increase in other plasma proteins. Because of the virtual absence of albumin the defect must be identified at the DNA level. In this study the mutation causing analbuminemia in an Italian family was investigated by analysis of DNA from a mother and her daughter. The mother was homozygous for the trait and had a serum albumin value of < 0.01 g/dl (about 1/500th normal); the daughter was heterozygous for the trait and had a nearly normal albumin value. Molecular cloning and sequence analysis of DNA from both mother and daughter showed that the mutation is caused by a nucleotide insertion in exon 8; this produces a frameshift leading to a premature stop, seven codons downstream. The methods of heteroduplex hybridization and single-strand conformation polymorphism were used to compare the DNA of the mother and daughter to the DNA of two unrelated analbuminemic individuals (one Italian and one American). This showed that all three analbuminemic individuals had different mutations; these also differed from the mutation in the only human case previously studied at the DNA level, which was a splicing defect affecting the ligation of the exon 6-exon 7 sequences. Thus, analbuminemia may result from a variety of mutations and is genetically heterogeneous.

Furin has the proalbumin substrate specificity and serpin inhibitory properties of an in situ hepatic convertase

Furin, a KEX2 protease homolog with high RNA expression in the liver is an excellent candidate as a hepatic proprotein convertase. Here we show that purified recombinant furin has the same proalbumin specificity and serpin inhibitory properties as the in situ hepatic convertase. There was rapid cleavage at the -RRD- site of normal human proalbumin but there no significant cleavage of natural unprocessed variants with cleavage site sequences of -RRV-, -HRD-, -RQD-, or -CRD-. Cleavage of the latter was not increased by S-aminoethylation. Furin was specifically inhibited by alpha 1-antitrypsin Pittsburgh (358 Met-->Arg), (K1/2 = 3 microM) but not by 50 microM normal antitrypsin M or by antithrombin, however, antithrombin/heparin was a good inhibitor (K1/2 = 9 microM). The pH optimum for proalbumin cleavage was between pH 5.5 and 6.0, indicating that furin is potentially fully active within secretory vesicles, the site of proalbumin cleavage.

The structural characterization and bilirubin-binding properties of albumin Herborn, a [Lys240-->Glu] albumin mutant

We report the molecular defect of albumin Herborn, a new genetic variant of human serum albumin which has been found in Germany. Isoelectric focusing analysis of CNBr fragments from the purified variant allowed us to localize the mutation in fragment CNBr 3 (residues 124-298). This fragment was isolated on a preparative scale and subjected to tryptic and V8 protease digestion. Sequence determination of the abnormal tryptic and V8 peptides revealed that the variant arises from the substitution Lys240-->Glu. The -2 charge change of albumin Herborn, which is probably due to a A-->G transition in the first position of the corresponding codon in the structural gene, has no significant effect on its electrophoretic mobility under non-denaturating conditions. Therefore we have assumed that residue 240, which has been implicated in the bilirubin primary binding site (Jacobsen, C. (1978) Biochem. J. 171, 453-459), is buried. The binding of bilirubin and biliverdin by albumin Herborn was quantified using the fluorescence quenching method. The apparent equilibrium association constants (Ka +/- SD) and the number of high-affinity binding sites (n) of the defatted variant for bilirubin and biliverdin were Ka = 1.03 +/- 0.18 x 10(8) M-1, n = 1.07; and Ka = 7.48 +/- 1.10 x 10(6) M-1, n = 1.01, respectively. The Ka values are about 93.3% and 99.1% of the values found for the normal protein under the same conditions. These results strongly suggest that Lys240 of human serum albumin is not the basic residue involved in ion pairing with one of the carboxylate groups of bilirubin at its high-affinity site.

Protein and gene structure analysis of an albumin genetic variant: proalbumin Wu Yang (-2 Arg-->His)

An electrophoretically slow genetic variant of human serum albumin was isolated from a patient with bis-albuminemia in Xin Jiang who originated in the province of He Nan, county Wu Yang. Based on the FPLC-CNBr peptide profiles of the variant and normal albumins, the mutated CNBr peptide of the variant was found. Both the mutated and corresponding normal CNBr peptides were further subjected to trypsin digestion. An additional peptide peak from the mutated CNBr peptide was found in the HPLC mapping. The amino acid sequence of this peptide was determined to be Gly-Val-Phe-His-Arg corresponding to the fragment of albumin propeptide from position -5 to -1 with an -2 Arg-->His mutation. The N-terminal sequence of the mutated CNBr peptide was also determined to be Arg-Gly-Val-Phe-His*-Arg-Asp-Ala-His-Lys-, namely the N-terminal part of the mutated proalbumin. According to the known genomic structure of proalbumin, the gene fragments neighbouring the propeptide DNA sequences of the normal and variant albumins were amplified in vitro by polymerase chain reaction. The PCR products with 390 bp were then cloned into M13 vector, respectively. The nucleotide sequence analyses demonstrated that the codon CGT for -2 Arg was mutated to CAT for His. Thus, the point substitution was confirmed both at protein and gene levels. The above genetic variant of albumin was named proalbumin Wu Yang (-2 Arg-->His) being the first instance of the Lille type found in China.

Alloalbuminemia in Sweden: structural study and phenotypic distribution of nine albumin variants

Plasma samples exhibiting alloalbuminemia on electrophoresis at pH 8.6 were requested from clinical laboratories throughout Sweden. Nine variants, each representing a different single point mutation, were found in 100 apparently unrelated Swedes. The overall prevalence of alloalbuminemia was estimated at 1:1700. Mutations were identified by protein-structural analysis followed by allele-specific DNA hybridization to verify the most common types. Slightly retarded (+1) mobility was seen in 80 cases. Of these, 71 had the Arg(-2)----Cys proalbumin variant previously called Malmö I proalbumin. Thirteen examples of the second most frequent type, the substitution Lys313----Asn and a mobility change of -1 charge unit, were found, as well as six cases of Glu570----Lys (albumin B) and a single case of Arg-1----Gln (proalbumin Christchurch). Five previously unreported types of alloalbuminemia were identified: four instances of Glu376----Gln, which is the second known mutation at this site; two examples of Asp550----Ala, the second mutation reported at this site; and one example each of Asp63----Asn, Gln268----Arg, and Asn318----Lys. Other mutations were identified among eight subjects of foreign descent. The high frequency and relatively uniform geographic distribution of the Arg-2----Cys mutation suggest that it may have occurred in a founder individual many generation ago in Sweden.

Genetic variants of serum albumin in Americans and Japanese

A collaborative search for albumin genetic variants (alloalbumins) was undertaken by cellulose acetate and agarose electrophoresis at pH 8.6 of the sera of patients at two major medical centers in the United States and of nearly 20,000 blood donors in Japan. Seventeen instances of alloalbuminemia were ascertained, and seven different alloalbumin types were characterized by structural study. Two previously unreported alloalbumin types were identified. In one type, which was present in a Caucasian family and designated Iowa City-1, aspartic acid at position 365 was replaced by valine (365 Asp----Val); this is the second reported mutation at this position. The other type present in a Japanese blood donor had the mutation 128 His----Arg. An unexpected finding was the presence in a single Japanese of a Naskapi-type alloalbumin (372 Lys----Glu), a variant that had previously been described only for certain Amerindian tribes in whom it occurs with a polymorphic frequency (greater than 1%) and in Eti Turks. An arginyl-albumin (-1 Arg, 1 Asp----Val) occurred in an American family. The other alloalbumin types identified were proalbumins Lille and Christchurch and albumin B that have a cumulative frequency of about 1:3500 in Caucasians probably because of the hypermutability of CpG dinucleotides at the mutated sites. All of the variants characterized in this study are point mutants, and the sites are spread throughout the albumin gene. However, about one-fourth of all known albumin mutations are clustered in the sequence segment from position 354 through 382.

A donor splice mutation and a single-base deletion produce two carboxyl-terminal variants of human serum albumin

At least 35 allelic variants of human serum albumin have been sequenced at the protein level. All except two COOH-terminal variants, Catania and Venezia, are readily explainable as single-point substitutions. The two chain-termination variants are clustered in certain locations in Italy and are found in numerous unrelated individuals. In order to correlate the protein change in these variants with the corresponding DNA mutation, the two variant albumin genes have been cloned, sequenced, and compared to normal albumin genomic DNA. In the Catania variant, a single base deletion and subsequent frameshift leads to a shortened and altered COOH terminus. Albumin Venezia is caused by a mutation that alters the first consensus nucleotide of the 5' donor splice junction of intron 14 and the 3' end of exon 14, which is shortened from 68 to 43 base pairs. This change leads to an exon skipping event resulting in direct splicing of exon 13 to exon 15. The predicted Venezia albumin product has a truncated amino acid sequence (580 residues instead of 585), and the COOH-terminal sequence is altered after Glu-571. The variant COOH terminus ends with the dibasic sequence Arg-Lys that is apparently removed through stepwise cleavage by serum carboxypeptidase B to yield several forms of circulating albumin.

Mutations in genetic variants of human serum albumin found in Italy

A long-term electrophoretic survey of genetic variants of serum albumin has identified an alloalbumin in 589 unrelated individuals in Italy. The alloalbumins were classified electrophoretically into 17 types. The number of unrelated carriers for each type varied from 1 for several variants reported here to 103 for albumin B. The structural change in 8 of these types has previously been determined, and the amino acid substitutions in 3 additional types are reported here. Albumin Varese has a substitution, -2 arginine to histidine (-2 Arg----His), the same as that reported for proalbumin Lille; albumin Torino has the substitution 60 Glu----Lys; and albumin Vibo Valentia has the substitution 82 Glu----Lys. The ability to distinguish so many alloalbumin types by electrophoresis at several pH values indicates that similar substitutions at different sites produce variants with different electrophoretic mobilities. Except for chain terminations in two Italian variants, all the mutations thus far determined for alloalbumins are attributable to a single-base change in the structural gene, and there is a preponderance of transitions and purine mutations. Seven alloalbumins for which the structural change has been established have been ascertained only in Italy. Several of these are clustered in specific geographic regions of Italy, which suggests an origin through a founder individual. Other variants that occur worldwide are nonetheless clustered in geographic regions within Italy. In these cases an independent mutation probably occurred at a hypermutable site such as a CpG dinucleotide.