Albumin Canterbury (313 Lys----Asn). A point mutation in the second domain of serum albumin

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.

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.

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.

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.

Albumin Hawkes Bay; a low level variant caused by loss of a sulphydryl group at position 177

A slow migrating minor albumin component, representing 5% of total circulating albumin, was detected by routine serum protein electrophoresis and immunofixation. After treatment with 5 mM dithiothreitol the abnormal component was found to migrate normally suggesting the attachment of some component to the free thiol at position 34. However, purification and analysis by SDS-PAGE showed that the abnormal component had a slightly lower apparent molecular weight than normal albumin. Limited tryptic cleavage indicated the abnormal site to be in the N-terminal third of the molecule. HPLC analysis of tryptic peptides from this domain showed the presence of a new peptide of sequence Ala-Ala-Phe-Leu- Leu-Pro-Lys, indicating either a point mutation of 177 Cys-->Phe or the deletion of residues 166-177. DNA sequencing of PCR-amplified DNA confirmed the former Cys-->Phe substitution by indicating a point mutation of C to A at nucleotide position 5185. It appears that the aberrant electrophoretic mobility of the variant might be due to a gross conformational change associated with the formation of a new disulphide bond between Cys-168 and Cys-124.

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.

Beta 141 Leu is not deleted in the unstable haemoglobin Atlanta-Coventry but is replaced by a novel amino acid of mass 129 daltons

Reinvestigation of the structure of the beta-chain of Hb Atlanta-Coventry (beta 75 Leu----Pro, beta 141 Leu deleted) confirmed the presence of two abnormalities; however, analysis of the aberrant beta Co14 tryptic peptide by liquid secondary ion mass spectrometry indicated that the beta 141 Leu (mass 113 daltons) was not deleted but replaced by a novel amino acid of mass 129 daltons. The new amino acid in peptide beta Co14 was uncharged at pH 6.5, more hydrophillic than leucine and susceptible to cleavage by both chymotrypsin and carboxypeptidase A. We propose that the new residue is likely to be hydroxyleucine and that it results from post-translational oxidation of beta 141 Leu as a consequence of perturbation of the haem environment caused by the beta 75 Leu----Pro mutation in the E helix (E19). This proposal is entirely consistent with recent DNA analysis which showed that beta At-Co was not the product of a third beta-globin gene and that neither of the two beta-globin genes, beta A nor beta Atlanta, contained a deletion of the beta 141 Leu codon. We have subsequently found this modified amino acid at position beta 141 in two other unstable haemoglobins, both of which involve mutations on the haem side of the E helix.

Structural characterization of a glycoprotein variant of human serum albumin: albumin Casebrook (494 Asp----Asn)

Albumin Casebrook is an electrophoretically slow genetic variant of human albumin with a relative molecular mass 2.5 kDa higher than normal albumin. It constitutes about 35% of total serum albumin in heterozygous carriers. The decrease in negative charge observed on incubation with sialidase suggested the presence of a carbohydrate moiety and the normalization of molecular weight following treatment with Endo-F indicated that this was an N-linked oligosaccharide. Partial acid hydrolysis and limited tryptic digestion established that the oligosaccharide was located in the C-terminal domain, between residues 367 and 585. Tryptic, chymotryptic and S. aureus V8 proteinase digestions were carried out and the resulting glycopeptides were purified on concanavalin A-Sepharose. Peptide mapping of bound and unbound fractions followed by amino acid composition and sequence analysis, established a point mutation of 494 Asp----Asn. This introduces an Asn-Glu-Thr N-linked oligosaccharide attachment sequence centered on Asn-494 and explains the increase in molecular mass. There was no apparent pathology associated with the presence of this new glycosylated albumin, which was detected in two unrelated individuals of Anglo-Saxon descent.

Hormone binding to natural mutants of human serum albumin

High-affinity binding of progesterone, testosterone, prostaglandin F2 alpha and L-thyroxine to five genetic variants of human serum albumin with defined point mutations was investigated by equilibrium dialysis (pH 7.4). Endogenous albumin A (Alb A) from each individual and commercial human serum albumin were used as controls in each case. The association constant for binding of progesterone to Alb Canterbury (Lys313----Asn) was 1.5 times that calculated for binding to the corresponding, endogenous Alb A. In contrast, the variants Alb Niigata (Asp269----Gly), Alb Roma (Glu321----Lys), Alb Parklands (Asp365----His) and Alb Verona (Glu570----Lys) all had normal progesterone binding properties. Specificity with respect to the type of mutation was also found for the binding of testosterone and prostaglandin F 2 alpha. Testosterone binding to Alb Roma was only 0.7 of that determined for endogenous Alb A, whereas prostaglandin F 2 alpha binding to Alb Niigata was increased by a factor 2.4. In the case of L-thyroxine normal binding properties were found for all the variants. Steric effects and/or conformational changes of the protein, introduced by the amino acid substitutions, probably account for the altered hormone binding. However, in the case of the increased binding of prostaglandin F2 alpha to Alb Niigata electrostatic effects could also be involved. The experimental findings suggest different high-affinity sites for the four hormones. Progesterone, testosterone and prostaglandin F2 alpha are apparently bound within the middle third (domain II) of the protein molecule. The possible position of the primary L-thyroxine site is discussed.

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

An electrophoretically slow albumin variant was detected with a phenotype frequency of about 1:1000 in Sweden and was also found in a family of Scottish descent from Kaikoura, New Zealand, and in five families in Tradate, Italy. Structural study established that the major variant component was arginyl-albumin, in which arginine at the -1 position of the propeptide is still attached to the processed albumin. A minor component with the amino-terminal sequence of proalbumin was also present as 3-6% of the total albumin. After amplification of the gene segment encoding the prepro sequence of albumin, specific hybridization of DNA to an oligonucleotide probe encoding cysteine at position -2 indicated the mutation of arginine at the -2 position to cysteine (-2 Arg----Cys). This produced the propeptide sequence Arg-Gly-Val-Phe-Cys-Arg. This was confirmed by sequence analysis after pyridylethylation of the cysteine. This mutation produces an alternate signal peptidase cleavage site in the variant proalbumin precursor of arginyl-albumin giving rise to two possible products, arginyl-albumin and the variant proalbumin. Another plasma from Bremen had an alloalbumin with a previously described substitution (1 Asp----Val), which also affects propeptide cleavage. Hypermutability of two CpG dinucleotides in the codons for the diarginyl sequence may account for the frequency of mutations in the propeptide. Mutation at these two sites results in a series of recurrent proalbumin variants that have arisen independently in diverse populations.

Alloalbumin in cerebrospinal fluid: report of a case and remarks on physiopathology

Alloalbumin Tagliacozzo (313 Lys------Asn) was detected in the cerebrospinal fluid of a 27-yr-old woman. We studied this rare finding by means of high-resolution electrophoresis, isoelectric focusing, and immunofixation. We herein discuss the importance of molecular mass versus superficial electrical charge in the transfer of plasma proteins through the blood-brain barrier.

Point substitutions in albumin genetic variants from Asia

Despite their rarity and physiologically neutral character, more inherited structural variants of serum albumin (alloalbumins) are known than for any other human protein except hemoglobin. Including three previously unreported examples described here, we have identified 13 different point substitutions in alloalbumins of Japanese origin. Of these only albumin B and two proalbumins have been reported in other ethnic groups, and these are the most common variants of European origin. Some alloalbumins of Asiatic origin, but not yet identified in Japanese, are present in diverse ethnic groups. An alloalbumin found in indigenes of New Guinea (lysine----asparagine at position 313) is also present in Caucasians of various European descents. Albumin Lambadi, occurring in a tribal group in south India, has a mutation (glutamic acid----lysine at position 501) also found as a rare variant in individuals of diverse ethnic origin resident on four continents. These results suggest that some alloalbumins with the same substitution may have originated by independent mutations in various populations. This, together with the apparent clustering of point substitutions in the protein structure, may reflect hypermutability of the albumin gene.

Albumin Redhill (-1 Arg, 320 Ala----Thr): a glycoprotein variant of human serum albumin whose precursor has an aberrant signal peptidase cleavage site

Albumin Redhill is an electrophoretically slow genetic variant of human serum albumin that does not bind 63Ni2+ and has a molecular mass 2.5 kDa higher than normal albumin. Its inability to bind Ni2+ was explained by the finding of an additional residue of Arg at position -1. This did not explain the molecular basis of the genetic variation (since proalbumin contains adjacent Arg residues at -1 and -2) or the increase in apparent molecular mass. Fractionation of tryptic digests on concanavalin A-Sepharose followed by peptide mapping of the bound and unbound fractions and sequence analysis of the glycopeptides identified a mutation of 320 Ala----Thr. This introduces an Asn-Tyr-Thr oligosaccharide attachment sequence centered on Asn-318 and explains the increase in molecular mass. This, however, did not satisfactorily explain the presence of the additional Arg residue at position -1. DNA sequencing of polymerase chain reaction-amplified genomic DNA encoding the prepro sequence of albumin indicated an additional mutation of -2 Arg----Cys. This introduces a prepro sequence, Met-Lys-Trp-Val-Thr-Phe-Ile-Ser-Leu-Leu-Phe-Leu-Phe-Ser-Ser-Ala-Tyr- Ser-Arg-Gly-Val-Phe-Cys-Arg (cf.-Tyr-Ser-Arg-Gly-Val-Phe-Arg-Arg- in normal human pre-proalbumin). We propose that the new Phe-Cys-Arg sequence in the propeptide is an aberrant signal peptidase cleavage site and that the signal peptidase cleaves the propeptide of albumin Redhill in the lumen of the endoplasmic reticulum before it reaches the Golgi vesicles, the site of the diarginyl-specific proalbumin convertase.