Elsevier

Clinical Biochemistry

Volume 41, Issue 18, December 2008, Pages 1482-1485
Clinical Biochemistry

Case Report
Novel frameshift in the serum albumin gene results in analbuminemia through premature truncation and post translational modification

https://doi.org/10.1016/j.clinbiochem.2008.08.082Get rights and content

Abstract

Objectives

To identify the molecular lesion in a patient with analbuminemia.

Design and methods

DNA sequencing, genome-wide SNP microarray and synthetic peptide assays to investigate DNA and protein aberrations.

Results

A homozygous frameshift deletion in exon 12 of HSA is predicted to cause truncation of the albumin protein. A proalbumin-like sequence identified in the novel C-terminal sequence has the potential to be post-translationally modified.

Conclusions

The truncated albumin molecule is potentially edited by proteolytic cleavage before it enters the circulation.

Introduction

Serum albumin is encoded on chromosome 4q13.3 by a single autosomal gene (ALB; GenBank accession number EF649953) composed of 15 exons, the last of which is untranslated [1]. The protein is initially synthesised in the liver as preproalbumin and the prepeptide is cleaved cotranslationally in the lumen of the endoplasmic reticulum [1]. The N-terminal propeptide (RGVFRR-) is subsequently cleaved by furin in the trans Golgi network just prior to secretion [1]. The mature serum protein consists of a single unglycosylated chain of 585 amino acids and accounts for ∼ 60% (reference range 35–45 g/L) of total serum protein. Albumin functions primarily as a regulator of plasma oncotic pressure and as a transport protein for fatty acids, metal ions and a variety of metabolites and pharmaceuticals [1].

Analbuminemia (OMIM 103600) is a spectacular visual finding in the protein laboratory as it involves the complete absence of an albumin band on serum protein electrophoresis. However, albumin concentrations in the range 0.001–1.0 g/L are generally considered indicative of the condition [2]. Surprisingly, the absence of this major protein does not result in a particularly severe phenotype; patients display only mild edema, fatigue and hypercholesterolemia. In the absence of albumin, increased production of serum globulins facilitate the maintenance of osmotic pressure and lipoproteins can stand-in as transporters of free fatty acids [1]. Analbuminemia results from the inheritance of two defective gene copies, with a high frequency of cases resulting from consanguinity. Remarkably, heterozygous carriers display near normal levels of albumin despite the loss of one allele [1].

Analbuminemia is a rare disease and to date only 43 cases have been reported in the analbuminemia register [2], with an additional case recently published [3]. Of these 44 cases, only 12 causative mutations have been elucidated; 4 nonsense mutations, 4 mutations affecting splicing, 1 frameshift/deletion, 1 frameshift/insertion, and the report of a compound heterozygous nonsense/splice site mutation. Here we report the identification of the thirteenth causative analbuminemic mutation in the albumin Locust Valley proband [4] and explore the possibility that it creates a second propeptide sequence within the protein.

A 34 year old paraplegic man of Italian descent was found to have an abnormal serum protein profile which exhibited a distinct lack of the albumin peak. Subsequent investigation revealed raised serum globulins, low total serum protein and an albumin concentration < 1 g/L, as determined by several methods [4]. The investigators were unable to identify a causative mutation after sequencing the coding regions and intron/exon boundaries of the albumin gene, but named the ‘bona fide case of analbuminemia’ albumin Locust Valley. They speculated that the condition may be the result of a mutation affecting a regulatory sequence within the introns, or regions more peripheral to the gene. We recently identified and characterised a seemingly benign mutation deep within an intronic region of the fibrinogen beta gene as the cause of afibrinogenemia [5]. In an effort to identify other similar mutations we offered to reinvestigate the Locust Valley case.

Section snippets

Methods

Only a small amount of the original serum (0.5 mL), from the previous study [4], was available for analysis. Electrophoresis confirmed the absence of an albumin band and radial immunodiffusion indicated an albumin concentration ∼ 0.1% of normal. The entire coding region and the intron/exon boundaries of the albumin gene were amplified from genomic DNA and sequenced on an ABI 3130xl genetic analyser (Applied Biosystems, Foster City, CA, USA) using the Big Dye Terminator v3.1 cycle sequencing kit.

Results

DNA sequencing revealed a novel homozygous single nucleotide deletion, c.1610delT (numbering from the initiator codon) in exon 12 (Fig. 1), which was confirmed in two separate genomic DNA samples. This deletion of thymine from codon 537 (or residue 513 of the mature protein) causes a frameshift with premature termination after the translation of 20 new amino acids (p.Ile537AsnfsX21) (Fig. 1). This truncation would eliminate nearly half of the third domain of the 585-residue protein and

Discussion

In vivo cleavage at the RR-D motif would shorten albumin Locust Valley from 532 to 520 residues. However, with the slower cleavage rate, it is likely that both products would be produced and contribute to the small pool of plasma albumin. Individuals with heterozygous mutations causing variable length truncations of the C-terminus have been found to express low levels of mutant albumin in serum [1], [2]. However to date, no mutant albumin species have been isolated from analbuminemic plasma.

Acknowledgments

R.L.D is the recipient of a University of Otago Postgraduate scholarship.

We thank Dr Kazuhisu Nakayama, Institute of Biological Sciences, University of Tsukuba, Japan, for providing the recombinant furin.

References (9)

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Cited by (5)

  • Congenital analbuminaemia: Molecular defects and biochemical and clinical aspects

    2013, Biochimica et Biophysica Acta - General Subjects
    Citation Excerpt :

    The truncation would eliminate nearly all of the second half of the mature protein. A 34-year-old paraplegic man of Italian descent was found to be homozygous for a single nucleotide deletion, c.1610delT in exon 12 [47]. The deletion of thymine from codon 537 (or residue 513 of the mature protein) causes a frame-shift with premature termination after the translation of 20 new amino acids (p.Ile537Asnfs*21).

  • Human serum albumin: From bench to bedside

    2012, Molecular Aspects of Medicine
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    All the HSA variants are localized within either the coding regions or the intron-exon junctions of ALB (Dolcini et al., 2007; Minchiotti et al., 2008; http://www.albumin.org). Overall, these mutations can be classified as follows: 60 missense mutations leading to the synthesis of a HSA variant having one amino acid changed (Peters, 1996; Caridi et al., 2008; Minchiotti et al., 2008; Kim et al., 2010a); 7 nonsense mutations creating a premature stop codon (Papi et al., 1983; Watkins et al., 1994a,b; Campagna et al., 2005; Campagnoli et al., 2005a,b; Caridi et al., 2009; Ruhoff et al., 2010); 11 frameshift mutations leading to premature stop codons (Galliano et al., 1986a, 2002; Minchiotti et al., 1989, 2001; Watkins et al., 1991, 1994a; Madison et al., 1994; Campagna et al., 2005; Dolcini et al., 2007; Davis et al., 2008; Dagnino et al., 2010a,b); one insertion leading to a defective intron splicing and to the creation of a premature stop codon (Alb Rugby Park; Gly596_Leu609 delins7) (Peach et al., 1992); one insertion leading to the activation of a cryptic acceptor splice site, the deletion of 15 bp and the consequent synthesis of a shorter HSA variant (Alb Banks Peninsula; Gly596_Leu609delinsSerLeuCysSerGly) (Brennan et al., 1999); and three undetected splicing variants (Alb Baghdad, Alb Zonguldak, Analb Vancouver) (Ruffner and Dugaiczyk, 1988; Campagnoli et al., 2002; Caridi et al., 2008) (Table 1). The majority of the HSA variants results from single residue substitutions and almost all have net charges which differ from normal HSA at physiological pH: Alb Vibo Valentia (Glu82Lys), Alb Roma (Glu321Lys), Alb Sondrio (Glu333Lys), Alb Vancouver (Glu501Lys), Alb Ortonovo (Glu505Lys), and Alb Verona (Glu570Lys) are all +2 variants (i.e., they have two positive charges more than wild type HSA); Alb Blenheim (Asp1Val), Alb Niigata (Asp269Gly), Alb Brest (Asp314Val), Alb Parklands (Asp365His), Alb Milano Slow (Asp375His), and Alb Fukuoka-1 (Asp563Asn) are +1 variants (i.e., they have one positive charge more than wild type HSA); Alb Tregasio (Val122Glu), Alb Caserta (Lys276Asn), Alb Canterbury (Lys313Asn), and Alb Trieste (Lys359Asn) are −1 variants (i.e., they have one negative charge more than wild type HSA); and, Alb Tradate-2 (Lys225Gln), Alb Herborn (Lys240Glu), Alb Maku (Lys541Glu), Alb Church Bay (Lys560Glu), and Alb Krapina (Lys573Glu) are −2 variants (i.e., they have two negative charges more than wild type HSA) (Kragh-Hansen et al., 2005) (Table 1).

  • A novel frameshift deletion in the albumin gene causes analbuminemia in a young Turkish woman

    2010, Clinica Chimica Acta
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    A possible hypermutable region was identified in the exon 11–intron 11 junction, as the Fondi allele (c.1427A>G) and the Bartin mutation (c.1428 + 2 T>C) lie in close proximity within this junction [2]. The Safranbolu two base deletion starts 3 nucleotides downstream the single base deletion c.1610delT identified in analbuminemia Locust Valley [14], suggesting that also this region of exon 12 might be prone to deletions resulting in analbuminemia. The majority of the mutations identified as the genetic cause of analbuminemia are unique, i.e. they have been found in only a single individual or in the members of the same family.

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