Open Access Highly Accessed Research article

Detailed investigations of proximal tubular function in Imerslund-Gräsbeck syndrome

Tina Storm1, Christina Zeitz234, Olivier Cases234, Sabine Amsellem234, Pierre J Verroust1234, Mette Madsen1, Jean-François Benoist6, Sandrine Passemard78, Sophie Lebon8, Iben Møller Jønsson9, Francesco Emma10, Heidi Koldsø11, Jens Michael Hertz12, Rikke Nielsen1, Erik I Christensen1* and Renata Kozyraki2345*

Author Affiliations

1 Department of Biomedicine, Aarhus University, Aarhus, Denmark

2 INSERM UMR S968, Institut de la Vision, Paris, 75012, France

3 UPMC Univ Paris 06, UMR_S 968, Institute de la Vision, Paris, F-75012, France

4 CNRS, UMR_7210, Paris, F-75012, France

5 Service de Biochimie B, Hôpital Saint-Antoine, Paris, France

6 Laboratoire de Biochimie, Hôpital Robert Debré, Paris, France

7 Service de Génétique, Hôpital Robert Debré, Paris, France

8 Inserm U676, Hôpital Robert Debré, Paris, France

9 Department of Pediatrics, Aarhus University Hospital, Aarhus, Denmark

10 Department of Nephrology and Urology, Division of Nephrology and Dialysis, Ospedale Bambino Gesù, IRCCS, Rome, Italy

11 Centre for Insoluble Protein Structures (inSPIN) and Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, Aarhus University, Aarhus, Denmark

12 Department of Clinical Genetics, Odense University Hospital, Odense, Denmark

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BMC Medical Genetics 2013, 14:111  doi:10.1186/1471-2350-14-111

Published: 24 October 2013

Abstract

Background

Imerslund-Gräsbeck Syndrome (IGS) is a rare genetic disorder characterised by juvenile megaloblastic anaemia. IGS is caused by mutations in either of the genes encoding the intestinal intrinsic factor-vitamin B12 receptor complex, cubam. The cubam receptor proteins cubilin and amnionless are both expressed in the small intestine as well as the proximal tubules of the kidney and exhibit an interdependent relationship for post-translational processing and trafficking. In the proximal tubules cubilin is involved in the reabsorption of several filtered plasma proteins including vitamin carriers and lipoproteins. Consistent with this, low-molecular-weight proteinuria has been observed in most patients with IGS. The aim of this study was to characterise novel disease-causing mutations and correlate novel and previously reported mutations with the presence of low-molecular-weight proteinuria.

Methods

Genetic screening was performed by direct sequencing of the CUBN and AMN genes and novel identified mutations were characterised by in silico and/or in vitro investigations. Urinary protein excretion was analysed by immunoblotting and high-resolution gel electrophoresis of collected urines from patients and healthy controls to determine renal phenotype.

Results

Genetic characterisation of nine IGS patients identified two novel AMN frameshift mutations alongside a frequently reported AMN splice site mutation and two CUBN missense mutations; one novel and one previously reported in Finnish patients. The novel AMN mutations were predicted to result in functionally null AMN alleles with no cell-surface expression of cubilin. Also, the novel CUBN missense mutation was predicted to affect structural integrity of the IF-B12 binding site of cubilin and hereby most likely cubilin cell-surface expression. Analysis of urinary protein excretion in the patients and 20 healthy controls revealed increased urinary excretion of cubilin ligands including apolipoprotein A-I, transferrin, vitamin D-binding protein, and albumin. This was, however, only observed in patients where plasma membrane expression of cubilin was predicted to be perturbed.

Conclusions

In the present study, mutational characterisation of nine IGS patients coupled with analyses of urinary protein excretion provide additional evidence for a correlation between mutation type and presence of the characteristic low-molecular-weight proteinuria.

Keywords:
Imerslund-Gräsbeck syndrome; Cubilin; Amnionless; Proximal tubules; Tubular proteinuria