A novel deletion mutation within the carboxyl terminus of the copper-transporting ATPase gene causes Wilson disease

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Abstract

In patients with Wilson disease (WD), an autosomal recessive disorder, toxic accumulation of copper results in fatal liver disease and irreversible neuronal degeneration. ATP7B, the gene mutated in WD, contains 21 exons and encodes a copper-transporting ATPase. In this study, all exons of the ATP7B gene of nine WD patients were screened for alterations by conventional mutation detection enhancement (MDE) heteroduplex analysis, followed by direct sequencing of the regions that showed heteroduplex formation. For the first time, a novel deletion mutation (4193delC) in exon 21, causing a frameshift leading to premature truncation of the protein was detected in four of nine patients. The 4193delC removes several signals within the carboxyl terminal domain that may disrupt trafficking of ATP7B protein through trans-Golgi network at the cellular level.

Introduction

Wilson disease (WD), an autosomal recessive disorder of copper transport, is characterized by impaired biliary excretion and deficient incorporation of copper into ceruloplasmin [1]. Toxic accumulation of copper causes tissue damage, primarily in the liver (hepatic cirrhosis) and secondarily in the brain (neuronal degeneration). Patients can present with liver disease, with neurological impairment, or with both [2]. Patients with liver disease generally present in childhood or adolescence. Neurological symptoms begin at the age of 12 years or later [3]. WD occurs in the population of every geographic and ethnic origin. The worldwide prevalence of WD is estimated to be one in 30 000, with a corresponding gene frequency of 0.56% and a carrier frequency of about 1/90 [2].

ATP7B, the gene mutated in WD, has 21 exons and encodes a protein of 1465 amino acids. The protein is a copper transporting P-type ATPase homologous to the gene product (ATP7A) responsible for Menkes disease, an X-linked disorder of copper transport [4], [5]. Many mutations have been detected within the first 20 exons of the ATP7B gene; some of the mutations are relatively frequent and occur in different populations [6].

Although mutations in the ATP7B gene have been reported for many ethnic groups, until now a mutational analysis of this gene has not been extensively investigated in WD patients of Arab origin, especially Saudis. Such studies are important in view of the closed structure of the Arab population and the high prevalence of consanguineous marriage among the Arabic tribes [7]. In this paper, we describe the outcome of our initial effort to identify mutations in the ATP7B gene in a representative group of Saudi patients with WD.

Section snippets

Patients

The study included nine Saudi WD patients selected from seven different Arab tribes from different regions of Saudi Arabia. Diagnosis of WD was based on clinical and radiological features, liver biopsy results, low ceruloplasmin, low copper serum level and high urinary copper elimination [2], [6]. Five patients had both the liver and neurological hallmarks of the disease and four patients had only the liver abnormalities. In this series, eight patients were born of consanguineous marriage. The

Results

We performed MDE-heteroduplex analysis of the ATP7B gene of Saudi control subjects and patients. We detected polymorphism in several exons of the ATP7B gene derived from normal control subjects, as expected (data not shown). To rule out high degree of variability in each exon of the ATP7B gene [1], [6], proper control (wild type) DNA for each of the 21 exons was initially screened for alterations by MDE-heteroduplex analysis and DNA sequencing. Thus the selected control samples, whose

Discussion

MDE heteroduplex analysis, a general mutation screening method that detects multiple sequence changes, is an elegant technique for the detection of mutations in a large gene with multiple exons and a very large number of mutations such as the ATP7B gene. We detected a novel frameshift mutation, 4193delC, in exon 21 of the ATP7B gene. Furthermore, this deletion mutation was not detected in 55 normal Saudi subjects (i.e., 110 chromosomes) unrelated to patients, indicating that the mutation in

Acknowledgements

This study was supported by King Fahad National Guard Hospital and a grant from the King Abdulaziz City for Science and Technology (KACST), Saudi Arabia.

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    Citation Excerpt :

    This mutation abrogated the copper-induced trafficking of ATP7B in cells and conferred defective copper transport activity, which explained the hepatic copper accumulation in the tx mice (44, 50). The ATP7B-4193del human WD patient mutation results from a base pair deletion at nucleotide position 4193, which causes a frameshift, with the addition of 5 new amino acids before a premature stop codon (51). This mutation results in the truncation of the last 61 amino acids of the protein, which includes the trileucine sequence (51) that is important for localization and recycling of ATP7B (6).

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