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A “null allele” mutation is responsible for erythropoietic protoporphyria in an Israeli patient who underwent liver transplantation: relationships among biochemical, clinical, and genetic parameters

https://doi.org/10.1016/S1079-9796(03)00040-8Get rights and content

Abstract

Mutations in the human ferrochelatase gene (FECH) are the primary cause of the inborn disorder erythropoietic protoporphyria (EPP). While the majority of the EPP patients exhibit only photosensitivity, a small percentage of patients (∼2%) develop liver complications in addition to the cutaneous symptoms. In this study, the FECH gene of an Israeli EPP patient who suffered from EPP-related liver complications was sequenced. A splicing defect IVS10+1, g→t, which is known to cause the deletion of exon 10, was identified in the index patient as well as in his symptomatic older sister and his asymptomatic mother. Like the other 12 known FECH mutations associated with liver complications, IVS10+1, g→t is a “null-allele” mutation. Although the two siblings with overt EPP share an identical genotype with respect to both the mutation on one FECH allele and three intragenic single nucleotide polymorphisms, −251G, IVS1-23T, and IVS3-48C on the other allele, the sister of the index patient has so far shown no signs of liver involvement, suggesting that additional factors might account for the liver disease in EPP.

Introduction

Erythropoietic protoporphyria (EPP, OMIM 177000) is caused by a partial deficiency of ferrochelatase (EC 4.99.1.1), the last enzyme in the heme biosynthesis pathway [1]. It is an autosomal dominant inherited disorder with a low penetrance of 10–20%. According to a recent study, the phenotypic expression of EPP is modulated by an intronic single nucleotide polymorphism (SNP) of the ferrochelatase (FECH) gene [2]. Clinically, the disorder is expressed mainly as painful photosensitivity due to the accumulation of free protoporphyrin (PP) in the skin. A small percentage of patients (∼2%) develop terminal liver failure, probably resulting from the intrahepatic accumulation of PP [3].

A total of 56 different mutations have been described in the FECH gene of EPP patients [4]. Most of them (∼80%) are “null-allele” mutations, including nonsense and frameshift mutations, which lead to the formation of a shortened mRNA. The rest (∼20%) are missense mutations. All 16 patients with liver complications who have been described to date carried a null-allele mutation. Statistical analysis points to a genotype–phenotype correlation between the type of mutation and the risk for liver complications. Carriers of missense mutations are at significantly lower risk for liver disease than carriers of null-allele mutations [5]. The fact that identical null-allele mutations are found in patients with and without liver complications illustrates the need for extensive genetic studies among EPP families and patients in order to gain in-depth knowledge of the cause(s) of the liver complications [6]. For this purpose, we studied the FECH gene in an eight-member Israeli EPP family. The index patient of this family suffered from progressive EPP-related liver insufficiency that eventually required liver transplantation.

Section snippets

Case report

The index patient (II-3; Fig. 3) was a 32-year-old male Caucasian Ashkenazi Jew, who had suffered since childhood from severe photosensitivity. EPP was diagnosed when he was 10 years old. Three years ago, at the age of 29, the patient underwent liver transplantation for chronic liver failure. One of his sisters (II-1), who was also diagnosed as having EPP, suffered from mild photosensitivity and anemia. The parents (I-1 and I-2) and two other sisters (II-2 and II-5) of the index patient had

Results

In the index patient, all 11 exons as well as a portion of the intron (≥35 bp) adjacent to the exons of the FECH gene were sequenced. A g to t transversion was identified at position +1 in intron 10 (IVS10+1, g→t). The patient was found to be heterozygous for this mutation as shown by two overlapping peaks of “g” and “t,” roughly equal in height, in the electropherogram (Fig. 1). No additional mutations were found in the FECH gene.

All members of the family were subjected to DGGE analysis of

Discussion

Molecular study of the FECH gene identified an IVS10+1, g→t mutation in both the index patient and his symptomatic sister (II-1), as well as in their asymptomatic mother. This splicing defect causes exon 10 skipping and hence an in-frame deletion of 20 amino acids in the ferrochelatase enzyme [10]. The same null-allele mutation was previously described in a French EPP patient who had shown no liver complications up to the time of study [10].

Although the three family members mentioned above

Acknowledgements

We are grateful to the family for their cooperation. We thank the doctors, Prof. A. Atsmon and Dr. M. Sztern (Rabin Medical Center, Petah-Tikva) and Dr. R. Safadi and Prof. E. Galun (Hadassa University Hospital, Jerusalem), who treated the patient during various stages of his illness. We thank Sulejman Ahmetovic and Livia Buonanno for their excellent technical assistance.

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