A NOVEL MUTATION IN THE FERROCHELATASE GENE AMONG FINNISH EPP PATIENTS

Patients and clinical manifestations: We have studied altogether 14 Finnish EPP families including 40 patients, of whom 24 experienced photosensitivity. The pattern of inheritance is autosomal dominant in these families. The proband with a novel mutation was a 7-year-old boy experiencing stinging sensations and painful edema of the sun-exposed skin accompanied by severe hemorrhagic bullae on the toes after strong sun-exposure. The same mutation was identified in his father, who was symptom-free, and in his 72-year-old grandfather, who had experienced characteristic skin symptoms since childhood without a proper diagnosis. The activities of reticulocyte ferrochelatase in the proband, the father and the grandfather were considerably higher (90%, 80% and 50 % of the normal) compared to the mean activity of Finnish EPP patients (20% of the normal), while erythrocyte protoporphyrin levels (28 000, 1000, 42 000 nmol/l, respectively) correlated well with the clinical condition and were parallel to those of other EPP patients. Molecular genetics: To date we have identified a novel mutation (E413X) in the ferrochelatase gene in addition to eight previously described mutations (K86E, Q96X, 751delGAGAA, R115X, H157R, 1122delT, IVS8+1g-a, IVS4+1delg). Two mutations cause a splicing defect resulting exon skipping, two point mutations cause an amino acid change and four mutations cause early stop codon and truncated polypeptide. The outcome of the mutations have been determined by sequencing of the RT-PCR product amplified from total RNA extracted from the patients' lymphoblast cell lines. E413X is predicted to interrupt dimerization of ferrochelatase by disrupting α-helix number 17 at the C-terminus. Even in prokaryote expression, where dimerization of human ferrochelatase cannot occur, the remarkable difference in the activity between the wild-type (100%) and mutants (0%) indicates the complete loss of function of the mutated allele. The genotype frequencies of the IVS3-48 T/C polymorphism in different Finnish EPP patient groups (symptomatic EPP patients T/T 0.0; T/C 0.67; C/C 0.33, asymptomatic carriers 0.5; 0.5; 0.0, healthy relatives 0.22; 0.78; 0.0, controls 0.86; 0.14; 0.0, respectively) demonstrated over expression of a C-allele among symptomatic patients.