A genotype-phenotype correlation between null-allele mutations in the ferrochelatase gene and liver complication in patients with erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP), an inborn error of heme metabolism, causes in the majority of the patients only a symptom of photosensitivity. However, around 2% of the EPP sufferers develop liver complication in the form of liver cirrhosis and progressive liver failure. Mutations in the human ferrochelatase (FECH) gene causing EPP are highly heterogeneous and mostly family-specific. Actually, 62 FECH mutations have been published, 48 of them are "null allele" mutations inducing the formation of a truncated protein. The remaining 14 are missense mutations. In contrast to the null allele mutations, the latter lead to substitution of a single amino acid residue in the protein molecule and generate an enzyme that, although functionally impaired, is in its full length. In order to study the association between "null allele" mutation and liver complication, we combined our data with those in the literature. A total of 112 EPP patients were counted among 93 EPP families with a known FECH mutation. All 18 EPP patients who had severe liver complication carried a "null allele" mutation. In contrast, none of the 20 patients who carried a missense mutation had developed liver complication till the time of study (Fisher's exact test, p<0.05). High protoporphyrin blood concentration are considered to be a sign of an increased risk of liver disease. No correlation of protoporphyrin blood level with the type of mutation, was founds if patients with overt liver disease were excluded from the sample. Furthermore, no significant association of the liver complication with the location of the mutation within the FECH gene was found (Fisher exact test p= 0.46). These available data indicate a significant genotype-phenotype correlation between "null allele" mutation and protoporphyrin related liver disease in EPP. Although the risk for a EPP patient with a missense mutation to develop liver disease cannot be totally eliminated based on these data, it is comparably low.