Acute Intermittent Porphyria: Laboratory Diagnosis by Molecular Methods

Hereditary coproporphyria: exon screening by heteroduplex analysis detects three novel mutations in the coproporphyrinogen oxidase gene

Hereditary coproporphyria is a dominantly inherited disorder of porphyrin metabolism caused by a partial deficiency of coproporphyrinogen oxidase, the sixth enzyme in the heme synthetic pathway. We investigated the molecular basis of hereditary coproporphyria in three unrelated patients, amplifying each exon of the coproporphyrinogen oxidase gene and performing heteroduplex analysis to look for mutations. Unique heteroduplex patterns were noted in exons 2, 3, and 6. Sequencing revealed different mutations in each patient: a G-->A point mutation encoding a glutamic acid to lysine substitution at codon 101 (E101K), a C-->T point mutation encoding a proline to serine substitution at codon 149 (P149S), and a one base-pair insertion in exon 6 (968insT). No other mutations were found on sequencing the remaining exons and their intron-exon junctions. The two point mutations affect amino acids that are conserved in all species studied to date. The one base-pair insertion in exon 6 is the first frameshift mutation to be described in the coproporphyrinogen oxidase gene. This study adds three new mutations to those that have been previously reported, and all have been restricted to single families. These results indicate that hereditary coproporphyria is a genetically heterogeneous disease.

A splicing mutation in the hydroxymethylbilane synthase gene in a Japanese family with acute intermittent porphyria

OBJECTIVES

Acute intermittent porphyria (AIP) is an autosomal dominant inherited disease caused by a decreased activity of hydroxymethylbilane synthase (HMBS). As far as the gene abnormalities of the HMBS, many different mutations have been reported. In this work, we investigated the presence of mutations in a Japanese family with AIP.

DESIGN AND METHODS

A 44-year-old Japanese male and nine members of his family were investigated. All of them were screened by traditional biochemical markers. Mutational analysis was performed using polymerase chain reaction-single strand conformation polymorphism method followed by DNA sequencing. A reliable restriction enzyme cleavage assay was established for the pedigree analysis.

RESULTS

The mutation was a splicing mutation, a C to G transversion at position -3 of the acceptor site of intron 11 of the HMBS gene, resulting in the exon 12 skipping. The patient is heterozygous for the mutation, and his father appeared to be the source of the mutant allele. This mutation created a new cleavage site of the Nla III restriction enzyme and could be screened by a amplified fragment from genomic DNA with digestion. Using this cleavage assay, an asymptomatic carrier in the family was definitively identified.

CONCLUSIONS

This mutation was first found among Japanese AIP patients, but happened to be the same as reported previously from Europe. A similarity of gene abnormality may suggest that those European and Japanese AIP families have a common ancestor. Molecular investigations on the family members should be applied not only for more accurate diagnosis, but also for understanding the molecular genetic heterogeneity underlying this dominantly inherited enzymopathy.

Detection of a R173W mutation in the porphobilinogen deaminase gene in the Nova Scotian "foreign Protestant" population with acute intermittent porphyria: a founder effect

OBJECTIVES

Acute intermittent porphyria (AIP) is caused by mutations in the porphobilinogen deaminase (PBGD) gene that disrupt the function of the enzyme. Many mutations that lead to decreased PBGD activity have been described. An Arg to Trp substitution at codon 173 (CGG-->TGG in exon 10) and designated R173W, which leads to a CRIM-negative phenotype, has been reported in Swedish, Finnish, Scottish, and South African kindreds, and in a Nova Scotian proband with fatal AIP. In this work, we investigated the presence of this mutation in a Nova Scotian patient population presenting with AIP.

DESIGN AND METHODS

Single-strand conformation polymorphism analysis and DNA sequencing by TA cloning and Sanger's dideoxy chain termination method, were used to confirm the maternal transmission of this mutation to the proband. The mutation also eliminates an Ncil (also Mspl) endonuclease restriction site, which allows for detection of the mutant allele by polymerase chain reaction amplification and restriction enzyme digestion.

RESULTS

The family of the Nova Scotian proband and four other AIP kindreds showed the presence of the same mutation. These five families are descendants of German, Swiss, and French immigrants historically known as the "Foreign Protestants," who were recruited to Nova Scotia in the 1750s.

CONCLUSION

In all these families, descent from one couple that settled in Nova Scotia in 1751 has been identified by genealogy research, consistent with a founder effect within this population. This is the first identified mutation in PBGD causing AIP that has been linked to a founder effect in descendants of an immigrant population to North America, and which could be traced to such a distant background, similar to the South African variegate porphyria mutation.