AN AIP FAMILY WITH NO HMBS SEQUENCE VARIANT

Skadberg O1, Espetvedt L2, Boman H2, Sandberg S1

 

1Norwegian Porphyria Centre, Laboratory of Clinical Biochemistry, 2Norwegian Porphyria Centre, Center for Medical Genetics and Molecular Medicine, Haukeland University Hospital, Bergen, Norway

 

 

In a Norwegian family with typical clinical and biochemical acute intermittent porphyria, no mutations were found by sequencing the entire HBMS gene. While awaiting a mutation find, haplotype analysis was employed as a diagnostic tool. DNA was isolated from blood and all coding exons of HMBS of the index case were sequenced (capillary electrophoresis, ABI 3100). No sequence variants, thought to be associated with AIP, were found. Also the non-coding exon 2, as well as segments of 580 and 660 bp at the 5' and 3' UTR, respectively, were sequenced without suspicious findings. To reduce the possibility of interfering polymorphisms in the primer sites, a second set of sequencing primers was designed. No mutations were found. The sequence variants observed were apparent homozygosity for 5'UTR-430G>A, IVS2-96T>G, IVS3-14A>G, and V202V. Proof of hemizygosity was not found. We also observed heterozygosity for 3`UTR+337G>A. We tested family members for five chromosome 11 dinucleotide markers covering 1.8 Mb surrounding the HMBS locus (ABI 310, markers given in Fig.1). The affected haplotype was established among the definitely affected family members, and was subsequently used for identifying those sharing the HBMS locus carrying the putative AIP-allele. Apart from unconfirmed reports of a "Chester locus", all patients with AIP identified so far has been related to deficits of the HMBS locus. The affected haplotype co-segregated with AIP in this pedigree, a finding supporting the diagnosis. Thus, even though no DNA abnormalities have been found, true AIP families may be worked up and individual family members may be reliably classified, using indirect genetic haplotyping tools. By sequencing, we have so far identified the causative AIP mutation in 22 different Norwegian families. However, the putative mutation in this family defies revelation. Sequencing may not readily disclose whole exon deletions and large structural rearrangements or mutations afflicting control elements on the same (or other) chromosomes. We will continue to search for the putative AIP mutation in this family.