A new missense mutation in the L ferritin coding sequence associated with elevated levels of glycosylated ferritin in serum and absence of iron overload

Abstract

Background: Elevated serum ferritin levels are frequently encountered in clinical situations and once iron overload or inflammation has been ruled out, many cases remain unexplained. Genetic causes of hyperferritinemia associated to early cataract include mutations in the iron responsive element in the 5' untranslated region of the L ferritin mRNA, responsible for the hereditary hyperferritinemia cataract syndrome.

Design and methods: We studied 91 probands with hyperferritinemia comprising 25 family cases belonging to families with at least two cases of unexplained hyperferritinemia, and 66 isolated cases. In the families, we also analyzed 30 relatives. Hyperferritinemia was considered as unexplained when transferrin saturation was below 45% and/or serum iron below 25 mumol/L and/or no tissue iron excess was detected, when inflammation had been ruled out and when iron responsive element mutation was absent. We carried out sequencing analysis of the FTL gene coding the L ferritin.

Results: A novel heterozygous p.Thr30Ile mutation in the NH2 terminus of L ferritin subunit was identified in 17 probands out of the cohort. The mutation was shown to cosegregate with hyperferritinemia in all the 10 families studied. No obvious clinical symptom was found associated with the presence of the mutation. This unique mutation is associated with an unusually high percentage of ferritin glycosylation.

Conclusions: This missense mutation of FTL represents a new cause of genetic hyperferritinemia without iron overload. We hypothesized that the mutation increases the efficacy of L ferritin secretion by increasing the hydrophobicity of the N terminal "A" alpha helix.

Figures

Figure 1.

Family trees of 10 probands with hyperglycosylated serum ferritin. Families are numbered F1 to F10. Black symbols denote the presence of both hyperferritinemia and the heterozygous FTL mutation (p.Thr30Ile). The half-filled black symbols denote hyperferritinemia but no information on the FTL sequence. A question mark in the symbols indicates that the value of serum ferritin and the genotype are unknown. The genotype is indicated as p.Thr30Ile for the presence of the heterozygous FTL mutation, wild type (w.t.) for the normal sequence and a question mark for absence of genotyping. Serum ferritin values (Ft) are indicated in μg/L. The probands are indicated with an arrow.

Figure 2.

Position of the Thr30 in the L ferritin subunit crystal model and consequences of the mutation on the hydrophobicity of the N terminus. (A) Position of the Thr30 in the L ferritin crystal and details of its environment. Aminoacid residues Thr 30 is on the A helix, Trp90 is on the loop between the B and the C helices (B-C loop) and Leu 103 is on helix C. The glycosylated Asn is also indicated. The crystallographic data of L ferritin subunit from reference 18 were analyzed using the Pymol software. (B) Hydrophobicity cluster analysis of the A helix in the wild type L and the p.Thre30Ile mutant L ferritin subunits was carried out using the HCA software. (http://bioserv.rpbs.jussieu.fr/RPBS/cgi-bin/Ressource.cgi?chzn_lg=fr&chzn_rsrc=HCA). The replacement of the threonine residue by isoleucine at position 30 extends the hydrophobic cluster (hydrophobic residues are indicated in green and the hydrophobic cluster is encircled).