Juvenile hemochromatosis caused by a novel combination of hemojuvelin G320V/R176C mutations in a 5-year old girl

Abstract

During a screening program we identified a 5-year old girl with elevated iron parameters. The child was found to have a combination of a novel R176C mutation together with the G320V mutation in the juvenile hemochromatosis gene (HJV). The girl was also homozygous for the H63D mutation in HFE. The possibility of detecting juvenile hemochromatosis before the onset of clinical manifestations raises questions about the management of such young children in order to prevent iron overload.

Juvenile hemochromatosis (JH) is a severe form of hereditary hemochromatosis (HH) and is characterized by an early age of onset.1 The disease is genetically heterogeneous since it can be associated with mutations in two distinct genes: HAMP2 and HJV3 encoding hepcidin and hemojuvelin, respectively. The majority of cases of JH are due to mutations in HJV.4,5 Digenic inheritance of mutations in HFE6 and HAMP may result in either JH or adult HH depending upon the severity of the mutation in HAMP.7 A cascade screening program designed for family studies when a family member is affected by a major complication possibly due to hemochromatosis detected a 5-year girl with elevated iron indices and allowed us to characterize the genetic defect responsible.

The proband was referred in 2003 for investigation of repeatedly raised transferrin saturation (90%) associated with elevated serum ferritin (199 μgL). Iron parameters had initially been measured in all family members following the death of the paternal grandfather with a diagnosis of hepatocellular carcinoma. Serum iron parameters of the grandfather were not available.The proband’s parents and two brothers had normal iron indices (Table 1). Written informed consent to all the investigations was obtained from the parents for themselves and their children. The girl had a normal blood count, no elevated liver transaminases and no clinical signs of hemochromatosis. Her father originated from north-east France and her mother had Spanish and Italian ancestry. The girl was found to be homozygous for the H63D mutation in HFE as was her older brother 2 who did not, however, have any abnormal iron indices (Table 1). We then sequenced the HAMP gene, the only gene known to be related to JH in early 2003. None of the exons or exon-intron junctions were mutated. In an attempt to determine a possible digenic inheritance of mutations in HAMP and HFE, the latter was also sequenced and found to be normal. We then added TFR2 and SLC40A1 to the panel. A new TFR2 c.2085G→C variant was identified, although with no resulting amino acid change (p. S695S). It is very unlikely that this nucleotide change could have played a role in the elevated iron parameters in the proband as it was also present in her father who had normal serum iron parameters. We did not identify variants in the coding or flanking intronic sequences of the gene encoding for ferroportin from the proband. With the discovery of the HJV gene we asked for fresh informed consent from the parents for HJV analysis in the family. The proband was heterozygous for a new missense mutation at position c.526 (C→T) in exon 3 of HJV resulting in a substitution R176C. The other allele carried the G→T common transition at position 959 in exon 4 that leads to the G320V substitution in hemojuvelin (Figure 1). Both of the mutations were confirmed by restriction fragment length polymorphism analysis. The c.526 (C→T) and the c.959 (G→T) substitutions abolish HhaI and NlaIV enzyme restriction sites respectively (data not shown). The c.526 (C→T) transition was not detected in a control group of 61 individuals.

Figure 1.HJV sequencing results. A. Proband’s HJV mutation in position nt 526. B. Proband’s HJV mutation in position nt 959.

Table 1.Biochemical data and genotypic results in the family members.

Now aged 8 years old, the girl is still clinically asymptomatic with an elevated transferrin saturation of 100% and a ferritin concentration of 235 ng/L. Her hepatic iron concentration was found to be 260 μmol/g by magnetic resonance imaging (normal <36 μmol/g).

The new R176C substitution is located in the partial von Willebrand type D domain of the protein; this amino acid is highly conserved phylogenetically.3 The mutation occurred two amino acids downstream from a highly conserved GDPH motif, a consensus site for hemojuvelin and mRGMc (mouse repulsive guidance molecule c), which undergoes acid sensitive autocatalytic cleavage.8 By introducing the C176 residue in a molecular modeling program as already described9 we found that this substitution destabilizes a short helix made of the RSF residues (from 176 to 178). The proband was also homozygous for H63D in HFE, but HJV-related JH does not seem to be aggravated when associated with HFE genotypes, including H63D homozygosity.4,10 This confirms the need to search for other causes of disease in HFE-H63D homozygotes with clinical forms of HH.

In HJV-related JH clinical manifestations arise rapidly in teenagers. As it becomes possible to make an early diagnosis of JH before the onset of clinical manifestations, questions are being raised about the follow-up of such young patients in order to prevent them developing iron overload. Because of the increased iron accumulation in JH compared to type 1 hemochromatosis we have taken into account the projected severity of the disease and devised a phlebotomy program for the proband.

Acknowledgments

The authors are indebted to Dr. Jean-Marie Epeirier. We also thank Hélène Igual, Sabrina Julien, Karine Foulon and Alison Merryweather-Clarke for their expertise.

Footnotes

• Funding: this work was supported in part by grants from "la recherche clinique", CHU de Montpellier, AOI 2004 and INSERM grant n° GISMR0403 CHU d’Amiens.