HFE related-HH (OMIM 235200), classified as type 1, is the most frequent form of the disease and the most common autosomal recessive disorder in Northern European populations. HH is characterized by enhanced intestinal absorption of iron leading to multiple organ damage, such as cirrhosis, hepatoma, diabetes mellitus, arthritis, cardiomyopathy, and hypogonadism [20–23].

HFE gene (613609), constituted by 6 exons, encodes a membrane protein that is similar to major histocompatibility class I-like proteins, called HFE.

Most HH patients carry homozygosity for p.Cys282Tyr or p.Cys282Tyr/p.His63Asp compound heterozygous genotypes. Besides the missense mutation at position 282, where cysteine is replaced by tyrosine (p.Cys282Tyr, c.845G>A, rs1800562) and the common substitution of histidine for aspartic acid at position 63 (p.His63Asp, c.187C>G, rs1799945), a third mutation is also commonly assessed: the substitution of cysteine for serine at amino acid position 65 (p.Ser65Cys, c.193A>T, rs1800730). However, recent reports have suggested that rare HFE variants, such as p.Gly43Ala, p.Leu46Trp, p.Val53Met, p.Gly93Arg, p.Ile105Thr, p.Gln127His, p.Asp129Asn, p.Glu168Gln, p.Glu168del, p.Leu183Pro, p.Glu277Lys, p.Gln283Pro, p.Val284Met, p.Arg330Met, and a deletion in the 6p chromosome region containing HFE could also be linked to HH thus contributing to genetic and phenotypic heterogeneity of the disease [6,7,9,15,24,25].

The first proposed pathogenic mechanism for explaining HH was the disruption of a disulfide bond in HFE that is critical for its binding to β2 microglobulin. This complex interacts with transferrin receptor 1, decreasing the affinity with transferrin and consequently modulating iron absorption in enterocytes [26]. However, in recent years, evidences indicating HFE protein as a hepcidin modulator have emerged. The functional loss of HFE in mice and humans has been shown to reduce hepcidin synthesis [27–29] and that HFE loss seems to be associated with blunted signaling responses to BMP6 (bone morphogenetic protein 6), a key regulator of hepcidin, in vitro and in vivo [30,31]. Indeed, HFE related-HH has been associated with reduced hepcidin levels (Figure 1) [24,28,29].

Juvenile hemochromatosis (JH), also classified as type 2, is a rare autosomal recessive disorder of iron overload that leads to organ damage before the age of 30, and usually causes cardiomyopathy, hypogonadotrophic hypogonadism, liver damages and endocrine dysfunctions. Types 2A (OMIM 602390) and 2B (OMIM 613313) are caused by mutations in HJV and HAMP genes, respectively [32,33].

HJV (608374) gene, constituted by 4 exons, was identified in 2004 and encodes a protein called hemojuvelin [32]. Patients with type 2A JH and knockout mice models demonstrate low hepcidin levels implying that hemojuvelin is involved in the hepcidin synthesis [34]. HAMP gene (606464), constituted by 3 exons, encodes hepcidin, a peptide known as iron hormone. Hepcidin is produced by hepatocytes and it plays a role in iron absorption related to ferroportin degradation of the enterocytes [33,35].

Several HJV mutations have been found in patients: p.Arg54del, p.Cys80Arg, p.Ser85Pro, p.Gly99Arg, p.Gly99Val, p.Leu101Pro, p.Gly116del, p.Cys119Phe, p.Ile222Asn, p.Arg131fs, p.Asp149fs, p.Leu165del, p.Ala168Asp, p.Phe170Ser, p.Asp172Glu, p.Arg176Cys, p.Trp191Cys, p.Asn196Lys, p.Ser205Arg, p.Ile222Asn, p.Lys234del, p.Asp249His, p.Gly250Val, p.Asn269fs, p.Ile281Thr, p.Arg288Trp, p.Cys321Trp, p.Cys321del, p.Arg326del, p.Ser328fs, p.Cys361fs, and p.Arg385del. However, the HJV p.Gly320Val is the most frequent mutation and has been reported in JH patients in several populations around the world [22,32,36–39]. In contrast, mutations in HAMP are a very rare cause of JH: p.Met31fs, p.Met50fs, p.Arg56del, p.Arg59Gly, p.Cys70Arg, p.Gly71Asp, and p.Cys78Thr [22,33,40,41]. In addition, some studies support the concept that digenic inheritance of HFE and HJV or of HFE and HAMP mutations can lead to iron overload or may aggravate the phenotype [37,39,41–44].

For both type 2A and 2B JH, it is well established that the cause of iron overload may be explained by decreases in the synthesis and, consequently depressed hepcidin levels (Figure 1) [34,45]. Cell-surface expression of hemojuvelin was associated with increased expression of hepcidin; likewise, loss of hemojuvelin expression, as in juvenile hemochromatosis, was associated with reduced hepcidin expression [22,46].

HJV seems to play a role in iron absorption and release from cells and has anti-inflammatory properties [47]. An important study revealed that HJV acts as a BMP co-receptor and signals via the SMAD pathway to regulate hepcidin expression [46,48]. A BMP6 dependent signaling pathway has been shown to play a key role in regulation of hepcidin expression [24]. BMPs bind to type I and type II serine threonine kinase receptors, which phosphorylate specific intracellular SMAD proteins (SMAD1,5,8). Phosphorylated SMAD1,5,8 (P-SMAD1,5,8) binds to the common mediator SMAD4, and the SMAD complex translocates to the nucleus to affect transcription of target genes HAMP (encoding hepcidin) is transcriptionally up-regulated by BMPs [46,49–52]. Impaired hepatic signaling through mutations in genes encoding either the ligand BMP6, the BMP coreceptor hemojuvelin or Smad4 leads to low hepcidin levels and iron overload in mice. Collectively, these data show that BMP-SMAD signaling is an important regulatory pathway for hepcidin expression and thus iron metabolism [53–56].

Type 3 HH (OMIM 604250) is an autosomal recessive disease caused by mutations in TFR2 gene and iron overload is similar to HFE related-HH phenotype. TFR2 gene (604720), constituted by 18 exons, encodes transferrin receptor 2 protein (TFR2). TFR2 is involved with uptake of transferrin bound iron by hepatocytes and it is also involved in the hepcidin synthesis [57–60]. One possibility is that it operates in the pathway discussed above for HFE (or in a parallel pathway of its own) facilitating the BMP/SMAD signaling that activates hepcidin expression. Another possibility is that TFR2, which is also able to interact with HFE, forms an iron-sensing complex that modulates hepcidin expression in response to blood levels of diferric transferrin [24,61–63].

This disorder seems to be rare and few TFR2 mutations have been reported: p.His33Asn, p.Glu60del, p.Arg105del, p.Met172Lys, p.Tyr250del, p.Gln317del, p.Arg396del, p.Ala444Thr, p.Arg455Gln, p.Arg481His, p.Leu490Arg, p.Val561del, p.Gln690Pro, and p.Gly792Arg. In both animal models and patients with TFR2 related-HH decreased hepcidin levels were observed (Figure 1) [22,42,57,64–67].

Type 4 HH (OMIM 606069) has an autosomal dominant pattern and it is caused by mutations in the SLC40A1 gene. This rare disease can present peculiar clinical features such as high serum ferritin levels plus low or normal transferrin saturation values until the end stage of the disease. It may also be the presence of a mild iron-deficient anemia in the initial stage and a reduced tolerance to therapeutic phlebotomy [45,68,69].

SLC40A1 (604353) gene, constituted by 8 exons, encodes a membrane transporter called ferroportin that modulates iron efflux [70]. SLC40A1 mutations, such as p.His32Arg, p.Tyr64Asn, p.Val72Asp, p.Ala77Asp, p.Gly80Val, p.Arg88Thr, p.Asn144His, p.Asp157Gly, p.Asp157Asn, p.Val162del, p.Asn174Ile, p.Arg178Gly, p.Ile180Thr, p.Asp181Val, p.Gln182His, p.Asn185Asp, p.Gln248His, p.Gly267Asp, p.Gly323Val, p.Cys326Ser, p.Cys326Tyr, p.Gly330del, p.Ser338Arg, p.Arg489Ser, p.Gly490Asp, and p.Gly490Val were associated with type 4 HH. Two hypotheses have been proposed to account for this disease: the trapping of iron in macrophages that are unable to export iron and the failure to be degraded by interaction with hepcidin [22,42,69–74].

HFE testing for the two main mutations (p.Cys282Tyr and p.His63Asp) should be performed in all patients with unexplained increased TS and/or serum ferritin values (Figure 2). In these cases, the molecular diagnostic of HFE related-HH is usually associated with the presence of the p.Cys282Tyr homozygosity and p.Cys282Tyr/p.His63Asp compound heterozygous genotypes. However, p.His63Asp homozygous and p.His63Asp/p.Ser65Cys compound heterozygous genotypes have been associated with HH phenotype [3,15,23].

In the absence of the mentioned HFE genotype combinations, other HH types could be considered. When there is genetic iron overload in a patient with less than 30 years and cardiac or endocrine manifestations, JH diagnostic is suggestive (Figure 2). Thus, the evaluation of the p.Gly320Val mutation in the HJV gene must be the molecular test of choice. According to several studies, this procedure would confirm the majority of JH cases [5,29]. Early diagnosis is paramount. If result is negative, sequencing should be performed to evaluate the HJV and HAMP genes (Figure 2). Our group reported a case with both clinical and molecular diagnostic of JH, and the use of deferasirox therapy adjunct to venesections during the initial treatment presented significant improvements as cardiomyopathy and liver disease were prevented, and endocrine functions were normalized [5].

Mutations in the TFR2 and SLC40A1 genes are rare compared with HFE mutations and they have also been reported in children, adolescents, and adults. These genes should be sequenced after negative results for other genes (Figure 2). Nowadays, the costs for sequencing have come down, especially if it evaluates the number of bases per dollar of the next generation sequencings. However, for the most part of clinical practice around the world, screening of HFE, HJV, HAMP, TFR2 and SLC40A1 through direct sequencing is not widely available. This approach is usually reserved for scientific studies and for very specific cases such as patients who are not responsive to treatment and had more severe complications due to iron overload. In addition and for the most part of cases, the treatment is not dependent on molecular diagnosis. Our group reported that direct sequencing does not significantly improve the diagnostic throughput as when it was compared to the sole HFE main mutations testing [15,19,79,80].

Genotyping of HFE p.Cys282Tyr and p.His63Asp mutations is one of the most requested molecular assays in the laboratorial routine. The analysis of single nucleotide polymorphisms, for example HFE p.Cys282Tyr, HFE p.His63Asp, and HJV p.Gly320Val, can be performed by several available methods for genotyping (Figure 2), such as restriction fragment length polymorphisms (RFLP), allele-specific amplification analysis real time-polymerase, denaturing HPLC, sequencing strategies, TaqMan assay, multiplex amplification followed by reverse hybridization [79,82–84]. A high-resolutionmelting (HRM) assay was developed by our group for genotyping HFE p.Cys282Tyr and p.His63Asp mutations in a unique procedure being capable of ensuring the result in approximately 112 minutes and, with cost-effectiveness especially in a large-scale demand, compared to methods cited above. The advantages of genotyping with this procedure were the non-dependence on gel electrophoresis and on mutagenic reagents for visualization of fragments, and the reduction of the chances for contamination due to sample preparation compared to RFLP and sequencing strategies. There are disadvantages for the HRM method: interference from another genetic variant that may be present in the amplicon leading to misdiagnosis by altering the curve pattern of the target-mutation. The amplicon for the HFE p.Cys282Tyr mutation may present the following known genetic variants: p.Thr281Thr, p.Gln283Pro, p.Val284Met; and the amplicon for the HFE p.His63Asp mutation may present the p.Ser65Cys known genetic variants. Moreover, non-described mutations may also be present [83,85,86].