Identification of novel mutations in hemochromatosis genes by targeted next generation sequencing in Italian patients with unexplained iron overload

Diagnosis and management of hereditary hemochromatosis: lifestyle modification, phlebotomy, and blood donation

The term hemochromatosis refers to a group of genetic disorders characterized by hepcidin insufficiency in the context of normal erythropoiesis, iron hyperabsorption, and expansion of the plasma iron pool with increased transferrin saturation, the diagnostic hallmark of the disease. This results in the formation of toxic non-transferrin-bound iron, which ultimately accumulates in multiple organs, including the liver, heart, endocrine glands, and joints. The most common form is HFE-hemochromatosis (HFE-H) due to p.Cys282Tyr (C282Y) homozygosity, present in nearly 1 in 200 people of Northern European descent but characterized by low penetrance, particularly in females. Genetic and lifestyle cofactors (especially alcohol and dysmetabolic features) significantly modulate clinical expression so that HFE-H can be considered a multifactorial disease. Nowadays, HFE-H is mostly diagnosed before organ damage and is easily treated by phlebotomy, with an excellent prognosis. After iron depletion, maintenance phlebotomy can be usefully transformed into a blood donation program. Lifestyle changes are important for management. Non-HFE-H, much rarer but highly penetrant, may lead to early and severe heart, liver, and endocrine complications. Managing severe hemochromatosis requires a comprehensive approach optimally provided by consultation with specialized centers. In clinical practice, a proper diagnostic approach is paramount for patients referred for hyperferritinemia, a frequent finding that reflects hemochromatosis only in a minority of cases.

Hemochromatosis classification: update and recommendations by the BIOIRON Society

Hemochromatosis (HC) is a genetically heterogeneous disorder in which uncontrolled intestinal iron absorption may lead to progressive iron overload (IO) responsible for disabling and life-threatening complications such as arthritis, diabetes, heart failure, hepatic cirrhosis, and hepatocellular carcinoma. The recent advances in the knowledge of pathophysiology and molecular basis of iron metabolism have highlighted that HC is caused by mutations in at least 5 genes, resulting in insufficient hepcidin production or, rarely, resistance to hepcidin action. This has led to an HC classification based on different molecular subtypes, mainly reflecting successive gene discovery. This scheme was difficult to adopt in clinical practice and therefore needs revision. Here we present recommendations for unambiguous HC classification developed by a working group of the International Society for the Study of Iron in Biology and Medicine (BIOIRON Society), including both clinicians and basic scientists during a meeting in Heidelberg, Germany. We propose to deemphasize the use of the molecular subtype criteria in favor of a classification addressing both clinical issues and molecular complexity. Ferroportin disease (former type 4a) has been excluded because of its distinct phenotype. The novel classification aims to be of practical help whenever a detailed molecular characterization of HC is not readily available.

Non-invasive diagnosis and follow-up of hyperferritinaemia

Increased serum ferritin is a very frequent cause of referral for which thorough evaluation is required to avoid unnecessary exploration and inaccurate diagnosis. Clinicians must thus know factors and tools that are relevant in this setting. Several biochemical and radiological tools drastically improved the diagnosis work-up of increased serum ferritin. Because serum ferritin value can be altered by many cofounding factors, scrutiny in the initial clinical evaluation is crucial. Alcohol consumption, and the metabolic syndrome are the most frequent causes of secondary increased ferritin. Serum transferrin saturation level is a pivotal test, and if increased prompt testing for HFE C282Y patients in Caucasian population. In most cases further tests are require to establish whether increased ferritin is associated or not to iron overload. Magnetic resonance imaging is the reference method allowing to accurately establish liver iron content which indirectly reflect body iron load. Second line genetic testing for rare forms of iron overload or increased serum ferritin are available in reference center and should be discussed if diagnosis is equivocal or remain uncertain after careful evaluation. Definite genetic diagnosis is worthwhile as it allows family screening and refining long term management of the patient. Liver biopsy remains seldom useful to assess liver fibrosis, mostly in patients with severe iron overload.

Identification of Novel Mutations by Targeted NGS Panel in Patients with Hyperferritinemia

BACKGROUND

Several inherited diseases cause hyperferritinemia with or without iron overload. Differential diagnosis is complex and requires an extensive work-up. Currently, a clinical-guided approach to genetic tests is performed based on gene-by-gene sequencing. Although reasonable, this approach is expensive and time-consuming and Next Generation Sequencing (NGS) technology may provide cheaper and quicker large-scale DNA sequencing.

METHODS

We analysed 36 patients with non-HFE-related hyperferritinemia. Liver iron concentration was measured in 33 by magnetic resonance. A panel of 25 iron related genes was designed using SureDesign software. Custom libraries were generated and then sequenced using Ion Torrent PGM.

RESULTS

We identified six novel mutations in SLC40A1, three novel and one known mutation in TFR2, one known mutation and a de-novo deletion in HJV, and a novel mutation in HAMP in ten patients. In silico analyses supported the pathogenic role of the mutations.

CONCLUSIONS

Our results support the use of an NGS-based panel in selected patients with hyperferritinemia in a tertiary center for iron metabolism disorders. However, 26 out of 36 patients did not show genetic variants that can individually explain hyperferritinemia and/or iron overload suggesting the existence of other genetic defects or gene-gene and gene-environment interactions needing further studies.

MRI-Based Iron Phenotyping and Patient Selection for Next-Generation Sequencing of Non-Homeostatic Iron Regulator Hemochromatosis Genes

BACKGROUND AND AIMS

High serum ferritin is frequent among patients with chronic liver disease and commonly associated with hepatic iron overload. Genetic causes of high liver iron include homozygosity for the p.Cys282Tyr variant in homeostatic iron regulator (HFE) and rare variants in non-HFE genes. The aims of the present study were to describe the landscape and frequency of mutations in hemochromatosis genes and determine whether patient selection by noninvasive hepatic iron quantification using MRI improves the diagnostic yield of next-generation sequencing (NGS) in patients with hyperferritinemia.

APPROACH AND RESULTS

A cohort of 410 unselected liver clinic patients with high serum ferritin (defined as ≥200 μg/L for women and ≥300 μg/L for men) was investigated by HFE genotyping and abdominal MRI R2*. Forty-one (10%) patients were homozygous for the p.Cys282Tyr variant in HFE. Of the remaining 369 patients, 256 (69%) had high transferrin saturation (TSAT; ≥45%) and 199 (53%) had confirmed hepatic iron overload (liver R2* ≥70 s^-1 ). NGS of hemochromatosis genes was carried out in 180 patients with hepatic iron overload, and likely pathogenic variants were identified in 68 of 180 (38%) patients, mainly in HFE (79%), ceruloplasmin (25%), and transferrin receptor 2 (19%). Low spleen iron (R2* <50 s^-1 ), but not TSAT, was significantly associated with the presence of mutations. In 167 patients (93%), no monogenic cause of hepatic iron overload could be identified.

CONCLUSIONS

In patients without homozygosity for p.Cys282Tyr, coincident pathogenic variants in HFE and non-HFE genes could explain hyperferritinemia with hepatic iron overload in a subset of patients. Unlike HFE hemochromatosis, this type of polygenic hepatic iron overload presents with variable TSAT. High ferritin in blood is an indicator of the iron storage disease, hemochromatosis. A simple genetic test establishes this diagnosis in the majority of patients affected. MRI of the abdomen can guide further genetic testing.

Genetic Diagnosis in Hereditary Hemochromatosis: Discovering and Understanding the Biological Relevance of Variants

BACKGROUND

Hereditary hemochromatosis (HH) is a genetic disease, leading to iron accumulation and possible organ damage. Patients are usually homozygous for p. Cys282Tyr in the homeostatic iron regulator gene but may have mutations in other genes involved in the regulation of iron. Next-generation sequencing is increasingly being utilized for the diagnosis of patients, leading to the discovery of novel genetic variants. The clinical significance of these variants is often unknown.

CONTENT

Determining the pathogenicity of such variants of unknown significance is important for diagnostics and genetic counseling. Predictions can be made using in silico computational tools and population data, but additional evidence is required for a conclusive pathogenicity classification. Genetic disease models, such as in vitro models using cellular overexpression, induced pluripotent stem cells or organoids, and in vivo models using mice or zebrafish all have their own challenges and opportunities when used to model HH and other iron disorders. Recent developments in gene-editing technologies are transforming the field of genetic disease modeling.

SUMMARY

In summary, this review addresses methods and developments regarding the discovery and classification of genetic variants, from in silico tools to in vitro and in vivo models, and presents them in the context of HH. It also explores recent gene-editing developments and how they can be applied to the discussed models of genetic disease.

Identification of Variants Associated With Rare Hematological Disorder Erythrocytosis Using Targeted Next-Generation Sequencing Analysis

An erythrocytosis is present when the red blood cell mass is increased, demonstrated as elevated hemoglobin and hematocrit in the laboratory evaluation. Congenital predispositions for erythrocytosis are rare, with germline variants in several genes involved in oxygen sensing (VHL, EGLN1, and EPAS1), signaling for hematopoietic cell maturation (EPOR and EPO), and oxygen transfer (HBB, HBA1, HBA2, and BPGM) that were already associated with the eight congenital types (ECYT1–8). Screening for variants in known congenital erythrocytosis genes with classical sequencing approach gives a correct diagnosis for only up to one-third of the patients. The genetic background of erythrocytosis is more heterogeneous, and additional genes involved in erythropoiesis and iron metabolism could have a putative effect on the development of erythrocytosis. This study aimed to detect variants in patients with yet unexplained erythrocytosis using the next-generation sequencing (NGS) approach, targeting genes associated with erythrocytosis and increased iron uptake and implementing the diagnostics of congenital erythrocytosis in Slovenia. Selected 25 patients with high hemoglobin, high hematocrit, and no acquired causes were screened for variants in the 39 candidate genes. We identified one pathogenic variant in EPAS1 gene and three novel variants with yet unknown significance in genes EPAS1, JAK2, and SH2B3. Interestingly, a high proportion of patients were heterozygous carriers for two variants in HFE gene, otherwise pathogenic for the condition of iron overload. The association between the HFE variants and the development of erythrocytosis is not clearly understood. With a targeted NGS approach, we determined an actual genetic cause for the erythrocytosis in one patient and contributed to better management of the disease for the patient and his family. The effect of variants of unknown significance on the enhanced production of red blood cells needs to be further explored with functional analysis. This study is of great significance for the improvement of diagnosis of Slovenian patients with unexplained erythrocytosis and future research on the etiology of this rare hematological disorder.

A Novel ALAS2 Missense Mutation in Two Brothers With Iron Overload and Associated Alterations in Serum Hepcidin/Erythroferrone Levels

Iron loading anemias are characterized by ineffective erythropoiesis and iron overload. The prototype is non-transfusion dependent ß-thalassemia (NTDT), with other entities including congenital sideroblastic anemias, congenital dyserythropoietic anemias, some hemolytic anemias, and myelodysplastic syndromes. Differential diagnosis of iron loading anemias may be challenging due to heterogeneous genotype and phenotype. Notwithstanding the recent advances in linking ineffective erythropoiesis to iron overload, many pathophysiologic aspects are still unclear. Moreover, measurement of hepcidin and erythroferrone (ERFE), two key molecules in iron homeostasis and erythropoiesis, is scarcely used in clinical practice and of uncertain utility. Here, we describe a comprehensive diagnostic approach, including next-generation sequencing (NGS), in silico modeling, and measurement of hepcidin and erythroferrone (ERFE), in two brothers eventually diagnosed as X-linked sideroblastic anemia (XLSA). A novel pathogenic ALAS2 missense mutation (c.1382T>A, p.Leu461His) is described. Hyperferritinemia with high hepcidin-25 levels (but decreased hepcidin:ferritin ratio) and mild-to-moderate iron overload were detected in both patients. ERFE levels were markedly elevated in both patients, especially in the proband, who had a more expressed phenotype. Our study illustrates how new technologies, such as NGS, in silico modeling, and measurement of serum hepcidin-25 and ERFE, may help in diagnosing and studying iron loading anemias. Further studies on the hepcidin-25/ERFE axis in additional patients with XLSA and other iron loading anemias may help in establishing its usefulness in differential diagnosis, and it may also aid our understanding of the pathophysiology of these genetically and phenotypically heterogeneous entities.

Novel mutations in the bone morphogenetic protein 6 gene in patients with iron overload and non-homozygous genotype for the HFE p.Cys282Tyr mutation

BACKGROUND

Five main genes are associated with hemochromatosis; however, current studies show that, in addition to these genes, others may be associated with primary iron overload (IO). One of these is the bone morphogenetic protein 6 (BMP6), which encodes a protein that modulates hepcidin synthesis and, consequently, iron homeostasis.

AIM

To identify BMP6 gene pathogenic variants in patients with IO and non-homozygous genotype for the HFE p.Cys282Tyr mutation.

MATERIALS AND METHODS

Fifty-three patients with primary IO and non-homozygous genotype for the HFE p.Cys282Tyr were selected. Subsequent bidirectional DNA sequencing of BMP6 exons was performed.

RESULTS

Two novel variants were found. One at homozygous state p.Gln158Ter (c.472C>T) was pathogenic, the other one at heterozygous state p.Val394Met (c.1180G>A) was of uncertain significance (VUS); the third variant at heterozygous state p.Arg257His (c.770G>A) has already been described and associated with IO. No BMP6 pathogenic variants that would explain iron overload phenotypes were detected in 94% of the studied patients.

CONCLUSION

Identification of the BMP6 pathogenic variants in Brazilian patients with primary IO might contribute to the genetic understanding of this phenotype.

Inherited iron overload disorders

Hereditary iron overload includes several disorders characterized by iron accumulation in tissues, organs, or even single cells or subcellular compartments. They are determined by mutations in genes directly involved in hepcidin regulation, cellular iron uptake, management and export, iron transport and storage. Systemic forms are characterized by increased serum ferritin with or without high transferrin saturation, and with or without functional iron deficient anemia. Hemochromatosis includes five different genetic forms all characterized by high transferrin saturation and serum ferritin, but with different penetrance and expression. Mutations in HFE, HFE2, HAMP and TFR2 lead to inadequate or severely reduced hepcidin synthesis that, in turn, induces increased intestinal iron absorption and macrophage iron release leading to tissue iron overload. The severity of hepcidin down-regulation defines the severity of iron overload and clinical complications. Hemochromatosis type 4 is caused by dominant gain-of-function mutations of ferroportin preventing hepcidin-ferroportin binding and leading to hepcidin resistance. Ferroportin disease is due to loss-of-function mutation of SLC40A1 that impairs the iron export efficiency of ferroportin, causes iron retention in reticuloendothelial cell and hyperferritinemia with normal transferrin saturation. Aceruloplasminemia is caused by defective iron release from storage and lead to mild microcytic anemia, low serum iron, and iron retention in several organs including the brain, causing severe neurological manifestations. Atransferrinemia and DMT1 deficiency are characterized by iron deficient erythropoiesis, severe microcytic anemia with high transferrin saturation and parenchymal iron overload due to secondary hepcidin suppression. Diagnosis of the different forms of hereditary iron overload disorders involves a sequential strategy that combines clinical, imaging, biochemical, and genetic data. Management of iron overload relies on two main therapies: blood removal and iron chelators. Specific therapeutic options are indicated in patients with atransferrinemia, DMT1 deficiency and aceruloplasminemia.

Genetic and Clinical Heterogeneity in Thirteen New Cases with Aceruloplasminemia. Atypical Anemia as a Clue for an Early Diagnosis

Aceruloplasminemia is a rare autosomal recessive genetic disease characterized by mild microcytic anemia, diabetes, retinopathy, liver disease, and progressive neurological symptoms due to iron accumulation in pancreas, retina, liver, and brain. The disease is caused by mutations in the Ceruloplasmin (CP) gene that produce a strong reduction or absence of ceruloplasmin ferroxidase activity, leading to an impairment of iron metabolism. Most patients described so far are from Japan. Prompt diagnosis and therapy are crucial to prevent neurological complications since, once established, they are usually irreversible. Here, we describe the largest series of non-Japanese patients with aceruloplasminemia published so far, including 13 individuals from 11 families carrying 13 mutations in the CP gene (7 missense, 3 frameshifts, and 3 splicing mutations), 10 of which are novel. All missense mutations were studied by computational modeling. Clinical manifestations were heterogeneous, but anemia, often but not necessarily microcytic, was frequently the earliest one. This study confirms the clinical and genetic heterogeneity of aceruloplasminemia, a disease expected to be increasingly diagnosed in the Next-Generation Sequencing (NGS) era. Unexplained anemia with low transferrin saturation and high ferritin levels without inflammation should prompt the suspicion of aceruloplasminemia, which can be easily confirmed by low serum ceruloplasmin levels. Collaborative joint efforts are needed to better understand the pathophysiology of this potentially disabling disease.

Impact of natural neuromedin-B receptor variants on iron metabolism

Iron overload heritability remains partly unexplained. By performing whole exome sequencing in three patients with a clinical phenotype of hemochromatosis not accounted by known genetic risk factors, we identified in all patients rare variants predicted to alter activity of Neuromedin-B receptor (NMBR). Coding NMBR mutations were enriched in 129 patients with hereditary hemochromatosis or iron overload phenotype, as compared to ethnically matched controls, including 100 local healthy blood donors and 1000Genomes project participants (15.5% vs 5%, P = .0038 at burden test), and were associated with higher transferrin saturation in regular blood donors (P = .04). Consistently, in 191 patients with nonalcoholic fatty liver, the most common low-frequency p.L390 M variant was independently associated with higher ferritin (P = .03). In 58 individuals, who underwent oral iron challenge, carriage of the p.L390 M variant was associated with higher transferrin saturation and lower hepcidin release. Furthermore, the circulating concentration of the natural NMBR ligand, Neuromedin-B, was reduced in response to iron challenge. It was also decreased in individuals carrying the p.L390 M variant and with hemochromatosis in parallel with increased transferrin saturation. In mice, Nmbr was induced by chronic dietary iron overload in the liver, gut, pancreas, spleen, and skeletal muscle, while Nmb was downregulated in gut, pancreas and spleen. Finally, Nmb amplified holo-transferrin dependent induction of hepcidin in primary mouse hepatocytes, which was associated with Jak2 induction and abolished by the NMBR antagonist PD168368. In conclusion, NMBR natural variants were enriched in patients with iron overload, and associated with facilitated iron absorption, possibly related to a defect of iron-induced hepcidin release.

Diagnosis and Management of Genetic Iron Overload Disorders

Iron overload disorders lead to excess iron deposition in the body, which can occur as a result of genetic or secondary causes. Genetic iron overload, referred to as hereditary hemochromatosis, may present as a common autosomal recessive mutation or as one of several uncommon mutations. Secondary iron overload may result from frequent blood transfusions, exogenous iron intake, or certain hematological diseases such as dyserythropoietic syndrome or chronic hemolytic anemia. Iron overload may be asymptomatic, or may present with significant diseases of the liver, heart, endocrine glands, joints, or other organs. If treated appropriately prior to end-organ damage, life expectancy has been shown to be similar compared to matched populations. Alongside clinical assessment, diagnostic studies involve blood tests, imaging, and in some cases liver biopsy. The mainstay of therapy is periodic phlebotomy, although oral chelation is an option for selected patients.

Causes of iron overload in blood donors - a clinical study

BACKGROUND AND OBJECTIVES

Despite the obligate iron loss from blood donation, some donors present with hyperferritinaemia that can result from a wide range of acute and chronic conditions including hereditary haemochromatosis (HH). The objective of our study was to investigate the causes of hyperferritinaemia in the blood donor population and explore the value of extensive HH mutational analyses.

MATERIALS AND METHODS

Forty-nine consecutive donors (f = 6, m = 43) were included prospectively from the Capital Regional Blood Center. Inclusion criteria were a single ferritin value >1000 μg/l or repeated hyperferritinaemia with at least one value >500 μg/l. All donors were questioned about their medical history and underwent a physical examination, biochemical investigations and next-generation sequencing of HH-related genes, including the HFE gene, the haemojuvelin gene (HFE2/HJV), the hepcidin gene (HAMP), the ferroportin 1 gene (SLC40A1) and the transferrin receptor 2 gene (TFR2).

RESULTS

Forty of 49 donors were mutation positive with a combined 69 mutations, 54 of which were located in the HFE gene. There were 11 mutations in the TFR2 gene, two mutations in the HFE2 gene and two mutations in the HAMP gene. Only four donors had apparent alternative causes of hyperferritinaemia.

CONCLUSION

HH-related mutations were the most frequent cause of hyperferritinaemia in a Danish blood donor population, and it appears that several different HH-genotypes can contribute to hyperferritinaemia. HH screening in blood donors with high ferritin levels could be warranted. HH-related iron overload should not in itself result in donor ineligibility.

Identification of new BMP6 pro-peptide mutations in patients with iron overload

Hereditary Hemochromatosis (HH) is a genetically heterogeneous disorder caused by mutations in at least five different genes (HFE, HJV, TFR2, SLC40A1, HAMP) involved in the production or activity of the liver hormone hepcidin, a key regulator of systemic iron homeostasis. Nevertheless, patients with an HH-like phenotype that remains completely/partially unexplained despite extensive sequencing of known genes are not infrequently seen at referral centers, suggesting a role of still unknown genetic factors. A compelling candidate is Bone Morphogenetic Protein 6 (BMP6), which acts as a major activator of the BMP-SMAD signaling pathway, ultimately leading to the upregulation of hepcidin gene transcription. A recent seminal study by French authors has described three heterozygous missense mutations in BMP6 associated with mild to moderate late-onset iron overload (IO). Using an updated next-generation sequencing (NGS)-based genetic test in IO patients negative for the classical HFE p.Cys282Tyr mutation, we found three BMP6 heterozygous missense mutations in four patients from three different families. One mutation (p.Leu96Pro) has already been described and proven to be functional. The other two (p.Glu112Gln, p.Arg257His) were novel, and both were located in the pro-peptide domain known to be crucial for appropriate BMP6 processing and secretion. In silico modeling also showed results consistent with their pathogenetic role. The patients' clinical phenotypes were similar to that of other patients with BMP6-related IO recently described. Our results independently add further evidence to the role of BMP6 mutations as likely contributing factors to late-onset moderate IO unrelated to mutations in the established five HH genes.