Novel mutation in the TMPRSS6 gene with iron-refractory iron deficiency anemia

The role of TMPRSS6 gene polymorphism in iron resistance iron deficiency anaemia (IRIDA): a systematic review

Iron resistance iron deficiency anaemia is a rare autosomal recessive disorder characterized by hypochromic microcytic anaemia, low transferrin saturation and inappropriately high hepcidin levels. The aetiology of this condition is rooted in genetic variations within the transmembrane serine protease 6 (TMPRSS6) genes, responsible for encoding matriptase-2, a pivotal negative regulator of hepcidin. We conducted a systematic search across four electronic databases, yielding 538 articles in total out of which 25 were finally included and were preceded further, aiming to prognosticate prevalent single nucleotide polymorphisms (SNPs) and detrimental genetic alterations. This review aims to elucidate the effects of various SNPs and pathogenic mutations on both haematological and biochemical parameters, as well as their potential interethnic correlation. Employing bioinformatics tools, we subjected over 100 SNPs to scrutiny, discerning their potential functional ramifications. We found rs1373272804, rs1430692214 and rs855791 variants to be most frequent and were having a significant impact on haematological and biochemical profile. We found that individuals of European ancestry were more prone to have these variants compared to other ethnic groups. In conclusion, this review not only sheds light on the association of TMPRSS6 polymorphism in iron resistance iron deficiency anaemia (IRIDA), but also highlights the critical need for further investigations involving larger sample size and more diverse ethnic groups around the globe. These future studies will be vital for gaining a stronger and more reliable understanding of how these genetic differences are linked to the development of IRIDA.

Response to Prolonged Duration of Therapeutic Dose Oral Iron Therapy in a Girl With Novel TMPRSS6 Gene Variants: A Case Report and Review Literature

Iron-refractory iron deficiency anemia (IRIDA) is an autosomal recessive disorder caused by mutations in the TMPRSS6 gene, which impair iron homeostasis. We reported a 4-year-old girl who presented with a 1-year history of iron deficiency anemia. Her hemoglobin level increased from 6.5 g/dL to 12.6 g/dL with a prolonged duration of therapeutic dose oral iron therapy (5 mg/kg/d), and the level remained quite stable during the therapy. Genetic analysis of the TMPRSS6 gene revealed compound heterozygotes of 2 novel pathogenic variants: c.811C> T (NM_153609.3) in exon 7 (NP_705837: p.R271Ter) and c.1254C> G in exon 11 (p.Y418Ter). The results highlight the significance of genetic investigation and long-term iron therapy in iron-refractory iron deficiency anemia patients.

The ectodomain of matriptase-2 plays an important nonproteolytic role in suppressing hepcidin expression in mice

Matriptase-2 (MT2), encoded by TMPRSS6, is a membrane-anchored serine protease that plays a key role in suppressing hepatic hepcidin expression. MT2 is synthesized as a zymogen and undergoes autocleavage for activation. Previous studies suggest that MT2 suppresses hepcidin by cleaving hemojuvelin and other components of the bone morphogenetic protein-signaling pathway. However, the underlying mechanism is still debatable. Here we dissected the contributions of the nonproteolytic and proteolytic activities of Mt2 by taking advantage of Mt2 mutants and Tmprss6-/- mice. Studies of the protease-dead full-length Mt2 (Mt2S762A) and the truncated Mt2 that lacks the catalytic domain (Mt2mask) indicate that the catalytic domain, but not its proteolytic activity, was required for Mt2 to suppress hepcidin expression. This process was likely accomplished by the binding of Mt2 ectodomain to Hjv and Hfe. We found that Mt2 specifically cleaved the key components of the hepcidin-induction pathway, including Hjv, Alk3, ActRIIA, and Hfe, when overexpressed in hepatoma cells. Nevertheless, studies of a murine iron-refractory iron-deficiency anemia-causing mutant (Mt2I286F) in the complement protein subcomponents C1r/C1s, urchin embryonic growth factor, and bone morphogenetic protein 1 domain indicate that Mt2I286F can be activated, but it exhibited a largely compromised ability to suppress hepcidin expression. Coimmunoprecipitation analysis revealed that Mt2I286F, but not Mt2S762A, had reduced interactions with Hjv, ActRIIA, and Hfe. In addition, increased expression of a serine protease inhibitor, the hepatocyte growth factor activator inhibitor-2, in the liver failed to alter hepcidin. Together, these observations support the idea that the substrate interaction with Mt2 plays a determinant role and suggest that the proteolytic activity is not an appropriate target to modulate the function of MT2 for clinical applications.

Genetic analysis of TMPRSS6 gene in Saudi female patients with iron deficiency anemia

OBJECTIVE/BACKGROUND

Mutations in transmembrane protease serine 6 (TMPRSS6) gene induce high hepcidin level, which causes iron-refractory iron deficiency anemia (IRIDA) by preventing duodenal iron absorption. This study aims to identify the common genetic variations of the TMPRSS6 gene that affect iron levels among Saudi female patients with iron deficiency anemia (IDA).

METHODS

All study participants were Saudi females (12-49 years old): 32 patients with IDA, 32 patients with IRIDA, and 34 healthy individuals comprising the control group. Hematological investigations, iron profile, serum hepcidin level, and TMPRSS6 gene transcription were determined. The TMPRSS6 gene was amplified, sequenced, and analyzed among all study participants.

RESULTS

The mean hepcidin and TMPRSS6 RNA transcription levels in IDA and IRIDA groups were significantly lower than those in the control group. TMPRSS6 gene sequence analysis detected 41 variants: two in the 5' untranslated region (5'UTR), 17 in introns, and 22 in exons. Thirty-three variants were previously reported in the Single Nucleotide Polymorphism Database, and eight variants were novel; one novel variant was in 5'UTR (g.-2 T > G); five novel variants were detected in exons (p.W73X, p.D479N, p.E523K, p.L674L, and p.I799I). At the time of the sequence analysis of our samples, two variants-p.D479N and p.674L-were novel. However, these variants are present at a very low allele frequency in other populations (L674L, 0.00007761 and D479N, 0.000003980).

CONCLUSION

This is the first study to investigate the genetic variants of TMPRSS6 gene in Saudi female patients with IDA. The generated data will serve as a reference for future studies on IDA in the Arab population.

Natural selection on TMPRSS6 associated with the blunted erythropoiesis and improved blood viscosity in Tibetan pigs

Tibetan pigs, indigenous to Tibetan plateau, are well adapted to hypoxia. So far, there have been not any definitively described genes and functional sites responsible for hypoxia adaptation for the Tibetan pig. The whole genome-wide association studies in human suggested that genetic variations in TMPRSS6 was associated with hemoglobin concentration (HGB) and red cell counts (RBC). Here we conducted resequencing of the nearly entire genomic region (40.1 kb) of the candidate gene TMPRSS6 in 40 domestic pigs and 40 wild boars along continuous altitudes and identified 708 SNPs, in addition to an indel (CGTG/----) in the intron 10. We conduct the CGTG indel in 838 domestic pigs, both the CGTG deletion frequency and the pairwise r² linkage disequilibrium showed an increase with elevated altitudes, suggesting that TMPRSS6 has been under Darwinian positive selection. As the conserved core sequence of hypoxia-response elements (HREs), the deletion of CGTG in Tibetan pigs decreased the expression levels of TMPRSS6 mRNA and protein in the liver revealed by real-time quantitative PCR and western blot, respectively. We compared domestic pigs and Tibetan pigs living continuous altitudes, found that the blood-related traits with the increase of altitude, however, the HGB did not increase with the elevation in Tibetan pigs. Genotype association analysis results dissected a genetic effect on reducing HGB by 13.25 g/L in Gongbo'gyamda Tibetan pigs, decreasing mean corpuscular volume (MCV) by 4.79 fl in Diqing Tibetan pigs. In conclusion, the CGTG deletion of TMPRSS6 resulted in lower HGB and smaller MCV, which could reflect a blunting erythropoiesis and improving blood viscosity as well as erythrocyte deformability. It remains to be determined whether a blunting of erythropoiesis for TMPRSS6 or others genetic effects are the physiological adaptations among Tibetan pigs.

The catalytic, stem, and transmembrane portions of matriptase-2 are required for suppressing the expression of the iron-regulatory hormone hepcidin

Matriptase-2 (MT2) is a type-II transmembrane, trypsin-like serine protease that is predominantly expressed in the liver. It is a key suppressor for the expression of hepatic hepcidin, an iron-regulatory hormone that is induced via the bone morphogenetic protein signaling pathway. A current model predicts that MT2 suppresses hepcidin expression by cleaving multiple components of the induction pathway. MT2 is synthesized as a zymogen that undergoes autocleavage for activation and shedding. However, the biologically active form of MT2 and, importantly, the contributions of different MT2 domains to its function are largely unknown. Here we examined the activities of truncated MT2 that were generated by site-directed mutagenesis or Gibson assembly master mix, and found that the stem region of MT2 determines the specificity and efficacy for substrate cleavage. The transmembrane domain allowed MT2 activation after reaching the plasma membrane, and the cytoplasmic domain facilitated these processes. Further in vivo rescue studies indicated that the entire extracellular and transmembrane domains of MT2 are required to correct the low-hemoglobin, low-serum iron, and high-hepcidin status in MT2 ^-/- mice. Unlike in cell lines, no autocleavage of MT2 was detected in vivo in the liver, implying that MT2 may also function independently of its proteolytic activity. In conjunction with our previous studies implicating the cytoplasmic domain as an intracellular iron sensor, these observations reveal the importance of each MT2 domain for MT2-mediated substrate cleavage and for its biological function.

Hepcidin is an endogenous protective factor for osteoporosis by reducing iron levels

Postmenopausal osteoporosis is a global health issue. Although a lack of estrogen is considered the major reason for postmenopausal osteoporosis, other factors might also contribute the etiology of the disease. In previous reports, we and others proposed that iron accumulation after menopause accelerates osteoporosis, and here, we genetically modified the expression of an endogenous hormone, hepcidin, to modulate iron status in a mouse model. Our results show that hepcidin levels negatively correlate with bone loss in both knockout and overexpression (with ovariectomy) murine models. In addition, iron overload enhances reactive oxygen species (ROS) activity and attenuates the functions of primary osteoblasts, while iron depletion could reverse this phenomenon through inhibiting the functions of primary osteoclasts. Therefore, our results provide more evidence of the 'iron accumulation' hypothesis, which suggests that high iron levels are risk factors for osteoporosis, and the 'Huang's hypothesis' that hepcidin is a potential drug target for the prevention of postmenopausal osteoporosis.

Hepcidin: Homeostasis and Diseases Related to Iron Metabolism

Iron is an essential metal for cell survival that is regulated by the peptide hormone hepcidin. However, its influence on certain diseases is directly related to iron metabolism or secondary to underlying diseases. Genetic alterations influence the serum hepcidin concentration, which can lead to an iron overload in tissues, as observed in haemochromatosis, in which serum hepcidin or defective hepcidin synthesis is observed. Another genetic imbalance of iron is iron-refractory anaemia, in which serum concentrations of hepcidin are increased, precluding the flow and efflux of extra- and intracellular iron. During the pathogenesis of certain diseases, the resulting oxidative stress, as well as the increase in inflammatory cytokines, influences the transcription of the HAMP gene to generate a secondary anaemia due to the increase in the serum concentration of hepcidin. To date, there is no available drug to inhibit or enhance hepcidin transcription, mostly due to the cytotoxicity described in the in vitro models. The proposed therapeutic targets are still in the early stages of clinical trials. Some candidates are promising, such as heparin derivatives and minihepcidins. This review describes the main pathways of systemic and genetic regulation of hepcidin, as well as its influence on the disorders related to iron metabolism.

Iron Refractory Iron Deficiency Anemia in Dizygotic Twins Due to a Novel TMPRSS6 Gene Mutation in Addition to Polymorphisms Associated With High Susceptibility to Develop Ferropenic Anemia

Iron refractory iron deficiency anemia (IRIDA) is an autosomal recessive ferropenic anemia. Its hypochromic microcytic pattern is associated with low transferrin saturation, normal-high ferritin, and inappropriately high hepcidin level. This entity is caused by mutants of the TMPRSS6 gene that encodes the protein matriptase II, which influences hepcidin expression, an iron metabolism counterregulatory protein. We report two 29-year-old dizygotic female twins with ferropenic, hypochromic microcytic anemia with 20 years of evolution, refractory to oral iron therapy. After exclusion of gastrointestinal etiologies, IRIDA diagnosis was suspected and a novel mutation in the TMPRSS6 gene was identified. It was found in intron 11 (c.1396+4 A>T) and seems to affect the gene expression. In addition, 3 polymorphisms already associated with a higher risk of developing iron deficiency anemia were also found (D521D, V736A, and Y739Y). Our case reports an undescribed mutation causing IRIDA and supports the hypothesis that this clinical syndrome may be more common than previously thought and its genetics more heterogeneous than initially described.

Hepcidin: A Promising Therapeutic Target for Iron Disorders: A Systematic Review

Iron is required for most forms of organisms, and it is the most essential element for the functions of many iron-containing proteins involved in oxygen transport, cellular respiration, DNA replication, and so on. Disorders of iron metabolism are associated with diverse diseases, including anemias (e.g., iron-deficiency anemia and anemia of chronic diseases) and iron overload diseases, such as hereditary hemochromatosis and β-thalassemia. Hepcidin (encoded by Hamp gene) is a peptide hormone synthesized by hepatocytes, and it plays an important role in regulating the systematic iron homeostasis. As the systemic iron regulator, hepcidin, not only controls dietary iron absorption and iron egress out of iron storage cells, but also induces iron redistribution in various organs. Deregulated hepcidin is often seen in a variety of iron-related diseases including anemias and iron overload disorders. In the case of iron overload disorders (e.g., hereditary hemochromatosis and β-thalassemia), hepatic hepcidin concentration is significantly reduced.Since hepcidin deregulation is responsible for iron disorder-associated diseases, the purpose of this review is to summarize the recent findings on therapeutics targeting hepcidin.Continuous efforts have been made to search for hepcidin mimics and chemical compounds that could be used to increase hepcidin level. Here, a literature search was conducted in PubMed, and research papers relevant to hepcidin regulation or hepcidin-centered therapeutic work were reviewed. On the basis of literature search, we recapitulated recent findings on therapeutic studies targeting hepcidin, including agonists and antagonists to modulate hepcidin expression or its downstream signaling. We also discussed the molecular mechanisms by which hepcidin level and iron metabolism are modulated.Elevating hepcidin concentration is an optimal strategy to ameliorate iron overload diseases, and also to relieve β-thalassemia phenotypes by improving ineffective erythropoiesis. Relative to the current conventional therapies, such as phlebotomy and blood transfusion, therapeutics targeting hepcidin would open a new avenue for treatment of iron-related diseases.