Radical S-Adenosylmethionine Enzymes in Human Health and Disease.

Radical S-adenosylmethionine (SAM) enzymes catalyze an astonishing array of complex and chemically challenging reactions across all domains of life. Of approximately 114,000 of these enzymes, 8 are known to be present in humans: MOCS1, molybdenum cofactor biosynthesis; LIAS, lipoic acid biosynthesis; CDK5RAP1, 2-methylthio-N(6)-isopentenyladenosine biosynthesis; CDKAL1, methylthio-N(6)-threonylcarbamoyladenosine biosynthesis; TYW1, wybutosine biosynthesis; ELP3, 5-methoxycarbonylmethyl uridine; and RSAD1 and viperin, both of unknown function. Aberrations in the genes encoding these proteins result in a variety of diseases. In this review, we summarize the biochemical characterization of these 8 radical S-adenosylmethionine enzymes and, in the context of human health, describe the deleterious effects that result from such genetic mutations.

[Preimplantation genetic testing for monogenic/single gene disorders in a family with Molybdenum co-factor deficiency].

OBJECTIVE: To carry out preimplantation genetic testing for monogenic/single gene disorders (PGT-M) for a Chinese family affected with Molybdenum co-factor deficiency due to pathogenic variant of MOCS2 gene. METHODS: A family with molybdenum co-factor deficiency who attended to the Maternal and Child Health Care Hospital of Guangxi Zhuang Autonomous Region in April 2020 was selected as the research subject. Trophoblast cells were biopsied from blastocysts fertilized by intracytoplasmic sperm injection. Embryos carrying the MOCS2 gene variant and chromosome copy number variation (CNV) of more than 4 Mb were detected by single-cell whole genome amplification, high-throughput sequencing and single nucleotide polymorphism typing. Embryos without or carrying the heterozygous variant and without abnormal chromosome CNV were transplanted. During mid-pregnancy, amniotic fluid sample was collected for prenatal diagnosis to verify the results of PGT-M. RESULTS: Eleven oocytes were obtained, among which three blastocysts were formed through culturing. Results of genetic testing suggested that one embryo was heterozygous for the maternally derived MOCS2 gene variant and without chromosomal CNV. Following embryo transfer, intrauterine singleton pregnancy was attained. Prenatal diagnosis by amniocentesis at 18 weeks of gestation revealed that the MOCS2 gene variant and chromosomal analysis results were both consistent with that of PGT-M, and a healthy male infant was born at 37+5 weeks of gestation. CONCLUSION: PGT-M has helped the couple carrying the MOCS2 gene variant to have a healthy offspring, and may become an important method for couples carrying other pathogenic genetic variants.

Genotype-Phenotype Dissociation in Two Taiwanese Children with Molybdenum Cofactor Deficiency Caused by MOCS2 Mutation.

BACKGROUND: To describe the genotype-phenotype dissociation in two Taiwanese patients with molybdenum cofactor deficiency (MoCoD) caused by MOCS2 gene mutations. PATIENT DESCRIPTION: Patient 1 exhibited early-onset neurological symptoms soon after birth, followed by subsequent myoclonic seizures and movement disorder. The brain magnetic resonance imaging (MRI) showed diffuse brain injury with cystic encephalomalacia along with bilateral globus pallidi involvement, hypoplasia of corpus callosum, and cerebellar atrophy. Patient 2 had a mild phenotype with prominent movement disorder after intercurrent illness, and the brain MRI showed selective injury of the bilateral globus pallidi and the cerebellum. Both patients had markedly low levels of plasma uric acid and harbored the same MOCS2 homozygous c.16C > T mutation. Patient 1 showed chronic regression of developmental milestones and died of respiratory failure at the age of 8 years, whereas patient 2 demonstrated improvement in motor function. CONCLUSION: Genotype-phenotype dissociation could be noted in patients with MoCoD due to MOCS2 mutation. Patients with neonatal seizures, developmental delay, movement disorder, and motor regression after an illness, as well as focal or bilateral involvement of the globus pallidi on the neuroimages, should undergo biochemical testing of plasma uric acid. A pronounced plasma uric acid level is a good indicator of MoCoD. Early diagnosis can allow early provision of adequate genetic counseling.

Molybdenum cofactor deficiency type B knock-in mouse models carrying patient-identical mutations and their rescue by singular AAV injections.

Molybdenum cofactor deficiency is an autosomal, recessively inherited metabolic disorder, which, in the absence of an effective therapy, leads to early childhood death due to neurological deterioration. In type A of the disease, cyclic pyranopterin monophosphate (cPMP) is missing, the first intermediate in the biosynthesis of the cofactor, and a biochemical substitution therapy using cPMP has been developed. A comparable approach for type B of the disease with a defect in the second step of the synthesis, formation of molybdopterin, so far has been hampered by the extreme instability of the corresponding metabolites. To explore avenues for a successful and safe gene therapy, knock-in mouse models were created carrying the mutations c.88C>T (p.Q30X) and c.726_727delAA, which are also found in human patients. Recombinant adeno-associated viruses (rAAVs) were constructed and used for postnatal intrahepatic injections of MoCo-deficient mice in a proof-of-concept approach. Singular administration of an appropriate virus dose in 60 animals prevented the otherwise devastating phenotype to a variable extent. While untreated mice did not survive for more than 2 weeks, some of the treated mice grew up to adulthood in both sexes.

The Persistent Generalized Muscle Contraction in Siblings with Molybdenum Cofactor Deficiency Type A.

Molybdenum cofactor deficiency (MoCD) is a rare autosomal recessive metabolic disease with severe neurological symptoms. Most disease-causing mutations are found in the MOCS1 gene, corresponding to MoCD type A (MoCD-A). There have been few reports describing the long-term detailed neurological features with MoCD-A because most patients do not survive childhood. We describe the clinical, radiologic, biochemical, and genetic data of two patients (female siblings aged 26 and 22 years) with MoCD-A. Both patients presented with feeding difficulties, neurological deterioration, and persistent generalized muscle contraction which can be easily confused with status dystonicus. Biochemical tests revealed low serum uric acid, elevated urinary sulfocysteine, and xanthine. Brain magnetic resonance imaging (MRI) revealed distinctive abnormalities in the bilateral caudate nucleus, putamen, globus pallidus, and cerebral white matter adjacent to the cortex. The thalamus was relatively unaffected. Genetic testing identified a novel homozygous variant in the MOCS1 gene (c.949C > T p.Arg317Cys). Biochemical results supported the hypothesis that this genetic variant is a pathological mutation. When there are symptoms of persistent generalized muscle contraction and characteristic MRI findings, MoCD should be considered as a differential diagnosis.

Whole genome microarray expression analysis in blood identifies pathways linked to signs and symptoms of a patient with hypercalprotectinaemia and hyperzincaemia.

A child, 2 years with the 'hypercalprotectinaemia with hyperzincaemia' clinical syndrome, presented with atypical symptoms and signs, notably persistent fever of approximately 38°C, thrombocythaemia of > 700 × 109 /l and a predominance of persistent intestinal symptoms. In an effort to find a cure by identifying the dysregulated pathways we analysed whole-genome mRNA expression by the Affymetrix HG U133 Plus 2·0 array in blood on three occasions 3-5 months apart. Major up-regulation was demonstrated for the Janus kinase/signal transducer and activators of transcription (JAK/STAT) pathway including, in particular, CD177, S100A8, S100A9 and S100A12, accounting for the thrombocytosis; a large number of interleukins, their receptors and activators, accounting for the febrile apathic state; and the high mobility group box 1 (HMBG1) gene, possibly accounting for part of the intestinal symptoms. These results show that gene expression array technology may assist the clinician in the diagnostic work-up of individual patients with suspected syndromal states of unknown origin, and the expression data can guide the selection of optimal treatment directed at the identified target pathways.

Novel founder intronic variant in SLC39A14 in two families causing Manganism and potential treatment strategies.

Congenital disorders of manganese metabolism are rare occurrences in children, and medical management of these disorders is complex and challenging. Homozygous exonic mutations in the manganese transporter SLC39A14 have recently been associated with a pediatric-onset neurodegenerative disorder characterized by brain manganese accumulation and clinical signs of manganese neurotoxicity, including parkinsonism-dystonia. We performed whole exome sequencing on DNA samples from two unrelated female children from the United Arab Emirates with progressive movement disorder and brain mineralization, identified a novel homozygous intronic mutation in SLC39A14 in both children, and demonstrated that the mutation leads to aberrant splicing. Both children had consistently elevated serum manganese levels and were diagnosed with SLC39A14-associated manganism. Over a four-year period, we utilized a multidisciplinary management approach for Patient 1 combining decreased manganese dietary intake and chelation with symptomatic management of dystonia. Our treatment strategy appeared to slow disease progression, but did not lead to a cure or reversal of already established deficits. Clinicians should consider testing for noncoding mutations in the diagnosis of congenital disorders of manganese metabolism and utilizing multidisciplinary approaches in the management of these disorders.

A mild case of molybdenum cofactor deficiency defines an alternative route of MOCS1 protein maturation.

Molybdenum cofactor deficiency is an autosomal recessive inborn error of metabolism, which results from mutations in genes involved in Moco biosynthesis. Moco serves as a cofactor of several enzymes, including sulfite oxidase. MoCD is clinically characterized by intractable seizures and severe, rapidly progressing neurodegeneration leading to death in early childhood in the majority of known cases. Here we report a patient with an unusual late disease onset and mild phenotype, characterized by a lack of seizures, normal early development, a decline triggered by febrile illness and a subsequent dystonic movement disorder. Genetic analysis revealed a homozygous c.1338delG MOCS1 mutation causing a frameshift (p.S442fs) with a premature termination of the MOCS1AB translation product at position 477 lacking the entire MOCS1B domain. Surprisingly, urine analysis detected trace amounts (1% of control) of the Moco degradation product urothione, suggesting a residual Moco synthesis in the patient, which was consistent with the mild clinical presentation. Therefore, we performed bioinformatic analysis of the patient's mutated MOCS1 transcript and found a potential Kozak-sequence downstream of the mutation site providing the possibility of an independent expression of a MOCS1B protein. Following the expression of the patient's MOCS1 cDNA in HEK293 cells we detected two proteins: a truncated MOCS1AB protein and a 22.4 kDa protein representing MOCS1B. Functional studies of both proteins confirmed activity of MOCS1B, but not of the truncated MOCS1AB. This finding demonstrates an unusual mechanism of translation re-initiation in the MOCS1 transcript, which results in trace amounts of functional MOCS1B protein being sufficient to partially protect the patient from the most severe symptoms of MoCD.

Molybdenum cofactor deficiency: Identification of a patient with homozygote mutation in the MOCS3 gene.

We describe the clinical presentation and 17 years follow up of a boy, born to consanguineous parents and presenting with intellectual disability (ID), autism, "marfanoid" dysmorphic features, and moderate abnormalities of sulfite metabolism compatible with molybdenum cofactor deficiency, but normal sulfite oxidase activity in cultured skin fibroblasts. Genomic exome analysis revealed a homozygous MOCS3 missense mutation, leading to a p.Ala257Thr substitution in the highly conserved ubiquitin-like-domain of the protein. MOCS3 is the third protein, besides MOCS1 and MOCS2, involved in the biosynthesis of the molybdenum cofactor and has a dual ubiquitin-like function in tRNA thiolation. It is plausible that the phenotype results from deficiency of this dual function, not only from defective synthesis of molybdenum cofactor, which would explain similarities and differences from the MOCS1 and MOCS2-related disorders. This observation should encourage testing of additional ID patients with mild abnormalities of sulfite metabolism for MOCS3 mutations.

Critical appraisal of genotype assessment in molybdenum cofactor deficiency.

INTRODUCTION: Molybdenum cofactor deficiency (MoCD) is an ultra-orphan, life-threatening disease. Substrate substitution therapy has successfully been performed in single cases of MoCD type A and clinical trials are underway for drug registration. We present an innovative approach for classification of genotype severity to test the hypothesis that milder sequence variants in MoCD result in a less severe disease phenotype quantitated by patient survival. METHODS: All available worldwide published cases with clinical and genetic data were included (n = 40). We stratified the already published disease causing sequence variants as mild or severe with the use of in silico prediction programs, where possible and assessed the possible impact of the variants on the expression of the gene or function of the expressed protein. In a compound heterozygous situation the mildest sequence variant determined the genotype. Subsequently, clinical manifestations and outcomes of both groups were compared. RESULTS: Patients with a severe genotype showed a median survival of 15 months and had a lower probability of survival compared to patients with mild genotypes who were all alive at last reported follow-up (p = 0.0203, Log-rank test). DISCUSSION: The severity of the genotype assessed by in silico prediction and further classification explained survival in molybdenum cofactor deficiency and may therefore be considered a confounder for the outcome of therapeutic clinical trials requiring adjustment in the clinical trial design or analysis. These results should further be investigated by future in vitro or in vivo functional studies. Caution should be taken with this approach for the classification of variants in molecular genetic diagnostics or genetic counseling.

Haematological involvement associated with a mild autoinflammatory phenotype, in two patients carrying the E250K mutation of PSTPIP1.

OBJECTIVES: Hyperzincaemia/hypercalprotectinemia (Hz/Hc) syndrome is a recently described condition caused by a specific de novo mutation (E250K) affecting PSTPIP1 gene. It has a phenotype distinct from classical pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome that includes severe systemic and cutaneous inflammation, hepatosplenomegaly, arthritis without sequelae, pancytopenia and failure to thrive. METHODS: We describe an 8-year-old boy who presented recurrent right knee swelling mimicking septic arthritis and persistent bone marrow involvement, without cutaneous involvement. RESULTS: Molecular analysis of the PSTPIP1 gene revealed the presence of a heterozygous E250K mutation. No growth failure was detected nor in the patient neither in his mother, carrying the same variant. Blood zinc and calprotectin MRP8/14 concentrations of the patient were found to be markedly increased. Therapy with anakinra was started with rapid disappearance of clinical symptoms and normalization of CRP levels in 24 hours, but persistence of bone marrow involvement. CONCLUSIONS: The patient described has a milder phenotype, with no skin features, minor episodes of arthritis with no sequelae and normal growth. Compared to the patients with de novo mutations described in the literature, familial cases seem to have a milder phenotype. Our case further confirms the lack of efficacy of anakinra on bone marrow involvement.

Drosophila melanogaster Models of Metal-Related Human Diseases and Metal Toxicity.

Iron, copper and zinc are transition metals essential for life because they are required in a multitude of biological processes. Organisms have evolved to acquire metals from nutrition and to maintain adequate levels of each metal to avoid damaging effects associated with its deficiency, excess or misplacement. Interestingly, the main components of metal homeostatic pathways are conserved, with many orthologues of the human metal-related genes having been identified and characterized in Drosophila melanogaster. Drosophila has gained appreciation as a useful model for studying human diseases, including those caused by mutations in pathways controlling cellular metal homeostasis. Flies have many advantages in the laboratory, such as a short life cycle, easy handling and inexpensive maintenance. Furthermore, they can be raised in a large number. In addition, flies are greatly appreciated because they offer a considerable number of genetic tools to address some of the unresolved questions concerning disease pathology, which in turn could contribute to our understanding of the metal metabolism and homeostasis. This review recapitulates the metabolism of the principal transition metals, namely iron, zinc and copper, in Drosophila and the utility of this organism as an experimental model to explore the role of metal dyshomeostasis in different human diseases. Finally, a summary of the contribution of Drosophila as a model for testing metal toxicity is provided.

Mouse model for molybdenum cofactor deficiency type B recapitulates the phenotype observed in molybdenum cofactor deficient patients.

Molybdenum cofactor (MoCo) deficiency is a rare, autosomal-recessive disorder, mainly caused by mutations in MOCS1 (MoCo deficiency type A) or MOCS2 (MoCo deficiency type B) genes; the absence of active MoCo results in a deficiency in all MoCo-dependent enzymes. Patients with MoCo deficiency present with neonatal seizures, feeding difficulties, severe developmental delay, brain atrophy and early childhood death. Although substitution therapy with cyclic pyranopterin monophosphate (cPMP) has been successfully used in both Mocs1 knockout mice and in patients with MoCo deficiency type A, there is currently no Mocs2 knockout mouse and no curative therapy for patients with MoCo deficiency type B. Therefore, we generated and characterized a Mocs2-null mouse model of MoCo deficiency type B. Expression analyses of Mocs2 revealed a ubiquitous expression pattern; however, at the cellular level, specific cells show prominent Mocs2 expression, e.g., neuronal cells in cortex, hippocampus and brainstem. Phenotypic analyses demonstrated that Mocs2 knockout mice failed to thrive and died within 11 days after birth. None of the tested MoCo-dependent enzymes were active in Mocs2-deficient mice, leading to elevated concentrations of purines, such as hypoxanthine and xanthine, and non-detectable levels of uric acid in the serum and urine. Moreover, elevated concentrations of S-sulfocysteine were measured in the serum and urine. Increased levels of xanthine resulted in bladder and kidney stone formation, whereas increased concentrations of toxic sulfite triggered neuronal apoptosis. In conclusion, Mocs2-deficient mice recapitulate the severe phenotype observed in humans and can now serve as a model for preclinical therapeutic approaches for MoCo deficiency type B.

Iron homeostasis: transport, metabolism, and regulation.

PURPOSE OF REVIEW: Iron is essential for normal cellular function and many diseases result from disturbances in iron homeostasis. This review describes some of the recent key advances in iron transport and its regulation, how this relates to iron-related disorders, and emerging therapies for these diseases. RECENT FINDINGS: The iron-regulatory hormone hepcidin and its target, the iron exporter ferroportin (FPN), play central roles in iron homeostasis. Recent studies have expanded our understanding of how hepcidin is regulated in response to stimulated erythropoiesis and have added some new players to the complex network of factors that influences hepcidin expression. Novel structural insights into how FPN transports iron have been an important addition to the field, as has the recognition that some zinc transporters such as ZIP14 can transport iron. Investigations into cardiac iron homeostasis have revealed a key role for FPN, and transferrin receptor 1, which is essential for cellular iron uptake, has been shown to be critical for normal immune function. SUMMARY: The increased understanding of mechanisms of iron homeostasis that has resulted from recent research has greatly improved our ability to diagnose and manage iron-related disorders, and has offered new therapies for this important class of human diseases.

Single amino acid charge switch defines clinically distinct proline-serine-threonine phosphatase-interacting protein 1 (PSTPIP1)-associated inflammatory diseases.

BACKGROUND: Hyperzincemia and hypercalprotectinemia (Hz/Hc) is a distinct autoinflammatory entity involving extremely high serum concentrations of the proinflammatory alarmin myeloid-related protein (MRP) 8/14 (S100A8/S100A9 and calprotectin). OBJECTIVE: We sought to characterize the genetic cause and clinical spectrum of Hz/Hc. METHODS: Proline-serine-threonine phosphatase-interacting protein 1 (PSTPIP1) gene sequencing was performed in 14 patients with Hz/Hc, and their clinical phenotype was compared with that of 11 patients with pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome. PSTPIP1-pyrin interactions were analyzed by means of immunoprecipitation and Western blotting. A structural model of the PSTPIP1 dimer was generated. Cytokine profiles were analyzed by using the multiplex immunoassay, and MRP8/14 serum concentrations were analyzed by using an ELISA. RESULTS: Thirteen patients were heterozygous for a missense mutation in the PSTPIP1 gene, resulting in a p.E250K mutation, and 1 carried a mutation resulting in p.E257K. Both mutations substantially alter the electrostatic potential of the PSTPIP1 dimer model in a region critical for protein-protein interaction. Patients with Hz/Hc have extremely high MRP8/14 concentrations (2045 ± 1300 µg/mL) compared with those with PAPA syndrome (116 ± 74 µg/mL) and have a distinct clinical phenotype. A specific cytokine profile is associated with Hz/Hc. Hz/Hc mutations altered protein binding of PSTPIP1, increasing interaction with pyrin through phosphorylation of PSTPIP1. CONCLUSION: Mutations resulting in charge reversal in the y-domain of PSTPIP1 (E→K) and increased interaction with pyrin cause a distinct autoinflammatory disorder defined by clinical and biochemical features not found in patients with PAPA syndrome, indicating a unique genotype-phenotype correlation for mutations in the PSTPIP1 gene. This is the first inborn autoinflammatory syndrome in which inflammation is driven by uncontrolled release of members of the alarmin family.

Hereditary hypotransferrinemia can lead to elevated transferrin saturation and, when associated to HFE or HAMP mutations, to iron overload.

As our understanding of iron metabolism improves through the more accurate description of iron metabolism actors, new causes of iron overload are identified. We, here, report 16 cases of hereditary hypotransferrinemia related to 4 previously undescribed TF (transferrin) mutations (p.Val221Gly, p.Arg609Trp, p.Glu370Lys, p.Tyr533X and p.Cys421Arg). We show that, besides increasing serum transferrin saturation without iron overload, hypotransferrinemia, when associated to mutations in HFE or HAMP or to acquired factors, can lead to clinically relevant iron burden. These cases emphasize the usefulness of serum transferrin determination in the diagnostic evaluation of iron overload and the importance for clinicians to be aware of this syndrome.

Manganese transport disorder: novel SLC30A10 mutations and early phenotypes.

BACKGROUND: SLC30A10 mutations cause an autosomal recessive disorder, characterized by hypermanganesaemia, polycythemia, early-onset dystonia, paraparesis, or late-onset parkinsonism, and chronic liver disease. This is the first identified inborn error of Mn metabolism in humans, reported in 10 families thus far. METHODS: Methods for this study consisted of clinical examination, neuroimaging studies (MRI), serum dosages, and SLC30A10 genetic analysis. RESULTS: We describe early disease manifestations (including videos) in 5 previously unreported Indian children, carrying novel homozygous SLC30A10 mutations. Gait and speech disturbances, falls, dystonias, and central hypotonia were the presenting neurological features, starting within the first 5 years of life. All children also had severe hypermanganesemia, polycythemia, variable degree of liver disease, and marked brain MRI T1 hyperintensities. CONCLUSIONS: Our findings expand the mutational and clinical spectra of this recently recognized disorder. An early diagnosis is warranted, because treatment with manganese-chelating agents, iron supplementation, or their combination might improve symptoms and prevent progression of this otherwise potentially fatal disease. © 2015 International Parkinson and Movement Disorder Society.

A nonsense mutation in PLD4 is associated with a zinc deficiency-like syndrome in Fleckvieh cattle.

BACKGROUND: Bovine hereditary zinc deficiency (BHZD) is an autosomal recessive disorder of cattle, first described in Holstein-Friesian animals. Affected calves suffer from severe skin lesions and show a poor general health status. Recently, eight calves with the phenotypic appearance of BHZD have been reported in the Fleckvieh cattle population. RESULTS: In spite of the similar disease phenotypes, SLC39A4, the gene responsible for BHZD in Holstein-Friesian was excluded as underlying gene for the disorder in the affected Fleckvieh calves. In order to identify the disease-associated region, genotypes of eight affected calves obtained with the Illumina BovineHD BeadChip comprising 777,962 SNPs were contrasted with the genotypes of 1,339 unaffected animals. A strong association signal was observed on chromosome 21 (P = 5.87 × 10(-89)). Autozygosity mapping in the eight affected animals revealed a common segment of extended homozygosity encompassing 1,023 kb (BTA 21: 70,550,045 - 71,573,501). This region contains 17 genes/transcripts, among them two genes encoding gastro-intestinal zinc transporters (CRIP1, CRIP2). However, no mutation that was compatible with recessive inheritance could be detected in these candidate genes. One of the affected calves was re-sequenced together with 42 unaffected Fleckvieh animals. Analysis of the sequencing data revealed a nonsense mutation (p.W215X) in a phospholipase encoding gene (PLD4) as candidate causal polymorphism. To confirm the causality, genotypes of the p.W215X-mutation were obtained from 3,650 animals representing three different breeds. None of the unaffected animals was homozygous for the defect allele, while all eight affected calves were homozygous. The deleterious effect of the mutation is manifested in a significantly lower survival rate of descendants from risk matings when compared with the survival rate of descendants from non-risk matings. The deleterious allele has an estimated frequency of 1.1% in the Fleckvieh population. CONCLUSION: Our results provide strong evidence that a newly identified recessive disorder in the Fleckvieh population is caused by a nonsense mutation in PLD4, most likely resulting in an impaired function of the encoded protein. Although the phenotype of affected calves strongly resembles BHZD, a zinc deficiency resulting from malabsorption is unlikely to be responsible for the diseased Fleckvieh calves.

Inherited disorders of renal hypomagnesaemia.

The kidney plays a key role in the maintenance of normal magnesium balance. The distal tubule of the kidney, namely the thick ascending limb of the loop of Henle and the distal convoluted tubule, is crucial for the regulation of serum magnesium levels and body magnesium content. The identification of molecular defects related to rare inherited magnesium losing disorders has contributed greatly to a better understanding of the process of renal magnesium handling. Since the number of genetic defects related to magnesium metabolism is still increasing, it might be expected that our knowledge on magnesium physiology will further improve. This knowledge will hopefully lead to therapeutic strategies that enable specific therapies for patients suffering from the symptoms and possible sequelae of chronic magnesium depletion.