The dual loss and gain of function of the FPN1 iron exporter results in the ferroportin disease phenotype.

Heterozygous mutations in SLC40A1, encoding a multi-pass membrane protein of the major facilitator superfamily known as ferroportin 1 (FPN1), are responsible for two distinct hereditary iron-overload diseases: ferroportin disease, which is associated with reduced FPN1 activity (i.e., decrease in cellular iron export), and SLC40A1-related hemochromatosis, which is associated with abnormally high FPN1 activity (i.e., resistance to hepcidin). Here, we report three SLC40A1 missense variants with opposite functional consequences. In cultured cells, the p.Arg40Gln and p.Ser47Phe substitutions partially reduced the ability of FPN1 to export iron and also partially reduced its sensitivity to hepcidin. The p.Ala350Val substitution had more profound effects, resulting in low FPN1 iron egress and weak FPN1/hepcidin interaction. Structural analyses helped to differentiate the first two substitutions, which are predicted to cause local instabilities, and the third, which is thought to prevent critical rigid-body movements that are essential to the iron transport cycle. The phenotypic traits observed in a total of 12 affected individuals are highly suggestive of ferroportin disease. Our findings dismantle the classical dualism of FPN1 loss versus gain of function, highlight some specific and unexpected functions of FPN1 transmembrane helices in the molecular mechanism of iron export and its regulation by hepcidin, and extend the spectrum of rare genetic variants that may cause ferroportin disease.

Clinical versus molecular diagnosis of Gorlin syndrome: Relevance of diagnostic criteria depends on the age of patients.

BACKGROUND: Gorlin Syndrome (GS) is an autosomal dominant disorder characterised by a predisposition to basal cell carcinoma and developmental defects, and caused by pathogenic variants in PTCH1 or SUFU genes. OBJECTIVES: To ascertain the efficiency of molecular screening in a cohort of patients with a suspicion of GS and to describe patients' clinical and genetic characteristics. METHODS: 110 patients with a suspicion of GS, addressed to the Genetic Department of Bichat University Hospital for molecular screening were studied. Patients' clinical and paraclinical data were collected and analysed according to Evan's diagnosis criteria, and compared to molecular information. RESULTS: Among 110 probands, only 56% fulfill Evan's diagnosis criteria. 75% of patients who fulfill those criteria carry a PTCH1/SUFU pathogenic variation. We compared clinical and paraclinical data of 54 probands carrying a PTCH1/SUFU mutation with 56 probands without identified mutation. Among patients carrying a pathogenic variation in PTCH1 or SUFU genes, 30 years of age appears to be the cut off age after which all patients have a clear clinical GS. Indeed, after 30 years, all patients carrying a PTCH1/SUFU mutation fulfill the diagnosis criteria of Evans (82% meet the clinical criteria, and we reach 100% with complementary exams such as X-rays and ultrasound). Before 30 years of age, only 37% of mutated patients fulfilled the clinical diagnosis criteria's and we only reach 62% with simple complementary exams. Furthermore, we report 22 new mutations in PTCH1. CONCLUSIONS: Molecular screening of patients with GS who do not fulfill Evan's diagnostic criteria should only be offered in first intention to patients under 30 years of age. After 30 years, a careful clinical examination and complementary radiological exams should be enough to eliminate the diagnosis of GS among patients who do not fulfill diagnostic criteria.

ABCB6 polymorphisms are not overly represented in patients with porphyria.

The Mendelian inheritance pattern of acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria is autosomal dominant, but the clinical phenotype is heterogeneous. Within the general population, penetrance is low, but among first-degree relatives of a symptomatic proband, penetrance is higher. These observations suggest that genetic factors, in addition to mutation of the specific enzyme of the biosynthetic pathway of heme, contribute to the clinical phenotype. Recent studies by others suggested that the genotype of the transporter protein ABCB6 contribute to the porphyria phenotype. Identifying the molecule(s) that are transported by ABCB6 has been problematic and has led to uncertainty with respect to how or if variants/mutants contribute to phenotypic heterogeneity. Knockout mouse models of Abcb6 have not provided a direction for investigation as homozygous knockout animals do not have a discrete phenotype. To address the proposed link between ABC6 genotype and porphyria phenotype, a large cohort of patients with acute hepatic porphyria and erythropoietic protoporphyria was analyzed. Our studies showed that ABCB6 genotype did not correlate with disease severity. Therefore, genotyping of ABCB6 in patients with acute hepatic porphyria and erythropoietic protoporphyria is not warranted.

A Variant of Peptide Transporter 2 Predicts the Severity of Porphyria-Associated Kidney Disease.

CKD occurs in most patients with acute intermittent porphyria (AIP). During AIP, δ-aminolevulinic acid (ALA) accumulates and promotes tubular cell death and tubulointerstitial damage. The human peptide transporter 2 (PEPT2) expressed by proximal tubular cells mediates the reabsorption of ALA, and variants of PEPT2 have different affinities for ALA. We tested the hypothesis that PEPT2 genotypes affect the severity and prognosis of porphyria-associated kidney disease. We analyzed data from 122 individuals with AIP who were followed from 2003 to 2013 and genotyped for PEPT2 At last follow-up, carriers of the PEPT2*1*1 genotype (higher affinity variant) exhibited worse renal function than carriers of the lower affinity variants PEPT2*1/*2 and PEPT2*2/*2 (mean±SD eGFR: 54.4±19.1, 66.6±23.8, and 78.1±19.9 ml/min per 1.73 m2, respectively). Change in eGFR (mean±SD) over the 10-year period was -11.0±3.3, -2.4±1.9, and 3.4±2.6 ml/min per 1.73 m2 for PEPT2*1/*1, PEPT2*1*2, and PEPT*2*2*2 carriers, respectively. At the end of follow-up, 68% of PEPT2*1*1 carriers had an eGFR<60 ml/min per 1.73 m2, compared with 37% of PEPT2*1*2 carriers and 15% of PEPT2*2*2 carriers. Multiple regression models including all confounders indicated that the PEPT2*1*1 genotype independently associated with an eGFR<60 ml/min per 1.73 m2 (odds ratio, 6.85; 95% confidence interval, 1.34 to 46.20) and an annual decrease in eGFR of >1 ml/min per 1.73 m2 (odds ratio, 3.64; 95% confidence interval, 1.37 to 9.91). Thus, a gene variant is predictive of the severity of a chronic complication of AIP. The therapeutic value of PEPT2 inhibitors in preventing porphyria-associated kidney disease warrants investigation.

Heterozygous Mutations in BMP6 Pro-peptide Lead to Inappropriate Hepcidin Synthesis and Moderate Iron Overload in Humans.

BACKGROUND & AIMS: Hereditary hemochromatosis is a heterogeneous group of genetic disorders characterized by parenchymal iron overload. It is caused by defective expression of liver hepcidin, the main regulator of iron homeostasis. Iron stimulates the gene encoding hepcidin (HAMP) via the bone morphogenetic protein (BMP)6 signaling to SMAD. Although several genetic factors have been found to cause late-onset hemochromatosis, many patients have unexplained signs of iron overload. We investigated BMP6 function in these individuals. METHODS: We sequenced the BMP6 gene in 70 consecutive patients with a moderate increase in serum ferritin and liver iron levels who did not carry genetic variants associated with hemochromatosis. We searched for BMP6 mutations in relatives of 5 probands and in 200 healthy individuals (controls), as well as in 2 other independent cohorts of hyperferritinemia patients. We measured serum levels of hepcidin by liquid chromatography-tandem mass spectrometry and analyzed BMP6 in liver biopsy specimens from patients by immunohistochemistry. The functions of mutant and normal BMP6 were assessed in transfected cells using immunofluorescence, real-time quantitative polymerase chain reaction, and immunoblot analyses. RESULTS: We identified 3 heterozygous missense mutations in BMP6 (p.Pro95Ser, p.Leu96Pro, and p.Gln113Glu) in 6 unrelated patients with unexplained iron overload (9% of our cohort). These mutations were detected in less than 1% of controls. p.Leu96Pro also was found in 2 patients from the additional cohorts. Family studies indicated dominant transmission. Serum levels of hepcidin were inappropriately low in patients. A low level of BMP6, compared with controls, was found in a biopsy specimen from 1 patient. In cell lines, the mutated residues in the BMP6 propeptide resulted in defective secretion of BMP6; reduced signaling via SMAD1, SMAD5, and SMAD8; and loss of hepcidin production. CONCLUSIONS: We identified 3 heterozygous missense mutations in BMP6 in patients with unexplained iron overload. These mutations lead to loss of signaling to SMAD proteins and reduced hepcidin production. These mutations might increase susceptibility to mild-to-moderate late-onset iron overload.

Hepcidin regulates intrarenal iron handling at the distal nephron.

Hepcidin, the key regulatory hormone of iron homeostasis, and iron carriers such as transferrin receptor1 (TFR1), divalent metal transporter1 (DMT1), and ferroportin (FPN) are expressed in kidney. Whether hepcidin plays an intrinsic role in the regulation of renal iron transport is unknown. Here, we analyzed the renal handling of iron in hemochromatosis Hepc(-/-) and Hjv(-/-) mouse models, as well as in phenylhydrazine (PHZ)-treated mice. We found a marked medullary iron deposition in the kidneys of Hepc(-/-) mice, and iron leak in the urine. The kidneys of Hepc(-/-) mice exhibited a concomitant decrease in TFR1 and increase in ferritin and FPN expression. Increased FPN abundance was restricted to the thick ascending limb (TAL). DMT1 protein remained unaffected despite a significant decrease of its mRNA level, suggesting that DMT1 protein is stabilized in the absence of hepcidin. Treatment of kidney sections from Hepc(-/-) mice with hepcidin decreased DMT1 protein, an effect confirmed in renal cell lines where hepcidin markedly decreased (55)Fe transport. In the kidneys of Hjv(-/-) mice exhibiting low hepcidin expression, the iron overload was similar to that in the kidneys of Hepc(-/-) mice. However, in PHZ mice, iron accumulation resulting from hemoglobin leak was detected in the proximal tubule. Thus, kidneys exhibit a tissue-specific handling of iron that depends on the extra iron source. Hepcidin may control the expression of iron transporters to prevent renal iron overload.

The TNFRSF1A R92Q mutation is frequent in rheumatoid arthritis but shows no evidence for association or linkage with the disease.

OBJECTIVE: TNFRSF1A mutations cause TNFRSF1A-associated periodic syndrome (MIM#142680). A recent study suggested that the R92Q mutation was associated with polyarthritis. We aimed to search for this and other TNFRSF1A mutations in rheumatoid arthritis (RA), and to test for linkage. METHODS: The DNA of 100 trio families and 86 index cases of RA-affected sib-pair (ASP) families from the French Caucasian population were investigated by denatured high-performance liquid chromatography for TNFRSF1A mutations in exons 2 to 4. The test for association compared cases and controls (derived from un-transmitted parental chromosomes). The test for linkage relied on the transmission disequilibrium test (TDT) in trio families and cosegregation in ASP families. RESULTS: Only the R92Q mutation was detected--in 2 of the 100 index cases of trio families and in 5 (5.8%) of the index cases of ASP families, but also in 5% of the controls, showing no association with the disease. No RA linkage evidence was found in TDT and ASP RA families. CONCLUSION: This TNFRSF1A investigation in RA from the French Caucasian population showed only the R92Q mutation, with a frequency of 4.5%, but no evidence for RA association or linkage to the disease. The R92Q mutation could be considered to be a low-penetrance variant.

Unexpected high frequency of P46L TNFRSF1A allele in sub-Saharan West African populations.

TNF receptor-associated periodic syndrome (TRAPS) is an autosomal dominant disorder characterized by recurrent attacks of fever and serositis. To date, more than 30 mutations have been reported in TNFRSF1A, the responsible gene. In Caucasian populations, the P46L (c.224C>T) TNFRSF1A sequence variation is considered as a low-penetrance mutation because its allele frequency is similar in patients and controls ( approximately 1%). Whereas the spectrum of TNFRSF1A gene mutations has been well established in Caucasian and several Mediterranean populations, it remains unknown in sub-Saharan African populations. In this study, we found an unexpected high P46L allele frequency ( approximately 10%) in two groups from West Africa - a group of 145 patients with sickle cell anaemia and a group of 349 healthy controls. These data suggest that the P46L variant is rather a polymorphism than a TRAPS causative mutation. We propose that the P46L high frequency in West African populations could be explained by some biological advantage conferred to carriers.

In silico prediction of the deleterious effect of a mutation: proceed with caution in clinical genetics.

When a sequence variation is found in a candidate gene for a disease, it is important to establish whether this change is neutral or responsible for the observed disorders in a patient. To answer this question, in the absence of further experimental investigations, several simulation programs have been proposed to predict whether a nonsynonymous single-nucleotide polymorphism is likely to have or not have a deleterious effect on the phenotype. In this work, we tested two such programs, PolyPhen and SIFT, using two kinds of targets. The first ones concerned the products of the hemoglobin and glucose-6-phosphate dehydrogenase genes, which are abundantly documented. The second concerned two systems for which much less information is available: (a) the TNFRSF1A gene, implicated in tumor necrosis factor receptor-associated periodic syndrome, and (b) the MEFV gene, which is believed to be involved in familial Mediterranean fever. Our data suggest that, from a practical point of view, these programs should not be used to decide, in the absence of other tests or arguments, whether the sequence variation found in a patient is or is not responsible for the disease. The consequence of an erroneous prediction may be disastrous in the perspective of genetic counseling.

The E148Q MEFV allele is not implicated in the development of familial Mediterranean fever.

Familial Mediterranean fever (FMF) is an autosomal recessive disorder characterized by recurrent attacks of fever and serositis, common in populations of Armenian, Arab, Sephardic Jewish and Turkish origin. Early diagnosis is crucial to start colchicine therapy that prevents the occurrence of attacks and renal amyloidosis. In the absence of functional test for FMF, the diagnosis remains clinical and is generally confirmed by molecular analysis of the MEFV gene. More than 40 missense mutations and two in-frame deletions have been reported, most of them being located in exon 10 of the gene. The M694V (c.2080A>G) mutation, the most frequent defect, is responsible for a severe phenotype when present in the homozygous state. The E148Q (c.442G>C) sequence variant, which is situated in exon 2, is also common, but its role in FMF is controversial. In order to assess the implication of the E148Q variation in FMF, we investigated 233 patients of Sephardic Jewish origin living in France and 213 disease-free relatives of these patients. The frequency of the E148Q allele was found to be similar in the two groups (3.62% and 3.75%, respectively, p=0.93). Most importantly, the frequency of the M694V/E148Q compound heterozygous genotype was comparable between the patients group (3.9%) and the healthy relatives group (4.2%, p=0.85). This population-based study, therefore, strongly supports the hypothesis that E148Q is a just a benign polymorphism and not a disease-causing mutation. Considering this variant as a mutation may lead to set false positive diagnoses and to neglect the likely existence of genetic heterogeneity in FMF.

Expression of the two mRNA isoforms of the iron transporter Nramp2/DMTI in mice and function of the iron responsive element.

Nramp2/DMT1 is a transmembrane proton-coupled Fe(2+) transporter. Two different mRNAs are generated by alternative splicing; isoform I contains an iron responsive element (IRE), whereas isoform II does not. They encode two proteins differing at their C-terminal end and by their subcellular localization. IRE-mediated stabilization of isoform I mRNA is thought to stimulate DMT1 expression in response to iron deficiency. We have measured the two mRNAs by real-time quantitative PCR in several mouse tissues, in normal conditions or following injection of phenylhydrazine, a potent haemolytic agent. Isoform I mRNA is expressed in the duodenum and is induced by stimulation of erythropoiesis, whereas the non-IRE isoform is mostly induced in erythropoietic spleen. Surprisingly, both isoforms are highly expressed in the kidney and are not regulated by erythropoiesis. To evaluate the role of the IRE in regulating isoform I mRNA stability, in response to variations in cell iron status, several constructs were made in pCDNA3 with either a normal or a mutated IRE placed at the 3' end of a stable mRNA. These constructs were transfected into HT29 cells and mRNAs were analysed after growing cells in the presence or absence of exogenous iron. There was no difference in the level of expression of the different messages, suggesting that the IRE does not regulate stability of isoform I mRNA. The half-life of the endogenous IRE-mRNA was also measured following actinomycin D addition in iron- or desferrioxamine-treated cells. Decay of the mRNA was very similar in both conditions. These results suggest that additional transcriptional regulations at the promoter level, or iron-dependent regulation of alternative splicing are likely to participate in the induction of isoform I mRNA by iron deficiency.