Iron-deficiency anemia secondary to mutations in genes controlling hepcidin.

The discovery of the peptide hormone hepcidin in 2001 has shed light on the control of iron metabolism. Studies in animal models over the past few years have demonstrated its key role in regulating iron homeostasis. It was found that hepcidin deficiency leads to iron overload, and that its overexpression leads to severe iron-deficiency anemia. Since then, other genes regulating hepcidin expression have been discovered, and defects in them mostly resulted in iron overload. In 2008, a new gene, TMPRSS6, was identified that encodes a negative regulator of hepcidin expression. This discovery has been of great relevance because TMPRSS6 is the first gene regulating hepcidin, mutations in which cause chronic iron-deficiency anemia. Recently, genome-wide association studies identified common TMPRSS6 variants associated with hematological parameters, suggesting that TMPRSS6 is crucial in the control of iron homeostasis and normal erythropoiesis.

C329X in KRIT1 is a founder mutation among CCM patients in Sardinia.

Cerebral cavernous malformations (CCMs) are CNS vascular anomalies associated with seizures, headaches and hemorrhagic strokes and represent 10-20% of cerebral lesions. CCM is present in 0.1-0.5 of the population. This disorder most often occurs sporadically but may also be familial. Familial cases are inherited as a dominant trait with incomplete penetrance and are estimated to account for KRIT1 10-40% of the patients. The identification of the genes involved in such disorders allows to characterize carriers of the mutations without clear symptoms. The first gene involved in CCM1 is KRIT1. In addition to two other genes have been described: MGC4607 (CCM2) and PDCD10 (CCM3). We selected 13 patients belonging to seven Sardinian families on the basis of clinical symptoms and Magnetic Resonance results. In MGC4607 gene an undescribed exon five deletion likely producing a truncated protein was identified in one family. In two patients with clear phenotype and in three asymptomatic relatives a 4 bp deletion in exon 9 of KRIT1 gene, leading to a premature stop codon, was detected. A unique nonsense mutation (C329X) has been found in seven patients and two asymptomatic subjects belonging to four unrelated families. Haplotype analysis revealed a common origin of this mutation. These data suggest a "founder effect" in Sardinia for the C329X mutation, similar to other mutations described in different populations.

A mutation in the TMPRSS6 gene, encoding a transmembrane serine protease that suppresses hepcidin production, in familial iron deficiency anemia refractory to oral iron.

BACKGROUND: Hepcidin plays a key role in body iron metabolism by preventing the release of iron from macrophages and intestinal cells. Defective hepcidin synthesis causes iron loading, while overproduction results in defective reticuloendothelial iron release and iron absorption. DESIGN AND METHODS: We studied a Sardinian family in which microcytic anemia due to defective iron absorption and utilization is inherited as a recessive character. Five members showed iron deficiency anemia that was not responsive to oral iron and only partially responsive to parenteral iron administration. To investigate the involvement of known genes implicated in iron metabolism we carried out linkage analysis with microsatellite markers mapping close to these genes. Afterwards, a genome-wide search was performed. RESULTS: No linkage was found between the phenotype of the patients and several known human genes involved in iron metabolism (DMT1, TF, TFRC, ZIRTL, HAMP, HJV). Genome-wide scanning by microsatellites and single nucleotide polymorphisms showed a multipoint LOD score of 5.6 on chromosome 22q12.3-13.1, where the matriptase-2 (also known as transmembrane protease, serine 6 or TMPRSS6) gene is located. Its murine counterpart (Tmprss6) has recently been found to be an essential component of a pathway that detects iron deficiency and suppresses hepcidin production. Sequencing analysis of TMPRSS6 revealed a homozygous causal mutation, predicting a splicing error and a truncated TMPRSS6 protein in affected members. Homozygous subjects had inappropriately elevated levels of serum and urinary hepcidin. CONCLUSIONS: The findings of this study suggest that the observed TMPRSS6 mutation leads to overproduction of hepcidin and, in turn, to defective iron absorption and utilization. More generally, they confirm in humans the inhibitory effect of matriptase-2 on hepcidin synthesis already demonstrated in mice.

A novel interstitial deletion in Xq25, identified by array-CGH in a patient with Lowe syndrome.

The oculocerebrorenal syndrome of Lowe (OCRL) (MIM:309000) is an X-linked multisystemic disorder affecting the eyes, nervous system and kidneys due to mutations in OCRL1 gene. The gene contains 24 exons, and encodes a 105kDa phosphatydylinositol 4,5-biphosphate [PtdIns(4,5)P(2)] 5-phosphatase localized primarily in the trans-Golgi network and the lysosomes. The large majority of the OCRL1 mutations producing Lowe syndrome are either missense mutations localized mainly in the catalytic domain or non-sense/frameshift mutations resulting in truncated proteins. Rarely, in about 6% of the cases, the disease results from large gene deletions occurring in the 5' part of the gene. Here we report a new case of a patient with Lowe syndrome due to a deletion of about 4Mb, encompassing the OCRL1 gene, detected by PCR and CGH array. The mother was carrier of the same deletion.

A locus for familial skewed X chromosome inactivation maps to chromosome Xq25 in a family with a female manifesting Lowe syndrome.

In mammals, X-linked gene products can be dosage compensated between males and females by inactivation of one of the two X chromosomes in the developing female embryos. X inactivation choice is usually random in embryo mammals, but several mechanisms can influence the choice determining skewed X inactivation. As a consequence, females heterozygous for X-linked recessive disease can manifest the full phenotype. Herein, we report a family with extremely skewed X inactivation that produced the full phenotype of Lowe syndrome, a recessive X-linked disease, in a female. The X chromosome inactivation studies detected an extremely skewed inactivation pattern with a ratio of 100:0 in the propositus as well as in five out of seven unaffected female relatives in four generations. The OCRL1 "de novo" mutation resides in the active paternally inherited X chromosome. X chromosome haplotype analysis suggests the presence of a locus for the familial skewed X inactivation in chromosome Xq25 most likely controlling X chromosome choice in X inactivation or cell proliferation. The description of this case adds Lowe syndrome to the list of X-linked disorders which may manifest the full phenotype in females because of the skewed X inactivation.

Frequency of hemochromatosis C282Y and H63D mutations in Sardinia.

Hereditary hemochromatosis (HH) is one of the most common autosomal recessive disorders of iron metabolism among Caucasians, and it is associated with C282Y mutation of the HFE gene in populations of Celtic origins. A second mutation, H63D, shows a very high widespread frequency, although its role in iron metabolism is still inconclusive. There are no data on the frequencies of these two mutations in Sardinia, an island in the Mediterranean sea that has not been invaded by Celtic peoples. We examined 836 chromosomes from Sardinian subjects and tested for the mutation by restriction enzyme digestion of PCR products. Among the 836 analyzed chromosomes, we found a C282Y allele frequency of 0.0036 and an H63D allele frequency of 0.173. These data could explain the observed rarity of HH in Sardinia. The high allele frequency of H63D and the rarity of HH in Sardinia is suggestive that this mutation is not a major contributor to this disease.