Rare genetic iron overload diseases are an evolving field due to major advances in genetics and molecular biology. Genetic iron overload has long been confined to the classical type 1 hemochromatosis related to the HFE C282Y mutation. Breakthroughs in the understanding of iron metabolism biology and molecular mechanisms led to the discovery of new genes and subsequently, new types of hemochromatosis. To date, four types of hemochromatosis have been identified: HFE-related or type1 hemochromatosis, the most frequent form in Caucasians, and four rare types, named type 2 (A and B) hemochromatosis (juvenile hemochromatosis due to hemojuvelin and hepcidin mutation), type 3 hemochromatosis (related to transferrin receptor 2 mutation), and type 4 (A and B) hemochromatosis (ferroportin disease). The diagnosis relies on the comprehension of the involved physiological defect that can now be explored by biological and imaging tools, which allow non-invasive assessment of iron metabolism. A multidisciplinary approach is essential to support the physicians in the diagnosis and management of those rare diseases.

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Smith-Magenis syndrome (SMS) is an intellectual disability syndrome with sleep disturbance, self-injurious behaviors and dysmorphic features. It is estimated to occur in 1/25,000 births, and in 90% of cases it is associated with interstitial deletions of chromosome 17p11.2. RAI1 (retinoic acid induced 1; OMIM 607642) mutations are the second most frequent molecular etiology, with this gene being located in the SMS locus at 17p11.2. Here, we report 9 new RAI1-truncating mutations in nonrelated individuals referred for molecular analysis due to a possible SMS diagnosis. None of these patients carried a 17p11.2 deletion. The 9 mutations include 2 nonsense mutations and 7 heterozygous frameshift mutations leading to protein truncation. All mutations map in exon 3 of RAI1 which codes for more than 98% of the protein. RAI1 regulates gene transcription, and its targets are themselves involved in transcriptional regulation, cell growth and cell cycle regulation, bone and skeletal development, lipid and glucide metabolisms, neurological development, behavioral functions, and circadian activity. We report the clinical features of the patients carrying these deleterious mutations in comparison with those of patients carrying 17p11.2 deletions.

p.Cys282Tyr (C282Y) homozygosity explains most cases of HFE-related hemochromatosis, but a significant number of patients presenting with typical type I hemochromatosis phenotype remain unexplained. We sought to describe the clinical relevance of rare HFE variants in non-C282Y homozygotes. Patients referred for hemochromatosis to the National Reference Centre for Rare Iron Overload Diseases from 2004 to 2010 were studied. Sequencing was performed for coding region and intronic flanking sequences of HFE, HAMP, HFE2, TFR2, and SLC40A1. Nine private HFE variants were identified in 13 of 206 unrelated patients. Among those, five have not been previously described: p.Leu270Argfs*4, p.Ala271Valfs*25, p.Tyr52*, p.Lys166Asn, and p.Asp141Tyr. Our results show that rare HFE variants are identified more frequently than variants in the other genes associated with iron overload. Rare HFE variants are therefore the most frequent cause of hemochromatosis in non-C282Y homozygote HFE patients. Am. J. Hematol. 91:1202-1205, 2016. © 2016 Wiley Periodicals, Inc.