Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability syndrome characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. Cells from NBS patients are hypersensitive to ionizing radiation with cytogenetic features indistinguishable from ataxia telangiectasia. We describe the positional cloning of a gene encoding a novel protein, nibrin. It contains two modules found in cell cycle checkpoint proteins, a forkhead-associated domain adjacent to a breast cancer carboxy-terminal domain. A truncating 5 bp deletion was identified in the majority of NBS patients, carrying a conserved marker haplotype. Five further truncating mutations were identified in patients with other distinct haplotypes. The domains found in nibrin and the NBS phenotype suggest that this disorder is caused by defective responses to DNA double-strand breaks.

DNA methylation is an important regulator of genetic information in species ranging from bacteria to humans. DNA methylation appears to be critical for mammalian development because mice nullizygous for a targeted disruption of the DNMT1 DNA methyltransferase die at an early embryonic stage. No DNA methyltransferase mutations have been reported in humans until now. We describe here the first example of naturally occurring mutations in a mammalian DNA methyltransferase gene. These mutations occur in patients with a rare autosomal recessive disorder, which is termed the ICF syndrome, for immunodeficiency, centromeric instability, and facial anomalies. Centromeric instability of chromosomes 1, 9, and 16 is associated with abnormal hypomethylation of CpG sites in their pericentromeric satellite regions. We are able to complement this hypomethylation defect by somatic cell fusion to Chinese hamster ovary cells, suggesting that the ICF gene is conserved in the hamster and promotes de novo methylation. ICF has been localized to a 9-centimorgan region of chromosome 20 by homozygosity mapping. By searching for homologies to known DNA methyltransferases, we identified a genomic sequence in the ICF region that contains the homologue of the mouse Dnmt3b methyltransferase gene. Using the human sequence to screen ICF kindreds, we discovered mutations in four patients from three families. Mutations include two missense substitutions and a 3-aa insertion resulting from the creation of a novel 3 splice acceptor. None of the mutations were found in over 200 normal chromosomes. We conclude that mutations in the DNMT3B are responsible for the ICF syndrome.DNA methylation ͉ somatic cell complementation ͉ consanguinity ͉ RNA splicing

Patient registries are instrumental for clinical research in rare diseases. They help to achieve a sufficient sample size for epidemiological and clinical research and to assess the feasibility of clinical trials. The European Society for Immunodeficiencies (ESID) registry currently comprises information on >25,000 patients with inborn errors of immunity (IEI). The prerequisite of a patient to be included into the ESID registry is an IEI either defined by a defect in a gene included in the disease classification of the international union of immunological societies (IUIS), or verified by applying clinical criteria. Because a relevant number of patients, including those with common variable immunodeficiency (CVID), representing the largest group of patients in the registry, remains without a genetic diagnosis, consensus on classification of these patients is mandatory. Here, we present clinical criteria for a large number of IEI that were designed in expert panels with external review. They were implemented for novel entries and verification of existing datasets from 2014, yielding a substantial refinement. For instance, 8% of adults and 27% of children with CVID (176 out of 1704 patients) were reclassified to 22 different immunodeficiencies, illustrating progress in genetics, but also the previous lack of standardized disease definitions. Importantly, apart from registry purposes, the clinical criteria are also helpful to support treatment decisions in the absence of a genetic diagnosis or in patients with variants of unknown significance.

A 10-year-old boy with microcephaly, stunted growth, mental retardation, café-au-lait spots and immunodeficiency is described. An older brother of the patient had the same clinical symptoms and a more severe immunodeficiency. Cytogenetic studies in the proband revealed a typical form of chromosome instability with multiple rearrangements of chromosomes 7 and 14. Such abnormalities were also present, though in very low frequencies, in the father and three of the phenotypically normal sibs. The similarity of the symptoms in the two sibs, the close consanguinity of their parents and the results of the cytogenetic studies in the family favour the hypothesis that the disorder is an inherited one. The clinical features and the chromosome aberrations as present in the proband are usually found in chromosomal breakage syndromes, but it was possible to exclude each of the classical chromosomal breakage syndromes on clinical and/or cytogenetic grounds.

Immunodeficiency with centromeric instability and facial anomalies (ICF) syndrome is a primary immunodeficiency, predominantly characterized by agammaglobulinemia or hypoimmunoglobulinemia, centromere instability and facial anomalies. Mutations in two genes have been discovered to cause ICF syndrome: DNMT3B and ZBTB24. To characterize the clinical features of this syndrome, as well as genotype-phenotype correlations, we compared clinical and genetic data of 44 ICF patients. Of them, 23 had mutations in DNMT3B (ICF1), 13 patients had mutations in ZBTB24 (ICF2), whereas for 8 patients, the gene defect has not yet been identified (ICFX). While at first sight these patients share the same immunological, morphological and epigenetic hallmarks of the disease, systematic evaluation of all reported informative cases shows that: (1) the humoral immunodeficiency is generally more pronounced in ICF1 patients, (2) B-and T-cell compartments are both involved in ICF1 and ICF2, (3) ICF2 patients have a significantly higher incidence of intellectual disability and (4) congenital malformations can be observed in some ICF1 and ICF2 cases. It is expected that these observations on prevalence and clinical presentation will facilitate mutation-screening strategies and help in diagnostic counseling.