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Trisomy 21 (Down syndrome) is the most common autosomal aneuploidy among newborns. About 90% result from meiotic nondisjunction during oogenesis, which occurs around conception, when also the most profound epigenetic modifications take place. Thus, maternal meiosis is an error prone process with an extreme sensitivity to endogenous factors, as exemplified by maternal age. This contrasts with the missing acceptance of causal exogenous factors. The proof of an environmental agent is a great challenge, both with respect to ascertainment bias, determination of time and dosage of exposure, as well as registration of the relevant individual health data affecting the birth prevalence. Based on a few exemplary epidemiological studies the feasibility of trisomy 21 monitoring is illustrated. In the nearer future the methodical premises will be clearly improved, both due to the establishment of electronic health registers and to the introduction of non-invasive prenatal tests. Down syndrome is a sentinel phenotype, presumably also with regard to other congenital anomalies. Thus, monitoring of trisomy 21 offers new chances for risk avoidance and preventive measures, but also for basic research concerning identification of relevant genomic variants involved in chromosomal nondisjunction.

PALB2 was recently identified as a nuclear binding partner of BRCA2. Biallelic BRCA2 mutations cause Fanconi anemia subtype FA-D1 and predispose to childhood malignancies. We identified pathogenic mutations in PALB2 (also known as FANCN ) in seven families affected with Fanconi anemia and cancer in early childhood, demonstrating that biallelic PALB2 mutations cause a new subtype of Fanconi anemia, FA-N, and, similar to biallelic BRCA2 mutations, confer a high risk of childhood cancer.

We report on a consanguineous family with two infertile sisters with oocyte arrest and prematurely condensed sperm chromosomes. A genome-wide linkage scan and exome sequencing revealed a homozygous variant in the gene for the thyroid receptor interacting protein 13 (TRIP13), c.518G˃A (p.Arg173Gln), affecting an evolutionary highly conserved amino acid within an ATP binding motif. Just recently, compound heterozygosity for this variant was described in a Chinese proband as pathogenic, confirming that the homozygous mutation is causative for the oocyte arrest. The TRIP13 gene and the orthologous yeast pch2 gene are, amongst others, involved in a meiotic checkpoint control. This checkpoint defect is obviously responsible for the premature condensation of the sperm chromosomes. TRIP13 and pch2 are involved in meiotic recombination. To exclude that it is involved in reciprocal somatic exchanges, we analyzed the rate of sister chromatid exchanges (SCEs) in the proband´s lymphoblastoid cells. Obviously, TRIP13 is not involved in this type of somatic recombination. Moreover, we tested whether TRIP13 can complement the defect of the yeast pch2 gene. Using a yeast deletion strain lacking pch2, we integrated plasmids containing either the yeast pch2 or the human TRIP13 gene, both harboring the wild-type or the mutant allele and assessed the crossingover rate between marker genes lys2 and leu2 as a measure of complementation. Evidence is presented that the human plasmids, unexpectedly also that with the mutation, could complement the pch2 deficient yeast strain, underlining that the evolutionary conservation at the molecular level obviously extends to the functional level.

Entry into mitosis is accompanied by dramatic changes in cellular architecture, metabolism and gene expression. Many viruses have evolved cell cycle arrest strategies to prevent mitotic entry, presumably to ensure sustained, uninterrupted viral replication. Here we show for human cytomegalovirus (HCMV) what happens if the viral cell cycle arrest mechanism is disabled and cells engaged in viral replication enter into unscheduled mitosis. We made use of an HCMV mutant that, due to a defective Cyclin A2 binding motif in its UL21a gene product (pUL21a), has lost its ability to down-regulate Cyclin A2 and, therefore, to arrest cells at the G1/S transition. Cyclin A2 up-regulation in infected cells not only triggered the onset of cellular DNA synthesis, but also promoted the accumulation and nuclear translocation of Cyclin B1-CDK1, premature chromatin condensation and mitotic entry. The infected cells were able to enter metaphase as shown by nuclear lamina disassembly and, often irregular, metaphase spindle formation. However, anaphase onset was blocked by the still intact anaphase promoting complex/cyclosome (APC/C) inhibitory function of pUL21a. Remarkably, the essential viral IE2, but not the related chromosome-associated IE1 protein, disappeared upon mitotic entry, suggesting an inherent instability of IE2 under mitotic conditions. Viral DNA synthesis was impaired in mitosis, as demonstrated by the abnormal morphology and strongly reduced BrdU incorporation rates of viral replication compartments. The prolonged metaphase arrest in infected cells coincided with precocious sister chromatid separation and progressive fragmentation of the chromosomal material. We conclude that the Cyclin A2-binding function of pUL21a contributes to the maintenance of a cell cycle state conducive for the completion of the HCMV replication cycle. Unscheduled mitotic entry during the course of the HCMV replication has fatal consequences, leading to abortive infection and cell death.

Biallelic mutations in MCPH1 cause primary microcephaly (MCPH) with the cellular phenotype of defective chromosome condensation. MCPH1 encodes a multifunctional protein that notably is involved in brain development, regulation of chromosome condensation, and DNA damage response. In the present studies, we detected that MCPH1 encodes several distinct transcripts, including two major forms: full-length MCPH1 (MCPH1-FL) and a second transcript lacking the six 3' exons (MCPH1Δe9-14). Both variants show comparable tissue-specific expression patterns, demonstrate nuclear localization that is mediated independently via separate NLS motifs, and are more abundant in certain fetal than adult organs. In addition, the expression of either isoform complements the chromosome condensation defect found in genetically MCPH1-deficient or MCPH1 siRNA-depleted cells, demonstrating a redundancy of both MCPH1 isoforms for the regulation of chromosome condensation. Strikingly however, both transcripts are regulated antagonistically during cell-cycle progression and there are functional differences between the isoforms with regard to the DNA damage response; MCPH1-FL localizes to phosphorylated H2AX repair foci following ionizing irradiation, while MCPH1Δe9-14 was evenly distributed in the nucleus. In summary, our results demonstrate here that MCPH1 encodes different isoforms that are differentially regulated at the transcript level and have different functions at the protein level.

Mutations in the human gene MCPH1 cause primary microcephaly associated with a unique cellular phenotype with premature chromosome condensation (PCC) in early G2 phase and delayed decondensation post-mitosis (PCC syndrome). The gene encodes the BRCT-domain containing protein microcephalin/BRIT1. Apart from its role in the regulation of chromosome condensation, the protein is involved in the cellular response to DNA damage. We report here on the first mouse model of impaired Mcph1-function. The model was established based on an embryonic stem cell line from BayGenomics (RR0608) containing a gene trap in intron 12 of the Mcph1 gene deleting the C-terminal BRCT-domain of the protein. Although residual wild type allele can be detected by quantitative real-time PCR cell cultures generated from mouse tissues bearing the homozygous gene trap mutation display the cellular phenotype of misregulated chromosome condensation that is characteristic for the human disorder, confirming defective Mcph1 function due to the gene trap mutation. While surprisingly the DNA damage response (formation of repair foci, chromosomal breakage, and G2/M checkpoint function after irradiation) appears to be largely normal in cell cultures derived from Mcph1(gt/gt) mice, the overall survival rates of the Mcph1(gt/gt) animals are significantly reduced compared to wild type and heterozygous mice. However, we could not detect clear signs of premature malignant disease development due to the perturbed Mcph1 function. Moreover, the animals show no obvious physical phenotype and no reduced fertility. Body and brain size are within the range of wild type controls. Gene expression on RNA and protein level did not reveal any specific pattern of differentially regulated genes. To the best of our knowledge this represents the first mammalian transgenic model displaying a defect in mitotic chromosome condensation and is also the first mouse model for impaired Mcph1-function.

Background Nijmegen breakage syndrome is an autosomal recessive disorder characterized by microcephaly, immunodeficiency, hypersensitivity to X-irradiation, and a high predisposition to cancer. Nibrin, the product of the NBN gene, is part of the MRE11/RAD50 (MRN) complex that is involved in the repair of DNA double strand breaks (DSBs), and plays a critical role in the processing of DSBs in immune gene rearrangements, telomere maintenance, and meiotic recombination. NBS skin fibroblasts grow slowly in culture and enter early into senescence. Case presentation Here we present an incidental finding. Skin fibroblasts, derived from a 9 year old NBS patient, showed a mosaic of normal diploid cells (46,XY) and those with a complex, unbalanced translocation. The aberrant karyotype was analysed by G-banding, comparative genomic hybridization, and whole chromosome painting. The exact breakpoints of the derivative chromosome were mapped by whole genome sequencing: 45,XY,der(6)(6pter → 6q11.1::13q11 → 13q21.33::20q11.22 → 20qter),-13. The deleted region of chromosomes 6 harbors almost 1.400 and that of chromosome 13 more than 500 genes, the duplicated region of chromosome 20 contains about 700 genes. Such unbalanced translocations are regularly incompatible with cellular survival, except in malignant cells. The aberrant cells, however, showed a high proliferation potential and could even be clonally expanded. Telomere length was significantly reduced, hTERT was not expressed. The cells underwent about 50 population doublings until they entered into senescence. The chromosomal preparation performed shortly before senescence showed telomere fusions, premature centromere divisions, endoreduplications and tetraploid cells, isochromatid breaks and a variety of marker chromosomes. Inspection of the site of skin biopsy 18 years later, presented no evidence for abnormal growth. Conclusions The aberrant cells had a significant selective advantage in vitro. It is therefore tempting to speculate that this highly unbalanced translocation could be a primary driver of cancer cell growth.

Interstitial deletions of chromosome band 10q22 are rare. We report on the characterization of three overlapping de novo 10q22 deletions by high-resolution array comparative genomic hybridization in three unrelated patients. Patient 1 had a 7.9 Mb deletion in 10q21.3–q22.2 and suffered from severe feeding problems, facial dysmorphisms and profound mental retardation. Patients 2 and 3 had nearly identical deletions of 3.2 and 3.6 Mb, the proximal breakpoints of which were located at an identical low-copy repeat. Both patients were mentally retarded; patient 3 also suffered from growth retardation and hypotonia. We also report on the results of breakpoint analysis by array painting in a mentally retarded patient with a balanced chromosome translocation 46,XY,t(10;13)(q22;p13)dn. The breakpoint in 10q22 was found to disrupt C10orf11 , a brain-expressed gene in the common deleted interval of patients 1–3. This finding suggests that haploinsufficiency of C10orf11 contributes to the cognitive defects in 10q22 deletion patients.

The evolutionary conserved Polo-like kinase 4 (PLK4) is essential for centriole duplication, spindle assembly, and de novo centriole formation. In man, homozygous mutations in PLK4 lead to primary microcephaly, altered PLK4 expression is associated with aneuploidy in human embryos. Here, we report on a consanguineous four-generation family with 8 affected individuals compound heterozygous for a novel missense variant, c.881 T > G, and a deletion of the PLK4 gene. The clinical phenotype of the adult patients is mild compared to individuals with previously described PLK4 mutations. One individual was homozygous for the variant c.881G and phenotypically unaffected. The deletion was inherited by 14 of 16 offspring and thus exhibits transmission ratio distortion (TRD). Moreover, based on the already published families with PLK4 mutations, it could be shown that due to the preferential transmission of the mutant alleles, the number of affected offspring is significantly increased. It is assumed that reduced expression of PLK4 decreases the intrinsically high error rate of the first cell divisions after fertilization, increases the number of viable embryos and thus leads to preferential transmission of the deleted/mutated alleles.

BackgroundAssisted reproductive technologies (ART) such as in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) are believed to destabilise genomic imprints. An increased frequency of Beckwith–Wiedemann syndrome in children born after ART has been reported. Other, mostly epidemiological, studies argue against this finding.ObjectiveTo examine the effect of ART on the stability of DNA methylation imprints, DNA was extracted from maternal peripheral blood (MPB), umbilical cord blood (UCB) and amnion/chorion tissue (ACT) of 185 phenotypically normal children (77 ICSI, 35 IVF, and 73 spontaneous conceptions). Using bisulfite based technologies 10 differentially methylated regions (DMRs) were analysed, including KvDMR1, H19, SNRPN, MEST, GRB10, DLK1/MEG3 IG-DMR, GNAS NESP55, GNAS NESPas, GNAS XL-alpha-s and GNAS Exon1A.ResultsMethylation indices (MI) do not reveal any significant differences at nine DMRs among the conception groups in neither MPB, UCB nor in ACT. The only slightly variable DMR was that of MEST. Here the mean MI was higher in UCB and MPB of IVF cases (mean MI±SD: 0.41±0.03 (UCB) and 0.40±0.03 (MPB)) compared to the ICSI (0.38±0.03, p=0.003 (UCB); 0.37±0.04, p=0.0007 (MPB)) or spontaneous cases (0.38±0.03, p=0.003 (UCB); 0.38±0.04, p=0.02 (MPB)). Weak but suggestive correlations between DMRs were, however, found between MPB, UCB and ACT.ConclusionThis study supports the notion that children conceived by ART do not show a higher degree of imprint variability and hence do not have an a priori higher risk for imprinting disorders.