Atypical clinical picture of the Nijmegen breakage syndrome associated with developmental abnormalities of the brain

Impaired p53-Mediated DNA Damage Response Contributes to Microcephaly in Nijmegen Breakage Syndrome Patient-Derived Cerebral Organoids

Nijmegen Breakage Syndrome (NBS) is a rare autosomal recessive genetic disorder caused by mutations within nibrin (NBN), a DNA damage repair protein. Hallmarks of NBS include chromosomal instability and clinical manifestations such as growth retardation, immunodeficiency, and progressive microcephaly. We employed induced pluripotent stem cell-derived cerebral organoids from two NBS patients to study the etiology of microcephaly. We show that NBS organoids carrying the homozygous 657del5 NBN mutation are significantly smaller with disrupted cyto-architecture. The organoids exhibit premature differentiation, and Neuronatin (NNAT) over-expression. Furthermore, pathways related to DNA damage response and cell cycle are differentially regulated compared to controls. After exposure to bleomycin, NBS organoids undergo delayed p53-mediated DNA damage response and aberrant trans-synaptic signaling, which ultimately leads to neuronal apoptosis. Our data provide insights into how mutations within NBN alters neurogenesis in NBS patients, thus providing a proof of concept that cerebral organoids are a valuable tool for studying DNA damage-related disorders.

Diabetes, Oxidative Stress, and DNA Damage Modulate Cranial Neural Crest Cell Development and the Phenotype Variability of Craniofacial Disorders

Craniofacial malformations are among the most common birth defects in humans and they often have significant detrimental functional, aesthetic, and social consequences. To date, more than 700 distinct craniofacial disorders have been described. However, the genetic, environmental, and developmental origins of most of these conditions remain to be determined. This gap in our knowledge is hampered in part by the tremendous phenotypic diversity evident in craniofacial syndromes but is also due to our limited understanding of the signals and mechanisms governing normal craniofacial development and variation. The principles of Mendelian inheritance have uncovered the etiology of relatively few complex craniofacial traits and consequently, the variability of craniofacial syndromes and phenotypes both within families and between families is often attributed to variable gene expression and incomplete penetrance. However, it is becoming increasingly apparent that phenotypic variation is often the result of combinatorial genetic and non-genetic factors. Major non-genetic factors include environmental effectors such as pregestational maternal diabetes, which is well-known to increase the risk of craniofacial birth defects. The hyperglycemia characteristic of diabetes causes oxidative stress which in turn can result in genotoxic stress, DNA damage, metabolic alterations, and subsequently perturbed embryogenesis. In this review we explore the importance of gene-environment associations involving diabetes, oxidative stress, and DNA damage during cranial neural crest cell development, which may underpin the phenotypic variability observed in specific craniofacial syndromes.

Update on DNA-Double Strand Break Repair Defects in Combined Primary Immunodeficiency

PURPOSE OF REVIEW

The most serious DNA damage, DNA double strand breaks (DNA-dsb), leads to mutagenesis, carcinogenesis or apoptosis if left unrepaired. Non-homologous end joining (NHEJ) is the principle repair pathway employed by mammalian cells to repair DNA-dsb. Several proteins are involved in this pathway, defects in which can lead to human disease. This review updates on the most recent information available for the specific diseases associated with the pathway.

RECENT FINDINGS

A new member of the NHEJ pathway, PAXX, has been identified, although no human disease has been associated with it. The clinical phenotypes of Artemis, DNA ligase 4, Cernunnos-XLF and DNA-PKcs deficiency have been extended. The role of haematopoietic stem cell transplantation, following reduced intensity conditioning chemotherapy, for many of these diseases is being advanced. In the era of newborn screening, urgent genetic diagnosis is necessary to correctly target appropriate treatment for patients with DNA-dsb repair disorders.

Prenatal Microcephaly and Hydrocephalus and Normal Heart Anatomy, Postnatal Diagnosis of Nijmegen Syndrome - Case Report

Nijmengen breakage syndrome is a rare autosomal condition mainly characterized by microcephaly. Patients are predisposed to malignancies due to combined immunodeficiency. The presented patient had prenatally diagnosed microcephaly with atypical ventriculomegaly of occipital horns. Fetal echocardiography showed a normal fetal heart anatomy. Diagnosis of Nijmengen syndrome was confirmed postnatally. The differential diagnosis of fetal microcephaly should take into account intrauterine infections, perinatal brain injury, congenital malformations or biological variants.

Nijmegen breakage syndrome detected by newborn screening for T cell receptor excision circles (TRECs)

Purpose

Severe combined immunodeficiency (SCID) encompasses a group of disorders characterized by reduced or absent T-cell number and function and identified by newborn screening utilizing T-cell receptor excision circles (TRECs). This screening has also identified infants with T lymphopenia who lack mutations in typical SCID genes. We report an infant with low TRECs and non-SCID T lymphopenia, who proved upon whole exome sequencing to have Nijmegen breakage syndrome (NBS).

Methods

Exome sequencing of DNA from the infant and his parents was performed. Genomic analysis revealed deleterious variants in the NBN gene. Confirmatory testing included Sanger sequencing and immunoblotting and radiosensitivity testing of patient lymphocytes.

Results

Two novel nonsense mutations in NBN were identified in genomic DNA from the family. Immunoblotting showed absence of nibrin protein. A colony survival assay demonstrated radiosensitivity comparable to patients with ataxia telangiectasia.

Conclusions

Although TREC screening was developed to identify newborns with SCID, it has also identified T lymphopenic disorders that may not otherwise be diagnosed until later in life. Timely identification of an infant with T lymphopenia allowed for prompt pursuit of underlying etiology, making possible a diagnosis of NBS, genetic counseling, and early intervention to minimize complications.

Nijmegen breakage syndrome (NBS)

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive syndrome of chromosomal instability mainly characterized by microcephaly at birth, combined immunodeficiency and predisposition to malignancies. Due to a founder mutation in the underlying NBN gene (c.657_661del5) the disease is encountered most frequently among Slavic populations. The principal clinical manifestations of the syndrome are: microcephaly, present at birth and progressive with age, dysmorphic facial features, mild growth retardation, mild-to-moderate intellectual disability, and, in females, hypergonadotropic hypogonadism. Combined cellular and humoral immunodeficiency with recurrent sinopulmonary infections, a strong predisposition to develop malignancies (predominantly of lymphoid origin) and radiosensitivity are other integral manifestations of the syndrome. The NBN gene codes for nibrin which, as part of a DNA repair complex, plays a critical nuclear role wherever double-stranded DNA ends occur, either physiologically or as a result of mutagenic exposure. Laboratory findings include: (1) spontaneous chromosomal breakage in peripheral T lymphocytes with rearrangements preferentially involving chromosomes 7 and 14, (2) sensitivity to ionizing radiation or radiomimetics as demonstrated in vitro by cytogenetic methods or by colony survival assay, (3) radioresistant DNA synthesis, (4) biallelic hypomorphic mutations in the NBN gene, and (5) absence of full-length nibrin protein. Microcephaly and immunodeficiency are common to DNA ligase IV deficiency (LIG4 syndrome) and severe combined immunodeficiency with microcephaly, growth retardation, and sensitivity to ionizing radiation due to NHEJ1 deficiency (NHEJ1 syndrome). In fact, NBS was most commonly confused with Fanconi anaemia and LIG4 syndrome. Genetic counselling should inform parents of an affected child of the 25% risk for further children to be affected. Prenatal molecular genetic diagnosis is possible if disease-causing mutations in both alleles of the NBN gene are known. No specific therapy is available for NBS, however, hematopoietic stem cell transplantation may be one option for some patients. Prognosis is generally poor due to the extremely high rate of malignancies.

Zespół Nijmegen (Nijmegen breakage syndrome; NBS) jest rzadkim schorzeniem z wrodzoną niestabilnością chromosomową dziedziczącym się w sposób autosomalny recesywny, charakteryzującym się przede wszystkim wrodzonym małogłowiem, złożonymi niedoborami odporności i predyspozycją do rozwoju nowotworów.

Choroba występuje najczęściej w populacjach słowiańskich, w których uwarunkowana jest mutacją założycielską w genie NBN (c.657_661del5). Do najważniejszych objawów zespołu zalicza się: małogłowie obecne od urodzenia i postępujące z wiekiem, charakterystyczne cechy dysmorfii twarzy, opóźnienie wzrastania, niepełnosprawność intelektualną w stopniu lekkim do umiarkowanego oraz hipogonadyzm hipogonadotropowy u dziewcząt. Na obraz choroby składają się także: niedobór odporności komórkowej i humoralnej, który jest przyczyną nawracających infekcji, znaczna predyspozycja do rozwoju nowotworów złośliwych (zwłaszcza układu chłonnego), a także zwiększona wrażliwość na promieniowanie jonizujące. Wyniki badań laboratoryjnych wykazują: (1) spontaniczną łamliwość chromosomów w limfocytach T krwi obwodowej, z preferencją do rearanżacji chromosomów 7 i 14, (2) nadwrażliwość na promieniowanie jonizujące lub radiomimetyki, co można wykazać metodami in vitro, (3) radiooporność syntezy DNA, (4) hipomorficzne mutacje na obu allelach genu NBN, oraz (5) brak w komórkach pełnej cząsteczki białka, nibryny. Małogłowie i niedobór odporności występują także w zespole niedoboru ligazy IV (LIG4) oraz w zespole niedoboru NHEJ1. Rodzice powinni otrzymać poradę genetyczną ze względu na wysokie ryzyko (25%) powtórzenia się choroby u kolejnego potomstwa. Możliwe jest zaproponowanie molekularnej diagnostyki prenatalnej jeżeli znane są obie mutacje będące przyczyną choroby. Nie ma możliwości zaproponowania specyficznej terapii, ale przeszczep szpiku może być alternatywą dla niektórych pacjentów. Generalnie prognoza nie jest pomyślna z uwagi na wysokie ryzyko rozwoju nowotworu.

An utter refutation of the "fundamental theorem of the HapMap"

The International HapMap Project was proposed in order to quantify linkage disequilibrium (LD) relationships among human DNA polymorphisms in an assortment of populations, in order to facilitate the process of selecting a minimal set of markers that could capture most of the signal from the untyped markers in a genome-wide association study. The central dogma can be summarized by the argument that if a marker is in tight LD with a polymorphism that directly impacts disease risk, as measured by the metric r(2), then one would be able to detect an association between the marker and disease with sample size that was increased by a factor of 1/r(2) over that needed to detect the effect of the functional variant directly. This "fundamental theorem" holds, however, only if one assumes that the LD between loci and the etiological effect of the functional variant are independent of each other, that they are statistically independent of all other etiological factors (in exposure and action), that sampling is prospective, and that the estimates of r(2) are accurate. None of these are standard operating assumptions, however. We describe the ramifications of these implicit assumptions, and provide simple examples in which the effects of a functional variant could be unequivocally detected if it were directly genotyped, even as markers in high LD with the functional variant would never show association with disease, even in infinite sample sizes. Both theoretical and empirical refutation of the central dogma of genome-wide association studies is thus presented.

NBS1, the Nijmegen breakage syndrome gene product, regulates neuronal proliferation and differentiation

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder, characterized by progressive microcephaly, growth retardation, immunodeficiency, and pre-disposition to tumor formation. To investigate the functions of the NBS gene product, NBS1, on neurons, PC12 cells overexpressing NBS1 and related mutants and primary cortical neuronal culture were used in the present study. Small interfering RNA (siRNA) was applied to repress the expression of endogenous Nbs1 in PC12 cells and primary cortical neurons. We demonstrated that overexpression of NBS1 increases cellular proliferation and decreases the apoptosis of PC12 cells in serum withdrawal and ionizing irradiation, through the activation of phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Overexpression of NBS1 also decreases neurite elongation on PC12 cells under nerve growth factor stimulation. Transfection of NBS1-overexpressing PC12 cells with a dominant negative Akt mutant attenuates the neuroprotection and cellular proliferation effects of NBS1 while having no effect on neurite elongation. PC12 cells overexpressing NBS657del5 and NBS653 mutants, in which the major NBS1 protein in cells are truncated proteins, have decreased cellular proliferation, increased cell death, and decreased neurite elongation compared with those of control PC12 cells. Repression of Nbs1 by siRNA decreases the PI 3-kinase activity and Akt phosphorylation levels, and induces neurite elongation in PC12 cells even without nerve growth factor stimulation. Repression of Nbs1 by siRNA in primary cortical neurons also increased neurite elongation, but increased neuronal death. We conclude that NBS1 can regulate neuronal proliferation and neuroprotection via PI 3-kinase/Akt pathway while regulating neuronal differentiation in a different pathway. Excessive accumulation of truncated protein secondary to 657del5 mutation may be detrimental to neurons, leading to defective neuronal proliferation and differentiation.

Nijmegen breakage syndrome (NBS) with neurological abnormalities and without chromosomal instability

BACKGROUND

Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability disorder with hypersensitivity to ionising radiation. The clinical phenotype is characterised by congenital microcephaly, mild dysmorphic facial appearance, growth retardation, immunodeficiency, and greatly increased risk for lymphoreticular malignancy. Most NBS patients are of Slavic origin and homozygous for the founder mutation 657del5. The frequency of 657del5 heterozygotes in the Czech population is 1:150. Recently, another NBS1 mutation, 643C>T(R215W), with uncertain pathogenicity was found to have higher frequency among tumour patients of Slavic origin than in controls. This alteration results in the substitution of the basic amino acid arginine with the non-polar tryptophan and thus could potentially interfere with the function of the NBS1 protein, nibrin.

METHODS AND RESULTS

Children with congenital microcephaly are routinely tested for the 657del5 mutation in the Czech and Slovak Republics. Here, we describe for the first time a severe form of NBS without chromosomal instability in monozygotic twin brothers with profound congenital microcephaly and developmental delay who are compound heterozygotes for the 657del5 and 643C>T(R215W) NBS1 mutations. Both children showed reduced expression of full length nibrin when compared with a control and a heterozygote for the 657del5 mutation. Radiation response processes such as phosphorylation of ATM and phosphorylation/stabilisation of p53, which are promoted by NBS1, are strongly reduced in cells from these patients.

CONCLUSIONS

Interestingly, the patients are more severely affected than classical NBS patients. Consequently, we postulate that homozygosity for the 643C>T(R215W) mutation will also lead to a, possibly very, severe disease phenotype.