Truncation of NHEJ1 in a patient with polymicrogyria
ABSTRACT Polymicrogyria (PMG) is a common malformation of the human cerebral cortex for which both acquired and genetic causes are known. Although genetic heterogeneity is documented, only one gene is currently known to cause isolated PMG. To clone new genes involved in this type of cerebral malformation, we studied a fetus presenting a defect of cortical organization consisting of a polymicrogyric cortex and neuronal heterotopia within the white matter. Karyotype analysis revealed that the fetus was carrier of a balanced, de novo, chromosomal translocation t(2;7)(q35;p22). Cloning and sequencing of the two translocation breakpoints reveals that the chromosomal rearrangement disrupts the coding region of a single gene, called NHEJ1, Cernunnos, or XLF, in 2q35. The NHEJ1 gene was recently identified as being responsible for autosomal recessive immunodeficiency with microcephaly. Using quantitative PCR experiments, we show that a truncated transcript is expressed in the polymicrogyric patient cells, suggesting a potential dominant negative effect possibly leading to a different phenotype. We performed in situ hybridization on human embryos and showed that the NHEJ1 transcript is preferentially expressed in the telencephalic ventricular and subventricular zones, consistent with the phenotype of the affected individual. In the human adult central nervous system (CNS), NHEJ1 is mainly expressed in the cerebral cortex and in the cerebellum. The association of PMG with the disruption of its transcript suggests that, in addition to its recently uncovered function in the immune system, the NHEJ1 protein may also play a role during development of the human cerebral cortex.
SourceAvailable from: Marie Reine Haddad[Show abstract] [Hide abstract]
ABSTRACT: DNA double-strand breaks (DSBs) frequently occur in rapidly dividing cells such as proliferating progenitors during central nervous system development. If they cannot be repaired, these lesions will cause cell death. The non-homologous end joining (NHEJ) DNA repair pathway is the only pathway available to repair DSBs in post-mitotic neurons. The non-homologous end joining factor 1 (Nhej1) protein is a key component of the NHEJ pathway. Nhej1 interacts with Xrcc4 and Lig4 to repair DSBs. Loss of function of Xrcc4 or Lig4 is embryonic lethal in the mouse while the loss of Nhej1 is not. Surprisingly, the brains of Nhej1-deficient mice appear to be normal although NHEJ1 deficiency in humans causes severe neurological dysfunction and microcephaly. Here, we studied the consequences of Nhej1 dysfunction for the development of the cerebral cortex using in utero electroporation of inactivating small hairpin RNAs (shRNAs) in the developing rat brain. We found that decreasing Nhej1 expression during neuronal migration phases causes severe neuronal migration defects visualized at embryonic stages by an accumulation of heterotopic neurons in the intermediate zone. Knocked-down cells die by 7 days after birth and the brain regions where RNA interference was achieved are structurally abnormal, suffering from a reduction of the width of the external cortical layers. These results indicate that the Nhej1 protein is necessary for proper rat cortical development. Neurons unable to properly repair DNA DSBs are unable to reach their final destination during the development and undergo apoptosis, leading to an abnormal cortical development.Molecular Neurobiology 10/2014; DOI:10.1007/s12035-014-8919-y · 5.29 Impact Factor
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ABSTRACT: Polymicrogyria (PMG) is one of the most common malformations of cortical development. It is characterized by overfolding of the cerebral cortex and abnormal cortical layering. It is a highly heterogeneous malformation with variable clinical and imaging features, pathological findings, and etiologies. It may occur as an isolated cortical malformation, or in association with other malformations within the brain or body as part of a multiple congenital anomaly syndrome. Polymicrogyria shows variable topographic patterns with the bilateral perisylvian pattern being most common. Schizencephaly is a subtype of PMG in which the overfolded cortex lines full-thickness clefts connecting the subarachnoid space with the cerebral ventricles. Both genetic and non-genetic causes of PMG have been identified. Non-genetic causes include congenital cytomegalovirus infection and in utero ischemia. Genetic causes include metabolic conditions such as peroxisomal disorders and the 22q11.2 and 1p36 continguous gene deletion syndromes. Mutations in over 30 genes have been found in association with PMG, especially mutations in the tubulin family of genes. Mutations in the (PI3K)-AKT pathway have been found in association PMG and megalencephaly. Despite recent genetic advances, the mechanisms by which polymicrogyric cortex forms and causes of the majority of cases remain unknown, making diagnostic and prenatal testing and genetic counseling challenging. This review summarizes the clinical, imaging, pathologic, and etiologic features of PMG, highlighting recent genetic advances. © 2014 Wiley Periodicals, Inc.American Journal of Medical Genetics Part C Seminars in Medical Genetics 06/2014; 166(2). DOI:10.1002/ajmg.c.31399 · 3.54 Impact Factor
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ABSTRACT: DNA non-homologous end-joining (NHEJ) is the major DNA double strand break (DSB) repair pathway in mammalian cells. Defects in NHEJ proteins confer marked radiosensitivity in cell lines and mice models, since radiation potently induces DSBs. The process of V(D)J recombination functions during the development of the immune response, and involves the introduction and rejoining of programmed DSBs to generate an array of diverse T and B cells. NHEJ rejoins these programmed DSBs. Consequently, NHEJ deficiency confers (severe) combined immunodeficiency – (S)CID – due to a failure to carry out V(D)J recombination efficiently. NHEJ also functions in class switch recombination, another step enhancing T and B cell diversity. Prompted by these findings, a search for radiosensitivity amongst (S)CID patients revealed a radiosensitive sub-class, defined as RS-SCID. Mutations in NHEJ genes, defining human syndromes deficient in DNA ligase IV (LIG4 Syndrome), XLF-Cernunnos, Artemis or DNA-PKcs, have been identified in such patients. Mutations in XRCC4 or Ku70,80 in patients have not been identified. RS-SCID patients frequently display additional characteristics including microcephaly, dysmorphic facial features and growth delay. Here, we overview the clinical spectrum of RS-SCID patients and discuss our current understanding of the underlying biology.DNA repair 05/2014; 16. DOI:10.1016/j.dnarep.2014.02.011 · 3.36 Impact Factor