Clinical Spectrum of LIG4 Deficiency Is Broadened with Severe Dysmaturity,Primordial Dwarfism,and Neurological Abnormalities

DNA double‐strand break repair via non‐homologous end joining (NHEJ) is involved in recombination of immunoglobulin and T‐cell receptor genes. Mutations in NHEJ components result in syndromes that are characterized by microcephaly and immunodeficiency. We present a patient with lymphopenia, extreme radiosensitivity, severe dysmaturity, corpus callosum agenesis, polysyndactily, dysmorphic appearance, and erythema, which are suggestive of a new type of NHEJ deficiency. We identified two heterozygous mutations in LIG4. The p.S205LfsX29 mutation results in lack of the nuclear localization signal and appears to be a null mutation. The second mutation p.K635RfsX10 lacks the C‐terminal region responsible for XRCC4 binding and LIG4 stability and activity, and therefore this mutant might be a null mutation as well or have very low residual activity. This is remarkable since Lig4 knockout mice are embryonic lethal and so far in humans no complete LIG4 deficiencies have been described. This case broadens the clinical spectrum of LIG4 deficiencies.     

Knockout of the DNA ligase IV homolog gene in the sphingoid base producing yeast <Emphasis Type="Italic">Pichia ciferrii</Emphasis> significantly increases gene targeting efficiency

The yeast Pichia ciferrii produces large quantities of the sphingoid base tetraacetyl phytosphingosine (TAPS) and is an interesting platform organism for the biotechnological production of sphingolipids and ceramides. Ceramides have attracted great attention as a specialty ingredient for moisture retention and protection of the skin in the cosmetics industry. First attempts have been started to metabolically engineer P. ciferrii for improved production of TAPS and other sphingoid bases. However, rational metabolic engineering of P. ciferrii is difficult due to a low gene targeting efficiency. In eukaryotes, two major pathways coexist, which are responsible for genomic DNA integration, homologous recombination (HR) and non-homologous end joining (NHEJ). Integration via HR is targeted, while NHEJ involves ectopic (non-targeted) integration depending on a ligation step mediated by DNA ligase IV (Lig4). Here, we demonstrate a dramatical increase in gene targeting efficiency in a P. ciferrii lig4 knockout strain, deficient in NHEJ. Furthermore, a quick and easy to use freeze–thaw method was developed to transform P. ciferrii with high efficiency. Owing to the ability of targeting genomic DNA integration our results pave the way for further genetic and metabolic engineering approaches with P. ciferrii by means of knocking out or overexpressing predestinated genes.     

E1B 55k-independent dissociation of the DNA ligase IV/XRCC4 complex by E4 34k during adenovirus infection

The ligase IV/XRCC4 complex plays a central role in DNA double-strand break repair by non-homologous end joining (NHEJ). During adenovirus infection, NHEJ is inhibited by viral proteins E4 34k and E1B 55k, which redirect the Cul5/Rbx1/Elongin BC ubiquitin E3 ligase to polyubiquitinate and promote degradation of ligase IV. In cells infected with E1B 55k-deficient adenovirus, ligase IV could not be found in XRCC4-containing complexes and was observed in a novel ligase IV/E4 34k/Cul5/Elongin BC complex. These observations suggest that dissociation of the ligase IV/XRCC4 complex occurs at an early stage in E4 34k-mediated degradation of ligase IV and indicate a role for E4 34k in dissociation of the ligase IV/XRCCC4 complex. Expression of E4 34k alone was not sufficient to dissociate the ligase IV/XRCC4 complex, which indicates a requirement for an additional, as yet unidentified, factor in E1B 55k-independent dissociation of the ligase IV/XRCC4 complex.     

The role of WRN in DNA repair is affected by post-translational modifications

Werner syndrome (WS) is an autosomal recessive progeroid disease characterized by genomic instability. WRN gene encodes one of the RecQ helicase family proteins, WRN, which has ATPase, helicase, exonuclease and single stranded DNA annealing activities. There is accumulating evidence suggesting that WRN contributes to the maintenance of genomic integrity through its involvement in DNA repair, replication and recombination. The role of WRN in these pathways can be modulated by its post-translational modifications in response to DNA damage. Here, we review the functional consequences of post-translational modifications on WRN as well as specific DNA repair pathways where WRN is involved and discuss how these modifications affect DNA repair pathways.     

DNA repair deficiency in neurodegeneration

Deficiency in repair of nuclear and mitochondrial DNA damage has been linked to several neurodegenerative disorders. Many recent experimental results indicate that the post-mitotic neurons are particularly prone to accumulation of unrepaired DNA lesions potentially leading to progressive neurodegeneration. Nucleotide excision repair is the cellular pathway responsible for removing helix-distorting DNA damage and deficiency in such repair is found in a number of diseases with neurodegenerative phenotypes, including Xeroderma Pigmentosum and Cockayne syndrome. The main pathway for repairing oxidative base lesions is base excision repair, and such repair is crucial for neurons given their high rates of oxygen metabolism. Mismatch repair corrects base mispairs generated during replication and evidence indicates that oxidative DNA damage can cause this pathway to expand trinucleotide repeats, thereby causing Huntington's disease. Single-strand breaks are common DNA lesions and are associated with the neurodegenerative diseases, ataxia-oculomotor apraxia-1 and spinocerebellar ataxia with axonal neuropathy-1. DNA double-strand breaks are toxic lesions and two main pathways exist for their repair: homologous recombination and non-homologous end-joining. Ataxia telangiectasia and related disorders with defects in these pathways illustrate that such defects can lead to early childhood neurodegeneration. Aging is a risk factor for neurodegeneration and accumulation of oxidative mitochondrial DNA damage may be linked with the age-associated neurodegenerative disorders Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Mutation in the WRN protein leads to the premature aging disease Werner syndrome, a disorder that features neurodegeneration. In this article we review the evidence linking deficiencies in the DNA repair pathways with neurodegeneration.     

Activation-induced cytidine deaminase structure and functions: a species comparative view

In the ten years since the discovery of activation-induced cytidine deaminase (AID) there has been considerable effort to understand the mechanisms behind this enzyme's ability to target and modify immunoglobulin genes leading to somatic hypermutation and class switch recombination. While the majority of research has focused on mouse and human models of AID function, work on other species, from lamprey to rabbit and sheep, has taught us much about the scope of functions of the AID mutator. This review takes a species-comparative approach to what has been learned about the AID mutator enzyme and its role in humoral immunity.     

Reversion of the ErbB malignant phenotype and the DNA damage response

The ErbB or HER family is a group of membrane bound tyrosine kinase receptors that initiate signal transduction cascades, which are critical to a wide range of biological processes. When over-expressed or mutated, members of this kinase family form homomeric or heteromeric kinase assemblies that are involved in certain human malignancies. Targeted therapy evolved from studies showing that monoclonal antibodies to the ectodomain of ErbB2/neu would reverse the malignant phenotype. Unfortunately, tumors develop resistance to targeted therapies even when coupled with genotoxic insults such as radiation.