CsA can induce DNA double-strand breaks: implications for BMT regimens particularly for individuals with defective DNA repair

A novel role for the peptidyl-prolyl cis-trans isomerase Cyclophilin A in DNA-repair following replication fork stalling via the MRE11-RAD50-NBS1 complex

Cyclosporin A (CsA) induces DNA double-strand breaks in LIG4 syndrome fibroblasts, specifically upon transit through S-phase. The basis underlying this has not been described. CsA-induced genomic instability may reflect a direct role of Cyclophilin A (CYPA) in DNA repair. CYPA is a peptidyl-prolyl cis-trans isomerase (PPI). CsA inhibits the PPI activity of CYPA. Using an integrated approach involving CRISPR/Cas9-engineering, siRNA, BioID, co-immunoprecipitation, pathway-specific DNA repair investigations as well as protein expression interaction analysis, we describe novel impacts of CYPA loss and inhibition on DNA repair. We characterise a direct CYPA interaction with the NBS1 component of the MRE11-RAD50-NBS1 complex, providing evidence that CYPA influences DNA repair at the level of DNA end resection. We define a set of genetic vulnerabilities associated with CYPA loss and inhibition, identifying DNA replication fork protection as an important determinant of viability. We explore examples of how CYPA inhibition may be exploited to selectively kill cancers sharing characteristic genomic instability profiles, including MYCN-driven Neuroblastoma, Multiple Myeloma and Chronic Myelogenous Leukaemia. These findings propose a repurposing strategy for Cyclophilin inhibitors.

Allogeneic hematopoietic stem cell transplantation corrects ligase IV deficiency

BACKGROUND

Mutations in the DNA ligase IV (LIG4) gene cause a rare autosomal recessive disorder called LIG4 deficiency syndrome. The LIG4 deficiency is featured by severe disorders, including combined immunodeficiency disease, special face ("bird-head-like" face), developmental delays, pancytopenia, and radiosensitivity. Currently there are no curative treatment options except potentially by performing a hematopoietic stem cell transplantation (HSCT).

CASE PRESENTATION

Here we reported the clinical course of a 4 and 1/2-year-old Chinese female with LIG4-deficiency featured with pancytopenia, severe growth retardation (weight of 13.5 kg, < 3rd percentile), length of 100 cm (<2d percentile), head circumference of 46 cm (<3rd percentile), and mild microcephaly. Despite regular IVIG administrations (5 g, once a month), the patient's thrombocytopenia had progressed. Eventually, the patient received HSCT that successfully normalized the LIG4 syndrome associated pancytopenia and corrected the LIG4 mutation. Despite progress the patient succumbed to thrombotic microangiopathy more than 3 months after HSCT.

CONCLUSIONS

This case reports an example of partially successful HSCT as a treatment option for LIG4 syndrome. It is possible that individual factors influence the therapeutic effect of HSCT in LIG4 deficiency.

DNA ligase IV dificiency with elevated serum IgG levels suspected to have myelodysplastic syndrome: a case report

BACKGROUND

Ligase IV (LIG4) dificiency is a very rare clinical syndrome with around 50 cases reported to date. This syndrome is caused by biallelic pathogenic variants in the LIG4 gene, which cause DNA damage repair disorders, mainly manifesting as severe immunodeficiency.

CASE PRESENTATION

We report the case of a 15-month-old male child with pancytopenia, growth retardation, microcephaly, history of vaccine-related rubella, elevated immunoglobulin G, and decreased T- and B lymphocytes. Next-generation sequencing revealed LIG4 pathogenic genes and compound heterozygous mutations, namely the missense mutation c.833G > T (p.Arg278Leu) and deletion mutation c.1271_1275del (p.Lys424Argfs*20).

CONCLUSION

This case suggests that LIG4 dificiency can manifest not only as immunodeficiency but also with increased serum IgG levels and pancytopenia, which constitutes an additional clinical phenotype. Furthermore, this case suggests that LIG4 deficiency should be considered upon differential diagnosis of myelodysplastic syndrome in children.

LIG4 syndrome: clinical and molecular characterization in a Chinese cohort

Background

DNA Ligase IV (LIG4) syndrome is a rare disease with few reports to date. Patients suffer from a broad spectrum of clinical features, including microcephaly, growth retardation, developmental delay, dysmorphic facial features, combined immunodeficiency, and malignancy predisposition. There may be a potential association between genotypes and phenotypes. We investigated the characteristics of LIG4 syndrome in a Chinese cohort.

Results

All seven patients had growth restriction. Most patients (6/7) had significant microcephaly (< − 3 SD). Recurrent bacterial infections of the lungs and intestines were the most common symptoms. One patient had myelodysplastic syndromes. One patient presented with an inflammatory bowel disease (IBD)-like phenotype. Patients presented with combined immunodeficiency. The proportions of naïve CD4+ and naïve CD8+ T cells decreased notably in five patients. All patients harbored compound heterozygous mutations in the LIG4 gene, which consisted of a missense mutation (c.833G > T, p.R278L) and a deletion shift mutation, primarily c.1271_1275delAAAGA (p.K424Rfs*20). Two other deletion mutations, c.1144_1145delCT and c.1277_1278delAA, were novel. Patients with p.K424Rfs*20/p.R278 may have milder dysmorphism but more significant IgA/IgM deficiency compared to the frequently reported genotype p.R814X/p.K424Rfs*20. One patient underwent umbilical cord blood stem cell transplantation (UCBSCT) but died.

Conclusions

The present study reported the clinical and molecular characteristics of a Chinese cohort with LIG4 syndrome, and the results further expand the phenotypic and genotypic spectrum and our understanding of genotype-to-phenotype correlations in LIG4 syndrome.

Allogeneic hematopoietic stem cell transplantation in two brothers with DNA ligase IV deficiency: a case report and review of the literature

BACKGROUND

DNA ligase IV deficiency is a rare autosomal recessive disorder caused by hypomorphic mutations in the DNA ligase IV (LIG4) gene. DNA ligase IV is an essential protein for the development of a healthy immune system as well as for the protection of genomic integrity. Apart from typical stigmata, patients with DNA ligase IV deficiency are characterized by progressive bone marrow failure and a predisposition to malignancy. To our knowledge this reported case is the first description of two brothers with ligase IV deficiency who are treated with different hematopoietic stem cell transplantation (HSCT) regimens resulting in vastly divergent outcomes.

CASE PRESENTATION

The cases of two brothers suffering from severe recurrent infections and growth retardation are described. The laboratory findings showed pancytopenia with significant lymphopenia. The two boys were diagnosed with DNA ligase IV deficiency, associated with severe combined immunodeficiency (SCID). Both patients received HSCT from two different matched unrelated donors (MUD) at the age of 33 and 18 months. The older brother succumbed post-transplant due to fatal side-effects 143 days after allogeneic HSCT. The younger brother - conditioned with a different regimen - received a T cell depleted graft 4 months later. No severe side-effects occurred, neither post-transplant nor in the following years. Ten years after HSCT the patient is well off, living a normal life and attending a regular high school. His immune system is fully reconstituted, resulting in a maximum of T cell receptor (TCR) diversity, which is a prerequisite for immune competence. However, he still suffers from microcephaly, dwarfism and dystrophy.

CONCLUSIONS

This case report gives an example of a successful HSCT as a treatment option in a genetic disorder such as ligase IV deficiency, using a rather mild conditioning regimen. Further studies are required to determine the viability and efficacy of this treatment option.

A Novel Missense LIG4 Mutation in a Patient With a Phenotype Mimicking Behçet's Disease

DNA ligase IV (LIG4) syndrome is a rare autosomal recessive disorder, manifesting with variable immune deficiency, growth failure, predisposition to malignancy, and cellular sensitivity to ionizing radiation. The facial features are subtle and variable, as well. Herein, we described an 18-year-old boy, the first child of consanguineous parents who presented with Behçet's disease (BD)-like phenotype, developmental delay, and dysembryoplastic neuroepithelial tumor (DNET). Whole-exome sequencing revealed a homozygous p.Arg871His (c.2612G > A) mutation in LIG4. To date, 35 cases have been reported with LIG4 syndrome. Peripheral blood mononuclear cells of the patient displayed notable sensitivity to ionizing radiation. Flow cytometric annexin V-propidium iodide (PI) and eFluor670 proliferation assays showed accelerated radiation-induced apoptosis and diminished proliferation, respectively. To our knowledge, this is the first case presenting with a BD-like phenotype. This case provides further evidence that rare monogenic defects could be the underlying cause of atypical presentations of some well-described disorders. Moreover, this clinical report further expands the phenotypical spectrum of LIG4 deficiency.

Failing to Make Ends Meet: The Broad Clinical Spectrum of DNA Ligase IV Deficiency. Case Series and Review of the Literature

DNA repair defects are inborn errors of immunity that result in increased apoptosis and oncogenesis. DNA Ligase 4-deficient patients suffer from a wide range of clinical manifestations since early in life, including: microcephaly, dysmorphic facial features, growth failure, developmental delay, mental retardation; hip dysplasia, and other skeletal malformations; as well as a severe combined immunodeficiency, radiosensitivity, and progressive bone marrow failure; or, they may present later in life with hematological neoplasias that respond catastrophically to chemo- and radiotherapy; or, they could be asymptomatic. We describe the clinical, laboratory, and genetic features of five Mexican patients with LIG4 deficiency, together with a review of 36 other patients available in PubMed Medline. Four out of five of our patients are dead from lymphoma or bone marrow failure, with severe infection and massive bleeding; the fifth patient is asymptomatic despite a persistent CD4+ lymphopenia. Most patients reported in the literature are microcephalic females with growth failure, sinopulmonary infections, hypogammaglobulinemia, very low B-cells, and radiosensitivity; while bone marrow failure and malignancy may develop at a later age. Dysmorphic facial features, congenital hip dysplasia, chronic liver disease, gradual pancytopenia, lymphoma or leukemia, thrombocytopenia, and gastrointestinal bleeding have been reported as well. Most mutations are compound heterozygous, and all of them are hypomorphic, with two common truncating mutations accounting for the majority of patients. Stem-cell transplantation after reduced intensity conditioning regimes may be curative.

A new phosphorylated form of Ku70 identified in resistant leukemic cells confers fast but unfaithful DNA repair in cancer cell lines

Ku70-dependent canonical nonhomologous end-joining (c-NHEJ) DNA repair system is fundamental to the genome maintenance and B-cell lineage. c-NHEJ is upregulated and error-prone in incurable forms of chronic lymphocytic leukemia which also displays telomere dysfunction, multiple chromosomal aberrations and the resistance to DNA damage-induced apoptosis. We identify in these cells a novel DNA damage inducible form of phospho-Ku70. In vitro in different cancer cell lines, Ku70 phosphorylation occurs in a heterodimer Ku70/Ku80 complex within minutes of genotoxic stress, necessitating its interaction with DNA damage-induced kinase pS2056-DNA-PKcs and/or pS1981-ATM. The mutagenic effects of phospho-Ku70 are documented by a defective S/G2 checkpoint, accelerated disappearance of γ-H2AX foci and kinetics of DNA repair resulting in an increased level of genotoxic stress-induced chromosomal aberrations. Together, these data unveil an involvement of phospho-Ku70 in fast but inaccurate DNA repair; a new paradigm linked to both the deregulation of c-NHEJ and the resistance of malignant cells.

Ageing in relation to skeletal muscle dysfunction: redox homoeostasis to regulation of gene expression

Ageing is associated with a progressive loss of skeletal muscle mass, quality and function—sarcopenia, associated with reduced independence and quality of life in older generations. A better understanding of the mechanisms, both genetic and epigenetic, underlying this process would help develop therapeutic interventions to prevent, slow down or reverse muscle wasting associated with ageing. Currently, exercise is the only known effective intervention to delay the progression of sarcopenia. The cellular responses that occur in muscle fibres following exercise provide valuable clues to the molecular mechanisms regulating muscle homoeostasis and potentially the progression of sarcopenia. Redox signalling, as a result of endogenous generation of ROS/RNS in response to muscle contractions, has been identified as a crucial regulator for the adaptive responses to exercise, highlighting the redox environment as a potentially core therapeutic approach to maintain muscle homoeostasis during ageing. Further novel and attractive candidates include the manipulation of microRNA expression. MicroRNAs are potent gene regulators involved in the control of healthy and disease-associated biological processes and their therapeutic potential has been researched in the context of various disorders, including ageing-associated muscle wasting. Finally, we discuss the impact of the circadian clock on the regulation of gene expression in skeletal muscle and whether disruption of the peripheral muscle clock affects sarcopenia and altered responses to exercise. Interventions that include modifying altered redox signalling with age and incorporating genetic mechanisms such as circadian- and microRNA-based gene regulation, may offer potential effective treatments against age-associated sarcopenia.

Novel compound heterozygous DNA ligase IV mutations in an adolescent with a slowly-progressing radiosensitive-severe combined immunodeficiency

We herein describe a case of a 17-year-old boy with intractable common warts, short stature, microcephaly and slowly-progressing pancytopenia. Simultaneous quantification of T-cell receptor recombination excision circles (TREC) and immunoglobulin κ-deleting recombination excision circles (KREC) suggested very poor generation of both T-cells and B-cells. By whole exome sequencing, novel compound heterozygous mutations were identified in the patient's DNA ligase IV (LIG4) gene. The diagnosis of LIG4 syndrome was confirmed by delayed DNA double-strand break repair kinetics in γ-irradiated fibroblasts from the patient and their restoration by an introduction of wild-type LIG4. Although the patient received allogeneic hematopoietic stem cell transplantation from his haploidentical mother, he unfortunately expired due to an insufficiently reconstructed immune system. An earlier definitive diagnosis using TREC/KREC quantification and whole exome sequencing would thereby allow earlier intervention, which would be essential for improving long-term survival in similar cases with slowly-progressing LIG4 syndrome masked in adolescents.

Evaluation of Severe Combined Immunodeficiency and Combined Immunodeficiency Pediatric Patients on the Basis of Cellular Radiosensitivity

Pediatric patients with severe or nonsevere combined immunodeficiency have increased susceptibility to severe, life-threatening infections and, without hematopoietic stem cell transplantation, may fail to thrive. A subset of these patients have the radiosensitive (RS) phenotype, which may necessitate conditioning before hematopoietic stem cell transplantation, and this conditioning includes radiomimetic drugs, which may significantly affect treatment response. To provide statistical criteria for classifying cellular response to ionizing radiation as the measure of functional RS screening, we analyzed the repair capacity and survival of ex vivo irradiated primary skin fibroblasts from five dysmorphic and/or developmentally delayed pediatric patients with severe combined immunodeficiency and combined immunodeficiency. We developed a mathematical framework for the analysis of γ histone 2A isoform X foci kinetics to quantitate DNA-repair capacity, thus establishing crucial criteria for identifying RS. The results, presented in a diagram showing each patient as a point in a 2D RS map, were in agreement with findings from the assessment of cellular RS by clonogenic survival and from the genetic analysis of factors involved in the nonhomologous end-joining repair pathway. We provide recommendations for incorporating into clinical practice the functional assays and genetic analysis used for establishing RS status before conditioning. This knowledge would enable the selection of the most appropriate treatment regimen, reducing the risk for severe therapy-related adverse effects.

Reprint of "The clinical impact of deficiency in DNA non-homologous end-joining"

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.

The clinical impact of deficiency in DNA non-homologous end-joining

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.

Extreme growth failure is a common presentation of ligase IV deficiency

Ligase IV syndrome is a rare differential diagnosis for Nijmegen breakage syndrome owing to a shared predisposition to lympho-reticular malignancies, significant microcephaly, and radiation hypersensitivity. Only 16 cases with mutations in LIG4 have been described to date with phenotypes varying from malignancy in developmentally normal individuals, to severe combined immunodeficiency and early mortality. Here, we report the identification of biallelic truncating LIG4 mutations in 11 patients with microcephalic primordial dwarfism presenting with restricted prenatal growth and extreme postnatal global growth failure (average OFC -10.1 s.d., height -5.1 s.d.). Subsequently, most patients developed thrombocytopenia and leucopenia later in childhood and many were found to have previously unrecognized immunodeficiency following molecular diagnosis. None have yet developed malignancy, though all patients tested had cellular radiosensitivity. A genotype-phenotype correlation was also noted with position of truncating mutations corresponding to disease severity. This work extends the phenotypic spectrum associated with LIG4 mutations, establishing that extreme growth retardation with microcephaly is a common presentation of bilallelic truncating mutations. Such growth failure is therefore sufficient to consider a diagnosis of LIG4 deficiency and early recognition of such cases is important as bone marrow failure, immunodeficiency, and sometimes malignancy are long term sequelae of this disorder.

SCID patients with ARTEMIS vs RAG deficiencies following HCT: increased risk of late toxicity in ARTEMIS-deficient SCID

A subgroup of severe combined immunodeficiencies (SCID) is characterized by lack of T and B cells and is caused by defects in genes required for T- and B-cell receptor gene rearrangement. Several of these genes are also involved in nonhomologous end joining of DNA double-strand break repair, the largest subgroup consisting of patients with T(-)B(-)NK(+)SCID due to DCLRE1C/ARTEMIS defects. We postulated that in patients with ARTEMIS deficiency, early and late complications following hematopoietic cell transplantation might be more prominent compared with patients with T(-)B(-)NK(+)SCID caused by recombination activating gene 1/2 (RAG1/2) deficiencies. We analyzed 69 patients with ARTEMIS and 76 patients with RAG1/2 deficiencies who received transplants from either HLA-identical donors without conditioning or from HLA-nonidentical donors without or with conditioning. There was no difference in survival or in the incidence or severity of acute graft-versus-host disease regardless of exposure to alkylating agents. Secondary malignancies were not observed. Immune reconstitution was comparable in both groups, however, ARTEMIS-deficient patients had a significantly higher occurrence of infections in long-term follow-up. There is a highly significant association between poor growth in ARTEMIS deficiency and use of alkylating agents. Furthermore, abnormalities in dental development and endocrine late effects were associated with alkylation therapy in ARTEMIS deficiency.

The repair and signaling responses to DNA double-strand breaks

A DNA double-strand break (DSB) has long been recognized as a severe cellular lesion, potentially representing an initiating event for carcinogenesis or cell death. The evolution of DSB repair pathways as well as additional processes, such as cell cycle checkpoint arrest, to minimize the cellular impact of DSB formation was, therefore, not surprising. However, the depth and complexity of the DNA damage responses being revealed by current studies were unexpected. Perhaps the most surprising finding to emerge is the dramatic changes to chromatin architecture that arise in the DSB vicinity. In this review, we overview the cellular response to DSBs focusing on DNA repair pathways and the interface between them. We consider additional events which impact upon these DSB repair pathways, including regulated arrest of cell cycle progression and chromatin architecture alterations. Finally, we discuss the impact of defects in these processes to human disease.

Statistical analysis of kinetics, distribution and co-localisation of DNA repair foci in irradiated cells: cell cycle effect and implications for prediction of radiosensitivity

Detection of γ-H2AX foci as a measure of DNA double strand break induction and repair provides the basis of a rapid approach to establish individual radiosensitivity. However, the assignment of criteria to define increased radiosensitivity is not straightforward. Experimental end points, analytical methods and proliferative status of the cells sampled for analysis are important. All these issues are addressed in the present study, which was prompted by a clinical request to assess the radiosensitivity status of an SCID paediatric patient being considered for bone marrow transplantation. We investigated the kinetics of repair of radiation-induced γ-H2AX foci in proliferating and confluent cultures of skin fibroblasts obtained from the patient, and from normal and radiosensitive (Artemis-deficient) controls. As well as the standard approach of averaging foci per cell over the entire population ("standard average"), we also examined foci per cell frequency distributions and calculated average foci per cell values in the major Poisson-distributed subpopulation ("principal average"). This approach allowed to avoid distortions such as that due to the S/G2 population in proliferating cells, with focus numbers approaching twice the normal, and to detect subpopulations of cells with defects in focus formation and repair. From the "standard average" analysis and co-localisation of γ-H2AX foci with 53BP1 we assigned the patient's repair status as close-to-normal. However, analysis of "principal average", foci per cell frequency distributions and survival curves challenged this initial conclusion. These studies indicate new dimensions of the γ-H2AX assay that, with further elaboration and exemplification, have the potential to augment its power to predict radiosensitivity.

A positive feedback signaling loop between ATM and the vitamin D receptor is critical for cancer chemoprevention by vitamin D

Both epidemiologic and laboratory studies have shown the chemopreventive effects of 1α,25-dihydroxyvitamin D(3) (1,25-VD) in tumorigenesis. However, understanding of the molecular mechanism by which 1,25-VD prevents tumorigenesis remains incomplete. In this study, we used an established mouse model of chemical carcinogenesis to investigate how 1,25-VD prevents malignant transformation. In this model, 1,25-VD promoted expression of the DNA repair genes RAD50 and ATM, both of which are critical for mediating the signaling responses to DNA damage. Correspondingly, 1,25-VD protected cells from genotoxic stress and growth inhibition by promoting double-strand break DNA repair. Depletion of the vitamin D receptor (VDR) reduced these genoprotective effects and drove malignant transformation that could not be prevented by 1,25-VD, defining an essential role for VDR in mediating the anticancer effects of 1,25-VD. Notably, genotoxic stress activated ATM and VDR through phosphorylation of VDR. Mutations in VDR at putative ATM phosphorylation sites impaired the ability of ATM to enhance VDR transactivation activity, diminishing 1,25-VD-mediated induction of ATM and RAD50 expression. Together, our findings identify a novel vitamin D-mediated chemopreventive mechanism involving a positive feedback loop between the DNA repair proteins ATM and VDR.

Haploinsufficiency of DNA Damage Response Genes and their Potential Influence in Human Genomic Disorders

Genomic disorders are a clinically diverse group of conditions caused by gain, loss or re-orientation of a genomic region containing dosage-sensitive genes. One class of genomic disorder is caused by hemizygous deletions resulting in haploinsufficiency of a single or, more usually, several genes. For example, the heterozygous contiguous gene deletion on chromosome 22q11.2 causing DiGeorge syndrome involves at least 20-30 genes. Determining how the copy number variation (CNV) affects human variation and contributes to the aetiology and progression of various genomic disorders represents important questions for the future. Here, I will discuss the functional significance of one form of CNV, haploinsufficiency (i.e. loss of a gene copy), of DNA damage response components and its association with certain genomic disorders. There is increasing evidence that haploinsufficiency for certain genes encoding key players in the cells response to DNA damage, particularly those of the Ataxia Telangiectasia and Rad3-related (ATR)-pathway, has a functional impact. I will review this evidence and present examples of some well known clinically similar genomic disorders that have recently been shown to be defective in the ATR-dependent DNA damage response. Finally, I will discuss the potential implications of a haploinsufficiency-induced defective DNA damage response for the clinical management of certain human genomic disorders.

Irradiation induced foci (IRIF) as a biomarker for radiosensitivity

It has long been known that the level of radiosensitivity between individuals covers a considerable range. This range is reflected in analysis of patient cell lines with some cell lines showing significantly reduced sensitivity to in vitro radiation exposure. Our increased exposure to radiation from diagnostic medical procedures and other life style changes has raised concerns that there may be individuals who are at an elevated risk from the harmful impact of acute or chronic low dose radiation exposure. Additionally, a subset of patients show an enhanced normal tissue response following radiotherapy, which can cause significant discomfort and, at the extreme, be life threatening. It has long been realised that the ability to identify sensitive individuals and to understand the mechanistic basis underlying the range of sensitivity within the population is important. A reduced ability to efficiently repair DNA double strand breaks (DSB) and/or activate the DSB damage response underlies some, although not necessarily all, of this sensitivity. In this article, we consider the utility of the recently developed γH2AX foci analysis to provide insight into radiation sensitivity within the population. We consider the nature of sensitivity including the impact of radiation on cell survival, tissue responses and carcinogenesis and the range of responses within the population. We overview the current utility of the γH2AX assay for assessing the efficacy of the DNA damage response to low and high dose radiation and its potential future exploitation.

Radiosensitive severe combined immunodeficiency disease

Inherited defects in components of the nonhomologous end-joining DNA repair mechanism produce a T-B-NK+ severe combined immunodeficiency disease (SCID) characterized by heightened sensitivity to ionizing radiation. Patients with the radiosensitive form of SCID may also have increased short- and long-term sensitivity to the alkylator-based chemotherapy regimens that are traditionally used for conditioning before allogeneic hematopoietic cell transplantation (HCT). Known causes of radiosensitive SCID include deficiencies of Artemis, DNA ligase IV, DNA-dependent protein kinase catalytic subunit, and Cernunnos-XLF, all of which have been treated with HCT. Because of these patients' sensitivity to certain forms of chemotherapy, the approach to donor selection and the type of conditioning regimen used for a patient with radiosensitive SCID requires careful consideration. Significantly more research needs to be done to determine the long-term outcomes of patients with radiosensitive SCID after HCT and to discover novel nontoxic approaches to HCT that might benefit those patients with intrinsic radiosensitivity and chemosensitivity as well as potentially all patients undergoing an HCT.

A novel mutation in a family with DNA ligase IV deficiency syndrome

DNA ligase IV deficiency syndrome (LIG4 syndrome) is a rare autosomal recessive disorder characterized by microcephaly, growth retardation, low birth weight, dysmorphic facial findings, immunodeficiency, pancytopenia, and radiosensitivity due to impaired repair of DNA double-strand breaks by non-homologous end-joining. Herein, we report two siblings with LIG4 syndrome with a novel mutation. One of the siblings, who had normocellular marrow, had autologous reconstitution after initial non-myeloablative conditioning and underwent successful second hematopoietic stem cell transplantation after conditioning with busulfan, cyclophosphamide, and anti-thymocyte globulin. Our findings indicate that transplantation with myeloablative conditioning can be used successfully in LIG4 syndrome patients.

Long-term outcome after hematopoietic stem cell transplantation of a single-center cohort of 90 patients with severe combined immunodeficiency

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for severe combined immunodeficiency (SCID). Detailed assessment of the long-term outcome of HSCT, ie, the occurrence of clinical events and the quality and stability of immune reconstitution, is now required. We performed a single-center retrospective analysis of the long-term outcome of HSCT in 90-patient cohort followed for between 2 and 34 years (median, 14 years). Clinical events and immune reconstitution data were collected. Almost half the patients have experienced one or more significant clinical events, including persistent chronic graft-versus-host disease (GVHD), autoimmune and inflammatory manifestations, opportunistic and nonopportunistic infections, chronic human papilloma virus (HPV) infections, and a requirement for nutritional support. With the notable exception of severe HPV infection, these complications tend to become less common 15 years later after HSCT. A multivariate analysis showed that the occurrence of these events correlated with non-genoidentical donors, diagnosis of Artemis SCID, and quality of immune reconstitution. In most cases, HSCT enables long-term survival with infrequent sequelae. However, the occurrence of relatively late-onset complications is a concern that requires specific means of prevention and justifies careful patient follow-up.