The RIDDLE syndrome protein mediates a ubiquitin-dependent signaling cascade at sites of DNA damage.

The biological response to DNA double-strand breaks acts to preserve genome integrity. Individuals bearing inactivating mutations in components of this response exhibit clinical symptoms that include cellular radiosensitivity, immunodeficiency, and cancer predisposition. The archetype for such disorders is Ataxia-Telangiectasia caused by biallelic mutation in ATM, a central component of the DNA damage response. Here, we report that the ubiquitin ligase RNF168 is mutated in the RIDDLE syndrome, a recently discovered immunodeficiency and radiosensitivity disorder. We show that RNF168 is recruited to sites of DNA damage by binding to ubiquitylated histone H2A. RNF168 acts with UBC13 to amplify the RNF8-dependent histone ubiquitylation by targeting H2A-type histones and by promoting the formation of lysine 63-linked ubiquitin conjugates. These RNF168-dependent chromatin modifications orchestrate the accumulation of 53BP1 and BRCA1 to DNA lesions, and their loss is the likely cause of the cellular and developmental phenotypes associated with RIDDLE syndrome.

Integrative analysis of spontaneous CLL regression highlights genetic and microenvironmental interdependency in CLL.

Spontaneous regression is a recognized phenomenon in chronic lymphocytic leukemia (CLL) but its biological basis remains unknown. We undertook a detailed investigation of the biological and clinical features of 20 spontaneous CLL regression cases incorporating phenotypic, functional, transcriptomic, and genomic studies at sequential time points. All spontaneously regressed tumors were IGHV-mutated with no restricted IGHV usage or B-cell receptor (BCR) stereotypy. They exhibited shortened telomeres similar to nonregressing CLL, indicating prior proliferation. They also displayed low Ki-67, CD49d, cell-surface immunoglobulin M (IgM) expression and IgM-signaling response but high CXCR4 expression, indicating low proliferative activity associated with poor migration to proliferation centers, with these features becoming increasingly marked during regression. Spontaneously regressed CLL displayed a transcriptome profile characterized by downregulation of metabolic processes as well as MYC and its downstream targets compared with nonregressing CLL. Moreover, spontaneous regression was associated with reversal of T-cell exhaustion features including reduced programmed cell death 1 expression and increased T-cell proliferation. Interestingly, archetypal CLL genomic aberrations including HIST1H1B and TP53 mutations and del(13q14) were found in some spontaneously regressing tumors, but genetic composition remained stable during regression. Conversely, a single case of CLL relapse following spontaneous regression was associated with increased BCR signaling, CLL proliferation, and clonal evolution. These observations indicate that spontaneously regressing CLL appear to undergo a period of proliferation before entering a more quiescent state, and that a complex interaction between genomic alterations and the microenvironment determines disease course. Together, the findings provide novel insight into the biological processes underpinning spontaneous CLL regression, with implications for CLL treatment.

Genotype, extrapyramidal features, and severity of variant ataxia-telangiectasia.

OBJECTIVE: Variant ataxia-telangiectasia is caused by mutations that allow some retained ataxia telangiectasia-mutated (ATM) kinase activity. Here, we describe the clinical features of the largest established cohort of individuals with variant ataxia-telangiectasia and explore genotype-phenotype correlations. METHODS: Cross-sectional data were collected retrospectively. Patients were classified as variant ataxia-telangiectasia based on retained ATM kinase activity. RESULTS: The study includes 57 individuals. Mean age at assessment was 37.5 years. Most had their first symptoms by age 10 (81%). There was a diagnostic delay of more than 10 years in 68% and more than 20 years in one third of probands. Disease severity was mild in one third of patients, and 43% were still ambulant 20 years after disease onset. Only one third had predominant ataxia, and 18% had a pure extrapyramidal presentation. Individuals with extrapyramidal presentations had milder neurological disease severity. There were no significant respiratory or immunological complications, but 25% of individuals had a history of malignancy. Missense mutations were associated with milder neurological disease severity, but with a higher risk of malignancy, compared to leaky splice site mutations. INTERPRETATION: Individuals with variant ataxia-telangiectasia require malignancy surveillance and tailored management. However, our data suggest the condition may sometimes be mis- or underdiagnosed because of atypical features, including exclusive extrapyramidal symptoms, normal eye movements, and normal alpha-fetoprotein levels in some individuals. Missense mutations are associated with milder neurological presentations, but a particularly high malignancy risk, and it is important for clinicians to be aware of these phenotypes. ANN NEUROL 2019;85:170-180.

Regulation of DNA-end resection by hnRNPU-like proteins promotes DNA double-strand break signaling and repair.

DNA double-strand break (DSB) signaling and repair are critical for cell viability, and rely on highly coordinated pathways whose molecular organization is still incompletely understood. Here, we show that heterogeneous nuclear ribonucleoprotein U-like (hnRNPUL) proteins 1 and 2 play key roles in cellular responses to DSBs. We identify human hnRNPUL1 and -2 as binding partners for the DSB sensor complex MRE11-RAD50-NBS1 (MRN) and demonstrate that hnRNPUL1 and -2 are recruited to DNA damage in an interdependent manner that requires MRN. Moreover, we show that hnRNPUL1 and -2 stimulate DNA-end resection and promote ATR-dependent signaling and DSB repair by homologous recombination, thereby contributing to cell survival upon exposure to DSB-inducing agents. Finally, we establish that hnRNPUL1 and -2 function downstream of MRN and CtBP-interacting protein (CtIP) to promote recruitment of the BLM helicase to DNA breaks. Collectively, these results provide insights into how mammalian cells respond to DSBs.

USP7 inhibition alters homologous recombination repair and targets CLL cells independently of ATM/p53 functional status.

The role of deubiquitylase ubiquitin-specific protease 7 (USP7) in the regulation of the p53-dependent DNA damage response (DDR) pathway is well established. Whereas previous studies have mostly focused on the mechanisms underlying how USP7 directly controls p53 stability, we recently showed that USP7 modulates the stability of the DNA damage responsive E3 ubiquitin ligase RAD18. This suggests that targeting USP7 may have therapeutic potential even in tumors with defective p53 or ibrutinib resistance. To test this hypothesis, we studied the effect of USP7 inhibition in chronic lymphocytic leukemia (CLL) where the ataxia telangiectasia mutated (ATM)-p53 pathway is inactivated with relatively high frequency, leading to treatment resistance and poor clinical outcome. We demonstrate that USP7 is upregulated in CLL cells, and its loss or inhibition disrupts homologous recombination repair (HRR). Consequently, USP7 inhibition induces significant tumor-cell killing independently of ATM and p53 through the accumulation of genotoxic levels of DNA damage. Moreover, USP7 inhibition sensitized p53-defective, chemotherapy-resistant CLL cells to clinically achievable doses of HRR-inducing chemotherapeutic agents in vitro and in vivo in a murine xenograft model. Together, these results identify USP7 as a promising therapeutic target for the treatment of hematological malignancies with DDR defects, where ATM/p53-dependent apoptosis is compromised.

A Hypomorphic PALB2 Allele Gives Rise to an Unusual Form of FA-N Associated with Lymphoid Tumour Development.

Patients with biallelic truncating mutations in PALB2 have a severe form of Fanconi anaemia (FA-N), with a predisposition for developing embryonal-type tumours in infancy. Here we describe two unusual patients from a single family, carrying biallelic PALB2 mutations, one truncating, c.1676_1677delAAinsG;(p.Gln559ArgfsTer2), and the second, c.2586+1G>A; p.Thr839_Lys862del resulting in an in frame skip of exon 6 (24 amino acids). Strikingly, the affected individuals did not exhibit the severe developmental defects typical of FA-N patients and initially presented with B cell non-Hodgkin lymphoma. The expressed p.Thr839_Lys862del mutant PALB2 protein retained the ability to interact with BRCA2, previously unreported in FA-N patients. There was also a large increased chromosomal radiosensitivity following irradiation in G2 and increased sensitivity to mitomycin C. Although patient cells were unable to form Rad51 foci following exposure to either DNA damaging agent, U2OS cells, in which the mutant PALB2 with in frame skip of exon 6 was induced, did show recruitment of Rad51 to foci following damage. We conclude that a very mild form of FA-N exists arising from a hypomorphic PALB2 allele.

Targeting the Ataxia Telangiectasia Mutated-null phenotype in chronic lymphocytic leukemia with pro-oxidants.

Inactivation of the Ataxia Telangiectasia Mutated gene in chronic lymphocytic leukemia results in resistance to p53-dependent apoptosis and inferior responses to treatment with DNA damaging agents. Hence, p53-independent strategies are required to target Ataxia Telangiectasia Mutated-deficient chronic lymphocytic leukemia. As Ataxia Telangiectasia Mutated has been implicated in redox homeostasis, we investigated the effect of the Ataxia Telangiectasia Mutated-null chronic lymphocytic leukemia genotype on cellular responses to oxidative stress with a view to therapeutic targeting. We found that in comparison to Ataxia Telangiectasia Mutated-wild type chronic lymphocytic leukemia, pro-oxidant treatment of Ataxia Telangiectasia Mutated-null cells led to reduced binding of NF-E2 p45-related factor-2 to antioxidant response elements and thus decreased expression of target genes. Furthermore, Ataxia Telangiectasia Mutated-null chronic lymphocytic leukemia cells contained lower levels of antioxidants and elevated mitochondrial reactive oxygen species. Consequently, Ataxia Telangiectasia Mutated-null chronic lymphocytic leukemia, but not tumors with 11q deletion or TP53 mutations, exhibited differentially increased sensitivity to pro-oxidants both in vitro and in vivo. We found that cell death was mediated by a p53- and caspase-independent mechanism associated with apoptosis inducing factor activity. Together, these data suggest that defective redox-homeostasis represents an attractive therapeutic target for Ataxia Telangiectasia Mutated-null chronic lymphocytic leukemia.

Identification of the first ATRIP-deficient patient and novel mutations in ATR define a clinical spectrum for ATR-ATRIP Seckel Syndrome.

A homozygous mutational change in the Ataxia-Telangiectasia and RAD3 related (ATR) gene was previously reported in two related families displaying Seckel Syndrome (SS). Here, we provide the first identification of a Seckel Syndrome patient with mutations in ATRIP, the gene encoding ATR-Interacting Protein (ATRIP), the partner protein of ATR required for ATR stability and recruitment to the site of DNA damage. The patient has compound heterozygous mutations in ATRIP resulting in reduced ATRIP and ATR expression. A nonsense mutational change in one ATRIP allele results in a C-terminal truncated protein, which impairs ATR-ATRIP interaction; the other allele is abnormally spliced. We additionally describe two further unrelated patients native to the UK with the same novel, heterozygous mutations in ATR, which cause dramatically reduced ATR expression. All patient-derived cells showed defective DNA damage responses that can be attributed to impaired ATR-ATRIP function. Seckel Syndrome is characterised by microcephaly and growth delay, features also displayed by several related disorders including Majewski (microcephalic) osteodysplastic primordial dwarfism (MOPD) type II and Meier-Gorlin Syndrome (MGS). The identification of an ATRIP-deficient patient provides a novel genetic defect for Seckel Syndrome. Coupled with the identification of further ATR-deficient patients, our findings allow a spectrum of clinical features that can be ascribed to the ATR-ATRIP deficient sub-class of Seckel Syndrome. ATR-ATRIP patients are characterised by extremely severe microcephaly and growth delay, microtia (small ears), micrognathia (small and receding chin), and dental crowding. While aberrant bone development was mild in the original ATR-SS patient, some of the patients described here display skeletal abnormalities including, in one patient, small patellae, a feature characteristically observed in Meier-Gorlin Syndrome. Collectively, our analysis exposes an overlapping clinical manifestation between the disorders but allows an expanded spectrum of clinical features for ATR-ATRIP Seckel Syndrome to be defined.

Classical ataxia telangiectasia patients have a congenitally aged immune system with high expression of CD95.

Ataxia-telangiectasia (A-T) is a rare neurodegenerative immunodeficiency disorder caused by mutations in the ataxia telangiectasia mutated gene. Patients commonly have lymphopenia and Ig-production abnormalities. We used multicolor flow cytometry and IL-7 ELISA to investigate the effect of A-T and age on the proportions of major lymphocyte subsets and their pattern of CD95 expression in relation to IL-7 levels in 15 classical A-T patients. We also analyzed the sensitivity of T cells from four classical A-T patients to CD95-mediated apoptosis using TUNEL and caspase-activation assays. Our results confirmed lymphopenia and a deficiency in naive T and B cells in A-T patients. In contrast to controls, the proportions of naive and memory T and B cell subsets in A-T patients did not vary in relation to age. There was no evidence of a deficiency in plasma IL-7 or IL-7R expression, and IL-7 concentration correlated positively with CD95 expression on CD4(+) T cells. CD95 expression on unstimulated A-T lymphocytes was high, and the apoptotic sensitivity of activated naive and central memory T cells was increased. These findings show that the immunodeficiency in A-T patients may be described as congenitally aged and is not progressive. The naive cell deficiency is not related to a deficiency in IL-7 or its receptor. However, IL-7 may upregulate CD95 on A-T lymphocytes. High CD95 expression and increased apoptotic sensitivity of activated naive and central memory T cells may result in an increased level of CD95-mediated apoptosis, which could contribute to the congenital lymphopenia in A-T.

Mre11 modulates the fidelity of fusion between short telomeres in human cells.

The loss of telomere function can result in the fusion of telomeres with other telomeric loci, or non-telomeric double-stranded DNA breaks. Sequence analysis of fusion events between short dysfunctional telomeres in human cells has revealed that fusion is characterized by a distinct molecular signature consisting of extensive deletions and micro-homology at the fusion points. This signature is consistent with alternative error-prone end-joining processes. We have examined the role that Mre11 may play in the fusion of short telomeres in human cells; to do this, we have analysed telomere fusion events in cells derived from ataxia-telangiectasia-like disorder (ATLD) patients that exhibit hypomorphic mutations in MRE11. The telomere dynamics of ATLD fibroblasts were indistinguishable from wild-type fibroblasts and they were proficient in the fusion of short telomeres. However, we observed a high frequency of insertion of DNA sequences at the fusion points that created localized sequence duplications. These data indicate that Mre11 plays a role in the fusion of short dysfunctional telomeres in human cells and are consistent with the hypothesis that as part of the MRN complex it serves to stabilize the joining complex, thereby controlling the fidelity of the fusion reaction.

Very mild presentation in adult with classical cellular phenotype of ataxia telangiectasia.

BACKGROUND: The major clinical feature of ataxia telangiectasia (A-T) is severe progressive neurodegeneration with onset in infancy. This classical A-T phenotype is caused by biallelic null mutations in the ATM gene, leading to the absence of ATM protein and increased cellular radiosensitivity. We report an unusual case of A-T in a 41-year-old mother, A-T210, who had very mild neurological symptoms despite complete loss of ATM protein. METHODS: A neurological examination was performed, cellular radiosensitivity was assessed, and the ATM gene was sequenced. Skin fibroblasts and a lymphoblastoid cell line (LCL) were assayed for ATM protein expression and kinase activity. RESULTS: Patient A-T210 showed mild chorea, dystonia, and gait ataxia, walked independently, and drove a car. LCL and skin fibroblasts were radiosensitive and did not express ATM protein. Two ATM-null mutations were identified. CONCLUSIONS: The severe neurodegeneration resulting from loss of ATM can be mitigated in some circumstances.

Adenovirus 12 E4orf6 inhibits ATR activation by promoting TOPBP1 degradation.

Activation of the cellular DNA damage response is detrimental to adenovirus (Ad) infection. Ad has therefore evolved a number of strategies to inhibit ATM- and ATR-dependent signaling pathways during infection. Recent work suggests that the Ad5 E4orf3 protein prevents ATR activation through its ability to mislocalize the MRN complex. Here we provide evidence to indicate that Ad12 has evolved a different strategy from Ad5 to inhibit ATR. We show that Ad12 utilizes a CUL2/RBX1/elongin C-containing ubiquitin ligase to promote the proteasomal degradation of the ATR activator protein topoisomerase-IIbeta-binding protein 1 (TOPBP1). Ad12 also uses this complex to degrade p53 during infection, in contrast to Ad5, which requires a CUL5-based ubiquitin ligase. Although Ad12-mediated degradation of p53 is dependent upon both E1B-55K and E4orf6, Ad12-mediated degradation of TOPBP1 is solely dependent on E4orf6. We propose that Ad12 E4orf6 has two principal activities: to recruit the CUL2-based ubiquitin ligase and to act as substrate receptor for TOPBP1. In support of the idea that Ad12 E4orf6 specifically prevents ATR activation during infection by targeting TOPBP1 for degradation, we demonstrate that Ad12 E4orf6 can inhibit the ATR-dependent phosphorylation of CHK1 in response to replication stress. Taken together, these data provide insights into how Ad modulates ATR signaling pathways during infection.

The PARP inhibitor olaparib induces significant killing of ATM-deficient lymphoid tumor cells in vitro and in vivo.

The Ataxia Telangiectasia Mutated (ATM) gene is frequently inactivated in lymphoid malignancies such as chronic lymphocytic leukemia (CLL), T-prolymphocytic leukemia (T-PLL), and mantle cell lymphoma (MCL) and is associated with defective apoptosis in response to alkylating agents and purine analogues. ATM mutant cells exhibit impaired DNA double strand break repair. Poly (ADP-ribose) polymerase (PARP) inhibition that imposes the requirement for DNA double strand break repair should selectively sensitize ATM-deficient tumor cells to killing. We investigated in vitro sensitivity to the poly (ADP-ribose) polymerase inhibitor olaparib (AZD2281) of 5 ATM mutant lymphoblastoid cell lines (LCL), an ATM mutant MCL cell line, an ATM knockdown PGA CLL cell line, and 9 ATM-deficient primary CLLs induced to cycle and observed differential killing compared with ATM wildtype counterparts. Pharmacologic inhibition of ATM and ATM knockdown confirmed the effect was ATM-dependent and mediated through mitotic catastrophe independently of apoptosis. A nonobese diabetic/severe combined immunodeficient (NOD/SCID) murine xenograft model of an ATM mutant MCL cell line demonstrated significantly reduced tumor load and an increased survival of animals after olaparib treatment in vivo. Addition of olaparib sensitized ATM null tumor cells to DNA-damaging agents. We suggest that olaparib would be an appropriate agent for treating refractory ATM mutant lymphoid tumors.

Premature ageing of the immune system underlies immunodeficiency in ataxia telangiectasia.

ATM kinase modulates pathways implicated in premature ageing and ATM genotype predicts survival, yet immunodeficiency in ataxia telangiectasia is regarded as mild and unrelated to age. We address this paradox in a molecularly characterised sequential adult cohort with classical and mild variant ataxia telangiectasia. Immunodeficiency has the characteristics of premature ageing across multiple cellular and molecular immune parameters. This immune ageing occurs without previous CMV infection. Age predicts immunodeficiency in genetically homogeneous ataxia telangiectasia, and in comparison with controls, calendar age is exceeded by immunological age defined by thymic naïve CD4+ T cell levels. Applying ataxia telangiectasia as a model of immune ageing, pneumococcal vaccine responses, characteristically deficient in physiological ageing, are predicted by thymic naïve CD4+ T cell levels. These data suggest inherited defects of DNA repair may provide valuable insight into physiological ageing. Thymic naïve CD4+ T cells may provide a biomarker for vaccine responsiveness in elderly cohorts.

Independent and sequential recruitment of NHEJ and HR factors to DNA damage sites in mammalian cells.

Damage recognition by repair/checkpoint factors is the critical first step of the DNA damage response. DNA double strand breaks (DSBs) activate checkpoint signaling and are repaired by nonhomologous end-joining (NHEJ) and homologous recombination (HR) pathways. However, in vivo kinetics of the individual factor responses and the mechanism of pathway choice are not well understood. We report cell cycle and time course analyses of checkpoint activation by ataxia-telangiectasia mutated and damage site recruitment of the repair factors in response to laser-induced DSBs. We found that MRN acts as a DNA damage marker, continuously localizing at unrepaired damage sites. Damage recognition by NHEJ factors precedes that of HR factors. HR factor recruitment is not influenced by NHEJ factor assembly and occurs throughout interphase. Damage site retention of NHEJ factors is transient, whereas HR factors persist at unrepaired lesions, revealing unique roles of the two pathways in mammalian cells.

Modeling ATM mutant proteins from missense changes confirms retained kinase activity.

Ataxia-telangiectasia mutated (ATM) is the gene mutated in the cancer-predisposing disorder ataxia-telangiectasia (A-T). We modeled ATM sequence variants identified in UK A-T patients to determine the stability and kinase activity of the resulting proteins as well as the distribution of these mutations across the coding region. Of 20 missense changes modeled, 10 proteins showed ATM kinase activity and 10 showed none. In the majority of cases the mutant ATM protein was unstable, although this was variable. Reduction in ATM kinase activity can result either from the presence of low levels of unstable mutant protein with relatively normal specific kinase activity or from stable mutant protein with deficient ATM kinase activation. Indeed, ATM mutant proteins without kinase activity toward downstream targets were still able to autophosphorylate on serine 1981, although in a much less efficient manner, suggesting that this was not sufficient for ATM activation. In terms of function, green fluorescent protein (GFP)-tagged kinase inactive ATM proteins could form ionizing radiation (IR)-induced foci (IRIF), at least temporarily, which colocalized with the DNA double-strand break (DSB) marker gammaH2AX. Consistent with this, both kinase active and inactive mutant ATM proteins were able to interfere with phosphorylation of targets by endogenous ATM. Since the majority of missense mutations occurred C-terminal to aa1966, including all 10 mutations with absence of kinase activity, the implication was that mutations N-terminal to this, with exceptions, are less likely to result in loss of kinase activity and therefore, are less likely to be identified in A-T patients.

A role for E1B-AP5 in ATR signaling pathways during adenovirus infection.

E1B-55K-associated protein 5 (E1B-AP5) is a cellular, heterogeneous nuclear ribonucleoprotein that is targeted by adenovirus (Ad) E1B-55K during infection. The function of E1B-AP5 during infection, however, remains largely unknown. Given the role of E1B-55K targets in the DNA damage response, we examined whether E1B-AP5 function was integral to these pathways. Here, we show a novel role for E1B-AP5 as a key regulator of ATR signaling pathways activated during Ad infection. E1B-AP5 is recruited to viral replication centers during infection, where it colocalizes with ATR-interacting protein (ATRIP) and the ATR substrate replication protein A 32 (RPA32). Indeed, E1B-AP5 associates with ATRIP and RPA complex component RPA70 in both uninfected and Ad-infected cells. Additionally, glutathione S-transferase pull-downs show that E1B-AP5 associates with RPA components RPA70 and RPA32 directly in vitro. E1B-AP5 is required for the ATR-dependent phosphorylation of RPA32 during infection and contributes to the Ad-induced phosphorylation of Smc1 and H2AX. In this regard, it is interesting that Ad5 and Ad12 differentially promote the phosphorylation of RPA32, Rad9, and Smc1 during infection such that Ad12 promotes a significant phosphorylation of RPA32 and Rad9, whereas Ad5 only weakly promotes RPA32 phosphorylation and does not induce Rad9 phosphorylation. These data suggest that Ad5 and Ad12 have evolved different strategies to regulate DNA damage signaling pathways during infection in order to promote viral replication. Taken together, our results define a role for E1B-AP5 in ATR signaling pathways activated during infection. This might have broader implications for the regulation of ATR activity during cellular DNA replication or in response to DNA damage.

Prominent oromandibular dystonia and pharyngeal telangiectasia in atypical ataxia telangiectasia.

Ataxia telangiectasia (A-T) typically presents with early-onset progressive cerebellar ataxia, oculomotor apraxia and later, oculo-cutaneous telangiectasia. Extrapyramidal symptoms, apart from chorea, are rare. In this paper, we report a case of A-T with an atypically mild and slowly progressive disease course. Although by history there was mild gait clumsiness in early childhood, the leading problem was that of dystonia with onset at age 15, in the absence of gross gait imbalance or ocular motor apraxia. Dystonia was generalized and with prominent oromandibular involvement. Unusually, a leash of telangiectasia was present on the posterior pharyngeal wall, while other features frequently associated with A-T were absent.

Chromosome instability syndromes.

Fanconi anaemia (FA), ataxia telangiectasia (A-T), Nijmegen breakage syndrome (NBS) and Bloom syndrome (BS) are clinically distinct, chromosome instability (or breakage) disorders. Each disorder has its own pattern of chromosomal damage, with cells from these patients being hypersensitive to particular genotoxic drugs, indicating that the underlying defect in each case is likely to be different. In addition, each syndrome shows a predisposition to cancer. Study of the molecular and genetic basis of these disorders has revealed mechanisms of recognition and repair of DNA double-strand breaks, DNA interstrand crosslinks and DNA damage during DNA replication. Specialist clinics for each disorder have provided the concentration of expertise needed to tackle their characteristic clinical problems and improve outcomes. Although some treatments of the consequences of a disorder may be possible, for example, haematopoietic stem cell transplantation in FA and NBS, future early intervention to prevent complications of disease will depend on a greater understanding of the roles of the affected DNA repair pathways in development. An important realization has been the predisposition to cancer in carriers of some of these gene mutations.